Nuclear Magnetic Resonance Methods & Development Group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany.
Faculty of Physics and Earth Sciences, Felix Bloch Institute for Solid State Physics, Leipzig University, Leipzig, Germany.
Magn Reson Med. 2024 Mar;91(3):1239-1253. doi: 10.1002/mrm.29934. Epub 2023 Nov 27.
In any MR experiment, the bulk magnetization acts on itself, caused by the induced current in the RF receiver circuit that generates an oscillating damping field. This effect, known as "radiation damping" (RD), is usually weak and, therefore, unconsidered in MRI, but can affect quantitative studies performed with dedicated coils that provide a high SNR. The current work examined RD in a setup for investigations of small tissue specimens including a quantitative characterization of the spin-coil system.
A custom-made Helmholtz coil (radius and spacing 16 mm) was interfaced to a transmit-receive (Tx/Rx) switch with integrated passive feedback for modulation or suppression of RD similar to preamplifier decoupling. Pulse sequences included pulse-width arrays to demonstrate the absence/ presence of RD and difference techniques employing gradient pulses or composite RF pulses to quantify RD effects during free precession and transmission, respectively. Experiments were performed at 3T in small samples of MnCl solution.
Significant RD effects may impact RF pulse application and evolution periods. Effective damping time constants were comparable to typical T * times or echo spacings in multi-echo sequences. Measurements of the phase relation showed that deviations from the commonly assumed 90° angle between the damping field and the transverse magnetization may occur.
Radiation damping may affect the accuracy of quantitative MR measurements performed with dedicated RF coils. Efficient mitigation can be achieved hardware-based or by appropriate consideration in the pulse sequence.
在任何磁共振实验中，由于射频接收器电路中感应电流产生的振荡阻尼场，体磁化强度会对自身产生作用。这种效应称为“辐射阻尼”（RD），通常较弱，因此在 MRI 中不予考虑，但会影响使用提供高信噪比的专用线圈进行的定量研究。本研究在用于研究小组织标本的设置中检查了 RD，包括对自旋线圈系统进行定量表征。
定制的亥姆霍兹线圈（半径和间距 16mm）与具有集成无源反馈的发射/接收（Tx/Rx）开关接口，用于调制或抑制 RD，类似于前置放大器去耦。脉冲序列包括脉冲宽度阵列，以证明 RD 的存在/不存在，以及使用梯度脉冲或复合 RF 脉冲的差分技术，分别在自由进动和传输期间量化 RD 效应。实验在 3T 下对 MnCl 溶液的小样本进行。
RD 效应可能会影响 RF 脉冲的施加和演化周期。有效阻尼时间常数与多回波序列中的典型 T * 时间或回波间隔相当。相位关系的测量表明，阻尼场与横向磁化之间的角度可能偏离通常假设的 90°。
辐射阻尼可能会影响使用专用 RF 线圈进行的定量磁共振测量的准确性。可以通过硬件或在脉冲序列中进行适当考虑来实现有效的缓解。
