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Biomedical subjects

P Suetens

Publications and source records attributed to P Suetens.

At least 19 recordsLinked to original sources
PubMed1995

The use of magnetic resonance angiography in stereotactic neurosurgery.

The authors discuss the advantages and disadvantages of the use of magnetic resonance (MR) angiography images in stereotactic neurosurgery. Current computer programs designed to assist the neurosurgeon in the planning of stereotactic neurosurgical interventions use intraarterial digital subtraction angiography images to visualize the blood vessels. Magnetic resonance angiography is a recent technique with a number of advantages over the digital subtraction method: it is less invasive and less prone to complications; it provides truly three-dimensional data sets that can be viewed from any direction; and it can visualize both stationary and flowing tissues with the same imaging device and localizer frame. Although digital subtraction images are still superior in contrast and vascular detail, state-of-the-art high-resolution MR angiography sequences provide sufficient vascular detail for planning surgery. Contrast-enhanced MR angiography images were acquired using adapted gradient-echo sequences to compensate for flow-induced distortions; postacquisition distortion correction was not necessary. Five methods to integrate and inspect a possible trajectory in the MR angiography data are discussed. Initial clinical experience with eight patients led to the conclusion that MR angiography is a valuable imaging modality that can be integrated reliably into a stereotactic neurosurgery planning procedure.

Adult↗
PubMed1995

[Clinical application of three-dimensional spiral CT in skeletal disorders].

Three-dimensional (3D) computed tomographic reformations are used as an adjunct to standard axial computed tomography (CT) in the evaluation of disorders affecting areas of complex anatomy such as the skull, the spine, the pelvis, hands and feet. 3D-CT can provide a clear impression of lesion extent, pattern, shape and proximity to adjacent structures. This unique spatial information potentially has a significant impact on decisions regarding resectability, surgical approach and repair. The basic requirements for high-quality 3D-reformations are an absence of patient movement and narrow-width transaxial sections. Spiral CT significantly contributes to fulfilling these requirements. In addition, spiral CT scan data can be used to produce the ultimate representation of the third dimension: a life-size palpable medical model. In this review we describe the process of three-dimensional reconstruction and highlight potential problems arising during 3D-imaging processing. Useful clinical applications of spiral CT for 3D-visualization of skeletal pathology are discussed.

Bone Diseases↗
PubMed1995

A study of the liver-heart artifact in emission tomography.

UNLABELLED: With the introduction of 99mTc-teboroxime, a previously undocumented artifact has shown up in cardiac SPECT imaging. In the images, the uptake values near the inferior wall are lower than expected. The artifact has been reported in the literature, but an adequate explanation has not yet been provided. The high uptake of 99mTc-teboroxime in the liver has been demonstrated to be the cause of this artifact. METHODS: With simulations we show that an artifact can be reproduced by applying filtered backprojection (without corrections for attenuation) of attenuated and blurred projections. The conclusions from the simulations are validated with SPECT and PET phantom measurements. Maximum likelihood expectation maximization (ML-EM) reconstruction is applied to evaluate the effect of accurate attenuation correction. The influence of the high liver uptake on the convergence of ML-EM was also evaluated. RESULTS: The artifact results mainly when the photon attenuation during reconstruction is ignored. This results in a distorted reconstruction of the liver. These distortions affect the neighboring inferior wall of the myocardium. While the use of opposite projections reduces the effect, accurate attenuation correction nearly eliminates it. A small additional deformation is caused by the position dependence of the spatial resolution of the gamma camera. It was also noted that the presence of the liver slows down the convergence of ML-EM in the heart region. CONCLUSION: The liver-heart artifact is an attenuation effect and is eliminated by attenuation correction. The local convergence of ML-EM is affected by the total image content.

Animals↗
PubMed1994

Combining fast response and low cost in an intensive care unit viewing station.

This paper describes a prototype viewing station for the Intensive Care Unit (ICU) that combines a low cost with a fast response for the primary viewing tasks. A single monitor is used together with the possibility to switch to another image on the same screen instantaneously. To reduce the time for image retrieval and display, the viewing station anticipates the need for particular images, and preloads these from the disk into the internal memory whenever the computer has some idle time. Preliminary experience suggests that the use of a single monitor is no fundamental limitation for this application. Most of the time, the system can anticipate which images will be required shortly. This has an important impact on the user efficiency.

Data Display↗
PubMed1994

Acquisition and processing of the radio-frequency signal in echocardiography: a new global approach.

Recently there has been an increasing interest in the use of the "raw" radio-frequency (RF) signal generated in echocardiography for use in tissue-characterization to distinguish between normal and pathological myocardium, for automatic delineation of the endocardial border without being limited by the weak contrast of the traditional video images, and for use in contrast echography, where it could offer the possibility to visualize perfusion using intravenous contrast injections. One of the main problems in this kind of research is the acquisition of the signal having a high frequency and large bandwidth. We have developed a new global method for the acquisition of this RF signal. To digitize the data, a video sequencer is used. In this way it becomes possible to sample all available data generated by the echographic equipment during at least 1 s. This means that all data of the complete sector scan during a complete heart-cycle can be digitized without using any data reduction technique or triggering on the electrocardiogram. The advantage of this approach is that all characteristics of the signal can be studied, without being limited by data reduction techniques used during acquisition. This method enables us to calculate parameters such as "integrated backscatter," or to investigate the signal more extensively, e.g., by using spectrum analysis. We can compare different regions of the myocardium and examine them during the heart-cycle, all within the same beat. We have also written a software package for the processing of the large amount of data resulting from the acquisition.

Animals↗
PubMed1994

On the problem of geometric distortion in magnetic resonance images for stereotactic neurosurgery.

In this paper, we discuss the issue of geometric distortion in magnetic resonance (MR) images used to plan stereotactic neurosurgical interventions. We analyze the process for the case of Fourier transform imaging and demonstrate that spatial misregistrations are fundamentally due to two causes: deviations of the magnetic field from its ideal value and blood flow. This enables us to relate the causes of geometric distortion to the MR imaging system, the patient and the stereotactic localizer frame. Based on the general model, we propose model refinements and discuss methods for the quantification and correction of all causes. The results of our calculations and experiments indicate that, using the proposed corrections, MRI and MR angiography should be considered valuable and reliable acquisition modalities for the planning of stereotactic neurosurgical interventions.

Brain↗
PubMed1994

How does the stereotactic workstation help the neurosurgeon?

At the KUL University of Leuven a workstation for the planning of neurosurgical stereotactic procedures has been developed. Its benefits are illustrated in three exemplary cases. The CT and/or MR images, acquired under stereotactic conditions, are transmitted via a PACS network (picture archiving and communication systems) directly to the stereotactic workstation in the operating theater. Target and entry point can be accurately defined on zoomed images. The trajectory can be checked and modified on all registered data sets and on resliced images along any plane. Maximum intensity projection of magnetic resonance angiography data sets along any arbitrary direction show the relative position of the blood vessels and the trajectory. During the preceding 32 months 29 patients were operated on using the stereotactic workstation. Postoperatively no new neurological deficit was observed in any of these patients. The workstation improves patient safety and increases the accuracy of neurosurgical stereotactic operations, because it helps the neurosurgeon to avoid blood vessels and/or important functional areas.

Adult↗
PubMed1993

A new quantitative method for the analysis of cardiac perfusion tomography (SPET): validation in post-infarct patients treated with thrombolytic therapy.

In this study a new method for assessment of perfusion defects (PDs) derived from myocardial perfusion tomograms was evaluated in patients treated with thrombolytic therapy. Using global constraints and dynamic programming, a model-based delineation algorithm defined myocardial borders, the basal plane and absolute and relative PD size in 49 thallium-201 chloride (201TL CL) and 60 technetium-99m methoxyisobutylisonitrile (99mTc-MIBI) tomograms. Tomographic (single-photon emission tomography: SPET) and planar quantification of PDs was compared to enzymatic infarct size as well as to global (LVEF) and regional ventricular function (RWM) obtained by contrast angiography. The algorithm delineated the myocardium and the valve plane in most cases, even when large PDs were present. Manual correction of the automatic delineation of the basal plane was necessary in less than 20% of the studies. Using 201Tl Cl, LVEF correlated better with tomographic PD (r = -0.67) than with planar PD (r = -0.54). Comparing planar to tomographic imaging using 99mTc-MIBI, a higher correlation with enzymatic infarct size (r = 0.73 vs 0.57) and with global ventricular function (r = 0.64 vs -0.52) was found when tomographic techniques were used. No close correlation between PD and RWM was found. The beneficial effect of thrombolysis was shown by a significant difference of PD in patients with open versus occluded infarct-related vessels. It can be concluded that this new automated algorithm for quantification of SPET perfusion defect size provides a useful tool in evaluating thrombolytic therapy.

Algorithms↗
PubMed1993

Image segmentation: methods and applications in diagnostic radiology and nuclear medicine.

We review and discuss different classes of image segmentation methods. The usefulness of these methods is illustrated by a number of clinical cases. Segmentation is the process of assigning labels to pixels in 2D images or voxels in 3D images. Typically the effect is that the image is split up into segments, also called regions or areas. In medical imaging it is essential for quantification of outlined structures and for 3D visualization of relevant image data. Based on the level of implemented model knowledge we have classified these methods into (1) manual delineation, (2) low-level segmentation, and (3) model-based segmentation. Pure manual delineation of structures in a series of images is time-consuming and user-dependent and should therefore be restricted to quick experiments. Low-level segmentation analyzes the image locally at each pixel in the image and is practically limited to high-contrast images. Model-based segmentation uses knowledge of object structure such as global shape or semantic context. It typically requires an initialization, for example in the form of a rough approximation of the contour to be found. In practice it turns out that the use of high-level knowledge, e.g. anatomical knowledge, in the segmentation algorithm is quite complicated. Generally, the number of clinical applications decreases with the level and extent of prior knowledge needed by the segmentation algorithm. Most problems of segmentation inaccuracies can be overcome by human interaction. Promising segmentation methods for complex images are therefore user-guided and thus semi-automatic. They require manual intervention and guidance and consist of fast and accurate refinement techniques to assist the human operator.

Algorithms↗
PubMed1991

Knowledge-based system for the three-dimensional reconstruction of blood vessels from two angiographic projections.

A knowledge-based system for the three-dimensional reconstruction of blood vessels from wide-angle coronary and stereoscopic cerebral angiographic projections is developed. For the reconstruction of the coronary vessels, the left coronary artery (LCA) is automatically labelled on standard RAO and LAO projections, using anatomical models of the LCA. The labelling system succeeds in giving the most important coronary arteries a correct anatomical label. These labelling results enable us to find corresponding segments in both images. In the case of the reconstruction of the cerebral vessels however, such an anatomical model is clearly unavailable. To find corresponding segments, small-angle projections must be relied on, resulting in very similar images. Owing to the small angular separation between both projections, the three-dimensional reconstruction will be less accurate. Once the corresponding segments in both projections are obtained, the three-dimensional artery trajectory is reconstructed with dynamic programming techniques. The three-dimensional reconstructed coronary vessels are also used for an automatic quantification of stenotic lesions.

Blood Vessels↗
PubMed1990

An expert system for the labeling and 3D reconstruction of the coronary arteries from two projections.

In this paper we present a rule-based expert system for the automatic delineation and 3D reconstruction of the left coronary artery on standard RAO and LAO angiographic projections. The approach is based on the application of a general blood vessel model and on anatomical models which take into account the normal variations of the coronary artery structure. In a first step, the arteries are delineated by detecting the maximum intensity on the centerline of the vessels. Then, we label the blood vessel segments according to an anatomical model of the left coronary artery. In general, only 1-2 labels remain for each blood vessel segment. Finally, these results are used for an automatic 3D reconstruction of the left coronary artery from two projections. Results from clinical RAO and LAO angiograms will be presented.

Algorithms↗
PubMed1990

A prototype medical workstation for computer-assisted stereotactic neurosurgery.

We have developed a prototype display workstation for use in stereotactic neurosurgery. Patient image data from computed tomography, magnetic resonance imaging, and digital subtraction angiography are acquired with the stereotactic frame in place and subsequently transferred to the workstation for further processing. Target points may be identified on any image type and probe trajectories defined. Any point or line indicated on one set of images may be transferred immediately to other images, to determine, for instance, safe avascular probe paths. We present some general outlines for the use of computers for stereotactic neurosurgery and discuss the different components of the current system. Finally, we make some suggestions as to further developments.

Brain↗
PubMed1989

Model-based quantification of myocardial perfusion images from SPECT.

A system for quantitative analysis of myocardial perfusion tomograms is proposed. The system starts with an automated delineation of the total left ventricle, including possible perfusion defects, to determine the mass and shape of the myocardium. Next, polar maps or bulls-eyes are computed from the delineation, which can then be compared to reference bulls-eyes to detect perfusion defects. The proposed system differs in three main aspects from currently available bulls-eye algorithms. First, radial slices are used rather than short-axis slices. In this way three-dimensional gradient information is retained, in particular near the base and the apex of the left ventricle. Moreover, the reproducibility of this method is expected to be superior, since the interactive selection of short axis slices through the left ventricle is eliminated. Second, the left ventricle is automatically delineated using a flexible computer model in order to obtain higher reproducibility. The resulting delineation contains both mass and shape information. Third, in addition to the classic count rate bulls-eye, a mass bulls-eye is computed, which contains the myocardial mass corresponding to each bulls-eye pixel. Analysis of the count rate bulls-eye reveals perfusion defects, the quantification of the defects is carried out with the mass bulls-eye.

Algorithms↗
PubMed1987

Angiographic localizer ring for the BRW stereotactic system.

An accessory locating device to the existing BRW stereotactic system is presented. It can be used as a reference device to locate angiographic data with respect to the BRW stereotactic system. Hence, the projection of target points onto angiograms, visible on CT scans, are easily calculated, as well as the stereotactic coordinates of a set of points (e.g., AVM) indicated on at least two angiograms. As a final result integrated images of cerebral blood vessels and an outline of tumor lesions can be generated using more sophisticated computer equipment.

Cerebral Angiography↗
PubMed1987

Angiographic localizer for the BRW stereotactic system.

Preliminary experience with a newly constructed angiographic localizer system for use in stereotactic neurosurgery is reported. This localizer ring, mounted on the BRW head ring, allows for the transformation of target points with known stereotactic coordinates (e.g., visible on computerized tomography scans) onto angiograms, as well as the determination of stereotactic coordinates of a set of points (e.g., arteriovenous malformations) indicated on at least two angiograms.

Equipment Design↗
PubMed1986

A new thresholding method for volume determination by SPECT.

The quantification of organ volumes from SPECT images suffers from two major problems: image segmentation and imperfect system transfer function. Image segmentation defines the borders of an organ and allows volume measurements by counting the voxels inside this contour in all slices containing parts of this organ. A review of the literature, showed that several investigators use a fixed threshold (FT) to determine the organ pixels. It is our aim to demonstrate that the threshold has to be adapted to every single case because its value is dependent upon several factors, such as size and contrast. Therefore a threshold selection algorithm, based on the gray level histogram (GLH), is evaluated. It is nearly impossible to calculate and eliminate errors induced by the complex system response function. A correction method based on linear regression is proposed. By minimizing the relative error (sigma), a linear correlation (Y = AX + B) between the true volume (Y) and the measured volume (X) is established for three fixed thresholds (30%, 40%, 50%) and for the GLH method. The methods are evaluated on a series of nineteen phantoms with a volume range between 9.8 and 202.5 ml. The relative error is minimal for the GLH method. The whole procedure is semi-automated and virtually operator independent.

Algorithms↗
PubMed1980

Reconstruction of an arbitrary cross section from the serial drawings of a stereotaxic brain atlas.

A method is described for the reconstruction of arbitrary cross section from the serial line drawings of a stereotaxic brain atlas. Mathematically, the problem reduces to the intersection of a plane with a line pattern lying in another plane. However, the calculation yields a cloud of points, often confusing and unusable for the surgeon. Because it is impossible to link correctly all the calculated points in all cases [1,2], we propose a method where the number of points is drastically reduced: only regions of interest are depicted, i.e., the area where the electrode point is situated and the neighbouring regions. Instead of points, symbols are displayed, each symbol representing a different region. The efficiency of the method is shown using an atlas simulation of four intersecting spheres in the three dimensional space, Although the procedure is useful for all stereotaxic brain atlasses, a practical example is given where the poor quality of the reconstructed images does not allow good interpretation.

Analog-Digital Conversion↗