AND, HERE'S A LIST OF LINKS to collections of intriguing verses on other medical/dental topics that can now be found on various posts including:
Tuesday 5 July 2022
Selected Topics in DIAGNOSTIC IMAGING
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.
Tests of myocardial flow in the Nuclear laboratory, e.g. gated SPECT, require a stimulus so that the specific radiotracer can be injected at peak blood flow to image the state of heart muscle. Ideally, exercise stress, as with a submaximal treadmill procedure, provides this stimulus, with a high attained heart rate reflecting success. As a backup in those unable to exercise sufficiently, (musculoskeletal or breathing problems, reduced fitness), pharmacologic stress may be used. Dipyridamole ,tradename Persantine, is a commonly used agent for this purpose, increasing blood flow optimally, but with little change in heart rate or blood pressure.
Authors' Note: The verse above is a companion verse to ”dipyridamole”.
When doctors want to check whether all areas of heart muscle can increase their blood flow appropriately, they may use certain drugs as helpers; this is particularly true when the patient is unable to perform a submaximal exercise test, often called a Bruce treadmill test, after its inventor. Dobutamine (doh-BYOO-ta-meen), relative of adrenaline, raises heart rate and blood pressure. It's the only drug available for tests which use echocardiography to picture the heart while it is 'under stress', but is a second-choice drug for the nuclear imaging test called gated SPECT. The preferred drug for those tests is one whose primary effect is to dilate arterial blood vessels, increasing the flow to normal heart muscle. Such drugs include dipyridamole and adenosine.
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: In disease states, including those producing congestive heart failure, the ejection fraction of the left ventricle, a measure indicating the strength of contraction, provides important information concerning prognosis (potential outcome) and the need for treatment.
The ejection fraction can be measured by echocardiography, magnetic resonance imaging, or several different nuclear (radio-isotope) techniques. These techniques measure the volume of the ventricle at the end of diastolic (relaxation) and systolic (contraction) phases of the cardiac cycle. Despite the name, the change during systole is generally given in medical jargon as the percent relative change, rather than as a true fraction; e.g. 60% is good, 30% is bad.
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 above verse panders to the jargony use of the medical term biopsy,as a verb. The position mentioned in the verse would apply specifically to fine-needle biopsy of the prostate, a procedure discussed in a verse HERE.
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.
Patients and their Maladies (parts #1, #2 and #3)
and in 'brief sagas' including:
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Posted by Giorgio Coniglio at July 05, 2022