Radiation Protection Dosimetry (2015), Vol. 167, No. 4, pp. 532 – 541 Advance Access publication 11 November 2014
doi:10.1093/rpd/ncu331
EVALUATION OF THE CT DOSE INDEX FOR SCANS WITH AN ECG USING A 320-ROW MULTIPLE-DETECTOR CT SCANNER
*Corresponding author: [email protected] Received 12 June 2014; revised 7 October 2014; accepted 10 October 2014 The relationship between heart rate (HR) and computed tomography dose index (CTDI) was evaluated using an electrocardiogram (ECG) gate scan for scan applications such as prospective triggering, Ca scoring, target computed tomography angiography (CTA), prospective CTA and retrospective gating, continuous CTA/CFA (cardiac functional analysis) and CTA/CFA modulation. Even in the case of a volume scan, doses for the multiple scan average dose were similar to those for CTDI. Moreover, it was found that the ECG gate scan yields significantly different doses. When selecting the optimum scan, the doses were dependent on many factors such as HR, scan rotation time, active time, prespecified cardiac phase and modulation rate. Therefore, it is necessary to take these results into consideration when selecting the scanning parameters.
INTRODUCTION The Society of Cardiovascular Computed Tomography has published guidelines to ensure reliable scanning methods and outcomes for coronary computed tomography angiography (CTA)(1, 2). These guidelines recommend recording the dose length product and the effective dose for each patient and that the dose values at any given institution be revised if the dose values are high in comparison with the documented reference values. Moreover, the computed tomography dose index (CTDI) is regarded as inappropriate for use as an alternative to the total exposure dose for management of radiation exposure doses because the scan length is not taken into consideration(1, 2). However, various international organisations recommend diagnostic reference levels for computed tomography (CT) examinations based on the CTDI(3 – 7). The dose optimisation for X-ray CT scanning, including that for coronary CTA, must be evaluated in terms of the CTDI. The CTDI has been defined by a number of different international organisations, including the International Commission on Radiological Protection(3, 4) and the International Electrotechnical Commission (IEC)(5 – 7). The definition of the CTDI was partially modified in 2009 in IEC60601-2-44 Ed. 3(7), which was the first international standard to consider CT scanners with a larger number of detector rows, as follows: CTDI100 ¼
1 minfNT; 100g
ð þ50 mm DðzÞdz; 50 mm
ð1Þ
where N, T and D(z) indicate the number of detector rows, the nominal slice thickness and the dose distribution along the z-axis, respectively. When NT is greater than 100 mm, the physical meaning of CTDI100 changes from the average dose at the centre of a 100-mm scan length to the average dose over the central 100-mm region of a single axial scan. In addition, Mori et al.(8), Geleijns et al.(9) and Report No. 3 of the American Association of Physicists in Medicine(10) found that the CTDI value is underestimated when using standard lengths for the polymethylmethacrylate (PMMA) phantom and ionisation chamber. It is now necessary to describe the properties of broad beam width CT systems. The coronary CTA is an example of a CT examination that uses 320-row multiple-detector CT (MDCT) with a broad beam width. The diagnostic quality of a coronary CTA is generally accepted to be highly dependent on a number of technical factors, including the hardware, software and acquisition protocols, and the patient radiation exposure varies with the image acquisition protocol and settings. However, dose measurement methods for scan applications that are synchronised with the electrocardiogram (ECG) used during the coronary CTA have yet to be considered. In a 320-row MDCT scan without an ECG, an optimised image can be reconstructed from one axial scan. In contrast, when an ECG is used, an optimised image (fewer motion artefacts) cannot necessarily be reconstructed from one rotation of the X-ray tube. In a coronary CTA with an ECG, the heart rate (HR)
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Masanao Kobayashi1,2,*, Yasuki Asada3, Kosuke Matsubara2, Kichiro Koshida2, Shouichi Suzuki3, Yuta Matsunaga4, Ai Kawaguchi5, Tomonobu Haba1, Kazuhiro Katada6 and Hiroshi Toyama6 1 Department of Radiology, Fujita Health University Hospital, Toyoake, Japan 2 Graduate School of Medical Sciences, Division of Medical Sciences, Kanazawa University, Kanazawa, Japan 3 Graduate School of Health Sciences, Fujita Health University, Toyoake, Japan 4 Department of Imaging, Nagoya Kyoritsu Hospital, Nagoya, Japan 5 Department of Radiology, Toyota Memorial Hospital, Toyota, Japan 6 Department of Radiology, Fujita Health University School of Medicine, Toyoake, Japan
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
MATERIALS AND METHODS Multiple scan average dose The medical X-ray CT device used was a 320-row MDCT (Aquilion ONE, Toshiba Medical Systems, Otawara, Japan). The CT examinations involve exposures taken from multiple rotations of the X-ray source; thus, the total dose to the irradiated volume is equal to
the accumulated dose from adjacent scans. One quantity used to describe this accumulated dose is the MSAD, which is the dose from an examination consisting of multiple scans, averaged over one scan interval in the central region of the multiple scan dose profile. The MSAD value is determined by the extent of the overlap between scan slices and is valid only if the examination uses more than just a few scans. The MSAD is defined as the effective sum of the dose profiles for a scan series:
MSAD ¼
1 I
ð þI
2
2I
DN; I ðzÞdz;
ð2Þ
where N is the total number of scans in a clinical series, I is the distance increment that separates the scans and D(z) is the dose at position z, parallel to the z-axis(11). The centre of the PMMA phantom, which had a diameter of 320 mm, was placed on the patient support and was aligned with the centre of rotation. Forty-two thermoluminescence dosemeter (TLD) chips (MSO-S, Kyokko, Japan) were put into the 10-mm-diameter PMMA cylinder at intervals of 3.5 mm (Figure 1). The phantom was exposed to radiation using tube voltage of 135 kV, tube current of 200 mA, nominal beam width of 160 mm (0.5 mm` 320 rows), field of view of 320 mm and X-ray tube rotation times of 0.35, 0.375 and 0.4 s without an ECG. The MSAD was evaluated using the average values of the four TLD chips located from 25.25 mm to 5.25 mm along the z-axis. When the MSAD is measured for multiple slices and the distance between slices is equal to the beam width (or the pitch ¼ 1), the MSAD is equal to the CTDI that is
Figure 1. Schematic of the instrumentation used for measuring the radiation exposure in CT.
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and its variability are critically important for image reconstruction. At lower HRs [,65 beats min21 (bpm)], the optimal phase will generally be found in the late diastole, whereas at higher HRs (.65 –70 bpm), the optimal phase will more frequently (but not always) occur at the end systole(1, 2). For a coronary CTAwith an ECG, it is necessary to select the optimal heart cycle and cardiac phase for X-ray irradiation, and thus, it is necessary to consider the scanning situation when measuring the CTDI. In this study, the first report on the properties of large volume scans with an ECG is presented. The relation between HR and CTDI with an ECG may provide significant information for improving scanning protocols. For this study, it was first confirmed that the CTDI of a volume scan without an ECG corresponded to the multiple scan average dose (MSAD). Then, the relation between HR and CTDI was demonstrated using cardiac applications. For CTDI measurements, problems involving ionisation chambers and polymethylmethacrylate (PMMA) phantoms have been reported(8 – 10). However, in this work, the CTDI was assumed to be the appropriate method for evaluating dose optimisation.
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measured in the phantom at the same radial position as the MSAD(12, 13). CTDI For evaluating the CTDI, a pencil chamber was used with an effective length of 100 mm and volume of 3 cm3 (3CT, Radcal Corporation, Monrovia, CA, USA). A dosemeter (9015, Radcal) was used instead of TLD chips (Figure 1). The scan conditions were the same as those used for the MSAD measurement. The definition of CTDI100 is given in Equation 1(7). The dose measurement at the centre of the phantom is defined as CTDI100,c, and the average doses for the peripheral locations at the 0, 3, 6 and 9 o’clock positions are defined as CTDI100,p. The standard deviation (SD) was derived with respect to CTDI100,p (Table 1).
Calcium scoring Calcium scoring, which is used to evaluate the amount of Ca in coronary arteries, is reported to be an effective index for deciding the appropriate treatment programme for a given risk group, depending on the degree of risk (e.g. using the Framingham risk score)(14). In Ca scoring, the X-ray tube is activated only during a prespecified phase (a %) within the cardiac cycle (see Figure 2a). The potential disadvantage of prospective triggering lies in the fact that images can be reconstructed only during the prespecified phase of the cardiac cycle, and no image reconstruction is possible outside that interval. The irradiation times were standardised to occur at the following optimal cardiac phases: 75 % of the R–R
CTDI with an ECG The scanning conditions were the same as those used for the CTDI measurement. A simulated ECG waveform was utilised for the HR, and the HR was evaluated over 40 –120 bpm in increments of 10 bpm (IVY l 3000, Chronos Medical Devices, Inc., Chiba, Japan). The CTDI was measured using CTDI100,c, which is independent of the X-ray tube starting position. The scanning time was varied in accordance with the HR; however, dose measurements were performed for one or more rotations, depending on the
Table 1. CTDI100 without ECG gate scanning. Rotation time (s)
CTDI100,c (mGy)
CTDI100,0 (mGy)a
CTDI100,3 (mGy)a
CTDI100,6 (mGy)a
CTDI100,9 (mGy)a
5.1 + 0.1 5.7 + 0.1 6.0 + 0.1
10.1 + 2.5 9.7 + 2.6 11.3 + 1.1
9.3 + 0.6 11.1 + 2.2 11.0 + 0.7
8.4 + 0.4 9.5 + 1.3 11.0 + 1.6
10.1 + 1.6 11.4 + 1.4 11.1 + 0.6
0.35 0.375 0.4 a
Peripheral locations at the 0, 3, 6 and 9 o’clock positions.
Table 2. Prespecified cardiac phase for X-ray irradiation. HR (bpm)
Prespecified cardiac phase Ca scoring (%)
Target CTA (%)
Prospective CTA (%)
Continuous CTA/CFA (%)
CTA/CFA modulation (%)
40–60
75
75
0 –100
0–100
70–120
40
40
75–80; 5 % 70–80; 10 % 65–80; 15 % 60–80; 20 % 35–40; 5 % 35–45; 10 % 35–50; 15 % 35–55; 20 %
0 –100
0–100
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scan applications such as prospective triggering, Ca scoring, target CTA, prospective CTA and retrospective gating, continuous CTA/CFA (cardiac functional analysis) and CTA/CFA modulation. The results of all measurements are given by CTDI100,c,ECG. The details of each technique are presented below; the scan protocols are summarised in Table 2. A diagram illustrating the prospective ECG gating and retrospective ECG gating is shown in Figure 2, where two ECG waveforms are shown for each gating technique. The distance between the spikes represents one heartbeat, whereas the shaded areas represent the X-ray exposure and tube current modulation.
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Figure 3. The MSAD measurements obtained using the PMMA phantom with a diameter of 320 mm.
interval for 40 –60 bpm and 40 % for 70 –120 bpm (see Figure 2).
reconstruction. The CTDI100,c,ECG values were evaluated for one heartbeat (see Figure 2d).
Target CTA
CTA/CFA modulation
Target CTA permits scanning to be performed with shorter X-ray exposures irrespective of variations in the patient HR. In this technique, the X-ray tube is activated only during a prespecified cardiac phase (a %) within the cardiac cycle and covers a prespecified acquisition time (see Figure 2b). The CTDI100,c,ECG values were acquired at 0.35, 0.375, 0.4 and 0.5 s for tube rotation times of 0.35 and 0.375 s and were acquired at 0.4, 0.45 and 0.5 s for a tube rotation time of 0.4 s. The values were evaluated for various acquisition times. Irradiation occurred at the 75 % (late diastole) phase for 40 –60 bpm and the 40 % (end systole) phase for 70–120 bpm.
In CTA/CFA modulation, the X-ray tube is activated throughout the entire cardiac cycle: 0–100 %. Moreover, the intensity-modulated X-ray beam is activated for the entire R –R interval during imaging. The tube current was increased during the prespecified cardiac phase of the R –R interval and decreased during the remainder of the R–R interval. The CTDI100,c,ECG values were evaluated for the various cardiac phase ranges, rotation times. Outside of these prespecified cardiac phase ranges (a –b %), the tube current was decreased by 25 and 50 % (see Figure 2e). Then, the phase ranges of 70 –80 %; 10, 65 –80 %; 15, 60 –80 %; 20 and 35 –80 %; 45 % of the R–R interval for HR 40–60 bpm and 35 –50 %; 15, 35–80 %; 45 and 35 %–Next R; 65 % (i.e. the next R peak) of the R–R interval for HR 40–80 beats min21 were considered.
Prospective CTA Prospective CTA is used for morphological evaluation of the coronary artery and collects image data over a prespecified range of the cardiac phase. Currently, this technique is the most effective method for lowering the radiation dose from a coronary CTA(1, 2). In prospective CTA, the X-ray tube is activated only during a prespecified cardiac phase (a–b %) within the cardiac cycle (see Figure 2c). The CTDI100,c,ECG values were evaluated for various cardiac phase ranges, rotation times and acquisition times. Continuous CTA/CFA Continuous CTA/CFA (i.e. retrospective gating) is used for the morphological evaluation of the coronary artery and for cardiac function analysis. In continuous CTA/CFA, the X-ray tube is activated throughout the entire cardiac cycle, 0–100 %, and only the data acquired during the cardiac phase with the least amount of motion are used for image
RESULTS MSAD The dose profiles for the 42TLD chips located along the z-axis are shown in Figure 3. The MSAD was evaluated using the average values of the four TLD chips arranged from 25.25 to 5.25 mm, which were 5.4+1.0 mGy, 0.35 s; 5.5+0.6 mGy, 0.375 s; and 6.1+1.2 mGy, 0.4 s for the volume scan, with a scan width of 160 mm. CTDI without an ECG The CTDI100,c values calculated from expressions given in IEC60601-2-44 Ed.37 differed from the MSAD values by 5 %, 0.35 s; 4 %, 0.375 s; and 2 %, 0.4 s, respectively. In all cases, the SDs of the peripheral locations were higher than CTDI100,c. As a result,
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Figure 2. An illustration of the prospective ECG gating and retrospective ECG gating.
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the CTDI with an ECG was evaluated using only CTDI100,c (Table 1). Ca scoring The CTDI100,c,ECG values were measured to be 5.2 mGy for a rotation time of 0.35 s, 5.5 mGy for 0.375 s and 5.9 mGy for 0.4 s (Figure 4). These values were 0.1 mGy (2 %), 0.2 mGy (3 %) and 0.2 mGy (3 %) different from those for CTDI100,c without an ECG.
These results suggest that the CTDI is independent of HR and cardiac phase when the Ca scoring technique is used. Note that CTDI100,c,ECG increased with longer X-ray tube rotation times. Target CTA
Prospective CTA Figure 4. The CTDI100.c.ECG values for the Ca scoring mode.
For the 75 –80 and 35 –40 % cardiac phases, CTDI100,c,ECG was measured to be 5.1 mGy for a
Figure 5. The CTDI100.c.ECG values for target CTA mode, where the acquisition time is specified by the rotation time of the X-ray tube.
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The CTDI100,c,ECG values were measured to be 5.1, 5.5 and 5.8 mGy, respectively, for a rotation time of 0.35, 0.375 and 0.4 s and an acquisition time of 0.35, 0.375 and 0.4 s (Figure 5). These results are the same as those for CTDI100,c without an ECG and the CTDI100,ECG values for Ca scoring. In addition, the rotation times of 0.35 and 0.375 s were measured with acquisition times of 0.4 s. In these cases, CTDI100,c,ECG was 5.8 mGy, independent of the rotation time. Thus, these CTDI100,c,ECG results suggest that the CTDI is independent of HR, cardiac phase and X-ray tube rotation time for target CTA. Moreover, CTDI100,c,ECG increased when the acquisition time increased.
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Continuous CTA/CFA For an HR of 60 bpm, CTDI100,c,ECG was 17.4 mGy for a rotation time of 0.35 s, 17.9 mGy for 0.375 s, and 17.9 mGy for 0.4 s. These values are more than three times those found for the one scan rotation. Moreover, CTDI100,c,ECG was higher for low HR, declining with increasing HR (Figure 8). Because the difference in X-ray output due to different X-ray tube rotation speeds was extremely small, no significant difference in dose was observed as a result of the influence of factors such as output reproducibility and ionisation chamber sensitivity. That is, time to X-ray output even if rotation time changes is constant. Thus, CTDI100,c,ECG for continuous CTA/CFA was dependent on HR only.
CTA/CFA modulation Compared with continuous CTA/CFA, CTDII00,c,ECG for the CTA/CFA modulation technique (Figures 9, 10) was lower at all conditions. In addition, CTDII00,c,ECG values for the tube current modulation of 50 % was lower than that for modulation of 25 %. In particular, the CTDII00,c,ECG values with modulation of 25 % were ,19 % at 40 bpm, 14 % at 60 bpm and 6 % at
Figure 6. The CTDI100.c.ECG values for prospective CTA. The prospective CTA mode collects images concerning the prespecified cardiac phase range.
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rotation time of 0.35 s, 5.4 mGy for 0.375 s and 5.8 mGy for 0.4 s. These results are the same as those for CTDI100,c without an ECG and the CTDI100,ECG determined for Ca scoring (Figure 6). In addition, the results for the 70–80 and 35 –45 % cardiac phases were the same. For large prespecified cardiac phase ranges, the CTDI100,c,ECG values increased for low HRs. In general, for an HR of 60 bpm, CTDI100,c,ECG was 5.1 mGy for the cardiac phase range of 5 %, 5.1 mGy for 10 %, 5.5 mGy for 15 % and 6.1 mGy for 20 %. However, CTDI100,c,ECG was not influenced by the prespecified cardiac phase range and had the same value for an HR of 90 bpm or more. For prospective CTA, CTDI100,c,ECG was correlated with HR and the cardiac phase ranges. For 70 bpm or more, the value of CTDI100,c,ECG was constant regardless of cardiac phase ranges. On the other hand, when the HR was ,70 bpm, CTDI100,c,ECG increased with broadening cardiac phase ranges and also increased with longer X-ray tube rotation times (Table 3). The limitations of the cardiac phase range for reducing the patient dose as a function of HR are shown in Figure 7; the HR values and ranges are 40 bpm and 9 %, 50 bpm and 11 %, 60 bpm and 13 %, 70 bpm and 15 %, 80 bpm and 17 %, 100 bpm and 20 %, 110 bpm and 22 %, and 120 bpm and 24 %.
M. KOBAYASHI ET AL. Table 3. The time of cardiac phase for prospective CTA. The time of cardiac phasea (s)
Cardiac phase range (%)
5 10 15 20 a
40
50
60
70
80
90
100
110
120
0.08 0.15 0.23 0.30
0.06 0.12 0.18 0.24
0.05 0.10 0.15 0.20
0.04 0.09 0.13 0.17
0.04 0.08 0.11 0.15
0.03 0.07 0.10 0.13
0.03 0.06 0.09 0.12
0.03 0.05 0.08 0.11
0.03 0.05 0.08 0.10
The shaded area indicates the dose for one rotation in Figure 6.
Figure 8. The CTDI100.c.ECG values for the continuous CTA/CFA mode.
80 bpm for continuous CTA/CFA. With 50 % modulation, the CTDII00,c,ECG values were ,43, 33 and 19 %, respectively. The CTDI100,c,ECG for CTA/CFA modulation was dependent on the HR, modulation phase ranges and tube current modulation. DISCUSSION The dose has been shown to vary for different coronary CTA scan techniques using 320-row MDCT. These results indicate that using a narrow prespecified
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Figure 7. The limitation of the cardiac phase range for prospective CTA.
cardiac phase range may not reduce the patient dose. For example, in the case of prospective coronary CTA, the limitations of the cardiac phase range required to reduce the patient dose for a given HR were determined, as shown in Figure 4. If the narrow prespecified cardiac phase range exceeds these values, the patient dose is not reduced. To determine the preferred scanning conditions for a coronary CTA, the relation between the CTDI and the HR is useful. To obtain these results, the MSAD must be measured first. The MSAD is the dose measurement method that most simply and appropriately reflects the dose resulting from CT examinations using multiple scans. The average dose at the centre of the PMMA phantom resulting from multiple scans is evaluated. In volume scanning, the radiation exposure is accomplished using a single scan rotation. In this case, the MSAD evaluates the average dose at the centre position within the PMMA phantom resulting from a single scan. For actual measurements, the uncertainties involved in the TLD dose determination must be analysed, because the TLD has an energy dependence and non-uniform sensitivity. To obtain a correct measurement, the response of the TLD chips at an effective energy of 62.4 keV was applied as the calibration factor. However, compared with the error range for the measured dose values, the calibration factor for the TLD does not have a significant impact on the measurements. In addition, a more practicable method for evaluating the half-value layer (HVL) has been reported(15, 16). However, it appears that differences in the measuring methods do not impact the results. Because, even if a measuring method is changed, it has been reported that the measurement result of HVL is not affected(17). The relationship between the MSAD and the CTDI has been described by international organisations(3 – 7, 11 – 13). For a volume scan, the definition of CTDI was partially modified in 2009 in IEC60601-244 Ed.3(7), which was the first international standard to consider CT scanners with a larger number of detector rows. However, it should be verified whether the definition given in Ed. 3 produces similar values to those found using the MSAD. The results of this
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Figure 10. The CTDI100.c.ECG values for the CTA/CFA modulation mode, 50 %.
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Figure 9. The CTDI100.c.ECG values for the CTA/CFA modulation mode, 25 %.
M. KOBAYASHI ET AL.
time [i.e. active time (s)] for target CTA and for a relative time [i.e. prespecified cardiac phase (%)] for prospective CTA. To reduce the patient dose, the cardiac phase acquisition window should be kept as narrow as possible. In this study, the limitations of the cardiac phase range required for reducing the patient dose for a given HR were found as shown in Figure 4. These results were estimated for the case where a scan of one rotation plus a (i.e. over-beaming) could reconstruct the image of the cardiac phase range in 0.13 s. If the narrow prespecified cardiac phase range exceeds these values, the patient dose is not reduced. Although there is a report on the doses for cardiac phase ranges beyond those reported here, the mode used may not have led to a reduction of the patient dose(14). For retrospective ECG gating such as continuous CTA/CFA and CTA/CFA modulation, X-ray data are acquired throughout the entire cardiac cycle, and only the data acquired during the cardiac phase with the least motion are used for the image reconstruction. With these results, a prospective scan of two or more beats can be compared with a continuous retrospective scan. In continuous CTA/CFA, CTDI100,c,ECG was dependent on HR only because the time of one heart cycle changes with HR. For example, an HR of 60 bpm is 1.0 s per 1 heart cycle, 90 bpm is 0.75 s and 120 bpm is 0.5 s. In CTA/CFA modulation, CTDI100,c,ECG is evaluated over 40–80 bpm; however, this mode is not very effective for dose reduction at high HRs. Because the length of one heartbeat is short, the tube current is not modulated. Therefore, CTA/CFA modulation cannot be selected for HRs of 90 bpm or more. This study provides useful data for determining the optimal scanning conditions for a coronary CTA. When HR becomes high, CTDI100,c,ECG decreases when using the techniques of prospective CTA, continuous CTA/CFA and CTA/CFA modulation. However, to obtain an optimal image, X-ray irradiation spanning more than one heartbeat is required, and thus, CTDI100,c,ECG increases. For example in the authors’ hospital, coronary CTA examinations at 50 bpm, performed using prospective CTA for the prespecified cardiac phase ranges of 70–80 %, have a dose of 5.1 mGy, whereas for prespecified cardiac phase ranges of 35–50 % at 90 bpm, the dose is 5.1 mGy. However, the X-ray output is active during 35–50 % within three beats. Thus, these coronary CTA examinations at 90 bpm are approximately three times that for those at 50 bpm. In the case of atrial fibrillation at 90 bpm, coronary CTA examinations performed using continuous CTA/CFA within three beats have a dose of 52.0 mGy. This dose is 10 times that for a coronary CTA dose at 60 bpm. In addition, narrowly specifying the cardiac phase might not lead to a decrease in the dose. For example, prospective CTA for the prespecified cardiac phase ranges of 75–80 and 70–80 % for an HR of 60 bpm produced the same
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study indicate that the difference in the MSAD and CTDI values is 5 % or less; furthermore, this difference arises from the non-uniform sensitivity of the TLD. Even in the case of a volume scan, the MSAD measurement and the CTDI should produce similar results. The CTDI has been recommended for use by a number of different international organisations for CT scan dose management (3 – 7, 10 – 13). In these recommendations, the CTDI is defined as being measured over one axial scan (one rotation of the X-ray tube)(4). The authors’ study deviated from the official definition of CTDI by allowing more than one 360degree rotation. This measurement method is an appropriate way to examine dose management for ECG gate scanning using wide-area detectors. In addition, motion-free image reconstruction is crucial for obtaining high-quality diagnostic images for cardiac CTA. For many ECG-based examinations, X-ray irradiation over more than one rotation of X-ray tube is required. It is necessary to measure the dose using the same conditions as those used for the coronary CTA. The CTDI also encompasses CTDI100,p, CTDIw and CTDIvol; however, this study was restricted to the evaluation of CTDI00,c. The CTDI100,p values evaluate the average dose measurements for the 0, 3, 6 and 9 o’clock directions at 10 mm below the surface of the PMMA phantom. These values have large errors, arising from the position of the ionisation chamber, tube current modulation, scanning time and overbeaming. For this reason, the CTDI100,p values were not measured, and the characteristic dose was evaluated using CTDI100,c,ECG. For prospective ECG triggering, such as that used in Ca scoring, target CTA and prospective CTA, the X-ray tube is activated only during a prespecified cardiac phase within the cardiac cycle. The CTDI100,c,ECG values for Ca scoring are 5.2 –5.9 mGy (0.35 –0.4 s), which is 3 % or lesser than the CTDI100,c without an ECG. In addition, similar results were obtained for target CTA for scan conditions in which the rotation time and the acquisition time were the same. These results indicate that in order to reconstruct the image from a cardiac phase, X-ray irradiation spanning one rotation is needed. In Ca scoring, the X-ray tube is active for only one rotation in order to obtain the image data during a prespecified cardiac phase (35 or 75 %). In target CTA, the X-ray tube is active during the acquisition time. These scan modes are effective for lowering the radiation dose in coronary CT. However, to use these techniques, the cardiac phase of the patients must be carefully selected. The prospective CTA is the most practical scan mode because the HR of patient may change during the scan timing. In this case, X-ray irradiation was performed for one rotation scan regardless of the HR change for Ca scoring. In contrast, X-ray irradiation was performed for an absolute
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
CTDI100,c,ECG; this is a crucial result of this study. Therefore, it is necessary to take this result into consideration when selecting the scanning parameters. This study shows some potential weaknesses because the data were acquired from only one scanner, and there was no attempt to change X-ray tube voltage. Therefore, dose evaluation for cardiac applications of various scanners is desired. REFERENCES
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1. Raff, G. L., Abidov, A., Achenbach, S., Berman, D. S., Boxt, L. M., Budoff, M. J., Cheng, V., DeFrance, T., Hellinger, J. C. and Karlsberg, R. P. SCCT guidelines for the interpretation and reporting of coronary computed tomographic angiography. J. Cardiovasc. Comput. Tomogr. 3, 122– 136 (2009). 2. Abbara, S., Arbab-Zadeh, A., Callister, T. Q., Desai, M. Y., Mamuya, W., Thomson, L. and Weigold, W. G. SCCT guidelines for performance of coronary computed tomographic angiography: a report of the Society of Cardiovascular Computed Tomography Guidelines Committee. J. Cardiovasc. Comput. Tomog. 3, 190– 204 (2009). 3. ICRP Publication 87: Managing patient dose in computed tomography. Ann. ICRP. 30(4), (2002). 4. ICRP Publication 102: Managing patient dose in multidetector computed tomography (MDCT). Ann. ICRP. 37(1), (2007). 5. International Electrotechnical Commission. Medical electrical equipment—Part 2-44: particular requirements for the safety of X-ray equipment for computed tomography. IEC publication no. 60601-2-44, Ed. 1. IEC Central Office (2000). 6. International Electrotechnical Commission. Medical electrical equipment—Part 2-44: particular requirements for the safety of X-ray equipment for computed tomography. IEC publication no. 60601-2-44, Ed. 2. IEC Central Office (2003). 7. International Electrotechnical Commission. Medical electrical equipment—Part 2-44: particular requirements for the basic safety and essential performance of X-ray equipment for computed tomography. IEC publication no. 60601-2-44, Ed. 3. IEC Central Office (2009).
8. Mori, S., Endo, M., Nishizawa, K., Tsunoo, T., Aoyama, T., Fujiwara, H. and Murase, K. Enlarged longitudinal dose profiles in cone-beam CT and the need for modified dosimetry. Med. Phys. 32, 1061– 1069 (2005). 9. Geleijns, J., Salvado´, A. M., de Bruin, P. W., Matter, R., Muramatsu, Y. and McNitt-Gray, M. F. Computed tomography dose assessment for a 160 mm wide, 320 detector row, cone beam CT scanner. Phys. Med. Biol. 54, 3141–3159 (2009). 10. American Association of Physicists in Medicine (AAPM) Report of AAPM Task Group 111. Comprehensive methodology for the evaluation of radiation dose in X-ray computed tomography (2010). Available on http://www.aapm.org/pubs/reports/RPT_ 111.pdf (12 December 2010, date last accessed). 11. International Atomic Energy Agency. International basic safety standards for protection against ionizing radiation and for the safety of radiation sources. IAEA Safety Series No.115. Vienna (1996). 12. International Atomic Energy Agency. Optimization of the radiological protection of patients undergoing radiography, fluoroscopy and computed tomography. IAEATECDOC-1423. Vienna (2004). 13. European Commission. European guidelines on quality criteria for computed tomography. EUR 16262. Office for Official Publications of the European Communities (2000). 14. Chen, M. Y., Shanbhag, S. M. and Arai, A. E. Submillisievert median radiation dose for coronary angiography with a second-generation 320-detector row CT scanner in 107 consecutive patients. Radiology 267(1), 76– 85 (2013). 15. Maia, A. F. and Caldas, L. V. E. A simple method for evaluation of half-value layer variation in CT equipment. Phys. Med. Biol. 51, 1595–1601 (2006). 16. Matsubara, K., Ichikawa, K., Murasaki, Y., Hirosawa, A. and Koshida, K. Accuracy of measuring half- and quarter-value layers and appropriate aperture width of a convenient method using a lead-covered case in X-ray computed tomography. J. Appl. Clin. Med. Phys. 15(1), 309–316 (2014). 17. Kondo, H., Matsubara, K., Hirosawa, A. and Koshida, K. A comparison of several convenient methods of estimating effective energy in X-ray computed tomography. Jpn J. Radiol. Technol. 70(5), 453– 460 (2014).
doi:10.1093/rpd/ncu331
EVALUATION OF THE CT DOSE INDEX FOR SCANS WITH AN ECG USING A 320-ROW MULTIPLE-DETECTOR CT SCANNER
*Corresponding author: [email protected] Received 12 June 2014; revised 7 October 2014; accepted 10 October 2014 The relationship between heart rate (HR) and computed tomography dose index (CTDI) was evaluated using an electrocardiogram (ECG) gate scan for scan applications such as prospective triggering, Ca scoring, target computed tomography angiography (CTA), prospective CTA and retrospective gating, continuous CTA/CFA (cardiac functional analysis) and CTA/CFA modulation. Even in the case of a volume scan, doses for the multiple scan average dose were similar to those for CTDI. Moreover, it was found that the ECG gate scan yields significantly different doses. When selecting the optimum scan, the doses were dependent on many factors such as HR, scan rotation time, active time, prespecified cardiac phase and modulation rate. Therefore, it is necessary to take these results into consideration when selecting the scanning parameters.
INTRODUCTION The Society of Cardiovascular Computed Tomography has published guidelines to ensure reliable scanning methods and outcomes for coronary computed tomography angiography (CTA)(1, 2). These guidelines recommend recording the dose length product and the effective dose for each patient and that the dose values at any given institution be revised if the dose values are high in comparison with the documented reference values. Moreover, the computed tomography dose index (CTDI) is regarded as inappropriate for use as an alternative to the total exposure dose for management of radiation exposure doses because the scan length is not taken into consideration(1, 2). However, various international organisations recommend diagnostic reference levels for computed tomography (CT) examinations based on the CTDI(3 – 7). The dose optimisation for X-ray CT scanning, including that for coronary CTA, must be evaluated in terms of the CTDI. The CTDI has been defined by a number of different international organisations, including the International Commission on Radiological Protection(3, 4) and the International Electrotechnical Commission (IEC)(5 – 7). The definition of the CTDI was partially modified in 2009 in IEC60601-2-44 Ed. 3(7), which was the first international standard to consider CT scanners with a larger number of detector rows, as follows: CTDI100 ¼
1 minfNT; 100g
ð þ50 mm DðzÞdz; 50 mm
ð1Þ
where N, T and D(z) indicate the number of detector rows, the nominal slice thickness and the dose distribution along the z-axis, respectively. When NT is greater than 100 mm, the physical meaning of CTDI100 changes from the average dose at the centre of a 100-mm scan length to the average dose over the central 100-mm region of a single axial scan. In addition, Mori et al.(8), Geleijns et al.(9) and Report No. 3 of the American Association of Physicists in Medicine(10) found that the CTDI value is underestimated when using standard lengths for the polymethylmethacrylate (PMMA) phantom and ionisation chamber. It is now necessary to describe the properties of broad beam width CT systems. The coronary CTA is an example of a CT examination that uses 320-row multiple-detector CT (MDCT) with a broad beam width. The diagnostic quality of a coronary CTA is generally accepted to be highly dependent on a number of technical factors, including the hardware, software and acquisition protocols, and the patient radiation exposure varies with the image acquisition protocol and settings. However, dose measurement methods for scan applications that are synchronised with the electrocardiogram (ECG) used during the coronary CTA have yet to be considered. In a 320-row MDCT scan without an ECG, an optimised image can be reconstructed from one axial scan. In contrast, when an ECG is used, an optimised image (fewer motion artefacts) cannot necessarily be reconstructed from one rotation of the X-ray tube. In a coronary CTA with an ECG, the heart rate (HR)
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
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Masanao Kobayashi1,2,*, Yasuki Asada3, Kosuke Matsubara2, Kichiro Koshida2, Shouichi Suzuki3, Yuta Matsunaga4, Ai Kawaguchi5, Tomonobu Haba1, Kazuhiro Katada6 and Hiroshi Toyama6 1 Department of Radiology, Fujita Health University Hospital, Toyoake, Japan 2 Graduate School of Medical Sciences, Division of Medical Sciences, Kanazawa University, Kanazawa, Japan 3 Graduate School of Health Sciences, Fujita Health University, Toyoake, Japan 4 Department of Imaging, Nagoya Kyoritsu Hospital, Nagoya, Japan 5 Department of Radiology, Toyota Memorial Hospital, Toyota, Japan 6 Department of Radiology, Fujita Health University School of Medicine, Toyoake, Japan
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
MATERIALS AND METHODS Multiple scan average dose The medical X-ray CT device used was a 320-row MDCT (Aquilion ONE, Toshiba Medical Systems, Otawara, Japan). The CT examinations involve exposures taken from multiple rotations of the X-ray source; thus, the total dose to the irradiated volume is equal to
the accumulated dose from adjacent scans. One quantity used to describe this accumulated dose is the MSAD, which is the dose from an examination consisting of multiple scans, averaged over one scan interval in the central region of the multiple scan dose profile. The MSAD value is determined by the extent of the overlap between scan slices and is valid only if the examination uses more than just a few scans. The MSAD is defined as the effective sum of the dose profiles for a scan series:
MSAD ¼
1 I
ð þI
2
2I
DN; I ðzÞdz;
ð2Þ
where N is the total number of scans in a clinical series, I is the distance increment that separates the scans and D(z) is the dose at position z, parallel to the z-axis(11). The centre of the PMMA phantom, which had a diameter of 320 mm, was placed on the patient support and was aligned with the centre of rotation. Forty-two thermoluminescence dosemeter (TLD) chips (MSO-S, Kyokko, Japan) were put into the 10-mm-diameter PMMA cylinder at intervals of 3.5 mm (Figure 1). The phantom was exposed to radiation using tube voltage of 135 kV, tube current of 200 mA, nominal beam width of 160 mm (0.5 mm` 320 rows), field of view of 320 mm and X-ray tube rotation times of 0.35, 0.375 and 0.4 s without an ECG. The MSAD was evaluated using the average values of the four TLD chips located from 25.25 mm to 5.25 mm along the z-axis. When the MSAD is measured for multiple slices and the distance between slices is equal to the beam width (or the pitch ¼ 1), the MSAD is equal to the CTDI that is
Figure 1. Schematic of the instrumentation used for measuring the radiation exposure in CT.
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and its variability are critically important for image reconstruction. At lower HRs [,65 beats min21 (bpm)], the optimal phase will generally be found in the late diastole, whereas at higher HRs (.65 –70 bpm), the optimal phase will more frequently (but not always) occur at the end systole(1, 2). For a coronary CTAwith an ECG, it is necessary to select the optimal heart cycle and cardiac phase for X-ray irradiation, and thus, it is necessary to consider the scanning situation when measuring the CTDI. In this study, the first report on the properties of large volume scans with an ECG is presented. The relation between HR and CTDI with an ECG may provide significant information for improving scanning protocols. For this study, it was first confirmed that the CTDI of a volume scan without an ECG corresponded to the multiple scan average dose (MSAD). Then, the relation between HR and CTDI was demonstrated using cardiac applications. For CTDI measurements, problems involving ionisation chambers and polymethylmethacrylate (PMMA) phantoms have been reported(8 – 10). However, in this work, the CTDI was assumed to be the appropriate method for evaluating dose optimisation.
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measured in the phantom at the same radial position as the MSAD(12, 13). CTDI For evaluating the CTDI, a pencil chamber was used with an effective length of 100 mm and volume of 3 cm3 (3CT, Radcal Corporation, Monrovia, CA, USA). A dosemeter (9015, Radcal) was used instead of TLD chips (Figure 1). The scan conditions were the same as those used for the MSAD measurement. The definition of CTDI100 is given in Equation 1(7). The dose measurement at the centre of the phantom is defined as CTDI100,c, and the average doses for the peripheral locations at the 0, 3, 6 and 9 o’clock positions are defined as CTDI100,p. The standard deviation (SD) was derived with respect to CTDI100,p (Table 1).
Calcium scoring Calcium scoring, which is used to evaluate the amount of Ca in coronary arteries, is reported to be an effective index for deciding the appropriate treatment programme for a given risk group, depending on the degree of risk (e.g. using the Framingham risk score)(14). In Ca scoring, the X-ray tube is activated only during a prespecified phase (a %) within the cardiac cycle (see Figure 2a). The potential disadvantage of prospective triggering lies in the fact that images can be reconstructed only during the prespecified phase of the cardiac cycle, and no image reconstruction is possible outside that interval. The irradiation times were standardised to occur at the following optimal cardiac phases: 75 % of the R–R
CTDI with an ECG The scanning conditions were the same as those used for the CTDI measurement. A simulated ECG waveform was utilised for the HR, and the HR was evaluated over 40 –120 bpm in increments of 10 bpm (IVY l 3000, Chronos Medical Devices, Inc., Chiba, Japan). The CTDI was measured using CTDI100,c, which is independent of the X-ray tube starting position. The scanning time was varied in accordance with the HR; however, dose measurements were performed for one or more rotations, depending on the
Table 1. CTDI100 without ECG gate scanning. Rotation time (s)
CTDI100,c (mGy)
CTDI100,0 (mGy)a
CTDI100,3 (mGy)a
CTDI100,6 (mGy)a
CTDI100,9 (mGy)a
5.1 + 0.1 5.7 + 0.1 6.0 + 0.1
10.1 + 2.5 9.7 + 2.6 11.3 + 1.1
9.3 + 0.6 11.1 + 2.2 11.0 + 0.7
8.4 + 0.4 9.5 + 1.3 11.0 + 1.6
10.1 + 1.6 11.4 + 1.4 11.1 + 0.6
0.35 0.375 0.4 a
Peripheral locations at the 0, 3, 6 and 9 o’clock positions.
Table 2. Prespecified cardiac phase for X-ray irradiation. HR (bpm)
Prespecified cardiac phase Ca scoring (%)
Target CTA (%)
Prospective CTA (%)
Continuous CTA/CFA (%)
CTA/CFA modulation (%)
40–60
75
75
0 –100
0–100
70–120
40
40
75–80; 5 % 70–80; 10 % 65–80; 15 % 60–80; 20 % 35–40; 5 % 35–45; 10 % 35–50; 15 % 35–55; 20 %
0 –100
0–100
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scan applications such as prospective triggering, Ca scoring, target CTA, prospective CTA and retrospective gating, continuous CTA/CFA (cardiac functional analysis) and CTA/CFA modulation. The results of all measurements are given by CTDI100,c,ECG. The details of each technique are presented below; the scan protocols are summarised in Table 2. A diagram illustrating the prospective ECG gating and retrospective ECG gating is shown in Figure 2, where two ECG waveforms are shown for each gating technique. The distance between the spikes represents one heartbeat, whereas the shaded areas represent the X-ray exposure and tube current modulation.
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Figure 3. The MSAD measurements obtained using the PMMA phantom with a diameter of 320 mm.
interval for 40 –60 bpm and 40 % for 70 –120 bpm (see Figure 2).
reconstruction. The CTDI100,c,ECG values were evaluated for one heartbeat (see Figure 2d).
Target CTA
CTA/CFA modulation
Target CTA permits scanning to be performed with shorter X-ray exposures irrespective of variations in the patient HR. In this technique, the X-ray tube is activated only during a prespecified cardiac phase (a %) within the cardiac cycle and covers a prespecified acquisition time (see Figure 2b). The CTDI100,c,ECG values were acquired at 0.35, 0.375, 0.4 and 0.5 s for tube rotation times of 0.35 and 0.375 s and were acquired at 0.4, 0.45 and 0.5 s for a tube rotation time of 0.4 s. The values were evaluated for various acquisition times. Irradiation occurred at the 75 % (late diastole) phase for 40 –60 bpm and the 40 % (end systole) phase for 70–120 bpm.
In CTA/CFA modulation, the X-ray tube is activated throughout the entire cardiac cycle: 0–100 %. Moreover, the intensity-modulated X-ray beam is activated for the entire R –R interval during imaging. The tube current was increased during the prespecified cardiac phase of the R –R interval and decreased during the remainder of the R–R interval. The CTDI100,c,ECG values were evaluated for the various cardiac phase ranges, rotation times. Outside of these prespecified cardiac phase ranges (a –b %), the tube current was decreased by 25 and 50 % (see Figure 2e). Then, the phase ranges of 70 –80 %; 10, 65 –80 %; 15, 60 –80 %; 20 and 35 –80 %; 45 % of the R–R interval for HR 40–60 bpm and 35 –50 %; 15, 35–80 %; 45 and 35 %–Next R; 65 % (i.e. the next R peak) of the R–R interval for HR 40–80 beats min21 were considered.
Prospective CTA Prospective CTA is used for morphological evaluation of the coronary artery and collects image data over a prespecified range of the cardiac phase. Currently, this technique is the most effective method for lowering the radiation dose from a coronary CTA(1, 2). In prospective CTA, the X-ray tube is activated only during a prespecified cardiac phase (a–b %) within the cardiac cycle (see Figure 2c). The CTDI100,c,ECG values were evaluated for various cardiac phase ranges, rotation times and acquisition times. Continuous CTA/CFA Continuous CTA/CFA (i.e. retrospective gating) is used for the morphological evaluation of the coronary artery and for cardiac function analysis. In continuous CTA/CFA, the X-ray tube is activated throughout the entire cardiac cycle, 0–100 %, and only the data acquired during the cardiac phase with the least amount of motion are used for image
RESULTS MSAD The dose profiles for the 42TLD chips located along the z-axis are shown in Figure 3. The MSAD was evaluated using the average values of the four TLD chips arranged from 25.25 to 5.25 mm, which were 5.4+1.0 mGy, 0.35 s; 5.5+0.6 mGy, 0.375 s; and 6.1+1.2 mGy, 0.4 s for the volume scan, with a scan width of 160 mm. CTDI without an ECG The CTDI100,c values calculated from expressions given in IEC60601-2-44 Ed.37 differed from the MSAD values by 5 %, 0.35 s; 4 %, 0.375 s; and 2 %, 0.4 s, respectively. In all cases, the SDs of the peripheral locations were higher than CTDI100,c. As a result,
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Figure 2. An illustration of the prospective ECG gating and retrospective ECG gating.
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the CTDI with an ECG was evaluated using only CTDI100,c (Table 1). Ca scoring The CTDI100,c,ECG values were measured to be 5.2 mGy for a rotation time of 0.35 s, 5.5 mGy for 0.375 s and 5.9 mGy for 0.4 s (Figure 4). These values were 0.1 mGy (2 %), 0.2 mGy (3 %) and 0.2 mGy (3 %) different from those for CTDI100,c without an ECG.
These results suggest that the CTDI is independent of HR and cardiac phase when the Ca scoring technique is used. Note that CTDI100,c,ECG increased with longer X-ray tube rotation times. Target CTA
Prospective CTA Figure 4. The CTDI100.c.ECG values for the Ca scoring mode.
For the 75 –80 and 35 –40 % cardiac phases, CTDI100,c,ECG was measured to be 5.1 mGy for a
Figure 5. The CTDI100.c.ECG values for target CTA mode, where the acquisition time is specified by the rotation time of the X-ray tube.
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The CTDI100,c,ECG values were measured to be 5.1, 5.5 and 5.8 mGy, respectively, for a rotation time of 0.35, 0.375 and 0.4 s and an acquisition time of 0.35, 0.375 and 0.4 s (Figure 5). These results are the same as those for CTDI100,c without an ECG and the CTDI100,ECG values for Ca scoring. In addition, the rotation times of 0.35 and 0.375 s were measured with acquisition times of 0.4 s. In these cases, CTDI100,c,ECG was 5.8 mGy, independent of the rotation time. Thus, these CTDI100,c,ECG results suggest that the CTDI is independent of HR, cardiac phase and X-ray tube rotation time for target CTA. Moreover, CTDI100,c,ECG increased when the acquisition time increased.
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Continuous CTA/CFA For an HR of 60 bpm, CTDI100,c,ECG was 17.4 mGy for a rotation time of 0.35 s, 17.9 mGy for 0.375 s, and 17.9 mGy for 0.4 s. These values are more than three times those found for the one scan rotation. Moreover, CTDI100,c,ECG was higher for low HR, declining with increasing HR (Figure 8). Because the difference in X-ray output due to different X-ray tube rotation speeds was extremely small, no significant difference in dose was observed as a result of the influence of factors such as output reproducibility and ionisation chamber sensitivity. That is, time to X-ray output even if rotation time changes is constant. Thus, CTDI100,c,ECG for continuous CTA/CFA was dependent on HR only.
CTA/CFA modulation Compared with continuous CTA/CFA, CTDII00,c,ECG for the CTA/CFA modulation technique (Figures 9, 10) was lower at all conditions. In addition, CTDII00,c,ECG values for the tube current modulation of 50 % was lower than that for modulation of 25 %. In particular, the CTDII00,c,ECG values with modulation of 25 % were ,19 % at 40 bpm, 14 % at 60 bpm and 6 % at
Figure 6. The CTDI100.c.ECG values for prospective CTA. The prospective CTA mode collects images concerning the prespecified cardiac phase range.
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rotation time of 0.35 s, 5.4 mGy for 0.375 s and 5.8 mGy for 0.4 s. These results are the same as those for CTDI100,c without an ECG and the CTDI100,ECG determined for Ca scoring (Figure 6). In addition, the results for the 70–80 and 35 –45 % cardiac phases were the same. For large prespecified cardiac phase ranges, the CTDI100,c,ECG values increased for low HRs. In general, for an HR of 60 bpm, CTDI100,c,ECG was 5.1 mGy for the cardiac phase range of 5 %, 5.1 mGy for 10 %, 5.5 mGy for 15 % and 6.1 mGy for 20 %. However, CTDI100,c,ECG was not influenced by the prespecified cardiac phase range and had the same value for an HR of 90 bpm or more. For prospective CTA, CTDI100,c,ECG was correlated with HR and the cardiac phase ranges. For 70 bpm or more, the value of CTDI100,c,ECG was constant regardless of cardiac phase ranges. On the other hand, when the HR was ,70 bpm, CTDI100,c,ECG increased with broadening cardiac phase ranges and also increased with longer X-ray tube rotation times (Table 3). The limitations of the cardiac phase range for reducing the patient dose as a function of HR are shown in Figure 7; the HR values and ranges are 40 bpm and 9 %, 50 bpm and 11 %, 60 bpm and 13 %, 70 bpm and 15 %, 80 bpm and 17 %, 100 bpm and 20 %, 110 bpm and 22 %, and 120 bpm and 24 %.
M. KOBAYASHI ET AL. Table 3. The time of cardiac phase for prospective CTA. The time of cardiac phasea (s)
Cardiac phase range (%)
5 10 15 20 a
40
50
60
70
80
90
100
110
120
0.08 0.15 0.23 0.30
0.06 0.12 0.18 0.24
0.05 0.10 0.15 0.20
0.04 0.09 0.13 0.17
0.04 0.08 0.11 0.15
0.03 0.07 0.10 0.13
0.03 0.06 0.09 0.12
0.03 0.05 0.08 0.11
0.03 0.05 0.08 0.10
The shaded area indicates the dose for one rotation in Figure 6.
Figure 8. The CTDI100.c.ECG values for the continuous CTA/CFA mode.
80 bpm for continuous CTA/CFA. With 50 % modulation, the CTDII00,c,ECG values were ,43, 33 and 19 %, respectively. The CTDI100,c,ECG for CTA/CFA modulation was dependent on the HR, modulation phase ranges and tube current modulation. DISCUSSION The dose has been shown to vary for different coronary CTA scan techniques using 320-row MDCT. These results indicate that using a narrow prespecified
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Figure 7. The limitation of the cardiac phase range for prospective CTA.
cardiac phase range may not reduce the patient dose. For example, in the case of prospective coronary CTA, the limitations of the cardiac phase range required to reduce the patient dose for a given HR were determined, as shown in Figure 4. If the narrow prespecified cardiac phase range exceeds these values, the patient dose is not reduced. To determine the preferred scanning conditions for a coronary CTA, the relation between the CTDI and the HR is useful. To obtain these results, the MSAD must be measured first. The MSAD is the dose measurement method that most simply and appropriately reflects the dose resulting from CT examinations using multiple scans. The average dose at the centre of the PMMA phantom resulting from multiple scans is evaluated. In volume scanning, the radiation exposure is accomplished using a single scan rotation. In this case, the MSAD evaluates the average dose at the centre position within the PMMA phantom resulting from a single scan. For actual measurements, the uncertainties involved in the TLD dose determination must be analysed, because the TLD has an energy dependence and non-uniform sensitivity. To obtain a correct measurement, the response of the TLD chips at an effective energy of 62.4 keV was applied as the calibration factor. However, compared with the error range for the measured dose values, the calibration factor for the TLD does not have a significant impact on the measurements. In addition, a more practicable method for evaluating the half-value layer (HVL) has been reported(15, 16). However, it appears that differences in the measuring methods do not impact the results. Because, even if a measuring method is changed, it has been reported that the measurement result of HVL is not affected(17). The relationship between the MSAD and the CTDI has been described by international organisations(3 – 7, 11 – 13). For a volume scan, the definition of CTDI was partially modified in 2009 in IEC60601-244 Ed.3(7), which was the first international standard to consider CT scanners with a larger number of detector rows. However, it should be verified whether the definition given in Ed. 3 produces similar values to those found using the MSAD. The results of this
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
Figure 10. The CTDI100.c.ECG values for the CTA/CFA modulation mode, 50 %.
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Figure 9. The CTDI100.c.ECG values for the CTA/CFA modulation mode, 25 %.
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time [i.e. active time (s)] for target CTA and for a relative time [i.e. prespecified cardiac phase (%)] for prospective CTA. To reduce the patient dose, the cardiac phase acquisition window should be kept as narrow as possible. In this study, the limitations of the cardiac phase range required for reducing the patient dose for a given HR were found as shown in Figure 4. These results were estimated for the case where a scan of one rotation plus a (i.e. over-beaming) could reconstruct the image of the cardiac phase range in 0.13 s. If the narrow prespecified cardiac phase range exceeds these values, the patient dose is not reduced. Although there is a report on the doses for cardiac phase ranges beyond those reported here, the mode used may not have led to a reduction of the patient dose(14). For retrospective ECG gating such as continuous CTA/CFA and CTA/CFA modulation, X-ray data are acquired throughout the entire cardiac cycle, and only the data acquired during the cardiac phase with the least motion are used for the image reconstruction. With these results, a prospective scan of two or more beats can be compared with a continuous retrospective scan. In continuous CTA/CFA, CTDI100,c,ECG was dependent on HR only because the time of one heart cycle changes with HR. For example, an HR of 60 bpm is 1.0 s per 1 heart cycle, 90 bpm is 0.75 s and 120 bpm is 0.5 s. In CTA/CFA modulation, CTDI100,c,ECG is evaluated over 40–80 bpm; however, this mode is not very effective for dose reduction at high HRs. Because the length of one heartbeat is short, the tube current is not modulated. Therefore, CTA/CFA modulation cannot be selected for HRs of 90 bpm or more. This study provides useful data for determining the optimal scanning conditions for a coronary CTA. When HR becomes high, CTDI100,c,ECG decreases when using the techniques of prospective CTA, continuous CTA/CFA and CTA/CFA modulation. However, to obtain an optimal image, X-ray irradiation spanning more than one heartbeat is required, and thus, CTDI100,c,ECG increases. For example in the authors’ hospital, coronary CTA examinations at 50 bpm, performed using prospective CTA for the prespecified cardiac phase ranges of 70–80 %, have a dose of 5.1 mGy, whereas for prespecified cardiac phase ranges of 35–50 % at 90 bpm, the dose is 5.1 mGy. However, the X-ray output is active during 35–50 % within three beats. Thus, these coronary CTA examinations at 90 bpm are approximately three times that for those at 50 bpm. In the case of atrial fibrillation at 90 bpm, coronary CTA examinations performed using continuous CTA/CFA within three beats have a dose of 52.0 mGy. This dose is 10 times that for a coronary CTA dose at 60 bpm. In addition, narrowly specifying the cardiac phase might not lead to a decrease in the dose. For example, prospective CTA for the prespecified cardiac phase ranges of 75–80 and 70–80 % for an HR of 60 bpm produced the same
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study indicate that the difference in the MSAD and CTDI values is 5 % or less; furthermore, this difference arises from the non-uniform sensitivity of the TLD. Even in the case of a volume scan, the MSAD measurement and the CTDI should produce similar results. The CTDI has been recommended for use by a number of different international organisations for CT scan dose management (3 – 7, 10 – 13). In these recommendations, the CTDI is defined as being measured over one axial scan (one rotation of the X-ray tube)(4). The authors’ study deviated from the official definition of CTDI by allowing more than one 360degree rotation. This measurement method is an appropriate way to examine dose management for ECG gate scanning using wide-area detectors. In addition, motion-free image reconstruction is crucial for obtaining high-quality diagnostic images for cardiac CTA. For many ECG-based examinations, X-ray irradiation over more than one rotation of X-ray tube is required. It is necessary to measure the dose using the same conditions as those used for the coronary CTA. The CTDI also encompasses CTDI100,p, CTDIw and CTDIvol; however, this study was restricted to the evaluation of CTDI00,c. The CTDI100,p values evaluate the average dose measurements for the 0, 3, 6 and 9 o’clock directions at 10 mm below the surface of the PMMA phantom. These values have large errors, arising from the position of the ionisation chamber, tube current modulation, scanning time and overbeaming. For this reason, the CTDI100,p values were not measured, and the characteristic dose was evaluated using CTDI100,c,ECG. For prospective ECG triggering, such as that used in Ca scoring, target CTA and prospective CTA, the X-ray tube is activated only during a prespecified cardiac phase within the cardiac cycle. The CTDI100,c,ECG values for Ca scoring are 5.2 –5.9 mGy (0.35 –0.4 s), which is 3 % or lesser than the CTDI100,c without an ECG. In addition, similar results were obtained for target CTA for scan conditions in which the rotation time and the acquisition time were the same. These results indicate that in order to reconstruct the image from a cardiac phase, X-ray irradiation spanning one rotation is needed. In Ca scoring, the X-ray tube is active for only one rotation in order to obtain the image data during a prespecified cardiac phase (35 or 75 %). In target CTA, the X-ray tube is active during the acquisition time. These scan modes are effective for lowering the radiation dose in coronary CT. However, to use these techniques, the cardiac phase of the patients must be carefully selected. The prospective CTA is the most practical scan mode because the HR of patient may change during the scan timing. In this case, X-ray irradiation was performed for one rotation scan regardless of the HR change for Ca scoring. In contrast, X-ray irradiation was performed for an absolute
EVALUATION OF THE CTDI WITH AN ECG USING A 320-ROW MDCT
CTDI100,c,ECG; this is a crucial result of this study. Therefore, it is necessary to take this result into consideration when selecting the scanning parameters. This study shows some potential weaknesses because the data were acquired from only one scanner, and there was no attempt to change X-ray tube voltage. Therefore, dose evaluation for cardiac applications of various scanners is desired. REFERENCES
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