摘要:

Synthetic diamonds with controlled amounts of impurity atoms can be manufactured so that, as thermoluminescent dosimeters, they can be made to have sensitivities at least as good as presently available commercial thermoluminescent dosimeters. They also exhibit, for radiations normally found in therapy situations, a linearity of response that extends from less than 0.01 Gy (1 rad) to over 10 Gy (1000 rad). Their physical size and form, crystals which can have volumes of less than 1 mm3, make them ideal candidates forin vivomonitoring of radiation fields, particularly electron fields where high‐resolution measurements are essential for accurate isodose line determinations. Aspects of dose response from gamma‐ray beams in relation to the type and concentrations of the impurity atoms within the diamond are discussed, and some experimental values for gamma, x‐ray, and electron beams are presented.

ISSN:0269-3127    

DOI:10.1118/1.596024

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We investigated the problem of how to minimize the dose rate outside a spherical target volume for a given minimal dose rate inside the sphere for 1/r2photon emitters. An integral equation for an activity distribution is derived from the demand that the dose rate is constant inside the sphere. The solution of this equation is given and it is proven that the resulting activity distribution gives the smallest dose rate at every point outside the target volume. Some other activity distributions are discussed; they are assessed by the required integral activity, which is proportional to the dose rate at some distance from the target volume. Compared with a single point source at the center of the sphere, one can save 50% of the total activity when using the optimal activity distribution. The benefit reduces to about 30% for the other continuous activity distributions and to a few % for symmetrical arrangements of a few point sources.

ISSN:0269-3127    

DOI:10.1118/1.596025

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It is known that motion of an object imaged by magnetic resonance imaging leads to errors and artifacts in the image. This effect is especially important when a pulse sequence with long echo time (TE) and longTRis used to provideT2‐weighted images. We have modified the usual spin‐warp imaging pulse sequence in a way which greatly reduces the severity of such errors. The method was tested on both a moving phantom and a living rabbit. The pulsing of the scanner was gated in synchrony to the motion of the phantom; two pulses per mechanical cycle were used to create a special situation which served to verify the validity of a theoretical analysis of the loss of signal with echo time. Under such conditions it was shown that the conventional method led to a progressive loss of signal with increasingTEwhile the new method gave a pure exponential decay of signal in accordance with the knownT2of the sample. When either the phantom or the animal was scanned without gating, the new pulse sequence showed far less artifacts attributable to motion. The potential application of the technique to improved quantitative imaging of the abdomen in clinical situations is discussed.

ISSN:0269-3127    

DOI:10.1118/1.596026

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Presented is a straightforward derivation of the effective sample resistance presented to the input amplifier of a nuclear magnetic resonance imager when flat circular surface coils are used to image lossy media. The resistance may be used to determine sample noise following the method of Hoult and Lauterbur.

ISSN:0269-3127    

DOI:10.1118/1.596027

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous experiments have shown that murine ocular lenses immersed in salinein vitrosuffered microscopic physical damage (including cataractogenic changes) after exposure to pulsed microwaves under conditions in which the temperature rise was negligible. The suspected coupling mechanism is thermoelastic transduction (TET). To test this hypothesis, laser interferometry was used to measure the lens surface motion induced by pulsed microwaves. Although experimental constraints delayed the recording of surface motion data until 0.5 ms after the microwave pulse, damped oscillatory motion was measured over an interval of several milliseconds. A theoretical model is presented to explain the observed motion as natural oscillation modes of an elastic sphere excited by TET waves produced in the surrounding saline. Effects of the lens capsule are also discussed. The conclusions reached are that the observed motion is TET induced, but the lens damage occurs during the passage of the intial excitatory TET waves through the lens, an event which could not be monitored by this experiment.

ISSN:0269-3127    

DOI:10.1118/1.596028

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Ultrasonic waveforms backscattered from tissue simulating phantoms and from normal and cirrhotic human liversin vivowere digitized to a standard dynamic range prior to envelope detection and determination of envelope amplitude distributions. For 11 individual narrow‐band Gaussian‐shaped filters of −6 dB bandwidth 200 kHz, and of center frequencies from 2 to 4 MHz, envelope amplitude distributions were plotted and mean values of the various distributions computed. Analysis of data was performed for data from a phantom containing only relatively small graphite scatterers (<170 μ), and a similar phantom to which glass spheres 0.5 mm in diameter had been added homogeneously. For lower center frequency narrow‐band filters, significantly more high‐amplitude occurrences were observed for data from the phantom to which glass spheres had been added. Higher center frequency narrow‐band filters gave significantly more high‐amplitude occurrences for the phantom containing only small scatterers. Similar data analysis was performed forin vivohuman liver data from ten normal subjects and five patients with known cirrhosis of the liver. For the cirrhotic and normal livers, data analysis using narrow‐band filters of relatively low center frequency resulted in more high‐ amplitude occurrences for cirrhotic, compared to normal liver; the converse was true for narrow‐band filtration at relatively high center frequencies. Determination of mean amplitude following narrow‐band filtration with a filter centered at 3.4 MHz was found to be quite repeatable for the normal and cirrhotic liver data; analysis of variance showed the measurement was 94.1% a function of the subject examined, and 5.9% related to the data acquisition session. The same measurement of mean amplitude permitted discrimination (p<0.02) between the normals and the cirrhotic liver patients. It was noted that the amplitude distributions of the unfiltered envelope permitted no discrimination whatever between the two phantoms, or between normal and cirrhotic liver, in contradistinction to the technique involving narrow‐band filtering.

ISSN:0269-3127    

DOI:10.1118/1.596029

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An endoscopic instrument for normal optical inspection and simultaneous fluorescence characterization is described. The equipment is primarily intended for early cancer detection using characteristic fluorescence from the tumor‐seeking agent hematoporphyrin derivative. A dimensionless ratio of two fluorescence intensities is monitored making the equipment insensitive to target distance and surface topography. A measurement example is given and possible improvements are discussed.

ISSN:0269-3127    

DOI:10.1118/1.596030

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Early tumors can be diagnosed by the fluorescence of an injected, tumor‐specific agent such as dihematoporphyrin ether. However, brightness or intensity contrast is low because of the autofluorescence of tissue. Addition of color (hue) information aids in detection of tumors, and in elimination of false positives. Standard color video cameras are not sensitive enough to image the weak fluorescence. Thus an intensified monochrome video camera has been equipped with a synchronized color filter wheel, and the image displayed by multiplexing alternate lines to a red green blue (RGB) monitor.

ISSN:0269-3127    

DOI:10.1118/1.596031

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Gibbs artifact in magnetic resonance imaging results when band‐limited interpolation is used. This is typically done when there are more reconstructed pixels in the phase encoding direction of the image than corresponding phase encoding measurements. Such sampling is effectively an ideal (in a noise sense) low‐pass filter which provides a maximal improvement in contrast resolution at the expense of a decrease in spatial resolution. In this paper we demonstrate that an alternate low‐pass filter can be used to improve contrast resolution with a loss in spatial resolution and yet not result in Gibbs artifact. We show that the noise performance of this filter can be made to approach that of an ideal filter by properly specifying the number of samples averaged for each phase encoding index.

ISSN:0269-3127    

DOI:10.1118/1.596032

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new tissue‐equivalent phantom material has been developed which also acts as a dosimeter. The new phantom material has a similar elemental composition to that of soft tissue and has a density 1.1 g/cm3. The phantom has an agar‐gel base, and contains crystallizedL‐α‐alanine which traps radiation‐induced free radicals. Samples from the phantom were analyzed by an electron paramagnetic resonance (EPR) spectrometer and the intensity of the EPR signal was related to the absorbed dose. When calibrated, the phantom material acts as a dosimeter, with applications in radiation therapy.

ISSN:0269-3127    

DOI:10.1118/1.596033

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY