Some have also proposed the use of RF cushions to minimize RF shading in body and cardiac imaging at 3T. Various investigators have attempted to relate the extent of such RF inhomogeneity to the patient’s body habitus. 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. 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. A number of methods have been proposed in the literature to combat these effects. 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.
Roi transmit free#
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). This finding indicates the need for subject-specific RF shimming.ĭespite 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). For all body types studied, cardiac B 1 field homogeneity was significantly improved by performing local RF shimming with 2 independent RF-transmit channels. With or without RF shimming, cardiac B1 field homogeneity does not depend on body type, as characterized by BMI, BSA, and AP/RL. Subject specific RF (B 1) shimming with a dual-transmit system improved local RF homogeneity across all body types. B 1 homogeneity was independent of subject body type (body surface area, body mass index or anterior-posterior/right-left patient width ratio ). 81.2 ± 13.3% P = 0.0014), reduced the B 1 field variation by 42.2 ± 13%, and significantly improved the percentage of voxels closer to μ (39% and 82% more voxels were closer to ± 10% and ± 5% of μ, respectively) when compared with no RF shimming. Local RF shimming across the region encompassed by the heart increased the mean flip angle (μ) in that area (88.5 ± 15.2% vs. Metrics quantifying B 1 field homogeneity were calculated and compared with subject body habitus.
B 1 maps with and without subject-specific local RF shimming (exploiting the independent control of transmit amplitude and phase of the 2 RF transmitters) were obtained. We obtained cardiac images from 37 subjects (including 11 patients) by using dual-transmit 3T CMR. In this article, we sought to evaluate the effect of local RF (B 1 field) shimming by using a dual-source–transmit RF system for cardiac cine imaging and to systematically evaluate the effect of subject body type on the B 1 field with and without local RF shimming. Radiofrequency (RF) shading artifacts degrade image quality while performing cardiovascular magnetic resonance (CMR) at higher field strengths.