Computational protocol: Residual Effects of THC via Novel Measures of Brain Perfusion and Metabolism in a Large Group of Chronic Cannabis Users

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Protocol publication

[…] In terms of regional brain blood flow, we analyzed pCASL MRI data using routine processing (). The pCASL image series were realigned to the first volume for motion correction (SPM5’s realign function, University College London, UK). ASL images with motion of >3 mm and >3° were excluded from further processing. Next, an in-house MATLAB (Mathworks, Natick, MA) program was used to calculate the difference between averaged control and label images. Then, the difference image was corrected for imaging slice delay time to yield CBF-weight image that was normalized to the Montreal Neurological Institute (MNI) brain template. Finally, the absolute CBF (aCBF) was estimated in the units of ml blood/min/100 g of brain tissue. The voxel-wise analyses were performed on the relative CBF (rCBF) maps that included dividing the aCBF spatial maps by the whole-brain absolute CBF value. In a prior publication, we showed that such technique improves the sensitivity of regional differences by reducing physiological variations (). During voxel-based analyses (VBAs), the individual rCBF maps were spatially smoothed (with full-width half-maximum (FWHM) of 4 mm) to account for small differences in sulci/gyri location across subjects. To define a threshold for the VBA results, we used 3dClustsim (with -acf option) in AFNI (NIMH Scientific and Statistical Computing Core, Bethesda, MD) that controls for false positive activation clusters over the set of all activation clusters throughout the whole-brain volume given voxel size, whole-brain volume, and effective smoothness (inherent smoothness plus additional smoothness applied). We refer to this procedure in the Results section as cluster-level family-wise error correction (FWE corrected). The error correction was conditional on three criteria: smoothness of the voxel map, a magnitude threshold for defining clusters of contiguous voxels, and a minimum volume for each cluster. For the CBF VBA, we used p(FWE corrected) value of <0.05 that is based on an effective smoothness of 8.7 mm FWHM, a cluster-defining magnitude corresponding to the 99.5th percentile of t-statistic distribution at p<0.0002, and a minimum volume of 34 voxels (272 mm3). […]

Pipeline specifications

Software tools SPM, AFNI
Applications Magnetic resonance imaging, Functional magnetic resonance imaging
Diseases Marijuana Abuse
Chemicals Creatinine, Oxygen, Dronabinol