摘要:

Ionization chambers with wall materials of various electron densities have been employed to investigate the penumbral region of a cobalt‐60 beam of side length 10 cm. Measurements were made in air with chambers of various inside diameters. The homogeneous walls of the ionization chambers were 0.5 g/cm2in thickness. Extrapolation techniques to zero inside diameter and to infinite density were used, respectively, to eliminate the effect of the inside diameter of the chamber and to determine the penumbral photon‐fluence distribution. The method also quantitates both the penumbral primary‐dose profiles in water‐equivalent media and the degradation of these profiles in media of low electron density.

ISSN:0094-2405    

DOI:10.1118/1.595750

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Diaphanography is an imaging technique using transillumination with visible and near‐infrared radiation, and a video camera, to diagnose breast disease, including cancer. Originally based on luminance contrast only, there is now interest in false‐color multispectral imaging in selected spectral bands to improve tissue differentiation. Some success has been achieved, but the scientific basis for the results was unknown. This research is concerned with measurements of the diffuse transmittance of breast tissues as a function of wavelength in the 600–1060 nm range (and calculations of contrast with a one‐dimensional diffusion theory model). Carcinoma and glandular tissues were found to have similar spectral transmittances with an increase in transmittance between 750 and 900 nm, and an absorption window around 960 nm. Adipose tissue showed a distinct transmittance minimum at 930 nm.In vivomeasurement of an intact normal breast showed a minimum at about 825 nm, as yet unexplained. The transmittance data and reflectance data were used to derive the scattering and absorption coefficients. The diffusion length was also determined from radiance versus depth measurements. These coefficients were used for calculations of contrast in a one‐dimensional slab model, with and without a layer of cancerous tissue positioned between two slabs of normal tissue.

ISSN:0094-2405    

DOI:10.1118/1.595701

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Hepatic iron stores were measured noninvasively in 31 patients (thalassemia, hemodialysis, hemosiderosis, refractory anemia) with suspected iron overload, employing a nuclear resonance scattering (NRS) technique. The thalassemia patients were undergoing desferrioxamine chelation therapy during the NRS measurements. The hemodialysis patients were measured before chelation therapy. Iron levels measured by NRS were in general agreement with those determined in liver biopsies by atomic absorption spectroscopy. In addition, NRS measurements from the thorax of some of these patients suggest that this method may also prove useful for clinical assessment of cardiac iron.

ISSN:0094-2405    

DOI:10.1118/1.595751

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In cancer radioimmunotherapy nonuniform distributions of radionuclides in geometrically irregular tumor volumes will be encountered. If the tumor is surrounded by a homogeneous layer of soft tissue, the radiation dose distribution in the tumor due to a beta‐emitting nuclide can be calculated by convolution of the appropriate Loevinger's point source dose function,J, with the nuclide concentration distribution. Computer programs using fast Fourier transform techniques have been developed to evaluate the three‐dimensional spatial convolution accurately and efficiently. Equivalent techniques have also been applied to gamma‐emitting nuclides by replacingJin the convolution by the photon point isotropic specific absorbed fraction. Illustractive results are presented for the radiation dose distribution in a 1‐ml target volume with activity distribution which is (i) uniform, (ii) limited by diffusion into the target volume, or (iii) limited by diffusion and shedding of radionuclide in the peripheral layers of the target volume. Calculations for the three cases are presented for32P,131I, and 15‐, 30‐, and 100‐keV photon emitters.

ISSN:0094-2405    

DOI:10.1118/1.595752

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new framework for selective excitation that offers simpler design and better performance than conventional excitation methods is introduced. The guidelines for choosing the appropriate radiofrequency (rf) pulse envelope in a conventional selective excitation sequence often rely on Fourier analysis, leading to less than desirable results. Although providing useful insight, Fourier analysis of the rf pulse envelope determines the resultant slice shape accurately only for small flip‐angle excitations, and not for larger flip‐angle excitations owing to the generally nonlinear behavior of the spin system. In the new excitation framework, additional excitation pulses (typically one) are applied in sequence with the conventional pulse to improve the performance (in phase characteristics and slice definition) over that achieved by the conventional pulse alone. Given a desired spatial spin distribution and an associated rf pulse (e.g., Fourier transform pairs), the Bloch equation is solvedbackwardsto yield the starting distribution required for the conventional pulse to give exactly the desired output. If this residual distribution is a small flip angle away from the actual starting distribution, then Fourier analysis of the residual distribution leads to the necessary “setup” pulse. A gradient of opposite polarity during the setup obviates a refocusing interval after the setup pulse. Computer simulations have verified the efficacy of the multiple‐pulse excitation sequence for both 90° and 180° excitations.

ISSN:0094-2405    

DOI:10.1118/1.595702

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In a Therac‐20 linear accelerator, 6–20 MeV electron beams are normally produced by shaping a scanned electron beam through primary x‐ray collimators and secondary electron trimmers. The collimator settings range continuously from 2 to 30 cm. Depth dose and field flatness parameters were measured for small elongated fields of the various electron energies. Depth dose of narrow fields defined either by the machine's collimator or lead cutouts agreed with data predicted from small square fields using the “square‐root” method.

ISSN:0094-2405    

DOI:10.1118/1.595703

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The AAPM Task Group 21 protocol is the latest amongst the protocols introduced in recent years covering the dosimetry of high‐energy x‐ray and electron beams used in radiotherapy. We have found that theCλvalue for 15‐MV x rays requires 1.6% upward adjustment, whereas theCEvalues for electrons in the range 7–18 MeV require downward adjustment of 1%–3% compared to our previous values. The adjustment values given in this paper pertain to the specific measuring systems and high‐energy beams considered here and would not necessarily be applicable to other systems. The dose to medium, calculated for different modalities, was verified by thermoluminescent dosimetry and the results were found to be consistent. These values ofCλandCEhave also been compared with those derived using other protocols, particularly the Nordic Association of Clinical Physics 80 and 81, and the discrepancies discussed.

ISSN:0094-2405    

DOI:10.1118/1.595704

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new nonlinear reconstruction method for tomosynthesis is described. This method is suited for “dilute” objects, i.e., objects in which most of the voxels have negligibly small absorption. Images of blood vessels filled with contrast material approximate this condition if the background is subtracted. The technique has been tested experimentally using a wire phantom and a prepared human heart. The results show significantly less artifacts than the well‐known backprojection. It is possible to get diagnostic image quality with a few projections. The reconstruction algorithm can be realized with dedicated real‐time hardware.

ISSN:0094-2405    

DOI:10.1118/1.595705

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to determine the modulation transfer functions (MTF's) for x‐ray computed tomography (CT) scanners, a measurement must be performed to obtain either the point spread function (PSF) or the line spread function (LSF). Thereafter, the usual procedure is to interpolate between the measured points and to determine the Fourier transforms numerically in order to obtain the MTF. Since this must usually be done many times to evaluate various reconstruction kernels and scan modalities, the process is tedious. Fortunately, it can be greatly simplified by utilizing a mathematical function to describe the PSF or LSF. Measured data for five CT scanners indicates that the PSF can usually be described by a Gaussian function. Hence, the MTF can be written in a generalized form eliminating the necessity of performing Fourier transformations each time. The MTF is determined directly from a single performance characteristic related to the full width at half maximum. The accuracy of the approach is compared with detailed MTF calculations for five CT scanners and it is shown to agree favorably with this data.

ISSN:0094-2405    

DOI:10.1118/1.595706

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous computer modeling of radiographic heel effect has shown that a simple model can give an accurate representation of heel effect for flat anode surfaces. This paper details the extension of this model to allow curved anode surfaces. A fixed‐anode x‐ray tube with a demountable anode was constructed and used to test the predictions of the computational model at 25 kVp using stainless‐steel anodes. The computer model was also used to estimate the magnitude of the heel effect for a tungsten anode at 60 kVp. This work allows determination of the effect of anode curvature, which is an inherent feature of certain recently proposed tube designs for scanning slit radiography.

ISSN:0094-2405    

DOI:10.1118/1.595668

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY