JUL 5, 2022: Selected Topics in DIAGNOSTIC IMAGING [4]

CURRENT CONTENTS: (#1 -- July 5, 2022) 

Technetium generators

ECG-gated myocardial SPECT

Parathyroid scintigraphy

A.C. to improve SPECT imaging

Authors' Note:  SPECT (Single Photon Emission Computed Tomography) imaging of the myocardium (heart muscle), performed at rest and with stress (exercise or drug infusion) is currently the most frequent test performed in hospitals' nuclear imaging departments.

   The 3-dimensional images, a type of computed tomography, are produced with a camera which detects the emission of the single-energy gamma rays following an injection of a radionuclide. By connecting the patient to a system for recording the ECG (electrocardiogram), the images can be "gated", i.e. divided into segments of the cardiac cycle; these show contraction of the ventricles, following each of two injections of the imaging agent. A muscle region showing identically poor blood flow (perfusion) and contractile function at rest and stress represents prior heart damage, and is unlikely to respond to therapy.

Authors' Note:   Attacks of renal colic (spasmodic intermittent pain) may occur due to blockage of urine flow by stones in the ureters. In adult patients, stones consisting of calcium salts are most likely. If a high serum calcium level is found (this situation prevails in only a minority of cases of kidney stones), overactivity of the tiny parathyroid (PT) glands may be responsible. Milder cases of excessive parathyroid hormone secretion may also occur without symptoms, but can lead to loss of bone mass and increased risk of bone fracture.

   A single functioning adenoma (benign tumor)  of one of the PT glands is most commonly responsible, but hyperplasia (overgrowth) of all four glands may also result in inappropriate PT hormone secretion, detected by increased blood levels. The radiotracer Tc-99m sestamibi is taken up selectively by overactive PT glands, and may help plan surgery to explore the neck and remove the tumor.

Authors' Note: Absorption of rays by body tissues complicates the interpretation of medical imaging with nuclear techniques. ‘Hybrid' scanners combine the nuclear camera with a CT x-ray unit that provides maps of attenuation; this technique for correction of attenuation (known to workers in the field as A.C.), makes the nuclear scan more accurate in the detection of various lesions, in particular, abnormalities in blood flow to the heart muscle, and studies of deeply-located tumors, e.g. somatostatin scans. 

The combination of the two scanners (nuclear and X-ray CT) has been given the puzzling name “SPECT-CT”. 

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