NORMAL REGIONAL DISTRIBUTION OF CEREBRAL BLOOD FLOW IN DOGS: COMPARISON BETWEEN 99M TC-ETHYLCYSTEINATE DIMER AND 99M TC- HEXAMETHYLPROPYLENE AMINE OXIME SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY ANTITA ADRIAENS,INGEBORGH POLIS,TIM WAELBERS,EVA VANDERMEULEN,ANDR ´ E DOBBELEIR, BART DE SPIEGELEER,KATHELIJNE PEREMANS Functional imaging provides important insights into canine brain pathologies such as behavioral problems. Two 99m Tc-labeled single photon emission computed tomography (SPECT) cerebral blood flow tracers— ethylcysteinate dimer (ECD) and hexamethylpropylene amine oxime (HMPAO)—are commonly used in human medicine and have been used previously in dogs but intrasubject comparison of both tracers in dogs is lacking. Therefore, this study investigated whether regional distribution differences between both tracers occur in dogs as is reported in humans. Eight beagles underwent two SPECT examinations first with 99m Tc-ECD and followed by 99m Tc-HMPAO. SPECT scanning was performed with a triple head gamma camera equipped with ultrahigh resolution parallel hole collimators. Images were reconstructed using filtered backprojection with a Butterworth filter. Emission data were fitted to a template permitting semiquantification using predefined regions or volumes of interest (VOIs). For each VOI, perfusion indices were calculated by normalizing the regional counts per voxel to total brain counts per voxel. The obtained perfusion indices for each region for both tracers were compared with a paired Student’s T-test. Significant (P < 0.05) regional differences were seen in the subcortical region and the cerebellum. Both tracers can be used to visualize regional cerebral blood flow in dogs, however, due to the observed regional differences, they are not entirely interchangeable. C  2013 Veterinary Radiology & Ultrasound. Key words: cerebral blood flow, dogs, SPECT, 99m Tc-ECD, 99m Tc-HMPAO. Introduction P ROGRESS IN FUNCTIONAL brain imaging in dogs is of great importance since these types of brain imaging contribute to detecting brain dysfunctions, rather than anatomical abnormalities. This is valuable in the case of canine behavioral problems like anxiety and aggression, and some cases of epilepsy. 1–3 Using diffusible radiolabeled tracers, regional cerebral blood flow can be measured with single photon emission computed tomography (SPECT). Frequently used radioligands include 99m Tc-ECD (ethyl- From the Faculty of Veterinary Medicine, Department of Small Animal Medicine (Adriaens, Polis, Waelbers) Faculty of Veterinary Medicine, Department of Medical Imaging and Small Animal Orthopedics (Vandermeulen, Dobbeleir, Peremans), Ghent University, Merelbeke Belgium, Department of Nuclear Medicine, Ghent University Hospi- tal, Ghent Belgium, (Dobbeleir), and Drug Quality & Registration (DruQuaR) group, Faculty of Pharmaceutical Sciences, Ghent Univer- sity, Ghent Belgium (De Spiegeleer) Funding: Special Research Fund of the Ghent University (BOF n ◦ 01J06109). Address correspondence and reprint requests to Antita Adriaens, Faculty of Veterinary Medicine, Department of Small Animal Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium. E-mail: Antita.adriaens@UGent.be Received October 9, 2012; accepted for publication February 8, 2013. doi: 10.1111/vru.12028 cysteinate dimer) and 99m Tc-HMPAO (hexamethylpropy- lene amine oxime), both commonly used in investigations of neuropsychiatric disorders in human medicine. 4 SPECT is, next to positron emission tomography (PET), a nuclear functional imaging modality that permits the visualization of different neurotransmitter systems in vivo in a noninva- sive manner. The major advantages of SPECT over PET are that it is less expensive, more accessible, and better suited for the use in animals due to the longer half lives of the isotopes, which permits a longer injection–acquisition in- terval. In human medicine, the use of SPECT as a diagnos- tic tool and research modality is widespread. 4 In veterinary medicine, its use is mostly applied in research studies focus- ing either on animal disease or on the animal as a model for human disease, including research on behavioral problems, epilepsy, and the working mechanisms of several drugs. 1–3, 5 Even though the first-pass extraction in brain tissue of both radioligands is high, each radioligand has a differ- ent retention mechanism. Both tracers undergo intracel- lular metabolization from a lipophilic form that readily passes the blood–brain barrier, to a hydrophilic form that is trapped within the cell. However, this trapping mechanism Vet Radiol Ultrasound, Vol. 54, No. 4, 2013, pp 403–407. 403