Cardiovascular Nuclear Imaging
Perfusion physiology and stress pharmacology
Myocardial perfusion tracers distribute in proportion to regional blood flow at the moment of injection. Tc-99m sestamibi and tetrofosmin are lipophilic cations retained by mitochondrial membrane potential with minimal redistribution, so rest and stress require separate injections; Tl-201 enters via the Na/K-ATPase and redistributes, permitting stress-redistribution-reinjection protocols. Extraction plateaus at high flow (roll-off), which is why vasodilator stress shows relative rather than absolute flow differences and why balanced three-vessel disease can appear normal.
Vasodilator stress uses adenosine, regadenoson, or dipyridamole; caffeine and other methylxanthines must be withheld (about 12 h for caffeine, longer for theophylline), and severe reactive airway disease is a contraindication. Exercise or dobutamine is preferred in asthma. Gated SPECT adds ejection fraction, wall motion, and thickening, helping separate attenuation artifact from a true fixed defect.
Interpretation: a defect present on stress and absent at rest is ischemia; a fixed defect is scar or artifact; transient ischemic dilation and increased lung uptake suggest severe multivessel disease. Breast and diaphragmatic attenuation create fixed anterior and inferior defects, respectively, correctable with attenuation correction or prone imaging.
Traps: calling a fixed inferior defect infarction without considering diaphragmatic attenuation; forgetting caffeine withdrawal; and treating a normal perfusion scan in a patient with known severe disease as excluding left-main disease.
Worked example: summed stress score 20 with summed rest score 8 gives a summed difference score of 12 - a large reversible defect; a score of 7 or more is generally considered high-risk ischemia.