Variability in Heart-to-Mediastinum Ratio from Planar ¹²³I-MIBG Images of a Thorax Phantom for 6 Common γ-Camera...

A heart-to-mediastinum (H/M) ratio of 1.6 or greater on planar ¹²³I-iobenguane (¹²³I-MIBG) images identifies heart failure patients at low risk of experiencing an adverse cardiac event. This phase-4 study used standardized phantoms to assess the intercamera, intracamera, and interhead variability in H/M ratio determinations from planar cardiac ¹²³I-MIBG imaging using commercially available, dual-head γ-cameras. Methods: A fillable thorax phantom was developed to simulate the typical uptake of ¹²³I-MIBG. The phantom had a nominal H/M ratio of 1.6 on the reference camera. Commercial cameras used in the study were dual-head and capable of 90° configuration for cardiac imaging. The target sample size was 8 units (examples) per camera model. Two imaging technologists independently analyzed planar images of simulated ¹²³I-MIBG uptake from the thorax phantom. H/M was the ratio of the average counts per pixel of the heart and mediastinum regions of interest. The primary endpoint, intercamera variability in H/M ratio from head 1, was determined for each camera model via comparison with the H/M ratio on the reference camera. Only cameras with at least 8 units tested (n ≥ 8) were included in the primary analysis. Intracamera and interhead variability in the H/M ratio were also evaluated. Results: Nine camera models were studied. The mean H/M ratio ranged from 1.342 to 1.677. The primary analysis (6 camera models) using a mixed-model, repeated-measures analysis showed no significant difference in H/M ratio between any camera model and the reference camera. Intracamera variability (head 1) in the H/M ratio among camera models with 8 units or more was high, with SDs ranging from 0.0455 to 0.1193. Interhead variability was low (SDs of the interhead difference, 0.017-0.074). Conclusion: Commonly used γ-cameras produced H/M ratios from simulated ¹²³I-MIBG phantom images that were not significantly different from those on the reference camera. This finding indicates that the results of previous clinical trials of ¹²³I-MIBG, involving many different clinical sites and camera models, are valid. The assessment of the performance of a given camera unit using an ¹²³I planar phantom before H/M results from ¹²³I-MIBG imaging are used for classifying risk in heart failure patients is encouraged.