Application of PET-MRI registration techniques to cat brain imaging

In positron emission tomography (PET) studies of diseased animals, it is ve ry useful to have accurate anatomical information as a reference. In human studies, anatomical information is usually obtained from magnetic resonance imaging (MRI) of the subject with retrospective registration of the subjec t's PET image to the MRI. A number of PET-MRI registration techniques are u sed for this purpose. However, the utility of these methods has not been te sted for animals image registration. This paper studies the feasibility of applying two currently used human brain PET-MRI registration techniques to cat brain images. Methods: Three cats were anesthetized with isoflurane gas , and PET images were acquired with (H2O)-O-15, benzodiazepine receptor lig and C-11-flumazemil (FMZ), dopamine receptor ligand C-11-nemonapride (NEM) and fluorodeoxy glucose (F-18-FDG). The four PET scans were acquired consec utively within the same day while the cat remained fixed in the scanner. We also obtained T1-weighted and T2-weighted MRI of the cats in a 4.7 T unit. The PET images were registered to MRI using two human brain registration t echniques: a semi-automatic method (SAM), which is a two-step method based on the extraction of the midsagittal plane, and an automatic method (AMIR) method that minimizes PET pixel variance within spatially connected segment s determined by MRI. Results: T2-weighted MRI provided better structural in formation than T1, MRI. FMZ did, while FDG or H2O PET images did not, provi de a structural outline of the brain. The FMZ PET image was registered to M RI satisfactorily using SAM. The striatum visualized in nemonapride PET ima ge re-sliced with the same parameters matched the striatum identified in T2 -weighted MRI. Registration by AMIR was successful by inspection for FMZ, F DG or H2O PET images in only one of the three cats. The registration error of SAM was estimated to be less than 2 mm or 2 degrees. Conclusion: A satis factory registration of FMZ-PET to T2-weighted MRI of the cat brain was obt ained by a two-step manual registration technique. This will enhance the us efulness of PET in the field of cerebral pathophysiology. (C) 2000 Publishe d by Elsevier Science B.V. All rights reserved.