Diagnostic Testing

Why Order an Exam?

Diagnostic exams are intended to provide information beyond what can be gleaned from the patient’s history, symptoms, and physical examination.

An appropriate diagnostic test will increase the certainty of a positive or negative diagnosis, so that treatment can be planned, or an alternate diagnosis considered. The more dangerous the disease, the more important diagnostic accuracy becomes.

Diagnostic tests may provide prognostic information valuable for patient management. Depending on the modality chosen, it is possible to spare the patient pain, anxiety, and the expense of invasive tests such as coronary angiography.

The Exam - SPECT Imaging

The most common procedures in nuclear cardiology are those that produce single photon emission computed tomography (SPECT) images. These images are captured from the uniform emission of single photons—bursts of energy—emitted by the decay of radioisotopes. The SPECT imaging procedure includes perfusion imaging and assessment of ventricular function in one exam.1,2 (Kumita abstract) (Ban abstract)

Radiopharmaceuticals

T1-201 was one of the early radiopharmaceuticals used in cardiology. Because it is an analogue of potassium, it mimics the uptake of potassium in the heart. Its myocardial uptake is dependent on perfusion and active transport across the membrane of a living muscle cell.

Tc99m agents, such as MYOVIEW, are widely used for a number of reasons. The photons emitted by Tc99m agents have the optimal energy for detection by SPECT cameras. Compared with Tl-201, Tc99m agents have less scatter and attenuation, superior images, and a shorter physical half-life.3-5 (Jain/p221-423, 228-230,232) (Higley/p30) (Takahashi/p29,36)

Rest and Stress Imaging

Images taken when the patient is at rest show the baseline state of myocardial perfusion. However, rest images may appear within normal limits even when perfusion defects are present.5,6 (Braunwald/p 1313) (Takahashi/p 37)

On the other hand, stress images, derived from either exercise or a pharmacologic stress agent, increase myocardial blood flow requirements. Under these conditions, blockages greater than 50% will significantly reduce blood flow relative to the blood flow to normal regions. Stress images thus reveal the maximal potential difference in perfusion between normal and abnormal regions of the myocardium. Patients who cannot perform an adequate amount of exercise, can instead be stressed by a pharmacologic agent such as dipyridamole or adenosine. All of these agents provide diagnostic information comparable with that of exercise testing.5 (Takahashi/p29,36)

References: 1. Kumita S, Cho K, Mizumura S, et al. Assessment of left ventricular systolic function derived from ECG-gated myocardial SPECT with 99mTc-tetrofosmin: automatic determination of LV epi- and endocardial surface. Kaku Igaku. 1997;34:237-242. 2. Ban K, Nakajima T, Iseki H, Abe S, Handa S, Suzuki Y. Evaluation of global and regional left ventricular function obtained by quantitative gated SPECT using 99mTc-tetrofosmin for left ventricular dysfunction. Intern Med. 2000; 39:612-617. 3. Jain D. Technetium-99m labeled myocardial perfusion imaging agents. Semin Nucl Med. 1999;29:221-236. 4. Higley B, Smith FW, Smith T, et al. Technetium-99m-1,2-bis[bis(2-ethoxyethyl) phosphino] ethane: human biodistribution, dosimetry and safety of a new myocardial perfusion imaging agent. J Nucl Med. 1993;34:30-38. 5. Takahashi N, Tamaki N, Tadamura E, et al. Combined assessment of regional perfusion and wall motion in patients with coronary artery disease with technetium 99m tetrofosmin. J Nucl Cardiol. 1994;1:29-38. 6. Braunwald E. Disorders of the heart. In: Braunwald E, Fauci AS, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 15th ed. New York, NY: McGraw-Hill Companies, Inc; 2001;231:1309-1318.