摘要:

Proton radiography is investigated for its use as a quality control tool in proton therapy. Images were produced both with range and range uncertainty information of protons passing through phantoms (Alderson phantom and a sheep's head). With the range images the correct positioning of the patient with respect to the beam could be verified. The range uncertainty images were used to quantitatively detect range variations of protons passing through inhomogeneities in the patient. These measurements can be used to indicate critical situations during proton therapy or to determine the safety margin around the tumor volume. With the range information the precision of different calibrations of computer tomography Hounsfield values to relative proton stopping power, used for proton treatment planning, was determined. It is found that the precision in range can be improved by a detailed analysis of the calibration data obtained from tissue‐substitute measurements, by a factor of 2.5. The resulting range errors are in the order of the positioning precision (∼1 mm).

ISSN:0269-3127    

DOI:10.1118/1.597470

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This is a theoretical study of the inherent spatial resolution of the latent image on the surface of an amorphous selenium (a‐Se) plate used for diagnostic x‐ray imaging. The following effects are considered: (A) ranges of primary photoelectrons; (B) reabsorption of K fluorescence; (C) reabsorption of Compton scattered photons; (D) diffusion; (E) the geometric effect due to oblique incidence of x rays; (F) electrostatic effect; and (G) the space charge effect. The modulation transfer function ofa‐Se in the diagnostic x‐ray energy range has been estimated. In conclusion, (A) and (E) are the main factors limiting the resolution, and for diagnostic x rays, the inherent spatial resolution ofa‐Se plates is much better than that of CsI layers used in x‐ray image intensifiers.

ISSN:0269-3127    

DOI:10.1118/1.597471

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Phase‐contrast radiography using an x‐ray interferometer is presented for observing organic matter. High sensitivity of phase‐contrast radiography is demonstrated with a rat cerebellar specimen without staining it with a contrast medium. The layer structure of the cerebellum can be observed in the obtained image while there is no clear structure in the corresponding absorption‐contrast image. Quantitative image analysis is made possible by converting an x‐ray interference pattern to an x‐ray phase shift distribution. The lipid distribution in the cerebellum is discussed by evaluating x‐ray phase shifts before and after lipid removal.

ISSN:0269-3127    

DOI:10.1118/1.597472

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper presents a computerized method for the automated segmentation of individual microcalcifications in a region of interest (ROI) known to contain a cluster in digital mammograms. Mammographic parenchyma can be accurately modeled with the fractal approach, but not areas with microcalcifications. The digitized image is divided into 16×16‐pixel overlapping windows and those accurately modeled by the fractal model are eliminated. The next steps include local thresholding of the ROIs using an iterative method, the elimination of some of the artifacts and identification of the clustered microcalcifications using a clustering algorithm. The evaluation was performed on 81 simulated clusters superimposed on normal mammographic backgrounds and on a representative database of 408 real mammograms. Microcalcification locations were identified by two radiologists independently. These locations were compared to those found by the computer algorithm. An average of 59% of the simulated microcalcifications and 69% of the microcalcifications common to both radiologists were detected. The algorithm described provides a fully automated method for the segmentation of individual microcalcifications in an area of the mammogram known to contain a cluster.

ISSN:0269-3127    

DOI:10.1118/1.597473

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of phase‐encode grouping (PEG) allows acquisition of a complete cardiac cine in a single breath hold, eliminating respiratory artifacts and improving edge definition. One approach to quantitative magnetic resonance (MR) flow measurements in pulmonary, coronary, and renal arteries uses phase velocity mapping with a PEG, single breath‐hold, cine technique. Phantom studies were designed to evaluate the accuracy of MR phase velocity mapping (MRV) with PEG in continuous and pulsatile flow measurements in small vessels. Experiments were performed on a 1.5‐T MRI system and results were compared with those obtained by timed volume measurements and a transit‐time ultrasound flowmeter. Flow velocity, pulsatile flow wave form, pulsing rate, and PEG size were varied. For continuous flow, MRV measured velocities correlated well with those calculated from timed volume measurements (r>0.998) under mean spatial velocities ranging from 20–100 cm s−1. The temporal mean velocity difference between MRV and ultrasound flowmeter measurements was less than ±2.9 cm s−1under pulsatile flow conditions with PEG sizes of 5 or less, however, the mean velocity difference was>7 cm s−1for PEG sizes of 8 or more. Accurate MR velocity measurement with PEG in small vessels under conditions of pulsatile flow is possible if an appropriate PEG size is selected.

ISSN:0269-3127    

DOI:10.1118/1.597604

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A comprehensive performance testing program is an essential ingredient of high‐quality single‐photon emission computed tomography (SPECT). Many of the procedures previously published are complicated, time consuming, or require a special testing environment. This Task Group developed a protocol for evaluating SPECT imaging systems that was simple, practical, required minimal test equipment, and could be performed in a few hours using processing software available on all nuclear medicine computers. It was designed to test rotational stability of uniformity and sensitivity, tomographic spatial resolution, uniformity and contrast, and the accuracy of attenuation correction. It can be performed in less than three hours and requires only a Co‐57 flood source, a line source, and a tomographic cylindrical phantom. The protocol was used 51 times on 42 different cameras (seven vendors) by four different individuals. The results were used to establish acceptable ranges for the measured parameters. The variation between vendors was relatively small and appeared to reflect slight differences in basic camera performance, collimation, and reconstruction software. Individuals can use the tabulated values to evaluate the performance of individual systems.

ISSN:0269-3127    

DOI:10.1118/1.597605

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The pencil beam method of calculating dose distributions for electron‐beam radiotherapy has been very useful, however, several limitations in the approach have been recognized. One such limitation is the lack of a mechanism to model range straggling of electrons. For stationary electron‐beam calculations, range straggling is incorporated incompletely in the planar‐fluence‐to‐dose conversion factor, which uses measured percentage depth dose curves to force the calculated percentage depth dose to reproduce the measurement. When calculating the dose distribution for an arced beam using a pencil beam algorithm, insufficient modeling of the pencil beams leads to larger errors than when using a stationary beam algorithm. The calculated depth of maximum dose is systematically overestimated by the pencil beam calculations. We will show that the lack of a way to account for range straggling in the arc‐electron pencil beam calculation is primarily responsible for this discrepancy. Methods of incorporating range straggling into the electron pencil beam dose calculation have been presented before, but no data have been shown to support their use for heterogeneous phantoms (patients). This paper presents a similar range‐straggling modification, as well as data to show that this model can predict pencil beam width to within 20% for heterogeneous slab phantoms. For stationary electron‐beam calculations, the calculated isodose lines follows the measured isodose lines to within 1 mm down to the 10% dose level. Incorporating this modification into the arc‐electron pencil beam calculation improves the agreement between the depths of the calculated and measured maximum doses to within 2 mm; however, the magnitude of the maximum dose is as much as 3%–4% lower for the calculated arc‐electron beams. This study discusses reasons for these discrepancies and presents possible improvements.

ISSN:0269-3127    

DOI:10.1118/1.597606

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The photon beam dose calculation methods using fast Fourier transform (FFT) convolution require that the primary beam energy spectrum is accurately known; however, a quantitative assessment of the required accuracy is not yet provided. In this study, the sensitivity of various parameters such as central axis depth dose and dose under attenuators to the incident photon spectrum was simulated and evaluated for 6‐ and 18‐MV photon beams. The central axis depth–dose data using the modified spectrum were compared with those calculated using the original spectrum. Maximum errors on the central axis between these two sets of depth–dose data were found by changing the weighting of each energy bin of the primary spectrum. The maximum change in percent depth dose is proportional to the change in average incident energy relating to the quality of the x ray. To keep the maximum depth–dose error to less than 1%, the relative change in average energy brought about by changes in single spectral bins should be less than 2%. The accuracy required for each energy bin varies dramatically, dependent on its deviation from the original average energy and its relative weighting. For example, for the 6‐MV x‐ray spectrum, the energy bins centered around 0.76 MeV (first bin) and 6.8 MeV (fifth bin) need to have accuracies of 5% and 40%, respectively, to obtain a 1% accuracy in percent depth dose.

ISSN:0269-3127    

DOI:10.1118/1.597467

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Multirod arrays can be used to produce irregularly shaped irradiation fields for use in external beam photon and neutron radiation therapy. Two prototype multirod collimators have been built for use with high energy photon beams. A practical multirod collimator is in routine use with the fast neutron beam at a superconducting cyclotron based neutron therapy facility. A simple computer program has been written for calculating the transmission of photon and neutron beams through multirod arrays. The results of calculations for both close packed and spaced rod arrays are presented, and compared with available photon and neutron transmission data. The transmission through a regularly packed array exhibits a pattern of maxima and minima which occur with a spacing corresponding to the rod radius. The program predicts the positions and magnitudes of the transmission peaks. The rod diameter and spacing, the source size, and the position of the multirod collimator and the measurement plane relative to the source, all effect the exact nature of the measured transmission pattern. The transmission of 15‐MV photons through close packed and spaced rod arrays was calculated using the program and compared with measurements made in a close packed tungsten rod array and with a prototype multirod collimator. Calculations for the transmission of ap(42)‐Be neutron beam through a close packed tungsten rod array were compared with previously published data. Good agreement between calculations and measured data was obtained in all cases. The program was used to design a practical multirod collimator for ad(50)‐Be fast neutron beam. An improvement to the collimator design in which half of the rod rows in the array are displaced by one quarter rod diameter relative to the rest of the array leads to reduced peak transmission along the central beam axis.

ISSN:0269-3127    

DOI:10.1118/1.597468

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An insert for a well ionization chamber is described for quality assurance tests for high dose rate remote afterloaders. The use of this insert over time has shown its applicability for efficient QA tests for the high dose rate afterloader. The measurement results are six times greater than the uncertainties involved for the measurements. Checks on position can be made better than 1 mm and dwell times better than 1 s. The overall measurements can be done to better than 1.5%. QA checks can be accomplished in a shorter time period with as great or better accuracy than prior film techniques.

ISSN:0269-3127    

DOI:10.1118/1.597469

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY