Independent Implementation of AAPM TG-150 Draft Image Receptor Test Recommendations
T Greene*, T Nishino, C Willis, UT M D Anderson Cancer Center, Houston, TXSU-E-I-102 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: To determine whether a proposed suite of generic tests for digital radiography (DR) detectors could be reduced to practice.
Methods: MATLAB software was developed to analyze images according to descriptions in a document drafted by the TG150 Detector Subgroup. For-processing images were acquired directly from the acquisition stations of three DR and one Computed Radiography system. Images included flat-field exposures at the manufacturer's calibration condition, twice the exposure, ½ the exposure, and a low exposure, plus three images of a lead bar pattern in different orientations, also at the calibration condition. The flat field images were analyzed to determine Detector Response; Gain Correction; Signal, Noise, and Signal-to-noise (SNR) Uniformity; SNR Magnitude; and Anomalous Detector Element (del) Identification. The program also allowed visual inspection for evaluation of collimation and non-uniformity. Bar pattern images were analyzed to evaluate spatial resolution by a variance method.
Results: Acquisition revealed a number of pitfalls. Some manufacturers have multiple calibration points. For-processing images are not directly available from all systems, and PACS may modify them from their original state. The orientation of the flat field with respect to the anode-cathode axis may not be defined by the manufacturer. Care must be taken to ensure collimation outside the edges of detectors, or the software must exclude collimator shadows. The matrix size of images differs among manufacturers, so the size of the region of interest (ROI) for analysis varies from the default size of 100x100 dels, as does the number of ROIs. The approach for dealing with edges and ROIs may affect the numerical results. The detector response function may also affect the interpretation of results.
Conclusions: The software successfully implements most of the detector tests recommended by TG150. Comparison of these results with those of the parallel effort will validate the draft test definition.