ISSN:0269-3127    

DOI:10.1118/1.596435

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper compares two methods for determining the radio frequency absorbed power in tissue: from the measured electrical properties of the tissue, and from the induced temperature rise per unit time. In previous research, we measured the radio frequency electrical properties of muscle, mammary gland, and malignant mammary tissue (SMT‐2A mammary adenocarcinoma) in female W/Fu isogeneic rats. From those measurements we calculated for each tissue the power absorption versus frequency, and formed the ratio of malignant‐to‐normal power absorption. This ratio exhibited a peak within the 150 to 400 MHz range, indicating a selective absorption of power in this type of malignant tissue over that of the normal host tissue. In the present study, by an entirely different method, we have directly tested the results of our earlier research. We filled a 20‐cm‐long section of rigid coaxial line (ordinarily air filled) with either normal (beef muscle or fat) or malignant (SMT‐2A) tissue, and measured the temperature increase versus time at the irradiated tissue surface for the same absorbed power in each tissue type. We made the measurements from 50 to 915 MHz, and found that the initial temperature increase per second per watt absorbed (dT/dt/Pa) was greater in malignant tissue than in muscle or fat at each frequency tested, with the greatest differences occurring below 450 MHz. Power absorption based on the measured values ofdT/dt/Pawas again greatest for the malignant tissue (SMT‐2A mammary adenocarcinoma) within the 150 to 400 MHz range.

ISSN:0269-3127    

DOI:10.1118/1.596308

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Nowadays, digital subtraction angiography systems must be able to sustain real‐time acquisition (30 frames per second) of 512×512×8 bit images and store several sequences of such images on low cost and general‐purpose mass memories. Concretely, that means a 7.8 Mbytes per second rate and about 780 Mbytes disk space to hold a 100‐s cardiac examination. To fulfill these requirements at competitive cost, a distortionless compressor/decompressor system can be designed: during acquisition, the real‐time compressor transforms the input images into a lower quantity of coded information through a predictive coder and a variable‐length Huffman code. The process is fully reversible because during review, the real‐time decompressor exactly recovers the acquired images from the stored compressed data. Test results on many raw images demonstrate that real‐time compression is feasible and takes place with absolutely no loss of information. The designed system indifferently works on 512 or 1024 formats, and 256 or 1024 gray levels.

ISSN:0269-3127    

DOI:10.1118/1.596309

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The small‐angle (2°–10°) total‐differential‐scattering cross sections of water, methyl methacrylate, and nylon 6 have been measured at a photon energy of 59.54 keV. An annular target geometry has been used with an acceptance of ±0.5° and an experimental uncertainty of ∼4% achieved. The experimental methods and data corrections necessary to obtain this accuracy are discussed in detail. The results demonstrate clearly the contribution of intermolecular effects to the scattering process and allow critical comparison with calculations using molecular form factors for water obtained from interpolated x‐ray diffractometer data; agreement is obtained within the experimental uncertainties. They also support earlier Monte Carlo predictions of scattering in water at angles of the order of 2° which are in disagreement with certain published experimental data.

ISSN:0269-3127    

DOI:10.1118/1.596310

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An energy discriminating detector for dual‐energy radiography can be configured as a two‐layer sandwich, where the mean energy of photons detected by the two layers differs. To characterize the quantum noise of such a detector, the noise covariance between the two layers must be known in addition to the noise variance in each layer. A theory is presented which permits the calculation of the noise covariance, and it is found to be negligibly small. Experimental results, based on measurements with a Nal sandwich detector and an isotope gamma ray source, are reported and shown to confirm the theory. The quantum noise in each layer is independent and Poisson.

ISSN:0269-3127    

DOI:10.1118/1.596311

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A digital x‐ray photoelectronic imaging system has been constructed using an optically flat 152‐mm‐diam, 2.5‐mm‐thick CsI:Na scintillating crystal. X‐ray images formed by the scintillator are viewed by a single microchannel plate intensified charge injection device (ICID) camera and digitized at a real‐time rate by a computerized frame‐grabbing system. Video images are recorded and selected image frames are subjected to image processing and analysis schemes. Parameters governing the performance characteristics of the system are determined accordingly. For a 152‐mm field size at the crystal plane, a spatial resolution limit of 1.50±0.10 1p/mm (1σ) measured at the 4% level of the modulation transfer function of the system has been obtained. This result is consistent with the measurements of the full width at half‐maximum of the line spread function which is found to be 645±35 μm (1σ). Similarly, the intrinsic resolution of the CsI:Na scintillator only was found to be 6.5±0.5 1p/mm (1σ). Contrast ratio measurements, which are mainly determined by the quality of phosphor, have indicated a value of 12.1±0.6, whereas minimum visible details are observed at radiation exposure rates of 100 μR/s. This limit has been reduced to 10 μR/s using the single‐scan integrating option provided by the ICID camera. A maximum contrast resolution of 1% corresponding to 100 statistically significant meaningful gray levels is achieved at a maximum exposure rate of 1000 μR/s. Consequently, although the imaging capability of the present system compares favorably with that of conventional x‐ray video‐fluoroscopic systems, a better performance may be achieved by using a higher resolution cooled dual‐microchannel intensified CID camera in conjunction with a thinner CsI:Na crystal and a real‐time digital image processing subsystem.

ISSN:0269-3127    

DOI:10.1118/1.596312

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It has long been recognized that the problems of motion artifacts in conventional time subtraction digital subtraction angiography (DSA) may be overcome using energy subtraction techniques. Of the variety of energy subtraction techniques investigated, non‐k‐edge dual‐energy subtraction offers the best signal‐to‐noise ratio (SNR). However, this technique achieves only 55% of the temporal DSA SNR. Noise reduction techniques that average the noisier high‐energy image produce various degrees of noise improvement while minimally affecting iodine contrast and resolution. A more significant improvement in dual‐energy DSA iodine SNR, however, results when the correlated noise that exists in material specific images is appropriately cancelled. The correlated noise reduction (CNR) algorithm presented here follows directly from the dual‐energy computed tomography work of Kalender who made explicit use of noise correlations in material specific images to reduce noise. The results are identical to those achieved using a linear version of the two‐stage filtering process described by Macovski in which the selective image is filtered to reduce high‐frequency noise and added to a weighted, high SNR, nonselective image which has been processed with a high‐frequency bandpass filter. The dual‐energy DSA CNR algorithm presented here combines selective tissue and iodine images to produce a significant increase in the iodine SNR while fully preserving iodine spatial resolution. Theoretical calculations predict a factor of 2– 4 improvement in SNR compared to conventional dual‐energy images. The improvement factor achieved is dependent upon the x‐ray beam spectra and the size of blurring kernel used in the algorithm. In the limit of large blurring kernels, the noise‐reduced dual‐energy image has an iodine SNR approaching the maximum value achievable with a linear combination of the low‐ and high‐energy images, with the additional advantage that low spatial frequency tissue signals are substantially reduced. Phantom measurements confirm the predicted increase in SNR of the CNR technique while images processed both by conventional dual‐energy and CNR methods demonstrate the noise suppression of the CNR algorithm and the tissue edge artifacts which it may introduce.

ISSN:0269-3127    

DOI:10.1118/1.596436

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study has intercompared the predictions of Fermi–Eyges theory for the rms spatial spread (σ) of an electron pencil beam scattering in muscle‐, lung‐ and bone‐equivalent media with those of; two range straggling modifications to the theory, Monte Carlo simulations, and an empirical method based on broad beam penumbra. Systematic differences among the results obtained by these methods for the values of σ have been identified. Monte Carlo simulations are lower than the predictions of Fermi‐Eyges theory for σ at all depths whereas the broad beam penumbra method results are in reasonable agreement with Fermi–Eyges theory at depths less than ∼0.7 times the range of the incident electrons. All of the methods investigated have an increasing discrepancy from the predictions of Fermi–Eyges theory with depth, especially close to the end of the electron range. The two range‐straggling modifications to Fermi–Eyges theory developed for soft tissue do not agree with either measured or Monte Carlo results for σ in homogeneous scattering media of lung and bone.

ISSN:0269-3127    

DOI:10.1118/1.596437

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A solution to the convection–dispersion model of tracer transport in biological systems is presented. This solution provides for the characterization of tracer transport through a network of blood vessels based on the tracer transients recorded at the inlet and outlet points of the circulation under investigation. Fourier transformations of the transients are used to produce a transfer function from which the mean transit time as well as other transport characteristics can be calculated. The practical aspects of applying transfer function analysis to retrieve transport characteristics under experimental conditions are also presented. The application demonstrates the efficacy of the transfer function analysis method independent of tracer recirculation effects andaprioriknowledge of actual tracer concentrations.

ISSN:0269-3127    

DOI:10.1118/1.596313

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measured beam profiles and central‐axis depth‐dose data for 6‐ and 25‐MV photon beams are used to generate a dose matrix which represents the full beam. A corresponding dose matrix is also calculated using the modified CBEAM model. The calculational model uses the usual set of three parameters to define the intensity at beam edges and the parameter that accounts for collimator transmission. An additional set of three parameters is used for the primary profile factor, expressed as a function of distance from the central axis. An optimization program has been adapted to automatically adjust these parameters to minimize the χ2between the measured and calculated data. The average values of the parameters for small (6×6 cm2), medium (10×10 cm2), and large (20×20 cm2) field sizes are found to represent the beam adequately for all field sizes. The calculated and the measured doses at any point agree to within 2% for any field size in the range 4×4 to 40×40 cm2.

ISSN:0269-3127    

DOI:10.1118/1.596314

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY