摘要:

The effectiveness of film–screen mammography is limited by tradeoffs between latitude and contrast, film granularity, and the need to increase dose when antiscatter methods are used. We are currently developing a scanned‐projection digital mammography (SPDM) system to overcome these limitations. The system consists of a pair of scanning slits, a high‐resolution x‐ray image intensifier tube, a linear photodiode array, and a digital display. The detective quantum efficiency of the SPDM system at spatial frequencies up to 3 cycles/mm is similar to that of mammographic film–screen combinations, but is lower at high frequencies. For low‐contrast objects as small as 0.1 mm in diameter, the signal‐to‐noise ratio is currently equal to that of optimally exposed mammographic film–screen images for equal dose to the breast and superior for regions which would be underexposed or overexposed on film. This is achieved by the use of a low‐noise detector system, geometric magnification, and scatter elimination. Images of a contrast‐detail phantom and excised breast tissue illustrate the superior contrast sensitivity of SPDM.

ISSN:0094-2405    

DOI:10.1118/1.596147

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An energy discriminating x‐ray detector has been developed for dual‐energy, scan projection digital radiography. The detector is comprised of a pair of x‐ray intensifying screen/linear photodiode arrays, aligned one behind the other. Energy discrimination is achieved by employing a low atomic number phosphor in the front screen and a high atomic number phosphor in the back screen. The x‐ray response, modulation transfer function, and defective quantum efficiency of the detector are reported along with the experimental methodology utilized for the measurements. Also presented is an analysis which indicates that in a typical patient's lung field, the detector can resolve the projected density (g/cm2) of a 3‐mm‐thick, 1‐cm2area of bone to better than 1.5%.

ISSN:0094-2405    

DOI:10.1118/1.596047

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We are developing a digital x‐ray imaging system using a multiple slit assembly (MSA) and an image intensifier (II)–TV digital system. The advantage of this approach is that the scatter from an object and the veiling glare in the II–TV system can be reduced significantly while the x‐ray utilization is maintained much better than that with a single slit‐beam technique. The quality of reconstructed images is related to many parameters such as the slit width, the lead spacer, the number of image frames, and the reconstruction algorithm. In this study, reduction of scatter and veiling glare was measured quantitatively, and image artifacts were analyzed. It was found that the fraction of scatter and veiling glare can be reduced to approximately 0.01–0.1 by use of the MSA imaging technique, and that the magnitude of the fractions is strongly dependent upon the slit width and the lead spacer of the MSA used. The artifacts are caused by inaccuracies in the slit width, lead spacer, and scan motion, and by undersampling of image data. The overlap scanning technique was effective in reducing the magnitude of these artifacts in the reconstructed image.

ISSN:0094-2405    

DOI:10.1118/1.596148

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The imaging performance of a storage phosphor system (SPS) for digital projection radiography is studied in which the x‐ray image is temporarily stored in a light‐stimulable phosphor plate which is subsequently read out by a scanning laser beam. The imaging performance of this system has been analyzed for two types of imaging plates. The spatial resolution is described by the modulation transfer function and the signal‐to‐noise (S/N) ratios of the recorded image data are measured in terms of noise‐equivalent quanta and detective quantum efficiency. Their dependence on detector entrance dose and space and spatial frequency is discussed. A detailed analysis of the different sources of image noise is given to outline the intrinsic features and limits of the system. Finally, the S/N behavior of the SPS is compared with published data of screen–film systems.

ISSN:0094-2405    

DOI:10.1118/1.596127

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A systematic study has been carried out to assess the x‐ray attenuation properties of various elements when used as radiographic contrast enhancing media. This study examines the effects of solutions of molecules with effective atomic numbers from 40 to 92 on the signal–contrast ratio of radiographic images formed with various input x‐ray spectra on suitable phantoms. A variety of x‐ray spectra were used including monoenergetic spectra, constant potential x‐ray tube spectra, and computed tomography spectra. In addition, a computer model was used to predict the effects studied. In general, the computer model is able to accurately predict the resulting signal–contrast ratio for a given combination of contrast media, input x‐ray spectra, and phantom composition. From these data and calculations, it may be possible to design new contrast media which are tailored to specific diagnostic imaging tasks.

ISSN:0094-2405    

DOI:10.1118/1.595999

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new method is introduced in which the total photon interaction cross sections per electron of human tissues are used to define effective atomic numbers for blood, bone, brain, fat, heart, kidney, liver, lung, muscle, ovary, pancreas, spleen, and water. These effective atomic numbers are equal within 4% from 10 to 200 keV in each soft tissue, whereas for bones of different chemical compositions the variation ranges from 2.86% to 5.03%. This effective atomic number definition is less energy dependent than a previous definition based on the total photon interaction cross section per atom averaged over all elements in the tissue, from which the computed effective atomic numbers varied by as much as 50% (in bone) as a function of photon energy over the energy range from 10 to 200 keV.

ISSN:0094-2405    

DOI:10.1118/1.596000

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The forward production of high‐energy electrons from materials with various atomic numbers from carbon to lead has been measured for megavoltage photon beams from 4‐ to 25‐MV peak bremsstrahlung energy by placing a thin‐window parallel‐plate ionization chamber directly behind foils of the various materials. The relative forward production of electrons decreases with atomic number for energies ≤10 MV until aboutZ=50, after which it rises. For photon energies ≥15 MV, forward production increases with atomic number with a break point atZ∼50, beyond which the curve becomes steeper.

ISSN:0094-2405    

DOI:10.1118/1.596001

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central‐axis depth dose are adequate; (2) that a three‐dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two‐dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

ISSN:0094-2405    

DOI:10.1118/1.596002

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A treatment planning technique for calculation of dose distributions in dynamic stereotactic “radiosurgery” with a 10‐MV isocentrically mounted linear accelerator is presented. The treatment planning for dynamic radiosurgery is a three‐dimensional problem, since during treatment both the gantry and the couch rotate simultaneously, the gantry from 30° to 330° and the couch from 75° to −75°. The patient surface and anatomical information is obtained from a family of computed tomography or magnetic resonance scans, and a stereotactic frame is used for target localization, treatment setup, and patient immobilization during the treatment. The dose calculational algorithm follows the gantry and couch rotation in an incremental fashion, and relies on measured stationary beam central axis percentage depth doses and dose profiles to calculate the normalized tissue‐maximum‐ratio distributions over a matrix of points defined on one of three orthogonal planes (transverse, sagittal, or coronal). The dose calculation algorithm is discussed in detail and calculated dose distributions for single plane and dynamic radiosurgery compared with measured data.

ISSN:0094-2405    

DOI:10.1118/1.596003

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A flexible, temperature‐regulated, water calorimeter has been constructed which consists of three nested cylinders. The innermost “core” is a 10×10 cm right cylinder made of glass, the contents of which are isolated from the environment. It has two Teflon‐washered glass valves for filling, and two thermistors are supported at the center by glass capillary tubes. Surrounding the core is a “jacket” that provides approximately 2 cm of air insulation between the core and the “shield.” The shield surrounds the jacket with a 2.5‐cm layer of temperature‐regulated water flowing at 5 l/min. The core is filled with highly purified water the gas content of which is established prior to filling. Convection currents, which may be induced by dose gradients or thermistor power dissipation, are eliminated by operating the calorimeter at 4 °C. Depending upon the power level of the thermistors, 15–200 μW, and the insulation provided by the glass capillary tubing, the temperature of the thermistors is higher than that of the surrounding water. To minimize potential errors caused by differences between calibration curves obtained at finite power levels, the zero‐power‐level calibration curve obtained by extrapolation is employed. Also the calorimeter response is corrected for the change in power level, and therefore thermistor temperature, that follows the resistance change caused by irradiation. The response of the calorimeter to 4‐MV x rays has been compared to that of an ionization chamber irradiated in an identical geometry. For nitrogen‐saturated water, the grand mean of the calorimeter‐to‐ion chamber (Cal/Ion) dose ratio for five experiments conducted over a period of six months is 1.006±0.001. Three experiments with oxygen‐saturated water in the same core yeild a Cal/Ion ratio of 0.991±0.001. These results are consistent with radiochemical models and refined experiments that used water saturated with various gases, and suggest that nitrogen‐saturated water calorimeters of the type described may be used to determine the dose‐to‐water without reference to radiation‐dependent parameters or recourse to corrections for thermal defects.

ISSN:0094-2405    

DOI:10.1118/1.596004

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY