M e d i c a l P hy s i c s a n d I n f o r m a t i c s • O r i g i n a l R e s e a r c h
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Yamada et al. CT Dose Reduction for Intraabdominal Fat Measurement Medical Physics and Informatics Original Research
CT Dose Reduction for Visceral Adipose Tissue Measurement: Effects of Model-Based and Adaptive Statistical Iterative Reconstructions and Filtered Back Projection
Yoshitake Yamada1 Masahiro Jinzaki1 Yuki Niijima1 Masahiro Hashimoto1 Minoru Yamada2 Takayuki Abe 3 Sachio Kuribayashi1
OBJECTIVE. The objective of our study was to evaluate the effects of radiation dose reduction and the reconstruction algorithm used—filtered back projection (FBP), adaptive statistical iterative reconstruction (ASIR), or model-based iterative reconstruction (MBIR)—on the measurement of abdominal visceral fat using CT. SUBJECTS AND METHODS. Standard-dose and low-dose abdominal CT examinations were performed simultaneously with automatic exposure control in 59 patients; the noise index for a 5-mm slice thickness was 12 for routine-dose CT and 24 for low-dose CT. The routine-dose CT images were reconstructed using FBP (reference standard), and the low-dose CT images were reconstructed using FBP, ASIR (so-called hybrid iterative reconstruction [IR]), and MBIR (so-called pure IR). In the 236 image series obtained, the visceral fat area was measured. Data were analyzed by the Pearson correlation coefficient test and a Bland-Altman difference analysis. RESULTS. The radiation dose of the low-dose abdominal CT examinations was 73.0% (mean) lower than that of routine-dose CT examinations. Excellent correlations were observed between the visceral fat areas measured on the routine-dose FBP images and those measured on the low-dose FBP, low-dose ASIR, and low-dose MBIR images (r = 0.998, 0.998, and 0.998, respectively; p < 0.001). A Bland-Altman difference analysis revealed excellent agreements, with mean biases of –0.47, –0.41, and 0.18 cm2 for the visceral fat area between the routine-dose FBP images and the low-dose FBP, low-dose ASIR, and low-dose MBIR images, respectively. CONCLUSION. A 73.0% reduction of the radiation dose would be possible in CT for the measurement of the abdominal visceral fat regardless of which reconstruction algorithm is used (i.e., FBP, hybrid IR, or pure IR).
Yamada Y, Jinzaki M, Niijima Y, et al.
Keywords: abdomen, intraabdominal fat, MDCT, radiation dose DOI:10.2214/AJR.14.13411 Received July 3, 2014; accepted after revision October 7, 2014. M. Jinzaki and S. Kuribayashi received a research grant from GE Healthcare Japan. The remaining authors had complete unrestricted access to the study data at all stages of the study. Supported in part by the Japan Society for the Promotion of Science (JSPS KAKENHI grant no. 26861016). 1 Department of Diagnostic Radiology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. Address correspondence to M. Jinzaki ([email protected]). 2 Multidimension Biomedical Imaging & Information Laboratory in Research Park, Keio University School of Medicine, Tokyo, Japan. 3 Department of Preventive Medicine and Public Health, Center for Clinical Research, Keio University School of Medicine, Tokyo, Japan.
WEB This is a web exclusive article. AJR 2015; 204:W677–W683 0361–803X/15/2046–W677 © American Roentgen Ray Society
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ody fat tissue is traditionally distributed in two main compartments with different metabolic characteristics: the visceral adipose tissue and subcutaneous adipose tissue. Several studies have implicated abdominal visceral adipose tissue in the development of various metabolic abnormalities [1]; coronary artery disease [2, 3]; and cancers of the colon [4], breast [5], and prostate [6]. In addition, investigators have reported associations between abdominal visceral adipose tissue and coronary artery calcium score [7]; systemic markers of inflammation [8]; and predisposition to infections and noninfectious complications, greater length of hospital stay, and higher hospital mortality [9]. The reference standard modality for the quantitative measurement of the abdominal
visceral fat area is CT [10, 11]. MRI can be also used to assess abdominal fat distribution [11, 12]; however, the use of MRI is limited by its poor accessibility and high cost as compared with CT [10]. Providing excellent resolution for adipose tissue, CT represents a direct method for assessing the visceral fat deposition in both adult and pediatric populations [10]. On the other hand, the exposure to ionizing radiation limits its broad application in healthy subjects given that the widespread use of CT around the world has raised concern about the increase in the risk of cancer from medical radiation exposure [13]. Because radiation-induced carcinogenesis is a stochastic effect, the risk may be expected to decrease with a decrease of the radiation dose. Several approaches have been attempted to reduce the radiation dose to patients dur-
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Yamada et al. ing CT, such as the reduction of fixed tube current, use of automatic exposure control, reduction of peak kilovoltage, use of noise reduction filters, and use of a high helical pitch [14–18]; furthermore, the use of the recently introduced iterative reconstruction (IR) techniques has been shown to have the potential to replace the conventionally used filtered back projection (FBP) algorithm [19–23]. To our knowledge, no prospective study to date has conducted intraindividual comparisons of the visceral adipose areas measured between routine-dose CT and low-dose CT performed simultaneously using automatic exposure control and a standard tube voltage of 120 kVp. Furthermore, no studies have evaluated the effects of different reconstruction algorithms, including FBP, adaptive statistical iterative reconstruction (ASIR) (socalled hybrid IR), and model-based iterative reconstruction (MBIR) (so-called pure IR), on the quantitative measurement of visceral adipose tissue. The purpose of this study was to evaluate the effects of radiation dose reduction and the reconstruction algorithm used (FBP, ASIR, or MBIR) on the measurement of abdominal visceral fat by CT. Subjects and Methods Patients This study was conducted with the approval of our institutional review board, and written informed consent was obtained from each of the participating patients. From April 2011 to July 2011, 63 consecutive patients referred for unenhanced abdominal CT on a specific scanner were considered for inclusion in this prospective study. Patients were excluded if they were younger than 50 years old, were pregnant or potentially pregnant, or were lactating or if they did not provide written informed consent; four of the 63 patients did not want to participate in the study and refused to provide informed consent. Therefore, images of the remaining 59 patients (age range, 50–83 years; mean age, 66.5 ± 8.4 years) were included in the final analysis. The study group was composed of 30 men (age range, 56–80 years; mean age, 68.7 ± 7.8 years) and 29 women (age range, 50–83 years; mean age, 64.2 ± 8.5 years). The clinical indications for abdominal CT of the subjects in this study were staging or restaging of known malignancy (n = 47); diarrhea (n = 2); back pain (n = 2); chronic abdominal pain (n = 1); and follow-up for renal or ureteral stones (n = 2), giant hepatic cyst (n = 1), gallstones (n = 1), pancreatitis (n = 1), inflammatory bowel disease (n = 1), and abdominal aortic aneurysm (n = 1). The height and
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weight of the patients were measured, and the body mass index (BMI) was calculated as weight in kilograms divided by height squared in meters.
Imaging Protocol All unenhanced abdominal CT examinations were performed on the same scanner (Discovery CT750 HD scanner, GE Healthcare). All the patients were scanned in the craniocaudal direction while they were lying in the supine position. According to a previous report, because radiation dose decreases to 1 / x, the image noise increases by the square root of x [24]; thus, when the radiation dose is reduced to one quarter, the noise may be expected to increase by the square root of 4 (i.e., 2). We used this formula to calculate the probable noise index that would result in a dose reduction of approximately 75% [25]. Our standard noise index for a slice thickness of 5 mm is 12. Therefore, we considered that, theoretically, a noise index of 24 (12 × 2) would yield a dose reduction of approximately 75%. The noise index settings (12 and 24) were based on our initial experiences in the clinical setting, our phantom data, and vendor recommendations so that a dose reduction was expected without a loss of diagnostic information. Two helical acquisitions were obtained for each patient with the same length of helical run and same FOV to obtain two datasets for the abdomen including the top of the diaphragm and the level of the umbilicus. The first acquisition, which was routine-dose CT, was performed with automatic exposure control (tube current modulation) using a noise index of 12 for a slice thickness of 5 mm while the patient was breath-holding during inspiration. The second acquisition, which was low-dose CT, was performed with automatic exposure control using a noise index of 24 (for a slice thickness of 5 mm) while the patient was breath-holding during another inspiration. The tube current range for the routine-dose and low-dose CT acquisitions ranged from a minimum of 10 mA to a maximum of 750 mA. The other scanning parameters were the same for both the routine-dose and low-dose CT acquisitions: peak tube voltage, 120 kVp; rotation speed, 0.4 second; detector collimation, 0.625 × 64 mm; table feed, 55 mm/rotation; pitch factor, 1.375. Thus, we obtained two files of raw data of the same size for each patient.
Image Reconstruction The series of contiguous 5-mm-thick routinedose CT images were reconstructed with FBP using the standard kernel; these images served as the reference standard. The series of contiguous 5-mm-thick low-dose CT images were reconstructed using FBP, ASIR, MBIR with the stan-
dard kernel. Thus, the same kernels were used for routine-dose and low-dose CT data. We used 50% ASIR, which means that 50% of the ASIR image was blended with the FBP image, to obtain the low-dose ASIR images; we selected this ASIR level on the basis of previous studies [26, 27]. Finally, we obtained 236 sets of 5-mm-thick images—that is, 59 image sets of each reconstruction algorithm (i.e., routine-dose FBP, low-dose FBP, low-dose ASIR, and low-dose MBIR) (Fig. 1).
Objective Measurements Two board-certified radiologists, with 7 and 22 years of experience in interpreting abdominal CT, independently measured the visceral and subcutaneous fat (adipose) areas at the level of the umbilicus in low-dose FBP, low-dose ASIR, and low-dose MBIR image series using the body fat analysis function at an independent workstation (Advantage workstation 4.5, GE Healthcare). They measured the areas regardless of the movement of the intestines between the routine-dose and low-dose CT acquisitions, as described previously [3, 11, 28, 29]. One of the two radiologists (the one with 7 years of experience in interpreting abdominal CT) measured the visceral and subcutaneous fat areas twice to evaluate intraobserver agreement; the time interval between the first and second measurements was 4 weeks. The same two radiologists measured the visceral and subcutaneous fat areas at the level of umbilicus by consensus in the routine-dose FBP images, which served as the reference standard. The abdominal muscular wall separating the visceral compartment from the subcutaneous compartment was traced manually (Fig. 1E). The CT attenuation of fat was defined as ranging from –190 to –30 HU, as described previously [4, 11, 28–30]. Furthermore, the objective image noise (i.e., SD of the CT number) was measured in all 236 image series. The same two board-certified radiologists placed, by consensus, circular or ovoid ROIs in the subcutaneous fat in the anterior abdominal wall (midline), retroperitoneal fat (right side), abdominal aorta, and psoas major muscle (right side) at the level of umbilicus; the size of the ROIs was maintained constant, to the extent possible, at approximately 1.0 cm2 at all sites. The size, shape, and position of the ROIs were kept constant among the four protocols by applying a copy-and-paste function at the workstation. CT radiation dose descriptors were recorded for all the image datasets after completion of the CT examinations.
Statistical Analysis Results are expressed as means ± SD. The associations between the visceral or subcutaneous adipose (fat) areas on the routine-dose FBP images
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CT Dose Reduction for Intraabdominal Fat Measurement
A
B
D
E
and the low-dose FBP, low-dose ASIR, and lowdose MBIR images were analyzed by calculation of the Pearson correlation coefficient. A BlandAltman analysis [31] was performed to evaluate the interchangeability between the visceral or subcutaneous adipose area measured on the routinedose FBP images and the visceral or subcutaneous adipose area measured on the low-dose FBP, lowdose ASIR, and low-dose MBIR images. The inter- and intraobserver agreements were evaluated by measuring the intraclass correlation coefficients (ICCs). The differences in the median objective image noises among the four image reconstruction protocols were tested using the Wilcoxon signed rank sum test, and the Bonferroni correction was used for multiple comparisons. The significance level for all tests was 5% (two-sided). All data were analyzed using a commercially available software program (SPSS, version 19, IBM).
spectively (see Appendix 1 for details of CTDIvol); therefore, the radiation dose in low-dose CT was 73.0% (mean) lower than that in routine-dose CT. The estimated effective doses for a 5-mm slice thickness at the level of the umbilicus in routine-dose CT and low-dose CT were 0.064 ± 0.026 and 0.017 ± 0.009 mSv, respectively. These estimated effective doses were determined on the basis of previously reported appropriate normalized coefficients for abdominal CT (0.015 mSv/mGy ⋅ cm) [32] and estimates of doselength product measurements for a 5-mm slice thickness of 4.28 ± 1.75 mGy ⋅ cm for routine-dose CT and 1.16 ± 0.59 mGy ⋅ cm for low-dose CT.
Results Patient Characteristics The mean height, weight, and BMI of the 59 patients were 161.2 ± 8.4 cm (range, 146– 182 cm), 59.5 ± 11.2 kg (range, 38–91 kg), and 22.9 ± 3.6 (range, 16.0–31.9), respectively. Radiation Doses The volume CT dose index (CTDIvol) values for routine-dose CT and low-dose CT were 8.56 ± 3.50 and 2.31 ± 1.18 mGy, re-
Visceral and Subcutaneous Fat Measurements The measured visceral fat areas on the routine-dose FBP, low-dose FBP, low-dose ASIR, and low-dose MBIR images were 120.9 ± 65.3 cm2 (range, 25.2–346.9 cm2), 121.4 ± 64.2 cm2 (range, 30.6–342.6 cm2), 121.3 ± 64.9 cm2 (range, 27.7–344.6 cm2), and 120.7 ± 65.3 cm2 (range, 24.3–344.4 cm2), respectively. The measured subcutaneous fat areas on the routine-dose FBP, low-dose FBP, low-dose ASIR, and lowdose MBIR images were 155.7 ± 82.8 cm2 (range, 13.9–356.3 cm2), 157.3 ± 83.5 cm2 (range, 14.3–356.6 cm2), 157.5 ± 83.6 cm2
C Fig. 1—CT images at level of umbilicus of 75-year-old man (body weight = 72 kg, height = 170 cm, body mass index = 24.9). A–D, Routine-dose filtered back projection (FBP) image (A), low-dose FBP image (B), low-dose adaptive statistical iterative reconstruction (ASIR) image (C), and low-dose model-based iterative reconstruction (MBIR) image (D). E, Routine-dose FBP image with line for enclosing fat area shows methods of obtaining measurements of visceral fat area and subcutaneous fat area: Boundary of visceral fat area is marked with red line, and subcutaneous fat area is area between red line and yellow line. CT attenuation of fat was defined as ranging from –190 to –30 HU.
(range, 13.0–356.6 cm2), and 155.8 ± 83.3 cm2 (range, 12.4–353.9 cm2), respectively. Excellent correlations were observed between the measured visceral fat areas on the routine-dose FBP and low-dose FBP images (r = 0.998; p < 0.001), on the routine-dose FBP and low-dose ASIR images (r = 0.998; p < 0.001), and on the routine-dose FBP and lowdose MBIR images (r = 0.998; p < 0.001) (Fig. 2). Excellent correlations were also observed between the measured subcutaneous fat areas on the routine-dose FBP and low-dose FBP images (r = 0.992; p < 0.001), on the routine-dose FBP and low-dose ASIR images (r = 0.992; p < 0.001), and on the routine-dose FBP and lowdose MBIR images (r = 0.992; p < 0.001). The average differences in the measured visceral fat area, which are also an estimate of agreement, were very small between the routine-dose FBP and low-dose FBP images (difference, –0.47 ± 3.97 cm2; 95% CI, –1.51 to 0.56 cm2; limits of agreement, –8.25 and 7.31 cm2), routine-dose FBP and low-dose ASIR images (difference, –0.41 ± 3.96 cm2; 95% CI, –1.44 to 0.63 cm2; limits of agreement, –8.17 and 7.36 cm2), and routine-dose FBP and low-dose MBIR images (difference, 0.18 ± 3.86 cm2; 95% CI, –0.83 to 1.18 cm2; limits of agreement, –7.39 and 7.75 cm2) (Fig. 3). However, one patient showed an approximately 20-cm2 difference of the measured
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0 100 200 300 400 Visceral Fat Area on Low-Dose FBP Images (cm2)
y = 0.9982x R 2 = 0.9963
Visceral Fat Area on Routine-Dose FBP Images (cm2)
y = 1.0003x R2 = 0.9963 Visceral Fat Area on Routine-Dose FBP Images (cm2)
Visceral Fat Area on Routine-Dose FBP Images (cm2)
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0 0 100 200 300 400 Visceral Fat Area on Low-Dose ASIR Images (cm2)
A
y = 0.9984x R 2 = 0.9965
B
C
Fig. 2—Measurements of visceral fat area on routine-dose filtered back projection (FBP) images (reference standard) compared with measurements on low-dose CT images. A–C, Scatterplots show association between visceral fat areas measured on routine-dose FBP images and low-dose FBP (A), low-dose statistical iterative reconstruction (ASIR) (B), and low-dose model-based iterative reconstruction (MBIR) (C) images. Lines show estimated regression. All scatterplots show excellent correlations (p < 0.001).
visceral fat area between the routine-dose FBP images and the low-dose FBP, low-dose ASIR, or low-dose MBIR images (Fig. 3). This large difference in measurements was considered to be attributable to the relatively large movement of the intestines between the routine-dose and low-dose CT acquisitions. The average differences in the measured subcutaneous fat areas were also very small between the routine-dose FBP images and low-dose FBP images (difference, –1.56 ± 10.77 cm2; 95% CI, –4.37 to 1.25 cm2; limits of agreement, –22.67 and 19.55 cm2), routine-dose FBP and low-dose ASIR images (difference, –1.78 ± 10.76 cm2; 95% CI, –4.58 to 1.02 cm2; limits of agreement, –22.86 and 19.30 cm2), and routine-dose FBP and lowdose MBIR images (difference, –0.08 ± 10.62 cm2; 95% CI, –2.85 to 2.68 cm2; limits of agreement, –20.89 and 20.73 cm2). The intraobserver agreements (ICCs) were 0.975 or greater, and the interobserver
agreements (ICCs) between the two radiologists were 0.980 or greater, as summarized in Table 1. Objective Image Noise The low-dose MBIR images showed significantly reduced objective image noise in all the ROIs (p < 0.0001) as compared with the routine-dose FBP, low-dose FBP, and lowdose ASIR images (Table 2). As compared with the routine-dose FBP images, the lowdose FBP and low-dose ASIR images showed significantly increased objective image noise in all the ROIs (p < 0.0001) (Table 2). Discussion Our prospective study showed that the visceral and subcutaneous fat areas measured on low-dose FBP, low-dose ASIR, and low-dose MBIR images were almost equal to those measured on routine-dose FBP images. Considering our results, it appears that
quantitative CT assessment of visceral adipose tissue—which is important for evaluating the potential risk of the development of many diseases such as metabolic syndrome, cardiovascular diseases, and various malignancies [1–7] and for accurately estimating prognosis—is possible at a radiation dose approximately one fourth of that in routinedose CT. The estimated effective dose in lowdose CT for the measurement of visceral fat in this study was 0.017 ± 0.009 mSv, which is comparable to the radiation dose that a patient is exposed to during a conventional radiography study of the abdomen. Therefore, in terms of the radiation dose, the low-dose CT protocol in this study for the measurement of visceral fat could be applied also to healthy subjects for the assessment of the potential risk of various diseases or for the accurate estimation of prognosis. With regard to lowering the dose of CT for measuring the visceral adipose tissue, a previ-
TABLE 1: Intraobserver and Interobserver Agreements (Intraclass Correlation Coefficients [ICCs]) for Low-Dose CT by Reconstruction Algorithm: Filtered Back Projection (FBP), Adaptive Statistical Iterative Reconstruction (ASIR), and Model-Based Iterative Reconstruction (MBIR) ICC (95% CI) Agreement
Low-Dose FBP
Low-Dose ASIR
Low-Dose MBIR
Visceral fat area
0.995 (0.991–0.997)
0.995 (0.992–0.997)
0.995 (0.992–0.997)
Subcutaneous fat area
0.975 (0.959–0.985)
0.982 (0.970–0.989)
0.983 (0.972–0.990)
Intraobserver agreements
Interobserver agreements Visceral fat area
0.986 (0.976–0.992)
0.986 (0.976–0.992)
0.986 (0.976–0.992)
Subcutaneous fat area
0.983 (0.971–0.990)
0.980 (0.966–0.988)
0.982 (0.970–0.989)
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pa
Image Noise (HU), Mean ± SD
ROI
RoutineDose FBP
Routine-Dose Routine-Dose Routine-Dose Low-Dose FBP and FBP and FBP and FBP and Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose FBP ASIR MBIR FBP ASIR MBIR ASIR
Retroperitoneal fat
14.0 ± 2.2
25.8 ± 3.7
18.5 ± 2.7
8.9 ± 1.7
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
Subcutaneous fat
10.2 ± 1.7
17.3 ± 2.2
12.5 ± 1.8
7.9 ± 1.6
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
Abdominal aorta
14.5 ± 4.2
26.4 ± 3.4
18.3 ± 2.7
9.3 ± 2.5
< 0.0001b
Psoas major muscle
15.0 ± 2.0
28.9 ± 3.7
20.3 ± 2.5
8.4 ± 1.1
< 0.0001b
Low-Dose FBP and Low-Dose MBIR
Low-Dose ASIR and Low-Dose MBIR
aWilcoxon signed rank sum test (pairwise comparison). A p < 0.0083 was considered to be statistically significant with Bonferroni correction for multiple comparisons. bStatistically significant (p < 0.0083).
20 15 1.96 SD 7.31
5
Mean −0.47
0 −5
−1.96 SD −8.25
−10 −15
0
100 200 300 400 Average Visceral Fat Area (cm2)
A
25 20 15 10
1.96 SD 7.36
5
Mean −0.41
0 −5
−1.96 SD −8.17
−10 −15
0
the low-dose FBP images. Although our results are consistent with previous reports on image noise [19, 22, 23], we found that application of IR did not affect the quantitative values of the visceral or subcutaneous fat areas in the low-dose CT images as compared with the routine-dose CT images. MBIR currently requires between 30 and 60 minutes for reconstruction of the images of a single patient, which would limit its application in routine CT examinations [23]. Considering our results, FBP or ASIR (reconstruction time per patient, < 1–2 minutes) may be sufficient for reconstructing low-dose CT images for the measurement of visceral fat. With regard to the quantitative measurements, previous studies have shown that IR algorithms can affect quantitative measurements of the lung and airways [38] and can affect coronary artery calcium scoring [39]. Differences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose MBIR Images)
25
10
ventional FBP, conferring image noise and radiation dose reduction capabilities [19, 25, 34]. MBIR was developed to overcome the limitation in relation to dose reduction capability and a trade-off between spatial resolution and image noise. The major difference between MBIR and ASIR is that MBIR uses a full-system optics model and full-system statistical model [35–37], and calculation for each voxel is conducted one by one through a full IR process between the projection space and image space [23]. The MBIR allows improved image quality with a higher spatial resolution and much lower image noise and lower radiation dose as compared with FBP and ASIR. Our study showed that objective image noise was lowest in the lowdose MBIR images among the four imaging protocols and that objective image noise was lower in the low-dose ASIR images than in Differences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose ASIR Images)
ous study compared a 90-kVp protocol (CTDIvol, 5.0 ± 0.1 mGy [mean ± SD]) with a 140-kVp protocol (CTDIvol, 18.8 ± 0.2 mGy) [33]; however, the different tube voltage settings resulted in heterogeneous values for the CT numbers of fat, which could have introduced bias in their results. In our study, we used only the conventional 120-kVp setting to avoid the bias caused by different tube voltage settings. Furthermore, the CTDIvol in our low-dose CT protocol (2.31 ± 1.18 mGy) was lower than that in the previously reported 90-kVp protocol (CTDIvol, 5.0 ± 0.1 mGy) [33]. FBP uses ideal system optics, such as a point x-ray source, a pencil x-ray beam, and point of detector elements. These simplified mathematic assumptions allow very fast image reconstruction, which was an important consideration in the early days of CT [23]. ASIR introduced system statistics into conDifferences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose FBP Images)
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TABLE 2: Objective Image Noise in the Routine-Dose Filtered Back Projection (FBP), Low-Dose FBP, Low-Dose Adaptive Statistical Iterative Reconstruction (ASIR), and Low-Dose Model-Based Iterative Reconstruction (MBIR) Images
100 200 300 400 Average Visceral Fat Area (cm2)
B
25 20 15 10
1.96 SD 7.75
5
Mean 0.18
0 −5
−1.96 SD −7.39
−10 −15
0
100 200 300 400 Average Visceral Fat Area (cm2)
C
Fig. 3—Bland-Altman plots. A, Differences in measured visceral fat areas between routine-dose filtered back projection (FBP) and low-dose FBP images. Dotted lines delineate limits of agreement between two images. B, Differences in measured visceral fat areas between routine-dose FBP and low-dose statistical iterative reconstruction (ASIR) images. Dotted lines delineate limits of agreement between two images. C, Differences in measured visceral fat areas between routine-dose FBP and low-dose model-based iterative reconstruction (MBIR) images. Dotted lines delineate limits of agreement between two images.
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Yamada et al. Our study had several limitations. First, we included only 59 patients, and additional studies on larger patient populations are required to confirm these preliminary findings. Second, the average BMI of the patients in this study was 22.9, and a future study may be needed to confirm the results for more obese patients. Third, we compared only the routine dose with one fourth of the routine dose. Additional studies comparing routine-dose CT with CT using less than one fourth of the routine dose may be desirable, although we believe that the radiation dose in our low-dose CT protocol was sufficiently low. Low-dose CT acquired at less than one fourth of the routine dose may affect the quantitative measurements of visceral or subcutaneous fat, and application of IR techniques may also affect these values. In conclusion, a 73.0% (mean) reduction of the radiation dose is possible in CT for the measurement of the abdominal visceral fat regardless of which reconstruction algorithm—namely, FBP, hybrid IR, or pure IR—is used. References 1. Fox CS, Massaro JM, Hoffmann U, et al. Abdominal visceral and subcutaneous adipose tissue compartments: association with metabolic risk factors in the Framingham Heart Study. Circulation 2007; 116:39–48 2. Ohashi N, Yamamoto H, Horiguchi J, et al. Association between visceral adipose tissue area and coronary plaque morphology assessed by CT angiography. JACC Cardiovasc Imaging 2010; 3:908–917 3. Imai A, Komatsu S, Ohara T, et al. Visceral abdominal fat accumulation predicts the progression of noncalcified coronary plaque. Atherosclerosis 2012; 222:524–529 4. Oh TH, Byeon JS, Myung SJ, et al. Visceral obesity as a risk factor for colorectal neoplasm. J Gastroenterol Hepatol 2008; 23:411–417 5. Schapira DV, Clark RA, Wolff PA, Jarrett AR, Kumar NB, Aziz NM. Visceral obesity and breast cancer risk. Cancer 1994; 74:632–639 6. von Hafe P, Pina F, Perez A, Tavares M, Barros H. Visceral fat accumulation as a risk factor for prostate cancer. Obes Res 2004; 12:1930–1935 7. Rosito GA, Massaro JM, Hoffmann U, et al. Pericardial fat, visceral abdominal fat, cardiovascular disease risk factors, and vascular calcification in a community-based sample: the Framingham Heart Study. Circulation 2008; 117:605–613 8. Pou KM, Massaro JM, Hoffmann U, et al. Visceral and subcutaneous adipose tissue volumes are cross-sectionally related to markers of inflam-
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mation and oxidative stress: the Framingham Heart Study. Circulation 2007; 116:1234–1241 9. Tsujinaka S, Konishi F, Kawamura YJ, et al. Visceral obesity predicts surgical outcomes after laparoscopic colectomy for sigmoid colon cancer. Dis Colon Rectum 2008; 51:1757–1765; discussion, 1765–1767 10. Shuster A, Patlas M, Pinthus JH, Mourtzakis M. The clinical importance of visceral adiposity: a critical review of methods for visceral adipose tissue analysis. Br J Radiol 2012; 85:1–10 11. Klopfenstein BJ, Kim MS, Krisky CM, Szumowski J, Rooney WD, Purnell JQ. Comparison of 3 T MRI and CT for the measurement of visceral and subcutaneous adipose tissue in humans. Br J Radiol 2012; 85:e826–e830 12. Seidell JC, Bakker CJ, van der Kooy K. Imaging techniques for measuring adipose-tissue distribution: a comparison between computed tomography and 1.5-T magnetic resonance. Am J Clin Nutr 1990; 51:953–957 13. Brenner DJ, Hall EJ. Computed tomography: an increasing source of radiation exposure. N Engl J Med 2007; 357:2277–2284 14. Kalra MK, Rizzo S, Maher MM, et al. Chest CT performed with z-axis modulation: scanning protocol and radiation dose. Radiology 2005; 237:303–308 15. Sigal-Cinqualbre AB, Hennequin R, Abada HT, Chen X, Paul JF. Low-kilovoltage multi-detector row chest CT in adults: feasibility and effect on image quality and iodine dose. Radiology 2004; 231:169–174 16. Kubo T, Ohno Y, Gautam S, Lin PJ, Kauczor HU, Hatabu H; iLEAD Study Group. Use of 3D adaptive raw-data filter in CT of the lung: effect on radiation dose reduction. AJR 2008; 191:1071 17. Loeve M, Lequin MH, de Bruijne M, et al. Cystic fibrosis: are volumetric ultra-low-dose expiratory CT scans sufficient for monitoring related lung disease? Radiology 2009; 253:223–229 18. Baumueller S, Alkadhi H, Stolzmann P, et al. Computed tomography of the lung in the highpitch mode: is breath holding still required? Invest Radiol 2011; 46:240–245 19. Hara AK, Paden RG, Silva AC, Kujak JL, Lawder HJ, Pavlicek W. Iterative reconstruction technique for reducing body radiation dose at CT: feasibility study. AJR 2009; 193:764–771 20. Miéville FA, Gudinchet F, Brunelle F, Bochud FO, Verdun FR. Iterative reconstruction methods in two different MDCT scanners: physical metrics and 4-alternative forced-choice detectability experiments—a phantom approach. Phys Med 2013; 29:99–110 21. Yamada Y, Jinzaki M, Hosokawa T, et al. Dose reduction in chest CT: comparison of the adaptive iterative dose reduction 3D, adaptive iterative dose
reduction, and filtered back projection reconstruction techniques. Eur J Radiol 2012; 81:4185–4195 22. Singh S, Kalra MK, Do S, et al. Comparison of hybrid and pure iterative reconstruction techniques with conventional filtered back projection: dose reduction potential in the abdomen. J Comput Assist Tomogr 2012; 36:347–353 23. Yamada Y, Jinzaki M, Tanami Y, et al. Modelbased iterative reconstruction technique for ultralow-dose computed tomography of the lung: a pilot study. Invest Radiol 2012; 47:482–489 24. Boedeker KL, McNitt-Gray MF. Application of the noise power spectrum in modern diagnostic MDCT. Part II. Noise power spectra and signal to noise. Phys Med Biol 2007; 52:4047–4061 25. Sagara Y, Hara AK, Pavlicek W, Silva AC, Paden RG, Wu Q. Abdominal CT: comparison of lowdose CT with adaptive statistical iterative reconstruction and routine-dose CT with filtered back projection in 53 patients. AJR 2010; 195:713–719 26. Leipsic J, Labounty TM, Heilbron B, et al. Adaptive statistical iterative reconstruction: assessment of image noise and image quality in coronary CT angiography. AJR 2010; 195:649–654 27. Mitsumori LM, Shuman WP, Busey JM, Kolokythas O, Koprowicz KM. Adaptive statistical iterative reconstruction versus filtered back projection in the same patient: 64 channel liver CT image quality and patient radiation dose. Eur Radiol 2012; 22:138–143 28. Yoshizumi T, Nakamura T, Yamane M, et al. Abdominal fat: standardized technique for measurement at CT. Radiology 1999; 211:283–286 29. Blitman NM, Baron LS, Berkenblit RG, Schoenfeld AH, Markowitz M, Freeman K. Feasibility of using single-slice MDCT to evaluate visceral abdominal fat in an urban pediatric population. AJR 2011; 197:482–487 30. Sj str m L, Kvist H, Cederblad A, Tylén U. Determination of total adipose tissue and body fat in women by computed tomography, 40K, and tritium. Am J Physiol 1986; 250:E736–E745 31. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307–310 32. American Association of Physicists in Medicine website. The measurement, reporting, and management of radiation dose in CT. www.aapm.org/ pubs/reports/rpt_96.pdf. Published January 2008. Accessed February 15, 2014 33. Yoon DY, Moon JH, Kim HK, et al. Comparison of low-dose CT and MR for measurement of intraabdominal adipose tissue: a phantom and human study. Acad Radiol 2008; 15:62–70 34. Yamada Y, Jinzaki M, Hosokawa T, et al. An intra-individual comparison between virtual monochromatic spectral and polychromatic 120-kVp images obtained during the same examination.
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CT Dose Reduction for Intraabdominal Fat Measurement Eur J Radiol 2014; 83:1715–1722 35. Thibault JB, Sauer KD, Bouman CA, Hsieh J. A three-dimensional statistical approach to improved image quality for multislice helical CT. Med Phys 2007; 34:4526–4544 36. Yu Z, Thibault JB, Bouman CA, Sauer KD, Hsieh J. Fast model-based X-ray CT reconstruction using spatially nonhomogeneous ICD optimization.
IEEE Trans Image Process 2011; 20:161–175 37. Machida H, Tanaka I, Fukui R, et al. Improved delineation of the anterior spinal artery with modelbased iterative reconstruction in CT angiography: a clinical pilot study. AJR 2013; 200:442–446 38. Choo JY, Goo JM, Lee CH, Park CM, Park SJ, Shim MS. Quantitative analysis of emphysema and airway measurements according to iterative
reconstruction algorithms: comparison of filtered back projection, adaptive statistical iterative reconstruction and model-based iterative reconstruction. Eur Radiol 2014; 24:799–806 39. Gebhard C, Fiechter M, Fuchs TA, et al. Coronary artery calcium scoring: influence of adaptive statistical iterative reconstruction using 64-MDCT. Int J Cardiol 2013; 167:2932–2937
APPENDIX 1: Volume CT Dose Index (CTDIvol) Values for Routine-Dose CT and Low-Dose CT Abdominal CT scans were obtained using automatic exposure control in this study, and the scanning coverage may have affected the CTDIvol values used for the calculation of the effective dose at the level of the umbilicus. On this matter, we assessed the specific tube current at the level of the umbilicus in each patient and compared the actual CTDIvol (described in the Results section) with the estimated (virtual) CTDIvol. We calculated the CTDIvol values on the basis of the specific tube current at the level
of the umbilicus using the CT console and using the same values for the other scanning parameters as indicated in the Subjects and Methods section. The average tube current values at the level of the umbilicus for the routine-dose and low-dose CT acquisitions were 414.08 ± 174.79 (SD) mA and 110.95 ± 57.86 mA, respectively. Consequently, the estimated (virtual) CTDIvol values calculated on the basis of the tube current at the level of the umbilicus (using the CT console) for routine-dose CT and low-dose CT were
8.64 ± 3.65 and 2.30 ± 1.21 mGy, respectively, which are comparable to the actual CTDIvol values for routine-dose and low-dose CT (8.56 ± 3.50 and 2.31 ± 1.18 mGy, respectively; described in the Results section). We consider these results to be attributable to the fact that the average tube current for the entire scanning length of the abdomen was comparable to the specific tube current at the level of the umbilicus, even though the abdominal CT scans were obtained using automatic exposure control.
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Yamada et al. CT Dose Reduction for Intraabdominal Fat Measurement Medical Physics and Informatics Original Research
CT Dose Reduction for Visceral Adipose Tissue Measurement: Effects of Model-Based and Adaptive Statistical Iterative Reconstructions and Filtered Back Projection
Yoshitake Yamada1 Masahiro Jinzaki1 Yuki Niijima1 Masahiro Hashimoto1 Minoru Yamada2 Takayuki Abe 3 Sachio Kuribayashi1
OBJECTIVE. The objective of our study was to evaluate the effects of radiation dose reduction and the reconstruction algorithm used—filtered back projection (FBP), adaptive statistical iterative reconstruction (ASIR), or model-based iterative reconstruction (MBIR)—on the measurement of abdominal visceral fat using CT. SUBJECTS AND METHODS. Standard-dose and low-dose abdominal CT examinations were performed simultaneously with automatic exposure control in 59 patients; the noise index for a 5-mm slice thickness was 12 for routine-dose CT and 24 for low-dose CT. The routine-dose CT images were reconstructed using FBP (reference standard), and the low-dose CT images were reconstructed using FBP, ASIR (so-called hybrid iterative reconstruction [IR]), and MBIR (so-called pure IR). In the 236 image series obtained, the visceral fat area was measured. Data were analyzed by the Pearson correlation coefficient test and a Bland-Altman difference analysis. RESULTS. The radiation dose of the low-dose abdominal CT examinations was 73.0% (mean) lower than that of routine-dose CT examinations. Excellent correlations were observed between the visceral fat areas measured on the routine-dose FBP images and those measured on the low-dose FBP, low-dose ASIR, and low-dose MBIR images (r = 0.998, 0.998, and 0.998, respectively; p < 0.001). A Bland-Altman difference analysis revealed excellent agreements, with mean biases of –0.47, –0.41, and 0.18 cm2 for the visceral fat area between the routine-dose FBP images and the low-dose FBP, low-dose ASIR, and low-dose MBIR images, respectively. CONCLUSION. A 73.0% reduction of the radiation dose would be possible in CT for the measurement of the abdominal visceral fat regardless of which reconstruction algorithm is used (i.e., FBP, hybrid IR, or pure IR).
Yamada Y, Jinzaki M, Niijima Y, et al.
Keywords: abdomen, intraabdominal fat, MDCT, radiation dose DOI:10.2214/AJR.14.13411 Received July 3, 2014; accepted after revision October 7, 2014. M. Jinzaki and S. Kuribayashi received a research grant from GE Healthcare Japan. The remaining authors had complete unrestricted access to the study data at all stages of the study. Supported in part by the Japan Society for the Promotion of Science (JSPS KAKENHI grant no. 26861016). 1 Department of Diagnostic Radiology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. Address correspondence to M. Jinzaki ([email protected]). 2 Multidimension Biomedical Imaging & Information Laboratory in Research Park, Keio University School of Medicine, Tokyo, Japan. 3 Department of Preventive Medicine and Public Health, Center for Clinical Research, Keio University School of Medicine, Tokyo, Japan.
WEB This is a web exclusive article. AJR 2015; 204:W677–W683 0361–803X/15/2046–W677 © American Roentgen Ray Society
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ody fat tissue is traditionally distributed in two main compartments with different metabolic characteristics: the visceral adipose tissue and subcutaneous adipose tissue. Several studies have implicated abdominal visceral adipose tissue in the development of various metabolic abnormalities [1]; coronary artery disease [2, 3]; and cancers of the colon [4], breast [5], and prostate [6]. In addition, investigators have reported associations between abdominal visceral adipose tissue and coronary artery calcium score [7]; systemic markers of inflammation [8]; and predisposition to infections and noninfectious complications, greater length of hospital stay, and higher hospital mortality [9]. The reference standard modality for the quantitative measurement of the abdominal
visceral fat area is CT [10, 11]. MRI can be also used to assess abdominal fat distribution [11, 12]; however, the use of MRI is limited by its poor accessibility and high cost as compared with CT [10]. Providing excellent resolution for adipose tissue, CT represents a direct method for assessing the visceral fat deposition in both adult and pediatric populations [10]. On the other hand, the exposure to ionizing radiation limits its broad application in healthy subjects given that the widespread use of CT around the world has raised concern about the increase in the risk of cancer from medical radiation exposure [13]. Because radiation-induced carcinogenesis is a stochastic effect, the risk may be expected to decrease with a decrease of the radiation dose. Several approaches have been attempted to reduce the radiation dose to patients dur-
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Yamada et al. ing CT, such as the reduction of fixed tube current, use of automatic exposure control, reduction of peak kilovoltage, use of noise reduction filters, and use of a high helical pitch [14–18]; furthermore, the use of the recently introduced iterative reconstruction (IR) techniques has been shown to have the potential to replace the conventionally used filtered back projection (FBP) algorithm [19–23]. To our knowledge, no prospective study to date has conducted intraindividual comparisons of the visceral adipose areas measured between routine-dose CT and low-dose CT performed simultaneously using automatic exposure control and a standard tube voltage of 120 kVp. Furthermore, no studies have evaluated the effects of different reconstruction algorithms, including FBP, adaptive statistical iterative reconstruction (ASIR) (socalled hybrid IR), and model-based iterative reconstruction (MBIR) (so-called pure IR), on the quantitative measurement of visceral adipose tissue. The purpose of this study was to evaluate the effects of radiation dose reduction and the reconstruction algorithm used (FBP, ASIR, or MBIR) on the measurement of abdominal visceral fat by CT. Subjects and Methods Patients This study was conducted with the approval of our institutional review board, and written informed consent was obtained from each of the participating patients. From April 2011 to July 2011, 63 consecutive patients referred for unenhanced abdominal CT on a specific scanner were considered for inclusion in this prospective study. Patients were excluded if they were younger than 50 years old, were pregnant or potentially pregnant, or were lactating or if they did not provide written informed consent; four of the 63 patients did not want to participate in the study and refused to provide informed consent. Therefore, images of the remaining 59 patients (age range, 50–83 years; mean age, 66.5 ± 8.4 years) were included in the final analysis. The study group was composed of 30 men (age range, 56–80 years; mean age, 68.7 ± 7.8 years) and 29 women (age range, 50–83 years; mean age, 64.2 ± 8.5 years). The clinical indications for abdominal CT of the subjects in this study were staging or restaging of known malignancy (n = 47); diarrhea (n = 2); back pain (n = 2); chronic abdominal pain (n = 1); and follow-up for renal or ureteral stones (n = 2), giant hepatic cyst (n = 1), gallstones (n = 1), pancreatitis (n = 1), inflammatory bowel disease (n = 1), and abdominal aortic aneurysm (n = 1). The height and
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weight of the patients were measured, and the body mass index (BMI) was calculated as weight in kilograms divided by height squared in meters.
Imaging Protocol All unenhanced abdominal CT examinations were performed on the same scanner (Discovery CT750 HD scanner, GE Healthcare). All the patients were scanned in the craniocaudal direction while they were lying in the supine position. According to a previous report, because radiation dose decreases to 1 / x, the image noise increases by the square root of x [24]; thus, when the radiation dose is reduced to one quarter, the noise may be expected to increase by the square root of 4 (i.e., 2). We used this formula to calculate the probable noise index that would result in a dose reduction of approximately 75% [25]. Our standard noise index for a slice thickness of 5 mm is 12. Therefore, we considered that, theoretically, a noise index of 24 (12 × 2) would yield a dose reduction of approximately 75%. The noise index settings (12 and 24) were based on our initial experiences in the clinical setting, our phantom data, and vendor recommendations so that a dose reduction was expected without a loss of diagnostic information. Two helical acquisitions were obtained for each patient with the same length of helical run and same FOV to obtain two datasets for the abdomen including the top of the diaphragm and the level of the umbilicus. The first acquisition, which was routine-dose CT, was performed with automatic exposure control (tube current modulation) using a noise index of 12 for a slice thickness of 5 mm while the patient was breath-holding during inspiration. The second acquisition, which was low-dose CT, was performed with automatic exposure control using a noise index of 24 (for a slice thickness of 5 mm) while the patient was breath-holding during another inspiration. The tube current range for the routine-dose and low-dose CT acquisitions ranged from a minimum of 10 mA to a maximum of 750 mA. The other scanning parameters were the same for both the routine-dose and low-dose CT acquisitions: peak tube voltage, 120 kVp; rotation speed, 0.4 second; detector collimation, 0.625 × 64 mm; table feed, 55 mm/rotation; pitch factor, 1.375. Thus, we obtained two files of raw data of the same size for each patient.
Image Reconstruction The series of contiguous 5-mm-thick routinedose CT images were reconstructed with FBP using the standard kernel; these images served as the reference standard. The series of contiguous 5-mm-thick low-dose CT images were reconstructed using FBP, ASIR, MBIR with the stan-
dard kernel. Thus, the same kernels were used for routine-dose and low-dose CT data. We used 50% ASIR, which means that 50% of the ASIR image was blended with the FBP image, to obtain the low-dose ASIR images; we selected this ASIR level on the basis of previous studies [26, 27]. Finally, we obtained 236 sets of 5-mm-thick images—that is, 59 image sets of each reconstruction algorithm (i.e., routine-dose FBP, low-dose FBP, low-dose ASIR, and low-dose MBIR) (Fig. 1).
Objective Measurements Two board-certified radiologists, with 7 and 22 years of experience in interpreting abdominal CT, independently measured the visceral and subcutaneous fat (adipose) areas at the level of the umbilicus in low-dose FBP, low-dose ASIR, and low-dose MBIR image series using the body fat analysis function at an independent workstation (Advantage workstation 4.5, GE Healthcare). They measured the areas regardless of the movement of the intestines between the routine-dose and low-dose CT acquisitions, as described previously [3, 11, 28, 29]. One of the two radiologists (the one with 7 years of experience in interpreting abdominal CT) measured the visceral and subcutaneous fat areas twice to evaluate intraobserver agreement; the time interval between the first and second measurements was 4 weeks. The same two radiologists measured the visceral and subcutaneous fat areas at the level of umbilicus by consensus in the routine-dose FBP images, which served as the reference standard. The abdominal muscular wall separating the visceral compartment from the subcutaneous compartment was traced manually (Fig. 1E). The CT attenuation of fat was defined as ranging from –190 to –30 HU, as described previously [4, 11, 28–30]. Furthermore, the objective image noise (i.e., SD of the CT number) was measured in all 236 image series. The same two board-certified radiologists placed, by consensus, circular or ovoid ROIs in the subcutaneous fat in the anterior abdominal wall (midline), retroperitoneal fat (right side), abdominal aorta, and psoas major muscle (right side) at the level of umbilicus; the size of the ROIs was maintained constant, to the extent possible, at approximately 1.0 cm2 at all sites. The size, shape, and position of the ROIs were kept constant among the four protocols by applying a copy-and-paste function at the workstation. CT radiation dose descriptors were recorded for all the image datasets after completion of the CT examinations.
Statistical Analysis Results are expressed as means ± SD. The associations between the visceral or subcutaneous adipose (fat) areas on the routine-dose FBP images
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CT Dose Reduction for Intraabdominal Fat Measurement
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and the low-dose FBP, low-dose ASIR, and lowdose MBIR images were analyzed by calculation of the Pearson correlation coefficient. A BlandAltman analysis [31] was performed to evaluate the interchangeability between the visceral or subcutaneous adipose area measured on the routinedose FBP images and the visceral or subcutaneous adipose area measured on the low-dose FBP, lowdose ASIR, and low-dose MBIR images. The inter- and intraobserver agreements were evaluated by measuring the intraclass correlation coefficients (ICCs). The differences in the median objective image noises among the four image reconstruction protocols were tested using the Wilcoxon signed rank sum test, and the Bonferroni correction was used for multiple comparisons. The significance level for all tests was 5% (two-sided). All data were analyzed using a commercially available software program (SPSS, version 19, IBM).
spectively (see Appendix 1 for details of CTDIvol); therefore, the radiation dose in low-dose CT was 73.0% (mean) lower than that in routine-dose CT. The estimated effective doses for a 5-mm slice thickness at the level of the umbilicus in routine-dose CT and low-dose CT were 0.064 ± 0.026 and 0.017 ± 0.009 mSv, respectively. These estimated effective doses were determined on the basis of previously reported appropriate normalized coefficients for abdominal CT (0.015 mSv/mGy ⋅ cm) [32] and estimates of doselength product measurements for a 5-mm slice thickness of 4.28 ± 1.75 mGy ⋅ cm for routine-dose CT and 1.16 ± 0.59 mGy ⋅ cm for low-dose CT.
Results Patient Characteristics The mean height, weight, and BMI of the 59 patients were 161.2 ± 8.4 cm (range, 146– 182 cm), 59.5 ± 11.2 kg (range, 38–91 kg), and 22.9 ± 3.6 (range, 16.0–31.9), respectively. Radiation Doses The volume CT dose index (CTDIvol) values for routine-dose CT and low-dose CT were 8.56 ± 3.50 and 2.31 ± 1.18 mGy, re-
Visceral and Subcutaneous Fat Measurements The measured visceral fat areas on the routine-dose FBP, low-dose FBP, low-dose ASIR, and low-dose MBIR images were 120.9 ± 65.3 cm2 (range, 25.2–346.9 cm2), 121.4 ± 64.2 cm2 (range, 30.6–342.6 cm2), 121.3 ± 64.9 cm2 (range, 27.7–344.6 cm2), and 120.7 ± 65.3 cm2 (range, 24.3–344.4 cm2), respectively. The measured subcutaneous fat areas on the routine-dose FBP, low-dose FBP, low-dose ASIR, and lowdose MBIR images were 155.7 ± 82.8 cm2 (range, 13.9–356.3 cm2), 157.3 ± 83.5 cm2 (range, 14.3–356.6 cm2), 157.5 ± 83.6 cm2
C Fig. 1—CT images at level of umbilicus of 75-year-old man (body weight = 72 kg, height = 170 cm, body mass index = 24.9). A–D, Routine-dose filtered back projection (FBP) image (A), low-dose FBP image (B), low-dose adaptive statistical iterative reconstruction (ASIR) image (C), and low-dose model-based iterative reconstruction (MBIR) image (D). E, Routine-dose FBP image with line for enclosing fat area shows methods of obtaining measurements of visceral fat area and subcutaneous fat area: Boundary of visceral fat area is marked with red line, and subcutaneous fat area is area between red line and yellow line. CT attenuation of fat was defined as ranging from –190 to –30 HU.
(range, 13.0–356.6 cm2), and 155.8 ± 83.3 cm2 (range, 12.4–353.9 cm2), respectively. Excellent correlations were observed between the measured visceral fat areas on the routine-dose FBP and low-dose FBP images (r = 0.998; p < 0.001), on the routine-dose FBP and low-dose ASIR images (r = 0.998; p < 0.001), and on the routine-dose FBP and lowdose MBIR images (r = 0.998; p < 0.001) (Fig. 2). Excellent correlations were also observed between the measured subcutaneous fat areas on the routine-dose FBP and low-dose FBP images (r = 0.992; p < 0.001), on the routine-dose FBP and low-dose ASIR images (r = 0.992; p < 0.001), and on the routine-dose FBP and lowdose MBIR images (r = 0.992; p < 0.001). The average differences in the measured visceral fat area, which are also an estimate of agreement, were very small between the routine-dose FBP and low-dose FBP images (difference, –0.47 ± 3.97 cm2; 95% CI, –1.51 to 0.56 cm2; limits of agreement, –8.25 and 7.31 cm2), routine-dose FBP and low-dose ASIR images (difference, –0.41 ± 3.96 cm2; 95% CI, –1.44 to 0.63 cm2; limits of agreement, –8.17 and 7.36 cm2), and routine-dose FBP and low-dose MBIR images (difference, 0.18 ± 3.86 cm2; 95% CI, –0.83 to 1.18 cm2; limits of agreement, –7.39 and 7.75 cm2) (Fig. 3). However, one patient showed an approximately 20-cm2 difference of the measured
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Fig. 2—Measurements of visceral fat area on routine-dose filtered back projection (FBP) images (reference standard) compared with measurements on low-dose CT images. A–C, Scatterplots show association between visceral fat areas measured on routine-dose FBP images and low-dose FBP (A), low-dose statistical iterative reconstruction (ASIR) (B), and low-dose model-based iterative reconstruction (MBIR) (C) images. Lines show estimated regression. All scatterplots show excellent correlations (p < 0.001).
visceral fat area between the routine-dose FBP images and the low-dose FBP, low-dose ASIR, or low-dose MBIR images (Fig. 3). This large difference in measurements was considered to be attributable to the relatively large movement of the intestines between the routine-dose and low-dose CT acquisitions. The average differences in the measured subcutaneous fat areas were also very small between the routine-dose FBP images and low-dose FBP images (difference, –1.56 ± 10.77 cm2; 95% CI, –4.37 to 1.25 cm2; limits of agreement, –22.67 and 19.55 cm2), routine-dose FBP and low-dose ASIR images (difference, –1.78 ± 10.76 cm2; 95% CI, –4.58 to 1.02 cm2; limits of agreement, –22.86 and 19.30 cm2), and routine-dose FBP and lowdose MBIR images (difference, –0.08 ± 10.62 cm2; 95% CI, –2.85 to 2.68 cm2; limits of agreement, –20.89 and 20.73 cm2). The intraobserver agreements (ICCs) were 0.975 or greater, and the interobserver
agreements (ICCs) between the two radiologists were 0.980 or greater, as summarized in Table 1. Objective Image Noise The low-dose MBIR images showed significantly reduced objective image noise in all the ROIs (p < 0.0001) as compared with the routine-dose FBP, low-dose FBP, and lowdose ASIR images (Table 2). As compared with the routine-dose FBP images, the lowdose FBP and low-dose ASIR images showed significantly increased objective image noise in all the ROIs (p < 0.0001) (Table 2). Discussion Our prospective study showed that the visceral and subcutaneous fat areas measured on low-dose FBP, low-dose ASIR, and low-dose MBIR images were almost equal to those measured on routine-dose FBP images. Considering our results, it appears that
quantitative CT assessment of visceral adipose tissue—which is important for evaluating the potential risk of the development of many diseases such as metabolic syndrome, cardiovascular diseases, and various malignancies [1–7] and for accurately estimating prognosis—is possible at a radiation dose approximately one fourth of that in routinedose CT. The estimated effective dose in lowdose CT for the measurement of visceral fat in this study was 0.017 ± 0.009 mSv, which is comparable to the radiation dose that a patient is exposed to during a conventional radiography study of the abdomen. Therefore, in terms of the radiation dose, the low-dose CT protocol in this study for the measurement of visceral fat could be applied also to healthy subjects for the assessment of the potential risk of various diseases or for the accurate estimation of prognosis. With regard to lowering the dose of CT for measuring the visceral adipose tissue, a previ-
TABLE 1: Intraobserver and Interobserver Agreements (Intraclass Correlation Coefficients [ICCs]) for Low-Dose CT by Reconstruction Algorithm: Filtered Back Projection (FBP), Adaptive Statistical Iterative Reconstruction (ASIR), and Model-Based Iterative Reconstruction (MBIR) ICC (95% CI) Agreement
Low-Dose FBP
Low-Dose ASIR
Low-Dose MBIR
Visceral fat area
0.995 (0.991–0.997)
0.995 (0.992–0.997)
0.995 (0.992–0.997)
Subcutaneous fat area
0.975 (0.959–0.985)
0.982 (0.970–0.989)
0.983 (0.972–0.990)
Intraobserver agreements
Interobserver agreements Visceral fat area
0.986 (0.976–0.992)
0.986 (0.976–0.992)
0.986 (0.976–0.992)
Subcutaneous fat area
0.983 (0.971–0.990)
0.980 (0.966–0.988)
0.982 (0.970–0.989)
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CT Dose Reduction for Intraabdominal Fat Measurement
pa
Image Noise (HU), Mean ± SD
ROI
RoutineDose FBP
Routine-Dose Routine-Dose Routine-Dose Low-Dose FBP and FBP and FBP and FBP and Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose Low-Dose FBP ASIR MBIR FBP ASIR MBIR ASIR
Retroperitoneal fat
14.0 ± 2.2
25.8 ± 3.7
18.5 ± 2.7
8.9 ± 1.7
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
Subcutaneous fat
10.2 ± 1.7
17.3 ± 2.2
12.5 ± 1.8
7.9 ± 1.6
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
< 0.0001b
Abdominal aorta
14.5 ± 4.2
26.4 ± 3.4
18.3 ± 2.7
9.3 ± 2.5
< 0.0001b
Psoas major muscle
15.0 ± 2.0
28.9 ± 3.7
20.3 ± 2.5
8.4 ± 1.1
< 0.0001b
Low-Dose FBP and Low-Dose MBIR
Low-Dose ASIR and Low-Dose MBIR
aWilcoxon signed rank sum test (pairwise comparison). A p < 0.0083 was considered to be statistically significant with Bonferroni correction for multiple comparisons. bStatistically significant (p < 0.0083).
20 15 1.96 SD 7.31
5
Mean −0.47
0 −5
−1.96 SD −8.25
−10 −15
0
100 200 300 400 Average Visceral Fat Area (cm2)
A
25 20 15 10
1.96 SD 7.36
5
Mean −0.41
0 −5
−1.96 SD −8.17
−10 −15
0
the low-dose FBP images. Although our results are consistent with previous reports on image noise [19, 22, 23], we found that application of IR did not affect the quantitative values of the visceral or subcutaneous fat areas in the low-dose CT images as compared with the routine-dose CT images. MBIR currently requires between 30 and 60 minutes for reconstruction of the images of a single patient, which would limit its application in routine CT examinations [23]. Considering our results, FBP or ASIR (reconstruction time per patient, < 1–2 minutes) may be sufficient for reconstructing low-dose CT images for the measurement of visceral fat. With regard to the quantitative measurements, previous studies have shown that IR algorithms can affect quantitative measurements of the lung and airways [38] and can affect coronary artery calcium scoring [39]. Differences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose MBIR Images)
25
10
ventional FBP, conferring image noise and radiation dose reduction capabilities [19, 25, 34]. MBIR was developed to overcome the limitation in relation to dose reduction capability and a trade-off between spatial resolution and image noise. The major difference between MBIR and ASIR is that MBIR uses a full-system optics model and full-system statistical model [35–37], and calculation for each voxel is conducted one by one through a full IR process between the projection space and image space [23]. The MBIR allows improved image quality with a higher spatial resolution and much lower image noise and lower radiation dose as compared with FBP and ASIR. Our study showed that objective image noise was lowest in the lowdose MBIR images among the four imaging protocols and that objective image noise was lower in the low-dose ASIR images than in Differences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose ASIR Images)
ous study compared a 90-kVp protocol (CTDIvol, 5.0 ± 0.1 mGy [mean ± SD]) with a 140-kVp protocol (CTDIvol, 18.8 ± 0.2 mGy) [33]; however, the different tube voltage settings resulted in heterogeneous values for the CT numbers of fat, which could have introduced bias in their results. In our study, we used only the conventional 120-kVp setting to avoid the bias caused by different tube voltage settings. Furthermore, the CTDIvol in our low-dose CT protocol (2.31 ± 1.18 mGy) was lower than that in the previously reported 90-kVp protocol (CTDIvol, 5.0 ± 0.1 mGy) [33]. FBP uses ideal system optics, such as a point x-ray source, a pencil x-ray beam, and point of detector elements. These simplified mathematic assumptions allow very fast image reconstruction, which was an important consideration in the early days of CT [23]. ASIR introduced system statistics into conDifferences in Visceral Fat Areas (cm2) (Routine-Dose FBP Images – Low-Dose FBP Images)
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TABLE 2: Objective Image Noise in the Routine-Dose Filtered Back Projection (FBP), Low-Dose FBP, Low-Dose Adaptive Statistical Iterative Reconstruction (ASIR), and Low-Dose Model-Based Iterative Reconstruction (MBIR) Images
100 200 300 400 Average Visceral Fat Area (cm2)
B
25 20 15 10
1.96 SD 7.75
5
Mean 0.18
0 −5
−1.96 SD −7.39
−10 −15
0
100 200 300 400 Average Visceral Fat Area (cm2)
C
Fig. 3—Bland-Altman plots. A, Differences in measured visceral fat areas between routine-dose filtered back projection (FBP) and low-dose FBP images. Dotted lines delineate limits of agreement between two images. B, Differences in measured visceral fat areas between routine-dose FBP and low-dose statistical iterative reconstruction (ASIR) images. Dotted lines delineate limits of agreement between two images. C, Differences in measured visceral fat areas between routine-dose FBP and low-dose model-based iterative reconstruction (MBIR) images. Dotted lines delineate limits of agreement between two images.
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Yamada et al. Our study had several limitations. First, we included only 59 patients, and additional studies on larger patient populations are required to confirm these preliminary findings. Second, the average BMI of the patients in this study was 22.9, and a future study may be needed to confirm the results for more obese patients. Third, we compared only the routine dose with one fourth of the routine dose. Additional studies comparing routine-dose CT with CT using less than one fourth of the routine dose may be desirable, although we believe that the radiation dose in our low-dose CT protocol was sufficiently low. Low-dose CT acquired at less than one fourth of the routine dose may affect the quantitative measurements of visceral or subcutaneous fat, and application of IR techniques may also affect these values. In conclusion, a 73.0% (mean) reduction of the radiation dose is possible in CT for the measurement of the abdominal visceral fat regardless of which reconstruction algorithm—namely, FBP, hybrid IR, or pure IR—is used. References 1. Fox CS, Massaro JM, Hoffmann U, et al. Abdominal visceral and subcutaneous adipose tissue compartments: association with metabolic risk factors in the Framingham Heart Study. Circulation 2007; 116:39–48 2. Ohashi N, Yamamoto H, Horiguchi J, et al. Association between visceral adipose tissue area and coronary plaque morphology assessed by CT angiography. JACC Cardiovasc Imaging 2010; 3:908–917 3. Imai A, Komatsu S, Ohara T, et al. Visceral abdominal fat accumulation predicts the progression of noncalcified coronary plaque. Atherosclerosis 2012; 222:524–529 4. Oh TH, Byeon JS, Myung SJ, et al. Visceral obesity as a risk factor for colorectal neoplasm. J Gastroenterol Hepatol 2008; 23:411–417 5. Schapira DV, Clark RA, Wolff PA, Jarrett AR, Kumar NB, Aziz NM. Visceral obesity and breast cancer risk. Cancer 1994; 74:632–639 6. von Hafe P, Pina F, Perez A, Tavares M, Barros H. Visceral fat accumulation as a risk factor for prostate cancer. Obes Res 2004; 12:1930–1935 7. Rosito GA, Massaro JM, Hoffmann U, et al. Pericardial fat, visceral abdominal fat, cardiovascular disease risk factors, and vascular calcification in a community-based sample: the Framingham Heart Study. Circulation 2008; 117:605–613 8. Pou KM, Massaro JM, Hoffmann U, et al. Visceral and subcutaneous adipose tissue volumes are cross-sectionally related to markers of inflam-
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reduction, and filtered back projection reconstruction techniques. Eur J Radiol 2012; 81:4185–4195 22. Singh S, Kalra MK, Do S, et al. Comparison of hybrid and pure iterative reconstruction techniques with conventional filtered back projection: dose reduction potential in the abdomen. J Comput Assist Tomogr 2012; 36:347–353 23. Yamada Y, Jinzaki M, Tanami Y, et al. Modelbased iterative reconstruction technique for ultralow-dose computed tomography of the lung: a pilot study. Invest Radiol 2012; 47:482–489 24. Boedeker KL, McNitt-Gray MF. Application of the noise power spectrum in modern diagnostic MDCT. Part II. Noise power spectra and signal to noise. Phys Med Biol 2007; 52:4047–4061 25. Sagara Y, Hara AK, Pavlicek W, Silva AC, Paden RG, Wu Q. Abdominal CT: comparison of lowdose CT with adaptive statistical iterative reconstruction and routine-dose CT with filtered back projection in 53 patients. AJR 2010; 195:713–719 26. Leipsic J, Labounty TM, Heilbron B, et al. Adaptive statistical iterative reconstruction: assessment of image noise and image quality in coronary CT angiography. AJR 2010; 195:649–654 27. Mitsumori LM, Shuman WP, Busey JM, Kolokythas O, Koprowicz KM. Adaptive statistical iterative reconstruction versus filtered back projection in the same patient: 64 channel liver CT image quality and patient radiation dose. Eur Radiol 2012; 22:138–143 28. Yoshizumi T, Nakamura T, Yamane M, et al. Abdominal fat: standardized technique for measurement at CT. Radiology 1999; 211:283–286 29. Blitman NM, Baron LS, Berkenblit RG, Schoenfeld AH, Markowitz M, Freeman K. Feasibility of using single-slice MDCT to evaluate visceral abdominal fat in an urban pediatric population. AJR 2011; 197:482–487 30. Sj str m L, Kvist H, Cederblad A, Tylén U. Determination of total adipose tissue and body fat in women by computed tomography, 40K, and tritium. Am J Physiol 1986; 250:E736–E745 31. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307–310 32. American Association of Physicists in Medicine website. The measurement, reporting, and management of radiation dose in CT. www.aapm.org/ pubs/reports/rpt_96.pdf. Published January 2008. Accessed February 15, 2014 33. Yoon DY, Moon JH, Kim HK, et al. Comparison of low-dose CT and MR for measurement of intraabdominal adipose tissue: a phantom and human study. Acad Radiol 2008; 15:62–70 34. Yamada Y, Jinzaki M, Hosokawa T, et al. An intra-individual comparison between virtual monochromatic spectral and polychromatic 120-kVp images obtained during the same examination.
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APPENDIX 1: Volume CT Dose Index (CTDIvol) Values for Routine-Dose CT and Low-Dose CT Abdominal CT scans were obtained using automatic exposure control in this study, and the scanning coverage may have affected the CTDIvol values used for the calculation of the effective dose at the level of the umbilicus. On this matter, we assessed the specific tube current at the level of the umbilicus in each patient and compared the actual CTDIvol (described in the Results section) with the estimated (virtual) CTDIvol. We calculated the CTDIvol values on the basis of the specific tube current at the level
of the umbilicus using the CT console and using the same values for the other scanning parameters as indicated in the Subjects and Methods section. The average tube current values at the level of the umbilicus for the routine-dose and low-dose CT acquisitions were 414.08 ± 174.79 (SD) mA and 110.95 ± 57.86 mA, respectively. Consequently, the estimated (virtual) CTDIvol values calculated on the basis of the tube current at the level of the umbilicus (using the CT console) for routine-dose CT and low-dose CT were
8.64 ± 3.65 and 2.30 ± 1.21 mGy, respectively, which are comparable to the actual CTDIvol values for routine-dose and low-dose CT (8.56 ± 3.50 and 2.31 ± 1.18 mGy, respectively; described in the Results section). We consider these results to be attributable to the fact that the average tube current for the entire scanning length of the abdomen was comparable to the specific tube current at the level of the umbilicus, even though the abdominal CT scans were obtained using automatic exposure control.
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