Frank Rogge

Evaluation of the diagnostic value of a computed radiography system by comparison of digital hard copy images with screen–film mammography: results of a prospective clinical trial

The purpose of the study was to determine prospectively the diagnostic value of a computed radiography (CR) system by comparing mammographic hard copy images with screen–film mammography (SFM). A series of 100 patients, who came for diagnostic investigation, underwent two-view SFM (Lorad M-IV Platinum) and digital mammography with a CR system (AGFA CR system). The images were obtained by double exposure, i.e. same view without removing compression of the corresponding breast. The CR images were processed with dedicated processing for mammography. Six radiologists read sets of SFM and CR images. The primary efficacy parameter was the overall diagnostic value. The secondary efficacy parameters were lesion conspicuity and lesion details (for masses and micro-calcifications), tissue visibility at chest wall and at skin line, axillary details, overall density and sharpness impression and the overall noise impression. These parameters were scored by a 7-point scoring system. “CR non-inferior to SFM” was concluded if the lower confidence interval bound exceeded 80%. The confidence interval for the overall diagnostic value was between 96.4% and 100%. Pooled analysis of the ten features for image quality comparison demonstrated for all but one feature (lesion details of the calcifications) CR non-inferiority to SFM.

Preliminary validation of a new variable pattern for daily quality assurance of medical image display devices

This paper reports on a comparative study between the well-established test patterns for daily quality assurance (QA) of monitors of the American Association of Medical Physicists, Task Group 18 (AAPMtg18) and the Deutsches Institut für Normung e.V (DIN), and a newly proposed variable test pattern. A characteristic of the test patterns currently used for the QA of monitors is their static nature: The same test pattern is always used. This enables a learning effect that may bias the results over time. To address this problem we have developed a variable pattern for the quality assurance of monitors (MoniQA) that allows an evaluation of contrast visibility, geometric distortion, resolution, global image quality including uniformity, and artifacts. The test pattern includes randomly generated elements intended to prevent the observer from learning the test. Examples are random characters that have to be discriminated from the background to evaluate the threshold luminance difference and variable positions of different features in the test pattern. The newly proposed test patterns were generated and visualized on different viewing stations with a software tool developed in JAVA. In this study, we validated these patterns against the well-known AAPMtg18 and DIN test patterns on 22 monitors. The results showed that the MoniQA test can indicate the same monitor problems as the other well-known patterns and is significantly quicker to evaluate than the AAPMtg18 test patterns. The MoniQA pattern is a promising alternative for daily quality control of medical viewing stations.

Practical method for detected quantum efficiency (DQE) assessment of digital mammography systems in the radiological environment

X-ray detector systems can be characterized by their measured or estimated detective quantum efficiency (DQE). Assessment of DQE includes a measurement of the modulation transfer function (MTF) and the normalized noise power spectrum (NNPS). The incoming X-ray quantum flux has to be estimated. In this paper, the influence of the different possibilities regarding the measurement methods and phantoms, the X-ray quantum flux estimation models and the exposure geometry on the DQE of a full field digital mammography detector is assessed. Physical models were used to fit MTF measurements from bar-pattern and edge phantoms. The NNPS was calculated by 2D-FFT on a large number of flat-field subimages. The flux was calculated using anode spectra models (Boone, 1997) and attenuation data (NIST). We compared the influence of scattered radiation MTF calculations of both phantoms were similar. The edge method is preferred for practical reasons. NNPS data were similar to 1D synthetic-slit measurements. DQE data compared well with literature. Different exposure geometry conditions (with scattered radiation) showed similar results but a siginificantly lower DQE than in absence of scattered radiation. DQE assessment is feasible using normal exposure conditions, an edge phantom and calculated estimations of the flux.

Use of MTF calculation in global and local resolution assessment in digital mammography

The purpose of this study is to propose a test procedure for global and local resolution assessment in digital mammography to detect sharpness problems. The MTF calculation was based on the presampled edge method. In a first phase, we compared the effect of geometry and exposure conditions on the MTF. Results were: (1) the MTF was reproducible; (2) MTF data can be corrected for edge angle; (3) scatter conditions have significant influence; (4) edge position in the detector plane has negligible influence; (5) the required edge length for our algorithm is longer than the critical length to get rid of noise effects; (6) exposure conditions have no major influence except at very low dose levels. We propose to approximate clinical working conditions for the global MTF-check, with an edge-object embedded in 45mm PMMA and clinical exposures. Localized MTF calculations with this phantom and software can be automated for QA by the medical physicist. For sharpness analysis all over the detector, we designed a test-object with oblique, parallel bars and automatic software tools are being developed. By means of software simulations, local variations in the sharpness could be detected. Validation in practice and further automation of the software tools is ongoing.

Practical MTF calculation in digital mammography: a multicenter study

MTF is accepted as a measure for sharpness of a detector system, but analysis of one system by different researchers often results in differences. This can be due to differences in exposure setup or calculation algorithm. In this multicenter study, we investigate which options in the algorithm for the edge method result in differences in MTF. Three edge images were sent for analysis to nine participants, together with a questionnaire about different steps in their algorithm. One image was generated synthetically and is scatter-free and noise-free. The other images were created with an edge phantom between two slabs of 2 cm PMMA and were known to have a slight difference in MTF. The results were compared in both absolutely and relatively. All participants could calculate the MTF from the images. Although there are numerous differences between the different implementations, the results for the synthetical image are quite similar. This indicates that the algorithms perform similarly in noise-free and scatter-free conditions. With the real images, larger deviations are observed. The implementations can be divided in two groups according to their ability to reproduce a low frequency drop. The main difference between both groups was the use of data conditioning prior to the Fourier transform. In the group with low frequency drop, only slight absolute differences are observed. The other algorithms show larger differences. These differences underline the need for guidelines if the MTF curve gets a crucial role in the acceptance of a digital mammography system.

Characterization of a new generation of computed radiography system based on line scanning and phosphor needles

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.

A simulation framework for pre-clinical studies on dose and image quality : concept and first validation

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.

Threshold contrast visibility of microcalcifications in digital mammography

The purpose of this study is to describe a method that allows the calculation of a contrast-detail curve for a particular system configuration using simulated micro calcifications into clinical mammograms. We made use of simulated templates of micro calcifications and adjusted their x-ray transmission coefficients and resolution to the properties of the mammographic system under consideration (4). We expressed the thickness of the simulated micro calcifications in terms of Al equivalence. In a first step we validated that the thickness of very small Al particles with well known size and thickness can be calculated from their x-ray transmission characteristics at a particular X-ray beam energy. Then, micro calcifications with equivalent diameters in the plane of the detector ranging from 300 to 800 μm and thicknesses, expressed in Al equivalent, covering 77 to 800 μm were simulated into the raw data of real clinical images. The procedure was tested on 2 system configurations: the GE Senographe 2000 D and the Se based Agfa Embrace DM1000 system. We adapted the X-ray transmissions and spatial characteristics of the simulated micro calcifications such that the same physical micro calcification could be simulated into images with the specific exposure parameters (Senographe 2000D: 28 kVp-Rh/Rh, Embrace DM1000: 28 kVp-Mo/Rh), compressed breast thickness (42+/-5mm) and detector under consideration. After processing and printing, 3 observers scored the visibility of the micro calcifications. We derived contrast-detail curves. This psychophysical method allows to summarize the performance of a digital mammography detector including processing and visualization.