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Biomedical subjects

J Bijhold

Publications and source records attributed to J Bijhold.

8 recordsLinked to original sources
PubMed1992

Maximizing setup accuracy using portal images as applied to a conformal boost technique for prostatic cancer.

A design procedure of a patient setup verification protocol based upon frequent digital acquisition of portal images is demonstrated with an application for conformal prostatic boost fields. The protocol aims at the elimination of large systematic deviations in the patient setup and includes decision rules which indicate when correction of the patient setup is needed. The decision rules were derived from the results of a theoretical and quantitative analysis of patient setup variations measured in three pelvic fields (one anterior-posterior and two lateral fields) of 105 fractions for nine patients. Deviations in the patient positioning, derived from one field, were quantified as two-dimensional (2-D) displacement vectors in the plane perpendicular to the beam axis by alignment of anatomical features in the portal and the simulator image. The magnitude of the overall setup variations along the anterior-posterior, superior-inferior and lateral directions varied between 2.6 and 3 mm (1 S.D.). Inter- and intra-treatment variations could be separated, both having equal magnitudes of 1.7 to 2.2 mm (1 S.D.). In addition, intra-treatment variations appeared to be predictable which was a prerequisite for the development of the decision rules. The 2-D setup deviations, measured in the three fields of one fraction were strongly correlated and a 3-D displacement vector was calculated. Utilization of this 3-D vector in a setup verification protocol may lead to an early detection of systematic setup deviations.

Analysis of Variance↗
PubMed1991

Fast evaluation of patient set-up during radiotherapy by aligning features in portal and simulator images.

A new fast method is presented for the quantification of patient set-up errors during radiotherapy with external photon beams. The set-up errors are described as deviations in relative position and orientation of specified anatomical structures relative to specified field shaping devices. These deviations are determined from parameters of the image transformations that make their features in a portal image align with the corresponding features in a simulator image. Knowledge of some set-up parameters during treatment simulation is required. The method does not require accurate knowledge about the position of the portal imaging device as long as the positions of some of the field shaping devices are verified independently during treatment. By applying this method, deviations in a pelvic phantom set-up can be measured with a precision of 2 mm within 1 minute. Theoretical considerations and experiments have shown that the method is not applicable when there are out-of-plane rotations larger than 2 degrees or translations larger than 1 cm. Inter-observer variability proved to be a source of large systematic errors, which could be reduced by offering a precise protocol for the feature alignment.

Computer Simulation↗
PubMed1990

First clinical experience with a newly developed electronic portal imaging device.

In our institute an electronic portal imaging device (PID) has been developed and it recently became available for routine clinical practice. Images are available within 3 to 6 seconds after the start of irradiation; they are displayed on a video monitor next to the control console of the accelerator. The image quality is similar to the quality of images obtained with films. Because of its cassette-like shape and its low weight, the PID can easily be handled by technicians. An important advantage of the PID over conventional films is its pseudo-real time viewing facility. Typically, 5 to 10 images of each field can be made during one treatment session. In case a high accuracy in setup is demanded, the field edges of the first image, obtained with about 10% of the fraction dose, can be studied for acceptability before the rest of the dose is delivered. Using two prototype PID's first clinical experience has been obtained with patients treated for malignant tumors at various sites. Intra-treatment motion as a result of breathing, swallowing, or patient motion in a cast was seen. Motion of high contrast objects, for example, a field edge during irradiation, can be followed. This feature is important for future applications in computer controlled radiotherapy. Another advantage of the PID over film is that the image is digitally available. Therefore it can be further processed for quality improvement and quantitative analysis. Simple processing is done within seconds on the PID unit. A local network for the transfer of images from the accelerators to the evaluation room, where a detailed analysis of the field placement is performed, is under installation. Simulator film images are digitized in this room and are sent to the PID at the accelerator for a quick comparison with portal images during irradiation. We conclude that our device can replace the conventional film detector for portal imaging, that useful images are obtained within seconds during irradiation, and that the position of the field outline relative to the patient anatomy can be followed during dose delivery.

Electronics↗
PubMed1990

A method for the measurement of field placement errors in digital portal images.

Correct placement of radiation fields relative to patient anatomy is essential in radiotherapy in order to minimise serious side effects to reduce the probability of recurrence of the tumour. One way to determine patient setup accuracy is to analyse portal images obtained in the therapy beam distal to the patient. A field placement analysis (FPA) method has been developed for detailed evaluation of patient setup by comparing positions of corresponding radiation field edges in digitised simulator and portal images. A simulator image is matched to a portal image using similar anatomical landmarks in both images and mapping these landmarks against each other applying a least squares minimisation approach. Discrepancies between the simulator field edge (reference) and a portal field edge are determined by comparing the distances between the central axis of the beam and corresponding edge segments and the angles of these segments with a reference line. Uncertainties in these distances and angles are to a large extent determined by the magnification, rotation and translation procedure. Uncertainties due to the FPA method itself are of about 1.0 mm and 0.5 degrees in portal images of head and neck fields. These FPA uncertainties are in general smaller than the variations due to patient setups. Matching of simulator and portal images of lateral pelvic fields revealed larger uncertainties: 1.7 mm and 1.1 degrees. Setup variations in this kind of pelvic radiation field are usually also larger, and therefore meaningful results can be obtained with the new FPA method.

Computer Simulation↗
PubMedDate not supplied

Automatic verification of radiation field shape using digital portal images.

Two computer methods for matching digital line drawings have been tested for automatic on-line verification of the radiation field shape during radiotherapy. This work is part of a research program aiming at automated inspection of on-line acquired digital portal images. Both methods, moment normalization and point distance minimization, compare the field edge detected in the portal image with the intended field edge and the beam shaping devices marked in the simulator image. Tests showed that the methods should be used together. First, shape deviations in the detected field edge are classified quickly, in less than a second (25 MHz 386 + 387 PC), as large (e.g., missing blocks) or small (e.g., shifts of a few mm) by moment normalization. Then the portal image is mapped to the simulator image by field edge alignment with a translation and magnification obtained from moment normalization and a rotation from point distance minimization. The mapped portal image and the simulator image juxtaposed on a monitor screen for visual inspection. Finally, the small field shape deviations are detected by an analysis of the relationship between the radiation field shape and the positioning of field shaping devices using point distance minimization.

Humans↗
PubMedDate not supplied

Sampling methods for a matrix ionization chamber system.

To achieve increased image acquisition speed or better image quality, several read-out methods for a matrix ionization chamber system have been investigated. In this device, which is applied for portal imaging in radiation therapy, 256 x 256 small liquid-filled ionization chambers are scanned by switching the polarizing voltage applied to rows of chambers. The ionization current of each column is measured by a separate amplifier. In this approach, instead of measuring row by row, more complex switching schemes can be applied for the polarizing voltage. These schemes are useful either for varying speed and spatial resolution of the imaging device or for coded sampling of the ionization signal. The former option allows for doubling or quadrupling the acquisition speed with a small loss in image quality, or for obtaining a large improvement in signal-to-noise ratio at the cost of image resolution. In the latter option, coded sampling, the image is reconstructed mathematically from the measured signals. It is shown that in this case the application of Hadamard or derived matrices for sampling leads, under certain circumstances, to a noise reduction in the reconstructed image.

Humans↗