摘要:

An isospherical array of ultrasonic transducers designed for hyperthermia treatment of malignant tumors in humans was employed in studies of transient cavitation in tissues during ultrasonic heating. Bothin vitroandin vivostudies revealed the presence of transient cavitation within tissues during ultrasonic heating, as indicated by the detection of the first half‐order subharmonic of insonifying ultrasound with an interrogating transducer. The applied power to the focal region of the array required to exceed cavitation thresholds in tissue was 75±8 W/cm2, a level considerably higher than that required to heat tissues to the temperature range used for cancer treatment (43–45 °C.).

ISSN:0269-3127    

DOI:10.1118/1.595065

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For diagnostic ultrasound 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‐sectionalB‐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.

ISSN:0269-3127    

DOI:10.1118/1.595218

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A comparison has been made of central axis percent depth dose and absorbed dose in electron beams of 7.8 and 10.2 MeV, measured with devices of differing geometry and construction. Flat and cylindrical ionization chambers have been used as well as thin thermoluminescent dosimeters. The ionization chambers had walls of air equivalent or tissue equivalent plastic. Results indicate that central axis depth dose measurements are independent of measuring device. No significant difference was found among the various ionization chambers with air equivalent walls in the determination of absorbed dose. The dose determined by the tissue‐equivalent wall chamber was about 3% higher than the dose determined by the other ionization chambers. Dose maximum on the central axis in water is about 4% greater than when this same quantity is calculated from data measured in polystyrene.

ISSN:0269-3127    

DOI:10.1118/1.595055

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For megavoltage x‐ray beams, the percent depth dose increases considerably with field size in the buildup region, with a concomitant shift in the position of the maximum dose (dmax) to a shallower depth. Various authors disagree as to the cause of this effect. The radiation components contributing to absorbed dose in the buildup region of 10‐MV photon field were analyzed as a function of field size by placing an electromagnet next to the Clinac 18 treatment head. The percent depth dose curves in the buildup region, obtained with a parallel plate chamber downstream from the magnetic field at 85 cm SSD, exhibited nodmaxshift and a much reduced dependence on field size, in clear distinction with similar data taken with zero magnetic field. Confirmatory data were obtained at 100 and 120 cm SSD. These results clearly show that scattered electrons are the primary cause for thedmaxshift and the dose increase in the buildup region with increasing field size.

ISSN:0269-3127    

DOI:10.1118/1.595066

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo calculations have been made to determine the energies delivered by photons and neutrons to the human body irradiated by collimated photon beams. The beams were monoenergetic and ranged from 100 keV to 40 MeV. The energy deposition in the body was sorted into two regions: inside and outside the irradiated volume. Most of the results obtained were for a beam size of 100 cm2although some calculations were also made for 600 cm2beams. The effect of beam size on energy deposition in the two regions was investigated for60Co gamma rays. Graphs are presented which give the integral doses delivered by neutrons and photons to the two regions for therapy beams of various energies. These graphs can be used to calculate the integral doses which are delivered inside and outside the treatment volume for photon spectra from most medical accelerators. Calculations of energy deposition were also made for the spectra from two particular accelerators. These were done using Monte Carlo as well as by simply “folding” the spectra into the results for monoenergetic photons. The results obtained by both methods were in good agreement and indicated that the integral doses deposited outside the treatment volume by neutrons are more than two orders of magnitude smaller than those deposited by scattered photons.

ISSN:0269-3127    

DOI:10.1118/1.595137

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo calculations have been made to determine the energy delivered to a phantom by neutrons escaping from the head of a Varian Clinac 35 medical accelerator at an SSD of 1 m. The energy was sorted into two regions: inside and outside a volume defined by a circular beam of area 100 and 600 cm2. For the two beam sizes, the energies outside the treatment volume were 12 and 8.7 g‐rad (per photon rad) respectively. Room scattering increased these values by about 20%. These energies are smaller than those delivered by scattered photons by more than ten times.

ISSN:0269-3127    

DOI:10.1118/1.595067

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Absorbed dose in A‐150 plastic irradiated by 15‐MeV neutrons was measured using three dosimetric methods: an A‐150 plastic calorimeter, A‐150 ionization chambers, and activation of gold foils. The dose determinations by the calorimeter and ionization chambers agreed to better than 1%, while the activation method yielded a 3.5% lower dose. The agreement between the methods is within the experimental uncertainties.

ISSN:0269-3127    

DOI:10.1118/1.595051

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Various mathematical techniques have been used to express the uniformity of a flood field image from an Anger scintillation camera. We have defined uniformity in terms of the parameters of a collection of Poisson distributions representing a flood field. A method for estimating uniformity and for constructing a confidence set for uniformity has been derived using order statistics. The method is illustrated using the 5th order statistic.

ISSN:0269-3127    

DOI:10.1118/1.595219

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Dual Radiation Action Theory of Kellerer and Rossi (DRA), along with presently available microdosimetric techniques, is applied to the detrmination of radiation quality variation within tissue equivalent phantoms irradiated by collimated fast neutron beams. The neutron beams investigated were produced by the bombardment of 22.5 and 16 MeVd+ on beryllium and by theT(d,n)4He reaction (15‐MeV neutrons). Microdosimetric spectra were obtained at points of varying depth and lateral distance from the central axis within a tissue equivalent phantom, including points within the penumbra. From the microdosimetric spectra the parameterRQ, a first approximation to RBE derived from DRA theory, is calculated for each point. AllRQvalues are calculated for the same level of effect. For these three different beams the results show that theRQvalues for the total radiation spectrum of neutron and gamma radiation remain fairly constant with depth and with lateral distance from the beam axis at 2 and 10 cm depths. The largest central axis variation inRQis 8% for thed(16)+Be beam. The largest variation between a penumbra and an on‐axisRQvalue is 4% at 2 cm depth in thed(22.5)+Be beam. The results for thed(22.5)+Be beam disagree with previously reported radiobiological results while the 15 MeV beam results are in good agreement.

ISSN:0269-3127    

DOI:10.1118/1.595052

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method to predict square and rectangular field output factors from the measurement of selected fields of electron beams on the Therac 20 Saturne has been developed. A two parameter fit of the square field output factor data, based on the functional dependence as predicted by a pencil beam calculational model, has proven clinically acceptable. The pencil beam distributions are given by the Fermi–Eyges theory of multiple Coulomb scattering. For a rectangular field, the output factor can be calculated from the square root of the product of the two square field output factors with sides equal to those of the rectangular field. If however, there is a significant asymmetry between theXandYcollimator systems, then rectangular field output factors should be predicted from the product of theXandYone‐dimensional output factors. One‐dimensional output factors are defined as output factors of rectangular fields where one side remains constant and equal to the side of the square reference field. Measured data indicate either of the two methods of determining rectangular field output factors to be clinically acceptable for the Therac 20, the use of one‐dimensional output factors demonstrating greater accuracy. Data show agreement to within approximately 1.5% at electron energies of 6, 9, 13, and 17 MeV.

ISSN:0269-3127    

DOI:10.1118/1.595138

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY