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P K Kijewski

Publications and source records attributed to P K Kijewski.

15 recordsLinked to original sources
PubMed1998

Effects of lossy compression on lesion detection: predictions of the nonprewhitening matched filter.

The nonprewhitening matched filter (NPWMF) was used to study the effect of wavelet-based lossy image compression on lesion detectability. Two classes of images, representing lesion-present and lesion-absent cases, were generated by simulation techniques. The performance index, da, of the NPWMF was calculated from the reconstructed images of the two classes and was used as an objective measure for quantifying signal detectability. The effect of compression on detectability was analyzed by defining signal amplitude, signal size, and noise amplitude at a constant da. Since human observer studies directly correlate with the performance index da, the effect of signal parameters on compression ratios could be evaluated. For example, a signal with 7 pixels in diameter and an amplitude of 10 that was compressed at a ratio of 4.6:1, and a signal of the same size with an amplitude of 15 but compressed at a ratio of 24.3:1, both had a da of 4.1, implying identical detectability. Similar results for other combinations of signal and noise parameters, but at constant da, can be used to analyze the effect of compression on detectability without requiring human observer studies.

Biophysical Phenomena↗
PubMed1993

Gain in target dose from using Computer Controlled Radiation Therapy (CCRT) in the treatment of non-small cell lung cancer.

PURPOSE: A study was undertaken to evaluate the gain from Computer Controlled Radiation Therapy in the treatment of non-small cell lung cancer. The purpose of this study is two-fold: (a) to measure the gain in the minimum target dose provided by Computer Controlled Radiation Therapy and (b) to determine the change in the minimum target dose as a function of the lung tolerance limits. METHODS AND MATERIALS: Six cases of non-small cell lung cancer were studied by stimulating treatments with conventional and Computer Controlled Radiation Therapy techniques. For conventional treatments, a boost dose was delivered to the gross tumor volume via a pair of opposed oblique fields with a fixed gantry angle. For Computer Controlled Radiation Therapy, the gantry angle was allowed to change along the longitudinal axis of the patient. Prescriptions had to satisfy a bound on the maximum dose in the spinal cord and a limit on the amount of contralateral lung which could exceed 20 Gy. The boost dose was increased until either a tolerance limit was reached or minimum target dose of 80 Gy was delivered. RESULTS: A minimum target dose of 80 Gy could be given to three of four patients who could not receive 80 Gy with conventional therapy. A minimum target dose of 80 Gy could be given to the remaining two patients with either technique. CONCLUSION: The gain from computer controlled radiation therapy strongly depended on the chosen lung tolerance limit. A 10 to 20 Gy gain in minimum target dose could be found for some patients, but the gain was significantly reduced for a relatively small decrease in the amount of lung permitted a dose above 20 Gy.

Carcinoma, Non-Small-Cell Lung↗
PubMed1990

Acetabular dysplasia in the adolescent and young adult.

Hip dysplasia is a major cause of osteoarthrosis in adults. Early aggressive osteotomy has the potential of preventing the development of arthritis, but carries with it significant risks. The problem is further complicated because the surgeon has no means of quantifying the dysplastic deformity or of predicting what a particular combination of osteotomies would do to correct the deformity. This study describes methods of quantifying hip-joint geometry in three dimensions based on computed tomography and magnetic resonance studies, and of simulating pelvic osteotomy to correct the deformities. The study analyzes 49 normal hip joints and 20 dysplastic hip joints. The results show that the normal acetabulum is nearly a full hemisphere, which is anteverted 20 degrees and abducted 53 degrees. The normal lateral center-edge angle is 37 degrees. The dysplastic acetabulum is not anterolaterally maldirected, as has been assumed, but is globally dysplastic. Analysis of the individual dysplastic hip joints showed a wide variability. Some patients were deficient globally, some anterolaterally, and some posterolaterally. Methods of analyzing a patient's hip joint, quantifying abnormalities, simulating surgery, and predicting results are demonstrated in a case example.

Acetabulum↗
PubMed1988

Quadtrees as a representation for irregularly shaped fields in radiotherapy applications.

The choice of data representations in treatment planning software impacts on the accuracy of the treatment plan and the total computation time. We present the quadtree data structure as one such representation for irregularly shaped fields in a three-dimensional electron pencil-beam algorithm. The quadtree subdivides the irregular field into pencil-beams of unequal size and considerably reduces the number of pencil-beams needed in the dose calculation while preserving the calculational accuracy. Small pencil-beams are concentrated around the field edges to provide optimal resolution and computational accuracy in penumbra regions. The quadtree simplifies the evaluation of the pencil-beam fluence at a given depth, and removes systematic errors present in alternative implementations. The quadtree algorithm results in a ten-fold performance improvement when compared to alternative implementations for the irregular field.

Algorithms↗
PubMed1988

Radiosurgery for arteriovenous malformations of the brain using a standard linear accelerator: rationale and technique.

We have recently initiated a program for irradiating small, unresectable arteriovenous malformations (AVM's) in the brain. The treatments are delivered using a modified and carefully calibrated 6 MV linac. We are using high, single doses (15 to 25 Gy) with a goal of sclerosing the vessels and preventing hemorrhages. This technique, radiosurgery, is somewhat controversial in the radiotherapy community. Since the treatment is given in a single sitting, rather than in the more conventional pattern of multiple small daily fractions, there is some concern about late radiation damage to the normal brain tissue. However an extensive review of the literature leads us to the conclusion that if a technique is used that keeps the volume irradiated to high dose small, radiosurgery is a safe and efficacious treatment for small (less than 2.5 cm) AVM's. To decrease the risk of necrosis of normal brain tissue, it is important to confine the high dose region as tightly as possible to the target volume. Precise target localization and patient immobilization is achieved using a stereotactic head frame which is used during angiography, CT scanning, and during the radiation treatment. This minimizes the margin of safety that must be added to the target volume for errors in localization and set-up. The treatment is delivered using multiple noncoplanar arcs, with small, sharp edged X ray beams, and with the center of the AVM at isocenter. This produces a rapid dropoff of dose beyond the target volume. Early results in our first few patients are encouraging.

Humans↗
PubMed1987

A coordinate transfer of anatomical information from CT to treatment simulation.

Use of CT in treatment planning requires that the anatomical information obtained from the CT scans can be transferred accurately to the treatment geometry. We have developed a technique which transfers the CT data to the simulation data and vice versa. The method uses skin marks and digital radiographs reconstructed from CT scans to reproduce the patient's position. The technique has been studied with a special phantom. Based on 10 phantom set-ups, CT coordinates can be transferred to simulation coordinates with a standard deviation of 0.25 cm. This method has been applied on a number of clinical cases that involve different disease sites. For these cases, comparisons of set-up films taken at the simulator and the digital radiographs reconstructed from CT data indicate that patient position can be reproduced to within 0.5 cm.

Computer Simulation↗
PubMed1987

Femoral anteversion.

Biplane roentgenography, axial roentgenography, and fluoroscopy are the usual roentgenographic methods of measuring femoral anteversion. These methods use a strict geometrical definition of anteversion. The computerized tomography method of measuring anteversion that was developed recently, and is now widely used, does not adhere to the accepted definition of anteversion and has not been tested for accuracy in a large series. In the present study, the widely used computerized-tomography method of measuring anteversion was tested on thirty-two femoral specimens. With that method, anteversion was consistently underestimated by an average of 10 degrees compared with direct measurements and was reproducible only to within +/- 3.6 degrees. Therefore, a new method of measuring anteversion using computerized tomography was developed. It was shown to be accurate to +/- 1 degree, as tested on the same specimens. This study demonstrated geometrically why the currently practiced computerized-tomography method of selecting the points that are used to define the axis of the femoral neck is not consistent with geometrical definitions of anteversion. A more accurate method for both defining the axis of the femoral neck and measuring femoral anteversion is described and recommended for clinical use.

Cadaver↗
PubMed1986

Computer-aided simulation, analysis, and design in orthopedic surgery.

Three-dimensional computer reconstructions of bony anatomy based on computed tomographic images and radiographs may be used to analyze, simulate, and design certain orthopedic procedures. In osteotomy surgery, the computer-reconstructed models may be used to measure critical angles, surface area, and congruity of the joint surfaces. Computer reconstructions may be used in total joint replacement surgery to simulate the effect of surgical reamers and rasps, to select the geometrically optimum standard implant, or to design a custom implant. In allograft reconstructive surgery, computer reconstructions may be used to measure bony defects and to identify the appropriate allografts for the reconstruction. Plastic models may be sterilized and used as templates to sculpt the allografts immediately preoperatively. In all three applications in orthopedic surgery, three-dimensional, computer-aided reconstructions have the potential to improve results and reduce morbidity.

Bone Transplantation↗
PubMed1983

Dose optimization with computer-controlled gantry rotation, collimator motion and dose-rate variation.

The applications of a computer-controlled radiation therapy system to optimize dose distributions in two dimensions are explored. This study is limited to a target volume with constant cross-section along an axis parallel to the long axis of the patient. The machine components that are continuously varied during treatment are the dose rate, the gantry angle, and the four independent collimator jaws, two of which can cross the beam centerline. Basic control strategies, treatment planning and delivery techniques are illustrated with clinical examples. We conclude that the computer-controlled radiation therapy system can easily and reliably deliver dose distributions which are significantly better than those produced by conventional multiple-field techniques.

Computers↗
PubMed1978

Computer-controlled radiation therapy.

Radiation therapy is often hampered in important body regions by the need to transit sensitive normal tissues which act as dose-limiting barriers. Computer-controlled radiation therapy permits the simultaneous variation of multiple treatment parameters during irradiation of the patient, producing improved dose distributions with the potential for improved local control. Equipment used for this purpose includes a Mevatron XII linear accelerator, redesigned for automatic control, and a PDP 11/45 minicomputer. Dose distributions are shown and potential clinical gains discussed.

Computers↗
PubMedDate not supplied

The planning of orthopaedic reconstructive surgery using computer-aided simulation and design.

Three-dimensional reconstructions from computed tomographic (CT) images are currently being used clinically in a wide variety of orthopaedic surgical applications. The computer may be used to select the optimum standard artificial joint replacement or to design a custom artificial joint replacement for a particular patient. Where large bony defects exist, the computer may be used to design bone allografts for joint reconstruction and to manufacture models of the bones for use in planning the surgery. In cases where osteotomies are performed to improve the mechanics of the joint, each proposed osteotomy may be simulated on the computer to identify the surgical plan that will optimally normalize the diseased joint.

Adolescent↗
PubMedDate not supplied

Correction for beam hardening in computed tomography.

Corrections for beam-hardening artifacts in computed tomography can be made by using a model which assumes that water and bone mineral are the only constituents of tissue. With this model, a correction factor for the measured transmission values can be calculated such that the reconstructed attenuation coefficients have values corresponding to a monoenergetic source of known energy. Systematic errors in the uncorrected attenuation coefficients, which may be 5%, can be reduced to less than 1% if corrected transmission values are used.

Body Water↗
PubMedDate not supplied

Wedge-shaped dose distributions by computer-controlled collimator motion.

We have recently installed a linear accelerator, modified to allow computer control of several machine parameters during irradiation of the patient. As an initial feasibility study of computer-controlled radiation therapy, its application to produce wedge-shaped dose distributions by moving the collimator jaws has been evaluated. The required collimator motions have been calculated with an iterative technique. When these routines were used during irradiations of phantoms containing radiographic film, a good correspondence between calculated and measured dose distributions was observed. It is concluded that computer-controlled motion of the collimator jaws to shape the dose distribution is technically feasible. Additionally, this technique has the advantage that the wedge angle can be continuously adjusted and the isodose curves optimized for a particular depth and field size.

Computers↗
PubMedDate not supplied

Monte Carlo calculations of scatter dose for small field sizes in a 60Co beam.

Photon dose calculations are frequently based on the separation of primary and scatter dose components. For small field sizes it is difficult to make this separation from measured data. Such problems lend themselves to solution by calculational methods, which permit rigorous definition of primary and scatter dose. Using Monte Carlo methods, we verify an equation derived from O'Connor's scaling theorem relating scatter-primary ratios at small depths and field sizes to those at larger depths and fields. This equation may prove to be useful when separating primary and scatter dose components, and consequently in dose calculations involving irregular fields or inhomogeneities.

Cobalt Radioisotopes↗