The contraindications to the use of exercise and pharmacological (adenosine, dipyridamole, regadenoson, dobutamine) stress testing are presented in previous chapters. The contraindications specifically for nuclear cardiology include patients who received iodine I-131 therapy within 12 h or technetium-99 studies within 48 h. In addition, due to the radiation exposure, SPECT is contraindicated in pregnancy and breast-feeding. Finally, uncooperative patients who are unable to lie supine for at least 30 min are not encouraged to undergo the procedure. In rare instances, it is contraindicated if there is an allergic potential to the radiopharmaceutics.

The essential tool for performing a SPECT study is the gamma or scintillation camera that detects the gamma rays (photons) produced from the injected radiopharmaceutical agents [2].

A gamma camera consists of a single crystal, usually sodium iodide crystal (NaI), a collimator, and photomultiplier tubes (PMT). The NaI crystal is able to scintillate when subjected to ionizing radiation in form of photons. The collimator is designed to limit by attenuation, the detection of gamma rays to those traveling in certain directions. The photomultiplier tubes convert photon energy and scintillation within the crystal into electrical energy that is processed by the pulse height analyzer. The energy spectrum obtained consists of a photopeak and multiple Compton scatters. Only signals in the specific energy range of the radiotracer (the photopeak) are processed and the counts analyzed.

The basic components of the gamma camera have not changed much over the years except for upgrading from scintillation detectors to semi-conductor detectors such as Cadmium Zinc Telluride (CZT) detectors with improved count detection and thus images.

The radiopharmaceutical tracers [3] used are thallium-201 and technetium-99m based agents, Tc-99m sestamibi and Tc-99m tetrofosmin, Due to the energy spectrum, effective half life and more favorable dosimetry, the technetium agents are preferred for gated SPECT imaging. These agents emit photons with a single photopeak of 140 keV and have a half-life of 6 h. After injection, 40–60 % is extracted by the myocardium and myocardial washout/redistribution is minimal requiring two injections at rest and peak exercise.

After performing an exercise or a pharmacological stress test and the injection of the pharmaceutical radiotracer, images are obtained. The images obtained by gamma cameras are 2 dimensional (2D) images of a 3 dimensional (3D) object. Multiple 2D planar projections are acquired from multiple angles to reconstruct a 3D image using a reconstruction algorithm.

Imaging times post injection for technetium-based agents, scanning can start 15–20 min after exercise and 45–60 min after rest or pharmacological stress testing. The routine position to acquire images is supine with arms above the head. In cases of significant inferior wall attenuation, the addition of prone images helps eliminate the inferior wall attenuation, creates more uniform breast attenuation, less motion artifact, but may cause an artifactual anteroseptal defect. The gamma camera is placed as close to the patient as possible for improved resolution with either one of the orbital types: circular or non circular (elliptical or body contour). In a double headed camera, planar images are obtained in a 180° arc that extends from 45° right anterior oblique (RAO) to 45° left posterior oblique (LPO). The acquisition type could be a “step and shoot”, continuous, or continuous “step and shoot”. For a dual head camera, low dose studies usually takes 25–30 s/step and for high dose studies 20–25 s per step which translates to approximately 12–16 min total time for the study. Using a single head camera will yield double the time to acquire the images.

Hybrid SPECT-CT systems are in use and play an essential role in attenuation correction.

Radiopharmaceutical protocols [3]:

Single isotope usually technetium based one day protocol is the most widely used. A rest injection is performed with low dose 8–9 mCi followed by SPECT imaging 45–60 min afterwards. After a 3-h delay, stress testing is performed and high dose 25–30 mCi is injected followed by SPECT imaging 15–30 min for exercise and 45–60 min for pharmacological stress testing (Fig. 8.1). The advantages of a single day protocol is good validation, fast tract protocol, easier image interpretation, able to determine transient ischemic dilation (TID), and validated attenuation correction. The 2-day protocols advantage is high quality rest and stress images as a result of using high dose injections with rest and stress. This is beneficial in an obese patient but requires 2 days to complete the study. More recently, stress only images are being performed; if SPECT images are normal, the rest images are not required.