Rats are commonly used for their size (creating the animal-sized scanners is so expensive they are commonly not used in veterinary medicine even for dogs and cats) and the fact that rats breed quickly (PET, 2011, New World Encyclopedia). Lab rats have also been bred to ensure that they have similar enough genetic profiles to the humans the drugs will eventually be used upon, and even more specific populations have been bred to manifest the types of cancers detected by PET scans. Acute toxicity studies are ideally conducted using the means of transmission deployed with the eventual human subjects, which this new technology permits. According to FDA guidelines, the minimum amount of animals should be used to determine toxicity, contrary to previous ways of determining potential lethality. "Animals should be observed for 14 days after pharmaceutical administration. All mortalities, clinical signs, time of onset, duration, and reversibility of toxicity should be recorded. Gross necropsies should be performed on all animals, including those sacrificed moribund, found dead, or terminated at 14 days" (Guidance, 2006, CDER).

Prostate cancer is one of the drugs currently tested in imaging. "The androgen receptor may therefore play a key role in the biologic behavior of prostate cancer. For this reason, PET of androgen receptors, especially in the patient whose disease is progressing despite low androgen levels, may be highly revealing" (Zanzonico 2004). Given the high mortality rate and the need for additional testing and experimentation using PET scans upon this variety of cancer, using rats in drug trials for PET...

Not only is there suitable technology to replicate the procedure on humans -- genetically similar populations of rats designed to have a predisposition to develop prostate cancer can also be used.
References

CFR - Code of Federal Regulations Title 21. (2011). FDA. Retrieved August 19, 2011 at

21 http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?CFRPart=315&showFR=1

Guidance. (2009). FDA. Retrieved August 19, 2011 at http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM070306.pdf

Guidance for industry. (2006). Center for Drug Evaluation and Research (CDER).

Retrieved August 19, 2011 at http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/ucm079270.pdf

Kimball, J. (2011). Testing new drugs. Biology Pages. Retrieved August 19, 2011 at http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/DrugTrials.html

PET: Q&A. (2011). Retrieved August 19, 2011 at http://www.fda.gov/Drugs/DevelopmentApprovalProcess/Manufacturing/ucm193476.htm

Positron emission tomography (PET). (2011). New World Encyclopedia.

http://www.newworldencyclopedia.org/entry/Positron_emission_tomography

Zanzonico, Pat. (2004 et al.). PET-Based Radiation Dosimetry in Man of 18F

Fluorodihydrotestosterone, a New Radiotracer for Imaging Prostate Cancer. Journal of Nuclear Medicine, 45(11) 1966-1971

http://jnm.snmjournals.org/content/45/11/1966.full