摘要:

The saturation curve has been studied in xenon and in krypton up to a pressure of 8 atm. An empirical formula has been found that describes the fraction of current collected over a wide range of voltages, pressures, ionization intensities, and electrode spacings. This is of practical value in the design of ionography chambers. For krypton the collection fractionfKr= (1+0.25η−1.74)−1, and for xenonfXe= (1+0.16η−1.88)−1, where η=Fp−0.7Vd−2q−1/2withF=3.61×10−7and 2.50×10−7for krypton and xenon, respectively. The ranges of the variables covered in the experiments werep=1–8 atm,V=5–25 000 V,d=0.3–1.3 cm, andq=4×10−9–6×10−8A/cm3.

ISSN:0269-3127    

DOI:10.1118/1.594186

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Inherent in the line‐spread function approach in modulation transfer function (MTF) determinations is the use of a microdensitometer to scan the density pattern resulting from the line exposure. In such a procedure, loss of fidelity in the distribution being scanned is expected. This loss is due in part to the use of a finite‐width scanning slit. In spite of the central role of the scanning slits in the MTF determination, little if anything has appeared in the literature from which one can quantitate the effect of a scanning slit on the MTF determination. This paper analyzes the loss of response attendant with use of scanning slits of differing widths. Specifically, the MTF of a finite scanning slit is derived. A method for correcting for loss of response, given a specific slit width, is indicated on the basis of the chain‐multiplication property of MTFs. The current practice of using a 10‐μm width for film‐screen combinations is shown to be justified. Results can be generalized to any configuration scan slit. In addition, it may also be applied to any generalized sampling or display‐type slit.

ISSN:0269-3127    

DOI:10.1118/1.594187

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To determine the modulation transfer function (MTF) of a radiographic film or film‐screen system, it is common to expose the system to a line source and scan the result with a microdensitometer. The question arises as to how the MTF determination is affected by such factors as microdensitometer slit width and possible misalignment of the slit with the line pattern being scanned. In any experiment, some degree of misalignment can be expected. This paper analyzes the resulting problem and derives an expression for the MTF of slit misalignment and slit width: MTF={sin[2πf(L/2)]/2πf(L/2) } (sin{2πf [H sin(θ/2)]}/2πf [H sin(θ/2)]}), whereLis the slit width,Hthe slit height, θ the angle of misalignment, andfthe spatial frequency variable. The left‐hand factor of this product is the MTF of a finite slit width, while the right‐hand factor is the MTF of misalignment. It is shown that, for radiographic film‐screen combinations, small‐angle misalignment up to 0.25° is not serious with 1.5‐cm‐long slits, while such a misalignment may become serious in high‐bandwidth systems such as nonscreen films.

ISSN:0269-3127    

DOI:10.1118/1.594189

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

With monoenergetic x‐ray beams incident on polystyrene phantoms, the spectra of the transmitted x rays were measured with a scintillation spectrometer. The scattered and unscattered components of the transmitted x‐ray fluence at a point on the beam axis were determined as a function of (i) the incident x‐ray energy (18, 22, 32, 49, 58, 69, and 660 keV), (ii) the phantom thickness (5.3, 10, and 21 cm), (iii) the scatter solid angle determined by the exposed area of the phantom and the separation distance of the image plane (0.090, 0.31, 0.66, 1.8, 3.5, 4.3, 4.8, and 5.1 sr), and (iv) the beam diameter at the image plane (25, 17, and 10 cm). The results indicate that, as the incident x‐ray energydecreasesfrom 660 to 30 keV, the contribution of the scattered component to the transmitted fluenceincreasesfrom approximately 50% to 90% for the 21‐cm phantom and from 21% to 50% for the 5.3‐cm phantom. For typical cases, the data show the effect of the scatter component on the ratio of the image to the background signals. In addition, the examples show that optimum conditions for maximizing this signal ratio may be obtained by a careful selection of the incident x‐ray energy for low‐, medium‐, and high‐contrast objects.

ISSN:0269-3127    

DOI:10.1118/1.594188

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new device has been developed with which the focal spot distribution can be measured accurately. The alignment and localization of the focal spot relative to the device are accomplished by adjustment of three micrometer screws in three orthogonal directions and by comparison of red reference light spots with green fluorescent pinhole images at five locations. The standard deviations for evaluating the reproducibility of the adjustments in the horizontal and vertical directions were 0.2 and 0.5 mm, respectively. Measurements were made of the pinhole images as well as of the line‐spread functions (LSFs) and modulation transfer functions (MTFs) for an x‐ray tube with focal spots of 1‐mm and 50‐μm nominal size. The standard deviations for the LSF and MTF of the 1‐mm focal spot were 0.017 and 0.010, respectively.

ISSN:0269-3127    

DOI:10.1118/1.594190

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Readings made with condenser‐ionization chamber systems are frequently suspect because significant changes in calibration can occur without being evident in the normal use of the instruments. These changes can be traced to physical causes that alter the voltage sensitivity or electrical capacitance. Thus, measurement of these electrical parameters will indicate whether or not a change in calibration has occurred. Formulas are presented for calculating such changes and determining which parameter is responsible. Good agreement was obtained between the results of calculations with these formulas and the results of radiation measurements made with a Victoreen Condenser R‐Meter after several components had been deliberately maladjusted.

ISSN:0269-3127    

DOI:10.1118/1.594191

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Repositioning errors for given angles have been found to be as large as ±0.5°. Errors of this magnitude are undesirable and unnecessary. To overcome this difficulty, a small laser was mounted to the counterweight of our accelerator with its light beam projected at 90° to the axis of the x‐ray beam. The light beam is used as a lever arm to indicate large distances (0.125 in.) for small changes in gantry angle (0.1°). Gantry angles can easily be repositioned within 0.1°. Routine use of the laser beam indicator has reduced the time taken to reposition the gantry.

ISSN:0269-3127    

DOI:10.1118/1.594192

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A high‐atomic‐number rare gas has been incorporated into xeroradiographic cassettes in order to increase the sensitivity of the process to x radiation. Preliminary results indicate that krypton gas at 1‐atm pressure increases the sensitivity by approximately a factor of 2 in the mammography energy region.

ISSN:0269-3127    

DOI:10.1118/1.594193

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements of exposure accumulated in a one‐month period in and around a scanning x‐ray transaxial tomograph are reported. For the unit studied (the EMI neurological scanner) values measured indicate that the shielding required is “minimal.”

ISSN:0269-3127    

DOI:10.1118/1.594194

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.594195

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY