Editing Single-photon emission computed tomography (section)

===Functional brain imaging===
{{More citations needed section|date=January 2014}}
{{main|Neuroimaging}}
Usually, the gamma-emitting tracer used in functional brain imaging is [[Technetium (99mTc) exametazime]]. [[Technetium-99m|<sup>99m</sup>Tc]] is a metastable [[nuclear isomer]] that emits gamma rays detectable by a gamma camera. Attaching it to exametazime allows it to be taken up by brain tissue in a manner proportional to brain blood flow, in turn allowing [[cerebral blood flow]] to be assessed with the nuclear gamma camera.

Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the <sup>99m</sup>Tc-exametazime tracer (as well as the similar <sup>99m</sup>Tc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of [[dementia]]. Meta-analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease vs. 81% sensitivity for clinical exam ([[cognitive test]]ing, etc.). More recent studies have shown the accuracy of SPECT in Alzheimer's diagnosis may be as high as 88%.<ref name="pmid16785801">{{cite journal |vauthors=Bonte FJ, Harris TS, Hynan LS, Bigio EH, White CL |s2cid=39518497 |title=Tc-99m exametazime SPECT in the differential diagnosis of the dementias with histopathologic confirmation |journal=Clin Nucl Med |volume=31 |issue=7 |pages=376–8 |year=2006|pmid=16785801 |doi=10.1097/01.rlu.0000222736.81365.63}}</ref> In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias.<ref name="pmid15545324">{{cite journal |vauthors=Dougall NJ, Bruggink S, Ebmeier KP |title=Systematic review of the diagnostic accuracy of 99mTc-HMPAO-SPECT in dementia |journal=Am J Geriatr Psychiatry |volume=12 |issue=6 |pages=554–70 |year=2004 |pmid=15545324 |doi=10.1176/appi.ajgp.12.6.554}}</ref> This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease. Another recent review article showed that multi-headed SPECT cameras with quantitative analysis result in an overall sensitivity of 84-89% and an overall specificity of 83-89% in cross sectional studies and sensitivity of 82-96% and specificity of 83-89% for longitudinal studies of dementia.<ref>{{cite journal|last=Henderson|first=Theodore|title=The diagnosis and evaluation of dementia and mild cognitive impairment with emphasis on SPECT perfusion neuroimaging|journal=CNS Spectrums|date=December 2012|volume=17|issue=4|pages=188–89|doi=10.1017/S1092852912000636|pmid=22929226|s2cid=36441907}}</ref>

<sup>99m</sup>Tc-exametazime SPECT scanning competes with [[fludeoxyglucose]] (FDG) [[Positron emission tomography|PET]] scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes. SPECT is more widely available, because the radioisotope used  is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. While <sup>99m</sup>Tc is extracted from relatively simple [[technetium-99m generator]]s, which are delivered to hospitals and scanning centers weekly to supply fresh radioisotope, FDG PET relies on FDG, which is made in an expensive medical [[cyclotron]] and "hot-lab" (automated chemistry lab for radiopharmaceutical manufacture), and then   delivered immediately to scanning sites because of the natural short 110-minute half-life of [[Fluorine-18]].