Abstract Background The Z‐resolution evaluation in the ACR Digital Mammography (DM) QC manual measures a pseudo full width half max (pFWHM) from speck group signals through slices of the phantom for comparison to a baseline value. Purpose The evaluation requires measurement of seven ROIs on up to nine slices, for a potential of up to 63 ROIs. This work evaluates whether measuring fewer ROIs would produce similar results. Methods Z‐resolution data from 468 surveys from 147 mammography units were retrospectively analyzed. The pFWHM was calculated using the 6‐speck mean (pFWHM 6 ) as described in the ACR manual, as well as using each single speck (pFWHM 1 ), and all combinations of a 2‐speck mean (pFWHM 2 ) using the center and each peripheral speck. Deviation from the respective initial measured pFWHM initial was determined for all units with at least two surveys. Results The pFWHM 1 had an average absolute error of 6.0% relative to pFWHM 6 across all surveys, compared to 3.9% for pFWHM 2 . For the 111 units with longitudinal data, the mean absolute deviation from pFWHM initial was 7.8% for pFWHM 1 , compared to 5.6% for pFWHM 2 , and 5.1% for pFWHM 6 . For longitudinal testing, pFWHM 6 produced two failures that were related to major component replacement. Both pFWHM 1 and pFWHM 2 methods caught those failures for a false negative rate of 0%. However, pFWHM 1 had a false positive rate of 12.9%, while pFWHM 2 ’s was only 2.8%. No positive test from any method was attributable to actual equipment failure. Conclusion A single‐speck pFWHM for ACR DM QC resulted in high variability and frequent unit failures for the Z‐resolution evaluation. However, a 2‐speck pFWHM produced results clinically equivalent to the 6‐speck method. Using the 2‐speck method as an alternative methodology for Z‐resolution evaluation achieves similar results with 57% fewer measurements, increasing the efficiency of testing.
Fisher et al. (2026) studied this question.