This work examined the impact of the presampling Modulation Transfer Function (MTF) on detectability of lesion-like targets in digital mammography. Two needle CR plates (CR1 and CR2) with different MTF curves but identical detector response (sensitivity) were selected. The plates were characterized by MTF, normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE). Three image quality phantoms were applied to study the impact of the difference in MTF: first, the CDMAM contrast-detail phantom to give gold thickness threshold (T); second, a 3D structured phantom with lesion models (calcifications and masses), evaluated via a 4-alternative forced-choice study to give threshold diameter (d<sub>tr</sub>) and third, a detectability index (d') from a 50 mm PMMA flat field image and an 0.2 mm Al contrast square. MTF coefficient of variation was ~1%, averaged up to 5 mm<sup>-1</sup>. At 5 mm<sup>-1</sup>, a significant 24% reduction in MTF was observed. The lower MTF caused a 12% reduction in NNPS for CR2 compared to CR1 (at detector air kerma 117 μGy). At 5 mm<sup>-1</sup>, there was a drop in DQE of 34% for CR2 compared to CR1. For the test objects, there was a trend to lower detectability for CR2 (lower MTF) for all but one parameter, however none of the changes were significant. The MTF is a sensitive and easily applied means of tracking changes in sharpness before these changes are uncovered using lesion simulating objects in test objects.
The purpose of this study is comparing the detection performance in 2D full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) using a structured phantom with inserted target objects. The phantom consists of a semi-cylindrical PMMA container, filled with water and PMMA spheres of different diameters. Microcalcifications and 3D printed masses (spiculated and non-spiculated) were inserted. The phantom was imaged ten times in both modes of five systems, using automatic exposure control (AEC) and at half and double the AEC dose. Five readers evaluated target detectability in a four-alternative forced-choice study. The percentage of correct responses (PC) was assessed based on 10 trials of each reader for each object type, size, imaging modality and dose level. Additionally, detection threshold diameters at 62.5 PC were assessed via non-linear regression fitting of the psychometric curve. Evaluation of target detection in FFDM showed that spiculated masses were better detected compared to non-spiculated masses. In DBT, detection of both mass types increased significantly (p=0.0001) compared to FFDM. Microcalcification detection thresholds ranged between 110 and 118 μm and were similar for the five systems in FFDM while larger variations (106-158 μm) were found in DBT. Mass detection was independent of dose in FFDM while weak dependence was seen for DBT. Microcalcification detection increased with increasing dose for both modalities. The phantom was able to show detectability differences between FFDM and DBT mode for five commercial systems in line with the findings from clinical trials. We suggest to use the phantom for task-based assessment methods for acceptance and commissioning testing of DBT systems.