摘要:

The potential role of nonlinearities in the magnetic field gradient in magnetic resonance imaging for producing sharper boundaries for the excited spin slice region is investigated. A theoretical analysis based on solving the Bloch equations numerically is presented. Simple criteria for measuring the performance of selective excitation are defined and then used to evaluate the results for selective 90° and 180° pulses.

ISSN:0269-3127    

DOI:10.1118/1.595893

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Imaging parameters associated with a clinical magnetic resonance imaging (MRI) system, evaluated with the use of simple and inexpensive phantoms, are described in evaluation of a clinical MRI system. Images, obtained with a Diasonics 0.35‐T MRI system using an elliptical whole‐body radio frequency coil, are presented which demonstrate geometric distortion, inaccuracy of image dimensions, and artifacts. Measurements of reproducibility and uniformity of both signal intensity and spin‐lattice relaxation times are presented for uniform phantoms. Replicate measurements are analyzed by two‐way analyses of variance to determine the significance of variations as a function of position and slice. ApparentT1's are not significantly different among the slices for the images analyzed, but there is a significant effect due to position in the field. Direct measurement of the radio frequency field for the center slice follows a pattern which is similar. Spin‐lattice relaxation times are compared among two‐point calculations and nonlinear four‐point calculations, and the effects of pooling data are found to increase the precision ofT1measurements.

ISSN:0269-3127    

DOI:10.1118/1.595894

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The energy deposition pattern within an isolated human leg heated with a mini‐annular phased array (MAPA) hyperthermia applicator has been determined. The non‐tumor‐bearing lower portion of a human leg amputated at the hip due to the presence of a large tumor in the thigh was “fixed” in a 50% ethanol in 0.9% saline solution. Subsequent to this fixation process, the leg was rehydrated in 0.9% saline and heated four times using a MAPA operating at 122 MHz. Specific absorption rates and electric field strengths were calculated from the rates of change of temperature with time measured at 143 different anatomical locations within the leg. When the leg was coaxial with the MAPA and the MAPA was axially positioned midway between the knee and the ankle, the points of maximum heating were skewed away from the center of the MAPA, towards the ankle of the leg and along the central axis of the MAPA. Significant temperature rise was measured inside the bone and the fat as well as inside the muscle of the leg. Bone heating was reduced when the leg was shifted away from the MAPA axis.

ISSN:0269-3127    

DOI:10.1118/1.595945

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It has been known for some time that the irradiation of metallic thermometers by microwaves during clinical hyperthermia can lead to artifactual readings. We describe here a series of measurements in which this effect has been quantitatively studied. In particular, the data yield values for the conversion coefficient describing the rate of heat production per unit length of a thermocouple array per watt applied power which can be compared with the rate of heat production in the same volume of tissue. The degree of artifact in the temperature recording depends on the thermal resistance of the protective materials surrounding the array, and this thermal resistance has also been determined. It has been shown that measures taken to reduce the temperature artifact do not compromise the response time of the probe.

ISSN:0269-3127    

DOI:10.1118/1.595952

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In the previous paper we had developed a general thermodynamic equation describing a polymethylmethacrylate implant at the site of giant cell tumors. In this paper we consider various characteristics of bone and methylmethacrylate crucial to the analysis such as thermal conductivity, specific heat, density, and heat generation. Also, an estimation of the temperature at which adjacent cells die is analyzed from literature. Finally, using the physical constants measured in laboratory situations a temperature profile is developed at various depths of bone that could facilitate predicting the zone of necrosis. These analyses show the maximum temperature attained in the acrylic cement–bone system depends primarily on (a) the volume of the implant, (b) the relative proportion of polymerization of the monomer, (c) the temperature at which the monomer and polymer are mixed together, and (d) the time lapse between the beginning of polymerization and implantation into the bone cavity. The temperature profile is shown to be relatively insensitive to the geometry of the system, greatly simplifying the analysis.

ISSN:0269-3127    

DOI:10.1118/1.595852

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When the power spectrum of radiographic noise is estimated from scans obtained with a microdensitometer equipped with a rectangular slit, a bias is present due to the finite length of this slit. A method to correct for this bias was developed and was tested on both a simulated and a measured spectrum. No bias remains with our method, in situations where a conventional method shows a significantly biased result. Good agreement was found with the synthesized slit method, described by Sandrik and Wagner.

ISSN:0269-3127    

DOI:10.1118/1.595853

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two‐component cortical bone and water, and trabecular bone and water models were used to study the beam hardening errors associated with computed tomography (CT) bone densitometry and sizing methods. Specimens used included a femur, humerus, radius, ulna, and vertebral bodies. A second‐order correction algorithm was employed to improve the accuracy of these quantitative CT methods. Physical measurements of the bones were obtained to verify the CT results. Large discrepancies were found between the uncorrected and second‐order corrected CT assessments of cortical bone density, trabecular bone density, water density within the medullary canal, total bone area, medullary canal area, and cortical area. The interosseous lucent streak was also quantitatively studied. Results showed that second‐order correction significantly improved the accuracy of CT bone densitometry and sizing methods.

ISSN:0269-3127    

DOI:10.1118/1.595971

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Reprojection is the process by which projections are produced from an image, such that if these projections are filtered and backprojected, they yield the original image. In computed tomography, applications of reprojection include its use in algorithms for iterative beam hardening correction, metal artifact removal, and streak suppression. However, because of the computational expense of reprojection, algorithms that employ this process have never been widely used. A method will be presented that enables an unmodified backprojector to be used as a reprojector. Because backprojectors are designed to exploit the parallelism in the backprojection algorithm, the time required to obtain reprojections is significantly reduced.

ISSN:0269-3127    

DOI:10.1118/1.595950

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper presents an analysis of two cone beam configurations (having focal lengths of 40 and 60 cm) for the acquisition of single photon emission computed tomography (SPECT) projection data. A three‐dimensional filtered backprojection algorithm is used to reconstruct SPECT images of cone beam projection data obtained using Monte Carlo simulations. The mathematical analysis resulted in on‐axis point source sensitivities (calculated for a distance of 15 cm from the collimator surface) for cone beam configurations that were 1.4–3 times the sensitivities of parallel‐hole and fan beam geometries having similar geometric resolutions. Cone beam collimation offers the potential for improved sensitivity for SPECT devices using large‐field‐of‐view scintillation cameras.

ISSN:0269-3127    

DOI:10.1118/1.595854

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo methods have been used to simulate the scattering of x rays in polystyrene and water phantoms. In particular, the ratio of the scattered to total x‐ray fluence (scatter fraction) has been calculated for monoenergetic x‐ray beams in the energy region relevant to diagnostic radiology and nuclear medicine (30–660 keV). Simulations have been made for representative values of the pertinent geometrical factors; phantom thickness from 5 to 21 cm, x‐ray beam diameters of 10 and 25 cm, and scatterer‐to‐image‐plane separations from 0 to 20 cm. As a function of x‐ray energy, the scatter fraction was found to vary slowly between 30 and 100 keV, and to decrease between 100 and 660 keV. The present results were generated with a special transport code which included the effects of special geometries and the response of the x‐ray detector. With the inclusion of these effects, the results resolved inconsistencies and showed good agreement with previous measured and calculated data.

ISSN:0269-3127    

DOI:10.1118/1.595855

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY