Showing papers in "Medical Physics in 1985"

Fast calculation of the exact radiological path for a three-dimensional CT array.

Abstract: Ready availability has prompted the use of computed tomography (CT) data in various applications in radiation therapy. For example, some radiation treatment planning systems now utilize CT data in heterogeneous dose calculations algorithms. In radiotherapy imaging applications, CT data are projected onto specified planes, thus producing "radiographs," which are compared with simulator radiographs to assist in proper patient positioning and delineation of target volumes. All these applications share the common geometric problem of evaluating the radiological path through the CT array. Due to the complexity of the three-dimensional geometry and the enormous amount of CT data, the exact evaluation of the radiological path has proven to be a time consuming and difficult problem. This paper identifies the inefficient aspect of the traditional exact evaluation of the radiological path as that of treating the CT data as individual voxels. Rather than individual voxels, a new exact algorithm is presented that considers the CT data as consisting of the intersection volumes of three orthogonal sets of equally spaced, parallel planes. For a three-dimensional CT array of N3 voxels, the new exact algorithm scales with 3N, the number of planes, rather than N3, the number of voxels. Coded in FORTRAN-77 on a VAX 11/780 with a floating point option, the algorithm requires approximately 5 ms to calculate an average radiological path in a 100(3) voxel array.

Measurement of signal intensities in the presence of noise in MR images

Abstract: Power spectrum or magnitude images are frequently presented in magnetic resonance imaging. In such images, measurement of signal intensity at low signal levels is compounded with the noise. This report describes how to extract true intensity measurements in the presence of noise.

A convolution method of calculating dose for 15-MV x rays.

Abstract: Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting ‘‘dose spread arrays’’ were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three‐dimensional dose distributions. The method gives good agreement for the 15‐MV x‐ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low‐density heterogeneities.

Energy and angular distributions of photons from medical linear accelerators

Abstract: For accurate three‐dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (egs) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one‐third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half‐value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.

MR image artifacts from periodic motion

Abstract: Artifacts due to periodic motion during the acquisition of magnetic resonance (MR) images have been studied. A mechanical device was constructed to oscillate a small sample along any line within a 0.15-T Technicare imager. Two- and three-dimensional images were obtained using various frequencies and amplitudes of oscillation. Computer simulations of these experiments yielded images which agreed with the experiments. We demonstrated that movement influences MR images locally through blurring, and also generates ghost artifacts along the phase-encoding directions of the Fourier transform imaging technique.

Noise and filtration in magnetic resonance imaging.

Abstract: Noise in two-dimensional Fourier transform magnetic resonance images has been investigated using noise power spectra and measurements of standard deviation. The measured effects of averaging, spatial filtering, temporal filtering, and sampling have been compared with theoretical calculations. The noise of unfiltered images is found to be white, as expected, and the choice of the temporal filter and sampling interval affects the noise in a manner predicted by sampling theory. The shapes of the imager's spatial frequency filters are extracted using noise power spectra.

NMR blood flow imaging using multiecho, phase contrast sequences.

Abstract: Moving nuclei, in contrast to stationary nuclei, experience a phase shift in the presence of a balanced gradient. Monitoring of this phase shift can be used to measure the flow velocity of moving nuclei. A specific scan sequence for blood flow imaging is presented. This sequence uses multiple echoes as well as a phase contrast approach to generate both conventional anatomical images and blood flow images from the same data. Images of a phantom and a human volunteer demonstrating the accuracy of the method are presented.

A photon dose distribution model employing convolution calculations.

Abstract: A three-dimensional photon beam calculation is described which models the primary, first-scatter, and multiple-scatter dose components from first principles Three key features of the model are (1) a multiple-scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions The results of calculations for cobalt-60 in a homogeneous phantom are used to verify the accuracy of the model

Calculation of the uncertainty in the dose delivered during radiation therapy

Abstract: There is, inevitably, uncertainty in our knowledge of the dose at any point within an irradiated patient. A technique is presented for estimating this uncertainty by performing three parallel calculations, one using nominal values and the others extreme values of the parameters upon which the dose depends. Such calculations can be made with almost any algorithm for calculating dose. They result in an estimate, at some specified confidence level which is determined by the data used, of the range of dose likely at any point. Such calculations should help therapists to avert over- or underdosage which might not be evident in conventional calculations of the nominal dose.

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

Abstract: 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.

Calculation of the radiological depth

Abstract: The concept of the radiological depth is central to all algorithms which calculate radiation dose in a heterogeneous medium. For a discrete heterogeneous medium, consisting of regions of inhomogeneity, the radiological depth is usually presented as the sum over segments of the product of the segment length and the inhomogeneity density of the region corresponding to the segment. This paper illustrates that the usual formulation is inefficient because it requires the solution of the topological problem of which region corresponds to each segment. For simple heterogeneity problems involving just three regions of inhomogeneity, it is found that the topological problem constitutes at least 85% of the time required to calculate the radiological depth. It is shown in this paper that formulating the radiological depth as a sum over regions rather than as a sum over segments allows one to avoid this topological problem entirely.

Lung dose corrections for 6- and 15-MV x rays

Abstract: We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

A simplified approach for modulation transfer function determinations in computed tomography.

Abstract: In order to determine the modulation transfer functions (MTF's) for x-ray computed tomography (CT) scanners, a measurement must be performed to obtain either the point spread function (PSF) or the line spread function (LSF). Thereafter, the usual procedure is to interpolate between the measured points and to determine the Fourier transforms numerically in order to obtain the MTF. Since this must usually be done many times to evaluate various reconstruction kernels and scan modalities, the process is tedious. Fortunately, it can be greatly simplified by utilizing a mathematical function to describe the PSF or LSF. Measured data for five CT scanners indicates that the PSF can usually be described by a Gaussian function. Hence, the MTF can be written in a generalized form eliminating the necessity of performing Fourier transformations each time. The MTF is determined directly from a single performance characteristic related to the full width at half maximum. The accuracy of the approach is compared with detailed MTF calculations for five CT scanners and it is shown to agree favorably with this data.

NMR imaging in the presence of magnetic field inhomogeneities and gradient field nonlinearities

Abstract: The relative merits of the spin‐warp method and the multiple‐angle projection–reconstruction method are assessed for nuclear magnetic resonance imaging in the presence of magnetic field inhomogeneities. The results of computer modeling studies demonstrate the superiority of the spin‐warp method under these conditions. Because the form of the point spread function for the spin‐warp approach is insensitive to magnetic field irregularities, simple correction algorithms have been developed to reduce artifacts associated with bias field inhomogeneities. In addition, the formalism developed for magnetic field inhomogeneities can be easily extended to correct for gradient field nonlinearities.

Noise performance of surface coils for magnetic resonance imaging at 1.5 T.

Abstract: In this paper we analyze the signal-to-noise ratio (SNR) for surface coil magnetic resonance imaging at 1.5 T. We have applied the treatment of Hoult and Lauterbur to determine the factors that most affect coil performance. We have imaged lossy phantoms with 8-, 10-, and 14-cm-diam circular surface coils and compared the results to body and head coil images. Surface coils can improve SNR by a factor of 4 or more for regions close to the surface. Surface coils are effective for regions up to 6 cm deep in the head and about 12 cm deep in the body. Nonuniformity of image intensity is a necessary requirement for improved SNR in surface coils. Coil losses make only a small contribution to image noise compared to tissue losses at 1.5 T. Surface coils need not be placed in close contact with the patient at 1.5 T.

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

Abstract: 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.

Scatter rejection by air gaps: an empirical model.

Abstract: Scatter rejection by air gaps was analyzed in the context of a model in which scattered radiation was treated as if it originated from an "effective scatter point source" (ESPS), located between the focal spot of the x-ray tube and the exit surface of the phantom or patient. Excellent agreement was found between the ESPS model and scatter measurements performed on phantoms for a variety of experimental conditions. Values for Xs (distance from phantom exit surface to effective scatter point source) were consistently in the range 15-20 cm. Applications of the model for predictions of scatter rejection by air gaps and comparisons to other scatter rejection methods are presented.

Dual‐energy mammography: Initial experimental results

Abstract: 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.

Signal-to-noise properties of mammographic film-screen systems.

Abstract: The signal‐to‐noise ratio (SNR) and the detective quantum efficiency (DQE) have been experimentally determined as a function of spatial frequency for several mammographic film–screen systems. These two parameters were determined from our measurements of noise power spectra and sensitometric properties of each system along with modulation transfer function(MTF) data for the screens which were obtained from others. From the noise power spectra, it was found that film noise contributes significantly to the total noise of mammographic film–screen systems, comprising 30%–50% of the total noise at 1 cycle/mm and as much as 75% at 5 cycles/mm. All systems had approximately the same SNR below 1.5 cycles/mm, but differed at higher frequencies due to differences in screen MTF and in the gradient of the film’s sensitometric curve. The DQE curves varied between systems at all frequencies, however, due to differences in system speed, MTF, and gradient. Generally, the DQE of mammographic film–screen systems is between 10%–30% at frequencies below 1 cycle/mm and decreases to about 1% between 8 and 12 cycles/mm. Compared to film–screen systems used in general radiography,mammographic systems have similar DQE values at low frequencies, but are superior at higher frequencies.

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

Abstract: 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.

Spectral transmittance and contrast in breast diaphanography.

Abstract: Diaphanography is an imaging technique using transillumination with visible and near‐infrared radiation, and a video camera, to diagnose breast disease, including cancer. Originally based on luminance contrast only, there is now interest in false‐color multispectral imaging in selected spectral bands to improve tissue differentiation. Some success has been achieved, but the scientific basis for the results was unknown. This research is concerned with measurements of the diffuse transmittance of breast tissues as a function of wavelength in the 600–1060 nm range (and calculations of contrast with a one‐dimensional diffusion theory model).Carcinoma and glandular tissues were found to have similar spectral transmittances with an increase in transmittance between 750 and 900 nm, and an absorption window around 960 nm. Adipose tissue showed a distinct transmittance minimum at 930 nm. I n v i v o measurement of an intact normal breast showed a minimum at about 825 nm, as yet unexplained. The transmittance data and reflectance data were used to derive the scattering and absorption coefficients. The diffusion length was also determined from radiance versus depth measurements. These coefficients were used for calculations of contrast in a one‐dimensional slab model, with and without a layer of cancerous tissue positioned between two slabs of normal tissue.

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

Abstract: 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.

Two-dimensional dose distribution of 125I seeds

Abstract: Two‐dimensional dose distribution has been measured for the new (model 6711) 1 2 5I seeds used in interstitial implants. Two independent methods, using a silicon diode or thermoluminescent dosimeters, yielded identical results. At any given distance r from the seed center, the dose varies with θ, the angle relative to the seed’s axis. Similarly, the r dependence of the dose distribution is different at various θ values. These observations can be qualitatively understood in terms of several factors, namely, source encapsulation, geometrical relationship, and attenuation and scatter. Empirical expressions which approximate the measured results have been developed to facilitate clinical dose distribution calculations.

Photodynamic and photohyperthermic response of malignant tumors.

Abstract: This work presents an analytical dosimetry model for photodynamic therapy of malignant tumors. A hyperthermic contribution originating from the absorption of optical irradiation in the tissue may be equally taken into account. The model has been used to analyze tumor response during photodynamic therapy of an i n v i v o murine tumor. This tumor model was C3H/Tif mammary adenocarcinoma in C3D2F1/Bom mouse strain.

Stereotaxic radiotherapy technique for small intracranial lesions.

Abstract: A stereotaxic radiotherapy technique that permits accurate delivery of highly localized dose to a small intracranial target has been developed. The technique facilitates precise integration of the diagnostic and therapeutic procedures including target localization, treatment planning, simulation, repetitive patient irradiation, and daily treatment verification. A conventional linear accelerator and computed tomography scanner as well as special diagnostic and therapeutic guides are used. A suitable dosimetric distribution is achieved using arc therapy with small radiation fields and 10-MV x rays.

Dental enamel as an in vivo radiation dosimeter.

Abstract: The determination of the radiation exposure history of the population has become increasingly important in the study of the effects of low‐level radiation. The present work was started to try to obtain an i n v i v odosimeter that could give an indication of radiation exposure. Dental enamel is the only living tissue which retains indefinitely its radiation history, and electron spin resonance measurements have shown that the radiation signal can be resolved down to about 10 cGy. Measurements on samples from the general population give radiation exposure estimates that are reasonable, and one measurement on a patient who had radiotherapy to the mouth area showed a good correlation with tumordose.We believe that this is an important new indicator of radiationdose and taken together with exposure histories should provide important data for epidemiological studies as well as accidental exposures.

Two-film brachytherapy reconstruction algorithm.

Abstract: We have developed a new isocentric two‐film reconstruction algorithm for brachytherapy seed and needle implants. The algorithm has no requirements that the two films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the two films even have the same number of images. We have found removal of these usual constraints useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional two‐film techniques are minimized by considering the image end points, rather than just the image centroids. For two films, the new algorithm, which considers all image combinations at o n e time, matches all the end‐point images on one film with those on the other, and then reconstructs the end‐point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new two‐film image matching problem is shown to be equivalent to the well‐known assignment problem. For an implant of N seeds, this equivalence allows the two‐film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2 s on a VAX 11/780.

Acceptance testing computerized radiation therapy treatment planning systems: direct utilization of CT scan data.

Abstract: The availability of computerized radiation therapy treatment planning systems that utilize computed tomography (CT) scan data requires testing additional to that routinely needed for non-CT systems. These additional items include dimensioning verification, establishing CT number-to-tissue property conversions, verifying the accuracy of heterogeneity corrected dose predictions and autocontouring. One testing protocol is presented and sample results from an Atomic Energy of Canada Theraplan L system are presented.

Models in radiotherapy: Definition of decision criteria

Abstract: A method is presented by which dose distributions in radiotherapy may be judged. This method, based on statistical decision theory, combines the calculated probabilities of radiation induced complications (including failure to control disease) with the therapist's judgment of the morbidity of each complication to yield a single value representing the clinical utility of the dose distribution. Using this figure of merit, alternative dose distributions can be compared on a clinical basis. If the morbidities associated with each injury are satisfactorily estimated, and the dose response parameters are adjusted to match clinical experience, this method can be used to evaluate novel treatment techniques prior to their implementation. Automatic optimization algorithms can be used to maximize the figure of merit as a function of the physical treatment parameters so as to provide a statistically optimal treatment.