Biomedical subjects

K Doi

Publications and source records attributed to K Doi.

At least 937 records · Page 52Linked to original sources

PubMedDate not supplied

Experimental and theoretical energy and angular dependencies of scattered radiation in the mammography energy range.

Predicted scatter characteristics from a Monte Carlo code have been compared to an experiment for a 4.0-cm-thick acrylic resin (Plexiglas) phantom and 27.3-keV radiation. The results show generally good agreement, except that in the scattered spectra the predictions appear to underestimate the proportion of photons in the low-energy tail of the scattered radiation. The angular dependency of the energy-integrated scattered radiation fluence is in excellent agreement with the predictions.

Mammography↗

PubMedDate not supplied

Studies of x-ray energy absorption and quantum noise properties of x-ray screens by use of Monte Carlo simulation.

The imaging properties of the phosphor layer in fluorescent screens or image intensifiers are related to its x-ray absorption characteristics. In this study, we applied Monte Carlo methods for the simulation of x-ray photon diffusion in a phosphor layer. The K-reabsorption factor, absorbed x-ray energy, quantum absorption efficiency, statistical factor, and noise-equivalent absorption were determined as a function of the incident energy and angle of the x rays for eight commonly used phosphor layers. These basic physical quantities will be useful for the prediction of the information transfer properties of a phosphor layer.

Elementary Particles↗

PubMedDate not supplied

Radiation dose in diagnostic radiology: Monte Carlo simulation studies.

We applied Monte Carlo calculations to determine the radiation dose absorbed in water phantoms. Monoenergetic incident x-ray beams with energies from 15 to 100 keV and phantom thicknesses from 5 to 20 cm were considered in this study. We calculated the spatial distributions of energy absorption in the phantom, the rad/R conversion factors, the average rad/R conversion factors, and the scatter-to-primary ratios of absorbed dose. We also compared the relative absorbed doses under various imaging conditions when the transmitted radiation produced a given optical density on radiographic film. The information provided will be useful for the estimation of radiation doses in various radiographic procedures.

Humans↗

PubMedDate not supplied

Dual-film cassette technique for studying the effect of radiographic image quality on diagnostic accuracy.

In order to investigate the relationship between diagnostic accuracy and radiographic image quality, we have developed a dual-film cassette which produces two radiographs simultaneously with a single exposure. One of these radiographs is of standard quality; the second is of lower quality because of degraded spatial resolution and a significantly lower exposure. We have studied the basic physical properties of the standard and low-dose screen-film systems which we use in the dual-film cassette by comparing their beam hardening, scatter fractions, contrasts, modulation transfer functions (MTFs), and Wiener spectra. The x-ray spectrum incident on the low-dose system contained more high-energy photons than that incident on the standard system, and the scatter fractions for the low-dose system were slightly less than or comparable to those for the standard system. While the radiographic contrast produced by the two systems were generally comparable, the standard system had a slightly higher contrast than the low-dose system in some cases. The MTF of the low-dose system was considerably lower than that of the standard system, and the low-dose system had a noise level considerably greater than did the standard system. Phantom images and clinical radiographs indicated that, for the low-dose system, the image quality degraded significantly.

Humans↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 2. Noise Wiener spectrum.

We investigated theoretically the effects of various digital parameters, such as sampling aperture, sampling distance, number of quantization levels, and display aperture, on the noise Wiener spectrum of digital radiographic imaging systems. We also measured Wiener spectra for our digital image simulation/processing system, and the results agreed well with the theoretical predictions. Aliasing, which is an artifact caused by undersampling, and the use of a limited number of quantization levels were found to increase the Wiener spectrum for digital systems. The effects of image processing, including unsharp mask filtering, integration, and subtraction, on the Wiener spectrum were also demonstrated. Since noise influences the detectability of radiologic objects and thus diagnostic accuracy, knowledge of the effects of the various digital parameters on the noise spectrum will be useful in the evaluation and design of future digital imaging systems.

Analog-Digital Conversion↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. I. Modulation transfer function.

The effect of various digital parameters, such as the sampling aperture, sampling distance, and display aperture, on the modulation transfer function (MTF) of digital radiographic imaging systems was investigated by means of theoretical simulation studies. The MTFs were also determined experimentally to confirm the relationship used in the simulation studies. The results indicate that the overall MTF of a digital system cannot specify the resolution properties in the same way as can the MTFs of analog systems. The MTF of a digital system may include a "false" response due to aliasing, which could lead to an incorrect interpretation of the resolution properties. The magnitude of aliasing that will occur in a digitized signal depends on the sampling parameters chosen and on the frequency content of the radiologic object being imaged. Thus, the type of object to be detected as well as various digital parameters must be considered in the design and evaluation of digital imaging systems.

Radiography↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 6. MTFs of II-TV digital imaging systems.

We devised a new, simple technique for measuring the modulation transfer function (MTF) of a digital imaging system by using an image of an angulated slit. With this technique, the "presampling" analog MTF, which includes the geometric unsharpness, the detector unsharpness, and the unsharpness of the sampling aperture, can be measured even beyond the Nyquist frequency. A single-frame image of a slightly angulated slit was employed in order to obtain Fourier transforms of line spread functions at different alignments. The presampling MTF was determined by averaging the two Fourier transforms which we obtained from two extreme alignments (center and shifted) of the slit relative to the sampling coordinate. The presampling MTFs of our digital subtraction angiographic system were determined in two orthogonal directions for three different image-intensifier modes.

Fourier Analysis↗

PubMedDate not supplied

Performance of antiscatter grids in diagnostic radiology: experimental measurements and Monte Carlo simulation studies.

We have devised an experimental method with which one can accurately measure the transmission of primary radiation and the transmission of total radiation by an antiscatter grid in a setting similar to a practical radiographic examination. We measured the transmission values for 27 combinations of x-ray tube potentials, phantom thicknesses, screen-film systems, and grid parameters. The standard deviation of one measurement was estimated to be 2.9% and 1.5% for the total and primary transmissions, respectively. The measured grid transmission was compared with results predicted by our Monte Carlo calculations; 92% of the measured and calculated values agree within two standard deviations. This close agreement indicates that our Monte Carlo calculation can accurately predict the performance of antiscatter grids under diagnostic imaging conditions.

Biophysical Phenomena↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 3. Effect of pixel size on SNR and threshold contrast.

The effect of pixel size on the signal-to-noise ratio (SNR) and threshold detection of low-contrast radiologic patterns was investigated theoretically for digital radiographic systems. The SNR based on the perceived statistical decision theory model, together with the internal noise of the human eye-brain system, was calculated by using two-dimensional displayed digital signal spectra and noise Wiener spectra. Threshold contrasts were predicted from the calculated SNR for various combinations of object size and shape, pixel size, resolution, and noise. Predicted threshold contrasts agreed well with those determined experimentally in an observer performance study. The threshold contrast of small objects increased substantially as the pixel size increased beyond 0.2 mm. For pixel sizes of 0.1 and 0.2 mm, however, the threshold contrasts were similar. Since a digital system is not shift invariant, a range of threshold contrast results for a small object and a large pixel, depending on the alignment of the object position relative to the sampling coordinates.

Humans↗

PubMedDate not supplied

Physical characteristics of scattered radiation in diagnostic radiology: Monte Carlo simulation studies.

We applied Monte Carlo methods for the simulation of x-ray scattering in water phantoms. The phantom thickness was varied from 5 to 20 cm, and the monoenergetic incident x rays were varied from 15 to 100 keV. Eight screen pairs and a total absorption system were used as x-ray receptors. We determined the angular, spectral, and spatial distributions of the scattered radiation and the scatter fractions recorded in the image plane. The dependence of these properties on the incident x-ray energy, the phantom thickness, and the energy response of the recording system was examined. The results of this study provide useful information for the development of antiscatter techniques and for the evaluation of radiographic procedures.

Models, Theoretical↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 4. Effect of unsharp masking on the detectability of simple patterns.

The signal-to-noise ratios (SNRs) of simple radiologic patterns processed by the unsharp-masking technique were calculated on the basis of a statistical decision theory model that includes both the observer's visual transfer function and a noise component internal to his eye-brain system. For a variety of processing parameters, the contrast-detail diagrams predicted from this SNR agreed qualitatively with experimental results obtained in observer performance studies. Unsharp masking with a large mask and a large weighting factor improved the detection of square objects to a level comparable with that achieved by the overall contrast enhancement technique using a factor of 4. However, unsharp masking with a small mask and a large weighting factor can substantially degrade the detectability of these objects. The potential practical utility of the unsharp-masking technique is discussed.

Humans↗

PubMedDate not supplied

Some properties of photon scattering in water phantoms in diagnostic radiology.

Monte Carlo simulation was applied to study the histories of photon interactions in a soft-tissue-equivalent medium under diagnostic imaging conditions. We examined the dependence on incident x-ray energy and phantom thickness of the basic properties of photon scattering, including the probabilities of occurrence of the various interaction processes, and the frequency distributions of scattering events. We investigated the properties of scattered radiation for monoenergetic incident x rays, which provide a basis for deriving the physical properties of scattered radiation for any polyenergetic incident beam. We also included four incident x-ray beams with broad spectra; these represented the incident x rays typically used for diagnostic imaging.

Humans↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography 10. Structure mottle of II-TV digital imaging systems.

Single-frame images obtained with image intensifier (II)-TV digital systems contain a large amount of structure mottle. In the present study, we examined several II-TV digital systems by use of Wiener spectral analysis and noted considerable variation of the structure mottle over the wide spatial frequency range. We found that the structure mottle in these systems may originate in the input phosphor, the output phosphor, and/or the electronic components, and that the Wiener spectra of structure mottle seem to depend on the specific combination of these components. The results of observer performance studies indicated that structure mottle can significantly decrease the detection of low-contrast objects in a single-frame image when the exposure incident on the II is greater than approximately 0.1 mR. In addition, we showed that the structure mottle can be removed by subtraction of a uniformly exposed mask. This simple procedure will improve the quality of radiologic images obtained with II-TV digital systems. Note, however, that the structure mottle is largely eliminated by subtraction in digital subtraction angiography (DSA) images.

Humans↗

PubMedDate not supplied

Accurate measurement of characteristic curves of II-TV digital systems by use of aluminum stepwedge technique.

In order to determine accurately the relative x-ray intensities transmitted through the different steps of an aluminum stepwedge and incident on an image intensifier (II)-TV digital system, we used a 100-mm camera which was attached to the II output. The characteristic curves of an II-TV digital system, which relate the output pixel value to the input relative x-ray intensity, were determined at 65 and 90 kV by the use of the aluminum attenuation curves measured at the two energies. We found that the two curves agreed within the experimental errors; this result indicates that the characteristic curve does not depend on the x-ray beam quality used. Furthermore, for accurate measurement of the characteristic curve, it is necessary to collimate the x-ray beams to a narrow width, approximately 0.6 X 10 cm, so that the effect of veiling glare from the thinner to the thicker parts of the stepwedge is eliminated.

Aluminum↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 8. Detection of simulated low-contrast objects in digital subtraction angiographic images.

We investigated the effects of imaging and display conditions on the detectability of low-contrast objects in digital subtraction angiographic (DSA) images. The test images were produced by superimposition of low-contrast objects on a uniform noisy background obtained with a DSA system. We employed 18-alternative forced-choice (18-AFC) experiments and predictions based on statistical decision theory to study the dependence of the threshold contrasts of the test objects on the object size, incident x-ray exposure, display window width, and display medium. The results indicated that the threshold contrast decreased with increasing object size, and that the detectability of an object of a given size increased with increasing incident x-ray exposure and decreasing width of the display window. We found that the signal-to-noise ratio (SNR) obtained from the perceived statistical decision theory model, which includes the observer's internal noise, can accurately predict the detectability of low-contrast objects in DSA images. The threshold SNR corresponding to 50% correct detection in the 18-AFC experiments had a constant value of 3.8, in agreement with results reported previously for screen-film systems. The theoretical model will be useful for prediction of the performance of a DSA system based on its physical characteristics, and for evaluation of the tradeoff between patient exposure and diagnostic accuracy for a given DSA unit.

Angiography↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 9. Effect of displayed grey levels on signal detection.

Results of an 18-alternative forced-choice experiment have shown that observers were capable of detecting a signal with a contrast of 1 in terms of 10-bit data which were displayed on a CRT monitor with an 8-bit video generator and a window width setting of 1024. We investigated the conditions under which 10-bit signal data can be detected when displayed using an 8-bit video generator. Results show that the 10-bit digital quantum noise, which was approximately Gaussian distributed, can act as a carrier of the signal data, thus allowing a signal having a contrast of a fraction of a displayed grey level to be detected. We demonstrate the relationship between the rms value of the digital noise (obtainable with a clinical digital subtraction angiography system under various exposure levels), the number of bits available in the display video generator, and the "transmitted" signal contrast displayed on the CRT monitor.

Biometry↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 5. Characteristic curves of II-TV digital systems.

A simple method was devised to determine the characteristic curve of image intensifier (II)-TV digital imaging systems, which relates the output pixel value to the input relative x-ray intensity. To provide a wide range of x-ray intensities incident on the II, we used an aluminum stepwedge consisting of nine steps with thickness increments of 6.3 mm, together with an 0.81-mm-thick copper plate. The x-ray field was narrowly collimated to the area occupied by the stepwedge in order to reduce the effect of veiling glare. The relative x-ray intensities transmitted through each step of the stepwedge were determined by using screen-film systems. The gradient curve of the system was derived from the slope of the characteristic curve. Results obtained with a Siemens Digitron 2 system showed that its characteristic and gradient curves depended upon the matrix size used, but did not change with the II field size. The validity of the characteristic curve was demonstrated by measurement of iodine attenuation curves obtained with the II-TV digital system at different exposure levels.

Humans↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 7. Noise Wiener spectra of II-TV digital imaging systems.

We used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II-TV digital system. The digital Wiener spectra in terms of relative x-ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II-TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single-frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.

Angiography↗