Variability in Heart-to-Mediastinum Ratio From Planar ¹²³I-mIBG Images of a Thorax Phantom for 6 Common Gamma Camera Models

Abstract

A heart-to-mediastinum (H/M) ratio ≥1.6 on planar Iobenguane ¹²³I (¹²³I-mIBG; AdreView) images identifies heart failure patients at low risk of experiencing an adverse cardiac event. This phase 4 study used standardized phantoms to assess the inter-camera, intra-camera, and inter-head variability in H/M ratio determinations from planar cardiac ¹²³I-mIBG imaging using commercially-available, dual-head gamma 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 GE Millennium MG (reference camera). Commercial cameras used in the study were dual-head and capable of 90-degree 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/pixel of the heart and mediastinum regions of interest. The primary endpoint, inter-camera 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. Intra-camera and inter-head variability in 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. Intra-camera variability (Head 1) in H/M ratio among camera models with N≥8 was high, with standard deviations (SDs) ranging from 0.0455 to 0.1193. Inter-head variability was low (SDs of the inter-head difference, 0.017 to 0.074). Conclusion: Commonly-used gamma 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. 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.