ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1992.tb36357.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The production of acoustic waves following the absorption of energy deposited by ionizing radiation, with a consequent production of localized thermal spikes has been confirmed by a number of papers published in the physics literature. This paper reviews the basic theory and presents most of the supporting experimental data. Some of the experimental methods used and the results obtained are summarized. In addition to the rather straightforward and routine use of acoustic phenomena produced by ionizing radiation for the detection and measurements of such radiation, there are some special applications that appear to be especially attractive for medical physics. Some of these are unique to ionizing radiation in that the amplitude of the ultrasound wave is proportional to the energy deposited in small volumes at localized sites of these interactions, while others derive from methodologies already in use with nonionizing radiations. The detection and measurement of this ultrasonic radiation could possibly lead to methods for the study of such fundamental phenomenon as track structure, precision localization of therapeutic treatment beams, and even the possible imaging of internal anatomic structures to provide on‐line portal images.

ISSN:0269-3127    

DOI:10.1118/1.596842

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The generation of monoenergetic or quasimonoenergetic x‐ray spectra has been accomplished by several methods including the use of K‐edge filtration, characteristic radiation, crystal monochromators, and multilayer mirrors. In this paper some practical questions are discussed regarding the possibility of obtaining narrow‐band spectra using x‐ray reflection in glass capillary optics which have been reported recently in connection with the focusing and generation of parallel x‐ray beams. Derivation of formulas for double and triple reflection with and without additional K‐edge filtration imply that these methods are superior to the use of filtration alone. It is shown that the double reflection scheme is most sensitive to the angular divergence of the beam emanating from the capillary array used to generate the incident parallel beam. Simulations using three reflections predict output spectra which are relatively insensitive to blurring due to angular divergence. A small amount of K‐edge filtration in combination with three reflections provides considerable sharpening of the x‐ray spectrum. Aside from the spectral selectivity provided by the capillary array, the reduced divergence of the primary transmission may have advantages for scatter rejection in situations where the air gap between the patient and detector is increased by an amount consistent with the resolution requirements of the application. In parallel beam situations, the use of capillary arrays between the patient and detector may also be useful for the rejection of divergent scattered radiation.

ISSN:0269-3127    

DOI:10.1118/1.596843

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A novel digital radiographic method has been developed for measuring the thickness of a tissue sample by iodine displacement. This simple, accurate method is useful both in medical research and in the comparison of pathological and clinical findings. Radiographic measurements of tissue samples in air suffer from the limitation that the quantity measured depends on the product of both thickness and radiographic attenuation coefficient. This technique allows one to obtain thickness measurements from a digital radiograph of a tissue sample suspended in a bath of radio‐opaque contrast agent. The attenuation of the iodinated contrast agent is much higher than that of tissue or calcium. Thus the resulting image is determined largely by the contour of the surrounding bath, and is only slightly influenced by the composition of the tissue. This technique improves both the accuracy and precision of radiographic thickness determination. In this paper the iodine displacement technique is described and the accuracy and precision of thickness measurements in appropriate phantoms are quantified. This technique has been used with both image intensifier and screen‐film based imaging systems to obtain thickness maps of calcified human aorta, with precision better than 4% and spatial resolution of 2.5 mm−1.

ISSN:0269-3127    

DOI:10.1118/1.596844

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Artificial neural networks have been applied to the differentiation of actual “true” clusters from normal parenchymal patterns and also to the differentiation of actual clusters from false‐positive clusters as reported by a computerized scheme for the detection of microcalcifications in digital mammograms. The differentiation was carried out in both the spatial and frequency domains. The performance of the neural networks was evaluated quantitatively by means of receiver operating characteristic (ROC) analysis. It was found that the networks could distinguish clustered microcalcifications from normal nonclustered areas in the frequency domain, and that they could eliminate approximately 50% of false‐positive clusters of microcalcifications while preserving 95% of the positive clusters, when applied to the results of the automated detection scheme. A large, comprehensive training database is needed for neural networks to perform reliably in clinical situations.

ISSN:0269-3127    

DOI:10.1118/1.596845

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Setting up and maintaining video display monitors properly will help to reduce display variation and improve overall presentation of the radiological image. Display monitor gray‐scale characteristics were examined using the SMPTE test pattern. This test pattern may be used as a standard for adjusting brightness and contrast. The controls should be adjusted to display the full dynamic range so that the 5% and 95% signal levels in the pattern are visible. Measured luminance on a laboratory workstation used for radiological perceptual experiments, and on the Siemens CT gray‐scale monitor was determined to range from 0.17 to 76.0 nit, and 0.17 to 24.66 nit, respectively. These were compared with the range of approximately 17 to 514 nit for a typical film–viewbox combination. Characteristic curves were determined for both monitors, and CRT gammas were 3.34 and 2.48 for the perceptual workstation and CT console, respectively. The display gamma was determined from fitting luminance data to a log–log plot of luminance versus input gray level. The usefulness of the SMPTE test pattern for visual presentation as well as photometric measurement is demonstrated.

ISSN:0269-3127    

DOI:10.1118/1.596846

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The modulation transfer function (MTF) of an imaging system may be used as a figure of merit to assess its geometrical resolution. As such it is often derived from the point spread function (PSF) through a Fourier transformation, or through a FFT in case of digital images. Using the special features of the convolution‐backprojection algorithm and making some simplifying assumptions about the number of rays, it is shown that the desired MTF is also obtainable without a Fourier transformation of the PSF, different shapes and sizes of the beam‐profile are then analyzed, and finally it is shown that calculated and measured MTF functions are in good agreement. The effects of aliasing on the cutoff frequency are also discussed.

ISSN:0269-3127    

DOI:10.1118/1.596847

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes two simple thermal methods for measuring the energy fluence in J/cm2from a diagnostic x‐ray exposure. Both detectors absorb essentially 100% of the radiation and give a signal that is directly proportional to the energy fluence of the x‐ray beam. One detector measures the thermal effect when a pulse of x rays is totally absorbed in the pyroelectric detector of lead–zirconium–titanate (PZT). The other detector measures the expansion of a gas surrounding a lead disk detector in a photoacoustic chamber. The increased pressure of the gas is transmitted through a 1‐mm duct to a sensitive microphone. Both detectors have previously been used to measure the energy fluence rate of continuous x‐ray beams in the same energy region using a chopped beam and a lock‐in amplifier. Measurement of the energy fluence of a pulse of radiation eliminates the need for the beam chopper and lock‐in amplifier and results in a simple, rugged, and inexpensive dosimeter. Either method can be combined with the area of the beam to give an estimate of the imparted energy to the patient from a diagnostic x‐ray exposure.

ISSN:0269-3127    

DOI:10.1118/1.596912

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Models for the computer generation of tungsten target x‐ray spectra proposed by Birch and Marshall [Phys. Med. Biol.24, 505–517 (1979)] and recently by Tucker, Barnes, and Chakraborty [Med. Phys.18211–218 (1991)] are compared. Some basic differences in the equations for the number of bremsstrahlung photons of different energies in the spectra are discussed. The models are compared in terms of their ability to characterize x‐ray spectra from constant potential clinical units using three parameter equivalent spectra (EQSPEC) determined from the fit of model generated transmission curves through aluminum to measured data. The Kramers model for x‐ray generation is included for completeness. It is shown that two of the models generate very similar x‐ray spectra from given transmission curves although the fitting parameters in the EQSPEC characterization differ.

ISSN:0269-3127    

DOI:10.1118/1.596848

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596899

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY