Department of Radiology, Texas Heart Institute at St, Luke's Episcopal Hospital, 6720 Bertner Avenue, Houston, TX 77030, USA.
J Cardiovasc Magn Reson. 2013 Aug 6;15(1):68. doi: 10.1186/1532-429X-15-68.
Radiofrequency (RF) shading artifacts degrade image quality while performing cardiovascular magnetic resonance (CMR) at higher field strengths. In this article, we sought to evaluate the effect of local RF (B1 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 B1 field with and without local RF shimming.
We obtained cardiac images from 37 subjects (including 11 patients) by using dual-transmit 3T CMR. B1 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. Metrics quantifying B1 field homogeneity were calculated and compared with subject body habitus.
Local RF shimming across the region encompassed by the heart increased the mean flip angle (μ) in that area (88.5 ± 15.2% vs. 81.2 ± 13.3%; P = 0.0014), reduced the B1 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. B1 homogeneity was independent of subject body type (body surface area [BSA], body mass index [BMI] or anterior-posterior/right-left patient width ratio [AP/RL]). Subject specific RF (B1) shimming with a dual-transmit system improved local RF homogeneity across all body types.
With or without RF shimming, cardiac B1 field homogeneity does not depend on body type, as characterized by BMI, BSA, and AP/RL. For all body types studied, cardiac B1 field homogeneity was significantly improved by performing local RF shimming with 2 independent RF-transmit channels. This finding indicates the need for subject-specific RF shimming.
在更高场强下进行心血管磁共振（CMR）时，射频（RF）遮蔽伪影会降低图像质量。本文旨在评估使用双源发射 RF 系统进行心脏电影成像时局部 RF（B1 场）匀场的效果，并系统评估在有/无局部 RF 匀场的情况下，受检者体型对 B1 场的影响。
我们使用双发射源 3T CMR 从 37 名受检者（包括 11 名患者）中获得心脏图像。获得了有无受检者特异性局部 RF 匀场（利用 2 个 RF 发射器的发射幅度和相位的独立控制）的 B1 图谱。计算了量化 B1 场均匀性的指标，并与受检者体型进行了比较。
心脏区域内的局部 RF 匀场增加了该区域的平均翻转角（μ）（88.5±15.2%对 81.2±13.3%；P=0.0014），降低了 B1 场变化 42.2±13%，并显著改善了更接近μ的体素百分比（分别有 39%和 82%的体素更接近±10%和±5%的μ），与无 RF 匀场相比。B1 均匀性与受检者体型（体表面积[BSA]、体重指数[BMI]或前后/左右患者宽度比[AP/RL]）无关。双发射源系统的受检者特异性 RF（B1）匀场改善了所有体型的局部 RF 均匀性。
无论是否进行 RF 匀场，心脏 B1 场均匀性都不依赖于体型，以 BMI、BSA 和 AP/RL 为特征。对于所有研究的体型，通过使用 2 个独立的 RF 发射通道进行局部 RF 匀场，可以显著改善心脏 B1 场均匀性。这一发现表明需要进行受检者特异性 RF 匀场。
