Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Ave, Boston, MA 02215, USA.
Radiology. 2011 May;259(2):550-7. doi: 10.1148/radiol.11092445. Epub 2011 Feb 25.
To explore the lower limits for radiofrequency (RF) power-induced specific absorption rate (SAR) achievable at 1.5 T for brain magnetic resonance (MR) imaging without loss of tissue signal or contrast present in high-SAR clinical imaging in order to create a potentially viable MR method at ultra-low RF power to image tissues containing implanted devices.
An institutional review board-approved HIPAA-compliant prospective MR study design was used, with written informed consent from all subjects prior to MR sessions. Seven healthy subjects were imaged prospectively at 1.5 T with ultra-low-SAR optimized three-dimensional (3D) fast spin-echo (FSE) and fluid-attenuated inversion-recovery (FLAIR) T2-weighted sequences and an ultra-low-SAR 3D spoiled gradient-recalled acquisition in the steady state T1-weighted sequence. Corresponding high-SAR two-dimensional (2D) clinical sequences were also performed. In addition to qualitative comparisons, absolute signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) for multicoil, parallel imaging acquisitions were generated by using a Monte Carlo method for quantitative comparison between ultra-low-SAR and high-SAR results.
There were minor to moderate differences in the absolute tissue SNR and CNR values and in qualitative appearance of brain images obtained by using ultra-low-SAR and high-SAR techniques. High-SAR 2D T2-weighted imaging produced slightly higher SNR, while ultra-low-SAR 3D technique not only produced higher SNR for T1-weighted and FLAIR images but also higher CNRs for all three sequences for most of the brain tissues.
The 3D techniques adopted here led to a decrease in the absorbed RF power by two orders of magnitude at 1.5 T, and still the image quality was preserved within clinically acceptable imaging times.
探索在 1.5T 场强下实现射频（RF）功率诱导特定吸收率（SAR）下限，以保持高 SAR 临床成像中存在的组织信号或对比不失真，从而创建一种在超低 RF 功率下对包含植入设备的组织进行成像的潜在可行的磁共振方法。
采用机构审查委员会批准的符合 HIPAA 标准的前瞻性磁共振研究设计，所有受试者在进行磁共振检查前均签署书面知情同意书。7 名健康受试者前瞻性地在 1.5T 场强下进行成像，采用超低 SAR 优化的三维（3D）快速自旋回波（FSE）和液体衰减反转恢复（FLAIR）T2 加权序列以及超低 SAR 3D 扰相梯度回波稳态 T1 加权序列。还进行了相应的高 SAR 二维（2D）临床序列检查。除了定性比较外，通过蒙特卡罗方法生成多线圈、并行采集的绝对信噪比（SNR）和对比噪声比（CNR），以对超低 SAR 和高 SAR 结果进行定量比较。
使用超低 SAR 和高 SAR 技术获得的脑组织 SNR 和 CNR 值的绝对值以及脑图像的定性外观存在较小到中度差异。高 SAR 2D T2 加权成像产生稍高的 SNR，而超低 SAR 3D 技术不仅为 T1 加权和 FLAIR 图像产生更高的 SNR，而且为大多数脑区的所有三个序列产生更高的 CNR。
这里采用的 3D 技术使 1.5T 场强下吸收的 RF 功率降低了两个数量级，而在可接受的临床成像时间内仍保持了可接受的图像质量。
