What methods are available to correct for attenuation artifact?
Two common methods are used to correct for attenuation artifact. One method directly corrects this problem by using an external radioactive source that is transmitted through the patient. This source is used to map the patient’s body structures to assess their ability to attenuate photons. By creating an attenuation map, the computer can combine the attenuation map with the emission images to correct for missing counts of radioactivity. The resulting image appears to “fill in” the areas that have attenuation artifact and eliminate the defects altogether.
A second method involves performing an ECG-gated study, obtaining images throughout the cardiac cycle. The ECG is monitored, and the R-R interval is divided into 8 or 16 equal segments during which the scan is acquired. The result is a sequence of images that can be played back to back to observe the wall motion throughout the R-R cycle. By obtaining the end-diastolic and end-systolic volumes, ejection fraction and regional wall motion can be determined. If a nonreversible defect is observed to move and thicken normally, it is most likely normal myocardium that has an apparent defect related to attenuation. If a nonreversible defect does not appear to move, it is most likely related to a scar or prior myocardial infarct. ECG-gating helps to assess whether an apparent defect is related to attenuation artifact or whether it is a real perfusion defect.