... Quality control for radiological system and observer ...

Introduction Most definitions of image quality in radiology are based on characterizing the physical properties of the image chain. However, medical diagnosis is not made by the image alone. The perception by the observer is crucial for the result. A test of the observers perception is possible with so called Contrast-Detail (CD) phantoms. With a CD-phantom it is possible to quantify both, detail and contrast, as observed by the radiologist. The Artinis CDInverse phantom can be used within the entire range of diagnostic imaging systems, such as fluoroscopy and digital subtraction angiography. The phantom is based on the CDRAD phantom, and can be used in addition to it. In general this phantom mimics reality better, where denser spots in tissue indicate pathological areas.

Construction The CDInverse phantom consists of a Plexiglas tablet with cylindrical rods of exact diameter and height (tolerances: 0.02 mm). Together with additional Plexiglas tablets, to simulate the dimensions of the patient, the radiographic image of the phantom gives information about the imaging performance of the whole system (refs. 1 and 2). The image shows 225 squares, 15 rows and 15 columns. In each square either one or two spots are present, being the images of the rods. The first three rows show only one spot, while the other rows have two identical spots, one in the middle and one in a randomly chosen corner (figure 1). The optical densities of the spots are lower as compared to the uniform background. Due to the (exponentially) increasing height of the holes in horizontal direction, the image shows 15 columns of spots with increasing contrast. In the vertical direction the diameter of the rods increases stepwise and exponentially from 0.3 tot 8.0 mm. For the image this means 15 rows of spots with increasing spatial resolution.

Evaluation For evaluation of the phantom image the observer indicates the location of the second spot in each square. Correct indication proves that a contrast is really seen. At the transition from visible to invisible it is difficult to decide in which corner the second spot is located and the response equals pure chance. The line connecting the central spots with smallest visible diameter and contrast is called the Contrast-Detail (CD) curve. For comparison of the imaging performance of different systems, phantom images are made under identical conditions and evaluated by the same observer and at the same time. The better system will produce an image in which smaller contrasts and details are visible. This results in a shift of the CD-curve to the lower left part of the image. This is seen in the figure in the right which shows the CD curves of monitor image (red) and the hard copy image (blue) of the same DSA equipment.
Comparison of the performance of several observers is also possible. The better performing observer produces a CD-curve more to the lower left part of the image. 

References
1. Thijssen MAO, et al: Quality Analysis of DSA equipment, Neuroradiology (1988)30: 561-568.
2. Thijssen MAO, Rosenbush G, Gerlach H.J.: Reduction of exposure in fluoroscopy, Electromedica (198804) 56: 126.133.
3. Rose A: Vision, human and electronic, Plenum, New York, 1974.

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