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

✍ Scribed by Belinda S.Y. Li; Juleiga Regal; Oded Gonen

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 209 KB
• Volume: 46
• Category: Article
• ISSN: 0740-3194
• DOI: 10.1002/mrm.1297

✦ Synopsis

Abstract

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)^−1^, at high magnetic fields (B~0~), are dominated by regional inhomogeneities (Δ__B__~0~), i.e., T ≪ T~2~, reducing the voxel dimensions may increase T. 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 increase 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.