SPECT- Myocardial Perfusion Scintigrapy

uclear imaging of the heart is fast emerging as a valuable and widely used non-invasive procedure, that reveals vital information about cardiac structure, physiology, perfusion and metabolism. Commonly perfonned nuclear imaging techniques in cardiology are radionuclide ventriculography (RNV) done by labelling autologous red blood cells with Technetium-99m (Tc-. 99m) and myocardial perfusion imaging with isotopes like Thalliwn-20 I (TI-20 I) and Technetium-99m labelled isonitriles or diphosphine compounds (1). Lately studies with positron emitting radio-isotopes like Nitrogen13 labelled Ammonia CNH3 ) and Fluorine-18labelled lluorodeoxyglucose (8FDG) are fast emerging as the gold standard for assessment of myocardial viability and for other metabolic studies. The images are acquired on a computerized galIDna camera or more commonly on a single photon emission computerized tomography (SPECT) system. Positron emission tomography (PET) has a greater sensitivity and specificity and is perfomled for perfusion, metabolic and viability imaging (2). The basic procedure is similar in all the studies. A radiopharmaceutical agent is created by chelation of a radioactive isotope to a pharmaceutical agent. This radiopharmaceutical agent or radio-isotope in the case ofThallium-20 I is injected intravenously, and the emitted gamma rays are detected by a gamma camera, SPECT or PET imaging device. Images are acquired as per a pre-defined software progralIDne protocol. The signals are than digitized and stored in a computer and reproduced as an image or functional data. The principal advantages of nuclear imaging of the heart are its noninvasiveness and its ability to assess quantitativcly or semi-quantitatively myocardial flow and ventricular function. Its main disadvantages are its relative cost, thc special equipment and the expertise required for accurate interpretation of the study. In 1995, 3.3 million cardiac perfusion studies \Ycn: performed in the United States of America. Sixty percent studies were performed with Tc-99m labelled agents and forty percent with Thallium-20 1. Most of the major medical centres in India are currently performi ng Thallium-20 I or Tc-99m myocardial perfusion scintigraphy, using the state of the art imaging equipment, and computerized analysis (3). A dedicated PET system will soon be made operational at Radiation Mcdicine Centre, Mumbai. Radiopharmaceuticals and radio-isotopes commonly used in myocardial perfusion imaging A. Thallium-20t Thallium is a group-IlIA, cyclotron produced mctallic element with biokinetic properties similar to potassium. The initial distribution ofthalliwll is based on blood flow and cellular extraction by viable myocardium. The delayed phase of localization is based on redistribution or equilibriwn in the myocardium based on continued extraction of thallium from blood and ongoing washout from previously localized thalliUl11 from myocardial cells. B. Technetium-99m labelled compounds 1. Isonitriles (Tc-99m Sestamibi) 11. Diphosphines (Tc-99m Tetrofosmin)· 111. Q-class agent (Tc-99m Furifosmin)