Biodistribution of tantalum-178: a short-lived radiopharmaceutical for blood pool imaging.

178Ta is a short lived radionuclide (half life = 9.3 min), which results in favorable radiation exposure compared to 99mTc (half life 6 h). The energy spectrum of 178Ta consists of imageable photons in the 55-65 keV (61.2%) and 93 keV (33.7%) range but also 6% of disintegrations result in photons with energies greater than 500 keV. These high energy photons cause septal penetration in low energy collimators so that resolution is degraded. However, a medium energy collimator prevents the septal penetration of these higher energy photons. Serial blood samples obtained from dog and rabbit models indicate that 178Ta is retained in the blood pool for at least 20-30 min after intravenous injection. The 178Ta appears to be associated with the protein fraction of the plasma and not primarily with the red blood cell fraction as determined by centrifugation and column chromatography. Gated equilibrium blood pool images using 178Ta were comparable in quality to images using the 99mTc labelled red blood cell technique. Therefore, 178Ta may allow comparable equilibrium gated blood pool imaging with much more favorable radiation dosimetry. Thus, serial studies over prolonged periods of observation may be obtained.