摘要:

A frequency‐switchable, homogeneous‐field rf saddle coil has been designed for imaging both protons (1H, 59.1 MHz) and fluorine (19F, 55.6 MHz) on a 1.4‐T superconducting small‐bore nuclear magnetic resonance imager. Frequency and impedance switching is accomplished by external capacitance and cable length changes; these operations permit imaging of both nuclei without perturbing the sample. The coil is optimized for19F operation, yet performs better at the proton frequency than does the unswitched19F coil. The angular distribution of the coil's wires and the use of distributed capacitors are designed to optimize field homogeneity andQ. A quantitative image of field homogeneity is presented. The coil is suitable for imaging small animals (7‐cm‐diam bore) and couples far better to small samples than does our standard receiver coil (15.2 cm in diameter). Images of phantoms and rats injected with a perfluorinated blood substitute are presented.

ISSN:0094-2405    

DOI:10.1118/1.595648

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A therapeutic hyperthermia protocol using a radio frequency (rf) electrode placed adjacent to a bronchial wall tumor has been modeled using the finite element technique. Variable physical properties and variable blood perfusion have been assigned to the tumor and to the surrounding normal lung tissue. The Laplace equation was solved on a curvilinear grid for a single rf source electrode to determine the steady‐state electric field, which in turn governs the energy deposition function. The heat generation in the tumor and in the lung tissue is then calculated from the energy deposition profile, and the bioheat equation is solved on the same finite element mesh to determine the transient temperature history. The temperatures are displayed as isothermal contours at designated times during the protocol and as temperature histories at selected points. In addition, an Arrhenius‐type injury model has been implemented to predict thermally induced damage, from which injury contours and injury histories are produced. Several protocols are investigated in which equal total amounts of energy are deposited into the tissue using a constant power density for an appropriate time or using a cyclic heating pattern. The cyclic heating pattern consisted of a series of equal duration time periods during which the rf current source is alternately turned on and off (50% duty cycle). This study illustrates how a finite element model could be used to evaluate alternative protocols for heating a tumor of a specific geometry and to evaluate thermally induced damage to surrounding normal tissue.

ISSN:0094-2405    

DOI:10.1118/1.595649

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Calculations of transient tissue temperature distributions are enhanced by the use of an algorithm to simulate automatic temperature regulation by negative feedback. This feature is quite useful in treatment planning, enabling the physicist to predict the effects on the treatment of variations in temperature regulation system parameters, such as temperature sensor placement and feedback loop gain.

ISSN:0094-2405    

DOI:10.1118/1.595650

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Optimal visualization of moving structures such as the heart and coronary arteries using digital radiographic imaging systems is a difficult problem that can involve tradeoffs between temporal, spatial, and density resolution. The motion dependence of four angiographic measures of vessel dimensions is given and demonstrated experimentally. Although the densitometric cross‐sectional area is shown to be independent of motion, densitometric thickness (contrast) decreases and the apparent width (distance between edges) and densitometric width both increase with motion. Knowledge of the velocity allows the apparent width but not the densitometric width to be corrected. It is also shown that current limitations which result in tradeoffs between kVp and x‐ray pulse duration seriously compromise the advantages of using small focal spots in coronary artery imaging.

ISSN:0094-2405    

DOI:10.1118/1.595651

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper examines statistical errors in the measurement of arterial stenoses by digital videodensitometry. Images of vessel phantoms were acquired using digital subtraction angiographic techniques with low concentrations of an iodine contrast medium and low levels of x‐ray exposure. Effects of the spatial and temporal averaging of image information on signal‐to‐noise ratios in the stenosis measurement were of primary interest. The influences of iodine concentration, x‐ray scatter, veiling glare, x‐ray energy spectrum, x‐ray exposure, and detective quantum efficiency of the system were also included in the theoretical analysis. The agreement between theoretical calculation and experimental measurement of a simulated vessel was verified using measured values of the imaging system parameters. With a 14.2 mg/ml iodine concentration, using 20 mR per image at the entrance to a 13‐cm water phantom, and averaging over a 6‐mm length of a vessel 6.2 mm in diameter, the standard deviation in a measurement of a vessel's relative cross‐sectional area was about 0.05. The extension of these results to practical applicationsin vivois discussed.

ISSN:0094-2405    

DOI:10.1118/1.595652

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We devised a new, simple technique for measuring the modulation transfer function (MTF) of a digital imaging system by using an image of an angulated slit. With this technique, the “presampling” analog MTF, which includes the geometric unsharpness, the detector unsharpness, and the unsharpness of the sampling aperture, can be measured even beyond the Nyquist frequency. A single‐frame image of a slightly angulated slit was employed in order to obtain Fourier transforms of line spread functions at different alignments. The presampling MTF was determined by averaging the two Fourier transforms which we obtained from two extreme alignments (center and shifted) of the slit relative to the sampling coordinate. The presampling MTFs of our digital subtraction angiographic system were determined in two orthogonal directions for three different image‐intensifier modes.

ISSN:0094-2405    

DOI:10.1118/1.595653

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The variance derived from the images of cyclic bar patterns can be used to determine the modulation transfer function (MTF) of an imaging system. Unlike most MTF methods, it is applicable even when the imaging system undersamples the test object and generates aliasing errors. The validity of the variance method in the presence of aliasing is established theoretically and by computer simulation.

ISSN:0094-2405    

DOI:10.1118/1.595654

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The technique of separating dose into primary and scatter components for calculating photon dose distributions is widely used. The primary and scatter dose models ignore the fact that electrons have a finite range. This may be a good approximation for60Co photons but not for higher energies. For the latter, the range of electrons may be several centimeters in soft tissue and even longer in lungs and will lead to errors in computed dose in regions where electronic equilibrium does not exist. Ignoring the finite range of electrons will affect dose at points such as those near the beam boundaries, near inhomogeneity boundaries, and at bone–soft‐tissue interfaces. Other possible problems associated with the definition and use of “primary” and “scatter” dose in dose distribution calculations result from extrapolation of measured data to obtain data for zero and very large field sizes and from the use of these quantities, which are defined for central axis, for points at large distances from the central axis. This paper examines the limits of the validity of these assumptions.

ISSN:0094-2405    

DOI:10.1118/1.595655

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The complete distribution of high‐energy photon primary absorbed dose along the beam path within a semi‐infinite or finite water phantom can be expressed by one functional equation. The derivation is based on a technique of convoluting a primary collision kerma distribution function with a pair of forward and backward spread dose‐distribution functions. In particular, the characteristic patterns of primary dose in the regions near the beam entrance and exit surfaces can be obtained. The calculated ratio of primary dose to primary collision kerma in the electron‐equilibrium region in water for60Co radiation is 1.0060 and that for 25‐MV radiation is 1.0444.

ISSN:0094-2405    

DOI:10.1118/1.595656

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This report investigates the buildup region of the depth‐dose curve for broad parallel beams of60Co and lower energy photons incident on slabs of tissue as defined by the International Commission on Radiation Units and Measurements (ICRU) and other materials. The EGS Monte Carlo system was used to simulate the transport of photons and electrons in the materials. Calculated buildup regions of the depth‐dose curves are presented for photons incident on ICRU tissue. For60Co beams, the primary and scatter dose components are presented, both for the buildup region and at depth in the phantom. Effects of beam radius were investigated and found to have only a small effect in the buildup region of the central axis depth‐dose curve. The calculated effects of beam radius are larger at depth and in good agreement with experiment. Effects of scatter and attenuation in a small miniphantom of water were investigated as a function of photon energy and were found to cancel within 0.8%. The mean distance of electron transport was calculated for60Co beams incident on carbon, water, polystyrene, aluminum, and lead.

ISSN:0094-2405    

DOI:10.1118/1.595657

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY