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Abstract:
Objective: To optimize the post-processing pipeline of 7 T chemical exchange saturation transfer imaging for reproducibility. To prove this optimization for the detection of age differences and differences between Parkinson patients versus normal subjects.
Methods: The following 7 T CEST MRI experiments were analyzed: Repeated measurements of a healthy subject, subjects of two age cohorts (14 older, 7 younger subjects), and measurements of 12 Parkinson patients. A slab-selective, B+1
-homogeneous parallel transmit protocol was used. The post-processing consisting of motion correction, smoothing, B0 -correction, normalization, denoising, B+1
-correction and Lorentzian fitting was optimized regarding the intra- and inter-subject coefficient of variation (CoV) of the amplitudes of the amide pool and the aliphatic rNOE pool within the brain.
Results: Seven 'tricks' for post-processing accomplished an improvement of the mean voxel CoV of the amide pool and the aliphatic rNOE pool amplitudes below 5 % and 3 %, respectively. These post-processing steps are, image-based: 1)motion correction with interpolation of the motion of low-signal-offsets, 2)using the amide pool frequency offset image as reference; Z-spectrum-based, 3)normalization of the Z-spectrum using the outermost saturated measurements, 4) B0
correction of the Z-spectrum with moderate spline smoothing, 5)denoising using principal component analysis preserving the 11 highest intensity-components, 6) B+1 correction using a linear fit, and 7)Lorentzian fitting using the 5-pool fit model. It turns out that that with the optimized post-processing pipeline, a significant age effect in the amide pool can be detected. Additionally, for the first time, an aliphatic rNOE contrast between subjects suffering from Parkinson's disease and age-matched healthy controls in the substantia nigra is detected. Interpretation We propose an optimized post-processing pipeline for CEST multi-pool evaluation. It is shown that by the use of these seven 'tricks', the reproducibility and, thus, the statistical power of a CEST measurement can be greatly improved and subtle changes can be detected.
