bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance characteristics of an orthovoltage x‐ray therapy machine
Authors: Niroomand‐Rad Azam , Gillin Michael T. , Lopez Francisco , Grimm Daniel F.
Published: March 2003

Performance characteristics sufficient to provide physical data base specific to the Siemen's Stabilipan 2 orthovoltage x‐ray therapy machine are presented. Operating conditions covering the working range of the unit from 100 to 300 kVp are selected. Beam quality, output, the central axis depth dose, relative output factors, field flatness, uniformity index, and filtration characteristics of the beams are studied. Selected results are reported.
Analysis of the bremsstrahlung component in 6–18 MeV electron beams
Authors: Rustgi Surendra N. , Rodgers James E.
Published: March 2003

The magnitude of the bremsstrahlung component in the 6–18 MeV electron beams from a Varian Clinac 18 accelerator was measured directly using a 0.16 T magnetic field to deflect the electron beams. The central axis depth ionization curves were measured at 130‐ and 197‐cm source–surface distances in a Therados RFA‐3 water phantom with and without the magnet being placed between the 4×4 cm2 electron cone and the water phantom. Results obtained clearly demonstrate that the bremsstrahlung component is primarily (>90%) generated in the electron scattering‐foil/collimation system and not in the water phantom. The x‐ray central axis depth ionization curves exhibit buildup and attenuation regions characteristic of megavoltage x‐ray beams. Ionization profiles of the x‐ray component, measured in a plane perpendicular to the central axis, were also examined. Analysis of bremsstrahlung production as a function of photon collimation jaw opening shows a strong dependence, especially for the smaller openings.
An alternative to conventional shielding for megavoltage therapy rooms
Authors: Fitzgerald Lawrence T.
Published: March 2003

Dry silica sand as an “in‐wall” shielding medium for megavoltage teletherapy rooms is discussed. Shielding considerations, construction techniques, and survey results are presented for a 6‐MV facility using sand shielding for both primary and secondary barriers.
A heterogeneity model for photon beams incorporating electron transport
Authors: Petti Paula L. , Rice Roger K. , Mijnheer Ben J. , Chin Lee M. , Bjärngard Bengt E.
Published: March 2003

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15‐MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low‐density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.
A linear‐quadratic fit for saturation curves obtained under continuous irradiation of parallel‐plate ionization chambers
Authors: Fallone B. G. , Bencomo J. A.
Published: March 2003

An approximation to saturation curves of parallel‐plate ionization chambers for various gases under continuous irradiation is given. The proposed fit asymptotically agrees with Mie's set of equations for the low and the high collection efficiency regions. It is shown that this fit which, for gases containing electronegative atoms, is dependent upon a parameter obtained from physical constants agrees with measurement and with values calculated from Mie's set of equations. However, for gases containing nonelectronegative atoms, it is shown that an additional experimental parameter is required to fit the measurements.
Errors in cerebral blood flow determinations by xenon‐enhanced computed tomography due to estimation of arterial xenon concentrations
Authors: Good Walter F. , Gur David , Yonas Howard , Herron John M.
Published: March 2003

Errors in the determination of xenon concentrations in arterial blood during inhalation of xenon–oxygen mixtures are used to assess errors in the derivation of regional cerebral blood flow by the xenon‐enhanced computed tomography (CT) method. The results of this study indicate that approximating the arterial buildup by a single exponential introduces relatively small errors in estimated flow values. The most significant systematic error is introduced by errors in estimation of the xenon arrival time to the brain in relationship to sequential (CT) scanning times.
Imaging of spatial radiation dose distribution in agarose gels using magnetic resonance
Authors: Appleby A. , Christman E. A. , Leghrouz A.
Published: March 2003

Radiation dose distributions are conventionally measured using ionization chambers or diodes in liquid phantoms, or in two dimensions using film. This work describes a new application of magnetic resonance imaging to radiation dose planning. Agarose gels containing ferrous sulfate, sulfuric acid, and benzoic acid have been irradiated with 137Cs gamma rays and 6–14 MeV electrons, to doses of up to 20 Gy. The dose distributions have been imaged by magnetic resonance, making use of the effect on the T1 proton relaxation times of the radiolytic Fe3+. The image intensity was proportional to doses of up to 10 Gy, and images were stable for at least 24 h postirradiation. The G value for Fe3+ production was about 100 (molecules per 100 eV absorbed).
Megavoltage portal films using computed radiographic imaging with photostimulable phosphors
Authors: Wilenzick Raymond M. , Merritt Christopher R. B. , Balter Stephen
Published: March 2003

Conventional portal films taken with high‐energy x rays are severely impaired by poor image quality, and many attempts have been made to overcome this problem. A new technology, computed radiographic imaging, has been investigated as a means of producing images taken with 6‐ and 10‐MV x rays. The computed radiographic imaging (CRI) system is based on the principle of laser‐stimulated luminescence induced in an image receptor plate exposed to ionizing radiation, and direct conversion of the light into digital form. Image processing using contrast enhancement and unsharp masking yields images at least as good as the best portal films obtainable with conventional commercial radiotherapy cassettes. Additionally, CRI offers advantages of reduced image size and digital storage. The system is capable of producing normal film densities over an extremely wide range of exposure.
Dose buildup for obliquely incident photon beams
Authors: Gerbi Bruce J. , Meigooni Ali S. , Khan Faiz M.
Published: March 2003

For obliquely incident photon beams, the buildup of dose with depth is markedly different from normally incident beams. However, relatively little data on this topic exists for high‐energy photon beams of energy greater than 6  MV. Measurements of dose in the buildup region were made using a plane‐parallel ionization chamber in a polystyrene phantom with obliquely incident 6‐, 10‐, 18‐, and 24‐MV x‐ray beams angled 0° to 84°. Buildup curves at these angles were plotted and from these an obliquity factor, defined as the ratio of ionization charge collected at a point for a particular angle of incidence to that collected at the same point at normal incidence, was determined. For each energy, the obliquity factor as a function of depth, field size, and source–chamber distance was studied. Results indicate that the obliquity factor is highly dependent on the beam energy, angle of incidence, the collimator opening, and the source–skin distance. A mathematical expression has been developed to predict the dose in the buildup region of high‐energy photon beams for various angles of beam incidence, field size, and chamber distance.
Relative electron beam measurements: Scaling depths in clear polystyrene to equivalent depths in water
Authors: Ten Haken Randall K. , Fraass Benedick A.
Published: March 2003

For purposes of mean incident energy determination and the accrual of consistent treatment planning data, measurements of relative ionization or dose made in clear polystyrene must be scaled in depth to produce depth‐ionization or depth‐dose curves equivalent to what would have been measured in water. Recommendations from various protocols for clear polystyrene to water depth scaling factors differ by as much as 5%. Here, central axis measurements of relative ionization as a function of depth have been made with parallel‐plate chambers both in a popular clear polystyrene phantom of density 1.045 g/cm3 and in water. Perturbation and displacement corrections were thus minimized. Both depth‐ionization and depth‐dose curves were formed for electron beams with nominal incident energies between 6 and 20 MeV and field sizes from 6 cm×6 cm to 15 cm×15 cm. Comparisons of the depths for 50% relative reading and practical range between the two phantoms yield average empirical scaling factors of 0.990 and 1.002, respectively.
Monte Carlo simulation of the dose distribution around 125I seeds
Authors: Burns G. S. , Raeside D. E.
Published: March 2003

The Monte Carlo method was used to investigate dose distributions around the 3M Company model 6701 and model 6702 125I brachytherapy seeds. The transverse axis dose distributions of the two seed models were found to be nearly identical, but the longitudinal axis dose distributions differed significantly. Seed design influences upon dose distributions also were investigated.
Temperature dependence of x‐ray induced luminescence from medical x‐ray screens
Authors: Shearer D. R. , Rowe L. E.
Published: March 2003

Texts in diagnostic radiography give rules for adjustment of exposure parameters to compensate for the effect of temperature on the efficiency of film–screen combinations. These rules are appropriate for traditional phosphors, such as calcium tungstate (CaWO4) and lead barium sulfate (BaSO4:Pb) but may not be applicable to newer screens in current use such as those of the rare‐earth type. The effect of temperature on x‐ray induced luminescence from modern and traditional screens has been compared over a temperature range of ∼−130 to ∼60 °C and has been found to differ markedly. Typically, the luminescent emission from rare‐earth phosphors varies by less than 10% over this temperature range compared to the 60% change in efficiency for conventional screens over the same temperature range. One type of modern screen, however, shows a continuous decrease in luminescent efficiency as the temperature is lowered. Qualitative physical explanations for these effects are presented.
A semianalytical method for the design of a linac x‐ray beam flattening filter
Authors: Flock Stephen T. , Shragge Peter C.
Published: March 2003

The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18‐MV x‐ray beam produced by this accelerator measured along the diagonal of a 40×40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within ±1% out to 24 cm along the diagonal of a 40×40 cm field at a depth of 5 cm.
The polarity effect for commercially available plane‐parallel ionization chambers
Authors: Gerbi Bruce J. , Khan Faiz M.
Published: March 2003

The polarity effect was investigated for three different commercially available plane‐parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30‐329 chamber manufactured by PTW, Frieburg, and the Capintec PS‐033 thin‐window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6‐, 10‐, 18‐, and 24‐MV x‐ray beams, and 9‐ and 22‐MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane‐parallel ionization chambers that we studied, we found a polarity effect of only 1%–2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high‐energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.
Image intensifier distortion correction
Authors: Chakraborty Dev P.
Published: March 2003

A method is described for determining and correcting the spatial distortion affecting images acquired with image‐intensified video systems. The distortion is separated into two physically distinct components, a predominant one originating from the projection of the x‐ray image onto the curved input phosphor, and a smaller component corresponding to the mapping from the input phosphor to the output phosphor and the digital image matrix. The former is determined geometrically and the latter is modeled with four empirical parameters. A method for determining the two components from calibration images of a grid phantom is presented. The model of the image intensifier distortion was found to be in good agreement with the measured distortion in the images. The correction is applicable to arbitrary angulations of the x‐ray beam with respect to the image intensifier tube, and consequently is directly applicable to digital tomosynthesis. The correction allows one to obtain accurate positional information with fluorography and may also be useful in radiation therapy treatment planning and quantitative digital subtraction angiography.
Performance of high‐resolution monitors for digital chest imaging
Authors: Suddarth S. A. , Johnson G. A. , Sherrier R. H. , Ravin C. E.
Published: March 2003

High‐resolution cathode‐ray tubes (CRT's) are currently the most viable soft‐copy display for digital radiography. We present here methods for measuring large‐area contrast ratio and detail contrast ratio. A two‐dimensional charge coupled device (ccd) array signal‐averaged with a video frame buffer permits linear microradiometric measure of individual beam lines. Results from three different 1000‐line monitors demonstrate the shift variance of resolution. The detail contrast ratio (or modulation depth) was found to vary from 100% to <10% across the face of one CRT. Dynamic focus in both the horizontal and vertical deflection circuitry proved effective in reducing this shift variance. Comparisons of three phosphors demonstrate the utility of long persistence phosphors (P164) for static display in producing brighter images with less flicker. Recommendations for CRT design and selection for high‐resolution digital radiography are included.
Noninvasive x‐ray tube current measurement
Authors: Atherton J. V. , Nickoloff E. L.
Published: March 2003

A clamp‐on current probe utilizing the Hall effect was used to determine x‐ray tube current. This noninvasive technique was compared to two other methods of mA measurement: the Machlett Dynalyzer and the mR/mAs linearity method. Three diagnostic x‐ray units were used in the comparison; two modern three‐phase rooms and one 25 year old single‐phase room. The Dynalyzer and current probe measurements agreed to within ±3% and showed mA miscalibration at several technique settings. The mR/mAs linearity method failed to detect any miscalibration. One disadvantage of using the current probe is its susceptibility to electronic noise when making measurements of tube current below 100 mA. These results justify the use of the current probe in routine quality control calibration checks.
Modification of the fault logic circuit of a high‐energy linear accelerator to accommodate selectively coded, large‐field wedges
Authors: Miller R. W. , van de Geijn J.
Published: March 2003

A modification to the fault logic circuit that controls the collimator (COLL) fault is described. This modification permits the use of large‐field wedges by adding an additional input into the reference voltage that determines the fault condition. The resistor controlling the amount of additional voltage is carried on board each wedge, within the wedge plug. This allows each wedge to determine its own, individual field size limit. Additionally, if no coding resistor is provided, the factory‐supplied reference voltage is used, which sets the maximum allowable field size to 15 cm. This permits the use of factory‐supplied wedges in conjunction with selected, large‐field wedges, allowing proper sensing of the field size maximum in all conditions.
Dosimetry of a dual photon energy linear accelerator
Authors: Findley D. O. , Forell B. W. , Wong P. S.
Published: March 2003

We describe the dosimetric characteristics of a newly introduced dual photon energy linear accelerator, the Varian Clinac 1800. Depth doses are compared with other accelerators of the same nominal accelerating potentials (6 and 10 MV). Field flatness at dmax and at 10 cm depth, depth of dmax, wedge characteristics, output factors, and doses in the buildup region are presented.
Dose distribution for 125I implants due to anisotropic radiation emission and unknown seed orientation
Authors: Prasad Satish C. , Bassano Daniel A. , Fear Peter I.
Published: March 2003

Variations in dose distribution due to anisotropic radiation emission around 125I seeds and a lack of knowledge about the orientation of the implanted seeds have been investigated. Upper and lower bounds for dose distributions have been calculated for planar implants using the experimentally determined angular dose distribution around a typical 125I seed. Results of our study suggest that significant dose variations in the center and the periphery of the implanted area are possible.
A prototype amorphous selenium imaging plate system for digital radiography
Authors: Papin Patrick J. , Huang H. K.
Published: March 2003

This paper summarizes the results of research and development of a prototype amorphous selenium digital imaging system. The first phase of this project consists of the preliminary design and fabrication of the system. In this system the conventional film–screen photon receptor is replaced by a charged amorphous selenium imaging plate. After exposure, the latent electrostatic image on the selenium surface is scanned with multiple microelectrometer probes forming a 1024×1024×12 bit digital image. The second phase investigates the system's physical imaging characteristics and clinical feasibility. X‐ray exposure latitude comparable to 200‐speed calcium–tungstate film–screen system are shown for three typical diagnostic kVp settings with total filtration of 3 mm aluminum and 9 cm Lucite. Using the modulation transfer function (MTF), resolving power of approximately 1.0 line pair per millimeter and detective quantum efficiency values of approximately 5% have been measured. The clinical evaluation consists of preliminary images of a 16‐kg female dog and a 4.5‐kg rabbit and comparisons to film–screen images are offered.
Pulse sequence design for volume selective excitation in magnetic resonance
Authors: Jensen Dye J. , Narayana Ponnada A. , Delayre Jean L.
Published: March 2003

The design of a pulse sequence for volume localization in magnetic resonance spectroscopy is described in detail. The sequence is based on the volume selective excitation technique (VSE) proposed by Aue et al. [J. Magn. Reson. 56, 350 (1984)] and overcomes the high rf power requirements of VSE. The implications of various design stages are demonstrated experimentally and by computer simulations.
The effects of random directional distributed flow in nuclear magnetic resonance imaging
Authors: Ahn C. B. , Lee S. Y. , Nalcioglu O. , Cho Z. H.
Published: March 2003

Capillary flow or microscopic random directional coherent flow as a model of perfusion is investigated both theoretically and experimentally. In the model, we assumed that molecular motion within a finite resolvable volume element (voxel) is a superposition of flow of randomly oriented small capillaries. In such a case, the observed signal from the capillary flow within a voxel will be attenuated in signal amplitude without any change in phase. Although this attenuation effect is similar to the diffusion phenomenon, it differs basically in the following aspects: since the motion in each capillary segment is coherent, phase cancellation occurs at even echoes due to spin rephasing, while the diffusion phenomenon is a purely random Brownian motion of the thermally agitated molecules, changing both in direction and speed during the measurement period. Because of the random character of diffusion, even‐echo rephasing cannot be observed. Thus capillary flow or perfusionlike microscopic flow can be measured based on the above distinct flow characteristics, i.e., signal restoration at even echoes versus signal amplitude attenuation at odd echoes. By applying a suitable mathematical algorithm, information on the capillary flow alone can be extracted from the two separate distinct measurements, i.e., one with a single echo and the other with a double echo. Both a theoretical calculation of the capillary flow, as well as the experimental results with a human volunteer by a 0.6‐T nuclear magnetic resonance imager, are presented.
Electronic scanning‐slit fluorography: Design and performance of a prototype unit
Authors: Plenkovich Dinko , Kruger Robert A. , Sorenson James A.
Published: March 2003

Electronic scanning‐slit fluorography involves replacing paired fore and aft slits for scatter rejection with only one beam‐defining tantalum fore aperture. Since the video signal within the projection of the aperture on the image intensifier is much more intense than behind the tantalum, one can discriminate electronically between these two signals and thus eliminate the unwanted x‐ray scatter and veiling glare. The general features of a prototype unit are described along with the rationale for the choice of design factors employed. Imaging time of 1–2 s has been achieved using multiple scanning slits. Small focal‐spot size and large number of pixels are favored for higher dose utilization, shorter imaging time, and lower x‐ray tube loading, as well as for better spatial resolution. Images of a chest phantom show better visibility of low‐contrast details, especially in poorly penetrated areas, when compared with the image obtained and displayed under the same conditions, but using a conventional grid to reject scattered radiation.
Digital image motion correction by spatial warp methods
Authors: Pickens David R. , Price Ronald R. , Erickson Jon J. , James A. Everette
Published: March 2003

A technique for correction of motion between images which are obtained in high‐speed digital subtraction or cine angiographic acquisitions is being tested. The method is based on the application of quadratic polynomial equations which transform one image so that it matches a reference image. Images which have been processed in this manner can be summed to improve the signal‐to‐noise ratios over individual images. The technique for motion correction currently being tested uses operator interaction to establish the appropriate polynomial transformation. An operator selects fiducial (reference) points on an image which will be the reference. Then he selects the corresponding fiducial points on the image to be processed. The algorithm calculates the coefficients of a pair of quadratic polynomial equations and applies them to each pixel in the image. Results demonstrate the application of the technique in phantoms and in digitized cine angiograms.
Systematic errors in bone‐mineral measurements by quantitative computed tomography
Authors: Rao Gopala U. , Yaghmai Issa , Wist Abund O. , Arora Ganesh
Published: March 2003

Bone‐mineral measurements using quantitative computed tomography (QCT) are commonly based on comparisons with solutions in water of known concentrations of K2HPO4. In this paper are described theoretical and experimental studies that have led to the conclusion that large systematic errors can arise in these measurements, depending on the soft‐tissue and fat concentrations in the vertebral spongiosa. In the case of single energy scanning, such large errors have been identified to be due to the varying water content (displacement effect) in the calibration samples and the varying fat content in the region of interest (ROI) within the patient. In the case of dual energy scanning, the error arises because when normalized to that of water, the mass attenuation coefficient of fat increases with photon energy while the reverse is true for K2HPO4. Our studies have also revealed that total trabecular bone density (which includes the mineral, soft tissue, and fat) can be much more accurately determined by the dual energy QCT method than bone mineral alone. This finding is especially interesting since there have been several reports in the literature suggesting that bone density rather than bone‐mineral content is a better predictor of the risk of osteoporosis‐related fractures.
The extended net fractional depth dose: Correction for inhomogeneities, including effects of electron transport in photon beam dose calculation
Authors: van de Geijn J.
Published: March 2003

The extended net fractional depth dose (ENFD) is developed from the net fractional depth dose (NFD) previously described for unit‐density media, basically by scaling the two geometric parameters, the side of the equivalent square field, and the depth along the ray by the relative electron density. Specifically, in the analytical description for the NFD, the geometric depth is replaced by the radiologic depth and, along the ray path, the geometric field side is scaled by the relative electron density. Interface effects on the electron and scattered‐photon fluences are accounted for. In addition, a simple function is developed to correct for the effect of lateral as well as longitudinal electron transport at the central ray. In the present work the inhomogeneities are assumed to be of planar parallel shape and to extend across the entire beam. The treatment of smaller inhomogeneities is outlined but will be treated in detail separately. Calculated results are compared to measured and calculated data from the literature for 60Co and 10‐MV x rays, and to 15‐MV data measured at the NCI.
Velocity imaging by rapid cycle tagging
Authors: Saloner D. , Moran P. R. , Tsui B. M. W.
Published: March 2003

The unique abilities of magnetic resonance imaging (MRI) to provide detailed images of blood flow in the body, and without resorting to the injection of contrast agents, has provided the stimulus for the keen interest in this subject. In this paper we discuss a technique aimed at providing quantitative information on the distribution of velocity of blood flow across the lumen of a vessel. The technique is designed to reduce the signal from stationary material allowing for a much more accurate determination of the signal from moving material. A particular feature is that the physical phenomenon sensitized is time‐of‐flight displacements or “fresh spin inflow,” and the technique does not rely upon phase‐sensitive modulations. Further this technique produces a steady‐state spatial distribution of magnetization, and hence a signal, which reflects both the time at which material enters the selected slice and how long the material remains in the slice. In this way, a single image will provide the information necessary to extract information on the velocities of interest.
The effects of simultaneous pulsing in different gradient coils on the nuclear magnetic resonance imaging of oblique slices
Authors: Zur Y. , Bendel P.
Published: March 2003

The definition and mapping of oblique planes by magnetic resonance imaging (MRI) requires the simultaneous application of two or three orthogonal gradients to define the desired intermediate direction of the frequency encoding or “readout” gradient. Each of the three main gradient coils produces different patterns of eddy currents. Consequently, the application of dephasing and rephasing lobes of these gradients will produce echoes at slightly different times for each gradient. If two or three gradients are applied simultaneously to create an arbitrary view direction, the resulting echo will therefore be shifted in time and considerably reduced in intensity. In this article, we present an analysis of the behavior of the magnetization in a typical two‐dimensional Fourier transform pulse sequence for the imaging of oblique slices. The theoretical displacements in time and reduction in intensity of the echo amplitudes are calculated and compared to the experimental behavior. We show that, in spite of this phenomenon, the final image suffers only marginally in signal‐to‐noise ratio, provided the slice width is small compared to the field of view. This is due to the fact that there always exists a cycle in the sequence in which the phase‐encoding gradient almost completely compensates for the above described effect.
A fast dual‐energy computational method using isotransmission lines and table lookup
Authors: Chuang Keh‐Shih , Huang H. K.
Published: March 2003

An efficient method has been developed to speed up the computational time required for tissue separation after the dual‐energy scan in computerized tomography and digital radiography. This method consists of two steps. First, an isotransmission line method is used to replace the iterative algorithm for solving the two nonlinear equations, which is computationally noisy and sensitive. Second, two tables, one for the aluminum component and the other for the plastic component, are derived from calibration data in order to replace calculation by a table lookup. Results from this method are compared with the conventional method.
A review of 1H nuclear magnetic resonance relaxation in pathology: Are T1 and T2 diagnostic?
Authors: Bottomley P. A. , Hardy C. J. , Argersinger R. E. , Allen‐Moore G.
Published: March 2003

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation times of pathological human and animal tissues in the frequency range 1–100 MHz are archived, reviewed, and analyzed as a function of tissue of origin, NMR frequency, temperature, species, and in vivo versus in vitro status. T1 data from specific disease states of the bone, brain, breast, kidney, liver, muscle, pancreas, and spleen can be characterized by simple dispersions of the form T1=AνB in the range 1–100 MHz with A and B empirically determined pathology‐dependent constants. Pathological tissue T2 values are essentially independent of NMR frequency. Raw relaxation data, best‐fit T1 parameters A and B, and the mean T2 values, are tabulated along with standard deviations and sample size to establish the normal range of pathological tissue relaxation times applicable to NMR imaging or in vitro NMR examination. Statistical analysis of relaxation data, assumed independent, reveals that most tumor and edematous tissue T1 values and some breast, liver, and muscle tumor T2 values are significantly elevated (p≥0.95) relative to normal, but do not differ significantly from other tumors and pathologies. Statistically significant abnormalities in the T1 values of some brain, breast, and lung tumors, and most pathological tissue T2 values could not, however, be demonstrated in the presence of large statistical errors. Both T1 and T2 in uninvolved tissue from tumor‐bearing animals or organs do not demonstrate statistically significant differences from normal when considered as a group, suggesting no appreciable systemic effects associated with the presence of tumors compared to the statistical uncertainty. Statistical prediction analysis for both T1 and T2 indicates that of all the tissues studied, only liver hepatoma can be reliably distinguished from normal liver based on a single T1 measurement (p≥0.95) given the scatter in the current published data. Indeed, data scatter, not easily attributable to temperature, species, in vivo versus in vitro status, the inclusion of implanted or chemical induced tumors, or the possible existence of multiple component relaxation, is recognized as the major factor inhibiting the diagnostic utility of quantitative NMR relaxation measurements. Malignancy indexes that combine T1 and T2 data as a diagnostic indicator suffer similar problems of uncertainty. The literature review reveals a dearth of information on the temperature and frequency dependence of pathological tissue relaxation and the possible existence of multiple relaxation components. The causes of differences in pathological tissue relaxation times are presently ambiguous, although increases in tissue water content, growth rate, Na+ and K+ concentrations, and reductions in tissue glycogen and protein content have been correlated with elevated tumor T1 values.
Effect of tissue inhomogeneity on beta dose distribution of 32P
Authors: Kwok Cheuk S. , Irfan Muhammad , Woo Milton K. , Prestwich William V.
Published: March 2003

In a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft‐tissue–bone planar interface by a polystyrene (PST)–aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of 32P using 7LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0–31, 125–156, and 283–314 mg/cm2 separations from the interface were increased by (12±3)%, (8±2)%, and (3±2)%, respectively, compared with homogeneous PST. With the point source at a PST–air planar interface to simulate a soft‐tissue–air junction, the dose rates at 0–31, 139–170, and 283–314 mg/cm2 from the interface were decreased by (25±4)%, (11±7)%, and (5±2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of 32P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two‐group method of calculation for a plane source is also presented.
A direct modeling approach to the early renal vascular transit of 99mTc chelates
Authors: Wesolowski Carl A. , Conrad Gary R. , Kirchner Peter T.
Published: March 2003

Delay channel integral methods are developed and applied for analyzing human renal scintiangiographic data produced after peripheral venous injection of a 99mTc chelete. The results include information for two renal vascular compartments (C1, C2) and a residual compartment (Cr). Each vascular compartment is associated with a fraction of the total renal plasma flow (A1, A2) and a mean transit time (T1, T2). The residual compartment has the residual plasma flow fraction Ar. The results obtained in 40 normal kidneys are presented and compared theoretically to the results from other methods.
Constrained least‐squares restoration of nuclear medicine images: Selecting the coarseness function
Authors: Penney Bill C. , King Michael A. , Schwinger Ronald B. , Baker Stephen P. , Stritzke Peter , Doherty Paul W.
Published: March 2003

Image restoration using the constrained least‐squares (CLS) method theoretically adapts to the image being processed. In addition, it only requires knowing the modulation transfer function of the imaging system when applied to nuclear medicine images. Prompted by these observations, a systematic evaluation of the effects of the form of the “coarseness function” [C( f )] used by the CLS method has been conducted. Nine C( f )'s are evaluated using an observer preference and a normalized mean‐squared error (NMSE) criterion. This evaluation is conducted for three modulation transfer functions and a wide range of count levels. The results of the subjective studies support using the form of C( f ) which has been most widely employed in previous studies, i.e., the form designed to minimize the energy in the second derivative of the restored image. A different form of C( f ) is generally found to be optimal by the mean‐squared error criterion. The CLS method is then compared to: (1) no processing, (2) count‐dependent smoothing, and (3) count‐dependent Metz restoration. When evaluated using objective measurements of error and contrast, the CLS method is found to be slightly inferior to the best method, Metz restoration. However, CLS restoration is found to be equal to or better than the other methods when judged by the results of observer preference studies.
Quantitative evaluation of a portal film contrast enhancement technique
Authors: Reinstein L. E. , Alquist L. , Amols H. I. , Lagueux B.
Published: March 2003

A study was conducted to evaluate the subjective improvement in portal film image quality resulting from the contact copy contrast enhancement technique which was introduced six years ago. Five observers were asked to identify and orient polyvinyl chloride cylinder images on both original and contrast‐enhanced portal films taken with a 10‐MeV linear accelerator. Fixed reviewing periods (T) were alloted of 20, 40, and 60 s as well as unlimited viewing time in order to increase the clinical relevance of this comparison. A scoring system and a probability representation were used to compare the original and enhanced films as a function of T. The results show a substantial increase in object detectability for the enhanced films at the short viewing times (T=20, 40, and 60 s). For longer times (T greater than or equal to 80 s) the object detectability for enhanced and original films is not statistically different.
Imaging performance of a digital storage phosphor system
Authors: Hillen W. , Schiebel U. , Zaengel T.
Published: March 2003

The imaging performance of a storage phosphor system (SPS) for digital projection radiography is studied in which the x‐ray image is temporarily stored in a light‐stimulable phosphor plate which is subsequently read out by a scanning laser beam. The imaging performance of this system has been analyzed for two types of imaging plates. The spatial resolution is described by the modulation transfer function and the signal‐to‐noise (S/N) ratios of the recorded image data are measured in terms of noise‐equivalent quanta and detective quantum efficiency. Their dependence on detector entrance dose and space and spatial frequency is discussed. A detailed analysis of the different sources of image noise is given to outline the intrinsic features and limits of the system. Finally, the S/N behavior of the SPS is compared with published data of screen–film systems.
The solution of Bloch equations for flowing spins during a selective pulse using a finite difference method
Authors: Yuan Chun , Gullberg Grant T. , Parker Dennis L.
Published: March 2003

The movement of spins during periods of selective pulses result in a modulation of the signal intensity and phase of the received magnetic resonance imaging (MRI) signal, and is a major cause of signal loss from vessels imaged with slice‐selective pulses. Methods are well developed for compensation of phase perturbations for spins flowing at constant velocity during the time of applied gradients. However, for spins flowing during selective pulses, the magnitude of the amplitude and phase perturbations has not been understood nor to this time has any method of flow compensation been proposed. This is due in part to the difficulty in using the Bloch equations to quantify the amplitude and phase modulation during radiofrequency (rf) excitation since solutions cannot be obtained analytically. In this paper a finite difference method is used to solve Bloch equations for flowing spins during a 90° selective pulse. Compared with stationary spins, the magnetization distribution for flowing spins exhibits a shift of the slice profile in the direction of the flow, an expansion of the profile, phase shifts, and changes in profile shape. The profiles show residual phase errors which become more severe with higher flow velocities, with flow compensation schemes which apply in the case of spins flowing during applied gradients, and in the absence of an rf pulse. The measurement and understanding of the magnetization distribution is important to designing pulse sequences that compensate for flow. Flow compensated pulse sequences are necessary to reduce image flow artifacts and to increase signal of vessels in MR angiographic images.
An analytical approach to quantify uniformity artifacts for circular and noncircular detector motion in single photon emission computed tomography imaging
Authors: Gullberg Grant T.
Published: March 2003

Uniformity artifacts in rotating gamma camera tomography will result if there are errors in the correction factors which are routinely calculated from a static uniformity flood image. The accuracy of the correction factors is a function of the statistics in the collected flood image. Since the factors are applied to each projection view, an error in a correction factor will propagate as a projection error at the same pixel location for each view. For circular detector motion, the error in each projection is reconstructed as a ring whose maximum amplitude varies approximately inversely proportional to the square root of the distance of the projection error from the center of rotation. For noncircular detector motion the artifacts are not rings but are more complicated geometric curves. Simulations show that statistical fluctuations in the reconstructed image will mask the uniformity artifacts provided the correction flood satisfies minimum count requirements. An analytical expression is derived for the percent root‐mean‐square (% rms) error in the reconstruction and is compared with the percent relative amplitude error (% RAE) of the reconstructed artifacts in order to obtain expressions for uniformity flood counting statistics. For an elliptical source distribution with total counts equal to CT, the uniformity statistics required to reconstruct elliptical disks is inversely proportional to the square root of the area: Ū≥KCT/area1/2. The constant K depends on the filter function and type of detector motion.
Analysis of vessel absorption profiles in retinal oximetry
Authors: Roberts D. Aaron
Published: March 2003

The optical densities of vessel absorption features in small bandwidth fundus photographs have been used by many people for finding the oxygenation of the blood in retinal vessels. The purpose of this article is to show that the information found in this way can be made more accurate, and more information can be found by considering the details of the absorption profile (intensity as a function of distance across the vessel). The “wings” of the profile are primarily determined by the line‐spread function of the video camera or film. A four‐parameter curve fitting procedure using the line‐spread function and an “ideal” vessel profile can be used to eliminate the effects both of the line‐spread function and of any central reflection (vessel reflex) that may be present, yielding estimates of the vessel diameter and central absorption. The conclusions concerning the reflection are supported by studies of reflections from capillary tubes and wires in a model eye.
The effect of collimator backscatter radiation on photon output of linear accelerators
Authors: Huang Pin‐Hua , Chu James , Bjärngard Bengt E.
Published: March 2003

The field‐size dependence of the photon output of linear accelerators in air has been attributed in part to changes in the amount of radiation backscattered from the collimator jaws into the dose monitor chamber. This possible effect was investigated for a variety of accelerators with energies from 4 to 15 MV by measuring the monitor unit rate (MU/min) for different collimator openings. This measurement was made without dose rate feedback control, i.e., with constant electron beam current in the accelerator. The monitor unit rate was independent of collimator setting for all machines tested. Hence, it is concluded that backscattered radiation from the collimator jaws into the dose monitor chamber does not contribute to the variation of output with field size.
Characteristic parameters of 6–22 MeV electron beams from a 25‐MeV linear accelerator
Authors: Jamshidi Abolghassem , Kuchnir Franca T. , Reft Chester S.
Published: March 2003

Depth‐ionization measurements were performed using a thin wall parallel plate chamber in water at nominal electron energies of 6, 9, 12, 15, 18, and 22 MeV for the standard available square field sizes. The characteristic parameters of the central axis depth‐dose distributions were derived and compared to corresponding values for other accelerators. Vacuum packed therapy‐verification films were used in water to obtain isodose distributions in a plane containing the central axis of the beam. The uniformity index and penumbra of the beams were measured from isodose distributions obtained in planes perpendicular to the beam central axis, at depths of R85 in water.
Calculations of total activity and relative radiolysis product yields for selected photonuclear reaction target systems used for production of clinically useful, short‐lived, positron‐emitting radionuclides
Authors: Piltingsrud Harley V. , Hoops Richard G.
Published: March 2003

The potential performance of certain photonuclear target systems designed to produce short‐lived positron emitters for nuclear medicine applications was evaluated. Included were calculations of photonuclear target activity yields, relative radiolysis product yields and relative specific activities of products as a function of photonuclear target shapes, dimensions, spacing from the bremsstrahlung photon source, and the energy of the electron beam using the reactions 12C(γ,n)11C, 14N(γ,n)13N, and 16O(γ,n)15O. Results indicate that substantial activity yields can be achieved in these photonuclear target systems and that specific activity of the product varies widely as a function of the energy of the electron beam used to produce the bremsstrahlung and the included angle from the central beam axis that the photonuclear target occupies. Results indicate that several commercial radiotherapy accelerators appear to have electron beam energy and current capabilities suitable for producing multiple mCi (1 mCi=3.7×107 Bq) amounts of the above radionuclides.