Ingrid Dypvik Landmark MSC, MSC, Presenter: Nothing to Disclose

Ingrid Helen Ryste Hauge MSC, Abstract Co-Author: Nothing to Disclose

Metta Hauge, Abstract Co-Author: Nothing to Disclose

Kristin Pedersen, Abstract Co-Author: Nothing to Disclose

Evaluate the utility of tests in a proposed protocol for constancy control of digital mammography systems.

The protocol contains tests for image acquisition, mechanical function and safety, and monitors. Sites with digital systems from five vendors were recruited. Dedicated PMMA test objects and Excel spreadsheets were developed. Quantitative measurements were generally done on processed images, measurements on raw images was done by some sites for comparison. For daily assessment of the Automatic Exposure Control system, a homogeneous PMMA phantom was exposed under clinical conditions. The mAs and signal to noise ratio (SNR) was recorded, the deviation from a baseline value calculated, and the resulting image inspected for artefacts. For thickness tracking, the signal difference to noise ratio obtained for three thicknesses was calculated. Detector homogeneity was assessed through comparison of SNR values for regions of interest in the centre and corners of a homogeneous image. Mechanical function and safety control included a compression test, a check list for mechanical aspects, and control of field alignment. Monitor performance was evaluated by visual inspection of the AAPM TG 18 QC test image.

For quantitative parameters, operational values and control limits were established. Test results exceeding the limits were recorded. All systems exhibited stable mAs values, indicating that the control limit of +/- 10% was readily achievable. The SNR also showed little variation, indicating that the control limit of +/- 20% is too wide. At one site the test for artefacts confirmed the existence of dead pixels, the problem was solved by changing the detector. At another site a defective grid lead to artefacts that were visible in images from the same test. The monitor controls proved more difficult to implement due to both difficulties importing and displaying the test image, and the radiographers not getting necessary access to the monitors.

The proposed tests could easily be performed by trained radiographers and within a time frame comparable to similar programs for analog systems. Based on this study, which tests to include in a vendor-independent constancy protocol can be chosen and a revised set of control limits established.

Mammograhy is going digital, and there is a need to replace the analog quality control protocols with protocols for digital systems.

Landmark, I, Hauge, I, Hauge, M, Pedersen, K, Trial of a Proposed Protocol for Constancy Control of Digital Mammography Systems.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6009706.html