Single Photon Computerized Tomography SPECT neuroimaging Seyed Kazem Malakouti, MD Faculty of Iran University of Medical Sciences Seyed Kazem Malakouti, MD

Radio pharmaceuticals (radiolegands, radiotracer) used to visualize and quantify the blood flow Emit gamma (photon) detected by camera TC 98, I 123 for high energy ( KeV) detectable by camera and long half live of hours Gamma photons converted to light photon, scintigraphy phenomenon Seyed Kazem Malakouti, MD

For PET imaging others radionuclides are used than for SPECT imaging. The PET tracers emit (when they decay) positrons (positively charged analogs of electrons). After a positron has been emitted it will meet soon an electron in the surrouding tissue, and annihilate As a consequence, 2 gamma-emitting photons will be emitted in opposite direction (180 degrees), both having an energy of 511 keV. The ring of crystals of a PET camera is able to detect these 2 photons. Because they are detected almost simultaneously, its origin can be calculated (line of response; line between the 2 detectors that detected the 2 photons simultaneously). Seyed Kazem Malakouti, MD

Therefore, as compared to the SPECT technique, a collimator is not needed consequently PET is a more sensitive technique to detect gamma photons spatial resolution of clinical PET systems is better than for SPECT systems. Seyed Kazem Malakouti, MD

PET brain imaging frequently tracers are used with short half-lives (2 to 20 min). both PET and SPECT are molecular imaging techniques which can be used to visualize and quantify dopamine transporters Seyed Kazem Malakouti, MD

PET is a more sensitive technique than SPECT clinical PET systems have a better resolution than SPECT systems. brain SPECT imaging studies, commonly 123I-labelled tracers are used. Due to the relatively long half-live of 123 I, it is not necessary to have a cyclotron available in the near environment. PET brain imaging frequently tracers are used with short half-lives, and such tracers are only available for centers with an in-house cyclotron. Seyed Kazem Malakouti, MD

The most commonly employed SPECT tracer is Exametazime, also called Hexamethyl- propylenamine-oxim (HMPAO) or CeretecTM, which is highly lipophilic, passes the blood brain barrier and is taken up into brain cells on first pass Seyed Kazem Malakouti, MD

After being taken up into brain cells it is transformed, using glutathione, into a hydrophilic compound, trapped and does not move any further. Brain activity thus represents the distribution of blood flow patterns within a minute or two after injection. Up to 15% of the activity is washed out from the brain by two minutes, after this the distribution pattern remains more or less constant for the following 24 hours. Fifty percent of the tracer is excreted through the intestinal tract over 48 hours and 40% via the kidney and urine. Seyed Kazem Malakouti, MD

Diagnostic Use of SPECT: In a systematic review of 48 SPECT studies Dougall et al (2004)1 determined the pooled sensitivity for Exametazime SPECT to detect patients with Alzheimer ’s disease as 71%, while its specificity against other dementias ranged from 76 to 78%, and was 90% for controls. Seyed Kazem Malakouti, MD

[usefulness in] discriminating Alzheimer’s disease from vascular dementia, dementia with Lewy bodies and frontotemporal dementia has been demonstrated.” ( index.html) Seyed Kazem Malakouti, MD