... Quality control in mammography ...

Introduction In mammography it is essential that objects with very small contrast and diameter can be distinguished. Therefore the quality of the technical aspects of the mammography equipment should be monitored at regular time-intervals. Usually this is performed by measurement of the physical parameters of the X-ray equipment, screen-film combination, developing process and observation conditions. With nowadays digital systems and pre-processing of data it becomes even more important  to monitor the equipment on a regular basis. The main item in quality control however should be the assurance of information transfer from the tissue under examination to the radiologist. Therefore the information content of an image should be monitored. 
The Artinis CDMAM 3.4 phantom is specially developed to facilitate this task in mammography systems, i.e. detecting very low contrast and detecting very small details. With the phantom the "threshold contrast" as a function of the object diameter can be determined and plotted in a Contrast-Detail curve.

Construction The Artinis CDMAM phantom consists of an aluminum base with gold discs of various thickness and diameter (figure). The aluminium base is attached to a Plexiglas (PMMA) cover. Under normal mammography-radiation conditions (Mo anode, 30 mm Mo filter, 28 kV) the aluminum base and PMMA cover together has a equivalent PMMA thickness of 10 mm. The phantom is delivered with 4 PMMA plates, each 10 mm thickness. Every plate has an engraved marker with lead inlet for identification. The gold discs are arranged in a matrix of 16 rows by 16 columns. Within a row the disc diameter is constant, with (partly) logarithmic increasing thickness and within a column the thickness of the discs is constant and the diameter increases logarithmic. Each square contains two identical discs (same thickness, same diameter), one in the center and one in a randomly chosen corner. Easily recognizable patterns have been avoided. The total matrix is rotated by 45 degrees and the corners of the matrix are skipped. This is done for reasons of getting a better focus on the interesting part (low contrast, small diameter) and making the recognition of the patterns more difficult.

Evaluation  To make a X-ray image, the Artinis CDMAM phantom (in combination with one or more Plexiglas plates) should be positioned on the bucky with the smallest disc diameters at the thorax side. After the film has been processed, the density of the film should be checked. In a series of Contrast-Detail images, all the images should have approximately the same densities in a reference position on the film. After this pre-processing, the following measurements are possible:

• Comparison of image quality with various film-screen combinations

• Optimization of digital mammography system

• Determination of the optimum background 

The Artinis CDMAM 3.4 phantom is a result of the project: "Quality Assurance in Mammography, Department of Radiology, University Medical Centre Nijmegen, the Netherlands." By M.A.O. Thijssen, Ph.D., K.R. Bijkerk, M.Sc. and J.M. Lindeyer, B.Sc.

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