Utility of reprojected tomograms

Abstract

Why do clinicians continue to perform gated blood pool imaging when other methods are available to assess left ventricular (LV) ejection fraction (EF)? One reason is that blood pool scanning continues to be the method favored for serial LV EF evaluations to detect cardiotoxicity caused by anthracycline therapy for several cancers in adults, 1,2 and for Hodgkins disease in children. 3 Echocardiography is considered inadequate for this purpose, 3,4 cardiac CT delivers considerably higher radiation dose than would be desirable for serial evaluations, and cardiac MRI is not widely available. For evaluation of cardiotoxicity, simultaneous perfusion assessment is not required, because anthracyclines cause generalized myocyte failure throughout the myocardium in a uniform fashion, manifest as global hypokinesis. 5 Planar blood pool scans have the advantage of simplicity and enable reproducible EF values. 6 EF reproducibility to within 5% has been considered a minimal requirement due to observations that a drop of more than 5 EF points from rest to stress is a significant prognostic indicator, signaling a high likelihood of future adverse cardiac events. 7-9 Building on those technical foundations, most clinicians consider a drop of more than 5 EF points due to chemotherapy to be clinically significant, with an overall decline below LVEF of 50% to be of serious concern. 3,10 Many patients undergoing chemotherapy are followed for several years after baseline assessment of cardiac function. Due to recent advances in technology, in the current nuclear imaging environment this can present referring physicians with a dilemma. They may have a long history of interpreting the implications of changes in planar blood pool EF values for patients in general, and have serial measurements for their own individual patients. Now they are confronted with an increasing prevalence of imaging facilities that are incapable of performing planar blood pool imaging, but instead have solid state SPECT capability, of which the referring physicians are told are superior to planar cameras. How are they going to interpret new EF results, and how will they subsequently manage their patients, when they are no longer able to obtain planar blood pool EF measurements in patients for whom they have successive planar blood pool EF values determined over many years? In ‘‘Assessment of an Intermediate Reprojection Technique Transitioning from Planar to SPECT Radionuclide Ventriculography’’ in the current issue of the Journal, 11 O’Doherty et al report similar EF values obtained by reprojected SPECT blood pool tomograms when compared to data in the same patients who also had conventional planar blood pool acquisitions. Why would there be any question that a reprojected tomogram would produce different EF values than if planar data had been acquired instead? One reason is that the reconstruction process requires multiple choices from among several available options. True maximum likelihood estimation maximization (ML-EM) iterative reconstruction algorithms involve lengthy computations and are not commercially available 12 ; rather, accelerated