Sources of variability in quantitative thallium analysis.

Nineteen patients undergoing exercise thallium scanning had two separate redistribution 40 degree LAO acquisitions obtained; the gamma camera was repositioned between the views. Data were analyzed by generating 60-point circumferential profile curves for each of the 38 images and the variability of individual points and 18 degrees and 36 degrees segments was determined. Intraobserver variability, assessed by reanalyzing the scans one week later, was approximately 5.0%. The interobserver variability was of similar magnitude (approximately 6%). The total reimaging variability was, however, significantly larger (approximately 13%). It is concluded that quantitative image analysis is highly reproducible with low intra- and interobserver variabilities, but the act of repositioning and reimaging introduces significant additional variability. This may limit the ability of quantitative thallium scanning to document changes in the size of defects.