Parathyroid imaging: The importance of pinhole collimation

Objectives To rigorously compare pinhole and parallel hole collimation in an intrapatient intrastudy design in 2 imaging protocols: 1) dual phase Tc-mibi imaging & 2) dual phase Tc-mibi + dual tracer, Tc-99m-mibi & I-123, simultaneous acquisition subtraction imaging.

Methods 37 surgically proven non ectopic parathyroid adenomas were evaluated. Anterior pinhole and parallel hole images of the neck were available for Tc-mibi at 15 min and 3 hr, and for simultaneously acquired Tc-mibi and I-123 subtraction at 15 min, all from a single study. The images were modified so that all images had a square border and so that the thyroid filled three quarters of the image. The observers indicated the location of any identified adenoma and graded the certainty of diagnosis on a 3 point scale.

Results The localization success rate for the 2 observers combined for the single tracer dual phase images was 66.2% with pinhole collimation and 43.2% with parallel hole collimation (P < 0.01). The localization success rate with the addition of the dual tracer simultaneous acquisition subtraction image was 83.8% with pinhole collimation and 62.2% with parallel hole collimation (P = 0.01). The degree of certainty of localization was greater with pinhole collimation with both imaging protocols (P < 0.01 in both cases).

Conclusions In the anterior projection pinhole collimation is superior to parallel hole collimation for parathyroid imaging with either dual phase Tc-mibi imaging or dual phase Tc-99m-mibi plus dual tracer, Tc-mibi and I-123, simultaneous acquisition subtraction imaging.