摘要:

A new theoretical network model for evaluating discharge hematocrits, explicitly based on plasma skimming at branches, is introduced. The particular network geometry chosen simulates bat wing microvasculature. Blood in vessels is approximated to be a two fluid with red cell suspension as the core and a plasma layer surrounding it. The plasma layer width depends on hematocrit, which leads to nonlinear hydrodynamic equations solved by iteration. Discharge hematocrit distributions are calculated by a computer for five generations of vessels. Dispersion of hematocrit values was found to be correlated to plasma skimming at branches. Contrary to previous suggestions, plasma skimming did not result in lowered mean hematocrit towards the capillaries. Network structure was found to be an important factor affecting the hematocrits. Mean discharge hematocrit remained steady against changes in vessel dimensions, capillary resistances, red cell concentration in plasma layer, and shape of the separation surface defining the streamlines entering the side branch. This high stable mean hematocrit is based on symmetry of the model network. Enhanced asymmetry tended to lower the hematocrit.

ISSN:0269-3127    

DOI:10.1118/1.596341

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The optical characteristics of 39 excised human tumors are determined. Then liquid phantoms, representative of these tumors, are used to measure the space irradiance in a plane containing the optical axis for two irradiation geometries (external and interstitial) and two wavelengths (514 and 630 nm); a rotating miniature probe allows a spatial resolution of 0.1 mm. It is found that, on the optical axis, the space irradiance at the surface of a phantom is higher than the irradiance applied externally and exhibits a maximum prior to reaching an exponential decrease. A zone where green light is expected to provide better photochemical efficiency than red light is determined and the space irradiances obtained with external and interstitial irradiation are compared quantitatively. The clinical usefulness of the results is limited by the fact that individual samples belonging to the same type of tumor differ considerably in their optical characteristics, at least unless a more detailed classification (based on a greater number of tumor samples) is available.

ISSN:0269-3127    

DOI:10.1118/1.596342

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple two‐dimensional analytic model is evaluated for transmitted cardiac motion in fetal lung. The model treats the latter as being an incompressible viscoelastic medium. The mean radial deformation in an elastic medium is demonstrated to depend on a length parameterl∼=(μ/ρω2)1/2, where μ, ρ, and ω corespond to elastic shear modulus, mass density, and frequency of cardiac motion, respectively. DigitizedM‐mode images are demonstrated as a feasible method to measure such deformationsinvivo. Data for two patients are presented to illustrate the technique.

ISSN:0269-3127    

DOI:10.1118/1.596441

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An autoradiographic technique incorporating a new imaging system was used to detect pion‐induced radioactivity in Plexiglass and the results were compared with aluminium activation and PET imaging. The activity distribution in the region of the pion‐stopping peak was similar in all three cases. Another large signal in the entrance region due to in‐flight interactions [12C(π−,π−n)11C] was detected by autoradiography and by PET but was not reflected in the aluminium activation measurements. This new technique is capable of defining the stopping region in phantoms with a better resolution than PET scanning and is useful as a complementary technique to other methods of pion dosimetry.

ISSN:0269-3127    

DOI:10.1118/1.596426

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have developed a method of compensation for fluctuations in beam intensity that may occur during measurement ofPion, the ion‐recombination correction factor, by the two‐voltage technique. The method requires signals proportional to beam intensity during measurement. We used a parallel‐plate ionization chamber, whosePionwas known, and a vacuum chamber to obtain signals that were proportional to the beam intensity. Experiments were conducted using pulsed proton beam providing doses that ranged from 0.16 to 0.01 cGy/pulse. The value ofPionof a thimble ionization chamber was measured. With these measurements, the validity of the method which we proposed for pulsed beam was verified experimentally.

ISSN:0269-3127    

DOI:10.1118/1.596343

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When a treatment planning system uses an empirical or semianalytical approach to describe the influence of a wedge filter on a photon beam, a number of experimentally determined parameters are required. These may be found from direct measurement. However, if the beam model is sensitive to the parameters, it will be necessary to optimize the parameter values to obtain better correspondence between dose profiles calculated by the model and actual measured profiles. The procedure is time consuming if optimization is done manually. We have developed an optimization scheme, using a personal computer, to find the set of wedge parameters which will result in the best fit of calculated wedge profiles (using the beam model of the treatment planning system) to measured wedge profiles. The procedure is efficient and calculated profiles were found to match measured profiles to within 2% of the central axis value.

ISSN:0269-3127    

DOI:10.1118/1.596344

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Planning radiation therapy using three‐dimensional patient data is a very time consuming process with current hardware and software. When calculating a three‐dimensional dose distribution, the standard technique is to cover the volume of interest with a uniformly spaced matrix of points at which the dose is calculated. It is obvious that the dose is usually quite slowly varying in a large proportion of the region of interest; namely, in those regions which are either well inside or well outside the geometrical boundaries of the field. We have developed an algorithm which allows us to reduce the number of calculation points, and hence the time of calculation of the entire dose distribution, manyfold. We use a nonuniform grid of calculated points, based on the fact that the only regions which are troublesome for accurate dose interpolation are those in which large values of the second derivative of the dose as a function of position occur. We demonstrate that, at most grid points, the dose can be determined without decreasing accuracy below acceptable limits by simple linear interpolation between grid points much further apart than is usual in conventional techniques. We investigated our algorithm for one‐, two‐, and three‐dimensional examples and for Co‐60, 25‐MV photon, and 160‐MV proton beams. In situations for which an accuracy of about 1% in dose and 1.6 mm in position was desired, we found gain factors for the number of points needing direct calculation of approximately 3 (one‐dimension), 6 to 10 (two‐dimensions) and 16 (three‐dimensions).

ISSN:0269-3127    

DOI:10.1118/1.596425

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Most computer algorithms used clinically for photon beam treatment planning are unable to predict the effect of electron backscattering on dose distribution from high atomic number materials. It has been observed that there is a significant dose enhancement at such an interface. We define the dose enhancement in terms of backscatter dose factor (BSDF), which depends on the energy of the photon beam, thickness and width of the inhomogeneity, distance from the interface, and the atomic number of the inhomogeneity. For all energies studied, the dose fall‐off is initially very rapid and disappears beyond a few millimeters upstream from the interface. Empirically derived equations are presented for dose calculation at the interfaces of various media, including bone and soft tissue, for photon energies in the range of Co‐60 gamma rays to 24 MV x rays.

ISSN:0269-3127    

DOI:10.1118/1.596345

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Use of I‐125 in breast implants may be therapeutically beneficial due to the 40% higher dose to the tumor relative to the normal adipose tissue. The dose distribution in the interface between adipose and nonadipose tissues is studied with mathematical models and radiobiological measurements. The dose transition zone is narrow, about 10μ, which is roughly the diameter of a cell. The dimension of the transition zone is weakly sensitive to the geometry of the interface. In most geometries, cells on either side of the interface receive doses that are significantly different (25%–35%). Radiobiological measurements are consistent with calculated results, providing a check on the theoretical model.

ISSN:0269-3127    

DOI:10.1118/1.596346

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We perform gamma camera imaging to generate data for estimation of internal radiation dose in our radioimmunotherapy candidates. Because of the inability of single photon emission computed tomography (SPECT) to account for patient attenuation variability without serious error, quantitative planar imaging was performed to estimate the radioactive content of normal organs. We undertook the following studies to further validate this method. A realistic fillable human phantom was used to determine the accuracy of I‐131 filled organ estimation. A transmission scan of the unfilled phantom was produced with an I‐131 filled flat flood source. Anterior and posterior planar images of the filled organs were acquired with region of interest determination of the activity in the organ. Correction by the attenuation factor and a camera calibration factor yielded the MBq in the organ. The procedure was also performed in a simple phantom. Three dogs were imaged and sacrificed to validate the techniqueinvivo. A high degree of accuracy in estimation of organ radioactive content was found to be present using the phantom and dog models. Use of this method requires further validation but provides a solid basis for estimation of internal radiation dosimetry in radioimmunotherapy planning.

ISSN:0269-3127    

DOI:10.1118/1.596347

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY