THE EFFECT ON DIAGNOSTIC QUALITY OF USING DUAL-ISOTOPE IMAGING FOR KR-81(M) VENTILATION AND TC-99(M)-MAA PERFUSION LUNG-SCANNING

It is the practice in some centres to use dual isotope imaging to redu ce imaging times in lung ventilation and perfusion studies with Kr-81( m) gas and Tc-99(m)-macroaggregated albumin (Tc-99(m)-MAA) by simultan eous acquisition of the two images. The resulting loss of image caused by cross-talk between the two energy windows was investigated using t wo phantoms, one with cold Tc-99(m) lesions of varying size and contra st, and the other a uniform field of Kr-81(m). It was found that, unde r scatter conditions typical of a patient study, the use of dual isoto pe acquisition and a krypton generator of 470 MBq or greater resulted in a perceptible loss of image quality with lesions up to 4 cm in diam eter being missed. On an older camera system, without modern energy an d linearity correction facilities, a lower generator activity of only 120 MBq was sufficient to cause image degradation even under very low scatter conditions. Seventy-five patient studies were performed using both single and dual isotope imaging with generator activities ranging from 80 to 282 MBq. At these low generator activities, the studies di d not demonstrate any differences between the images that would result in a different diagnosis. We conclude that the use of dual isotope V/ Q scanning reduces the diagnostic value of the perfusion image if the activity of the Kr-81(m) generator is too high, although at generator activities of 300 MBq or less no loss of image quality will occur on m odern camera systems.