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.
As part of an EC funded project, the design for a new phantom has been proposed that consists of a smaller contrast-detail part than the CDMAM phantom and that contains items for other parts of an acceptance protocol for digital mammography. A first prototype of the "DIGIMAM" has been produced. Both the CDMAM phantom and the DIGIMAM phantom were then used on a series of systems and read out as a part of a multi centre study.
The results with the new phantom were very similar to results obtained with the CDMAM phantom: readers scored different from each other and there was an overlap in the scores for the different systems. A system with a poor score in CDMAM had also the worst score for DIGIMAM. Reading time was significantly reduced however. There was promising agreement between automated reading of CDMAM and the scores of the DIGIMAM phantom. In order to reduce the subjectivity of the readings, computerized reading of the DIGIMAM should be developed. In a second version of the phantom, we propose to add more disks of the same size and contrast in each square to improve the statistical power of each reading.
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.
KEYWORDS: Modulation transfer functions, Detection and tracking algorithms, Digital mammography, Sensors, Spatial frequencies, Data modeling, Analytical research, Algorithm development, Fourier transforms, Imaging systems
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.
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.
KEYWORDS: Modulation transfer functions, Sensors, Data modeling, Digital mammography, Lead, Aluminum, Detection and tracking algorithms, X-rays, Tumor growth modeling, Polymethylmethacrylate
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.
We evaluated the visibility of simulated subtle microcalcifications in real digital mammograms acquired with a flat-panel system (GE) and a CR system (Fuji). Ideal templates of microcalcifications were created, based on the attenuation characteristics of subtle microcalcifications from biopsied specimen in magnified images. X-ray transmission coefficients were expressed in Al-equivalent thickness. In this way, the X-ray transmission of a particular lesion could be re-calculated for other X-ray beams, different mammography systems and for different breast thickness. Extra corrections for differences in spatial resolution were based on the pre-sampled MTF. Zero to 10 simulated microcalcifications were randomly distributed in square frames. These software phantoms were then inserted in sets of raw mammograms of the modalities under study. The composed images were compressed, processed and printed as in clinical routine. Two experienced radiologists indicated the locations of the microcalcifications and rated their detection confidence. It is possible to assess the visibility of 'well controlled’ microcalcifications in digital clinical mammograms. Microcalcifications were better visible in the CR images than in the flat panel images. This psychophysical method comes close to the radiologists’ practice. It allows fpr including processing and visualization in the analysis and was well appreciated by our radiologists.
KEYWORDS: Modulation transfer functions, Sensors, X-rays, Digital mammography, Signal attenuation, Quantum efficiency, X-ray detectors, Data modeling, Fourier transforms, Polymethylmethacrylate
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.
KEYWORDS: Electrodes, Silicon, Metals, Dielectrics, Electroplating, Aluminum, Photoresist materials, Deep reactive ion etching, Semiconducting wafers, Chemical species
This paper reports on the fabrication of innovative counter electrodes for the development of a new Scanning Atom Probe (SAP) instrument. A process using thick spin-on dielectrics, deep reactive ion etching and photolithography has been developed for the realization of the counter electrodes. The novel structure is a two-terminal device in the form of a hollow cone shape, with tow electrodes separated by a dielectric layer. Different counter electrode design versions are presented, with the focus on the results for the first iteration. Electrical testing of the insulating layer is performed to investigate the material suitability for the operating conditions of the SAP instrument. Details regarding the design and fabrication procedure for the different designs, with emphasis on the process flow for the non standard steps, are also presented.
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