Purpose: To investigate the influence of physician-selectable equipment variables on image quality for a cardiac X-ray
system equipped with flat panel detector.
Materials and Methods: Two contrast phantoms (Leeds TO.10 and CDRAD) were imaged in fluorography and
fluoroscopy mode. Three variables are studied: the detector entrance dose, patient thickness and antiscatter grid. In
fluorography mode, the detector entrance dose was 100, 120, 140, 170, 200 and 240 nGy/image. Patient thickness was
simulated with Perspex blocks of 8, 12, 16 and 20cm. The detectability of contrast details was visually evaluated by five
observers (subjective method). An alternative objective method of image quality evaluation was used. It consists on
determining a simple "figure of merit" parameter based on signal-to-noise and dose measurements.
Results: The threshold contrast values were determined for different settings. Contrast-detail curves are presented.
Conversion of curve data in single numerical values and comparison with the "figure of merit" are discussed.
Conclusion: Contrast detail objects are sensitive to variables changed and can be used in optimization process of new
systems. The change of detector entrance dose from a superior to a next inferior setting does not change dramatically the
image quality. Consequently, a saving of about 15% in patient "skin" dose is achievable.
Purpose:
The purposes of the study were to set-up and validate a simulation framework for dose and image quality optimization
studies. In a first phase we have evaluated whether CDRAD images as obtained with computed radiography plates could
be simulated.
Material and Methods:
The Monte Carlo method is a numerical method that can be used to simulate radiation transport. It is in diagnostic
radiology often used in dosimetry, but in present study it is used to simulate X-ray images. With the Monte Carlo
software, MCNPX, the successive steps in the imaging chain were simulated: the X-ray beam, the attenuation and scatter
process in a test object and image generation by an ideal detector.
Those simulated images were further modified for specific properties of CR imaging systems. The signal-transfer-properties
were used to convert the simulated images into the proper grey scale. To account for resolution properties the
simulated images were convolved with the point spread function of the CR systems. In a last phase, noise, based on
noise power spectrum (NPS) measurements, was added to the image.
In this study, we simulated X-ray images of the CDRAD contrast-detail phantom. Those simulated images, modified for
the CR-system, were compared with real X-ray images of the CDRAD phantom. All images were scored by computer
readings.
Results:
First results confirm that realistic CDRAD images can be simulated and that reading results of series of simulated and
real images have the same tendency. The simulations also show that white noise has a large influence on image quality
and CDRAD analyses.
A new generation CR system that is based on phosphor needles and that uses a digitizer with line scan technology was compared to a clinically used CR system. Purely technical and more clinically related tests were run on both systems. This included the calculation of the DQE, signal-to-noise and contrast to noise ratios from Aluminum inserts, contrast detail analysis with the CDRAD phantom and the use of anthropomorphic phantoms (wrist, chest and skull) with scoring by a radiologist. X-ray exposures with various dose levels and 50kV, 70kV and 125kV were acquired. For detector doses above 0.8 μGy, all noise related measurements showed the superiority of the new technology. The MTF confirmed the improvement in sharpness: between 1 and 3 lp/mm increases ranged from 20 to 50%. Further work should be devoted to the determination of the required dose levels in the plate for the different radiological applications.
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