Showing papers on "Imaging phantom published in 1985"

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.

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.

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.

Detector for dual-energy digital radiography.

Abstract: A detection scheme is described that allows one to accomplish dual-energy scanned projection digital radiography without switching the x-ray tube voltage. The method employs a high/low atomic number detector sandwich that simultaneously separates the x-ray beam transmitted by the patient into low and high energy components. To test the method, the response of a scanning linear array of energy-sensitive detectors was simulated, and bone and soft tissue images of an anthropomorphic chest phantom were obtained at 140 kVp. These were compared with similar images obtained by switching the x-ray tube voltage from 80 kVp to a heavily filtered 140 kVp. For comparable entrance skin exposures, the dual-energy detector images required a lower tube load and resulted in higher noise levels. The latter is attributable to the fact that the separation in energy between the high and low energy components is smaller with the dual-energy detector than with the voltage switching technique, and to misregistration problems ass...

MR velocity imaging by phase display

Abstract: The ability of the nuclear magnetic resonance signal to encode information about macroscopic motion has been recognized since the works of Hahn and Carr and Purcell. In the medical imaging setting this ability has led to a variety of ingenious magnetic resonance flow imaging schemes that ultimately may become competitive with X-ray angiography in sensitivity and specificity while remaining radically noninvasive. This work demonstrates that conventional spin-echo Fourier transform image acquisitions naturally encode a component of flow velocity that lies within the image plane. By displacing just the real part of the complex image data (phase display), the velocity distribution within the subject is revealed in image form. This method of flow imaging requires neither special pulse sequences nor image reconstruction and format software for its implementation. Further, images that intersect a flow channel longitudinally, demonstrating in-plane flow, yield an unusually large quantity of physiologic information per image. Phantom and in vivo flow images are presented. Also described is a phantom based on a rotating disk that enables calibration of the velocity/phase-shift constant for an untested pulse sequence.

Ion chamber response and Awall correction factors in a 60Co beam by Monte Carlo simulation

Abstract: The responses and wall correction factors for ion chambers in broad parallel 60Co beams have been calculated using Monte Carlo techniques. The calculated responses are in good agreement with Bragg-Gray cavity theory. In particular, the response divided by the wall correction factor Awall is found to be independent of the detector's shape but dependent on the material used for the chamber wall in a manner predicted by Bragg-Gray cavity theory. A simple theory is given which predicts the increase in response of a Farmer ion chamber due to an electrode of an arbitrary material and radius. The change in chamber response as a function of build-up cap composition is in good agreement with analytic expressions. The effect of guard regions in pancake chambers is found to be negligible. Embedding a pancake chamber in a flat phantom during calibration is shown to increase the response by 1.0+or-0.2%. Calculated values of Awall are in good agreement with most experimental results and with those given in the AAPM protocol but with a considerably lower uncertainty of +or-0.2%. When using these values with the AAPM protocol, the beta cep factor should not be used since it is included in these calculated values.

In vivo 19F NMR imaging of the cardiovascular system.

Abstract: A technique is described that permits the 19F nuclear magnetic resonance (NMR) imaging of the vascular system in animals that have been infused with certain perfluorocarbon blood substitutes. The essence of the technique is the selection for imaging of a narrow group of spectral lines from the fluorocarbon using a combination of selective excitation and selective spin echo. For perfluorotributylamine (FTBA), this uses 56% of the available fluorine signal. Both slice and whole body projection images were obtained. The method was studied on an experimental NMR imaging system based on a 1.4 T, 31 cm diameter magnet. Phantom tests showed that misregistration artifacts, due to the imaging of unwanted spectral lines, were negligible. The spatial resolution obtained in a scan time of 8.5 min was approximately 1 mm. The technique was tested with living rats in which 50% of the blood volume was replaced with an emulsion of FTBA. Short echo times, less than 15 ms, were essential to avoid losing signal from flow effects in the larger vessels. Structures identified included the heart, liver, spleen, lungs, kidneys, and major veins. A diffuse fluorine signal seen in most tissues is indicative of tissue perfusion. The problems and possibilities of potential clinical applications are discussed.

Α phantom marriage law

Abstract: At first sight, nothing could bo more straightforward. Cynthia was pleased that an old law had been repealed which, whon it was issued, had made both her and her lover weep for a long time in fears that it would separate them. Later in the poem, wo gather that there would have been pressure on Propertius to many and begot children, which lie now safely vows he would never have done. But both for historians and for those literary critics who are old-faeliioned enough to believe that a poem is not merely a collection of topoi and imitations, but that at least its references to public facts should be taken as real, the lines have caused problems. What law was this? When passed? Precisely how was it edicta? When and how was it sublata? Why would it have forced Propertius to marry? (I omit the important question of why lie could not have married Cynthia — a question to which, for various reasons, I see no possibility of finding an answer and which I do not propose to treat.)

Performance characteristics of Positologica III: a whole-body positron emission tomograph.

Abstract: A whole-body, multislice positron emission tomograph, Positologica III, has been constructed based on a sampling method "positology" The scanner consists of four continuously rotating detector rings with 192 bismuth germanate (BGO) crystals (12 X 24 X 24 mm) arranged at irregular intervals along each ring, providing seven tomographic images at 16 mm intervals simultaneously, for a total axial coverage of 12 cm The patient aperture is 54 cm in diameter, and the field of view is 40 cm in diameter and 12 cm in depth Newly developed dual photomultiplier tubes are used with 12 mm wide BGO crystals to provide a packing ratio of 0894 The spatial resolution at the center of the field is 76 mm full width at half maximum, and 25 mm hot spots were delineated in the Derenzo phantom The sensitivity for a 20 cm diameter cylindrical phantom is 342 and 522 kcps/muCi/ml for true planes and cross planes, respectively In clinical studies the posterior papillary muscle of the heart was visualized in the myocardial scan, and the lung behind the pulmonary artery was also visualized in the pulmonary ventilation scan These results suggest that our machine has sufficient resolution for most of the clinical studies of the body

Frequency independent ultrasound contrast-detail analysis.

Abstract: An ultrasound contrast-detail phantom has been developed to evaluate the image quality of diagnostic ultrasound imaging systems. The phantom includes eight conical targets whose B-mode images show disk lesions such that the object contrast of each lesion relative to background is independent of the imaging device or transducer frequency/spectrum. By maintaining conditions for Rayleigh scattering and Rayleigh speckle statistics in the phantom gel, the object contrast of each lesion depends only on the scatterer concentration in the lesion relative to the scatterer concentration in the background. Experimental data confirmed this frequency independence. Results of contrast-detail performance of an ultrasound imaging system are shown, and a standard technique for error analysis of contrast-detail data is described.

The imaging revolution and radiation oncology: use of CT, ultrasound, and NMR for localization, treatment planning and treatment delivery.

Abstract: The explosion of new imaging technologies such as X ray computed tomography (CT), ultrasound (US), positron emission tomography (PET), and nuclear magnetic resonance imaging (NMR) has forced a major change in radiation therapy treatment planning philosophy and procedures. Modern computer technology has been wedded to these new imaging modalities, making possible sophisticated radiation therapy treatment planning using both the detailed anatomical and density information that is made available by CT and the other imaging modalities. This has forced a revolution in the way treatments are planned, with the result that actual beam configurations are typically both more complex and more carefully tailored to the desired target volume. This increase in precision and accuracy will presumably improve the results of radiation therapy.

Polyethylene-based water-equivalent phantom material for X-ray dosimetry at tube voltages from 10 to 100 kV

Abstract: A water-equivalent plastic material RW-1, in the form of thin foils as well as of thicker sheets, has been produced by melting powdered polyethylene together with CaCO3 and MgO. The mixture has been developed in a three-step procedure, starting with provisional mixtures and using data from their measured attenuation curves to determine the final composition. The attenuation curves of RW-1 and of water, as well as their backscatter factors, are in excellent agreement for x-ray tube voltages from 10 to 100 kV.

Hyperthermia dough: a fat and bone equivalent phantom to test microwave/radiofrequency hyperthermia heating systems

Abstract: The recipe described gives an easy to prepare, easy to handle and inexpensive fat phantom for the determination of absorbed-power distributions of microwave/radiofrequency radiation. If the phantom is used together with the high-water-content phantom (Guy 1971) the absorbed-power distributions can be determined in structured tissues. The phantom has been used in Utrecht to test the e-m boundary conditions at 434 MHz on interfaces between fat and muscle structures (Lagendijk and de Leeuw 1984) with experimental results in full agreement with theory. The low specific heat of the phantom is slightly compensated by the high density. However, to analyse absorbed-power distributions in structured phantoms from measured-temperature distributions, these values must be taken into account.

Coherent scatter in radiographic imaging: a Monte Carlo simulation study

Abstract: The significance of coherently scattered radiation in radiographic imaging is investigated using the Monte Carlo simulation technique. Recent data on the form factor of liquid water, which take into account intermolecular interference effects, have been used for the calculation of the coherent differential cross section. The spatial distribution of scattered radiation in the detection plane was calculated separately for coherent and incoherent single and multiple scattering. In the pencil beam geometry, it is found that coherent scattering leaving the object, is almost exclusively single scattering, is concentrated near but not exactly at the transmitted primary beam and dominates over multiple incoherent scattering in this region even for thick objects and polyenergetic radiation. Some consequences concerning the performance of grids, the choice of phantom materials and a new imaging method are given.

A spin echo chemical shift MR imaging technique.

Abstract: A new method is described that produces images of either the fat or water component in tissues in magnetic resonance imaging. Only a single scan is required, with scan times of a few minutes. Chemical shift selectivity is achieved in the spin echo process by controlling the spectral content of the 180 degree pulse that induces the spin echoes. A theoretical analysis of the selective spin echo process for the case of a radio frequency pulse of constant amplitude shows that spin echoes will be suppressed for certain values of offset frequency that are similar to, but different from, the frequencies at which the Fourier spectrum of the pulse vanishes. The theory was confirmed by experiment on a water phantom. The imaging technique was tested on both a phantom of oil and water and on a human forearm. Excellent suppression of the water signal was found in the fat images, and the small fat component seen in the water images is attributable to components of the triglyceride molecule for which spectral lines overlap those of water. The forearm images also showed blood flow effects in the water image that were not visible in the fat image. The relationship of this method to other proposed methods of chemical shift imaging is discussed.

Field inhomogeneity correction and data processing for spectroscopic imaging

Abstract: To obtain separate NMR images of spatially resolved high-resolution chemical-shift information the effects of magnetic field inhomogeneity must be accounted for. A suitable correction method is described which relies on first generating a plot of the magnetic field distribution. Using these data the spectroscopic imaging data can be processed to display the spatial distributions of separate resonances without distortion from field inhomogeneity. The field plot is obtained by using the same data acquisition sequence while imaging a phantom object or in particular cases the plot may be derived from the spectroscopic imaging data set itself. The correction procedure is illustrated using proton in vivo imaging of a cat. Some additional data processing techniques are presented which offer alternative methods of displaying spectroscopic imaging data.

A new digital tomosynthesis method with less artifacts for angiography

Abstract: A new nonlinear reconstruction method for tomosynthesis is described. This method is suited for ‘‘dilute’’ objects, i.e., objects in which most of the voxels have negligibly small absorption. Images of blood vessels filled with contrast material approximate this condition if the background is subtracted. The technique has been tested experimentally using a wire phantom and a prepared human heart. The results show significantly less artifacts than the well‐known backprojection. It is possible to get diagnostic image quality with a few projections. The reconstruction algorithm can be realized with dedicated real‐time hardware.

Reproducibility of T1 and T2 relaxation times calculated from routine MR imaging sequences: phantom study.

Abstract: Measurement of T1 and T2 relaxation times has been sought as one fundamental way to characterize tissue. Relaxation times can be calculated from routine spin-echo (SE) imaging sequences using two distinct repetition times (TRs), each with two SE samplings of signal intensity. Previous reports have quantified relaxation times without discussing the variation in their measurements. By imaging a phantom containing different samples with known T1 and T2 relaxation times on three separate occasions, the variation in relaxation time measurements inherent in different routine imaging sequences was studied. For the present study a more complete and accurate equation was used to calculate T1 values. The variation in T1 and T2 relaxation times for samples with relaxation times similar to solid tissue was 2%-4%. The amount of variability in calculated relaxation times was found to be dependent on the magnitude of the relaxation times themselves. However, the mean values were independent of the imaging sequences used to calculate the relaxation times. No significant differences were seen between left-to-right or section-to-section position within the same study or between studies performed on different occasions. The variability in the calculated T1 was dependent on the pair of TR sequences used to calculate T1. Samples with long T1 and T2 relaxation times, similar to many body fluids, had much larger variability. A computer simulation of measurement error was created to explain these results. This study indicates that properly performed routine imaging studies do yield reproducible T1 and T2 measurements.

Molecular aspects of rubberlike elasticity

Abstract: Molecular theories of rubberlike elasticity are surveyed, starting with the earliest attempts carried out in the 1930's and ending with work still very much in progress. The models are variously based on phenomenological arguments, affiinely-deforming networks, phantom networks, constrained junctions, constrained chains, slip-links, molecular tubes, van der Waals concepts, or rotational isomeric chains. Most of the theories are analytical, but some involve computer simulations based on Monte Carlo methods

Selective plane removal in limited angle tomographic imaging.

Abstract: A method of improving a planar image in limited angle tomography by removing blurred image information from selected out-of-focus planes is discussed. Focused tomosynthesized images rather than individual projections are used. The necessary equations for removing the information from either two or four adjacent planes, produced with a symmetrical, but otherwise arbitrary blurring function, are developed and specialized to the geometry of circular tomography. Results of a phantom experiment illustrating the technique are presented.

Echo Projection Imaging-A Method to Obtain NMR Images Undistorted by Magnetic Field Inhomogeneities

Abstract: It is proposed to use a modified form of Mansfield's echo planar imaging to obtain NMR images which are entirely undistorted by background inhomogeneities of the magnetic field. In the proposed method, a train of 180° pulses is applied in the presence of a periodically switched or sinusoidally modulated linear field gradient. The time-domain signal is sampled at half the distance between the 180° pulses. At these points the magnetization will be modulated by the gradient, but will be independent of any mechanisms of inhomogeneous broadening, such as static field inhomogeneities, local susceptibility effects, or chemical shifts. A Fourier transform of the function comprising these points will therefore yield a faithful projection of the spin density, although the magnitude of the superimposed gradient need not be large compared to the inhomogeneous broadenings. This paper demonstrates the application of the proposed pulse sequence to a small-scale one-dimensional phantom. The major problem in upscaling this technique to human-scale dimensions lies in the limited available and allowed RF power, which in turn limits the maximal tolerable field inhomogeneities as well as the maximal practical field strength. An analysis of the tolerance of the proposed technique to these factors is presented, based on numerical simulation of its performance, using the Bloch equations. It is concluded that its use may be feasible on low-field systems, providing the advantages of increased signal-to-noise, lower required gradient strength, and drastically reduced sensitivity to the homogeneity and stability of the magnetic field, at the expense of larger RF power.

The influence of phantom size on output, peak scatter factor, and percentage depth dose in large-field photon irradiation.

Abstract: Machine outputs, peak scatter factors, and central axis percentage depth dose distributions were measured for various phantom sizes in large radiation fields produced at extended distances by cobalt, 6-MV, and 10-MV photon beams. The results can be applied to practical total body irradiation procedures which usually involve treatment volumes smaller than the actual field sizes in order to provide a uniform total body exposure to radiation. Our study addresses the question of the appropriate phantom dimension to be used in the calibration of photon beams employed in total body irradiations. The measurements show that the machine outputs are only slightly dependent on phantom size; the percentage depth dose distributions, however, are strongly dependent on the phantom size, suggesting that machine data for total body irradiations should be measured in phantoms whose dimensions approximate the patient during the total body irradiation. Peak scatter factors measured in large-field/small-phantom configurations link up well with the published small-field/large-phantom data. The finite patient thickness lowers the dose to points close to the beam exit surface by a few percent, when compared to dose measured at the same depths in infinitely thick phantoms. The surface doses in large radiation fields are essentially independent of phantom cross sections and range from 40% for the 10-MV beam, to 65% for the 6-MV beam and 80% for the cobalt beam.

A beam-edge modifier for abutting electron fields

Abstract: Abutment of unmodified electron fields to irradiate large areas can lead to significant dose inhomogeneities in the region of junction of the fields. In this paper we describe the design and dosimetric characteristics of a device developed to broaden the electron beam penumbra and thereby to improve the dose uniformity in the overlap region. The device is a high‐density triangular‐toothed comb capable of reducing the beam intensity without seriously degrading the beam energy. The effect of the comb is such that a single device will generate a beam penumbra which is broad and very nearly linear at all depths for all clinically used beam energies. Results are shown for various field configurations and energies. With a gap of 5.0 cm between the treatment cone and phantom surface the dose ‘‘ripple’’ in the region beneath the teeth was found not to exceed ±5% at 0.5‐cm depth.

Nuclear magnetic resonance imaging apparatus

Abstract: A phantom for use in assessing the performance of an NMR imaging apparatus which enables quantitative assessments of image slice width and spatial resolution under different conditions of contrast to be made, and which enables an assessment of overall performance of the apparatus to be made to facilitate comparison of the performance of the apparatus when operating in different modes. The structure and methods of use of the phantom are described.

Contoured mammography phantom with skin

Abstract: A phantom formed of a non-biological material into the shape of a breast, said material having radiation characteristics of breast tissue, for use by medical personnel while in training to interpret mammographs, and as a tool to assess the quality of a radiological imaging system. The phantom material comprises one epoxy resin based tissue substitute which simulates the breast tissue, and another which simulates the skin tissue. A slot is formed in the phantom into which targets for simulating breast masses, fibers and calcifications can be placed.

A method of measuring refractive index profile of cylinder

Abstract: Parallel light rays are applied to a cylinder and true exit positions of the rays from the cylinder are measured, phantom exit positions for the incident rays are calculated by using a mathematical function representing a phantom refractive index profile, and the mathematical function is modified so as to minimize the sum of the squared differences between the true and phantom exit positions.

Reproducibility of T1 and T2 Relaxation Times Calculated from Routine MR Imaging Sequences: Phantom Study

Abstract: Measurement of T1 and T2 relaxation times has been sought as one fundamental way to characterize tissue. Relaxation times can be calculated from routine spin-echo (SE) imaging sequences using two distinct repetition times (TRs), each with two SE samplings of signal intensity. Previous reports have quantified relaxation times without discussing the variation in their measurements. By imaging a phantom containing different samples with known T1 and T2 relaxation times on three separate occasions, the variation in relaxation time measurements inherent in different routine imaging sequences was studied. For the present study a more complete and accurate equation was used to calculate T1 values. The variation in T1 and T2 relaxation times for samples with relaxation times similar to solid tissue was 2%–4%. The amount of variability in calculated relaxation times was found to be dependent on the magnitude of the relaxation times themselves. However, the mean values were independent of the imaging sequences used to calculate the relaxation times. No significant differences were seen between left-to-right or section-to-section position within the same study or between studies performed on different occasions. The variability in the calculated T1 was dependent on the pair of TR sequences used to calculate T1. Samples with long T1 and T2 relaxation times, similar to many body fluids, had much larger variability. A computer simulation of measurement error was created to explain these results. This study indicates that properly performed routine imaging studies do yield reproducible T1 and T2 measurements.

Breast mass detection by US: a phantom study.

Abstract: Phantoms were used to compare imaging of various masses in the glandular region for two types of breasts. In one type, normal glandular tissue contains no fat clumps; in the other type, randomly distributed spherical fat clumps exist. The size distribution and number of fat clumps per unit volume in the phantoms represented those found in actual normal glandular tissue. For a 5-cm path length between the scanning window of the phantom and the centers of the masses, the presence of the fat clumps caused distortions and deviations of the beam, resulting in image distortion. This degradation was reduced when successive slabs of the material containing the fat clumps were removed. Image degradation appeared to be less severe when a 2.25 (rather than a 3.5) MHz transducer was used. The results show that the thickness of tissue containing fat clumps between the scanning transducer and a mass plays a role in the detectability of masses and diagnostic usefulness of the image of a breast. This suggests that imaging of the breast in the compressed configuration should be performed regardless of whether the freely suspended breast is imaged. Also, transducers of lower nominal frequency may have an important role in breast imaging.