Gated myocardial SPECT imaging; true additional value in AMI?

Over the years, myocardial perfusion imaging has shown to be a very accurate approach in establishing the diagnosis myocardial ischemia and infarction [1-15]. It also allows identifying the site, size and the severity of perfusion defects in patients with acute chest pain. It is well known that perfusion abnormalities may persist for several hours after the acute event together with wall motion and wall thickening abnormalities very likely due to myocardial stunning. Analysis by gated SPECT imaging may detect these high risk patients who might otherwise be seen as low risk patients due to normal perfusion. This is particularly important in patients with inferolateral ischemia as perfusion artifacts are common in the inferolateral myocardial region [16-31]. Concentration and proximity of subdiaphragmatic radioactivity relative to myocardium comprise a major factor in the nature and severity of inferior wall artifacts. If the subdiaphragmatic radioisotope concentration is equivalent to that in the myocardium, complex, potentially uninterpretable hot and cold inferior wall artifacts are produced. Gating offers considerable additional value to SPECT myocardial perfusion imaging in characterizing fixed defects thereby potentially improving test specificity. Since perfusion, function and wall motion/thickening can be assessed simultaneously, gated SPECT imaging follows the concept of a one-stop shop such as propagated by MRI studies [15, 32–45]. This holds in particular for regions with diminished radioisotope uptake. In case of preserved wall motion and/or thickening in region with a perfusion defect there might be still remaining viability influencing the appropriate management strategy [46-50]. On the other hand, it might be very interesting to study wall motion/thickening in areas with apparent normal myocardial perfusion in the acute phase of myocardial ischemia.

In the current issue of the International Journal of Cardiovascular Imaging, Neill et al. [51] used gated SPECT imaging at rest to compared semi-quantitative visual scores of perfusion, motion and thickening with an automated hypoperfusion index in patients with suspected acute inferolateral perfusion defects. In the absence of perfusion defects motion and thickening abnormalities were assessed. The authors studied 68 patients (of whom 56 with acute myocardial infarction) with chest pain at rest with either ST depression ≥0.1 mV in ≥1 of leads I, aVL, V1-V6 on 12-lead ECG or ST elevation ≥0.05 mV in ≥1 posterior lead on the body surface map. A rest gated SPECT image was performed within 24 h of the origin of chest symptoms. The ECG gated images were obtained 45–60 min after intravenous administration of 350–400 MBq of Tc99–sestamibi. The myocardial images were semi-quantitatively evaluated for perfusion, motion and thickening. The scores were compared to the automated global hypoperfusion index. This index is equivalent to the product of the surface of the hypoperfused area and the mean severity of the hypoperfused area, and expressed as a percentage of the left ventricular mass. A hypoperfusion index >5 was considered abnormal. It was shown by the authors that the summed perfusion score correlated well with the hypoperfusion index. Summed thickening score correlated well with the hypoperfusion index and the agreement between the scorers was good. Of the 1,156 evaluated segments, 21% of normally perfused segments had a motion abnormality and 19% of normally perfused segments had a thickening abnormality. It was concluded that, using gated SPECT imaging, assessment of wall motion and thickening in addition to perfusion in acute myocardial ischemia improves the diagnostic accuracy for acute myocardial infarction in particular in patients with inferolateral wall ischemia.

Analysis of gated SPECT images for wall motion and thickening abnormalities adds important diagnostic and prognostic information to the assessment of myocardial perfusion. Using gated SPECT imaging, myocardial areas may be identified by the presence of wall motion or thickening abnormalities in the presence of normal perfusion. Wall motion and thickening defects are often present in a segment with abnormal perfusion and are useful for discerning real perfusion defects from artifacts. Occurrence of attenuation artifacts during non-gated SPECT perfusion imaging has been considered an important limitation of the technique [52-55]. Apical thinning due to the overlying diaphragm and the occurrence of anteroseptal defects as a result of breast attenuation are very common causes for unwanted perfusion deficits, leading to image misinterpretation and potentially a wrong diagnosis. In radionuclide myocardial perfusion SPECT imaging, successful attenuation correction programs have been developed in order to discriminate between true and false perfusion defects [56-58]. Nowadays, gated SPECT fulfills a pivotal role in discriminating true perfusion defects from artifacts. On the other hand, wall motion and/or thickening disturbances in normally perfused areas may unravel pathophysiological states such as myocardial stunning. Such a gated approach should be routinely used in evaluating myocardial SPECT images and this is validly underscored by the study of Neill et al. [51].