摘要:

The x‐ray luminescence of Y2O2S:Eu phosphor screens is studied both in transmission and in reflection mode observation. Detailed experimental data concerning the dependence of the absolute efficiency on the tube voltage and the screen's thickness are presented and comparisons to another rare earth phosphor material (Y2O2S:Tb) are given. With the help of theoretical calculations based on the Hamaker–Ludwig model all the experimental data are explained and the intrinsic efficiency and scattering coefficients of the phosphor are estimated.

ISSN:0269-3127    

DOI:10.1118/1.597065

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In recent years, as a solution to the well‐documented problems associated with visual interpretation of coronary arteriograms, more physiologic means of assessing coronary artery stenosis are being investigated. Absolute arterial blood flow assessed as a function of time can be a valuable aid in the analysis of functional significance of arterial lesions and obstructions. An absolute volumetric blood flow measurement technique using a motion immune dual‐energy subtraction technique is being investigated, where the kVp and filtration are switched at 30 Hz. The low‐ and high‐energy images are corrected for scatter and veiling glare before subtraction. In this technique, the absolute arterial blood flow is calculated by combining the videodensitometric analysis of spatial and temporal aspects concerning the contrast propagation through the arterial bed using tissue suppressed energy subtracted images. The blood flow measurement technique was validated using a pulsatile pump and a flow chamber imaged over a Humanoid chest phantom. A 20‐MHz Doppler flow probe was used to validate the measurement of phasic volumetric blood flow. The measured (M) and known (K) mean blood flow for the entrance vessel technique and the videodensitometric calibration technique were related byM=1.14K−0.12mL/s(r2=0.98) andM=1.12K−0.23mL/s(r2=0.90), respectively. The results indicate that phasic volumetric blood flow can be measured using a CCD camera in conjunction with real time dual‐energy subtraction.

ISSN:0269-3127    

DOI:10.1118/1.597066

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The increased emphasis on mammography acceptance testing and quality control make the star pattern test an essential component of the procedure for diagnosing poor resolution due to large focal spots. In this paper procedures for calculating magnification and the correct angle of the star pattern to use are described. The preferred placement of the star pattern in the field and proper exposure technique is also discussed.

ISSN:0269-3127    

DOI:10.1118/1.597124

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method of displaying portal images which permits the user to optimize the display contrast (windowing and leveling) without clipping parts of the image has been developed. The method finds the average signal (brightness) in small regions of the image, generally 1/64th of the total image area. The difference between the average signal in each small region and the global average over the entire image is subtracted from the original image. This eliminates changes in average signal that occur over large spatial dimensions, such as those due to varying patient thickness, without obscuring changes in signal that occur over small spatial dimensions, such as those due to anatomic structures. The technique is conceptually simple, does not introduce any visually distracting noise into the image, and is not computationally intensive. The display method has been shown to work well with images acquired from a TV camera‐based imaging system used in radiation therapy.

ISSN:0269-3127    

DOI:10.1118/1.597067

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The theory and operation of a resonator which produces a uniform oscillating magnetic field gradient is described. When the resonator is operated at audio frequency, it will produce the oscillating magnetic field gradient required for use in spatial encoding in rapid acquisition magnetic resonance imaging.

ISSN:0269-3127    

DOI:10.1118/1.597161

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this study, the feasibility of using magnetic resonance imaging (MRI) to detect tissue necrosis induced by focussed ultrasound beams was investigated. It was shown that lesions produced in dog's thigh muscleinvivowere clearly visible in T2‐weighted images and that the lesion dimensions measured from the images correlated with the postmortem measurements of the visible tissue damage. It was also shown that the sonications can be done in the magnet and that the lesions are visible immediately after the sonications with increasing image contrast as a function of time. These results showed that MRI can be used to direct and monitor on‐line noninvasive ultrasound surgery. This may have a major impact in future patient treatments.

ISSN:0269-3127    

DOI:10.1118/1.597093

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Results of a human observer performance study using a new ultrasound contrast‐detail (CD) phantom are reported. The new phantom allows estimates of the fundamental statistical uncertainties involved in CD analysis. Results of these experiments show that observers generally considered to be “trained” require experience at the specific task of low‐contrast target detection under signal known exactly (SKE) conditions, and that observers require an orientation period at the beginning of each observation session to obtain stable responses. The results obtained in this study can be used to estimate the number of independent images, observers, and repeat observation sessions required for a desired uncertainty in CD curves, and show that 10% fractional standard deviations are obtainable with modest observer effort.

ISSN:0269-3127    

DOI:10.1118/1.597147

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Preliminary clinical results indicate that some tumors can be heated well utilizing cylindrical ultrasound sources placed in body cavities. In this paper a simple method for measuring the acoustic power from cylindrical intracavitary transducers will be described. The radially propagating acoustic field was converted to a beam with a single propagation direction by a brass reflector, and the radiation force generated by this beam on an absorbing target was measured. The power output of several clinical intracavitary arrays varied significantly between identically shaped transducer elements. The results show that it is important to measure the acoustic power output from each element prior to its clinical use. The radiation force technique is simple and sensitive and can be easily adapted to be used as a routine clinical quality assurance method.

ISSN:0269-3127    

DOI:10.1118/1.597094

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using Bragg ionization curves, measurements of the average stopping power of organic polymers relative to that of water have been made for 70 MeV protons losing about 30 MeV in the absorbers. By comparing calculated curves to experimental ones, I‐values for the polymers were obtained, relative to the I‐value for Al. For repeated measurements with A‐150 tissue equivalent plastic, the standard deviation of the ratio of the average stopping powersqwas ±0.06%. It was found thatqdepends on the year of production of A‐150. For other absorbers, the uncertainty ofqis estimated to be ±0.2%, with a corresponding uncertainty of the I‐values of ±1.5%. Most measured I‐values exceed values calculated with the additivity rule by about 10%. During the course of the measurements, fluctuations of the proton energy from the cyclotron of up to 0.8% were found.

ISSN:0269-3127    

DOI:10.1118/1.597095

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The photon beams of the MM50 racetrack microtron have special characteristics which make them more suitable than conventional photon beams for precision radiation therapy with good dosimetric control. The beam flattening is obtained by the scanning of an elementary beam instead of using a flattening filter. This will give a number of advantages such as the possibility to optimize field flattening to individual field forms and field sizes. The radiation quality is the same across the whole beam, which gives smaller changes in dose profiles with depth and also makes it easier to perform careful dose planning. Beam collimation is mainly performed by a multileaf collimator and the special design of the treatment head gives nearly ideal characteristics for dose determination in an arbitrary point in the treatment fields. The output factor has been shown to depend almost solely on scattering within the treatment field. The conventional methods for beam quality characterization have been found less suitable at high energies and a new method based on HVL measurements in water is proposed.

ISSN:0269-3127    

DOI:10.1118/1.597162

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY