摘要:

Early developments of computed tomography are traced. (AIP)

ISSN:0269-3127    

DOI:10.1118/1.594708

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594709

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Specifying and evaluating the performance of a computed tomographic (CT) scanner is necessary in order to ensure that a “typical” level of performance is achieved as well as providing baseline values for a program of quality assurance. Performance to be specified and tested include parameters related to: (a) slice geometry, (b) patient dosage, (c) artifactual behavior and (d) contrast–detail performance. Measures of performance are defined and specified. Tests which provide for performance evaluation in these four areas are detailed and discussed.

ISSN:0269-3127    

DOI:10.1118/1.594811

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Because the magnetic susceptibility of the heart and the intracardiac blood is substantially greater than that of the surrounding lung tissue, the periodic redistribution of blood and tissue during the cardiac cycle will alter an externally applied magnetic field. These magnetic field changes can be detected by a magnetometer outside the body and have been termed magnetic susceptibility plethysmography (MSPG) signals. This paper presents a theoretical analysis of the MSPG technique. A single moving sphere model is used to explain the susceptibility induced field changes and the detection of the MSPG signals. The susceptibility distribution within the thorax and a more detailed model of the heart are then used to predict the MSPG waveform. Several other contributions to the MSPG signal are analyzed. This model has demonstrated that the time dependence of certain MSPG components closely resembles that of the total heart volume while other components have waveforms similar to the displacement of the cardiac center of mass. The magnitude and direction of the MSPG field has been shown to depend on the location of the field point, the volume change of the heart, and the motion of the heart's center of mass. It has also been shown that 90% or more of the MSPG signal recorded near the heart but outside the body will be produced by cardiac volume changes. For these reasons, the MSPG technique may provide a noninvasive method of measuring changes both in total cardiac volume and in the position of the cardiac center of mass throughout the cardiac cycle.

ISSN:0269-3127    

DOI:10.1118/1.594710

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The magnetic susceptibility of the heart and the intracardiac blood is substantially greater than that of the surrounding lung tissue. The periodic redistribution of blood and tissue during the cardiac cycle will thus alter an externally applied magnetic field. These magnetic field changes can be detected by a magnetometer outside the body, and have been termed magnetic susceptibility plethysmography (MSPG) signals. A differential magnetometer, a liquid helium dewar, and a superconducting persistent‐current magnet have been developed to detect thexcomponent of the MSPG signal. The theoretical analysis of this instrument is presented to understand the spatial sensitivity of an MSPG susceptometer with a nonuniform magnetizing field. Preliminary measurements are shown to be in fair agreement with the model calculations. Several possible improvements of the technique are discussed.

ISSN:0269-3127    

DOI:10.1118/1.594711

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A study of the factors affecting image quality and x‐ray sensitivity of ionography and xeroradiography is presented. First, the relative charge sensitivities (nC/cm2 mR) are compared. It is shown that high pressure xenon (10 atm cm) as used in ionography produces more than 2.5–3.5 times more charge than the selenium layers used in xeroradiography for the same x‐ray exposure. The influence of development time and toner deposition on the appearance of images is investigated theoretically and experimentally. A parameter describing development sensitivity is proposed. It is shown that an increase in development sensitivity (OD cm2/nC) with increasing development time is accompanied by a loss in edge enhancement. The development sensitivity of ionography is about twice that typical of xeroradiography. This makes the total sensitivity (OD/mR) of the ionography process more than 4.5–6.5 times that of xeroradiography. The total sensitivity of ionography is about 1/2 that of par speed film screen combinations.

ISSN:0269-3127    

DOI:10.1118/1.594712

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A pulse‐echo technique for measuring the velocity of ultrasound in the cortex of the femurin vivois presented. The average result for 27 asymptomatic volunteers is 3250±190 m/s. The results show no significant differences between proximal and distal femoral sites, right and left femora, males and females. There is also no significant difference between the velocity of sound in the femur and in the radius in normal volunteers. The reproducibility of the time of flight measurement for three volunteers is found to be about 2%, and the total uncertainty of the femoral velocity of ultrasound measurement is determined to be 4% for proximal sites and 7% for distal sites. As the femur is a load‐bearing bone, the site of frequent fracture both in the elderly and in the osteoporotic, the measurement of the velocity of sound at this new site may of great value in investigating these conditions and may ultimately be helpful in assessing ’'quality” of the femoral cortex in those patients undergoing total hip or knee replacement.

ISSN:0269-3127    

DOI:10.1118/1.594713

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Xeroradiography is a diagnostic modality whose image‐forming properties are fundamentally different from those of the conventional silver halide process. In this work, the xeromammographic process, representing the low‐contrast limit of xeroradiography, is analyzed in detail. A step by step approach is taken, whereby the limiting factors in the exposure and development stages are identified and quantified. The sensitivity of the Se layer is also computed and the concept of optimal exposure introduced. The paper concludes by considering a simple model for xeroradiographic noise.

ISSN:0269-3127    

DOI:10.1118/1.594689

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study compared air exposures during conventional dental and maxillofacial radiography and similar views using free‐focus radiography with conventional image receptors. The results show that periapical type surveys on nonscreen film placed extraorally or in the buccal fold may be carried out with an exposure to the surface tissues, which is similar to or less than conventional dental radiography. Extraoral survey type radiographs of the jaws may be carried out with significantly less surface exposure than lateral oblique views of the jaws. The least exposure was required, when the film was placed in the buccla fold instead of against the face during free‐focus radiography. The exposures with film screen combinations were reduced by an order of magnitude when compared to the nonscreen techniques. Proper filtration of the beam of the miniaturized x‐ray machines radiography in dentistry may thus be desirable and applications in other parts of the body encouraged.

ISSN:0269-3127    

DOI:10.1118/1.594690

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dosimetric data were obtained forp(90)+(Be+Ta) andp(101)+(Be+Al) radiation with field size 9×11 cm at a 125‐cm SSD impinging on a tissue‐equivalent liquid phantom with density 1.10. The radiation was unfiltered or filtered by 10 cm of polyethylene. Dose rates measured with a 0.5‐cm3ion chamber at a depth of 5 cm ranged from 0.046 Gy min−1 μA−1for filteredp(90)+(Be+Ta) to 0.11 Gy min−1μA−1for unfilteredp(101)+(Be+Al). Neutron time of flight energy spectra in air averaged above 10 MeV indicate mean neutron energies ranging from 42.0–51.4 MeV for the four beams under study. The central axis depths at which 50% dose attentuation occurred ranged from 15 to 20 cm for these beams. At 3 cm depth the gamma dose component of the totalp(101)+(Be+Al) dose was 14.5%, or 17% with filtration.

ISSN:0269-3127    

DOI:10.1118/1.594813

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY