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Biomedical subjects

R Mohan

Publications and source records attributed to R Mohan.

At least 73 records · Page 4Linked to original sources
PubMed1977

Oak poisoning of cattle in Ohio.

Fourteen cases of oak poisoning were diagnosed at the Ohio Veterinary Diagnostic Laboratory, Reynoldsburg, Oh, in the fall of 1976. The poisoning was attributed to ingestion of oak shrubs, oak leaves, and acorns. Clinical signs included anorexia, rumen atony, hemorrhagic diarrhea, subcutaneous edema, and abnormal renal function. Perirenal edema and hemorrhage, ascites, hydrothorax, and hemorrhagic enteritis were frequently encountered pathologic changes. Histologic examination of the kidney revealed multifocal necrosis of the proximal convoluted tubules, which is a characteristic feature of this type of poisoning.

Animals↗
PubMedDate not supplied

Pancreatic B-cell function in relation to diabetic retinopathy in Asian Indian NIDDM patients.

Pancreatic B-cell function in relation to diabetic retinopathy was studied in 195 NIDDM patients with long-standing diabetes. Background diabetic retinopathy (BDR) was present in 95 (48.7%) and proliferative retinopathy (PDR) in 17 (8.7%) of the subjects. There was no significant difference between the BDR, PDR, and non-retinopathy groups with respect to age, age at diagnosis of diabetes and HbA1 values. Mean duration of diabetes was higher in the PDR group (p less than 0.05). Serum C-peptide values showed no correlation with the presence of retinopathy or with the duration of diabetes. The C-peptide values were widely scattered in patients with BDR and PDR showing no association between pancreatic B-cell reserve and occurrence or severity of retinopathy in NIDDM patients. Thus, decreased pancreatic B-cell reserve does not appear to be a risk factor for diabetic retinopathy in NIDDM patients.

Blood Glucose↗
PubMedDate not supplied

Computer-compatible patient contour plotter.

The use of a compact, mobile, relatively inexpensive electromechanical patient-contouring mechanism is described. The system utilizes a Numonics Model 224 graphics digitizer suspended over the patient. The digital X and Y coordinates of the patient contour in either of multiple, parallel transverse, or sagittal planes are inherently accurate to within +/-0.25 mm over a total reading are of 60X90 cm. Display features include the interface to an analog 11-in.X17-in. X-Y plotter with a variable scale-down factor for those contours which would otherwise be larger than the plotting surface. The system is easy to use and may also be readily interfaced for on-line entry of patient contours to calculator- or minicomputer-based treatment-planning systems.

Computers↗
PubMedDate not supplied

Validity of the concept of separating primary and scatter dose.

The technique of separating dose into primary and scatter components for calculating photon dose distributions is widely used. The primary and scatter dose models ignore ignore the fact that electrons have a finite range. This may be a good approximation for 60Co photons but not for higher energies. For the latter, the range of electrons may be several centimeters in soft tissue and even longer in lungs and will lead to errors in computed dose in regions where electronic equilibrium does not exist. Ignoring the finite range of electrons will affect dose at points such as those near the beam boundaries, near inhomogeneity boundaries, and at bone-soft-tissue interfaces. Other possible problems associated with the definition and use of "primary" and "scatter" dose in dose distribution calculations result from extrapolation of measured data to obtain data for zero and very large field sizes and from the use of these quantities, which are defined for central axis, for points at large distances from the central axis. This paper examines the limits of the validity of these assumptions.

Cobalt Radioisotopes↗
PubMedDate not supplied

Energy and angular distributions of photons from medical linear accelerators.

For accurate three-dimensional treatment planning, new models of dose calculations are being developed which require the knowledge of the energy spectra and angular distributions of the photons incident on the surface of the patient. Knowledge of the spectra is also useful in other applications, including the design of filters and beam modifying devices and determination of factors to convert ionization chamber measurements to dose. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear accelerators. Both the target and the flattening filter have been accurately modeled. We find the mean photon energy to have a value lower than the generally perceived value of one-third the maximum energy. As expected, the spectra become softer as the distance from the central axis increases. Verification of the spectra is performed by computing dose distributions and half-value layers in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In currently used models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. Although the angular spread of photons with respect to the incident direction has been found to be small, its contribution to the diffuseness of the beam boundaries is significant.

Energy Transfer↗
PubMedDate not supplied

Off-center ratios for three-dimensional dose calculations.

A new method is proposed for computing the off-center ratios (OCR's) in three-dimensional dose calculations. For an open field, the OCR at a point is computed as the product of the primary OCR (POCR) and the boundary factors (BF's). The POCR describes the beam profile for an infinite field, that is, without the effect of the collimators. It is defined as the ratio of the dose at a point off the central ray to the dose at the point on the central ray at the same depth for an infinite field. The POCR is a function of radial distance from the beam central ray and depth. The BF describes the shape of the beam in the neighborhood of the field boundary defined by the collimators. It is defined as the ratio of the OCR at a point for a finite field to the OCR at the same point for an infinite field. The BF is a function of distance from the field boundary, depth, and field size. For a wedged field, we assume that the boundary factors remain the same as for open fields but the POCR's are altered. The changes in beam profiles are described by a factor called the wedge profile factor (WPF), defined as the ratio of the dose at a point for the largest wedged field to the dose at the same point for an open field of the same field size. The WPF is a function of lateral distance from the beam central plane and depth. Calculated OCR's using this new method are in agreement with the measured data along both the transverse and the diagonal directions of the field.

Biometry↗
PubMedDate not supplied

Differential pencil beam dose computation model for photons.

Differential pencil beam (DPB) is defined as the dose distribution relative to the position of the first collision, per unit collision density, for a monoenergetic pencil beam of photons in an infinite homogeneous medium of unit density. We have generated DPB dose distribution tables for a number of photon energies in water using the Monte Carlo method. The three-dimensional (3D) nature of the transport of photons and electrons is automatically incorporated in DPB dose distributions. Dose is computed by evaluating 3D integrals of DPB dose. The DPB dose computation model has been applied to calculate dose distributions for 60Co and accelerator beams. Calculations for the latter are performed using energy spectra generated with the Monte Carlo program. To predict dose distributions near the beam boundaries defined by the collimation system as well as blocks, we utilize the angular distribution of incident photons. Inhomogeneities are taken into account by attenuating the primary photon fluence exponentially utilizing the average total linear attenuation coefficient of intervening tissue, by multiplying photon fluence by the linear attenuation coefficient to yield the number of collisions in the scattering volume, and by scaling the path between the scattering volume element and the computation point by an effective density.

Bone and Bones↗
PubMedDate not supplied

A technique for computing dose volume histograms for structure combinations.

Graphical displays of three-dimensional dose distribution data are often too complex to be easily assimilated and interpreted for the evaluation of radiation treatment plans. Histograms showing dose versus volume are convenient and useful tools for summarizing dose distribution information throughout the entire volume of a given anatomic structure. They can quickly highlight characteristics such as dose uniformity and hot and cold spots, and can be used to produce statistics including tumor control and normal tissue complication probabilities. To obtain a dose volume histogram for a given structure, it may be necessary to examine its spatial relationships with neighboring structures. They may overlap, be completely disjoint, or one may be contained within another. To resolve potential ambiguities, a procedure has been developed that assigns hierarchies to anatomical structures for the purpose of histogram calculation. The hierarchy assigned to each structure is used to determine the structure within which a given dose matrix point is considered to lie. In this manner, regions of structure intersection are assigned to one object or another, and dose volume histograms can be calculated for each structure separately. From this framework, addition and subtraction of histograms can also be performed. Details of the algorithm are presented along with an example using patient data.

Algorithms↗