Delgado Paula Ramos, Kuehne Andre, Periquito João S, Millward Jason M, Pohlmann Andreas, Waiczies Sonia, Niendorf Thoralf
Berlin Ultrahigh Field Facility (B.U.F.F), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.
Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany.
Magn Reson Med. 2020 Nov;84(5):2684-2701. doi: 10.1002/mrm.28307. Epub 2020 May 24.
The use of surface radiofrequency (RF) coils is common practice to boost sensitivity in (pre)clinical MRI. The number of transceive surface RF coils is rapidly growing due to the surge in cryogenically cooled RF technology and ultrahigh-field MRI. Consequently, there is an increasing need for effective correction of the excitation field ( ) inhomogeneity inherent in these coils. Retrospective B correction permits quantitative MRI, but this usually requires a pulse sequence-specific analytical signal intensity (SI) equation. Such an equation is not available for fast spin-echo (Rapid Acquisition with Relaxation Enhancement, RARE) MRI. Here we present, test, and validate retrospective B correction methods for RARE.
We implemented the commonly used sensitivity correction and developed an empirical model-based method and a hybrid combination of both. Tests and validations were performed with a cryogenically cooled RF probe and a single-loop RF coil. Accuracy of SI quantification and T contrast were evaluated after correction.
The three described correction methods achieved dramatic improvements in B homogeneity and significantly improved SI quantification and T contrast, with mean SI errors reduced from >40% to >10% following correction in all cases. Upon correction, images of phantoms and mouse heads demonstrated homogeneity comparable to that of images acquired with a volume resonator. This was quantified by SI profile, SI ratio (error < 10%), and percentage of integral uniformity (PIU > 80% in vivo and ex vivo compared to PIU > 87% with the reference RF coil).
This work demonstrates the efficacy of three B correction methods tailored for transceive surface RF probes and RARE MRI. The corrected images are suitable for quantification and show comparable results between the three methods, opening the way for T measurements and X-nuclei quantification using surface transceiver RF coils. This approach is applicable to other MR techniques for which no analytical SI exists.
使用表面射频（RF）线圈是在（临床前）临床磁共振成像（MRI）中提高灵敏度的常用方法。由于低温冷却射频技术和超高场MRI的兴起，收发表面RF线圈的数量正在迅速增加。因此，越来越需要有效校正这些线圈固有的激发场（ ）不均匀性。回顾性B校正允许进行定量MRI，但这通常需要一个特定脉冲序列的分析信号强度（SI）方程。对于快速自旋回波（快速采集弛豫增强，RARE）MRI，这样的方程并不存在。在此，我们提出、测试并验证了适用于RARE的回顾性B校正方法。
我们实施了常用的灵敏度校正，并开发了一种基于经验模型的方法以及两者的混合组合。使用低温冷却RF探头和单环RF线圈进行测试和验证。校正后评估SI定量的准确性和T对比度。
所描述的三种校正方法在B均匀性方面取得了显著改善，并显著提高了SI定量和T对比度，在所有情况下校正后平均SI误差从>40%降低到>10%。校正后，体模和小鼠头部的图像显示出与使用体部谐振器采集的图像相当的均匀性。这通过SI剖面图、SI比率（误差<10%）以及积分均匀性百分比（体内和体外PIU>80%，与参考RF线圈的PIU>87%相比）进行了量化。
这项工作证明了三种为收发表面RF探头和RARE MRI量身定制的B校正方法的有效性。校正后的图像适用于定量分析，并且三种方法之间显示出可比的结果，为使用表面收发RF线圈进行T测量和X核定量开辟了道路。这种方法适用于不存在分析SI的其他MR技术。
