摘要:

In radiotherapy treatments with negative pion beams, an external bolus is often used to compensate for inhomogeneities within the body in order that the pions will have the proper stopping distribution within a tumor. However, angular beam divergence, multiple Coulomb scattering, and elastic and inelastic nuclear scattering limit the degree to which the pion stopping region can be controlled. We have used the Monte Carlo computer code PION‐I to calculate pion stopping distributions for several idealized cases in order to show explicitly the effects of a number of factors on stopping distributions. Calculations have also been made for the same geometrical configurations used in measurements of pion stopping distributions in the biomedical beam at the Los Alamos Meson Physics Facility, and the calculated results are compared with experiment.

ISSN:0269-3127    

DOI:10.1118/1.595575

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using the O'Connor density scaling theorem, two basic equations have been derived to express scatter dose calculations in both homogeneous and heterogeneous phantoms: (i) the differential scatter–air ratio for calculating the frontscatter, and (ii) the differential backscatter factor for calculating the backscatter. In the derivation of both equations the relative electron density along the line between the scattering element and the calculation point has been averaged to account for scatter attenuation. Each equation expresses the amount of front or backscatter at the point of calculation per unit primary dose, per unit relative electron density, per unit volume at the scattering element. Primary dose calculations in both a homogeneous and a heterogeneous phantom have been carried out using a simple exponential attenuation law. Except in the area of or near interfaces and the area around field borders where there is electron disequilibrium, satisfactory dose calculations have been obtained using the primary and scatter dose calculation procedure for experiments done with Co‐60 gamma rays in homogeneous soft tissue phantoms and in heterogeneous cork and aluminum phantoms.

ISSN:0269-3127    

DOI:10.1118/1.595576

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The nuclear magnetic resonance (NMR) spin‐lattice relaxation time (T1) of samples taken from human tumors was measuredin vitroat Larmor frequencies of 24 and 6.25 MHz. It was found that on the averageT1at 6.25 MHz was linearly related toT1at 24 MHz. An analogous set of measurements was performed on pieces of normal rat tissue. In this case, the relationship betweenT1at the two frequencies was similar to that found for the human tumor tissue.

ISSN:0269-3127    

DOI:10.1118/1.595577

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To reduce noise‐ and reconstruction‐related artifacts in ultrasound computed tomography, the use of combined median and Hamming‐weighted spatial low‐pass filtering is evaluated. The evaluation of the reconstruction filters uses both computer‐generated projections of a known object with the least mean‐square error criterion as well as a more subjective evaluation of conventional ultrasonic attenuation and speed of sound images.

ISSN:0269-3127    

DOI:10.1118/1.595578

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Most nuclear magnetic resonance (NMR) imaging systems require pulses whose frequency spectrum is shaped so as to selectively excite a given plane in the presence of a magnetic field gradient. We demonstrate by both computer simulation and experiment that linear Fourier transform theory is not a reliable guide to the uniformity of flip angle in the slice. We show by simulation that the nonuniformity can have serious implications for the measurement of relaxation timeT1if selective 180° pulses are used; the exact results depend also on the details of data analysis and criteria for adjusting the rephasing gradients. We describe an experiment and a phantom in which the axial nonuniformity can be demonstrated on clinical NMR imaging machines.

ISSN:0269-3127    

DOI:10.1118/1.595579

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have employed noninvasive, external counting techniques for quantitation of I‐131F(ab′)2fragment of mouse monoclonal antibody localized in metastatic lesions and surrounding liver tissue in humans. This method utilizes counts from diametrically opposed views of tumor deposits and surrounding normal tissue. Corrections were made for patient attenuation, lesion size, and surrounding tissue activity. The validity of this method was evaluated using a fillable, tissue‐equivalent organ‐scanning phantom with organs and tumors of selected size. Less than 10% error was found in quantitation of various activities of I‐131 in a 4‐cm‐diam lesion. Tumor activity ranged from 0.001% to 0.018% of administered dose per cm3of tissue compared with 0.000 12% to 0.0023% per cm3of liver. In addition, the vascular clearance of total I‐131 and protein‐bound I‐131 was found to follow a two‐compartment model with mean half lives of 3.8 and 21.4 h for total I‐131 and 3.9 and 24.4 h for protein‐bound I‐131.

ISSN:0269-3127    

DOI:10.1118/1.595580

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The net fractional depth dose (NFD) is defined as the fractional depth dose (FDD) corrected for inverse square law. Analysis of its behavior as a function of depth, field size, and source–surface distance has led to an analytical description with only seven model parameters related to straightforward physical properties. The determination of the characteristic parameter values requires only seven experimentally determined FDDs. The validity of the description has been tested for beam qualities ranging from60Co gamma rays to 18‐MV x rays, using published data from several different sources as well as locally measured data sets. The small number of model parameters is attractive for computer or hand‐held calculator applications. The small amount of required measured data is important in view of practical data acquisition for implementation of a computer‐based dose calculation system. The generating function allows easy and accurate generation of FDD, tissue–air ratio, tissue–maximum ratio, and tissue–phantom ratio tables.

ISSN:0269-3127    

DOI:10.1118/1.595581

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An experiment to determine the peak of the energy spectrum of the photon beam from a Toshiba LMR‐15 medical linear accelerator is described. It is found that the flattening filters removed much of the bremsstrahlung spectrum below approximately 1 MeV, resulting in a photon spectrum which peaks around 1.2 MeV.

ISSN:0269-3127    

DOI:10.1118/1.595582

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We investigated theoretically the effects of various digital parameters, such as sampling aperture, sampling distance, number of quantization levels, and display aperture, on the noise Wiener spectrum of digital radiographic imaging systems. We also measured Wiener spectra for our digital image simulation/processing system, and the results agreed well with the theoretical predictions. Aliasing, which is an artifact caused by undersampling, and the use of a limited number of quantization levels were found to increase the Wiener spectrum for digital systems. The effects of image processing, including unsharp mask filtering, integration, and subtraction, on the Wiener spectrum were also demonstrated. Since noise influences the detectability of radiologic objects and thus diagnostic accuracy, knowledge of the effects of the various digital parameters on the noise spectrum will be useful in the evaluation and design of future digital imaging systems.

ISSN:0269-3127    

DOI:10.1118/1.595583

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A comparison of the AAPM “Protocol for the determination of absorbed dose from high‐energy photon and electron beams” (TG21) with currently used protocols for electron and photon dosimetry is presented. These protocols are the International Commission on Radiation Units and Measurements Report 21, “Radiation Dosimetry: Electrons with Initial Energies Between 1 and 50 MeV” (ICRU21), and the AAPM “Protocol for the Dosimetry of X‐ and Gamma Ray Beams with Maximum Energies Between 0.6 and 50 MeV” (SCRAD). Assuming a given radiation exposure and chamber parameters, doses to water atdmaxfor electron beams and at 5 g/cm2for photon beams are calculated using the three protocols and then compared. The doses for photon beams calculated using the TG21 and SCRAD protocols are found to differ by 3% or less at energies below 10 MeV. The largest differences occur in photon doses at high energies where the dose calculated with the TG21 protocol is as much as 5.5% greater than that calculated with the SCRAD protocol for a typical thimble ionization chamber. For low electron beam energies, the doses calculated with the ICRU21 protocol are as much as 5% less than TG21 doses when using thimble chambers constructed of tissue‐equivalent materials in a water phantom. If dosimetry measurements are performed in polystyrene, the dose calculated using TG21 may be greater than the ICRU21 dose, depending on chamber size and composition. An explanation for some of the differences between the protocols is presented emphasizing the dependence on chamber geometry, chamber composition, and phantom composition.

ISSN:0269-3127    

DOI:10.1118/1.595568

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY