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Biomedical subjects

M van Herk

Publications and source records attributed to M van Herk.

8 recordsLinked to original sources
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↗
PubMed1988

A matrix ionisation chamber imaging device for on-line patient setup verification during radiotherapy.

It is very important to have a daily verification of patient setup during radiotherapy. Therefore, we have developed an on-line imaging device for high energy photons. It consists of a matrix of 128 x 128 liquid filled ionisation chambers and has a field of view of 320 mm x 320 mm. This device has an extremely flat cassette-like housing for easy handling and for application with existing radiotherapy equipment. A dedicated microcomputer is used to measure the currents of the 16384 ionisation chambers at high speed. The same computer is used to restore and process the images. With an imaging time of 3.1 s, an image quality comparable to film is obtained. Images of high and low contrast phantoms and of patients are presented. With this device, high quality portal images will be available within only a few seconds after the start of the treatment. This allows an almost instantaneous decision on the approval of patient setup. In addition, it enables observation of organ or patient motion during a single treatment. Analysis of these images at high speed will be an interesting new area of research.

Computers↗
PubMed1988

An inverse filter for digital restoration of portal images.

Portal images obtained of patients during irradiation with high energy photon beams have a poor sharpness and contrast due to the physical limitations of the equipment forming these images. Consequently, improvement of the images is desirable. An inverse filter method was investigated for this purpose and values of the parameters of the filter have been determined for various clinical irradiation conditions. Data on the line spread function (LSF) were obtained from the image of a 0.2 mm wide slit between tungsten blocks that were positioned at the isocentre in front of a polystyrene phantom. Slit images were made in a 60Co beam and in a number of 8 and 16 MV x-ray beams with a copper screen-film detector. These images were made for various isocentre to detector distances, field sizes and phantom thicknesses. All slit images and a number of clinical images were digitised both with a TV camera system and a densitometer which were connected to a computer. The main factors that affect the LSF are the radiation source size and the isocentre to film distance. The photon energy had only a minor influence. The thickness of the phantom and the field size, however, did influence the contrast in the image and therefore they determined, together with the digitising system, the noise to signal ratio parameter of the inverse filter to a great extent. Application of the inverse filter improved significantly the visual appearance of anatomical details in the portal images. In addition to image restoration, the contrast was enhanced by the application of a low frequency cut-off filter.

Filtration↗
PubMedDate not supplied

Physical aspects of a liquid-filled ionization chamber with pulsed polarizing voltage.

The charge collection in a liquid-filled matrix ionization chamber system has been investigated. This system, which is used for megavoltage radiography, is scanned electronically by switching the polarizing voltage. In such a system the charge collection cannot be described by the classical ionization chamber theory since this is only valid for a fixed polarizing voltage. A model is presented for an ideal plane-parallel ionization chamber with switched polarizing voltage that gives a good qualitative description of the physical phenomena in our nonideal system. The new model predicts the amount of charge collected and the signal-to-noise ratio as a function of several parameters such as electrode distance, polarizing voltage, radiation intensity, and some liquid characteristics. An important result is that the quantum noise contribution can be made quite small. This situation occurs in liquids with a low ion mobility and long ion lifetimes by a charge-integrating effect in the liquid. Experiments were performed to test various aspects of the model. A reasonable agreement is found between theoretical and experimental results. The possible use of ultrahigh-mobility liquids is also discussed.

Humans↗
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↗