Biomedical subjects

J M Sabol

Publications and source records attributed to J M Sabol.

9 recordsLinked to original sources

PubMed1999

A Monte Carlo study of x-ray fluorescence in x-ray detectors.

Advances in digital x-ray detector systems have led to a renewed interest in the performance of x-ray phosphors and other detector materials. Indirect flat panel x-ray detector and charged coupled device (CCD) systems require a more technologically challenging geometry, whereby the x-ray beam is incident on the front side of the scintillator, and the light produced must diffuse to the back surface of the screen to reach the photoreceptor. Direct detector systems based on selenium have also enjoyed a growing interest, both commercially and academically. Monte Carlo simulation techniques were used to study the x-ray scattering (Rayleigh and Compton) and the more prevalent x-ray fluorescence properties of seven different x-ray detector materials, Gd2O2S, CsI, Se, BaFBr, YTaO4, CaWO4, and ThO2. The redistribution of x-ray energy, back towards the x-ray source, in a forward direction through the detector, and lateral reabsorption in the detector was computed under monoenergetic conditions (1 keV to 130 keV by 1 keV intervals) with five detector thicknesses, 30, 60, 90, 120, and 150 mg/cm2 (Se was studied from 30 to 1000 mg/cm2). The radial distribution (related to the point spread function) of reabsorbed x-ray energy was also determined. Representative results are as follows: At 55 keV, more (31.3%) of the incident x-ray energy escaped from a 90 mg/cm2Gd2O2S detector than was absorbed (27.9%). Approximately 1% of the total absorbed energy was reabsorbed greater than 0.5 mm from the primary interaction, for 90 mg/cm2 CsI exposed at 100 kVp. The ratio of reabsorbed secondary (fluorescence + scatter) radiation to the primary radiation absorbed in the detectors (90 mg/cm2) (S/P) was determined as 10%, 16%, 2%, 12%, 3%, 3%, and 0.3% for a 100 kVp tungsten anode x-ray spectrum, for the Gd2O2S, CsI, Se, BaFBr, YTaO4, CaWO4, and ThO2 detectors, respectively. The results indicate significant x-ray fluorescent escape and reabsorption in common x-ray detectors. These findings suggest that x-ray fluorescent radiation redistribution should be considered in the design of digital x-ray imaging systems.

Biophysical Phenomena↗

PubMed1997

Scintillating fiber optic screens: a comparison of MTF, light conversion efficiency, and emission angle with Gd2O2S:Tb screens.

The widespread effort in developing digital imaging systems has led to large area high pixel density photodetectors such as charge coupled devices (CCDs), amorphous silicon photodiode arrays, and complementary metal oxide semiconductor (CMOS) imagers. These photodetectors have different capabilities, characteristics, and requirements than conventional silver-halide-based film, and this fact had led to a new generation of exotic scintillators, including fiber optic screens made from scintillating glass. The scintillator performance characteristics of five different scintillating fiber optic screens and two conventional Gd2O2S:Tb screens (one 34 mg/cm2 and the other 60 mg/cm2) were measured and compared. The measurements that were made included the angular dependence of light emission relative to the normal, the modulation transfer function (MTF), and the absolute effective conversion efficiency (light photons per absorbed x-ray photon). It was found that the light emission of scintillating fiber optic screens is markedly forward peaked (depending on the sample) compared to conventional screens or Lambertian emitters. The MTFs of the five scintillating fiber optic screens measured were comparable and fell approximately midway between the two conventional screen MTFs. One of the scintillating fiber optic screens demonstrated light efficiency similar to the thick (60 mg/cm2) conventional screen, another had light output capabilities similar to the thin (34 mg/cm2) conventional screen, and the three others were less efficient than the thin screen. The non-Lambertian characteristics of the fiber optic scintillators will cause errors of up to 75% in lens efficiency calculations if a Lambertian source is assumed. The conventional screens were found to conform within about 5% of an ideal Lambertian emitter.

Biophysical Phenomena↗

PubMed1996

Analytical description of the high and low contrast behavior of a scan-rotate geometry for equalization mammography.

Many solutions have been proposed to overcome the problem of imaging the radiographically dense breast with high contrast mammographic film of limited dynamic range. In previous works, we have proposed utilizing a modulated fan-beam in a scan-rotate geometry RSER (Rotary Scanning Equalization Radiography), as an efficient method for producing exposure equalized mammograms. The image quality of RSER is similar to that attained with the inefficient single beam, raster scanning SER (Scanning Equalization Radiography) geometry. RSER has the potential to be a practical, efficient method for improving the detection of cancer in the dense breast. In this work, we present a theoretical analysis of the imaging properties of the RSER geometry in two regimes defined by the variation of x-ray transmission within the object. For low contrast objects, the imaging geometry was analyzed as a linear system, whereas the high contrast regime was studied by determining the contrast limit at which RSER requires nonphysical (negative) exposure modulation for a breast-like object. The low contrast transfer function of the RSER system is shown to be very similar to that of the SER geometry. We show that RSER enables the use of wide scanning beam of approximately 4 cm and thereby significantly reduces x-ray tube heat loading. Analysis of the high contrast behavior shows that a wide range of object contrasts and sizes can be equalized. For example, RSER can equalize a region of 100% glandular tissue within a 4.0 cm thick compressed breast composed of 100% adipose tissue. Thus, the RSER geometry produces images very similar to the more inefficient SER geometry, and is able to produce entrance exposure distributions appropriate for equalization of the range of contrast typically encountered in mammography.

Biometry↗

PubMed1996

Practical application of a scan-rotate equalization geometry to mammography.

The presence of dense fibroglandular tissue within the breast is the most significant cause of failure to detect breast cancer with mammography. The dense tissue often produces a range of exposure which exceeds the useful dynamic range of film-screen mammography. It has been shown that equalization radiography overcomes the latitude limitations of film-screen imaging. Equalization compensates for regional variations in x-ray transmission within the patient through spatial modulation of the entrance exposure. We have proposed rotary scanning equalization radiography (RSER), a scan-rotate geometry for efficient equalization radiography. In RSER the image receptor is exposed by repeated scans of a source-modulated fan beam. The fan beam is rotated with respect to the patient between scans. Numerical simulations and theoretical analysis have shown that the superposition of exposure from appropriately modulated fan beams at a variety of angles is an entrance exposure that effectively equalizes the film exposure. The design and characteristics of a prototype RSER imaging system are described. Anthropomorphic breast phantom images are used to determine the improvement in image contrast obtained with RSER, the expected tube loading, and the presence of artifacts. RSER increases the fraction of the breast imaged with high contrast (at least 90% of peak gradient) from 46% (conventional mammography) to 80%. Subjective examination of the phantom images show that RSER achieves image quality very similar to that of much less efficient equalization geometries with only 2.7 times greater tube loading than conventional mammography. As predicted by theoretical analysis of exposure artifacts in RSER, the prototype RSER system is relatively immune to artifacts. Exposure artifacts were demonstrated for extreme variations in x-ray transmission within the patient. These results show that RSER is an efficient, practical means of overcoming the latitude limitations of film-screen mammography, and improving the detection of breast cancer.

Biophysical Phenomena↗

PubMed1995

Role of equalisation mammography of dense breasts.

Parenchymal patterns characteristic of dense breasts are known to degrade the mammographic detection of small breast cancers and microcalcifications. This arises from large variations in exposure of the film, resulting in reduced image contrast over areas of suboptimal exposure. Based on sensitometric measurements of mammograms from a typical patient population, it is shown that over 60% of a typical mammogram in Wolfe's DY classification was found to be exposed suboptimally, suggesting a significant margin for improving mammography for these patients. In order to address this problem, a prototype mammographic version of scanning equalisation radiography (MSER) has been developed, which delivers a patient-specific spatially non-uniform distribution of breast exposure, adjusted to maintain optimal film exposure and contrast over the entire mammogram. Anthropomorphic phantom MSER images show a marked improvement in subjective image quality relative to conventional mammograms, while exhibiting a similar radiation risk. The detection of small microcalcifications and fibrils over clinically significant breast densities is found to be improved by factors eight and four, respectively. Such a system may be clinically practical through the use of multiple-beam equalisation methods with available X-ray tube technology.

Breast Neoplasms↗

PubMed1995

A method for practical equalization mammography of the radiographically dense breast.

It has been shown that equalization radiography can overcome the well-known problem of limited film latitude encountered in mammography of the radiographically dense breast. Current equalization geometries based on single scanning beam (SER) or multiple-beam techniques approach the heat-loading limits of mammographic x-ray sources and require excessively long scan times. The authors have proposed an alternative geometry for equalization mammography, rotary scanning equalization radiography (RSER), which uses a slot beam in a translate-rotate geometry. RSER provides the simplicity of a single-beam geometry while offering improved tube efficiency over multiple-beam geometries. Numerical simulations and a prototype imaging system are used to show that equalized mammograms exhibiting high contrast throughout the breast can be obtained with a large scanning beam translated over the image at only four scanning angles. These results indicate that RSER is an efficient, simple, and practical means of imaging the dense breast.

Female↗

PubMed1994

Rotary scanning equalization radiography: an efficient geometry for equalization mammography.

The detection of cancer in the radiographically dense breast is problematic, since the breast will produce a range of exposure that exceeds the useful dynamic range of high contrast film-screen combinations. It has been shown previously that mammographic scanning equalization radiography (MSER) can be used to overcome the latitude limitations of film-screen mammography. However, the tube loading of MSER is orders of magnitude greater than conventional mammography. A new rotary geometry for equalization radiography is proposed, in which the image receptor is exposed by repeated scans of a modulated slot beam, oriented at a variety of scanning angles with respect to the object. The superposition of the exposure from appropriately modulated, rotated slot beams produces an entrance exposure that will effectively equalize the film exposure. The principle advantages of this geometry is its simplicity and reduced tube loading. To determine the effectiveness and feasibility of RSER the effect of conventional, MSER, and RSER have been numerically simulated on the appearance of clinical mammograms, the relative heat loading, and the fraction of the breast imaged with high contrast are calculated. It is found that RSER produces images that are free of artefacts, and exhibit a similar degree of equalization, as found in MSER images. RSER accomplishes this with only four scanning angles, and a beam that is approximately 4 cm wide. The resulting tube loading is only three times greater than that found in conventional imaging. Numerical simulations indicate that RSER is a simple, feasible means of overcoming the latitude limitations of film-screen mammography.

Computer Simulation↗

PubMedDate not supplied

Mammographic scanning equalization radiography.

It is well recognized that variations in breast thickness and parenchymal composition can produce a range of exposure which exceeds the latitude of high contrast mammographic film/screen combinations. Optimal imaging of the dense breast is desired since 30%-60% of women present with dense breasts, and they are believed to be at the highest relative risk of developing breast cancer. The application of scanning equalization radiography to mammography has been investigated through the construction and characterization of a prototype mammographic scanning equalization radiography (MSER) system, designed to image mammographic phantoms. The MSER system exposes a Min-R/MRH cassette by raster scanning a 2.0 x 1.6 cm beam of pulsed x-rays across the cassette. A scanning detector behind the cassette measures the local x-ray transmission of the breast. Feedback of the transmission information is used to modulate the duration of each x-ray pulse, to equalize the film exposure. The effective dynamic range of the MSER system is 25 times greater than that of conventional mammography. Artifact-free images of mammographic phantoms show that MSER effectively overcomes the latitude limitations of film/screen mammography, enabling high contrast imaging over a wide range of object x-ray transmission. Anthropomorphic phantom images show that MSER offers up to a sixfold increase in film contrast in the normally underexposed regions of conventional mammograms. Characterization of the entrance exposure shows that there is not a significant difference in exposure between MSER and conventional mammographic techniques, suggesting that both would pose comparable risk to the patient. Calculations show that the construction of a clinical multiple beam MSER system is feasible with minor changes to existing technology.

Adipose Tissue↗

PubMedDate not supplied

Observer performance and dose efficiency of mammographic scanning equalization radiography.

The detection of fibrils, microcalcifications, and low contrast lesions is vital to the detection of early breast cancer. It has been shown through sensitometric measures and anthropomorphic phantom images that mammographic scanning equalization radiography (MSER) overcomes the latitude limitations of conventional mammographic techniques. MSER increases image quality by regional modulation of the entrance exposure to suit the local variations in x-ray transmission within the patient. In order to assess the effect of equalization on the detection of breast lesions, we have compared observer performance in MSER and conventional imaging techniques. The observation tasks were the threshold visualization of fibrils, microcalcifications, and low contrast discs (simulating lesions), located on a uniform background. The performance of the observers was determined for a range of background x-ray transmission simulating the range of transmission generated by variations in breast composition and thickness. For the conventional images, the threshold visible diameter of the fibrils, microcalcifications, and low contrast discs, increased as the x-ray transmission of the phantom changed from that for which the film was optimally exposed. For the MSER images, the performance of the observers was almost independent of the background transmission of the object since MSER ensures that the film is optimally exposed for a large range of object transmission. Even with significant changes in object x-ray transmission, only minor changes in fibril, microcalcification, and disc detection were observed. Utilizing the results of the contrast-detail experiment, a dose efficiency comparison of conventional and MSER imaging techniques was performed. The dose efficiency analysis showed that MSER varied the incident exposure so as to maintain consistent performance of the observer, over the entire breast. These results suggest that MSER would improve the ability of radiologists to detect early breast cancer in women presenting with mammographically dense breasts, in a very dose efficient manner.

Biophysical Phenomena↗