摘要:

A method is described in some detail for measuring the magnitude and penetration of the electron contamination in photon beams using a pancake charge detector. It is shown that the response of the detector to a photon beam can be separated from the component due to the electron contamination. In the present work, the detector is used to measure the electron fluence in a60Co photon beam. This fluence is subsequently converted to dose by comparison with the fluence and dose measured from a pure electron beam (90Sr). This study proves, within experimental error, that the observed changes in the buildup region, with the collimator opening for both filtered and unfiltered60Co beams, are due to electron, rather than photon, contamination.

ISSN:0269-3127    

DOI:10.1118/1.595719

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

X‐ray images acquired with an image intensifier detector system suffer from veiling glare, a low‐frequency degradation described by a point spread function (PSF). The PSF has two experimentally determined parameters unique to a given image intensifier. This information is utilized to deconvolve the degradation from digitally acquired images. Results demonstrate a significant increase in contrast ratio of high‐contrast objects after deconvolution and image restoration.

ISSN:0269-3127    

DOI:10.1118/1.595720

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Detection of a target object in a radiological image is often impeded by an obscuring background “clutter” resulting from the contrast between various materials in the neighborhood of the target. Dual‐energy techniques can reduce or remove this clutter. In order for the target to be detectable in the image after dual‐energy processing, the signal‐to‐noise ratio (SNR), defined as the difference between the target and the background divided by the photon noise in the difference, must exceed some threshold. A given SNR may be obtained for a wide range of the energies of the two x‐ray beams and the ratio of their fluences. A theoretical model is developed which permits the choice of beams to be optimized with respect to some critical parameter—in this case, patient dose. The analysis is applied to the detection of calcifications in mammography. For an ideal imaging system, we predict that the optimum beam energies are 19 and 68 keV. A dose of 0.42 cGy is required to obtain an SNR of 5 for detection of a 0.02‐cm cubic calcification in the resulting clutter‐free image. This can be reduced to 0.16 cGy if the higher energy image is smoothed, prior to dual‐energy processing, such that its variance is reduced to one‐fourth of its unsmoothed value.

ISSN:0269-3127    

DOI:10.1118/1.595766

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dual‐energy x‐ray techniques may be able to enhance the detectability of calcifications in mammographic examinations by removing the background “clutter” caused by contrast between adipose and glandular tissue. This hypothesis is examined experimentally by implementation of dual‐energy imaging on a prototype digital scanned projection radiography system developed in our laboratory. A model of the propagation of signal and noise in dual‐energy processing for a given radiation dose is validated by measurements from phantom images. The experimental imaging system has low spatial resolution and cannot be operated at dose‐optimum energies; however, since both the single‐ and dual‐energy images are subject to the same technical limitations, a comparison of such images allows an assessment of the benefits of dual energy. Experimental images of breast tissue specimens, showing improved detectability of calcifications when obscuring background clutter is removed, are presented. The dose required for a given signal‐to‐noise ratio can be reduced by smoothing the higher energy image prior to dual‐energy processing. For practical implementation, it is reasonable to smooth the higher energy image such that its variance is reduced fourfold.

ISSN:0269-3127    

DOI:10.1118/1.595767

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The determination of the radiation exposure history of the population has become increasingly important in the study of the effects of low‐level radiation. The present work was started to try to obtain anin vivodosimeter that could give an indication of radiation exposure. Dental enamel is the only living tissue which retains indefinitely its radiation history, and electron spin resonance measurements have shown that the radiation signal can be resolved down to about 10 cGy. Measurements on samples from the general population give radiation exposure estimates that are reasonable, and one measurement on a patient who had radiotherapy to the mouth area showed a good correlation with tumor dose. We believe that this is an important new indicator of radiation dose and taken together with exposure histories should provide important data for epidemiological studies as well as accidental exposures.

ISSN:0269-3127    

DOI:10.1118/1.595768

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Scatter rejection by air gaps was analyzed in the context of a model in which scattered radiation was treated as if it originated from an “effective scatter point source” (ESPS), located between the focal spot of the x‐ray tube and the exit surface of the phantom or patient. Excellent agreement was found between the ESPS model and scatter measurements performed on phantoms for a variety of experimental conditions. Values forXs(distance from phantom exit surface to effective scatter point source) were consistently in the range 15–20 cm. Applications of the model for predictions of scatter rejection by air gaps and comparisons to other scatter rejection methods are presented.

ISSN:0269-3127    

DOI:10.1118/1.595690

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Presented is a solution to a differential equation governing the transverse magnetization of a fluid whose flow is governed by the process of convective dispersion. The solution describes the case of flow parallel to an applied magnetic field gradient with boundary conditions and velocity profiles consistent with those likely to be encountered in nuclear magnetic resonance imaging of arterial or venous blood flow.

ISSN:0269-3127    

DOI:10.1118/1.595757

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this paper, experiments and related theoretical deductions on coherent/Compton scattering of 59.5‐keV Am241gamma line by bonelike materials are described. In particular, we demonstrate that a photon scattering mineralometer (PSM) can attain the best working conditions when it operates in a backscattering geometry mode. In fact, the large scattering angle we chose, ϑ=135°, allowed us to assemble a very compact source–detector device. Further, the relative sensitivity at 135° turns out to be ≃1.7 and ≃6 times bigger than at 90° and 45°, respectively. The performances of the ϑ=135° PSM we assembled were experimentally investigated; i.e., in a measuring time of 103s, a ≃5‰ statistical precision for bonelike materials, such as K2HPO4–water solutions, was obtained. The large‐angle PSM device seems therefore to be very promising for trabecular bone mineral density measurementsin vivoin peripheral anatomic sites.

ISSN:0269-3127    

DOI:10.1118/1.595795

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue–air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer‐aided planning of radiation therapy, but have been validated primarily for cobalt‐60 radiation. Tests performed with 6‐ and 15‐MV x rays reveal that incorrect doses can be computed within or near to a low‐density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x‐ray interactions is considered directly.

ISSN:0269-3127    

DOI:10.1118/1.595691

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Computed tomography (CT) provides a noninvasive technique with high resolution cross‐sectional tomographic images which allow volume measurements of an object, independent of its geometric configuration. A phantom of known volume with controllable periodic motion was used to validate the CT method of volume determination. A good correlation (P<0.05) was achieved. Missing angle reconstruction algorithms for gating were applied to estimate left ventricular volumes and ejection fraction in an experimental animal, and the results compared with a standard angiographic method. Left ventricular volumes correlated poorly, whereas the ejection fraction obtained correlated well (r=0.9). The discrepancies may be attributed in part to the CT method in which difficulties were recognized in defining the left ventricular borders at the base of the heart and partial volume effect, and in part to inaccuracies in the standard angiographic method. Once validated, this method has been applied to the animal model in the form of a pilot study.

ISSN:0269-3127    

DOI:10.1118/1.595692

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY