Giovanni Borasi

On site evaluation of three flat panel detectors for digital radiography.

During a tender we evaluated the image performance of three commercially available active matrix flat panel imagers (AMFPI) for general radiography, one based on direct detection method (Se photoconductor) the other two on indirect detection method (CsI phosphor). Basic image quality parameters (MTF, NNPS, DQE) were evaluated with particular attention to dose and energy dependence. As it is known, presampling modulation transfer function (MTF) of selenium based detector is very high (at 70 kV, 2 cycles/mm, 2.5 microGy, about 0.80). Indirect detection panels exhibit a comparable (lower) resolution (at 70 kV, 2 cycles/mm, 2.5 microGy, MTF is about 0.34 for both the systems analyzed) and a more pronounced energy and dose dependence could also be noted in one of them. As a consequence of the very high resolution, the normalized noise power spectrum (NNPS) of the direct system is substantially flat, very similar to a white noise. Considering that the sensitive layer of all detectors is the same (0.5 mm), the relatively higher NNPS values are related to selenium absorption properties (lower Z respect to CsI:Tl) and detector inherent noise. NNPSs of the other systems, at low frequencies, are comparable but the frequency dependence is significantly different. At 70 kV, 2.5 microGy, 0.5 cycles/mm detective quantum efficiency (DQE) is about 0.35 for the direct detection system, and about the same (0.6) for the indirect ones. The combined effect of additive and multiplicative noise components makes DQE dependence on dose not monotonic. DQE present a maximum for an intermediate exposure. This complex behavior may be useful to characterize the systems in terms of the monodimensional integral over the frequency of DQE (IDQE). Both visual contrast-detail experiment and the direct evaluation of the signal-to-noise ratio confirmed, at least in a qualitative way, the system performances predicted by IDQE.

Determination of the detective quantum efficiency of a digital x-ray detector: Comparison of three evaluations using a common image data set

The detective quantum efficiency (DQE) of an x-ray digital imaging detector was determined independently by the three participants of this study, using the same data set consisting of edge and flat field images. The aim was to assess the possible variation in DQE originating from established, but slightly different, data processing methods used by different groups. For the case evaluated in this study differences in DQE of up to +/-15% compared to the mean were found. The differences could be traced back mainly to differences in the modulation transfer function (MTF) and noise power spectrum (NPS) determination. Of special importance is the inclusion of a possible low-frequency drop in MTF and the proper handling of signal offsets for the determination of the NPS. When accounting for these factors the deviation between the evaluations reduced to approximately +/-5%. It is expected that the recently published standard on DQE determination will further reduce variations in the data evaluation and thus in the results of DQE measurements.

Contrast-detail analysis of three flat panel detectors for digital radiography

In this paper we performed a contrast detail analysis of three commercially available flat panel detectors, two based on the indirect detection mechanism (GE Revolution XQ/i, system A, and Trixell/Philips Pixium 4600, system B) and one based on the direct detection mechanism (Hologic DirectRay DR 1000, system C). The experiment was conducted using standard x-ray radiation quality and a widely used contrast-detail phantom. Images were evaluated using a four alternative forced choice paradigm on a diagnostic-quality softcopy monitor. At the low and intermediate exposures, systems A and B gave equivalent performances. At the high dose levels, system A performed better than system B in the entire range of target sizes, even though the pixel size of system A was about 40% larger than that of system B. At all the dose levels, the performances of the system C (direct system) were lower than those of system A and B (indirect systems). Theoretical analyses based on the Perception Statistical Model gave similar predicted SNRT values corresponding to an observer efficiency of about 0.08 for systems A and B and 0.05 for system C.

Physical and psychophysical characterization of a novel clinical system for digital mammography

PURPOSE In recent years, many approaches have been investigated on the development of full-field digital mammography detectors and implemented in practical clinical systems. Some of the most promising techniques are based on flat panel detectors, which, depending on the mechanism involved in the x-ray detection, can be grouped into direct and indirect flat panels. Direct detectors display a better spatial resolution due to the direct conversion of x rays into electron-hole pairs, which do not need an intermediate production of visible light. In these detectors the readout is usually achieved through arrays of thin film transistors (TFTs). However, TFT readout tends to display noise characteristics worse than those from indirect detectors. To address this problem, a novel clinical system for digital mammography has been recently marketed based on direct-conversion detector and optical readout. This unit, named AMULET and manufactured by FUJIFILM, is based on a dual layer of amorphous selenium that acts both as a converter of x rays (first layer) and as an optical switch for the readout of signals (second layer) powered by a line light source. The optical readout is expected to improve the noise characteristics of the detector. The aim is to obtain images with high resolution and low noise, thanks to the combination of optical switching technology and direct conversion with amorphous selenium. In this article, the authors present a characterization of an AMULET system. METHODS The characterization was achieved in terms of physical figures as modulation transfer function (MTF), noise power spectra (NPS), detective quantum efficiency (DQE), and contrast-detail analysis. The clinical unit was tested by exposing it to two different beams: 28 kV Mo/Mo (namely, RQA-M2) and 28 kV W/Rh (namely, W/Rh). RESULTS MTF values of the system are slightly worse than those recorded from other direct-conversion flat panels but still within the range of those from indirect flat panels: The MTF values of the AMULET system are about 45% and 15% at 5 and 8 lp/mm, respectively. On the other hand, however, AMULET NNPS results are consistently better than those from direct-conversion flat panels (up to two to three times lower) and flat panels based on scintillation phosphors. DQE results lie around 70% when RQA-M2 beams are used and approaches 80% in the case of W/Rh beams. Contrast-detail analysis, when performed by human observers on the AMULET system, results in values better than those published for other full-field digital mammography systems. CONCLUSIONS The novel clinical unit based on direct-conversion detector and optical reading presents great results in terms of both physical and psychophysical characterizations. The good spatial resolution, combined with excellent noise properties, allows the achievement of very good DQE, better than those published for clinical FFDM systems. The psychophysical analysis confirms the excellent behavior of the AMULET unit.

Application of QC_DR Software for Acceptance Testing and Routine Quality Control of Direct Digital Radiography Systems: Initial Experiences using the Italian Association of Physicist in Medicine Quality Control Protocol

Ideally, medical x-ray imaging systems should be designed to deliver maximum image quality at an acceptable radiation risk to the patient. Quality assurance procedures are employed to ensure that these standards are maintained. A quality control protocol for direct digital radiography (DDR) systems is described and discussed. Software to automatically process and analyze the required images was developed. In this paper, the initial results obtained on equipment of different DDR manufacturers were reported. The protocol was developed to highlight even small discrepancies in standard operating performance.

Physical and psychophysical characterization of a GE senographe DS clinical system

Indirect-conversion FFDM systems usually present a lower spatial resolution, with respect to the direct-conversion one. This can put serious issues in mammography, since high resolution is required. Digital software has been developed for restoring the losses in spatial resolution caused by blurring in the scintillation phosphor. GE Senographe DS system gives users the possibility of using such restoration. Basically, a filtering can be performed on the acquired images, by activating the FineView software option. In this work we present a complete characterization of a clinical system, in terms of MTF, NPS, DQE, and contrast-detail analysis. Figures of merit have been calculated on images acquired with and without the FineView software. The effects of the restoration software are investigated, both on image quality parameters, and on contrast-detail visibility. The MTF of the FFDM system is improved when FineView is activated. On the other hand, NPS presents noticeably changes, especially at high frequencies. DQE is fairly independent from the exposure, when FineView filter is not activated, whereas it presents a clear spread over the exposures, when FineView is activated. CDMAM analysis does not show significant differences between images with or without the restoration filter. Besides, the Mo/Mo beam seems to provide slightly better results than the Rh/Rh one.

Comparison of human observers and CDCOM software reading for CDMAM images

Contrast-detail analysis is one the most common way for the assessment of the performance of an imaging system. Usually, the reading of phantoms, such as CDMAM, is obtained by human observers. The main drawbacks of this practice is the presence of inter-observer variability and the great amount of time needed. However, software programs are available, for reading CDMAM images in an automatic way. In this paper we present a comparison of human and software reading of CDMAM images coming from three different FFDM clinical units. Images were acquired at different exposures in the same conditions for the three systems. Once software has completed the reading, the interpretation of the results is achieved on the same way used for the human case. CDCOM results are consistent with human analysis, if we consider figures such as COR and IQF. On the other hand, we find out some discrepancies along the CD curves obtained by human observers, with respect to those estimated by automated CDCOM analysis.

SU-GG-I-71: Acceptance and Routine Quality Control in Direct Radiography Systems: Initial Experiences with the Italian Association of Physicist in Medicine Protocol

Medical x‐ray imaging systems must be designed to guarantee that maximum image quality is obtained for an acceptable radiation risk to the patient, and quality assurance (QA) procedures are used to ensure these standards are maintained. A quality control protocol for direct digital radiography (DDR) systems is described and discussed. A software to automatically process and analyze the required images for controls was developed, too. In this poster the initial results obtained on equipments of different DDR manufacturers were reported. In order to establish a generally acceptable baseline performance of the systems, fourteen different commercially available DDRs from four different manufactures were periodically tested and their results compared in the frame of the Digital Quality Assurance Task Group of the Italian Association of Physics in Medicine (AIFM). The protocol, designed to be performed in short time (all the tests were done in a clinical environment), seem to be able to highlight discrepancies from the standard operating performances.