Myocardial SPECT: what do we gain from attenuation correction (and when)?

This study aimed at assessing the impact of attenuation correction performed by means of a new hardware device (Beacon(tm)) in the assessment of coronary artery disease with myocardial SPECT and, afterwards, at identifying what specific risk class of patients would mostly benefit from the technique.

The first 500 consecutive patients, referred to our facility for coronary artery disease (CAD) assessment, were included in this study. Myocardial SPECT was performed after i.v. of 740 MBq of (99m)Tc Tetrofosmin (Myoview, Amersham Health) both after stress and at rest. Studies were acquired by means of a 3 head system (IRIX, Philips Medical System) equipped with the Beacontm option to correct for non-uniform attenuation. Out of the whole population studied, 130 patients underwent coronary angiography (CAG), 108 of which were proven to be affected by significant CAD, defined as a >50% coronary vessel lumen reduction. Attenuation corrected (AC) as well as non-attenuation corrected studies (NAC) were reconstructed, blindly read and qualified as: normal; borderline normal; borderline abnormal and definitely abnormal.

In the group of 130 patients who underwent CAG, sensitivity for CAD detection did not vary significantly (96% and 93%, for NAC and AC studies, respectively, p=ns) whereas specificity increased from 73% (NAC studies) to 91% (AC studies), p<0.01. Normalcy rate, assessed in a small subgroup (n=21) with a <5% likelihood for CAD, was found to be 80% for NAC studies vs 93% for AC studies (p<0.05). As regards synthetic clinical judgements, when we grouped normal and probably normal readings into a single ''normal'' category and, conversely, probably abnormal and definitely abnormal into a single ''pathological'' category, we see that after attenuation correction studies reported as ''pathological'' are reclassified as ''normal'' in 17.8% of the cases (25.2% in males and 6.9% in females, p<0.05). The opposite is seen in only 1.6% of the cases (1.3% in males and 4.4% in females). According to established criteria, 155 patients were classified at low risk for CAD (<15%), 115 at intermediate risk (from >15% to <50%) and 230 at high risk (>50%). The intermediate risk class showed the greatest impact: ''normal'' findings increased from 52% to 72%, thus reducing the rate of ''pathological'' reports from 48% to 28%.

This study shows that, while sensitivity for CAD is not affected by attenuation correction, specificity increases significantly, as well as normalcy rate. Out of the whole population studied, the main finding was that attenuation correction increases the rate of normal reports, more frequently in males than in females, and that studies carried out in patients at intermediate risk for CAD are more likely to be affected.