I was a little sloppy when I said that the LA pain doc. requires a brain CT scan to position the TMS. It's actually more of a functional CT scan, specifically a hybrid imaging of positron emission tomography (PET) together with a CT scan.

That said, not only is a combined PET/CT apparently less effective on the whole in studying brain tissue than would be an fMRI,** but the radiation output of a PET/CT scan appears to be somewhat higher that a stand alone CT. See, e.g., Estimated cumulative radiation dose from PET/CT in children with malignancies: a 5-year retrospective review, Chawla SC, Federman N, Zhang D, Nagata K, Nuthakki S, McNitt-Gray M, Boechat MI, Pediatr Radiol. 2010 May;40(5):681-6. Epub 2009 Dec 5, FULL TEXT @ http://www.ncbi.nlm.nih.gov/pmc/arti...ticle_1434.pdf

Department of Radiology, Olive View-UCLA Medical Center, 14445 Olive View Drive, 2 D115, Sylmar, CA, USA. chawlasoni@gmail.com

Abstract
BACKGROUND: The increasing use of serial PET/CT scans in the management of pediatric malignancies raises the important consideration of radiation exposure in children.

OBJECTIVE: To estimate the cumulative radiation dose from PET/CT studies to children with malignancy and to compare with the data in literature.

MATERIALS AND METHODS: Two hundred forty-eight clinical PET/CT studies performed on 78 patients (50 boys/28 girls, 1.3 to 18 years old from December 2002 to October 2007) were retrospectively reviewed under IRB approval. The whole-body effective dose (ED) estimates for each child were obtained by estimating the effective dose from each PET/CT exam performed using the ImPACT Patient Dosimetry Calculator for CT and OLINDA for PET.

RESULTS: The average number of PET/CT studies was 3.2 per child (range: 1 to 14 studies). The average ED of an individual CT study was 20.3 mSv (range: 2.7 to 54.2), of PET study was 4.6 mSv (range: 0.4 to 7.7) and of PET/CT study was 24.8 mSv (range: 6.2 to 60.7). The average cumulative radiation dose per patient from CT studies was 64.4 mSv (range: 2.7 to 326), from PET studies was 14.5 mSv (range: 2.8 to 73) and from PET/CT studies was 78.9 mSv (range: 6.2 to 399). [Emphasis added.]

CONCLUSION: The radiation exposure from serial PET/CT studies performed in pediatric malignancies was considerable; however, lower doses can be used for both PET and CT studies. The ALARA principle must be applied without sacrificing diagnostic information.

PMID: 19967534 [PubMed - indexed for MEDLINE] PMCID: PMC2847163

http://www.ncbi.nlm.nih.gov/pubmed/19967534



** See, PET/CT: form and function, Blodgett TM, Meltzer CC, Townsend DW, Radiology 2007 Feb;242(2):360-85 at 365, FULL TEXT @ http://radiology.rsna.org/content/242/2/360.full.pdf:

It is unclear what the role of hardware-based PET/CT is in the brain. CT has a limited role in the evaluation of this organ; MR imaging often provides more detail, as well as more useful information. As tracers become more specific and result in less background uptake, it will be more important to have accurately coregistered PET and CT images. At the University of Pittsburgh, all patients referred because of neurologic indications are still examined with a dedicated PET scanner rather than with a PET/CT scanner.