Zhenxue Jing

Scattered radiation in scanning slot mammography.

Monte Carlo simulations were used to quantify the amount of scattered radiation a scanning slot detector geometry designed for use in digital mammography. Ratios of the scatter to primary (S/P) x-ray photon energy absorbed in the detector were obtained for a Lucite phantom, and were investigated as a function of photon energy, phantom thickness, and slot detector width. Over a Lucite phantom thickness range of 2-6 cm, the S/P ratios range from about 0.10 to 0.17 for a 4 mm wide slot detector at the x-ray photon energies used in mammography. These ratios increased by a factor of approximately 1.8 when the slot width was increased to 10 mm. In general, 20 keV photons gave S/P ratios similar to those of a 30 kVp x-ray spectrum (Mo target + 30 microns Mo filtration). The use of a 3 cm air gap reduced the S/P ratios by a factor of between 2.5 and 3.4, depending on the phantom thickness. For a constant primary energy fluence, coherent scatter was reduced as photon energy increased, whereas Compton scatter increased with increasing photon energy. With no air gap, the contributions of coherent and Compton scatter were found to be equal at 25 keV, whereas the introduction of a 3 cm air gap resulted in equal contributions for the two scatter processes at 36 keV. A 10 mm wide slot detector consisting of a 36.7 mg/cm2 thick Gd2O2S:Tb phosphor screen was compared to an ideal detector absorbing all incident primary/scatter photons. Average differences in the S/P ratios for these two detectors were 7% with no air gap and approximately 4% with a 3 cm air gap. The results obtained in this study will assist in the design of an optimal slot detector for use in digital mammography.

Signal-to-noise ratio and radiation dose as a function of photon energy in mammography

This study investigated the signal to noise ratio (SNR) and radiation dose as a function of photon energy in screen-film mammography. An analytical expression was derived for the SNR for monoenergetic photons incident on simulated masses and microcalcifications embedded in uniform slabs of Lucite. The mean dose, D, was determined by dividing the total energy deposited in the Lucite phantom by the corresponding phantom mass. SNR and dose data for different photon energies were normalized to a constant value of energy absorbed by a 34 mg/cm2 Gd2O2S screen. A figure of merit (FOM), defined as SNR2/D, permitted the optimum photon energy to be determined for each imaging task. For microcalcifications, the optimum energy was dependent on the size of the microcalcification, and increased from 19 keV for 100 micrometer to 22 keV for 500 micrometer imaged in a 4 cm Lucite phantom. The optimal photon energy for microcalcifications was 16 - 19 keV for a 2 cm phantom, increasing to 24 - 26 keV for an 8 cm phantom. For simulated masses of all diameters (2 mm to 10 mm) and thickness (0.15 to 0.6 mm), the optimal photon energy was approximately 17 keV for a 2 cm phantom, and increased to approximately 32 keV for an 8 cm phantom.

Imaging characteristics of plastic scintillating fiber screens for mammography

A scanning slot digital mammography system using a plastic scintillating fiber screen (SFS) is currently being developed. To improve the x-ray interaction efficiency and absorption efficiency of an SFS, high Z elements can be added into the scintillating fiber core. In this paper, we investigate theoretically the zero spatial frequency detective quantum efficiency, DQE(0), and modulation transfer function, MTF(f), of three 2 cm thick SFSs made of polystyrene, polystyrene loaded with 5% by weight of lead, and polystyrene loaded with 10% by weight of tin scintillating fibers. X-ray interaction efficiency, scintillating light intensity distributions and line spread functions were generated using Monte Carlo simulation. DQE(0) and MTF(f) were computed for x-ray energies ranging from 15 to 50 keg. Loading high Z elements into the SFS markedly increased the DQE(0). For x-ray energies used for mammography, DQE(0) values of both high Z element loaded SFSs are about a factor of three higher than the DQE(0) of an Min-R screen. At mammographic x-ray energies, MTF(f) values of all three SFSs are greater than 50% at 25 lp/mm spatial frequency, and were found to be dominated by the 20 micrometer individual scintillating fiber diameter used. The results show that both high DQE(0) and spatial resolution can be achieved with the high Z element loaded SFSs, which make these SFS attractive for use in a scanning slot detector for digital mammography.

How does observer training affect imaging performance in digital mammography

Simulated mass lesions, superimposed onto an anthropomorphic breast phantom, were x-rayed using a small field of view digital mammography system. Eight radiologists and four scientists viewed the phantom images on a display monitor in a darkened room. Five readers had prior experience of reading these type of images. Readers assessed the probability of a simulated mass being present in each ROI, with the resultant data used to plot the corresponding Receiver Operating Characteristic (ROC) curves, and determine the corresponding area under the ROC curve (Az). Readers viewed the same set of images five successive times in a single session, and the time taken to read each image was recorded. The average time to complete the study for all twelve observers was 24 minutes (71 seconds/image). Experienced readers were quicker than novices, and radiologists were quicker than the scientists. The average Az value for the twelve readers for this detection task was 0.842 +/- 0.037 with coefficient of variations for individual readers ranging from 2.1% to 7.7%. Differences in imaging performance between the radiologists and scientists were very small. Analysis of the trends in measured imaging performance for each reader viewing successive (repeat) images showed that there was no improvement in imaging performance with increasing experience.

Detective quantum efficiency of a CsI:Tl scintallator-based scanning slot x-ray detector for digital mammography

An investigation was made of the key determinants of the detective quantum efficiency at zero spatial frequency [DQE(0)] of a CsI:Tl scintillator based scanning slot x- ray detector for digital mammography. The slot x-ray detector was made of a prismatic type thallium activated CsI scintillator (150 micrometer thick) optically coupled to CCDs by fiber optical image guides. Monte Carlo calculations were performed to generate the CsI:Tl scintillator Swank factor on the basis of the energy deposition from pencil beam x-ray sources and light transmission within the CsI:Tl scintillator. A theoretical expression for the detector DQE(0) was obtained which was used to investigate the detector imaging performance as a function of x-ray energy, x-ray exposure, CCD electronic noise level, and optical coupling efficiency of the fiber optic image guide. The Swank factor of the CsI:Tl scintillator was close to unity at x-ray energies below Iodine K-edge (33.2 keV), but decreased to approximately 0.8 at higher x-ray energies up to 40 keV. DQE(0) of the slot x-ray detector was approximately 75% at 15 keV but decreased to approximately 40% at 30 keV. Optimum DQE(0) performance of the slot x-ray detector was generally obtained at a detector x-ray exposure level above approximately 5 to 10 mR and an electronic noise level below approximately 50 electrons rms. A drop in the optical coupling efficiency of the image guide from 1.0 to 0.3 reduced the detector DQE(0) from approximately 75% to approximately 55% in the mammography x-ray energy range. The key finding in this study is that the choice of the x-ray energy has a major impact on the DQE(0) of a CsI:Tl scintillator based slot x-ray detector. Since the x-ray photon energy also affects x-ray tube loading, mean glandular dose and subject contrast, the choices of optimal x-ray spectra from current mammography x-ray tubes require further investigation.

Novel dual screen-dual film combination for mammography

A new dual screen-dual film mammography combination was constructed which made of two phosphor screens and two films loaded into a single x-ray cassette. The screens and films were combined so that a single emulsion film (Film #1, Kodak Min-R E film) was placed in direct contact with the phosphor side of the first screen (Screen #1). Screen #1 was made of the Kodak Min-R phosphor (34.0 mg/cm2 Gd2O2S:Tb) coated on a 4 mil transparent Mylar backing. A double emulsion film (Film #2, Kodak T-Mat G film) was sandwiched between Screen #1 backing and the phosphor side of the second screen (Screen #2). Screen #2 was a Kodak Insight ME screen that has a Gd2O2S:Tb coating weight of 43.1 mg/cm2 and a reflective coating between its phosphor layer and support layer. The relative sensitometric responses and spatial resolution properties of the two films were measured as a function of x-ray tube potential (kVp). A series of a contrast-detail phantom images was obtained by varying the x- ray exposure level at 28 kVp. An observer performance study was subsequently carried out to investigate the low contrast performance using the dual screen-dual film combination. The effective speeds of the two films differed by a factor of 2.12 to 2.67 between 25 to 30 kVp. Film #2 contrast was a factor of two or greater than Film #1 in the range where Film #1 optical density values were under 0.7. Measured MTF curves from digitized edge images showed that Film #1 MTF performance was comparable to a standard Kodak Min-R screen-Min-R E film combination. The limiting spatial resolution was found to be 19.5 lp/mm for Film #1 and approximately 11 lp/mm for Film #2. Observer performance studies showed that the threshold contrast in detecting small (less than 10 mm) breast lesions could be up to a factor of two lower on Film #2 images when regions of interest are underexposed on Film #1 images.

Computed radiography acceptance testing

Compact sized computed radiography (CR) systems based on photostimulable phosphor technology are now being more widely used in radiology departments. Measurement of imaging performance at time of installation is essential to ensure that the CR system is operating within the manufacturers specifications and producing a clinically acceptable image quality. At any given radiation dose, CR imaging performance primarily relates to image contrast, spatial resolution, and noise. Tests for these key aspects of CR performance are described and typical results, as obtained with a commercial system, are presented.

High-resolution digital x-ray imaging system based on the scintillating plastic microfiber technology

The design and construction of a prototype high resolution digital x-ray imaging system, employing a novel scintillating microfiber detector as the x-ray imaging sensor is described. The fiber detector has an active imaging area of 5 cm X 5 cm and thickness of 1 cm. The optical image formed in the fiber detector is read out by a high resolution image intensifier- CCD camera system. Investigations on the image quality of the prototype x-ray system are reported. MTF measurement shows >= 10 line pairs/mm spatial resolution of the scintillating fiber detector over the diagnostic x-ray energy range. The feasibility of the medical diagnostic application of the scintillating fiber detector is analyzed, and the new sensor is shown to be adaptable to imaging tasks where both high resolution and high detector sensitivity are required.

Significance of exposure data recognizer modes in computed radiography

To take advantage of the wide latitude of computed radiography (CR), the AC series manufactured by Fuji uses an exposure data recognizer (EDR) system which may be operated in one of three modes: automatic, semi-automatic (semi auto), and fixed. This study evaluated the performance of these EDR modes in comparison with a conventional screen-film combination. Multiple images of foot and skull phantoms were obtained and measurements made of the resultant film density, density variability and contrast. The auto mode could correct for inaccurate exposures but introduced additional variability to CR film density. Semi auto mode had improved consistency but could result in suboptimal data processing parameters. For constant exposures, fixed mode had a consistency comparable to screen-film combinations but could not compensate for radiation dose variations. Selection of an optimal mode of operation requires an understanding of how the EDR affects CR performance and depends on the clinical problem at hand.

Performance of a CsI:TL-screen-based scanning slot detector for digital mammography

Thallium activated CsI scintillation screen (CsI:Tl) has advantages over rare-earth phosphor screens when used in a scanning slot x-ray detector for digital mammography. The scintillation decay time for CsI:Tl is only approximately 1 microsecond(s) which eliminates the afterglow effect associated with the use of rare-earth phosphor. The CsI needles serve to limit the spread of the scintillation light which permits the use of a relatively thick CsI:Tl screen to improve detector x-ray interaction efficiency without sacrificing resolution performance. A prototype scanning slot detector was made of a CsI:Tl screen and was optically coupled to two CCDs by plastic optical fiber image guides. The screen was composed of prismatic CsI crystals with a needle size of approximately 5 micrometers . Image guides used in the detector had an input to output ratio of 1:1, and were made of 10 micrometers diameter plastic optical fibers. Each CCD, operated in the time-delayed integration mode, was an 1100 X 330 pixel array with pixel size of 24 micrometers . A full scale scanning slot detector will contain eight rather than two such modules. The x-ray interaction efficiency of the slot x-ray detector was calculated to be approximately 84 percent at 20 keV for the CsI:Tl screen, and was approximately 20 percent greater than that of a 31.7 mg/cm2 thick Gd2O2S:Tb phosphor screen. Limiting spatial resolution was investigated by taking the images of a 1 degree star resolution test pattern as a function of detector scan speed. Limiting spatial resolution was approximately 15 1p/mm at a scan speed of 4 cm/s and improved only slightly to approximately 16 1p/mm as the scanning speed decreased to 1 cm/s.