Showing papers on "Imaging phantom published in 1982"

The unreliability of CT numbers as absolute values

Abstract: The use of CT numbers as absolute values was examined by scanning a standard phantom on five CT scanners under a variety of conditions simulating those encountered in routine body CT scanning. The results show that: (1) There are significant differences in absolute CT numbers between most scanners (only one scanner produced CT numbers that were equal to zero for water); (2) There are significant differences in absolute CT numbers between two scanners of the same manufacturer and model that were examined; (3) There is a significant difference in CT numbers in a single phantom scan, dependent on location in the scan, and the format of this variability is not constant from one scanner to another; and (4) There may be a significant difference in absolute CT numbers depending on various physical factors (e.g., kilovoltage, phantom orientation in scan aperture, and position of the phantom in the scan aperture). The findings suggest that there is a wide range of CT numbers observed for a given tissue type as a r...

The effects of scatter in x-ray computed tomography

Abstract: The effects of detection of scattered radiation in x-ray transmission CT are studied both theoretically and experimentally. It is shown that scatter induces nonlinear errors in the measurement of attenuation values which can lead to cupping, streaks, and CT number inaccuracies. It is shown that scatter effects predominate over beam spectrum hardening effects for large body parts, and that the artifact propensity is a direct function of the scatter-to-primary ratio. The presence of scatter induced streaks were demonstrated experimentally on both a third and a fourth-generation CT scanner using an appropriate water-equivalent phantom. A simple model with constant scatter background leads to a correction algorithm which was tested on several phantoms and one human pelvis. The algorithm worked well on the smaller phantoms but was less successful for the larger objects. Nevertheless, it still gave substantial improvement in the pelvic scan. We demonstrated that, at least in the pelvis, scatter is a more significant source of error than beam hardening and that improved scatter correction algorithms are needed. The consequences for the quantitative interpretation of CT numbers for clinical diagnoses are discussed.

A solid water phantom material for radiotherapy x-ray and γ-ray beam calibrations

Abstract: The formulation, manufacture and testing of an epoxy resin-based solid substitute for water is presented This "solid water" has radiation characteristics very close volumetrically to those of water When it is used as a dosimetry phantom for x- and gamma-ray beams in the radiotherapy range, phantom-to-water corrections and density corrections are eliminated Relative transmission measurements have shown that the transmission through 10 cm of solid water is within 02% of that through an equal thickness of water for x and gamma rays The use of this material for calibration phantoms can help achieve the goal of radiotherapy beam calibrations within +/- 10% of the true dose rate, easier to achieve

Scattered radiation in fan beam imaging systems.

Abstract: 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.

Experimental Assessment of the Gain Achieved by the Utilization of Time-of-Flight Information in a Positron Emission Tomograph (Super PETT I)

Abstract: The gain achieved in image quality by utilizing, in the image forming process, the time-of-flight information (TOF) of positron annihilation photons between their inception and detection was measured experimentally by means of a positron emission tomograph (PET)-Super PETT I. The measurements were carried out by imaging a 35 cm cylindrical uniform phantom containing different positron activity concentrations. The gain achieved through the incorporation of TOF information, defined as the ratio of variances in images reconstructed with and without TOF information, was found to be approximately 3 at the lowest activity concentration and 5-8 in the activity concentration range typically encountered in clinical studies especially in fast or dynamic studies. This increase in gain with activity was interpreted as resulting from the reduction of random coincidences when TOF information is used. Further image improvement is yielded by incorporating TOF information into the PET attenuation correction provided by the measurement of transmission of annihilation photons in the object imaged.

Determination of Organ Volume by Single-Photon Emission Tomography

Abstract: A method for estimation of organ volume is proposed, based on analysis of individual slices obtained from SPET images. In a phantom simulating clinical circumstances, the data show that the level a threshold at 46% of the maximum activity predicts most closely the true volume over a wide range above one liter. The level at 45% predicted better volumes of less than one liter. For phantoms of 839 ml or less, the error was 6.3 ml (one standard error of estimation). This level seems to be independent of the plane or position of the phantom and also independent of the amount of scattering material around it. Nonradioactive voids (''holes'') within a phantom may be included or excluded at will when their edges are not tangent to the edge of the phantom. In such cases, their edges are not distinguishable from the edge of the phantom and their volumes are excluded. Knowledge of organ volumes has both diagnostic and therapeutic importance and could lead to a more precisely quantitated total of the radioactivity contained in an organ or space.

A contrast‐detail analysis of diagnostic ultrasound imaging

Abstract: For diagnosticultrasound imaging, as in computed tomography, a feature of prime importance is the detection of focal lesions of varying size and contrast (echo amplitude) from surrounding tissue. This study describes a new tissue‐simulating phantom which has been used to measure the threshold detection of varying contrast, simulated lesions. The phantom consists of a block of tissue‐mimicking gelatin which contains a row of conical targets at a depth of 7 cm. Each cone contains a different tissue‐mimicking material so that the echo amplitude of the cones relative to the background material covers a dynamic range of 20 dB. Cross‐sectional B‐scans, perpendicular to the lengths of the cones, result in images of disks of constant diameter but varying contrast. Parallel cross‐sectional scans yield ‘‘lesions’’ varying in diameter from 20 to 1 mm. Relative contrast of the cones vs background tissue is obtained by varying scattering particle sizes from 90 to 300 μm. UltrasoundB‐scans of the phantom were examined by medical physicist observers to determine threshold detection of lesions as a function of size and contrast. The results indicate that detection of high contrast targets is limited by the imaging system’s spatial resolution. Detection of low contrast targets is limited by the image speckle, i.e., coherent noise.

Hyperthermia by magnetic induction: I. Physical characteristics of the technique

Abstract: Several electromagnetic techniques are currently used in hyperthermia therapy for cancer. This report discusses the magnetic induction technique, in which application of an alternating magnetic field to a conductor results in induction of eddy current flow and power deposition via ohmic losses. The power density in tissues depends upon the heterogeneous tissue conductivities and dielectric constants, magnetic field intensity and distribution, and eddy current radius. Using a newly developed magnetic field probe, the magnetic fields produced at 13.56 MHz by electrodes of a commercial magnetic induction device have been accurately mapped and used to calculate power densities in static phantoms. Calculated and observed temperature elevations in a cylindrically symmetric phantom agree well, confirming the simple formula in this case relating power density to magnetic field strength. The efficiencies of three commercially available electrodes have been accurately measured using calorimetric techniques and phantom loads modeling human anatomy and electrical conductivity. The effect of eccentric positioning of the load has been studied using magnetic field mapping techniques. Power densities in a very simplified model of human anatomy that retains cylindrical symmetry were calculated. Effects of inhomogeneities in conductivity have been investigated qualitatively using composite static phantoms modeling human cross-sectional anatomy and thermographic camera recordings of surface temperature distributions. The advantages and disadvantages of this heating technique are discussed from the point of view of the power density distributions in heterogeneous materials. Evaluation of power density distributions is essential for optimizing this heating technique using various electrode arrangements and/or load modifications, for providing part of the information needed for solution of the bioheat transfer equation in living subjects, and for predicting the ability of the technique to heat specific tumors.

Lesion detection with single-photon emission computed tomography (SPECT) compared with conventional imaging.

Abstract: We have evaluated analytically and experimentally the effectiveness of both conventional nuclear medicine imaging and single-photon emission computed tomography (SPECT) imaging to detect small photon-deficient areas (approximately the size of the system's resolution) with a relatively uniform background. The experimental model is based on the Tc-99m sulfur colloid study of the liver. The experimental data were obtained from a liver phantom containing two small photon deficient areas, nominally 1 and 1.5 cm in diameter. The liver phantom was placed in a water-filled Alderson body phantom and scanned with the cold defects located both centrally and peripherally. Lesion image contrast for both conventional and SPECT imaging is proportional to the lesion uptake ratio and is degraded by the system's finite spatial resolution and Compton-scattered photons. However, for conventional imaging the contrast is significantly degraded by the effect of radionuclide superposition (as modified by attenuation), while for SPECT imaging the contrast is essentially independent of these effects. This results in a significant increase in lesion-to-background contrast with SPECT as compared with conventional imaging. The measured SPECT image contrasts for the 1- and 1.5-cm areas of low uptake averaged more than five times the measured image contrasts for the conventional system.

Temperature distributions in tissues during local hyperthermia by stationary or steered beams of unfocused or focused ultrasound.

Abstract: Temperature distributions resulting from insonation with stationary or steered beams of unfocused or focused ultrasound were measured in tissue-equivalent phantom, beef muscle in vitro, dog muscle mass, and transplanted murine tumours in vivo. Arrays of 4 to 6 thermocouples stepped through the volume of interest under computer control were used to measure the steady-state temperatures at 600 to 800 locations in both in vitro and in vivo experiments. The results were confirmed in spontaneous tumours in dog patients using fewer multi-thermocouple probes. Plane wave ultrasound was found to result in spatially non-uniform hyperthermia even in superficial tumours. The region of maximum temperature rise was small in extent and was situated at a depth which varied in the different models from 0.5 to 1.0 cm. Neither its location nor its extent could be varied by spatial manipulations of the transducer or by changing the insonation parameters except the ultrasonic frequency. A second region of hyperthermia was produced at depth by reflective heating if an ultrasonically reflective target, such as bone or air-containing tissue, was located below the target tissue. On the other hand, using available steered, focused ultrasound techniques, tumours (whether situated superficially or at depth) could be heated to a uniform, controllable temperature without undesirable temperature elevation in surrounding normal tissues. The use of steered, focused ultrasound permits deposition of energy to be tailored to the specific needs of each individual tumour. The small size of the focal region enables heating of tumours even when located near ultrasound reflecting targets.

Magnetic field gradient coils for NMR imaging

Abstract: A new magnetic field gradient coil system is described for use in whole body NMR imaging experiments. The coil assembly has a low inductance and can therefore be used in structures where switching speed is important. Curves showing the performance of the coil system are given and an NMR image of a phantom object obtained with the assembly is presented.

Investigation of the performance of antiscatter grids: Monte Carlo simulation studies.

Abstract: Monte Carlo methods have been applied to the study of the performance of antiscatter grids in radiography. Photon histories for imaging with a 20 cm thick water phantom irradiated with an 80 kV X-ray spectrum were generated by Monte Carlo simulation. The scattering and absorption of the photons transmitted from the phantom were traced in the grid. Grids were evaluated with strip densities ranging from 10 to 67 lines/cm, grid ratios from 6 to 15, and lead-to-interspace ratios from 1/9 to 1/2. The contrast improvement factor and the Bucky factor were used as the benefit and cost indicators, respectively, for evaluation of the grids. The dependence of these factors on the grid ratio, lead-to-interspace ratio, strip density, lead content, and interspace material was investigated. The results indicate that high-strip-density grids with thin lead strips and high grid ratios can provide higher contrast improvement factors than are achieved with low-strip-density grids, without an increase in patient exposure. This theoretical approach is useful for the prediction and development of optimal antiscatter grids for radiographic procedures.

An anthropomorphic ultrasound breast phantom containing intermediate-sized scatteres

Abstract: An anthropomorphic breast phantom, built from materials which mimic tissue parenchymae with respect to attenuation coefficients, speeds of sound, densities and backscatter levels, is described in detail. One of the outstanding features of the phantom is that 30% of the volume of the glandular region consists of tissue-mimicking fat, the remainder being tissue-mimicking glandular (non-adipose) tissue. The presence of these tissue-mimicking fat globules should cause ultrasound beam distortions similar to those found in clinical scans of the breasts of younger women. Halliwell (1977) has presented direct evidence for such beam distortions in real breasts. Other tissues simulated in the phantom are: skin, subcutaneous and retromammary fat, Cooper's ligaments, ducts and (abnormal) masses. The potential areas of usefulness of the phantom are: aiding in the development of more effective ultrasound imaging machines, discovering— or verifying the causes of—various artifacts in breast imaging, routine testing of instruments being used clinically for breast imaging, and training of ultrasonographers. Ultrasound images of the phantom, made using three different instruments, are displayed and discussed.

Analytical study of the performance of a multilayer positron computed tomography scanner.

Abstract: The image-forming performance of multilayer positron tomographs for extended sources is evaluated analytically. The analysis is simplified by "rotation transform," by which three-dimensional photon detection problems are solved by two-dimensional treatment. Event rates of singles, unscattered true coincidence, and a single- and double-scattered coincidence are formulated for a uniform cylinder phantom as functions of various design parameters. Angle factors for Compton scattering and other parameters used in the evaluation are presented. Scatter components in projections and their effect on the reconstructed images are also evaluated. The scatter component in the reconstructed image depends critically on the detector ring radius, phantom radius, method of attenuation correction, etc. When the director radius is relatively small (40 or approximately 45 cm in diameter), the scatter/true ratio at the image center of a 20 cm diameter phantom may be larger than the scatter/true ratio in the event rates. Comparison with experimental data obtained with a head positron tomograph, POSITOLOGICA, showed reasonable agreement both in the total coincidence rates and in the scatter components in the images for a cylindrical phantom of 20 cm in diameter.

Comparison of 180 degrees and 360 degrees data collection in thallium-20 1 imaging using single-photon emission computerized tomography (SPECT): concise communication.

Abstract: Thallium-201 imaging using SPECT is being done with 180 degrees (RAO to LPO) data collection in some centers with single-gamma camera systems. Using our SPECT system with two gamma cameras, we have compared the effects of 180 degrees data collection without attenuation correction against 360 degrees collection with attenuation correction, using phantoms and patients. With a heart phantom in a chest phantom, TI-201 activities simulating "normal myocardium," "ischemia," "infarction," and "background" were placed in object contrast ratios (with respect to background) of 5.0, 2.0, and -1.0, respectively. The 180 degrees data gave image contrast ratios of 1.6, 0.2, and -0.8, and the 260 degrees data gave ratios of 1.5, 0.8, and -0.3, respectively. Uniform activity throughout the heart gave similar image contrast with both data-collection methods, but there was more variability with the 180 degrees collection than with 360 degrees collection. Since attenuation correction is available with the 260 degrees collection, the effects of attenuation are seen only on the 180 degrees collection images. In eight patients the image contrasts from the 180 degrees and 260 degrees collections are similar. For our two-camera SPECT system, the 360 degrees collection permits attenuation correction, has less variability in counting statistics, and gives contrast ratios like those of 180 degrees collection.

Quantitative multi-detector emission computerized tomography using iterative attenuation compensation.

Abstract: An iterative procedure to correct for attenuation has been developed for a multidetector, single-photon emission tomographic scanner. The difference between measured and estimated data projections is used at each iteration to form an error image which is used, in turn, to correct the image. A damping factor that minimizes chi/sup 2/ is applied after each iteration to speed convergence. Several phantoms of different size, with various concentration distributions, have been used to compare this method with a first-order multiplicative attenuation correction used previously with this scanner. The first-order correction is inadequate for most of the phantoms studied, whereas relative and absolute quantitative capability is demonstrated for the iterative attenuation correction. The reconstructed average number of counts per pixel is a linear function of activity concentration up to 5 muCi/ml for all regions of uniform activity whose size is greater than or equal to 5 cm. The importance of using an accurate attenuation distribution with this method is demonstrated with a torso-like phantom.

Phantom for nuclear magnetic resonance machine

Abstract: A phantom for an NMR machine is disclosed that includes a plurality of containers each filled with a material having a known spin density, T1, or T2 characteristic that differs a preselected amount from the spin density, T1, or T2 characteristics of the material in at least a portion of the other containers.

Properties of phantom networks and real networks

Abstract: The theory of phantom networks by James in 1947 and elaborated by Duiser and Staverman in 1965 and by Flory in 1976 is shown to lead generally to the frontfactor equation for the elastic free energy.

The use of phantom views to reduce CT streaks due to insufficient angular sampling

Abstract: An artefact that can occur in CT scans with too few views is the appearance of streaks at some distance from the sharp edge that causes them. The authors investigate, for parallel beam scanners, the effectiveness of the method of phantom views, as well as other methods of angular interpolation, in diminishing the view-limited streak artefact. The phantom view method consists of inserting data points obtained by interpolation in angle, along with the real data points. The authors show by a theoretical analysis, as well as by using simulated scans, that the phantom view method does tend to diminish the distant streaks more than other methods tested. Furthermore, they show that there is a point of diminishing returns in increasing the number of phantom views.

A simple tissue-like ultrasound phantom material

Abstract: Routine testing and calibration of modern grey-scale ultrasound scanning equipment and the training of operators in its proper application can often benefit from the use of tissue-like phantoms. For example, checking of display systems, simple registration checks or familiarization with the operation of scanner controls to achieve optimal results, can all be performed with this type of phantom. Another application is as a standard for ultrasonic tissue characterization studies.

A Monte Carlo Method for Patient Dosimetry from Dental X-Ray

Abstract: A Monte Carlo method has been developed for calculating dose distributions within a fluid phantom irradiated by an external heterogeneous x-ray beam. The method utilized a three-dimensional array of numerically-described cuboids (voxels) to provide phantom anatomy, with vector concepts to determine photon flight paths, locations, and interaction points, regardless of materials located within the phantom. Using a homogeneous water phantom and an 80 kVp x-ray beam, results of 196488 photon histories per cm2 were compared with a measured dose distribution. Discrepancies appeared to be the result of experimental error in the measured dose distribution. χ2 tests indicated that they were not statistically significant.

Digital Imaging For Radiation Therapy Verification

Abstract: A linear diode array was used to scan a radiation field for pulsed high-energy x-ray imaging With a signal of about 50 picocoulombs of charge from each diode for each beam pulse, the signal to noise ratio for an image of an open field was 119 An image of a porcine femur in a wax phantom was obtained by scanning the transmitted radiation The resultant electronic image was superior in quality when compared to the standard radiotherapy port film The simplicity of the detector system has demonstrated the feasibility of constructing an electronic portal imaging system for clinical use

Evaluating the Performance of Multiplane Positron Tomographs Designed for Brain Imaging

Abstract: The evaluation of a multiplane tomograph designed for brain imaging requires particular attention to certain design parameters that tend to be comprised in such designs These parameters include resolution uniformity, axial slice thickness uniformity with emphasis on the interplane slices, scatter fraction, accidentals fraction, and system dead time In addition, the final performance specifications should be relevant to the manner in which the system will actually be employed in the clinic These specifications should be measured under realistic scatter and count rate conditions using the same reconstruction techniques that are employed in patient imaging These parameters were evaluated on the NeuroECAT positron tomograph (1,2) and the effect of choice of manner of presentation and type of measurement condition on the apparent performance of the system is shown The results indicate that clearly defined conditions are a necessity in evaluating the true performance of a positron tomograph

Effect of variation in the energy spectrum of a cyclotron-produced fast neutron beam in a phantom relevant to its application in radiotherapy

Abstract: The fast neutron spectrum of the beam produced by the MRC cyclotron at the Hammersmith Hospital has been measured in air and at several depths in a water phantom using three field sizes. The neutron spectra were determined both by a set of six threshold detectors and by a liquid scintillator spectrometer. Where a direct comparison of the two methods was possible the agreement was satisfactory. It was found that the shape of the spectrum, above 3 MeV, is unchanged with depth in the phantom, but for each field size the neutron fluence between 0.5 and 3.5 MeV increases significantly and reaches a maximum and the mean energy a minimum. This minimum decreases with increase in field size. Values of the kerma ratio in ICRU muscle to both that in A–150 plastic and bone were calculated and found to vary by less than 0.5% with depth in the phantom; that of carbon to ICRU muscle varied by 5%, and values of W for methane-based tissue-equivalent gas were found to change by less than 1%. Variations in biologic...

Test body arrangement

Abstract: A test body or phantom for use in checking the performance of apparatus which detects the emission of radiations from a body includes an element of predetermined shape which incorporates a material having a known level of radiation intensity.

3D thin lead sheet compensating system

Abstract: The compensating filter system presented consists of a simulator mounted camera and light beam device. The light beam device produces a flash light beam 3‐mm wide perpendicular to the x‐ray axis. The distance between flashes can be varied from 0.5 to 2.5 cm in steps of 0.25 mm. An exposure of the patient in treatment position subject to a series of light flashes will produce a three‐dimensional representation of the patient surface. This map is then properly enlarged. Lead sheets are cut and fixed to a Perspex plate which fits into a slot in the head of the accelerator. Regardless of energy, all compensators are made from 0.5‐mm‐thick lead sheets. To determine the proper distance between light flashes, measurements were performed in a water phantom for each x‐ray energy to establish the equivalence of increasing numbers of layers of 0.5‐mm lead. These measurements were compared to theoretical calculating using tissue maximum ratios, and finally checked against a wedged water phantom of 25° as well as against a head and neck treatment field for the Alderson phantom. This way, a beam flattening of ±2% compared to the standard isodose lines was achieved.

Single-photon emission tomography with a 12-pinhole collimator.

Abstract: To assess the advantages of more complete angular sampling and of more views in the tomographic reconstruction process, tomographic imaging with a 12-pinhole (12PH) collimator has been compared with 7-pinhole tomography (7PH). The 12PH system gives a 50% increase in sensitivity but resolution degrades more rapidly with depth. The 7PH and 12PH systems provide similar accuracy of detection of lesions in a myocardial ring phantom. The 7PH images, however, demonstrated more noise and "ripple" artifacts. The 12PH system offers a larger reconstruction volume and generates fewer artifacts when the collimator is misaligned with the myocardial long-axis, thus making patient positioning less critical than with 7PH. A disadvantage is that individual views are minified by the 12PH collimator, and a 256 X 256 image matrix should be used during image acquisition to limit digital sampling errors.

Single photon tomographic imaging of a standard heart phantom with 201 Tl: A gamma camera based system

Abstract: A standard heart phantom (University of lowa design), including discrete myocardial walls, a central blood pool, and a 24-cc transmural “cold” defect, was studied with both planar and transverse tomographic imaging. The heart phantom was filled with 201Tl and placed within a cylindrical tank containing water and 201Tl to simulate nonmyocardial background activity from the thorax. The tomographic imaging system used was a commercially available, rotating, large field-of-view gamma camera. Image reconstruction from 64 sampling angles was performed in a nuclear medicine minicomputer system. The percentage activity in the region of the defect (actual activity of 0) contrasted to the normal wall was compared between planar and 1.25-cm transaxial tomographic slices. Defect activity fell to between 65% and 85% of that of the opposing normal wall in planar images, whereas it fell to between 26% and 49% of that of the normal wall in the tomographic images. In most cases, tomographic defect activity was half or less than that in the planar image. The geographic extent of the defect was seen in an appropriate number of tomographic slices; i.e., the geographic 3.2-cm defect length was predominantly seen in three 1.250cm transverse slices. We conclude that camera-based tomographic systems show promise for improved 201Tl myocardial defect detection and quantitation over conventional planar images.

Measurement of induced electric fields in a phantom model of man

Abstract: An implantable electric field probe with an interference-free lead wire system was constructed at a nominal cost for the purpose of measuring the induced electric fields in a phantom model of man irradiated by waves of various frequencies. The probe consisted of a short dipole loaded with a zero-bias microwave diode, and the interference-free lead wires were constructed with two series of lumped resistors of 3 k Ω. Characteristics of the probe were checked first by measuring the induced electric fields in irradiated, electrically small cubes filled with phantom material. The measured results were in good agreement with theoretical results obtained from the tensor integral equation method. A phantom model of man was constructed with thin Plexiglas filled with phantom material. The model was irradiated by 500- to 3000-MHz em waves in a microwave anechoic chamber. Induced electric fields were probed over 28 locations in one side of the model. The distribution of the measured electric fields was compared with the distribution of theoretical results obtained numerically from the tensor integral equation methods. A qualitative agreement was obtained between experiment and theory. It was found that the agreement between experiment and theory tended to deteriorate at lower frequencies. The reason for this discrepancy is explained.