摘要:

The basic partial differential formulation for small differences has been applied to the equations for the inversion recovery and saturation recovery sequences with spin‐echo rephase pulses and the spin‐echo sequence itself. The result is a generalized expression which is proportional to tissue discrimination expressed as the signal‐difference‐to‐noise ratio for a pair of tissues. The expression is a function of the fractional differences in the tissue relaxation times and proton densities,T1,T2, andn, respectively, and the sequence parametersTRandTI, orTE. From it can be derived the conditions which lead to optimum tissue discrimination for a given pulse sequence. It is also possible to predict the optimum pulse sequence to use for any pair of tissues of interest. The method requires a knowledge of the fractional differences inT1,T2, andnfor both tissues.

ISSN:0094-2405    

DOI:10.1118/1.595866

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Determination of the relaxation timesT1andT2which are important for tissue characterization generally requires the use of different pulse sequences in magnetic resonance imaging. In this study, a new pulse sequence which facilitates simultaneous determination of theT1andT2times is presented. Determination takes place in this case pixel by pixel from the measured images. The measuring time corresponds in this case approximately to that of a normal spin‐echo sequence with long repetition time and two data acquisitions. The functional dependence of the accuracy of theT1andT2determination upon external errors, e.g., angle of rotation errors, is discussed. The tissue contrast behavior of the individual echoes is shown and its dependence on pulse parameters is explained.

ISSN:0094-2405    

DOI:10.1118/1.595867

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A pulse sequence procedure for producing oblique magnetic resonance images is described. Using this procedure we present a new, accurate method to obtain true short‐axis views and true long‐axis views (both parallel and perpendicular to the septal plane) of the heart. The method is accurate regardless of the orientation of patient's heart. The method does not require the patient to be rotated, nor otherwise moved, and does not require any additional hardware. The method is experimentally verified with both human and phantom studies. The phantom study indicates accuracy of approximately 1° with a commerical scanner that reports angular measurements to a precision of 1°. Application of the short‐axis views to measurement of left ventricular volume, and possible advantages of Gauss–Legendre integration for this measurement are discussed. Finally, multiphase oblique cardiac images are presented.

ISSN:0094-2405    

DOI:10.1118/1.595868

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The images formed by many radiological systems are difficult to sample at spatial intervals small enough to avoid aliasing in the calculation of the system's modulation transfer function. However, if a system's response can be assumed to be symmetrical, this assumption can be used to effectively double the sampling density and to double the frequency limit before aliasing occurs. To accomplish this, a more complex algorithm is required. In this work, the formula for the calculation of the modulation transfer function from a symmetrical, one‐dimensional line spread function is derived and a similar result for a symmetrical, two‐dimensional point spread function is presented. The effect of noisy data and errors in the estimation of the offset of the center of the line spread function from a sampling point are investigated by simulation studies. For low noise (relative standard deviation of 1%) and an offset error of no more than 2% or 3% of a sampling interval, reasonable precision is obtained. These conditions appear to be achievable, especially when the noise is Poisson distributed.

ISSN:0094-2405    

DOI:10.1118/1.595869

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A spectrometer with a high‐resolution room‐temperature silicon detector was used to obtain spectra including characteristic x‐ray peaks produced after fluoroscopic beams were incident on W, Pb, and Th absorbers. Characteristic x‐ray yield points were obtained from spectra taken at several generator kVp settings which produced x rays with energy above theK‐shell absorption edge for each element. A line drawn through the yield points when projected to zero yield served to locate the console setting corresponding to the actualK‐absorption edge energy for each absorber. The technique is useful for precision measurement of generator kVp and absolute calibration of indirect kVp measuring devices under operational conditions.

ISSN:0094-2405    

DOI:10.1118/1.595870

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dose estimates were made (1) for Kodak Min‐R screens combined with Kodak Min‐R film and Kodak Ortho M film, both with and without a 5:1 Bucky grid; (2) for standard xeroradiographic techniques in negative development mode; and (3) for the new, higher sensitivity xeroradiographic process of the Xerox 175 System. The estimates were derived from exposure versus depth measurements in phantoms made of BR12 breast simulation material using thermoluminescent detectors. A molybdenum target source with molybdenum filtration, at a half‐value layer of 0.37‐mm Al, was used for the screen–film measurements. All xeroradiographic measurements were made with a tungsten target source with aluminum filtration at half‐value layers of 1.5 to 1.56 mm Al. Mean glandular dose estimates for the Min‐R screen/Ortho M film combination with Bucky grid and for the new xeroradiographic process were found to be similar. Dose reduction with the new xeroradiographic system was achieved through a more sensitive photoreceptor and more sensitive development, which also improved the unique imaging characteristics of xeroradiography.

ISSN:0094-2405    

DOI:10.1118/1.595871

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A parallel‐plate ion chamber has been investigated with a view to determiningNgas, the cavity‐gas calibration factor, introduced by the Task Group 21 protocol for dose determination. In‐air and in‐phantom irradiation of a Memorial Hospital parallel‐plate ion chamber in a60Co beam led to the determination of ion collecting volume and a calibration factor for the chamber. When these quantities were used to deduceNgasfactors for the chamber, an agreement within 1% was found. An average of the cavity‐gas factors obtained from60Co data agreed to within 0.5% with the factor determined through force fitting the doses received by the parallel‐plate chamber and a cylindrical chamber in a 22‐MeV (nominal) electron beam. Additionally, the parallel‐plate chamber was utilized to study the effectiveness, in electron beam dosimetry, of the electron fluence perturbation correction factors introduced when the phantom material is replaced by a cylindrical ion chamber.

ISSN:0094-2405    

DOI:10.1118/1.595872

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo calculations of radiation dosimetry usingmorsecode are performed for125I and60Co point sources in a cylindrical head phantom that simulates the geometry of eye plaque therapy for choroidal melanoma. We obtain the dose variation in the eye at submillimeter intervals over distances as close as 1 mm and up to 2.5 cm from the source. The calculations for125I are performed for the phantom media of water, protein, and a homogenized protein–water mixture simulating the composition of the eye. Relative dose functions for125I for these phantom media are fitted to second‐degree polynomials. Agreement is found with published results. The relative dose function for60Co at eye position in the water head phantom is fitted to a third‐degree polynomial and compared with that for60Co at the center of a large water sphere. A boundary effect due to the head phantom–air interface on the dose distribution for60Co is demonstrated. The dose falloff with distance is faster for the eye geometry compared with the bulk geometry. We also show that the relative dose distributions within the tumor are comparable for125I and60Co by comparing their relative dose functions. This result is consistent with the success of clinical trials of large melanoma treatments with125I plaques.

ISSN:0094-2405    

DOI:10.1118/1.595949

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Our studies have compared the “effective tissue–air ratio (TAR) method” (ICRU Report No. 24), “equivalent TAR method,” and the “generalized Batho method” (currently used by the TP‐11 computer treatment planning system) with measured results for different energy photon beams using two lung inhomogeneities to simulate a lateral chest field. Significant differences on the order of 3%–15% were found when comparing these various methods with measured values.

ISSN:0094-2405    

DOI:10.1118/1.595830

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For 1–50 MeV electrons incident on a water phantom there are systematic differences in the depth‐dose curves calculated by the Monte Carlo codes EGS and ETRAN (and its descendants SANDYL, CYLTRAN, ACCEPT, and the ITS system). Compared to ETRAN, the EGS code calculates a higher surface dose and a slightly slower dose falloff past the dose maximum. The discrepancy in the surface dose is shown to exist because the modified Landau energy‐loss straggling distribution used in ETRAN underestimates the mean energy loss by about 10% since it underestimates the number of large energy loss events. Comparison to experimental data shows a preference for the EGS depth‐dose curves at 10 and 20 MeV. Since various dosimetry protocols assign electron beam energies based on measured depth‐dose curves in water, formulas based on these more accurate EGS4 calculations are presented: (i) relating the mean energy of an incident electron beam toR50, the depth at which the dose in a water phantom falls to 50% of its maximum value; and (ii) relating the most probable energy of the incident beam to the projected range of the depth‐dose curve. A study is presented of the effects of the incident electron spectrum on the calculated depth‐dose curve.

ISSN:0094-2405    

DOI:10.1118/1.595831

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY