Li B S, Regal J, Gonen O
Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania, USA.
Magn Reson Med. 2001 Dec;46(6):1049-53. doi: 10.1002/mrm.1297.
It is commonly accepted that the signal-to-noise ratio (SNR = peak-signal/RMS-noise) per-unit-time of proton MR spectroscopy (1H-MRS) is linearly proportional to the voxel volume. Consequently, with a headcoil and 30-min acquisition, 1 cm3 is considered the SNR-limited spatial resolution barrier in the human brain. However, since local linewidths, Delta(upsilon*) = (piT2*)(-1), at high magnetic fields (B0), are dominated by regional inhomogeneities (DeltaB0), i.e., T2* << T2, reducing the voxel dimensions may increase T2*. This could compensate, in part, for signal loss with volume decrease. It is shown that for two cubic voxels of sides l1 and l2, l1 > l2, as the volume decreases by (l1/l2)3, their SNR ratio is reduced by only (l1/l2)2 due to a commensurate T2* increase of l1/l2. This is demonstrated in a phantom and the brains of volunteers, with 3D 1H-MRS in a headcoil at 4 T. It is shown that while the cubic voxels' dimensions were all halved, reducing their volume eightfold, their metabolites' SNR decreased only fourfold, due to their Delta(upsilon*s') twofold decrease. In other words, both spatial and spectral resolutions were doubled at a significantly, x2, smaller-than-expected SNR loss. This advantage was exploited to produce quality high spatial resolution, 0.75 x 0.75 x 0.75 cm3, metabolic maps in a 27-min acquisition.
人们普遍认为，质子磁共振波谱（1H-MRS）每单位时间的信噪比（SNR = 峰值信号/均方根噪声）与体素体积呈线性比例关系。因此，使用头部线圈并采集30分钟时，1 cm3被认为是人类大脑中受信噪比限制的空间分辨率屏障。然而，由于在高磁场（B0）下局部线宽Delta(υ*) = (πT2*)-1主要由区域不均匀性（DeltaB0）决定，即T2* << T2，减小体素尺寸可能会增加T2*。这可以部分补偿因体积减小而导致的信号损失。结果表明，对于边长分别为l1和l2（l1 > l2）的两个立方体素，当体积减小(l1/l2)3时，由于T2相应增加l1/l2，它们的信噪比仅降低(l1/l2)2。这在体模和志愿者大脑中得到了验证，使用4 T头部线圈进行3D 1H-MRS。结果表明，当立方体素尺寸均减半，体积减小八倍时，其代谢物的信噪比仅降低四倍，这是由于它们的Delta(υs')降低了两倍。换句话说，在信噪比损失显著小于预期两倍（x2）的情况下，空间分辨率和光谱分辨率都提高了一倍。利用这一优势，在27分钟的采集中生成了高质量的高空间分辨率（0.75 x 0.75 x 0.75 cm3）代谢图谱。
