Brain Perfusion Essay

Brain perfusion SPECT is a non-invasive nuclear medicine imaging technique that allows regional CBF evaluation. The technique includes four main components: the CBF radiotracers (radiopharmaceuticals), instrumentation, image reconstruction, and CBF quantification.
1.1.1 CBF radiotracers
The inert gas xenon 133 is one of the most important and classical radiotracers for CBF measurement in absolute units using SPECT []. Nevertheless, the poor spatial resolution and the requirement for dedicated instrumentation have limited its use. Thus, brain perfusion retention radiotracers using 99m technetium (99mTc) and iodine 123 (123I) labeled compounds are most common in clinical settings. Given the relatively high cost of 123I as a cyclotron-produced radioisotope, the use of radiotracers using 123I is limited as compared to 99mTc labeled compounds.
99mTc-hexamethyl propylene amine oxime (99mTc-HMPAO) and 99mTc-ethyl cysteine dimer (99mTc-ECD) are the 99mTc labeled compounds used as perfusion retention radiotracers. The commercialization of kits of HMPAO and ECD facilitates the labeling with 99mTc, which is a low-cost radioisotope available daily.
The regional distribution of both agents is nearly linear proportionally to regional CBF. To have a linear relationship, it is necessary to apply to the images the correction proposed by Lassen et al. [].The doses used in adults are between 555 – 1110 MBq for both 99mTc-ECD and 99mTc-HMPAO. These activities represent effective doses of 0.011 and 0.0093 mSv, respectively []. Although both agents performed equally well in the classification of patients with AD and healthy control subjects, 99mTc-ECD shows higher image contrast compared to 99mTc-HMPAO