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Single-photon emission computed tomography (SPECT scintigraphy) utilizes nuclear medicine along with gamma rays to get three-dimensional images (NLM, n.d.) of an internal part of the body such as heart with an objective of diagnosing a problem.
SPECT
In SPECT, a gamma-emitting radioisotope is administered into the bloodstream of a patient with the help of injection (Boyd, 2014; Hademenos, 1998). The radioisotope is, usually, a simple soluble ion as, for example, isotope of gallium(III) (Sharma, Prior, Belinsky, Kruh, & Piwnica-Worms, 2005; Wadas, Wong, Weisman, & Anderson, 2010). Most often, a marker radioisotope is also bound to a particular ligand to develop a radioligand that helps in binding to specific tissues in the body (Brahme, 2014; Innis et al., 2007). This binding of radioligand to the body part also helps in its appearance in gamma camera as SPECT images are taken with the help of a gamma camera (Mignon & Colombel, 1999).
Myocardial perfusion SPECT
Myocardial perfusion imaging (MPI) uses SPECT technique (Katlic, 2011) utilizing cardiac stress test for functional cardiac imaging (Bourque et al., 2011). MPI can help patients of chest discomfort in knowing the cause of discomfort, i.e. the discomfort may develop due to absence of blood flow to the heart muscle and the cause of this absence is blocked or narrowed heart arteries. Although MPI does not show the actual heart arteries, it can show the place of blockage of heart arteries and the extent of blockage. It can also give information about the previous heart attack (American Heart Association, 2015).
Cardiac stress test with MPI
Cardiac stress is commonly used to diagnose ischemic heart disease as it shows the flow of blood through the heart muscles (Vesely & Dilsizian, 2008). It is based on the principle of stress, i.e. under stressful conditions, pathologic myocardium gets reduced amount of blood flow as compared to normal myocardium. In MPI, a radiopharmaceutical is administered that specifically goes to heart. Radioactive thallium-201 ( 201 Tl) (Bailey, Griffith, Rouleau, Strauss, & Pitt, 1977) or various technetium-99m ( 99mTc) markers can be used in MPI (Baggish & Boucher, 2008). Among other radionuclides is iodine-123 (123 I)–labeled fatty acids (Knapp Jr & Kropp, 1995) that can produce cold spots at the place of ischemia. Gallium citrate-67 (67 Ga) can also be used as radionuclide (Bengalorkar, Bhuvana, Sarala, & Kumar, 2012) that can accumulate at different places of active inflammation such as in acute inflammatory cardiomyopathy. 123 I metaiodobenzylguanidine is a neurotransmitter analog accumulated in neurons of the sympathetic nervous system and can be utilized in the study of heart failure and some arrhythmias (Clements et al., 2015; Marshall, Cheetham, George, Mason, & Kelion, 2012).
After the administration, heart rate is increased resulting in myocardial stress. This increase in heart rate can be done with the help of drugs such as dobutamine or adenosine (Pharmacists, 1994), or with the help of exercise. After achieving specified stress levels, SPECT imaging shows the distribution of radiopharmaceutical, thereby showing the relative blood flow (Stolerman, 2010) to various areas of the myocardium. Heart disease is diagnosed by comparing the stress images to different sets of images obtained while the heart is at rest. In the diagnosis of heart disease with the help of SPECT technique, radionuclide redistributes slowly (John, 1993), so different sets of images are obtained on different days. In order to confirm the presence of a problem, stress imaging is performed initially and then the rest imaging is performed. The working of the myocardium can also be determined by computing the left ventricular ejection fraction (LVEF) of the heart, and this process is performed along with cardiac stress test (Foley et al., 2012; Munjal, Sharm, & India, 2012).
MPI was found to have an overall accuracy of nearly 83% with sensitivity of 85% and specificity of about 72% (Elhendy, Bax, & Poldermans, 2002). However, it is important to consider careful evaluation (Aziz, Javed, Alviar, & Herzog, 2011).
Indications of Myocardial Perfusion SPECT
MPI can be used to evaluate different problematic conditions of heart including hypertrophic cardiomyopathy (Dilsizian, Panza, & Bonow, 2010), coronary artery disease (CAD) (Lee, West, & Khafagi, 2013), and abnormalities of heart wall (Fathala, 2011) such as wall motion abnormalities. In case of CAD, MPI can also determine the location as well as degree of CAD in patients having a history of the problem. MPI can be used in the prognosis of patients having high risk of getting coronary or myocardial incident such as myocardial ischemia, myocardial infarction, and coronary aneurysm. It can evaluate the success of reperfusion interventions as, for example, percutaneous intervention, coronary artery bypass grafting (CABG), and thrombolysis. With the help of SPECT, interior and posterior abnormalities as well as small regions of infarction can be recognized. It can also identify occluded blood vessels and the mass of infarcted and viable myocardium.
Risks of MPI
MPI techniques are safe for many people as in these techniques, they are exposed to small dose of X-rays. Although those small doses are considered safe, still the possibilities are there that X-rays may result in cancer or other such bodily problems in small doses. It is important to consider that if the risks for a heart attack are high or there are other serious problems related to heart, people have to avoid MPI tests. Moreover, MPI tests can also harm pregnant women and nursing mothers, and their babies.
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