摘要:

Nonradioactive rubidium was used to label red cell in rabbits and its concentration was determined by employing a simple version of radioisotope‐induced x‐ray fluorescence analysis. About 10 ml of red cells were labeledin vitrowith 0.5–1 mg of RbCl and subsequently reinjected, resulting in a Rb concentration of about 50 ppm by the red cells. Four milliliters of blood were collected about every week, the concentrated erythrocytes deposited on filter paper disks, and the Rb content analyzed. Except for the first four to seven days, a function of the typeN=N0e−ktfitted very well the experimental Rb disappearance curve data. The mean half‐time value deduced from the experimental data was 310±15 days, which should be compared with the values ofT1/2=260±20 h andT1/2=206±30 h obtained, respectively, by these authors and by Priceet al., employing radioactive51Cr‐labeled red cells. The possibility of labeling human red cells with stable Rb is discussed.

ISSN:0269-3127    

DOI:10.1118/1.594671

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A 41‐channel xenon detector array has been built in our laboratory to be used in a fan‐beam CT scanner. Each channel is split into a front and rear detector producing two signals for each ray through the patient. Since the front and rear detectors measure different photon energies of the x‐ray spectra, dual energy information is obtained from a single CT scan. This information can be used to determine electron density and effective atomic number images of the patient and correct spectral artefacts. A 7.7‐cm‐diam plexiglas cylinder, filled with water and various plastics, was scanned. With this phantom, electron density was determined to an accuracy of 2% and effective atomic number to 4%. Details of the detector design and calibration are discussed and electron density, effective atomic number, and spectral artifact corrected images are presented.

ISSN:0269-3127    

DOI:10.1118/1.594672

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A phantom is presented which permits the evaluation of streaking artifacts produced in CT reconstructions by abrupt edges. Its application is demonstrated by results obtained from nine CT scanners. It is observed that even in regions where streaking is not readily apparent, edge‐induced artifacts can significantly increase the variance in the reconstruction.

ISSN:0269-3127    

DOI:10.1118/1.594673

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The feasibility of a new method forin vivoregional measurement of the loss of bone tissue in osteoporosis and bone mineral in osteomalacia has been studied analytically. A cross section in the patient's body containing the bone is excited by high energy x rays (hν>1.022 MeV) to produce positron–electron pairs. The positron distribution is imaged by detecting the resulting annihilations using two detectors placed on opposite sides of the irradiated slice and focused to receive annihilations produced inside the bone. Since the cross section for pair production is proportional to the square of the atomic number of the absorber, any pathological processes involving demineralization or loss of bone tissue are expected to strongly affect the number of counts received from the region imaged. This technique is unique among other radiological procedures for bone measurements in itsdirectthree‐dimensional imaging capability. It has no fundamental limit on precision, and higher precision is achieved by resorting to higher dose or greater bony volume irradiated. In addition, the measurement can be confined to an anatomically and physiologically appropriate bone site, thus maximizing the signal and minimizing interference from surrounding tissues. Using radiological models for osteoporosis and osteomalacia, it is shown that a high sensitivity to small changes (∼5%) in density or amount of mineral of cortical bone can be obtained with a relatively low radiation dose (<0.5 rad). With the same radiation dose, the method is less sensitive to changes in vertebral bone, due to the lower mineral fraction in this type of bone. The dependence of sensitivity on incident x‐ray energy is studied, and it is shown that the optimum energy, from a point of view of maximizing sensitivity while minimizing the loss of spatial resolution due to positron range lies at between 2 and 3 MeV. Finally, the application of the technique in the detection and monitoring of bone cancer and tumors metastatic to bone is suggested.

ISSN:0269-3127    

DOI:10.1118/1.594832

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The physical basis of deposition of radiation dose within a homogeneous phantom irradiated by a monoenergetic photon beam has been studied in terms of photon attenuation and energy‐absorption properties of the phantom material. A semi‐empirical model based on the Klein–Nishina formula for Compton scattering, and the ratio of multiply scattered to singly scattered photon fluences, has been developed for the scatter dose component within a realistic phantom to determine the central‐axial percent depth dose (PDD) and off‐central‐axis ratios (OCR). Differences between the predicted and measured values of PDD and OCR for cobalt‐60 and cesium‐137 beams are less than 3% for fields of equivalent‐square‐side less than 20 cm, and less than 5% for larger fields. Beam profiles of all field sizes can be well simulated by this model and reasonable agreement has been found between the predicted and tabulated values of scatter functions and the backscatter factor for cobalt‐60 beams. This formulation involves no variable parameters, and is valid for all values of the source‐to‐surface distance, field length and width, and field shape. However, the algorithm developed is not suitable for routine multiple‐field treatment planning because it requires large computer memory size.

ISSN:0269-3127    

DOI:10.1118/1.594674

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In conventional computed tomography images only the average CT number, which is a first‐order statistical parameter, is used to characterize the tisues by giving an estimate of tissue density. Second order statistical parameters such as the signal variance and cross‐correlation function have also been used to obtain additional information to discriminate between certain tissues and lesions. However, the contribution of quantum noise to the signal variance and cross‐correlation function creates, for the conventional CT patient dose, a background signal often larger than the signal containing the information about tissue structure. The misleading information, called “artifacts”, in second‐order image statistics caused by quantum noise, is studied.

ISSN:0269-3127    

DOI:10.1118/1.594771

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Seven coaxial cables which are in use for carrying currents generated in ionization chambers have been critically studied with reference to their suitability to this application. Included in this study are four low‐noise triaxial cables and three low‐noise two‐conductor cables. For each cable the following characteristics were considered: inherent noise currents, currents produced by cable movements, polarization currents, the degree of electrostatic shielding of the central signal‐carrying conductor, and radiation‐induced cable currents. The study indicated that of the seven cables, two low‐noise triaxial cables, both employing solid Teflon dielectric surrounding the central conductor, appear to offer the best overall performance for use with ionization chambers.

ISSN:0269-3127    

DOI:10.1118/1.594676

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The integral dose of accelerator‐produced leakage neutrons to patients undergoing high‐energy photon therapy is estimated and compared to other sources of integral dose. The leakage neutron component contributes about 5 g rad (1 rad=10−2Gy) for a typical treatment course of 5000 rad. When averaged over a 70‐kg tissue volume, the corresponding dose amounts to only 0.36 rad. From this, the risk of inducing fatal malignancies by leakage neutrons is estimated to be about 50×10−6per year following treatment. This is compared to other risks to which the patient is unavoidably exposed, and it is argued that the unwanted neutrons pose such small additional risk that regulatory intervention is not warranted. This assessment is performed without reference to neutron RBE or quality factor.

ISSN:0269-3127    

DOI:10.1118/1.594774

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effective energy of the x‐ray beam from linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. In this work, the transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL in water. Measurements were made on a variety of linear accelerators from 4 to 10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.

ISSN:0269-3127    

DOI:10.1118/1.594677

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The change in energy of linear accelerator x‐ray beams from the central ray to off‐axis points causes errors in the dose calculated by conventional techniques for large, irregularly shaped fields. A modification of conventional calculative methods to correct for the change in beam energy is presented. The results of measurements in irregular fields on a Clinac‐4 are reported which verify the validity of the calculative method. A discussion of the clinical significance will point out errors of 3% to 4% in conventional dose calculations.

ISSN:0269-3127    

DOI:10.1118/1.594663

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY