A case study on the impact of a reduction in MTF on test object detectability score in mammography

Abstract

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 (dtr) 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-1. At 5 mm-1, 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-1, 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.