Department of Diagnostic and Interventional Radiology, Leipzig University Hospital, Leipzig, Germany.
Med Phys. 2011 Nov;38(11):6327-35. doi: 10.1118/1.3655027.
MR-visible markers have already been used for various purposes such as image registration, motion detection, and device tracking. Inductively coupled RF (ICRF) coils, in particular, provide a high contrast and do not require connecting wires to the scanner, which makes their application highly flexible and safe. This work aims to thoroughly characterize the MR signals of such ICRF markers under various conditions with a special emphasis on fully automatic detection.
The small markers consisted of a solenoid coil that was wound around a glass tube containing the MR signal source and tuned to the resonance frequency of a 1.5 T MRI. Marker imaging was performed with a spoiled gradient echo sequence (FLASH) and a balanced steady-state free precession (SSFP) sequence (TrueFISP) in three standard projections. The signal intensities of the markers were recorded for both pulse sequences, three source materials (tap water, distilled water, and contrast agent solution), different flip angles and coil alignments with respect to the B(0) direction as well as for different marker positions in the entire imaging volume (field of view, FOV). Heating of the ICRF coils was measured during 10-min RF expositions to three conventional pulse sequences. Clinical utility of the markers was assessed from their performance in computer-aided detection and in defining double oblique scan planes.
For almost the entire FOV (±215 mm) and an estimated 82% of all possible RF coil alignments with respect to B(0), the ICRF markers generated clearly visible MR signals and could be reliably localized over a large range of flip angles, in particular with the TrueFISP sequence (0.3°-4.0°). Generally, TrueFISP provided a higher marker contrast than FLASH. RF exposition caused a moderate heating (≤5 °C) of the ICRF coils only.
Small ICRF coils, imaged at low flip angles with a balanced SSFP sequence showed an excellent performance under a variety of experimental conditions and therefore make for a reliable, compact, flexible, and relatively safe marker for clinical use.
磁共振（MR）可见标记物已经被用于各种目的，如图像配准、运动检测和设备跟踪。特别是感应耦合射频（ICRF）线圈，由于其对比度高，并且不需要与扫描仪连接电线，因此其应用具有高度的灵活性和安全性。本工作旨在全面描述各种条件下此类 ICRF 标记物的 MR 信号特征，特别强调全自动检测。
小标记由缠绕在含有 MR 信号源的玻璃管上的螺线管线圈组成，调谐到 1.5 T MRI 的共振频率。使用失相位梯度回波序列（FLASH）和平衡稳态自由进动（SSFP）序列（TrueFISP）在三个标准投影中对标记物进行成像。记录了两种脉冲序列、三种源材料（自来水、蒸馏水和造影剂溶液）、不同翻转角和线圈相对于 B0 方向的取向以及整个成像体积（视野，FOV）中不同标记物位置的标记物信号强度。在对三个常规脉冲序列进行 10 分钟 RF 曝光期间，测量了 ICRF 线圈的加热情况。从计算机辅助检测和定义双斜扫描平面的性能评估了标记物的临床应用价值。
在整个 FOV（±215 mm）内，并且在估计 B0 方向上所有可能的 RF 线圈取向的 82%范围内，ICRF 标记物产生了清晰可见的 MR 信号，并且可以在很大的翻转角范围内可靠地定位，特别是使用 TrueFISP 序列（0.3°-4.0°）。一般来说，TrueFISP 提供的标记物对比度高于 FLASH。RF 暴露仅导致 ICRF 线圈适度加热（≤5°C）。
在各种实验条件下，使用低翻转角的平衡 SSFP 序列成像的小型 ICRF 线圈表现出优异的性能，因此成为一种可靠、紧凑、灵活且相对安全的临床应用标记物。
