SNR versus resolution in 3D 1 H MRS of the human brain at high magnetic fields

It is commonly accepted that the signal‐to‐noise ratio (SNR = peak‐signal/RMS‐noise) per‐unit‐time of proton MR spectroscopy ( 1 H‐MRS) is linearly proportional to the voxel volume. Consequently, with a headcoil and 30‐min acquisition, 1 cm 3 is considered the SNR‐limited spatial resolution barrier in the human brain. However, since local linewidths, Δυ * = ( πT   * 2 ) −1 , at high magnetic fields ( B 0 ), are dominated by regional inhomogeneities (Δ B 0 ), i.e., T   * 2≪ T 2 , reducing the voxel dimensions may increase T   * 2 . This could compensate, in part, for signal loss with volume decrease. It is shown that for two cubic voxels of sides l 1 and l 2 , l 1 > l 2 , as the volume decreases by ( l 1 / l 2 ) 3 , their SNR ratio is reduced by only ( l 1 / l 2 ) 2 due to a commensurate T   * 2increase of l 1 / l 2 . This is demonstrated in a phantom and the brains of volunteers, with 3D 1 H‐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 Δυ*s' twofold decrease. In other words, both spatial and spectral resolutions were doubled at a significantly, ×2, smaller‐than‐expected SNR loss. This advantage was exploited to produce quality high spatial resolution, 0.75 × 0.75 × 0.75 cm 3 , metabolic maps in a 27‐min acquisition. Magn Reson Med 46:1049–1053, 2001. © 2001 Wiley‐Liss, Inc.