Background
Despite the increasing availability of 3T systems in the clinical setting, 1.5T remains the field-strength of choice for routine clinical cardiovascular magnetic resonance (CMR). Cardiac imaging at 3T is hampered by more pronounced off-resonance related artifacts in commonly used CMR sequences such as balanced steady state free precession (b-SSFP). It is also hampered by signal non-uniformity across the imaging slice for radiofrequency (RF) intensive sequences such as black-blood turbo-spin echo (BB-TSE), due to transmit RF field (B
1) inhomogeneity. A number of methods have been proposed in the literature to combat these effects [
1‐
8]. A particularly important problem is the transmit B
1 field variation across the slice at 3T in body and cardiac imaging, as the wavelength of the RF field at 3T approaches the size of the human body. Sung and coworkers demonstrated that the transmit B
1 field can vary by as much as 50% across the heart at 3T, and the resulting loss of contrast is irreversible [
4]. Various investigators have attempted to relate the extent of such RF inhomogeneity to the patient’s body habitus. Some have also proposed the use of RF cushions to minimize RF shading in body and cardiac imaging at 3T [
1,
9,
10]. Over the years, several groups have proposed that RF homogeneity can be improved by using independently controlled multiple-transmit sources and have demonstrated the feasibility of this approach in both phantom and human studies [
11,
12]. Recently, Willinek and associates [
12] showed that using a dual-transmit approach in a clinical scanner markedly improved the quality of body images. To our knowledge, however, the literature contains only 1 previous article regarding the use of dual-transmit systems for cardiac imaging at 3T [
13].
The purposes of our current study were 1) to observe the effect of local RF shimming for cardiac imaging using a dual-source transmit RF system that offers independent control of the RF phase and amplitude of the 2 channels, 2) to study the effect of subject body type on the B1 field with and without local RF shimming, and 3) to investigate the effect of local RF shimming on B1 field inhomogeneity in cardiac cine imaging using b-SSFP sequence.
Discussion
High-field imaging (at 3T) has gained widespread acceptance and is routinely used in higher-spatial-resolution brain imaging, neuroimaging (fMRI) [
17], and musculo-skeletal imaging [
18]. While the increased signal-to-noise ratio and contrast-to-noise ratio available with 3T could benefit signal-starved CMR acquisitions such as myocardial perfusion imaging, viability (late enhancement) imaging, and high-resolution coronary artery imaging, clinical acceptance of CMR at 3T has been challenging due to the presence of more-prominent artifacts in commonly used sequences such as b-SSFP (for cine imaging) and TSE (black-blood imaging). Whereas the need for greater control over B
1 high-field imaging has been well recognized for more than a decade, clinical 3T scanners with dual-transmit ability have been available on the market only since 2009. Commercial scanners that can perform B
1 shimming over the heart (cardiac gated B
1 shimming) became available only in 2011. Therefore, until recently, the preferred field strength for CMR was not 3T [
19]. In particular, many nonacademic radiologists remain strongly convinced that 3T is not suitable for clinical cardiovascular imaging, based on initial clinical experience with the first generation of 3T systems.
In this study, we sought to quantitatively evaluate the effect of local RF shimming using a commercially available dual-transmit system, and the effect of the subject body habitus on such shimming. Recently, Mueller and associates [
13] also evaluated the effect of a dual-transmit system on the cardiac B
1 field on 13 subjects, and their results also broadly confirm the findings of this study. To our knowledge, our current study is the first to systematically evaluate the effect local RF shimming using a dual-transmit system on subjects with varying body habitus, and to quantitatively measure the extent of the variation in B
1 amplitude/phase necessary to effect local RF shimming in a clinical system. Several important findings of our study merit discussion.
First, cine b-SSFP sequences pose a particular challenge for CMR at 3T. For a given degree of magnetic field inhomogeneity, for the cine b-SSFP sequence to have the same extent of off-resonance–induced effects as at 1.5T, the repetition time (TR) of the sequence has to be reduced by a factor of 2. The typical TR for a b-SSFP sequence at 1.5T is around 3.0ms. Reducing the already short TR by a factor of 2 places a severe burden on the gradient hardware and imposes a significant specific-absorption-rate (SAR) constraint by increasing the RF duty cycle. Conventional approaches for lowering the SAR by using longer RF pulses prolong the TR, thereby making the off-resonance induced artifacts more prominent. In addition, simulations show that the myocardial-to-blood signal contrast in b-SSFP sequences is significantly lower when the excitation flip angle is deliberately set to a value less than 46°, as compared to a higher flip angle, either to reduce SAR or due to RF inhomogeneity. In this regard, Sung and colleagues [
4] have shown that significant B
1 inhomogeneity exists even across a small region spanning the extent of the heart, and our results confirm their findings.
When the subject-specific local RF shimming is performed by using 2 RF transmit sources, the scalar metric (Cv) used for describing RF inhomogeneity decreases by 42%. Additionally, the number of voxels that fall within ± 10% of the mean flip angle increases significantly.
Our results show that with RF shimming, cardiac cine b-SSFP images show significantly fewer changes in the myocardial signal intensity across left ventricular regions than without RF shimming. The cine images acquired both with and without RF shimming were corrected to account for the spatial variation of receive-coil sensitivity, using the coil sensitivity maps acquired as a part of the parallel imaging acquisition. Therefore, our results suggest, at the very least, that a significant portion of the signal variation across the myocardium in b-SSFP sequences at 3T is due to transmit B
1 inhomogeneity, which is substantially reduced with local RF shimming.A couple of points are worth noting with respect to this analysis. First, the signal intensity variation across the segments of the left ventricular myocardium did not show any particular pattern, suggesting that B
1 field is subject-specific. Secondly, by calculating the IU values, we were able to determine the extent of myocardial signal intensity variation within a subject and across subjects. A higher value of IU variation in bSSFP images obtained without local RF shimming, when compared to those obtained with local RF shimming, points to the deleterious effect of B
1 field inhomogeneity and the need for RF field shimming. Furthermore, the variation in the B
1 field is subject-specific, as the fat, muscle, blood, and bone compartments of each person are different. The B
1 field distribution depends on the tissues’ electrical parameters, such as electric conductivity and permittivity, as well as the coupling of the RF field to the subject being imaged. Previous studies have shown that the B
1 field distribution within the body can be approximated with simple tissue models. However, it is challenging to predict the B
1 field in the clinical setting, because it is difficult to model the fat/muscle distribution for any given patient [
10,
20]. The results of our study show that there is no clear relationship between local B
1 field homogeneity and the subject body type (measured by BSA, BMI, or the AP/RL ratio), either with or without local RF shimming. Likewise μ, C
v, and the spread of the transmit amplitude and phase did not show any dependence on the BMI, BSA, and AP/RL ratio. These findings explain the inconsistent results obtained with the use of devices such as RF cushions. They also point to the need for subject-specific RF shimming. While, we did not perform an extensive study of the effect of the size of the shim volume, we did not find substantial changes in the mean flip angles or the Cv, as long as the RF shim volume was placed over the heart. We found this variation to be less than 2%, across the heart, even when the size of the shim box around the heart was changed by a factor of two along each encoding direction. Other pulse-sequence modifications, e.g., B
1-insensitive RF pulses have been proposed for combating the detrimental effect of B
1 inhomogeneity. When these B
1-insensitive RF pulses are used as preparation pulses, e.g., a T
2 preparation pulse for coronary artery imaging, their prolonged duration is not detrimental and can even be effective. However, the use of such pulses in sequences such as cine b-SSFP is more challenging for the reasons previously described. The ability to perform RF shimming by using multiple transmit sources provides additional flexibility in combating the deleterious effects of B
1 inhomogeneity without increasing scan time.
Researchers have found that having a greater number of transmit RF channels can provide greater flexibility and control over B
1field shimming [
21‐
23]. Nevertheless, the potential benefit of more RF channels must be balanced against increased system complexity. Preliminary theoretical evaluations indicate that the incremental benefit of having more than 2 transmit RF channels is rather modest [
11], but there have been few clinical evaluations of such systems to date, and these issues need to be evaluated further.
Although our study confirmed some overall conclusions of Mueller and colleagues [
13], substantial differences exist between their study and ours, including differences in analytical techniques. We believe that by clarifying the explicit relationship of the quantitative metrics to patient body habitus, our study makes an important contribution. Moreover, by providing information regarding the relative power between the 2 transmit channels and the phase difference between the channels as a function of body habitus, our results enable readers to make an informed decision about whether a dual-transmit system is needed for cardiac imaging at 3T or whether it might be just as effective to use a predetermined combination of RF amplitude/phase settings for the 2 channels, which might provide “optimal” B
1 shimming for a given anatomy across all patients. This information is not available elsewhere in the literature. Without it, the value of multi-transmit solutions will be influenced by commercial considerations.
Our study has some limitations. First is the relatively small number of subjects involved. Although the population was small, we included subjects with varying body types, resulting in a wide range of BSAs, BMIs, and AP/RL ratios. This wide range provided an idea of how the B1 field varies with body habitus, although a larger number of subjects would have added to the confidence of the study. In addition, we analyzed only the b-SSFP images clinically, although the difference between shimming and not shimming the local RF field could have been visually qualified in other commonly performed cardiac sequences such as BB-TSE, perfusion imaging, and delayed-enhanced viability imaging. As explained earlier in this section, we felt that b-SSFP images capture the cyclic constraint between TR and SAR, thereby severely limiting the other options (such as B1-insensitive RF pulses) available for RF field shimming. Also, the increased scan time limited our ability to perform these comparisons. Future studies that could add to our findings include investigations of B1 field homogeneity with other techniques, such as RF cushions and B1 field-insensitive RF pulses.
Competing interests
1) Dr. Amol Pednekar is an employee of Philips Health Care, Houston, TX, USA.
2) Dr. Marc Kouwenhoven is an employee of Philips Healthcare, Best, The Netherlands.
Authors’ contributions
Study Design: RK, AP, BC, RM. Image Acquisition: RK, AP, BC, RM. Post-Processing of Data: RK. First Draft: RK and RM. Draft editing: RK, AP, MK, BC, RM. All authors read and approved the final manuscript.