Selected Topics in DIAGNOSTIC IMAGING

CURRENT CONTENTS:

Ode to gamma rays

Technetium generators

ECG-gated myocardial SPECT

Parathyroid scintigraphy

A.C. for SPECT

DEXA for bone mineral assessment

Fluorine-18 PET (positron emission tomography)

A.C. for PET imaging

"DOG"-scans (computed axial DOG-graphy)

Authors' Note:   In measuring ionizing radiation such as gamma rays with an instrument such as a  dosimeter, terms such as exposure-rate, flux, and fluence relate to the strength of the source. With regard to possible harm to humans and other biologic creatures, the absorbed dose is more important. The many different units involved in scientific descriptions may, in fact, detract from comprehension by non-experts. A simple rule of thumb, adopted by most professional societies, is to keep exposure "as low as reasonably achievable", as summarized in the acronymic slogan ALARA.

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 theinterpretation 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”. 

 

Authors' Note:   DEXA, or Dual Energy X-ray Absorptiometry, is a simple standardized imaging test to assess bone mass in the context of osteoporosis. It actually measures bone mass in a peculiar form, i.e, mass per unit area (rather than volume) of tissue; differential attenuation (blocking by tissue) for the 2 photons of different energies is assessed for each spatial element in a planar field. The technique is however, excellent for serial tests in the same patient, and is now used  so widely that its results are regarded as synonymous with Bone Mineral Density (BMD).

Authors' Note:   Absorption of rays by body tissues complicates the interpretation of medical imaging with Positron Emission Tomography (PET). In equipment development since the year 2000, '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 PET more accurate in the detection of cancer. A potential limitation, the much lower energy of the photons used for x-ray CT, turns out to have little degrading effect in practical usage.

   Moreover, anatomic localization of the lesion can be obtained at the same session, enabling techniques such as superposition of the ‘hot’ focus on a 3D anatomic body-map. This technique has been given  the difficult and somewhat redundant term ‘PET-CT’.

Authors' Note:   The confusing terminology for advanced, i.e. 3D medical imaging, uses acronyms that may be historically based or poorly explained. The development of a method of imaging known as ‘DOGgraphy’ is apocryphal. 

CAT: computerized axial tomography, X-ray imaging of a body section; better described in modern terms as ‘x-ray CT’

PET: positron (dual-photon) emission tomography; a Nuclear Medicine technique involving prior injection of a positron-emitting radionuclide ('isotope'); becoming an important modality in cancer assessment

Holography: processing of fields of light or other radiation scattered from objects; well developed with lasers, but with limited current application in medical imaging.

Here's a LIST OF LINKS to collections of intriguing poems (over 160 of these!) on medical/dental topics that can now be found on various posts. 

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