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21 November 2009

Answer to Question #1035 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

What is the effective half-life of ^99mTc MAA when used for a lung V/Q scan in Nuclear Medicine?

If you are talking about injection of ^99mTc MAA (what is commonly suggested by the definition of the scan you mentioned), International Commission on Radiatiological Protection (ICRP) Publication 53 gives a rather complicated picture. The injected activity is taken up by the lungs, with 85 percent of the material having a six-hour biological half-time (three-hour effective half-time) and 15 percent having a three-day biological half-time (about a 5.5 hour effective half-time). There is some uptake of the released material by the liver (25 percent of the total, being held up there with a five-day biological, or a 5.7-hour effective, half-time). Some of the material entering the lung or liver, as well as any Tc that may have detached from the MAA, will be excreted from the body through the kidney/bladder pathway. Overall, the whole body effective half-time appears to be about 5.3 hours. If you are referring instead to inhalation of Tc aerosols (which could be done with DTPA, MAA, pertechnetate, or other compounds), the kinetic picture is quite different, but also covered in ICRP 53 if needed.

Michael G. Stabin, CHP, PhD

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