Direct determination of the attenuation coefficient for radionuclide volume measurements

Correcting for the attenuation of photons between the cardiac chambers and chest surface is crucial for accurate nongeometric ventricular volume determinations from equilibrium radionuclide angiograms. Previous techniques have assumed that the attenuation coefficient of water for 99mTc (0.15/cm) should be used for this correction. In this study, this assumption was tested directly by measuring attenuation of the activity of a radioactive source within the right and left cardiac chambers. The balloon of a flow-directed catheter, filled with 99mTc, was used as a source and its depth within the body was measured with biplane fluoroscopy. In ten patients, a total of 36 measurements of attenuation were made. With linear regression analysis, the overall calculated attenuation coefficient, mu, was 0.12/cm (standard error of slope = 0.01, R = 0.93). Although the mean value of mu varied from 0.08 to 0.13 for four different intracardiac locations these differences were not significant. These direct measurements indicate that the attenuation of photons in the heart is not equivalent to that of water and suggest that an attenuation coefficient of 0.12/cm should be used in analyzing ventricular activity.