摘要:

The two‐loop–one‐gap loop–gap resonator (planar pair) has been investigated as a local receiving coil for high‐resolution proton imaging at 1.5 T (63.8 MHz). Sensitivity contours were determined on the bench by measuring the voltage induced in the coil when it is placed on the surface of a tank containing 0.1‐M NaCl, as a function of the coordinates and orientation of a radiating dipole inside the tank. Contours were obtained as a function of the diameterDof the two loops, and their separationd. For comparison, considerable data also were obtained for single‐turn circular loops as a function of loop diameter. Coil quality factors,Q's, are reported in free space and the saline tank as a function of geometric parameters. Contours also were obtained using phantoms in a whole‐body imager. Planar‐pair coils couple to slabs of approximately (2D+d)×Dcross section, and are superior to circular coils for imaging anatomic structures that are somewhat elongated. A particularly important feature of planar‐pair local coils is that they are intrinsically isolated from the whole‐body coil of the imager, and no special passive or active circuits are required when the whole‐body coil is used for excitation. Planar‐pair local coils have been optimized for imaging of the temporomandibular joint, and an image is shown.

ISSN:0269-3127    

DOI:10.1118/1.595934

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The application of recursive filtering in digital radiology can yield incorrect blood flow estimates if blood flow is calculated using a transit time, defined as the difference in times to peak contrast over the length of a vessel segment. The transit time error is calculated here, from computer simulations, as a function of filter design, image noise and contrast medium dispersion over the vessel segment. An error reduction procedure based on prefiltering is suggested and tested by simulation. Results indicate that the transit time error can be substantial and the reduction procedure effective, depending on the filter design and system noise level.

ISSN:0269-3127    

DOI:10.1118/1.595928

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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

ISSN:0269-3127    

DOI:10.1118/1.595932

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Calculations of various physical quantities pertaining to scattered photons in diagnostic radiology are conveniently carried out using the Monte Carlo technique. Some quantities, e.g., the small‐angle distribution of scattered photons transmitted through the patient, are difficult to obtain with sufficient precision using straightforward simulation of physical experiments. By mixing the simulation of random trajectories with analytical calculations, the efficiency of deriving values for a particular field quantity may be drastically improved. This work describes a Monte Carlo collision density estimator that increases the efficiency of calculating the small‐angle distribution of transmitted scattered photons by a factor of more than 50. Examples of such distributions outside laterally infinite water slabs are given for x rays generated at 40–70 kV and for various slab thicknesses (10–200 mm). Comparison with experimental results from the literature shows that cross sections for coherent scattering which take diffraction phenomena in liquid water into account must be used to get accurate results. A discrepancy between the experimental and calculated distributions of photons transmitted at very small (<3°) angles to the normal to the slab may be interpreted in terms of experimental difficulties or insufficient accuracy in the differential scattering cross sections used in the calculations.

ISSN:0269-3127    

DOI:10.1118/1.595933

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A high‐pressure gas scintillation proportional chamber has been designed and constructed to image x and gamma rays for medical applications. The chamber contains 4 atm of pure xenon. Ultraviolet light emitted from excited xenon atoms within the detector is collected by a hexagonal array of seven UV‐sensitive photomultiplier tubes, which in turn are separated from the pressurized gas by 1‐cm‐thick fused‐silica windows. A model was used to predict the energy resolution of the device as a function of fill‐gas pressure, voltage within the detector, and light‐collection efficiency. The energy resolution improved with increasing scintillation region voltage from 17% full width at half maximum (FWHM) at 1.9 kV to 10% FWHM at 3.0 kV for 59.5‐keV photons; once above 1.5 kV, there was no improvement with increasing drift voltage. The addition of the signals from the peripheral phototubes to that of the center phototube did not substantially improve the energy resolution of the device. This was because the noise that was present yielded a high correlation between the phototubes; when this noise was incorporated into the model, the energy resolution of the multiphototube system was accurately estimated. The energy resolution of the gas scintillation proportional chamber was found to be superior to the sodium iodide Anger camera at 59.5 keV by a factor of 2. Further improvement can be obtained by increasing the scintillation region voltage and by increasing the light‐collection efficiency by moving the scintillation region closer to the phototubes.

ISSN:0269-3127    

DOI:10.1118/1.595921

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Past efforts at routine evaluation of gamma camera systems performance, using final product images, primarily resulted in qualitative assessments. This paper describes a system for quantitatively determining certain performance parameters of gamma camera systems using paper‐backed print and transparent film test images of the same type used for viewing by the nuclear medicine physician. By use of appropriate transmission and line source phantoms, the gamma camera system's spatial resolution, distortion, and flood field uniformity as a function of position in the image field, are determined using a semiautomated image scanning and data reduction system. This system consists of a high‐resolution scanning microdensitometer/reflectometer, interfaced to a computer, along with necessary programs to locate bar and line grid intersection points in the image. Also included are algorithms for calculating the various image parameters desired. This system may provide an analytical method for quantitatively assessing certain performance parameters of gamma camera systems, using the final‐product images from the cameras.

ISSN:0269-3127    

DOI:10.1118/1.595961

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique has been developed forin vivodetermination of lithium content in the head (and potentially in the whole body and in selected organs) of patients undergoing lithium therapy. It is based on the measurement of tritium induced by the6Li(n,α)T nuclear reaction after neutron irradiation of the body. The fraction of tritium exhaled in the expired air in the form of HT is collected, separated from the other gases, and counted in a high‐sensitivity beta counter. The feasibility of the technique was demonstrated by measurements of lithium in the head of a sheep and in the whole body of rats, following the administration of  6LiCl (enriched6Li isotope, 95.46% abundancy). The precision of the technique is acceptable for clinical applications based on a maximum propagated error of 8.4%. The sensitivity is 1 count/d (from T activity) per 10 mSv (total dose) and 1 μg of6Li. This indicates that studies on patients under6Li treatment can be performed successfully with a radiation dose to the head of 1–2 mSv.

ISSN:0269-3127    

DOI:10.1118/1.595922

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to predict the efficacy of boronated compounds for neutron capture therapy (NCT), it is mandatory that the boron concentration in tissues be known. Various techniques for measurement of trace amounts of boron (1–100 ppm) are available, including chemical and physical procedures. Experience has shown that, with the polyhedral boranes and carboranes in particular, the usual colorimetric and spark emission spectroscopic methods are not reliable. Although these compounds may be traced with additional radiolabels, direct physical detection of boron by nondestructive methods is clearly preferable. Boron analysis via detection of the prompt‐γ ray from the10B(n,α)7Li reaction has been shown to be a reliable technique. Two prompt‐γ facilities developed at Brookhaven National Laboratory are described. One, at the 60‐MW high flux beam reactor, uses sophisticated beam extraction techniques to enhance thermal neutron intensity and reduce fast neutron and γ contamination. The other was constructed at Brookhaven's 5‐MW medical research reactor and uses conventional shielding and electronics to provide an “on‐line” boron analysis facility adjacent to beams designed for NCT, thus satisfying one of the requisites for clinical application of this procedure. Technical restrictions attendant upon the synthesis and testing of boronated biomolecules often require the measurement of trace amounts of boron in extremely small (mg) samples. A track‐etching technique capable of detecting ng amounts of boron in mg liquid or cell samples is described. Thus it is possible to measure the boron content in small amounts (mg samples) of antibodies, or boron uptake in cells grown in tissue culture.

ISSN:0269-3127    

DOI:10.1118/1.595962

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When a beam‐modifying filter such as a wedge or a compensator is placed in an x‐ray beam, scattered photons are generated in the filter material. The magnitude of the dose contribution from these photons for a 4‐MV x‐ray beam was measured. At a distance of 30 cm from the filter, a copper sheet of 1‐cm thickness produced a dose contribution on the centerline of about 6% of the transmitted primary dose in a 20×20 cm2field. At the edges of the beam, this contribution was only about one‐half that on the centerline. The presence of these scattered photons leads to only minor dose errors in clinical applications if simple procedures are followed to account for their contribution.

ISSN:0269-3127    

DOI:10.1118/1.595923

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Differential pencil beam (DPB) is defined as the dose distribution relative to the position of the first collision, per unit collision density, for a monoenergetic pencil beam of photons in an infinite homogeneous medium of unit density. We have generated DPB dose distribution tables for a number of photon energies in water using the Monte Carlo method. The three‐dimensional (3D) nature of the transport of photons and electrons is automatically incorporated in DPB dose distributions. Dose is computed by evaluating 3D integrals of DPB dose. The DPB dose computation model has been applied to calculate dose distributions for60Co and accelerator beams. Calculations for the latter are performed using energy spectra generated with the Monte Carlo program. To predict dose distributions near the beam boundaries defined by the collimation system as well as blocks, we utilize the angular distribution of incident photons. Inhomogeneities are taken into account (1) by attenuating the primary photon fluence exponentially utilizing the average total linear attenuation coefficient of intervening tissue, (2) by multiplying photon fluence by the linear attenuation coefficient to yield the number of collisions in the scattering volume, and (3) by scaling the path between the scattering volume element and the computation point by an effective density.

ISSN:0269-3127    

DOI:10.1118/1.595924

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY