摘要:

The longitudinal (T1) and transverse (T2) hydrogen (1H) nuclear magnetic resonance (NMR) relaxation times of normal human and animal tissue in the frequency range 1–100 MHz are compiled and reviewed as a function of tissue type, NMR frequency, temperature, species,in vivoversusin vitrostatus, time after excision, and age. The dominant observed factors affectingT1are tissue type and NMR frequency (ν). All tissue frequency dispersions can be fitted to the simple expressionT1=AνBin the range 1–100 MHz, withAandBtissue‐dependent constants. This equation provides as good or better fit to the data as previous more complex formulas.T2is found to be multicomponent, essentially independent of NMR frequency, and dependent mainly on tissue type. Mean and raw values ofT1andT2for each tissue are tabulated and/or plotted versus frequency and the fitting parametersA,Band the standard deviations determined to establish the normal range of relaxation times applicable to NMR imaging. The mechanisms for tissue NMR relaxation are reviewed with reference to the fast exchange two state (FETS) model of water in biological systems, and an overview of the dynamic state of water and macromolecular hydrogen compatible with the frequency, temperature, and multicomponent data is postulated. This suggests that1H tissueT1is determined predominantly by intermolecular (possibly rotational) interactions between macromolecules and a single bound hydration layer, and theT2is governed mainly by exchange diffusion of water between the bound layer and a free water phase. Deficiencies in measurement techniques are identified as major sources of data irreproducibility.

ISSN:0269-3127    

DOI:10.1118/1.595535

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two recently developed A150 plastic‐equivalent gas mixtures have been tested for suitability in proportional counter applications. Methane‐ and propane‐based “tissue‐equivalent” gases were also included for comparison purposes. Event‐size weighted dose distributions were measured in a 14.8‐MeV neutron beam. Resolution was found to be independent of gas mixture. Moreover the gains of the two A150 mixtures were the same, and comparable to that of the methane‐based gas mixture. The ionization yield per event size was also independent of the hydrogenous gas mixture employed. Neutron doses determined with the proportional counter were in reasonable agreement with those obtained from an ionization chamber.

ISSN:0269-3127    

DOI:10.1118/1.595536

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Previous studies of the imaging performance of computed tomography (CT) scanners, and other imaging modalities, have failed to apply appropriate statistical methods to data analysis, thus impairing the accuracy and significance of results. Given that imaging performance involves a number of interrelated variables and an element of randomness, its empirical assessment requires multivariate regression analysis. This method is used here to analyze anew a set of contrast‐detail data from a previous study on CT scanners. The main issues considered are the specification of the proper functional form linking perceptibility, dose and contrast, the estimation of the contrast and dose coefficients, and of scanner‐specific constants to be used in computing indices of imaging quality. One of the main empirical findings is that the dose coefficient of the CT scanners studied is significantly less than that predicted by the theoretical model: 1/5 instead of 1/3. This result suggests that actual dose used in routine clinical studies could be reduced substantially without impairing much the quality of the images. On the other hand, the coefficient of contrast does correspond to its predicted value, i.e., 2/3. The methodology used here is not limited to the contrast‐detail framework, but is applicable to, and indeed essential in, empirical studies of the performance of any imaging modality.

ISSN:0269-3127    

DOI:10.1118/1.595537

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The two‐voltage method of determining the collection efficiency of an ionization chamber exposed to a pulsed and magnetically swept electron beam depends upon the form of the saturation curve agreeing closely with the formula based on ionic conduction and negligible space charge. The possible disturbing effects of charge transport by free electrons and of overlapping pulses are discussed and the conditions to be satisfied for the reliable use of the method are defined and demonstrated experimentally.

ISSN:0269-3127    

DOI:10.1118/1.595538

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Three sources of variability in a contrast‐detail (CD) experiment have been quantitated: within‐observer variance, between‐observer variance, and sample variance. It is concluded that (1) it is more efficient to increase the numbers of replicated images and observers than to increase the number of readings; (2) sampling and between‐observer variations are approximately equal; (3) one can expect approximately 10% standard errors in the measured value of threshold detail or threshold contrast in a CD experiment which employs four observers, four replicate image samples, and one reading per observer.

ISSN:0269-3127    

DOI:10.1118/1.595539

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Samples of potassium hexafluorosilicate (K2SiF6) and polytetrafluoroethylene (C2F4)nwere irradiated in the bremsstrahlung beam from a clinical linear electron accelerator at integral values of endpoint energy between 13 and 32 MeV. Sample positron activities produced by the reactions39K(γ,n)38K,19F(γ,n)18F, and12C(γ,n)11C, were determined by counting the gamma radiation. Saturation activities per target nucleus were calculated from these data. For the K2SiF6samples, the photonuclear activation ratio (PAR) of the 7.6 min39K saturation activity per nucleus divided by the18F result increased by more than 20% per MeV between 15 and 21 MeV. For the (C2F4)nsamples the ratio (PAR) of the11C saturation activity per nucleus to that for18F increased by more than 24% per MeV between 22 and 26 MeV. Quality changes caused by the lead flattening filter were easily detectable using the PAR technique. Because of the high sensitivity to bremsstrahlung spectrum changes these PAR values can serve as sensitive indices of quality for high‐energy beams.

ISSN:0269-3127    

DOI:10.1118/1.595540

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We applied Monte Carlo calculations to determine the radiation dose absorbed in water phantoms. Monoenergetic incident x‐ray beams with energies from 15 to 100 keV and phantom thicknesses from 5 to 20 cm were considered in this study. We calculated the spatial distributions of energy absorption in the phantom, the rad/R conversion factors, the average rad/R conversion factors, and the scatter‐to‐primary ratios of absorbed dose. We also compared the relative absorbed doses under various imaging conditions when the transmitted radiation produced a given optical density on radiographic film. The information provided will be useful for the estimation of radiation doses in various radiographic procedures.

ISSN:0269-3127    

DOI:10.1118/1.595541

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of dosemeter type and configuration on the measured penumbral distribution for Co‐60, 6‐MV, and 31‐MV x rays has been determined in air using equilibrium buildup caps for three commercial detection systems including a silicon diode and two ionization chambers. The diode is shown to be measuring a different parameter in the penumbral region than the ionization chambers. This fact in combination with the lateral spread of the secondary electrons and the difference in the inside diameters of the ionization chambers results in significant differences between the measured beam penumbra. The latter effect is studied in more detail with a series of specially designed ionization chambers of varying inside diameter from 0.3 to 1.4 cm. A theoretical model is described which resolves these differences, indicates a method to determine the true penumbral primary‐dose distribution and introduces the concept of an effective diameter for the ionometric measurement of high‐energy x‐ray penumbra. Recommendations are made concerning the dosemeters of choice for penumbral measurements over this range of photon energies.

ISSN:0269-3127    

DOI:10.1118/1.595542

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18‐ and 15‐MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18‐MV beam and a gamma counting technique for both the 18‐ and 15‐MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient‐treatment plane for different field sizes. The results show that the thermal‐neutron dose equivalent contributes only about 1%–2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18‐MV beam is approximately six times that of the 15‐MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.

ISSN:0269-3127    

DOI:10.1118/1.595543

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The spatial resolution and noise level of images produced by a commercial analog tomographic scanner have been measured and compared to those of images reconstructed digitally from projections from the same detector. The full width at half maximum of the line spread function was 3.6 mm for images from the analog scanner and 1.1 mm for the digitally reconstructed images. The standard deviation of the CT numbers over a 10‐cm2circular area at the center of a large water phantom, calculated as a percentage of the linear attenuation coefficient of water, was 3.5% for the analog images, 15.4% for high‐resolution digital images, and 3.2% for digital images reconstructed using a convolution filter which reduced the resolution to that of the analog images. The data contributing to each digital image were fewer than those contributing to each analog image by a factor of 10. The noise level did not depend on tube current in either the analog or the digital images. The utility of this analog device in radiation therapy planning will depend upon whether errors in contour localization resulting from transferring data from diagnostic CT scanners exceed the errors due to its poorer image quality.

ISSN:0269-3127    

DOI:10.1118/1.595519

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY