Abstract: A dual collimator designed for an Anger camera (gamma camera), to permit measurement of cardiac performance on a beat-to-beat basis. Special all-purpose (SAP) and special high-sensitivity (SHS) collimator sections can be interchanged without movement of the gamma camera or patient. Thus, left-ventricular regions of interest delineated on SAP multigated images can be transferred to SHS high sensitivity images to generate beat-to-beat volume curves. Preliminary balloon studies demonstrated an excellent correlation between ejection fractions calculated with the two collimators: 5>0.99, n=17, p<0.001. Varying the volume of an adjacent "right ventricle" balloon failed to alter significantly the count rate from the "left ventricle" balloon's region of interest. Preliminary results on 12 patients, comparing standard-camera ejection fractions with average beat-to-beat ejection fractions, showed that it is possible to measure cardiac function on a beat-to-beat basis with a single-crystal gamma camera.