摘要:

The imaging process has two fundamental stages: detection and display. The detection stage can be quantified rigorously using Shannon's information theory. This requires the contrast scale (CS), modulation transfer function (MTF), and noise power spectrum [N(f)] to be combined into a signal‐to‐noise ratio (SNR). This results in two fundamental summary figures of merit: the density of noise equivalent quanta (NEQ) in the image and the information bandwidth integral (IBWI). These algorithm‐independent measures are used to quantify the recording stage. The display stage is less well understood since it couples to an external observer. Several types of decision makers are treated. Examples are drawn from first and second generation CT, demonstrating that they are nearly quantum limited for large signals, indicating how their algorithms are matched or mismatched to the geometry, and calculating the contrast‐detail diagrams for those decision makers.

ISSN:0269-3127    

DOI:10.1118/1.594559

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The depth‐dose characteristics of the Toshiba LMR‐16 linear accelerator for 14‐MeV x rays have been measured at an SSD of 100 cm using diodes and ion chambers. The surface dose and build‐up depth both exhibit a considerable variation with field size. A new central axis model has been developed which takes account of these variations, and the agreement between the measured and computed data using this model is found to be excellent. Formulas are also presented to estimate the surface dose, buildup depth, and output factor as a function of field size.

ISSN:0269-3127    

DOI:10.1118/1.594560

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A stereo‐photographic system has been developed with which surface contours of a human subject may be obtained rapidly and objectively. Nonmetric cameras are used and the results are obtained from direct measurements of the photographs. Software has been developed for interpretation of the photographic data with the asistance of a small computer and desk‐top digitizer such as those routinely used in radiotherapy treatment planning. With this system the coordinates of a point on the subject may be determined with an accuracy of ±1–2 mm.

ISSN:0269-3127    

DOI:10.1118/1.594538

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Bone mineral content and bone width were measured at the distal and midshaft locations in the right radius and ulna with the arm in different orientations. The values of mineral content, width, and the ratio of bone mineral content divided by width, determined at each bone site were compared to establish the effect of orientation. Seven orientations were studied, two of which involved rotations of the forearm and five of which involved rotations of the upper arm about the axis of the forearm. Rotation of the forearm was found to affect the values of one or more of the measured quantities at all of the sites. Orientation of the upper arm was found to affect the values of all quantities at the midshaft radius, but not to affect the values of any of the quantities at the other sites.

ISSN:0269-3127    

DOI:10.1118/1.594539

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A systems analysis of reduced dose mammography with scatter removal has quantitatively specified the relationships between the several parameters in mammography systems. While the results are only preliminary in the sense that they need to be proven experimentally, they do seem to provide a good feel for the nature of the important trade‐offs. The major uncertainties in the analysis probably have to do with the formal equivalence, that is implicit in the aperture theory, between large area and detail contrast, as well as the output versus focal spot relationship that was adopted. Also, there are important questions about specifying motion unsharpness as a function of exposure time in mammography examinations.

ISSN:0269-3127    

DOI:10.1118/1.594540

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

With the recent introduction of fast rare‐earth screens into the market, the question of noise and how it influences image quality becomes particularly important. In this paper a compact formula involving familiar and measurable quantities is derived for the specification of the noise arising from the spatial fluctuations of the x‐ray quanta absorbed in the screens. Experimental evidence in support of this concept is also given.

ISSN:0269-3127    

DOI:10.1118/1.594542

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Zero‐degree neutron energy spectra for thep(41)‐Be andd(49)‐Be reactions were measured by time‐of‐flight for neutrons with energies above 1.9 and 1.4 MeV, respectively. Spectral changes resulting from the addition of copper, aluminum, and polyethylene filters to unfiltered beams were determined. Integral yields, average energies, filter material attenuation coefficients, and kerma fractions were computed for these spectra. Calculated spectra for neutron beams filtered by various thicknesses of polyethylene compared favorably with experimental results.

ISSN:0269-3127    

DOI:10.1118/1.594543

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Instrumentation and methodology for the determination of gallstone volume from standard roentgenographic views have been developed. Evaluation of size is made by using video viewing, an operator‐set electronic window, intercept registers, and the programming of a PDP/8E for handling the data. The programming makes corrections for geometric factors arising from anatomical variations, corrections for variations in technique, roentgenographic magnification, and provides for the use of several roentgenographic views. The precision of the measuring (video) technique is ∼0.5 mm as measured on the roentgenogram. Estimates and test data indicate that this system should be capable of an absolute accuracy within 10% of the true volume.

ISSN:0269-3127    

DOI:10.1118/1.594544

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements were made to determine if there was an optimal kVp that would maximize radiographic contrast for various materials while minimizing average patient dose. Radiographs were made of a 17‐cm‐thick plexiglass phantom (to simulate a typical abdomen) with added regions of plexiglass, hydroxyapatite, and iodine with peak kilovoltages from 40 to 150 kVp and 2 mm Al added filtration. Radiographs were also made of a 4‐cm‐thick plexiglass phantom to simulate a typical breast, with added regions of plexiglass, hydroxyapatite, and fat at a peak kilovoltage from 35 to 100 kVp and with 1 mm added filtration. For the thicker phantom it was found that the contrast per average absorbed dose for the added plexiglass, hydroxyapatite, and iodine increased by about 3, 2, and 0.4, respectively, when going from 50 to 150 kVp. Contrast was measured relative to a reference region on the phantom. Similar data for the thinner phantom showed an increase of about 35% in contrast per average absorbed dose for plexiglass and fat but a decrease by about 30% for hydroxyapatite as the kVp changed from 35 to 100 kVp.

ISSN:0269-3127    

DOI:10.1118/1.594541

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of secondary blocking on the output factor for 7–16 MeV electron beams is investigated for the two most widely used methods of field shaping, i.e.,(a) shield on surface, and (b) shield at end of cone with subsequent air gap. For these two methods differences in output on the order of 10% are observed. These effects are explained in terms of the blocking geometry and its influence on scattered radiation.

ISSN:0269-3127    

DOI:10.1118/1.594545

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY