Biomedical subjects

M A Viergever

Publications and source records attributed to M A Viergever.

At least 19 recordsLinked to original sources

PubMed2001

Quantitative analysis of vascular morphology from 3D MR angiograms: In vitro and in vivo results.

A 3D model-based approach for quantification of vascular morphology from several MRA acquisition protocols was evaluated. Accuracy, reproducibility, and influence of the image acquisition techniques were studied via in vitro experiments with ground truth diameters and the measurements of two expert readers as reference. The performance of the method was similar to or more accurate than the manual assessments and reproducibility was also improved. The methodology was applied to stenosis grading of carotid arteries from CE MRA data. In 11 patients, the approach was compared to manual scores (NASCET criterion) on CE MRA and DSA images, with the result that the model-based technique correlates better with DSA than the manual scores. Spearman's correlation coefficient was 0.91 (P < 0.001) for the model-based technique and DSA vs. 0.80 and 0.84 (P < 0.001) between the manual scores and DSA. From the results it can be concluded that the approach is a promising objective technique to assess geometrical vascular parameters, including degree of stenosis. Magn Reson Med 45:311-322, 2001.

Algorithms↗

PubMed2001

Correcting partial volume artifacts of the arterial input function in quantitative cerebral perfusion MRI.

To quantify cerebral perfusion with dynamic susceptibility contrast MRI (DSC-MRI), one needs to measure the arterial input function (AIF). Conventionally, one derives the contrast concentration from the DSC sequence by monitoring changes in either the amplitude or the phase signal on the assumption that the signal arises completely from blood. In practice, partial volume artifacts are inevitable because a compromise has to be reached between the temporal and spatial resolution of the DSC acquisition. As the concentration of the contrast agent increases, the vector of the complex blood signal follows a spiral-like trajectory. In the case of a partial-volume voxel, the spiral is located around the static contribution of the surrounding tissue. If the static contribution of the background tissue is disregarded, estimations of the contrast concentration will be incorrect. By optimizing the correspondence between phase information and amplitude information one can estimate the origin of the spiral, and thereupon correct for partial volume artifacts. This correction is shown to be accurate at low spatial resolutions for phantom data and to improve the AIF determination in a clinical example. Magn Reson Med 45:477-485, 2001.

Artifacts↗

PubMed2001

MRA of hemodialysis access grafts and fistulae using selective contrast injection and flow interruption.

MR is a potentially attractive modality for evaluating hemodialysis access anatomy and function. However, the wide range of flow rates in the hemodialysis access complicates interpretation of phase contrast, time-of-flight, and even contrast-enhanced MR angiograms. At high flow rates, signal voids may easily arise at mild narrowings or sharp-angled anastomoses. A method is proposed which visualizes hemodialysis accesses without flow artifacts. Diluted Gd-DTPA is hand-injected directly into the access, while a cuff is used to reduce and subsequently interrupt access flow. Filling of the access is monitored using a fast projection technique with complex subtraction. When filling is satisfactory, a 3D acquisition is started. The feasibility of this selective contrast-enhanced MR angiography technique is demonstrated in four Cimino-fistulae and four PTFE grafts. Magn Reson Med 45:557-561, 2001.

Arteriovenous Shunt, Surgical↗

PubMed2001

Background suppression using magnetization preparation for contrast-enhanced MR projection angiography.

In contrast-enhanced MR projection angiography, vessel conspicuity is determined by the T(1)-weighted signal difference between blood and surrounding tissues. For slice-selective excitation pulses, the excitation angle varies across the slice, leading to poor saturation of the background signal at the slice edge and reducing the blood-background signal difference. This work reports on the use of magnetization preparation to enhance the T(1)-weighted contrast between blood and background tissue. Applying the prepulse nonselectively reduces the influence of the slice profile imperfections of the excitation pulse by keeping the background tissue at the slice edge saturated. Analytical calculations and in vitro experiments show that a prepulse angle of 110 degrees -130 degrees and a delay time of 20-25 ms enhance the contrast between contrast-enhanced blood (T(1) < 50 ms) and background tissues (T(1) > 200 ms), and improve the slice weighting profile. Magnetization preparation is shown to effectively suppress signal from background tissue, resulting in a threefold increase of the vessel-to-background signal ratio. Magnetization preparation eliminates the need for subtraction at the cost of a slight increase in scan time. Possible applications, such as projection MRA, detection of contrast arrival, and test-bolus tracking are demonstrated in a pig model. Magn Reson Med 46:78-87, 2001.

Animals↗

PubMed2001

Automated separation of gray and white matter from MR images of the human brain.

A simple automatic procedure for segmentation of gray and white matter in high resolution 1.5T T1-weighted MR human brain images was developed and validated. The algorithm is based on histogram shape analysis of MR images that were corrected for scanner nonuniformity. Calibration and validation was done on a set of 80 MR images of human brains. The automatic method's values for the gray and white matter volumes were compared with the values from thresholds set twice by the best three of six raters. The automatic procedure was shown to perform as good as the best rater, where the average result of the best three raters was taken as reference. The method was also compared with two other histogram-based threshold methods, which yielded comparable results. The conclusion of the study thus is that automated threshold based methods can separate gray and white matter from MR brain images as reliably as human raters using a thresholding procedure.

Adult↗

PubMed2001

Automatic segmentation of the ventricular system from MR images of the human brain.

An algorithm was developed that automatically segments the lateral and third ventricles from T1-weighted 3-D-FFE MR images of the human brain. The algorithm is based upon region-growing and mathematical morphology operators and starts from a coarse binary total brain segmentation, which is obtained from the 3-D-FFE image. Anatomical knowledge of the ventricular system has been incorporated into the method in order to find all constituting parts of the system, even if they are disconnected, and to avoid inclusion of nonventricle cerebrospinal fluid (CSF) regions. A test of the method on a synthetic MR brain image produced a segmentation overlap of 0.98 between the simulated ventricles ("model") and those defined by the algorithm. Further tests were performed on a large data set of 227 1.5 T MR brain images. The algorithm yielded useful results for 98% of the images. The automatic segmentations had intra-class correlation coefficients of 0.996 for the lateral ventricles and 0.86 for the third ventricle, with manually edited segmentations. Comparison of ventricular volumes of schizophrenia patients compared with those of healthy control subjects showed results in agreement with the literature.

Algorithms↗

PubMed2001

MR imaging of vascular stents: effects of susceptibility, flow, and radiofrequency eddy currents.

PURPOSE: The purpose of this in vitro study was to examine the various sources of artifacts in magnetic resonance (MR) imaging and angiography of vascular stents. MATERIALS AND METHODS: Five low-artifact stents-Wallstent (cobalt alloy), Memotherm (nitinol), Perflex (stainless steel), Passager (tantalum), and Smart (nitinol)-were imaged in a vascular flow phantom, consisting of a thin-walled cellulose vessel model connected to a pump system. The echo time and the angulation of the stents with respect to the direction of the main magnetic field were varied. Spin echo and gradient echo images as well as three-dimensional MR angiograms were obtained to study the effects of flow, magnetic susceptibility, and radiofrequency-induced eddy currents. RESULTS: Susceptibility artifacts were restricted to the stents' direct environment and were mildest at short echo times and with the stents aligned with the main magnetic field. Nitinol stents showed less artifacts than steel stents did. Radiofrequency artifacts obscuring the stent lumen and flow-related lumen displacement were seen in all stents. The extent to which these occurred depended on strut geometry and orientation. CONCLUSIONS: For low-artifact stents, the material the stent is made of is not the only important factor in the process of artifact formation. Susceptibility artifacts, radiofrequency eddy currents and flow-related artifacts all contribute to the image distortion, and are dependent on the geometry and orientation of the struts and on the orientation of the stent in the main magnetic field.

Artifacts↗

PubMed2001

Quantitative evaluation of convolution-based methods for medical image interpolation.

Interpolation is required in a variety of medical image processing applications. Although many interpolation techniques are known from the literature, evaluations of these techniques for the specific task of applying geometrical transformations to medical images are still lacking. In this paper we present such an evaluation. We consider convolution-based interpolation methods and rigid transformations (rotations and translations). A large number of sinc-approximating kernels are evaluated, including piecewise polynomial kernels and a large number of windowed sinc kernels, with spatial supports ranging from two to ten grid intervals. In the evaluation we use images from a wide variety of medical image modalities. The results show that spline interpolation is to be preferred over all other methods, both for its accuracy and its relatively low computational cost.

Brain Mapping↗

PubMed2001

Ankle images digital analysis (AIDA): digital measurement of joint space width and subchondral sclerosis on standard radiographs.

OBJECTIVE: Reliable evaluation of joint space width and subchondral sclerosis of osteoarthritic joints is difficult. The present study describes a new digital method to analyse standard radiographs of the ankle. DESIGN: Standardized radiographs were taken of the ankle of 12 patients with severe osteoarthritis (OA) under full weight-bearing conditions, before treatment and 1 year after initiation of treatment. Treatment consisted of 3 months distraction of the tibio-talar joint, for which clinical benefit has been shown previously. The width of the joint space was measured on digitized images of the radiographs by means of the newly developed semi-automatic digital technique called AIDA (Ankle Images Digital Analysis) and by means of the most widely used conventional analogue measurements. In addition, AIDA was used to assess subchondral sclerosis by measuring the intensity of the radiograph at fixed positions at the bone-cartilage interface. RESULTS: AIDA appeared to be a reliable method for measuring small changes in joint space width and subchondral sclerosis because the intra- and interobserver variation was small. Mean JSW for two observers was 1.96 and 2.00 mm, with mean differences between two observations of 0.05 and -0.01, respectively. Mean subchondral sclerosis in the tibia was 1.52 and 1.61 with mean differences between two observations of, respectively, 0.00 and 0.03. In addition to conventional measurements, AIDA could demonstrate a decrease in subchondral sclerosis as a result of joint distraction of 71% and 69% after 1 year for talus and tibia, respectively. CONCLUSION: The use of AIDA is preferable to the conventional analogue method for evaluating the severity of ankle OA, because the method provides quantitative data not only for the joint space width but also for subchondral sclerosis.

Ankle Joint↗

PubMed2001

Three-dimensional modeling for functional analysis of cardiac images: a review.

Three-dimensional (3-D) imaging of the heart is a rapidly developing area of research in medical imaging. Advances in hardware and methods for fast spatio-temporal cardiac imaging are extending the frontiers of clinical diagnosis and research on cardiovascular diseases. In the last few years, many approaches have been proposed to analyze images and extract parameters of cardiac shape and function from a variety of cardiac imaging modalities. In particular, techniques based on spatio-temporal geometric models have received considerable attention. This paper surveys the literature of two decades of research on cardiac modeling. The contribution of the paper is three-fold: 1) to serve as a tutorial of the field for both clinicians and technologists, 2) to provide an extensive account of modeling techniques in a comprehensive and systematic manner, and 3) to critically review these approaches in terms of their performance and degree of clinical evaluation with respect to the final goal of cardiac functional analysis. From this review it is concluded that whereas 3-D model-based approaches have the capability to improve the diagnostic value of cardiac images, issues as robustness, 3-D interaction, computational complexity and clinical validation still require significant attention.

Diagnostic Imaging↗

PubMed2001

Reduction of patient motion artifacts in digital subtraction angiography: evaluation of a fast and fully automatic technique.

The performance of an automatic technique for the reduction of patient motion artifacts in digital subtraction angiography was evaluated. Four observers assessed the quality of 104 cerebral digital subtraction angiographic images that were processed by means of both the automatic technique and manual pixel shifting. The automatic technique resulted in better image quality and was considerably less time-consuming.

Adult↗

PubMed2001

Integrated volume visualization of functional image data and anatomical surfaces using normal fusion.

A generic method, called normal fusion, for integrated three-dimensional (3D) visualization of functional data with surfaces extracted from anatomical image data is described. The first part of the normal fusion method derives quantitative values from functional input data by sampling the latter along a path determined by the (inward) normal of a surface extracted from anatomical data; the functional information is thereby projected onto the anatomical surface independently of the viewpoint. Fusion of the anatomical and functional information is then performed with a color-encoding scheme based on the HSV model. This model is preferred over the RGB model to allow easy, rapid, and intuitive retrospective manipulation of the color encoding of the functional information in the integrated display, and two possible strategies for this manipulation are explained. The results first show several clinical examples that are used to demonstrate the viability of the normal fusion method. These same examples are then used to evaluate the two HSV color manipulation strategies. Furthermore, five nuclear medicine physicians used several other clinical cases to evaluate the overall approach for manipulation of the color encoded functional contribution to an integrated 3D visualization. The integrated display using the normal fusion technique combined with the added functionality provided by the retrospective color manipulation was highly appreciated by the clinicians and can be considered an important asset in the investigation of data from multiple modalities.

Adult↗

PubMed2000

Placement of an inferior vena cava filter in a pig guided by high-resolution MR fluoroscopy at 1.5 T.

Percutaneous placement of an inferior vena cava filter is a means for long-term prevention of pulmonary thromboembolism. In this study we investigated the magnetic resonance (MR) imaging properties of a Nitinol vena cava filter, in various anatomic and angiographic scans, as well as the feasibility of placing this filter under near real-time, high-resolution MR fluoroscopy. We made use of the passive tracking strategy, with on-line image processing and visualization, both in vitro and in a pig. The artifacts provoked by the metallic filter were such that the position and orientation of the filter were well depicted in all scans. Considerable radiofrequency caging obscured the interior of the filter. Our experiments showed that an MR-guided vena cava filter placement, with sufficient temporal and spatial resolution, is possible. Three-dimensional phase contrast MRA allowed direct evaluation of the filter placement procedure, without the use of contrast agent.

Alloys↗

PubMed2000

On-line flow quantification by low-resolution phase-contrast MR imaging and model-based postprocessing.

Over the past decade, magnetic resonance (MR) imaging has been developed toward a tool for guiding and evaluating diagnostic and therapeutic interventions. Within the field of vascular MR-guided interventions, MR has potential for providing on-line monitoring of the blood volume flow rate, which is relevant during procedures such as balloon angioplasty and stent placement. We recently reported a hardware and software environment for enabling flow quantification every 8 seconds using nontriggered phase-contrast imaging. In the present study, the objective was to increase temporal resolution further to one evaluation per 4 seconds. We achieve this by lowering spatial resolution to 3 pixels per lumen diameter. The accuracy of the measurements is preserved by applying model-based postprocessing for quantification of the volume flow rate. Phantom and volunteer studies are presented, demonstrating the accuracy of the model-driven approach for the applied short acquisitions. The capabilities of the presented approach are illustrated by the results of several hypercapnia experiments and carotid compression tests performed on healthy volunteers.

Blood Flow Velocity↗

PubMed2000

Simultaneous quantitative cerebral perfusion and Gd-DTPA extravasation measurement with dual-echo dynamic susceptibility contrast MRI.

Quantification of cerebral perfusion using dynamic susceptibility contrast MRI generally relies on the assumption of an intact blood-brain barrier. The present study proposes a method to correct the tissue response function that does not require this assumption, thus, allowing perfusion studies in, for example, high-grade brain tumors. The correction for contrast extravasation in the tissue during the bolus passage is based on a two-compartment kinetic model. The method separates the intravascular hemodynamic response and the extravascular component and returns the corrected tissue response function for perfusion quantification as well as the extravasation rate constant of the vasculature. Results of simulation experiments with different degrees of contrast extravasation are presented. The clinical potential is illustrated by determination of the perfusion and extravasation of a glioblastoma multiforme. The correction scheme proves to be fast and reliable even in cases of low signal-to-noise ratio. It is applicable whether extravasation occurs or not. When extravasation is present, application of the proposed method is mandatory for accurate cerebral blood volume measurements. Magn Reson Med 43:820-827, 2000.

Brain Neoplasms↗

PubMed2000

Selective contrast-enhanced MR angiography.

In this study the feasibility of intraarterial contrast administration was investigated. Its use for navigation and treatment evaluation during MR-guided intravascular interventions was explored in phantom and animal experiments. An injection protocol was developed, which accounts for sequence parameters and vessel flow rate. Tracking a bolus of contrast agent was useful to verify the catheter tip position and to assess flow conditions. Compared to intravenous contrast-enhanced magnetic resonance angiography (CE-MRA), selective contrast administration permitted a strongly reduced dose. In two-dimensional (2D) acquisitions overlap of vessels was prevented. Injection and acquisition were easily and accurately synchronized in selective 3D CE-MRA, and a high contrast concentration could be maintained during the entire acquisition. Selective injection is useful in the course of an intervention, to facilitate navigation, provide information on flow conditions, and to evaluate treatment progress repeatedly.

Animals↗

PubMed2000

Automatic morphology-based brain segmentation (MBRASE) from MRI-T1 data.

A method called morphology-based brain segmentation (MBRASE) has been developed for fully automatic segmentation of the brain from T1-weighted MR image data. The starting point is a supervised segmentation technique, which has proven highly effective and accurate for quantitation and visualization purposes. The proposed method automates the required user interaction, i.e., defining a seed point and a threshold range, and is based on the simple operations thresholding, erosion, and geodesic dilation. The thresholds are detected in a region growing process and are defined by connections of the brain to other tissues. The method is first evaluated on three computer simulated datasets by comparing the automated segmentations with the original distributions. The second evaluation is done on a total of 30 patient datasets, by comparing the automated segmentations with supervised segmentations carried out by a neuroanatomy expert. The comparison between two binary segmentations is performed both quantitatively and qualitatively. The automated segmentations are found to be accurate and robust. Consequently, the proposed method can be used as a default segmentation for quantitation and visualization of the human brain from T1-weighted MR images in routine clinical procedures.

Adolescent↗

PubMed2000

Repeated quantitative perfusion and contrast permeability measurement in the MRI examination of a CNS tumor.

This study reports on the results of quantitative MRI perfusion and contrast permeability measurement on two occasions in one patient. The measurements were separated 81 days in time. The tumor grew considerably in this period, but no change was found with respect to perfusion and contrast permeability. Non-involved white matter values were reproduced to demonstrate repeatability. The presented approach to dynamic susceptibility contrast MRI allows fast and repeatable quantitative assessment of perfusion and is easily integrated in a conventional brain tumor protocol.

Adult↗