Answer to Question #12387 Submitted to "Ask the Experts"

How long after receiving a nuclear medicine procedure using ^99mTc agent (a ventilation/perfusion or V/Q scan) should one wait before undergoing a cardiac stress test using ^99mTc -sestamibi or ^99mTc -tetrofosmin?

Your question raises important considerations regarding the order in which these two nuclear medicine tests should be performed, and the length of time between them.

First, there are clinical considerations. Pulmonary embolism is characterized by acute chest pain and shortness of breath. Although these symptoms could be consistent with cardiac ischemia, the mortality associated with acute pulmonary embolism is very high. If there is clinical suspicion of pulmonary embolism, a V/Q study should be performed first, as the questioner proposes.

Second, there are technical considerations. The concern is "contamination" of the second image due to residual activity from the first image. The delay required to preserve cardiac image quality would depend upon the imaging modality used and the nature of the first ^99mTc study.  For example, if the first study was a ^99mTc bone scan and a planar gamma camera is used for both the bone and cardiac studies, then a time delay of several half-lives of ^99mTc (18 –24 hours) would be required to ensure that activity in the ribs and sternum did not obscure visualization of the myocardium. If single-photon emission computerized tomography (SPECT) is used for the cardiac study, then a shorter delay might suffice. In the case of the perfusion lung scan, the stress myocardial scan could be performed immediately. This is because the residual activity from the lung perfusion study is so much lower than the administered activity for the stress cardiac study (less than 4 millicuries [mCi] vs. 30 mCi) that interpretation of the cardiac study would not likely be compromised. If the rest cardiac study is done first (only 8–10 millicuries are administered), then a delay of 12–18 hours after the V/Q scan may be required to preserve cardiac image quality. For the ventilation portion of the V/Q scan, no delay would be required if the agent were ¹³³Xe or ^81mKr, due to their very rapid clearance of from the body. However, if a ^99mTc-labeled aerosol is used, then the time delay before a cardiac scan would be required. Depending upon the amount of activity used in the aerosol study, the time delay would be 12–18 hours or more.

Third, there are considerations regarding radiation risk. There is a perception among patients—and some clinicians—that the risk of radiogenic cancer is higher if the two studies are performed together, and that a delay reduces that risk. Whether the risk of radiogenic cancer is cumulative is a subject of postulation, study, and debate among radiation risk experts. However, the low radiation doses from ^99mTc scans are likely to result in cancer risks that are negligible and are far outweighed by the risk introduced by a delay in the diagnosis and treatment of cardiac disease.

Thank you for using our service and we hope this is helpful. 

Dr. Robert Reiman Jr, MD