Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37232, USA; Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
MR Engineering, GE Healthcare, Waukesha, WI 53188, USA.
J Magn Reson. 2023 Jul;352:107479. doi: 10.1016/j.jmr.2023.107479. Epub 2023 May 26.
MR microscopy is in principle capable of producing images at cellular resolution (<10 µm), but various factors limit the quality achieved in practice. A recognized limit on the signal to noise ratio and spatial resolution is the dephasing of transverse magnetization caused by diffusion of spins in strong gradients. Such effects may be reduced by using phase encoding instead of frequency encoding read-out gradients. However, experimental demonstration of the quantitative benefits of phase encoding are lacking, and the exact conditions in which it is preferred are not clearly established. We quantify the conditions where phase encoding outperforms a readout gradient with emphasis on the detrimental effects of diffusion on SNR and resolution.
A 15.2 T Bruker MRI scanner, with 1 T/m gradients, and micro solenoid RF coils < 1 mm in diameter, were used to quantify diffusion effects on resolution and the signal to noise ratio of frequency and phase encoded acquisitions. Frequency and phase encoding's spatial resolution and SNR per square root time were calculated and measured for images at the diffusion limited resolution. The point spread function was calculated and measured for phase and frequency encoding using additional constant time phase gradients with voxels 3-15 µm in dimension.
The effect of diffusion during the readout gradient on SNR was experimentally demonstrated. The achieved resolutions of frequency and phase encoded acquisitions were measured via the point-spread-function and shown to be lower than the nominal resolution. SNR per square root time and actual resolution were calculated for a wide range of maximum gradient amplitudes, diffusion coefficients, and relaxation properties. The results provide a practical guide on how to choose between phase encoding and a conventional readout. Images of excised rat spinal cord at 10 µm × 10 µm in-plane resolution demonstrate phase encoding's benefits in the form of higher measured resolution and higher SNR than the same image acquired with a conventional readout.
We provide guidelines to determine the extent to which phase encoding outperforms frequency encoding in SNR and resolution given a wide range of voxel sizes, sample, and hardware properties.
磁共振显微镜原则上能够产生具有细胞分辨率（<10μm）的图像，但各种因素限制了实际中可达到的质量。自旋在强梯度中扩散引起的横向磁化弛豫是限制信噪比和空间分辨率的公认限制。可以通过使用相位编码而不是频率编码读出梯度来减少这种影响。然而，缺乏相位编码在定量上的优势的实验证明，并且尚未明确确定其优先的具体条件。我们量化了相位编码优于读出梯度的条件，重点是扩散对 SNR 和分辨率的有害影响。
使用 15.2T Bruker MRI 扫描仪，具有 1T/m 的梯度和直径<1mm 的微螺线管 RF 线圈，来量化扩散效应对分辨率和频率和相位编码采集的信噪比的影响。在扩散受限分辨率下计算并测量了频率和相位编码的空间分辨率和每平方根时间的 SNR。使用附加的具有 3-15μm 尺寸体素的恒定时相梯度，计算并测量了相位和频率编码的点扩散函数。
实验证明了在读出梯度期间扩散对 SNR 的影响。通过点扩散函数测量并显示了频率和相位编码采集的实际分辨率低于标称分辨率。针对各种最大梯度幅度、扩散系数和弛豫特性，计算并测量了每平方根时间的 SNR 和实际分辨率。结果为在相位编码和传统读出之间进行选择提供了实用指南。以 10μm×10μm 面内分辨率对切除的大鼠脊髓进行成像，证明了相位编码在更高的测量分辨率和比传统读出更高的 SNR 方面的优势。
我们提供了指南，用于确定在各种体素大小、样本和硬件特性下，相位编码在 SNR 和分辨率方面优于频率编码的程度。
