Biomedical subjects

P C Johns

Publications and source records attributed to P C Johns.

13 recordsLinked to original sources

PubMed2001

X-ray forward-scatter imaging: experimental validation of model.

In our research program we have investigated, through modeling and related numerical calculations, the potential use of scattered photons for medical x-ray imaging. In this work, we present an experimental validation of the primary and of the forward-scatter x-ray imaging models. Incident polyenergetic photon beams generated from a conventional rotating anode x-ray tube were used. To compare quantitatively the results between primary and forward-scatter imaging, an ionization chamber was used to record the incident air collision kerma, Kair(c). Plots of contrast (C) and the signal-to-noise ratio (SNR) as a function of the imaging task are presented. We have chosen to make measurements with plastics [polymethyl methacrylate (PMMA), polycarbonate, polystyrene, polyethylene, and nylon] placed at the center of a 15 cm diam spherical water phantom. Good agreement between experiment (expt) and prediction (pred) was obtained for many imaging tasks. For example, to image a 2 cm thick PMMA/polycarbonate combination using an 80 kV beam with the primary photons we obtain Cexpt = 0.01 +/- 0.02, Cpred = 0.008 +/- 0.002, SNRexp/square root Kair(c) = 0.86 +/- 1.6(mJ/kg)(-1/2) and SNRpred/square root K(air)c = 0.51 +/- 0.14(mJ/kg)(-1/2). The values obtained by using the theta = 4 degrees scattered field were Cexpt = 0.26 +/- 0.06, Cpred = 0.19 +/- 0.01, SNRexp/square root Kair(c) = 3.8 +/- 0.8(mJ/ kg)(-1/2), and SNRpred/square root K(air(c) = 3.2 +/- 0.3 (mJ/kg)(-1/2) We have, however, shown that using form factor data from different authors can have a significant effect on the predicted values of C and SNR. The use of our semianalytic expressions for the numbers of transmitted and scattered photons combined with our experimental measurements allowed us to quantify the amount of water contamination in our measurements. Some preliminary results in air with biological materials (liver, muscle, water) are also presented. We are confident that our model can be used as a tool for designing and optimizing an x-ray scatter imaging system.

Animals↗

PubMed1999

Photon-counting radiography with the gas microstrip detector.

We have built a proof of-concept photon-counting x-ray imaging system using a Xe:CH4 gas microstrip detector (GMD) as the image receptor, and have used this system to demonstrate several advantages of photon counting over energy integration. Our experiments spanned x-ray spectra from 10 to 50 kVp and Xe:CH4 pressures from 1 to 4 atm. When photon counting is done, the energy deposited in the detector by each incident photon can be measured on adjacent anode strips and a centroid calculation can be used to provide spatial resolution significantly better than the anode strip pitch. We measured > 11 lp mm(-1) at 13 kVp with our 200 microm pitch detector, and 8.2 lp mm(-1) at 50 kVp. The energy resolution of our GMD is 5.2% at 59.6 keV, and the space-charge limited counting rate is >2 x 10(6) mm(-2) s(-1) at 3 atm for a 30 kVp beam.

Biophysical Phenomena↗

PubMed1999

Analysis of spectral blur effects in x-ray scatter imaging.

Previous analysis in our research program investigating the potential use of scattered photons for medical x-ray imaging has been for monoenergetic beams. In practice, polyenergetic beams are almost always used due to their higher photon fluence rate. The effects of beam polychromaticity on x-ray scatter imaging are determined with the aid of our semianalytic model that images a target object against a background material of the same dimensions when both are situated within a water phantom. Our analysis involves four different photon beams with constant incident energy fluence: (1) a monoenergetic beam with photon energy E0, (2) a dual peak beam with two separate monoenergetic peaks of energies E1 and E2, (3) a clinical x-ray beam, and (4) a rectangular beam with uniform energy fluence between energies Emin and Emax. A comparison between the polyenergetic spectra is accomplished by matching the centroids and standard deviations of the dual peak and rectangular spectra to those of the clinical x-ray spectrum. For the task of imaging liver versus fat structures 1 cm thick in a 25-cm-diam spherical water phantom with the scattered photons between 2 degrees and 12 degrees, the predicted signal-to-noise ratio (SNR) obtained with a 100 kV beam is 87.5% of the SNR acquired with the optimum monoenergetic beam (SNRopt). The SNR for the corresponding dual peak beam is 84.4% of SNRopt and for the rectangular beam is 86.3%. Our analysis shows that monoenergetic x-ray beams are not necessary for x-ray scatter imaging.

Biophysical Phenomena↗

PubMed1998

A semianalytic model to investigate the potential applications of x-ray scatter imaging.

Although x-ray scatter is generally regarded as a nuisance that reduces radiographic contrast (C) and the signal-to-noise ratio (SNR) in conventional images, many technologies have been devised to extract useful information from the scattered x rays. A systematic approach, however, for analyzing the potential applications of x-ray scatter imaging has been lacking. Therefore, we have formulated a simple but useful semianalytic model to investigate C and SNR in scatter images. Our model considers the imaging of a target object against a background material of the same dimensions when both are situated within a water phantom. We have selected biological materials (liver, fat, bone, muscle, blood, and brain matter) for which intermolecular form factors for coherent scattering were available. Analytic relationships between C and SNR were derived, and evaluated numerically as the target object thickness (0.01-40 mm) and photon energy (10-200 keV) were systematically varied. The fundamental limits of scatter imaging were assessed via calculations that assumed that all first-order scatter exiting the phantom, over 4 pi steradians, formed the signal. Calculations for a restricted detector solid angle were then performed. For the task of imaging white brain matter versus blood in a 15 cm thick water phantom, the maximum SNR, over all energies, for images based on the detection of all forward scatter within the angular range 2 degrees-12 degrees is greater than that of primary images for target object thicknesses < or = 23 mm. Use of the backscattered x rays within the range 158 degrees-178 degrees to image objects 3 cm below the surface of a 25 cm thick water phantom allows the liver to be distinguished from fat with a SNR superior to that of primary imaging when the objects are < or = 22 mm thick. Our analysis confirms the usefulness of scattered x rays, and provides simple methods for determining the regimes of medical interest in which x-ray scatter imaging could outperform conventional imaging.

Biophysical Phenomena↗

PubMed1994

Incorporation of scattered radiation into dual-energy radiologic theory and application to mammography.

A previous analysis of dual-energy imaging is extended to incorporate scattered radiation. The analysis is general and can include polyenergetic beams and nonideal detectors. In the dual-material basis plane, scatter generates a displacement of the vectors representing the imaged materials. This changes the target versus background signal difference and the projection angle for cancellation of background contrast due to varying amounts of two materials. Complete cancellation is not possible, because vectors representing various mixtures of the two background constituents are displaced differently and thus no longer lie on a straight line in the dual-material plane. The effect can be minimized by allowing a typical amount of scatter to reach the image receptor when determining the parameters of the transformation from log attenuation to equivalent materials thicknesses. The analysis has been applied to dual-energy mammography. The presence of scatter has minimal effect on the optimum beam energies with an ideal detector, which are about 20 and 70 keV.

Female↗

PubMed1993

Radiation risk to patients from percutaneous transluminal coronary angioplasty.

OBJECTIVES: This retrospective study sought to estimate patient radiation exposure during percutaneous transluminal coronary angioplasty, the corresponding organ doses and the resulting cancer mortality risk. Patient demographic data were also examined. BACKGROUND: Coronary angioplasty is commonly used as an intervention for coronary atherosclerosis, and repeated application in the same patient is now common. The combined use of fluoroscopy and cineradiography in this complicated, delicate and, hence, lengthy procedure induced us to investigate the patient radiation exposures and resulting risks. METHODS: All complete records for angioplasty procedures performed over a 3-year period were entered into a data base. The data comprised 1,893 procedures performed in a total of 1,503 patients, of whom 21% had two or more procedures in the 3-year period. Fluoroscopy time was converted to entrance exposures, assuming a rate of 520 muC kg-1 min-1 (2.0 R min-1). Cineradiographic film lengths were determined for a smaller number of procedures (200) and converted to exposures at 7.7 muC kg-1 frame-1 (30 mR frame-1). In addition, fluoroscopy and cineradiographic times and, hence, exposures for 91 diagnostic angiograms performed in these patients were obtained. Exposures were converted to organ doses using the Monte Carlo results of the Rosenstein group and then to cancer mortality risks using the latest rates of the International Commission on Radiological Protection. RESULTS: The mean age was 56.0 years; men constituted 77.5% of the patients. Radiation doses varied considerably owing to a large spread in exposure times (e.g., fluoroscopy time per angioplasty case averaged 19 min but for some cases exceeded 1 h). The average patient skin entrance exposure per angioplasty procedure was 32.0 mC kg-1 (124 R), of which 69.7% was from cineradiography. The resulting cancer mortality risk per angioplasty procedure is approximately 8 x 10(-4). CONCLUSIONS: The skin exposures estimated for angioplasty are on average higher than for other X-ray procedures. The cancer mortality risk does not exceed the mortality risk of bypass surgery. Good professional practice requires maximization of the benefit/risk ratio through quality assurance in all aspects of the procedure.

Adult↗

PubMed1993

Relation of vertebral bone screw axial pullout strength to quantitative computed tomographic trabecular bone mineral content.

Noninvasive prediction of the maximum axial load that a spinal bone screw will be able to withstand after anterior surgical placement would be highly useful. To investigate if this is feasible, we first performed preliminary experiments to distinguish the trabecular and cortical contributions to overall stiffness; the trabecular component was found to dominate. We then used a commercial computed tomography bone mineral package to determine the mineral density of the trabecular region of 41 porcine vertebrae in terms of equivalent K2HPO4 concentration; values ranged from 104 to 343 mg/cm3. A 6.5-mm diameter cancellous bone screw was then inserted laterally in each vertebra, and the ultimate tensile strength (UTS) of the screw/bone interface was measured using a tensile testing machine. The UTS values ranged from 589 to 2,620 Newtons. A superlinear relation was found between UTS and the projected K2HPO4 concentration in the direction of the screw axis, expressed in units of mg/cm2.

Animals↗

PubMed1987

X-ray characterisation of normal and neoplastic breast tissues.

Normal and neoplastic breast tissues have been characterised in terms of x-ray attenuation. Samples of normal fat and fibrous tissue were obtained from reduction mammoplasty and autopsy, and infiltrating duct carcinoma specimens from mastectomy and lumpectomy. A high-purity germanium spectroscopy system and a beam of 120 kV constant potential x-rays were used to determine the linear attenuation coefficient from 18 to 110 keV. Densities were determined from buoyancy measurements and were used to obtain mass attenuation coefficients. Infiltrating duct carcinomas and fat are well distinguished by x-ray attenuation. For photon energies used for film-screen mammography, infiltrating duct carcinomas are more attenuating than fibrous tissue. Above 31 keV, the ranges of attenuation of the two tissue types overlap. The attenuation coefficients of tissues have been concisely represented by equivalent thicknesses of lucite and aluminium. Analysis based on the average attenuation properties of tissues indicates that dual-energy mammography, using an ideal imaging system, would require 0.06 cGy to provide images in which 1 cm infiltrating duct carcinomas are displayed with a signal to noise ratio of 5 against a background over which the fat/fibrous contrast has been suppressed. This dose is similar to that currently used in conventional film-screen mammography.

Adenofibroma↗

PubMed1986

Anthropomorphic radiologic phantoms.

A technique is being developed for the design and fabrication of anthropomorphic phantoms for diagnostic x-ray imaging. Anatomic information extracted from actual patient radiographs is incorporated into the phantoms using computer image processing and computer-assisted machining methods. In this paper, the technique is described as applied to a breast phantom, and preliminary images that closely mimic human anatomy on radiographs are shown.

Automation↗

PubMedDate not supplied

Scattered radiation in fan beam imaging systems.

Scatter-to-primary energy fluence ratios (S/P) have been studied for fan x-ray beams as used in CT scanners and slit projection radiography systems. The dependence of S/P on phantom diameter, distance from phantom to image receptor, and kilovoltage is presented. An empirical equation is given that predicts S/P over a wide range of fan beam imaging configurations. For CT body scans on a 4th-generation machine, S/P is approximately 5%. Scattered radiation can produce a significant cupping artefact in CT images which is similar to that due to beam hardening. When multiple slices are used in scanned slit radiography, they can be arranged such that the increase in S/P is negligible. Calculations of scatter-to-primary ratios for first order scattering showed that for fan beams the contribution of coherent scatter is comparable to or greater than that of incoherent first scatter.

Radiography↗

PubMedDate not supplied

Coherent scatter in diagnostic radiology.

Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation. Single-scatter calculations, however, show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast, and that they account for a significant fraction of the total scattered energy fluence at the image receptor. Grids and large air gaps are less effective in reducing coherent single scatter than incoherent and multiple scatter. For radiography of the abdomen, coherent first scatter comprises 10% of total scatter and 26% of the primary fluence before a grid, and on the order of 22% and 7.5%, respectively, behind a grid. Coherent first scatter comprises a higher fraction of the total amount of scatter for lower energy examinations such as mammography.

Mathematics↗

PubMedDate not supplied

Theoretical optimization of dual-energy x-ray imaging with application to mammography.

Detection of a target object in a radiological image is often impeded by an obscuring background "clutter" resulting from the contrast between various materials in the neighborhood of the target. Dual-energy techniques can reduce or remove this clutter. In order for the target to be detectable in the image after dual-energy processing, the signal-to-noise ratio (SNR), defined as the difference between the target and the background divided by the photon noise in the difference, must exceed some threshold. A given SNR may be obtained for a wide range of the energies of the two x-ray beams and the ratio of their fluences. A theoretical model is developed which permits the choice of beams to be optimized with respect to some critical parameter--in this case, patient dose. The analysis is applied to the detection of calcifications in mammography. For an ideal imaging system, we predict that the optimum beam energies are 19 and 68 keV. A dose of 0.42 cGy is required to obtain an SNR of 5 for detection of a 0.02-cm cubic calcification in the resulting clutter-free image. This can be reduced to 0.16 cGy if the higher energy image is smoothed, prior to dual-energy processing, such that its variance is reduced to one-fourth of its unsmoothed value.

Female↗

PubMedDate not supplied

Dual-energy mammography: initial experimental results.

Dual-energy x-ray techniques may be able to enhance the detectability of calcifications in mammographic examinations by removing the background "clutter" caused by contrast between adipose and glandular tissue. This hypothesis is examined experimentally by implementation of dual-energy imaging on a prototype digital scanned projection radiography system developed in our laboratory. A model of the propagation of signal and noise in dual-energy processing for a given radiation dose is validated by measurements from phantom images. The experimental imaging system has low spatial resolution and cannot be operated at dose-optimum energies; however, since both the single- and dual-energy images are subject to the same technical limitations, a comparison of such images allows an assessment of the benefits of dual energy. Experimental images of breast tissue specimens, showing improved detectability of calcifications when obscuring background clutter is removed, are presented. The dose required for a given signal-to-noise ratio can be reduced by smoothing the higher energy image prior to dual-energy processing. For practical implementation, it is reasonable to smooth the higher energy image such that its variance is reduced fourfold.

Computers↗