Magnetic Resonance Imaging 32 (2014) 523–528
Contents lists available at ScienceDirect
Magnetic Resonance Imaging journal homepage: www.mrijournal.com
Dual-source parallel radiofrequency transmission for magnetic resonance breast imaging at 3 T: Any added clinical value? Lei Jiang a, 1, Yiming Zhou b, 1, Cheng Zhou a, Min Chen a,⁎, Yongming Dai c,⁎, Yuan Fu a, Xuna Zhao c a b c
Radiology Department, Beijing Hospital, the Fifth Affiliated College of Peking University, Beijing, China Radiology Department, Beijing Chaoyang Hospital, the Affiliated College of Capital Medical University, Beijing, China Philips Healthcare, Andover, MA, USA
a r t i c l e
i n f o
Article history: Received 6 August 2013 Revised 12 January 2014 Accepted 14 January 2014 Keywords: Breast Magnetic resonance imaging Dual-source Parallel imaging 3T
a b s t r a c t Purpose: To investigate the influence of dual-source parallel radiofrequency (RF) excitation on clinical breast MR images. Methods: A 3 T MR system with both dual-source and conventional single-source RF excitations was used to examine 22 patients. Axial TSE-T2WI with fat suppression, TSE-T1WI without fat suppression, THRIVE (3D field echo) and DWI (SE-EPI) were obtained by using both excitation techniques. Image homogeneity, image contrast and lesion conspicuity were measured or independently scored by two radiologists and were compared by paired-sample t test or Wilcoxon test. Results: Both excitations revealed 24 lesions. For SE sequences using dual-source mode, image homogeneity was improved (P = 0.00), scan time was reduced, and ghost artifacts on DWI were significantly reduced (P = 0.00). However, image contrast was not increased and lesion conspicuity had no significant difference between two modes, except DWI on which lesion conspicuity was significantly improved (P = 0.00), due to less ghost artifacts. For field-echo sequence, image homogeneity, acquisition time, image contrast and lesion conspicuity had no significant difference between the two modes. Conclusions: Dual-source parallel RF transmission has some added value for improving breast image quality. However, its value is limited in terms of improving lesion detection and characterization. © 2014 Elsevier Inc. All rights reserved.
1. Introduction With the advance of magnetic resonance imaging (MRI), MRI systems with high field strength such as 1.5 and 3 T have been widely introduced and generally accepted for clinical routine in recent years [1,2]. MR imaging on 3 T system may benefit from higher spatial resolution and higher temporal resolution with its advantage of nearly doubled signal-to-noise ratio compared to 1.5 T system [3]. However, it is not simple to conclude that imaging at 3 T system is superior to that at 1.5 T system, because some artifacts exclusive to 3 T system may impair image quality. For example, dielectric shading effect, or known as standing wave artifact due to B1 field inhomogeneity, is a typical problem for imaging at 3 T, manifested with undesired spatially varying image contrast and signal intensity changes particularly for body imaging. This is due to the electromagnetic interaction of radiofrequency (RF) field with the examined object [4,5].
⁎ Corresponding authors. E-mail address: [email protected] (M. Chen). 1 These two authors contributed equally to this work as co-first authors. 0730-725X/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.mri.2014.01.010
To address this specific problem at 3 T, several potential solutions were proposed including variable phase and variable amplitude excitation, three-dimensional (3D) imaging, increased flip angles [6,7]. Besides these techniques, dual-source parallel RF excitation along with independent RF shimming (abbreviated to “dual source” hereafter) was presented, to reduce the degree of artifacts caused by B1 inhomogeneity [8,9]. With this technique, RF pulses are tailored to each patient and yield more uniform excitation and more homogeneous receive field. In addition, it also allows for optimization of the local and whole-body SAR using patient adaptive techniques, leading to reduced SAR deposition levels. To date, dual source has been shown to be useful in body and musculoskeletal imaging [10–14]. They concluded that it allowed for dielectric shading reduction, improved homogeneity of the RF magnetic induction field, and accelerated imaging at 3 T. Nevertheless, for breast imaging, there are only few reported studies [15,16], all of which focused on evaluating B1 map and quantitatively demonstrated the advantage of dual-source over conventional single-source excitation technique in terms of B1 inhomogeneity. However, corresponding assessment on clinical images or evaluation of its influence on clinical image interpretation is absent.
524
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
The aim of this study was to further investigate the influence of dualsource technique on image quality of clinical breast images by comparingwith single-sourcetechniquein aspectrum of imagehomogeneity, image contrast, lesion detection as well as lesion conspicuity. 2. Methods 2.1. Study population This study was performed with the approval of our local institutional review board. Written informed consent was obtained from all participants. We enrolled 22 patients (mean age, 45 years; age range, 30–66 years). Among the 22 patients, 2 patients had 2 cysts each and the other 20 patients each had a solitary lesion. In total, there were 9 cysts, 7 fibromas, 5 pathology-proved invasive ductal breast cancers and 3 pathology-proved focal hyperplasias. Patients less than 18 years of age and those who had the usual contraindications (MR incompatible metal implants, pacemakers, defibrillators, neurostimulators, cochlea implants, etc.) to MR imaging were excluded. All patients had at least one lesion that could be detected by our standard MR pre-contrast protocol.
quadrature body coil for RF excitation. The RF duty cycle of the scans was limited by the estimated local SAR. In case of RF shimming, an increase in the RF duty cycle was allowed according to the estimated reduced local SAR. The whole-body SAR was in any case within the normal whole-body limit (according to the International Electrotechnical Commission) [17]. Patients were scanned axially in the prone position with a set of MR sequences: axial T2-weighted fast spin echo with fat suppression (T2WI), T1-weighted fast spin echo without fat suppression (T1WI), 3D T1 fast field-echo high-resolution isotropic volume examination (THRIVE), and SE-EPI diffusion-weighted breast images (DWI). All these sequences were performed by dual-source and single-source modes without any change of patients. DWI was performed with diffusion gradient in three orthogonal directions with b values of 0 and 800 s/mm2. The following image assessments were made on images with b = 800 s/mm2. Spectral presaturation attenuated by inversion recovery (SPAIR) was used for fat suppression. After standard MR precontrast protocol, dynamic contrast enhanced imaging with dual source was also performed, but the data was not used in the study. Details about MR sequence protocols are listed in Table 1. 2.3. Image analysis
2.2. MRI All examinations were performed on a clinical 3 T whole-body MRI system (Achieva 3 T TX, Philips Healthcare, Best, the Netherlands), using a dedicated seven-channel bilateral breast coil. The MRI system was equipped with dual-source parallel RF excitation technology. The user was allowed to switch between dual-source and the standard single-source RF excitation mode without the need of additional adjustment for protocol and patients. By using dualsource mode, gradient system parameters were as follows: maximum achievable gradient amplitude, 80 mT/m; rise time, 0.2 msec; slew rate, 200 T/m/sec. The RF power of the two individual RF sources was distributed to the independent ports of the system body coil by using two RF transmit channels with software control. With this design, it was possible to independently control the phase and amplitude of each of the two RF waveforms supplied to the
Image quality (including image homogeneity on T2WI/T1WI/ THRIVE, ghost artifact on DWI and lesion conspicuity on all sequences) was visually judged by two readers (M.ZH., Y.T., 10 and 9 years' experience in breast MRI). The two readers did not know the difference between excitation techniques and were blinded to detailed sequence variants. They independently scored image quality according to the following grading system. Image homogeneity [13]: 5 = excellent image quality—uniform contrast over the entire field of view, with no standing wave artifacts; 4 = good image quality—mild standing wave artifacts without impaired image interpretation; 3 = moderate image quality—moderate standing wave artifacts that interfered with image interpretation; 2 = poor image quality— prominent standing wave artifacts, with questionable diagnostic quality; 1 = nondiagnostic image quality.
Table 1 MR image acquisition sequence parameters. Parameters
T2WI
T1WI
THRIVE
DWI
Sequence type Field of view (mm) Image matrix Section thickness (mm) Slice gap (mm) No. of sections No. of packages Sense factor Measured voxel size (mm) Water fat shift (Hz)/bandwidth (Hz) No. of acquired signals b value (sec/mm2) Echo time (msec) Repetition time (msec) Echo train length RF excitation (degrees) Refocusing angle (degrees) Fat saturation SPAIR TR (msec) Acquisition time
TSE 340 436 × 434 3 0 48 3a, 6b 2 0.78 × 0.78 × 3.0 1.743/249.3 1 NA 60 4199 15 90 120 SPAIR 262.44a, 524.88b 3 min 34 seca, 7 min 8 secb
TSE 340 308 × 295 3 0 48 16a, 48b 2 1.10 × 1.15 × 3.0 0.673/645.7 1 NA 13 500 7 90 120 no NA 2 min 16 seca, 6 min 48 secb
TFE 340 484 × 485 NA NA NA NA 2 0.7 × 0.7 × 4.0 1.105/393.0 1 NA 2.7 5.4 40 (TFE factor) 10 NA SPAIR 325.93 0 min 51 seca, 0 min 51 secb
SE-EPI 340 124 × 116 3 0 48 1 2.2 2.74 × 1.69 × 3.0 17.766/24.5 2 0, 800 62 7916a, 13,259b NA 90 NA SPAIR 164.91a, 276.22b 1 min 58 seca, 3 min 18 secb
Note: Given parameter was used with both single-source and dual-source RF excitations, unless otherwise noted. T2WI = T2-weighted image, T1WI = T1-weighted image, DWI = diffusion-weighted image, THRIVE = T1 turbo field-echo high-resolution isotropic volume examination, NA = nonapplicable, TSE = turbo spin echo, TFE = turbo field echo, SPAIR = spectral presaturation attenuated by inversion recovery, SPAIR TR ≈ inversion recovery time × 3. a Dual-source RF excitation. b Single-source RF excitation.
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
Fig. 1. Example of regions of interest (ROIs) drawn at the level of the nipple. Three circles were drawn on the anterior, lateral and medial fibroglandular tissue of each breast, respectively. Five circles were drawn on anterior–lateral, posterior–lateral, anterior–medial, posterior–medial, and posterior fat tissue surrounding fibroglandular tissue of each breast.
Ghost artifacts: 4 = no ghost artifacts; 3 = mild ghost artifacts without impaired image interpretation; 2 = moderate ghost artifacts that interfered with image interpretation; 1 = prominent ghost artifacts with questionable diagnostic quality. The readers were asked to record the number and location of lesions and to rate lesion conspicuity as follows: 3 = good, lesion is seen with definite internal structure and margin; 2 = moderate, lesion is probably seen with indistinct internal structure and margin; and 1 = poor, lesion is not seen. To assess the impact of different RF excitation modes on image homogeneity and hence tissue contrast, image quality was quantitatively evaluated by signal intensity ratios (SRs) and contrast ratios (CRs) [13], which are superior to conventional signal-to-noise ratio and contrast to noise ratio in evaluating image homogeneity and tissue contrast. First, signal intensities of both fat and fibroglandular tissue (FG) were measured for each breast. Signal intensities of fat were measured by defining five circular regions of interest (ROIs) in each breast at about the same axial slice as the nipple and signal intensities for FG were measured by defining three circular ROIs, as shown in Fig. 1. Mean
525
signal intensities for fat or FG in each breast were calculated by averaging the corresponding ROI values. The average area of ROI was 12.1 ± 1.5 mm2, avoiding the inclusion of blood vessels and the edge of tissue. To gain comparable measurements, by image registration, ROIs were placed in exactly corresponding regions on the two sets of images acquired with conventional and dual-source techniques. Subsequently, SRs of fat for both RF excitation modes were calculated. First, the average signal intensity of fat for each breast was obtained by averaging the corresponding five ROIs. SImax stands for the larger average signal intensity, while SImin stands for the other smaller one. Second, SRs were calculated as follows: SR = SImax/ SImin, indicating signal intensity difference between two intraindividual breasts. CRs were calculated between FG and fat, and also between lesion and ipsilateral FG for both excitations. CR = (SIa − SIb)/(SIa + SIb), where SIa and SIb are the signal intensity of two tissues, respectively. The acquisition time for the above sequences of both excitations was recorded. 2.4. Statistical analysis Interobserver agreement was calculated by kappa test. Kappa values higher than 0.75 were considered to be excellent agreement. In cases of excellent interobserver agreement, the data were pooled for the two readers. The score for each sequence and for each patient was taken as the average between the two readers. The average scores, CRs of FG to fat between two excitations were compared, using paired-sample t test, and CRs of lesion to FG and SRs, by Wilcoxon signed ranks test. To compare the conspicuity change of different lesion type, the conspicuity score of each lesion type for single-source technique was subtracted by that for dual source. The obtained final score for each lesion was compared by Kruskal–Wallis H test. P b 0.05 was considered to indicate statistical significance. 3. Results MR imaging was successfully performed with both excitation techniques for all 22 subjects. Interobserver agreements for scoring were excellent, with kappa values N0.75. 3.1. Image homogeneity and artifacts
Table 2 Image score comparison between dual- and single-source technique. Image quality and sequence Homogeneity T2WI T1WI THRIVE Ghost artifact DWI Conspicuity T2WI T1WI THRIVE DWI a b
Acquisition modea
Average scoreb
DS SS DS SS DS SS
4.89 3.84 4.89 4.30 4.84 4.80
0.31 0.36 0.31 0.45 0.36 0.40
9.62/0.00 0.776 0.831 4.54/0.00 0.776 0.891 0.53/0.61 0.831 0.861
DS SS
2.89 ± 0.31 1.50 ± 0.49
12.75/0.00 0.776 0.820
DS SS DS SS DS SS DS SS
2.71 2.75 2.38 2.35 2.42 2.33 2.44 1.42
DS = dual-source; SS = single-source. Data are mean ± standard deviation.
± ± ± ± ± ±
± ± ± ± ± ± ± ±
t/P
Interobserver agreement (kappa value)
0.44 −0.49/0.63 0.798 0.42 0.780 0.47 0.17/0.87 0.822 0.48 0.909 0.48 1.16/0.26 0.830 0.46 0.829 0.50 18.19/0.00 0.915 0.48 0.830
According to the scores, homogeneity on T2WI and T1WI with dual source was significantly better than that with single source (P = 0.00, 0.00), while homogeneity on THRIVE had no significant difference between the two excitation techniques (P = 0.61). Ghost artifact on DWI with dual source was significantly reduced, compared with single source (P = 0.00). The details are summarized in Table 2. On the other hand, quantitative analysis showed that SRs on T2WI and T1WI with dual source were significantly lower than those with Table 3 Signal intensity ratio (SR) comparison between dual- and single-source technique. MR sequence
Acquisition modea
SRb
T2WI
DS SS DS SS DS SS
1.08 1.44 1.05 1.09 1.09 1.11
T1WI THRIVE a
Z/P ± ± ± ± ± ±
0.06 0.24 0.04 0.08 0.06 0.09
−4.010/0.000 −2.062/0.039 −0.601/0.548
DS = dual-source; SS = single-source. SR = mean ± standard deviation, which were calculated as follows: SR = SImax/ SImin, where SImax and SImin mean the larger and smaller average signal intensity of fat between two intra-individual breasts, respectively. b
526
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
single source (P = 0.000, 0.039). SRs on THRIVE had no significant difference between the two excitation techniques (P = 0.548). The results are summarized in Table 3.
For different lesion types (breast cancer, adenoma, cyst and focal hyperplasia), conspicuity score change after using different excitation technique had no significant difference, shown in Table 4.
3.2. Lesion detection and conspicuity
3.3. Image contrast
All 24 lesions were detected by each mode. Fig. 2A–H shows a representative case of images acquired with both excitation modes. Lesions were more conspicuous on DWI with dual source (P = 0.00). However, lesion conspicuity on other sequences had no significant difference between the two excitation techniques, shown in Table 2.
On T2WI, CRs for right FG/fat, left FG/fat and lesion/FG with dual source were significantly lower than those with single source (P = 0.000, 0.000, and 0.000, respectively). On T1WI and THRIVE, CRs for right FG/fat, left FG/fat and lesion/FG had no significant difference between the two excitation techniques (P = 0.824, 0.820, and 0.732 for T1WI; P = 0.514, 0.570, and 0.710 for THRIVE). The signal
Fig. 2. A–H, Comparison between T2WI, T1WI, THRIVE and DWI (b = 800 s/mm2) obtained with dual-source parallel RF excitation (A, C, E, G) and those obtained with conventional single-source RF excitation (B, D, F, H). The window width and level were set to the same for each pair of images. Signal intensity was more homogeneous on images with dual source than that with single source, especially on T2WI (A, B) and T1WI (C, D). Fat on T2WI with dual source (A) was not suppressed as black as that with single source (B), and image contrast of the former was worse than the latter (the arrow indicates a lesion, which is focal hyperplasia, proved by pathology). Ghost artifact on DWI with dual source (E) was less than that with single source (F). The image homogeneity, fat suppression effect and image contrast had no significant difference between field-echo sequence with dual source (G) and that with single source (H).
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
concluded that 3D gradient-echo (or field-echo) sequences were less affected by B1 inhomogeneity, compared with TSE sequences, and could provide diagnostic image quality at 3 T [21,22]. The reason is that, unlike TSE sequence, gradient-echo sequence does not use a large number of RF pulses to generate image contrast, which will result in increased SAR and aggravate B1 non-uniformity. In addition, the ghost artifact on DWI with dual source was dramatically reduced, which has not ever been reported in the literature. In this study, dual-source technique provided limited value for improving breast lesion detection and characterization. For the two excitation modes, the same number of lesions was detected and had the similar conspicuity, except for DWI on which lesions were more conspicuous with dual source due to less interference of ghost artifacts. There was no conspicuity difference among the different evaluated lesion types, regarding to the effect of dual-source technique on improving lesion characterization. Image homogeneity improvement for T2WI and T1WI did not result in better lesion detection and characterization. This is different from a previous study about focal liver lesions [10], which concluded that lesion conspicuity on T2WI was improved with dual source. The reason for the different performance of dual source may be that breasts have different sizes and configuration from those of the abdomen and are not prone to shading artifacts as the abdomen. In addition, image contrast on all evaluated images with dual source did not improve. The image contrast on T2WI with dual source was even inferior to that with single source, mainly because background fat of the former was not suppressed as completely as that of the latter. The different performance of fat suppression on T2WI between two excitation technique is explained by different inversion recovery time (IR) used for SPAIR, which is reflected as SPAIR TR in Table 1. Although the same protocols, other than excitation mode, were set initially for both techniques, actual parameters may automatically change during scan. For dual-source technique, the duration of IR was shorter for its unique excitation mode. Therefore, with shorter IR, the longitudinal relaxation of fat tissue was not recovered to zero when readout frequency was administered, leading to poor fat suppression. This may benefit detection of fibrous lesion which has low signal on T2WI, but may also lead to decreased CR of high-signal lesion that is the case in our study. According to different clinical need, IR should be adjusted. Image acquisitions of SE sequence (including T2WI, T1WI and DWI) with dual source were accelerated, while scan time of fieldecho sequence did not change. The acceleration is related to improved RF uniformity, resulting in reduced whole-body and/or local energy deposition. As a consequence, more sections per TR cycle in combination with a fixed TR (on T2WI and T1WI) or a shorter TR (on DWI) were implemented and thus reduced the image acquisition time.
Table 4 Conspicuity score difference between two excitation techniques of different lesions. Sequence Breast cancer T2WI T1WI THRIVE DWI
−0.30 −0.10 −0.10 1.10
± ± ± ±
Adenoma
0.57 0.07 ± 0.19 0.22 −0.10 ± 0.22 0.22 0.07 ± 0.45 0.55 1.00 ± 0.00
Cyst 0.11 0.28 0.11 1.00
Focal hyperplasia ± ± ± ±
0.33 −0.33 ± 0.58 0.44 0.17 ± 0.29 0.22 0.33 ± 0.58 0.00 1.00 ± 0.50
527
P 0.192 0.296 0.379 1.000
intensity of fat between the two excitations had a significant difference on T2WI (P = 0.000), and no significant difference on T1WI and THRIVE. The results are summarized in Table 5. 3.4. Image acquisition time Image acquisitions of T2WI, T1WI and DWI with dual source were accelerated by 2, 3, and 1.68 times, compared with single source, while the acquisition time of THRIVE was the same for both excitations, shown in Table 1. 4. Discussion The main advantage of dual source is B1 field homogeneity improvement or reduced dielectric shading artifacts, and hence to improve final image quality. This has been proved by several studies focusing on liver MR imaging [11,13], since the abdomen is most vulnerable to B1 field inhomogeneity, for its comparable size to the wavelength of RF at 3 T. B1 field inhomogeneity may also be problematic in breast MRI imaging due to a variety of factors such as off-center positioning of breasts in the whole-body RF coil, respiration movement, heart pulsation, and so on. Unlike previous studies that employed only B1 map for analysis [15,16], our study performed a more complete investigation on both image quality and lesion visualization based on clinical images directly with a comprehensive set of routine MR sequences to probe the influence of B1 field homogeneity. Dual-source technique brought about artifacts reduction on SE sequences in this study. Most T2WI and T1WI with single source had mild dielectric shading effect that, however, did not interfere with image interpretation. By using dual-source technique, image homogeneity was significantly improved from the scores and SR measurement. As to images obtained by 3D field-echo sequence (THRIVE) with single-source technique, they had almost no dielectric shading artifacts for most patients. Dual-source RF excitation seems to have no prominent effect on 3D field-echo sequence in view of image homogeneity improvement. This is consistent with literatures [18–22]. Also, many reports have
Table 5 Contrast ratio (CR) and fat signal comparison between dual and single-source technique. Sequence and acquisition modea T2WI DS SS T1WI DS SS THRIVE DS SS a b
CR between right FG t/P and fatb 6.53 ± 3.88 29.41 ± 13.52
−14.90/0.000
CR between left FG t/P and fatb
CR between lesion and FGb
Z/P
9.03 ± 7.24 21.91 ± 10.26
−14.19/0.000 0.19 ± 0.15 0.32 ± 0.14
−4.286/0.000
Fat signal intensity 773.04 ± 469.58 231.09 ± 190.40
t/P
8.452/0.000
0.48 ± 0.09 0.48 ± 0.08
0.226/0.824
0.48 ± 0.08 0.49 ± 0.07
−0.230/0.820 0.11 ± 0.10 0.11 ± 0.10
−0.343/0.732 1712.58 ± 452.01 −.419/0.679 1722.69 ± 483,41
0.58 ± 0.05 0.59 ± 0.10
−0.617/0.514
0.58 ± 0.04 0.59 ± 0.08
−0.568/0.570 0.07 ± 0.07 0.09 ± 0.08
−0.371/0.710
287.53 ± 64.40 267.90 ± 65.57
DS = dual-source; SS = single-source. CR = mean ± standard deviation. CR = (SIa − SIb)/(SIa + SIb), where SIa and SIb are the signal intensities of two tissues. FG = fibroglandular tissue.
2632/0.061
528
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
This study has some limitations. First, the effect of dual-source technique on the apparent diffusion coefficients of DWI was not investigated further in this study for severe ghost artifacts on DWI with single source. Second, our study was performed on a single 3 T Philips MR scanner, so our results may be vendor specific. A generalization to MR systems from other vendors cannot safely be assumed. Third, the study population was small and further larger study was helpful to validate the results. 5. Conclusion In conclusion, dual-source parallel RF excitation will benefit SE sequences by improving image homogeneity, reducing scan time and increasing lesion conspicuity on DWI, however it has no obvious effect on 3D field-echo sequence. So it has some added but limited clinical value for improving clinical breast MR imaging. References [1] Kuhl CK. Current status of breast MR imaging. II. Clinical applications. Radiology 2007;244:672–91. [2] Peters NH, Borel Rinkes IH, Zuithoff NP, Mali WP, Moons KG, Peeters PH. Metaanalysis of MR imaging in the diagnosis of breast lesions. Radiology 2008;246: 116–24. [3] Soher BJ, Dale BM, Merkle EM. A review of MR physics: 3 T versus 1.5 T. Magn Reson Imaging Clin N Am 2007;15:277–90. [4] Gabriel C, Gabriel S, Corthout E. The dielectric properties of biological tissues. I. Literature survey. Phys Med Biol 1996;41:2231–49. [5] Mihara H, Iriguchi N, Ueno S. Imaging of the dielectric resonance effect in high field magnetic resonance imaging. J Appl Phys 2005;97:3051–3. [6] Collins CM, Liu W, Swift BJ, Smith MB. Combination of optimized transmit arrays and some receive array reconstruction methods can yield homogeneous images at very high frequencies. Magn Reson Med 2005;54:1327–32. [7] Partridge SC, Rahbar H, Lehman CD. Breast MR imaging. In: Kamel IR, Merkle EM, editors. Body MR imaging at 3 Tesla. New York: Cambridge University Press; 2011. [8] Zhu Y. Parallel excitation with an array of transmit coils. Magn Reson Med 2004;51:775–84. [9] Ullmann P, Junge S, Wick M, Seifert F, Ruhm W, Hennig J. Experimental analysis of parallel excitation using dedicated coil setups and simultaneous RF transmission on multiple channels. Magn Reson Med 2005;54:994–1001.
[10] Nelles M, König RS, Gieseke J, Guerand-van Battum MM, Kukuk GM, Schild HH, Willinek WA. Dual-source parallel RF transmission for clinical MR imaging of the spine at 3.0 T: intraindividual comparison with conventional single-source transmission. Radiology 2010;257:743–53. [11] Pazahr S, Fischer MA, Chuck N, Luechinger R, Schick F, Nanz D, Boss A. Liver: segment-specific analysis of B1 field homogeneity at 3.0-T MR imaging with single-source versus dual-source parallel radiofrequency excitation. Radiology 2012;265:591–9. [12] Bouvier J, Troprès I, Lamalle L, Grand S, Breil S, Le Bas JF, Krainik A. Evaluation of dual-source parallel RF excitation technology in MRI of thoraco-lumbar spine at 3.0 T. J Neuroradiol 2013;40:94–100. [13] Kukuk GM, Gieseke J, Weber S, Hadizadeh DR, Nelles M, Träber F, Schild HH, Willinek WA. Focal liver lesions at 3.0 T: lesion detectability and image quality with T2-weighted imaging by using conventional and dual-source parallel radiofrequency transmission. Radiology 2011;259:421–8. [14] Mueller A, Kouwenhoven M, Naehle CP, Gieseke J, Strach K, Willinek WA, Schild HH, Thomas D. Dual-source radiofrequency transmission with patient-adaptive local radiofrequency shimming for 3.0-T cardiac MR imaging: initial experience. Radiology 2012;263:77–85. [15] Rahbar H, Partridge SC, Demartini WB, Gutierrez RL, Parsian S, Lehman CD. Improved B1 homogeneity of 3 Tesla breast MRI using dual-source parallel radiofrequency excitation. J Magn Reson Imaging 2012;35:1222–6. [16] Trop I, Gilbert G, Ivancevic MK, Beaudoin G. Breast MR imaging at 3 T with dual source radiofrequency transmission offers superior B1 homogeneity: an intraindividual comparison with breast MR imaging at 1.5 T. Radiology 2013;267:602–8. [17] Mürtz P, Kaschner M, Träber F, Kukuk G, Skowasch D, Gieseke J, Schild HH, Willinek WA. Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T. J Magn Reson Imaging 2012;35: 456–61. [18] Kuhl CK, Tr ber F, Gieseke J, Drahanowsky W, Morakkabati-Spitz N, Willinek W, von Falkenhausen M, Manka C, Schild HH. Whole body high-field-strength (3.0-T) MR imaging in clinical practice. Part II. Technical considerations and clinical applications. Radiology 2008;247:16–35. [19] Merkle EM, Dale BM. Abdominal MRI at 3.0 T: the basics revisited. Am J Roentgenol 2006;186:1524–32. [20] Zapparoli M, Semelka RC, Altun E, Tsurusaki M, Pamuklar E, Dale BM, Gasparetto EL, Elias Jr J. 3.0-T MRI evaluation of patients with chronic liver diseases: initial observations. Magn Reson Imaging 2008;26:650–60. [21] Ramalho M, Herédia V, Tsurusaki M, Altun E, Semelka RC. Quantitative and qualitative comparison of 1.5 and 3.0 Tesla MRI in patients with chronic liver diseases. J Magn Reson Imaging 2009;29:869–79. [22] Tsurusaki M, Semelka RC, Zapparoli M, Elias Jr J, Altun E, Pamuklar E, Sugimura K. Quantitative and qualitative comparison of 3.0 T and 1.5 T MR imaging of the liver in patients with diffuse parenchymal liver disease. Eur J Radiol 2009;72: 314–20.
Contents lists available at ScienceDirect
Magnetic Resonance Imaging journal homepage: www.mrijournal.com
Dual-source parallel radiofrequency transmission for magnetic resonance breast imaging at 3 T: Any added clinical value? Lei Jiang a, 1, Yiming Zhou b, 1, Cheng Zhou a, Min Chen a,⁎, Yongming Dai c,⁎, Yuan Fu a, Xuna Zhao c a b c
Radiology Department, Beijing Hospital, the Fifth Affiliated College of Peking University, Beijing, China Radiology Department, Beijing Chaoyang Hospital, the Affiliated College of Capital Medical University, Beijing, China Philips Healthcare, Andover, MA, USA
a r t i c l e
i n f o
Article history: Received 6 August 2013 Revised 12 January 2014 Accepted 14 January 2014 Keywords: Breast Magnetic resonance imaging Dual-source Parallel imaging 3T
a b s t r a c t Purpose: To investigate the influence of dual-source parallel radiofrequency (RF) excitation on clinical breast MR images. Methods: A 3 T MR system with both dual-source and conventional single-source RF excitations was used to examine 22 patients. Axial TSE-T2WI with fat suppression, TSE-T1WI without fat suppression, THRIVE (3D field echo) and DWI (SE-EPI) were obtained by using both excitation techniques. Image homogeneity, image contrast and lesion conspicuity were measured or independently scored by two radiologists and were compared by paired-sample t test or Wilcoxon test. Results: Both excitations revealed 24 lesions. For SE sequences using dual-source mode, image homogeneity was improved (P = 0.00), scan time was reduced, and ghost artifacts on DWI were significantly reduced (P = 0.00). However, image contrast was not increased and lesion conspicuity had no significant difference between two modes, except DWI on which lesion conspicuity was significantly improved (P = 0.00), due to less ghost artifacts. For field-echo sequence, image homogeneity, acquisition time, image contrast and lesion conspicuity had no significant difference between the two modes. Conclusions: Dual-source parallel RF transmission has some added value for improving breast image quality. However, its value is limited in terms of improving lesion detection and characterization. © 2014 Elsevier Inc. All rights reserved.
1. Introduction With the advance of magnetic resonance imaging (MRI), MRI systems with high field strength such as 1.5 and 3 T have been widely introduced and generally accepted for clinical routine in recent years [1,2]. MR imaging on 3 T system may benefit from higher spatial resolution and higher temporal resolution with its advantage of nearly doubled signal-to-noise ratio compared to 1.5 T system [3]. However, it is not simple to conclude that imaging at 3 T system is superior to that at 1.5 T system, because some artifacts exclusive to 3 T system may impair image quality. For example, dielectric shading effect, or known as standing wave artifact due to B1 field inhomogeneity, is a typical problem for imaging at 3 T, manifested with undesired spatially varying image contrast and signal intensity changes particularly for body imaging. This is due to the electromagnetic interaction of radiofrequency (RF) field with the examined object [4,5].
⁎ Corresponding authors. E-mail address: [email protected] (M. Chen). 1 These two authors contributed equally to this work as co-first authors. 0730-725X/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.mri.2014.01.010
To address this specific problem at 3 T, several potential solutions were proposed including variable phase and variable amplitude excitation, three-dimensional (3D) imaging, increased flip angles [6,7]. Besides these techniques, dual-source parallel RF excitation along with independent RF shimming (abbreviated to “dual source” hereafter) was presented, to reduce the degree of artifacts caused by B1 inhomogeneity [8,9]. With this technique, RF pulses are tailored to each patient and yield more uniform excitation and more homogeneous receive field. In addition, it also allows for optimization of the local and whole-body SAR using patient adaptive techniques, leading to reduced SAR deposition levels. To date, dual source has been shown to be useful in body and musculoskeletal imaging [10–14]. They concluded that it allowed for dielectric shading reduction, improved homogeneity of the RF magnetic induction field, and accelerated imaging at 3 T. Nevertheless, for breast imaging, there are only few reported studies [15,16], all of which focused on evaluating B1 map and quantitatively demonstrated the advantage of dual-source over conventional single-source excitation technique in terms of B1 inhomogeneity. However, corresponding assessment on clinical images or evaluation of its influence on clinical image interpretation is absent.
524
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
The aim of this study was to further investigate the influence of dualsource technique on image quality of clinical breast images by comparingwith single-sourcetechniquein aspectrum of imagehomogeneity, image contrast, lesion detection as well as lesion conspicuity. 2. Methods 2.1. Study population This study was performed with the approval of our local institutional review board. Written informed consent was obtained from all participants. We enrolled 22 patients (mean age, 45 years; age range, 30–66 years). Among the 22 patients, 2 patients had 2 cysts each and the other 20 patients each had a solitary lesion. In total, there were 9 cysts, 7 fibromas, 5 pathology-proved invasive ductal breast cancers and 3 pathology-proved focal hyperplasias. Patients less than 18 years of age and those who had the usual contraindications (MR incompatible metal implants, pacemakers, defibrillators, neurostimulators, cochlea implants, etc.) to MR imaging were excluded. All patients had at least one lesion that could be detected by our standard MR pre-contrast protocol.
quadrature body coil for RF excitation. The RF duty cycle of the scans was limited by the estimated local SAR. In case of RF shimming, an increase in the RF duty cycle was allowed according to the estimated reduced local SAR. The whole-body SAR was in any case within the normal whole-body limit (according to the International Electrotechnical Commission) [17]. Patients were scanned axially in the prone position with a set of MR sequences: axial T2-weighted fast spin echo with fat suppression (T2WI), T1-weighted fast spin echo without fat suppression (T1WI), 3D T1 fast field-echo high-resolution isotropic volume examination (THRIVE), and SE-EPI diffusion-weighted breast images (DWI). All these sequences were performed by dual-source and single-source modes without any change of patients. DWI was performed with diffusion gradient in three orthogonal directions with b values of 0 and 800 s/mm2. The following image assessments were made on images with b = 800 s/mm2. Spectral presaturation attenuated by inversion recovery (SPAIR) was used for fat suppression. After standard MR precontrast protocol, dynamic contrast enhanced imaging with dual source was also performed, but the data was not used in the study. Details about MR sequence protocols are listed in Table 1. 2.3. Image analysis
2.2. MRI All examinations were performed on a clinical 3 T whole-body MRI system (Achieva 3 T TX, Philips Healthcare, Best, the Netherlands), using a dedicated seven-channel bilateral breast coil. The MRI system was equipped with dual-source parallel RF excitation technology. The user was allowed to switch between dual-source and the standard single-source RF excitation mode without the need of additional adjustment for protocol and patients. By using dualsource mode, gradient system parameters were as follows: maximum achievable gradient amplitude, 80 mT/m; rise time, 0.2 msec; slew rate, 200 T/m/sec. The RF power of the two individual RF sources was distributed to the independent ports of the system body coil by using two RF transmit channels with software control. With this design, it was possible to independently control the phase and amplitude of each of the two RF waveforms supplied to the
Image quality (including image homogeneity on T2WI/T1WI/ THRIVE, ghost artifact on DWI and lesion conspicuity on all sequences) was visually judged by two readers (M.ZH., Y.T., 10 and 9 years' experience in breast MRI). The two readers did not know the difference between excitation techniques and were blinded to detailed sequence variants. They independently scored image quality according to the following grading system. Image homogeneity [13]: 5 = excellent image quality—uniform contrast over the entire field of view, with no standing wave artifacts; 4 = good image quality—mild standing wave artifacts without impaired image interpretation; 3 = moderate image quality—moderate standing wave artifacts that interfered with image interpretation; 2 = poor image quality— prominent standing wave artifacts, with questionable diagnostic quality; 1 = nondiagnostic image quality.
Table 1 MR image acquisition sequence parameters. Parameters
T2WI
T1WI
THRIVE
DWI
Sequence type Field of view (mm) Image matrix Section thickness (mm) Slice gap (mm) No. of sections No. of packages Sense factor Measured voxel size (mm) Water fat shift (Hz)/bandwidth (Hz) No. of acquired signals b value (sec/mm2) Echo time (msec) Repetition time (msec) Echo train length RF excitation (degrees) Refocusing angle (degrees) Fat saturation SPAIR TR (msec) Acquisition time
TSE 340 436 × 434 3 0 48 3a, 6b 2 0.78 × 0.78 × 3.0 1.743/249.3 1 NA 60 4199 15 90 120 SPAIR 262.44a, 524.88b 3 min 34 seca, 7 min 8 secb
TSE 340 308 × 295 3 0 48 16a, 48b 2 1.10 × 1.15 × 3.0 0.673/645.7 1 NA 13 500 7 90 120 no NA 2 min 16 seca, 6 min 48 secb
TFE 340 484 × 485 NA NA NA NA 2 0.7 × 0.7 × 4.0 1.105/393.0 1 NA 2.7 5.4 40 (TFE factor) 10 NA SPAIR 325.93 0 min 51 seca, 0 min 51 secb
SE-EPI 340 124 × 116 3 0 48 1 2.2 2.74 × 1.69 × 3.0 17.766/24.5 2 0, 800 62 7916a, 13,259b NA 90 NA SPAIR 164.91a, 276.22b 1 min 58 seca, 3 min 18 secb
Note: Given parameter was used with both single-source and dual-source RF excitations, unless otherwise noted. T2WI = T2-weighted image, T1WI = T1-weighted image, DWI = diffusion-weighted image, THRIVE = T1 turbo field-echo high-resolution isotropic volume examination, NA = nonapplicable, TSE = turbo spin echo, TFE = turbo field echo, SPAIR = spectral presaturation attenuated by inversion recovery, SPAIR TR ≈ inversion recovery time × 3. a Dual-source RF excitation. b Single-source RF excitation.
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
Fig. 1. Example of regions of interest (ROIs) drawn at the level of the nipple. Three circles were drawn on the anterior, lateral and medial fibroglandular tissue of each breast, respectively. Five circles were drawn on anterior–lateral, posterior–lateral, anterior–medial, posterior–medial, and posterior fat tissue surrounding fibroglandular tissue of each breast.
Ghost artifacts: 4 = no ghost artifacts; 3 = mild ghost artifacts without impaired image interpretation; 2 = moderate ghost artifacts that interfered with image interpretation; 1 = prominent ghost artifacts with questionable diagnostic quality. The readers were asked to record the number and location of lesions and to rate lesion conspicuity as follows: 3 = good, lesion is seen with definite internal structure and margin; 2 = moderate, lesion is probably seen with indistinct internal structure and margin; and 1 = poor, lesion is not seen. To assess the impact of different RF excitation modes on image homogeneity and hence tissue contrast, image quality was quantitatively evaluated by signal intensity ratios (SRs) and contrast ratios (CRs) [13], which are superior to conventional signal-to-noise ratio and contrast to noise ratio in evaluating image homogeneity and tissue contrast. First, signal intensities of both fat and fibroglandular tissue (FG) were measured for each breast. Signal intensities of fat were measured by defining five circular regions of interest (ROIs) in each breast at about the same axial slice as the nipple and signal intensities for FG were measured by defining three circular ROIs, as shown in Fig. 1. Mean
525
signal intensities for fat or FG in each breast were calculated by averaging the corresponding ROI values. The average area of ROI was 12.1 ± 1.5 mm2, avoiding the inclusion of blood vessels and the edge of tissue. To gain comparable measurements, by image registration, ROIs were placed in exactly corresponding regions on the two sets of images acquired with conventional and dual-source techniques. Subsequently, SRs of fat for both RF excitation modes were calculated. First, the average signal intensity of fat for each breast was obtained by averaging the corresponding five ROIs. SImax stands for the larger average signal intensity, while SImin stands for the other smaller one. Second, SRs were calculated as follows: SR = SImax/ SImin, indicating signal intensity difference between two intraindividual breasts. CRs were calculated between FG and fat, and also between lesion and ipsilateral FG for both excitations. CR = (SIa − SIb)/(SIa + SIb), where SIa and SIb are the signal intensity of two tissues, respectively. The acquisition time for the above sequences of both excitations was recorded. 2.4. Statistical analysis Interobserver agreement was calculated by kappa test. Kappa values higher than 0.75 were considered to be excellent agreement. In cases of excellent interobserver agreement, the data were pooled for the two readers. The score for each sequence and for each patient was taken as the average between the two readers. The average scores, CRs of FG to fat between two excitations were compared, using paired-sample t test, and CRs of lesion to FG and SRs, by Wilcoxon signed ranks test. To compare the conspicuity change of different lesion type, the conspicuity score of each lesion type for single-source technique was subtracted by that for dual source. The obtained final score for each lesion was compared by Kruskal–Wallis H test. P b 0.05 was considered to indicate statistical significance. 3. Results MR imaging was successfully performed with both excitation techniques for all 22 subjects. Interobserver agreements for scoring were excellent, with kappa values N0.75. 3.1. Image homogeneity and artifacts
Table 2 Image score comparison between dual- and single-source technique. Image quality and sequence Homogeneity T2WI T1WI THRIVE Ghost artifact DWI Conspicuity T2WI T1WI THRIVE DWI a b
Acquisition modea
Average scoreb
DS SS DS SS DS SS
4.89 3.84 4.89 4.30 4.84 4.80
0.31 0.36 0.31 0.45 0.36 0.40
9.62/0.00 0.776 0.831 4.54/0.00 0.776 0.891 0.53/0.61 0.831 0.861
DS SS
2.89 ± 0.31 1.50 ± 0.49
12.75/0.00 0.776 0.820
DS SS DS SS DS SS DS SS
2.71 2.75 2.38 2.35 2.42 2.33 2.44 1.42
DS = dual-source; SS = single-source. Data are mean ± standard deviation.
± ± ± ± ± ±
± ± ± ± ± ± ± ±
t/P
Interobserver agreement (kappa value)
0.44 −0.49/0.63 0.798 0.42 0.780 0.47 0.17/0.87 0.822 0.48 0.909 0.48 1.16/0.26 0.830 0.46 0.829 0.50 18.19/0.00 0.915 0.48 0.830
According to the scores, homogeneity on T2WI and T1WI with dual source was significantly better than that with single source (P = 0.00, 0.00), while homogeneity on THRIVE had no significant difference between the two excitation techniques (P = 0.61). Ghost artifact on DWI with dual source was significantly reduced, compared with single source (P = 0.00). The details are summarized in Table 2. On the other hand, quantitative analysis showed that SRs on T2WI and T1WI with dual source were significantly lower than those with Table 3 Signal intensity ratio (SR) comparison between dual- and single-source technique. MR sequence
Acquisition modea
SRb
T2WI
DS SS DS SS DS SS
1.08 1.44 1.05 1.09 1.09 1.11
T1WI THRIVE a
Z/P ± ± ± ± ± ±
0.06 0.24 0.04 0.08 0.06 0.09
−4.010/0.000 −2.062/0.039 −0.601/0.548
DS = dual-source; SS = single-source. SR = mean ± standard deviation, which were calculated as follows: SR = SImax/ SImin, where SImax and SImin mean the larger and smaller average signal intensity of fat between two intra-individual breasts, respectively. b
526
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
single source (P = 0.000, 0.039). SRs on THRIVE had no significant difference between the two excitation techniques (P = 0.548). The results are summarized in Table 3.
For different lesion types (breast cancer, adenoma, cyst and focal hyperplasia), conspicuity score change after using different excitation technique had no significant difference, shown in Table 4.
3.2. Lesion detection and conspicuity
3.3. Image contrast
All 24 lesions were detected by each mode. Fig. 2A–H shows a representative case of images acquired with both excitation modes. Lesions were more conspicuous on DWI with dual source (P = 0.00). However, lesion conspicuity on other sequences had no significant difference between the two excitation techniques, shown in Table 2.
On T2WI, CRs for right FG/fat, left FG/fat and lesion/FG with dual source were significantly lower than those with single source (P = 0.000, 0.000, and 0.000, respectively). On T1WI and THRIVE, CRs for right FG/fat, left FG/fat and lesion/FG had no significant difference between the two excitation techniques (P = 0.824, 0.820, and 0.732 for T1WI; P = 0.514, 0.570, and 0.710 for THRIVE). The signal
Fig. 2. A–H, Comparison between T2WI, T1WI, THRIVE and DWI (b = 800 s/mm2) obtained with dual-source parallel RF excitation (A, C, E, G) and those obtained with conventional single-source RF excitation (B, D, F, H). The window width and level were set to the same for each pair of images. Signal intensity was more homogeneous on images with dual source than that with single source, especially on T2WI (A, B) and T1WI (C, D). Fat on T2WI with dual source (A) was not suppressed as black as that with single source (B), and image contrast of the former was worse than the latter (the arrow indicates a lesion, which is focal hyperplasia, proved by pathology). Ghost artifact on DWI with dual source (E) was less than that with single source (F). The image homogeneity, fat suppression effect and image contrast had no significant difference between field-echo sequence with dual source (G) and that with single source (H).
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
concluded that 3D gradient-echo (or field-echo) sequences were less affected by B1 inhomogeneity, compared with TSE sequences, and could provide diagnostic image quality at 3 T [21,22]. The reason is that, unlike TSE sequence, gradient-echo sequence does not use a large number of RF pulses to generate image contrast, which will result in increased SAR and aggravate B1 non-uniformity. In addition, the ghost artifact on DWI with dual source was dramatically reduced, which has not ever been reported in the literature. In this study, dual-source technique provided limited value for improving breast lesion detection and characterization. For the two excitation modes, the same number of lesions was detected and had the similar conspicuity, except for DWI on which lesions were more conspicuous with dual source due to less interference of ghost artifacts. There was no conspicuity difference among the different evaluated lesion types, regarding to the effect of dual-source technique on improving lesion characterization. Image homogeneity improvement for T2WI and T1WI did not result in better lesion detection and characterization. This is different from a previous study about focal liver lesions [10], which concluded that lesion conspicuity on T2WI was improved with dual source. The reason for the different performance of dual source may be that breasts have different sizes and configuration from those of the abdomen and are not prone to shading artifacts as the abdomen. In addition, image contrast on all evaluated images with dual source did not improve. The image contrast on T2WI with dual source was even inferior to that with single source, mainly because background fat of the former was not suppressed as completely as that of the latter. The different performance of fat suppression on T2WI between two excitation technique is explained by different inversion recovery time (IR) used for SPAIR, which is reflected as SPAIR TR in Table 1. Although the same protocols, other than excitation mode, were set initially for both techniques, actual parameters may automatically change during scan. For dual-source technique, the duration of IR was shorter for its unique excitation mode. Therefore, with shorter IR, the longitudinal relaxation of fat tissue was not recovered to zero when readout frequency was administered, leading to poor fat suppression. This may benefit detection of fibrous lesion which has low signal on T2WI, but may also lead to decreased CR of high-signal lesion that is the case in our study. According to different clinical need, IR should be adjusted. Image acquisitions of SE sequence (including T2WI, T1WI and DWI) with dual source were accelerated, while scan time of fieldecho sequence did not change. The acceleration is related to improved RF uniformity, resulting in reduced whole-body and/or local energy deposition. As a consequence, more sections per TR cycle in combination with a fixed TR (on T2WI and T1WI) or a shorter TR (on DWI) were implemented and thus reduced the image acquisition time.
Table 4 Conspicuity score difference between two excitation techniques of different lesions. Sequence Breast cancer T2WI T1WI THRIVE DWI
−0.30 −0.10 −0.10 1.10
± ± ± ±
Adenoma
0.57 0.07 ± 0.19 0.22 −0.10 ± 0.22 0.22 0.07 ± 0.45 0.55 1.00 ± 0.00
Cyst 0.11 0.28 0.11 1.00
Focal hyperplasia ± ± ± ±
0.33 −0.33 ± 0.58 0.44 0.17 ± 0.29 0.22 0.33 ± 0.58 0.00 1.00 ± 0.50
527
P 0.192 0.296 0.379 1.000
intensity of fat between the two excitations had a significant difference on T2WI (P = 0.000), and no significant difference on T1WI and THRIVE. The results are summarized in Table 5. 3.4. Image acquisition time Image acquisitions of T2WI, T1WI and DWI with dual source were accelerated by 2, 3, and 1.68 times, compared with single source, while the acquisition time of THRIVE was the same for both excitations, shown in Table 1. 4. Discussion The main advantage of dual source is B1 field homogeneity improvement or reduced dielectric shading artifacts, and hence to improve final image quality. This has been proved by several studies focusing on liver MR imaging [11,13], since the abdomen is most vulnerable to B1 field inhomogeneity, for its comparable size to the wavelength of RF at 3 T. B1 field inhomogeneity may also be problematic in breast MRI imaging due to a variety of factors such as off-center positioning of breasts in the whole-body RF coil, respiration movement, heart pulsation, and so on. Unlike previous studies that employed only B1 map for analysis [15,16], our study performed a more complete investigation on both image quality and lesion visualization based on clinical images directly with a comprehensive set of routine MR sequences to probe the influence of B1 field homogeneity. Dual-source technique brought about artifacts reduction on SE sequences in this study. Most T2WI and T1WI with single source had mild dielectric shading effect that, however, did not interfere with image interpretation. By using dual-source technique, image homogeneity was significantly improved from the scores and SR measurement. As to images obtained by 3D field-echo sequence (THRIVE) with single-source technique, they had almost no dielectric shading artifacts for most patients. Dual-source RF excitation seems to have no prominent effect on 3D field-echo sequence in view of image homogeneity improvement. This is consistent with literatures [18–22]. Also, many reports have
Table 5 Contrast ratio (CR) and fat signal comparison between dual and single-source technique. Sequence and acquisition modea T2WI DS SS T1WI DS SS THRIVE DS SS a b
CR between right FG t/P and fatb 6.53 ± 3.88 29.41 ± 13.52
−14.90/0.000
CR between left FG t/P and fatb
CR between lesion and FGb
Z/P
9.03 ± 7.24 21.91 ± 10.26
−14.19/0.000 0.19 ± 0.15 0.32 ± 0.14
−4.286/0.000
Fat signal intensity 773.04 ± 469.58 231.09 ± 190.40
t/P
8.452/0.000
0.48 ± 0.09 0.48 ± 0.08
0.226/0.824
0.48 ± 0.08 0.49 ± 0.07
−0.230/0.820 0.11 ± 0.10 0.11 ± 0.10
−0.343/0.732 1712.58 ± 452.01 −.419/0.679 1722.69 ± 483,41
0.58 ± 0.05 0.59 ± 0.10
−0.617/0.514
0.58 ± 0.04 0.59 ± 0.08
−0.568/0.570 0.07 ± 0.07 0.09 ± 0.08
−0.371/0.710
287.53 ± 64.40 267.90 ± 65.57
DS = dual-source; SS = single-source. CR = mean ± standard deviation. CR = (SIa − SIb)/(SIa + SIb), where SIa and SIb are the signal intensities of two tissues. FG = fibroglandular tissue.
2632/0.061
528
L. Jiang et al. / Magnetic Resonance Imaging 32 (2014) 523–528
This study has some limitations. First, the effect of dual-source technique on the apparent diffusion coefficients of DWI was not investigated further in this study for severe ghost artifacts on DWI with single source. Second, our study was performed on a single 3 T Philips MR scanner, so our results may be vendor specific. A generalization to MR systems from other vendors cannot safely be assumed. Third, the study population was small and further larger study was helpful to validate the results. 5. Conclusion In conclusion, dual-source parallel RF excitation will benefit SE sequences by improving image homogeneity, reducing scan time and increasing lesion conspicuity on DWI, however it has no obvious effect on 3D field-echo sequence. So it has some added but limited clinical value for improving clinical breast MR imaging. References [1] Kuhl CK. Current status of breast MR imaging. II. Clinical applications. Radiology 2007;244:672–91. [2] Peters NH, Borel Rinkes IH, Zuithoff NP, Mali WP, Moons KG, Peeters PH. Metaanalysis of MR imaging in the diagnosis of breast lesions. Radiology 2008;246: 116–24. [3] Soher BJ, Dale BM, Merkle EM. A review of MR physics: 3 T versus 1.5 T. Magn Reson Imaging Clin N Am 2007;15:277–90. [4] Gabriel C, Gabriel S, Corthout E. The dielectric properties of biological tissues. I. Literature survey. Phys Med Biol 1996;41:2231–49. [5] Mihara H, Iriguchi N, Ueno S. Imaging of the dielectric resonance effect in high field magnetic resonance imaging. J Appl Phys 2005;97:3051–3. [6] Collins CM, Liu W, Swift BJ, Smith MB. Combination of optimized transmit arrays and some receive array reconstruction methods can yield homogeneous images at very high frequencies. Magn Reson Med 2005;54:1327–32. [7] Partridge SC, Rahbar H, Lehman CD. Breast MR imaging. In: Kamel IR, Merkle EM, editors. Body MR imaging at 3 Tesla. New York: Cambridge University Press; 2011. [8] Zhu Y. Parallel excitation with an array of transmit coils. Magn Reson Med 2004;51:775–84. [9] Ullmann P, Junge S, Wick M, Seifert F, Ruhm W, Hennig J. Experimental analysis of parallel excitation using dedicated coil setups and simultaneous RF transmission on multiple channels. Magn Reson Med 2005;54:994–1001.
[10] Nelles M, König RS, Gieseke J, Guerand-van Battum MM, Kukuk GM, Schild HH, Willinek WA. Dual-source parallel RF transmission for clinical MR imaging of the spine at 3.0 T: intraindividual comparison with conventional single-source transmission. Radiology 2010;257:743–53. [11] Pazahr S, Fischer MA, Chuck N, Luechinger R, Schick F, Nanz D, Boss A. Liver: segment-specific analysis of B1 field homogeneity at 3.0-T MR imaging with single-source versus dual-source parallel radiofrequency excitation. Radiology 2012;265:591–9. [12] Bouvier J, Troprès I, Lamalle L, Grand S, Breil S, Le Bas JF, Krainik A. Evaluation of dual-source parallel RF excitation technology in MRI of thoraco-lumbar spine at 3.0 T. J Neuroradiol 2013;40:94–100. [13] Kukuk GM, Gieseke J, Weber S, Hadizadeh DR, Nelles M, Träber F, Schild HH, Willinek WA. Focal liver lesions at 3.0 T: lesion detectability and image quality with T2-weighted imaging by using conventional and dual-source parallel radiofrequency transmission. Radiology 2011;259:421–8. [14] Mueller A, Kouwenhoven M, Naehle CP, Gieseke J, Strach K, Willinek WA, Schild HH, Thomas D. Dual-source radiofrequency transmission with patient-adaptive local radiofrequency shimming for 3.0-T cardiac MR imaging: initial experience. Radiology 2012;263:77–85. [15] Rahbar H, Partridge SC, Demartini WB, Gutierrez RL, Parsian S, Lehman CD. Improved B1 homogeneity of 3 Tesla breast MRI using dual-source parallel radiofrequency excitation. J Magn Reson Imaging 2012;35:1222–6. [16] Trop I, Gilbert G, Ivancevic MK, Beaudoin G. Breast MR imaging at 3 T with dual source radiofrequency transmission offers superior B1 homogeneity: an intraindividual comparison with breast MR imaging at 1.5 T. Radiology 2013;267:602–8. [17] Mürtz P, Kaschner M, Träber F, Kukuk G, Skowasch D, Gieseke J, Schild HH, Willinek WA. Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T. J Magn Reson Imaging 2012;35: 456–61. [18] Kuhl CK, Tr ber F, Gieseke J, Drahanowsky W, Morakkabati-Spitz N, Willinek W, von Falkenhausen M, Manka C, Schild HH. Whole body high-field-strength (3.0-T) MR imaging in clinical practice. Part II. Technical considerations and clinical applications. Radiology 2008;247:16–35. [19] Merkle EM, Dale BM. Abdominal MRI at 3.0 T: the basics revisited. Am J Roentgenol 2006;186:1524–32. [20] Zapparoli M, Semelka RC, Altun E, Tsurusaki M, Pamuklar E, Dale BM, Gasparetto EL, Elias Jr J. 3.0-T MRI evaluation of patients with chronic liver diseases: initial observations. Magn Reson Imaging 2008;26:650–60. [21] Ramalho M, Herédia V, Tsurusaki M, Altun E, Semelka RC. Quantitative and qualitative comparison of 1.5 and 3.0 Tesla MRI in patients with chronic liver diseases. J Magn Reson Imaging 2009;29:869–79. [22] Tsurusaki M, Semelka RC, Zapparoli M, Elias Jr J, Altun E, Pamuklar E, Sugimura K. Quantitative and qualitative comparison of 3.0 T and 1.5 T MR imaging of the liver in patients with diffuse parenchymal liver disease. Eur J Radiol 2009;72: 314–20.
