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Clinical CMR at 3.0 Tesla using parallel RF transmission with patient-adaptive B1 shimming: initial experience
Journal of Cardiovascular Magnetic Resonance volume 12, Article number: O72 (2010)
The clinical implementation of high-field CMR systems has introduced new challenges for cardiac imaging due to B0 and B1 field inhomogeneities. TSE Black-Blood sequences (BB) are compromised by dielectric artifacts, whereas SSFP cine-sequences are known to suffer from dark-band artifacts. The flip-angle non-uniformity across the field of view affects image homogeneity of both sequences. The use of a multi-source RF transmission system may help reduce dielectric effects, improve flip-angle uniformity and avoid local SAR peaks, thus allowing a shorter minimum TR/TE in SSFP-sequences.
In this study we investigated the benefit of a novel multi-source RF transmission system with patient-adaptive RF shimming for cardiac imaging at 3.0 Tesla.
A clinical 3.0 T MRI system (Philips Achieva TX, Healthcare, Best), equipped with flexible dual-source RF transmission, was used. With this setup it is possible to independently control phase, amplitude and shape of the RF waveforms. A standard 6 element cardiac phased-array coil was used for signal reception.
The effect of single-source (SingleTransmit) vs. multi-source (MultiTransmit) RF-transmission with B1-shimming on SSFP cine-sequences and Black-Blood (BB) sequences was evaluated. Images were analyzed independently by two readers for homogeneity and off-resonance artifacts. Both were rated on a 4-point grading-scale: (4) uniform signal/no off-resonance artifacts, (3) mild signal inhomogeneity/off-resonance artifacts, (2) moderate signal inhomogeneity/off-resonance artifacts, (1) strong signal inhomogeneity/off-resonance artifacts. In addition diagnostic confidence was rated on a 4-point grading-scale: (4) very high confidence in diagnostic content, (3) good confidence in diagnostic content, (2) reservations about diagnostic content, (1) non-diagnostic. P-values were calculated using the Wilcoxon signed-ranks test. Percentage of overall interobserver agreement (Po) was calculated.
A total of 14 SSFP- and 7 BB-sequences were analyzed (patients = 7). For the SSFP-sequence the left and the right ventricle showed significant improvements with respect to homogeneity, off-resonance artifacts and diagnostic confidence (see Table 1 and Fig. 1).
In the BB-sequence homogeneity and diagnostic confidence significantly improved for the right ventricle (see Table 2 and Figure 2). Percentage of overall interobserver agreement for all ratings was good to excellent (0.66 < Po < 0.95).
Parallel multi RF-transmission with B1-shimming significantly improves the image homogeneity and contrast of cardiac SSFP and BB sequences and thus represents a major step forward in clinical CMR at 3.0 T.
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Mueller, A., Weisser-Thomas, J., Naehle, C.P. et al. Clinical CMR at 3.0 Tesla using parallel RF transmission with patient-adaptive B1 shimming: initial experience. J Cardiovasc Magn Reson 12 (Suppl 1), O72 (2010). https://doi.org/10.1186/1532-429X-12-S1-O72
- Diagnostic Confidence
- Image Homogeneity
- Diagnostic Content
- Dielectric Artifact