PubMed Health⌕ Search

Biomedical subjects

E Achten

Publications and source records attributed to E Achten.

At least 37 records · Page 2Linked to original sources
PubMed2000

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: I. Analysis and compensation of eddy currents.

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.

Acrylamide↗
PubMed2000

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: II. Analysis of B1-field inhomogeneity.

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to radiation dose that has been delivered to a gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B1-field inhomogeneities as a source of dose inaccuracy. First, the influence of B1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is used to map the B1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition, both experiments and computer simulations revealed the effects of B1 field inhomogeneities on the measured R2. This work offers a method to correct R2 maps for B1 -field inhomogeneities.

Acrylamide↗
PubMed1999

1H chemical shift imaging of the human brain at age 60-90 years reveals metabolic differences between women and men.

1H magnetic resonance spectroscopy was used to compare brain metabolism in 540 elderly persons, stratified by sex and age (60-90 years old). An 8 x 8 x 2 cm3 supraventricular brain volume, a transverse plane parallel to the canthomeatal line, was examined by automated 1H chemical shift imaging [point-resolved spectroscopy (PRESS), TE of 35 msec]. Regional choline (Cho), creatine (Cr), and N-acetyl aspartate (NAA) peak areas in the 518 successful examinations (96%) were studied by division through the total area of the particular metabolite in each spectral map. This procedure eliminated intersubject variance, maximized intervoxel variance (26 < or = F < or = 149, P < 0.0001) and reduced the standard deviations in the voxel metabolite signals threefold. Normalized signals in women (n = 257) and men (n = 261) differed in 9 (Cho/sigma Cho), 8 (Cr/sigma Cr), and 10 (NAA/sigma NAA) of 36 voxels examined (P < or = 0.001). In the cingulate gyrus Cho/sigma Cho, Cr/sigma Cr, and NAA/sigma NAA were reduced in men compared with women. These findings are consistent with a sex-related reduction of glucose metabolism in the same brain lobe revealed by positron emission tomography.

Aged↗
PubMed1999

Rupture of a cervical spinal cord arteriovenous malformation: a rare complication of endovascular embolization.

Patients suffering from a cervical spinal cord arteriovenous malformation (SCAVM) run high risk of devastating subarachnoid hemorrhage and hematomyelia. Therefore, cervical SCAVMs represent a compelling indication for surgical or endovascular therapy. The authors report on an acute life-threatening subarachnoid hemorrhage from a cervical SCAVM that ruptured during an embolization procedure. Causality and therapeutic management are discussed.

Acute Disease↗
PubMed1999

Presurgical evaluation of the motor hand area with functional MR imaging in patients with tumors and dysplastic lesions.

PURPOSE: To test an optimized functional magnetic resonance (MR) imaging procedure to depict the motor hand representation area (HRA) in patients with epilepsy lesions near the central sulcus. MATERIALS AND METHODS: Fast low-angle shot MR imaging was performed with an oblique single-section imaging technique in eight control subjects (10 hemispheres) and six patients (12 hemispheres). Three series of five activation images (obtained while subjects performed repetitive finger-to-thumb opposition movements) and five rest images were acquired. Each hemisphere was studied in three adjacent sections. Difference maps (obtained with simple subtraction between activation and rest images) were compared with t-test maps. RESULTS: In control subjects, the HRA was visible in 27 of 30 sections. Qualitatively, activation was seen better on t-test maps in 14 and on difference maps in four of these sections. In all patients, motor activation could be seen in the hemisphere that contained the lesion. This activation was considered normal in four patients. In two patients, the HRA was deformed. Functional MR imaging activation in the motor area was confirmed with Penfield stimulation in five patients. CONCLUSION: Functional MR imaging findings in the preoperative assessment of dysplastic lesions around the central sulcus are the same as for tumors. t-test maps are superior to difference maps in the treatment of motor functional MR imaging data.

Adult↗
PubMed1999

[Methyl-11C]thymidine positron emission tomography in tumoral and non-tumoral cerebral lesions.

BACKGROUND: No ideal radiopharmaceutical exists for positron emission tomography (PET) that fulfills all clinical requirements for the study of brain tumors. PURPOSE: The usefulness of a recently developed PET tracer, [methyl-11C]thymidine ([methyl-11C]TdR) is explored in brain tumors. PATIENTS AND METHODS: Twenty patients with confirmed tumoral and non-tumoral brain lesions were investigated with [methyl-11C] TdR PET. The 11C activity was visually and quantitatively assessed. In two patients, dynamic scans were performed. The PET findings were compared to those of magnetic resonance imaging (MRI) or computed tomography (CT) of the brain and to the final diagnosis. RESULTS: Eight out of ten patients with confirmed tumoral lesions or tumor recurrence had increased 11C activity within the lesion. In ten non-tumoral lesions no increased 11C uptake was found. The dynamic PET studies showed that [methyl-11C] TdR first acts as a blood flow tracer, but that later on the uptake of 11C activity is due to labeled metabolites, crossing the blood-brain barrier. Increased tracer activity was only observed in tumoral and not in non-tumoral contrast-enhanced lesions on MRI or CT. CONCLUSIONS: [Methyl-11C] TdR is not a selective PET radiopharmaceutical for brain tumors, but can be used as a tracer for tumoral blood-brain barrier disruption.

Adolescent↗
PubMed1999

Epilepsy surgery in Belgium, the experience in Gent.

Between January 1992 and July 1998, 320 patients were presurgically evaluated for medically refractory epilepsy at the University Hospital of Gent. All patients underwent a comprehensive presurgical evaluation, including extensive neurological history and examination, video-EEG monitoring of interictal EEG and habitual seizures, and optimum magnetic resonance (MR). In a large subgroup of these patients, a comprehensive neuropsychological examination and interictal 18FDG-PET were performed. Subsequently, a bilateral carotid angiography and intracarotid amytal procedure (Wada-test) were planned in 49 patients to establish hemispheric language dominance and bilateral memory function. After proper selection, 23 patients underwent invasive video-EEG monitoring with intracranial implantation of parenchymal and/or subdural electrodes to further document the area of seizure onset. From the initial group of 320 potential surgical candidates, 75 patients (42 males, 33 females) with mean age of 29 years (range: 2 months-55 years) and mean duration of uncontrolled seizures of 15 years (range: 2 weeks-38 years) eventually underwent a surgical procedure. Sixty of 75 patients were on high dose antiepileptic polytherapy. Optimum MR detected structural abnormalities, confined to a limited brain area, in 71 patients. These abnormalities were of space-occupying nature in 31 cases; an atrophic lesion was suspected in 39 patients; a combination of space-occupying and atrophic lesion was seen in 1 case. Structural abnormalities were most frequently located in the temporal lobe (n = 53) and the frontal lobe (n = 10). Video-EEG monitoring documented complex partial seizures in 67 patients with occasional secondary generalisation in 32. Most patients had complex partial seizures of temporal lobe as defined by clinical and EEG criteria. Two patients had only simple partial seizures. Ultimately, an area of seizure onset could be determined in all patients. Temporal lobectomy with hippocampectomy was the most commonly performed procedure (n = 42). In 13 patients, complete lesionectomies were performed for epileptogenic structural lesions in and outside the temporal lobe. In 2 patients, only partial lesionectomies were possible; in 5 patients, only biopsies in combination with partial lesionectomies could be performed. Anterior 2/3 callosotomy was performed in 4 patients and hemispherectomy was performed in 2 patients. Postsurgical seizure control, after average follow-up of 50 months (range: 12-98 months), was excellent in 49 patients who became seizure-free. In these patients, antiepileptic therapy was tapered 2 years after surgery. Patients in whom only biopsies or partial lesionectomies were performed have poor seizure control. Epilepsy surgery is a rewarding therapeutic alternative for patients with medically refractory epilepsy. Comprehensive presurgical evaluation and epilepsy surgery provide excellent neurological, neurophysiological, neuropsychological and imaging research opportunities.

Adolescent↗
PubMed1998

Intra- and interobserver variability of MRI-based volume measurements of the hippocampus and amygdala using the manual ray-tracing method.

We studied the intra- and interobserver variability of volume measurments of the hippocampus (HC) and the amygdala as applied to the detection of HC atrophy in patients with complex partial seizures (CPE), measuring the volumes of the HC and amygdala of 11 normal volunteers and 12 patients with presumed CPE, using the manual ray-tracing method. Two independent observers performed these measurements twice each using home-made software. The intra- and interobserver variability of the absolute volumes and of the normalised left-to-right volume differences (deltaV) between the HC (deltaV(HC)), the amygdala (deltaV(A)) and the sum of both (deltaV(HCA)) were assessed. In our mainly right-handed normals, the right HC and amygdala were on average 0.05 and 0.03 ml larger respectively than on the left. The interobserver variability for volume measurements in normal subjects was 1.80 ml for the HC and 0.82 ml for the amygdala, the intraobserver variability roughly one third of these values. The interobserver variability coefficient in normals was 3.6% for deltaV(HCA), 4.7% for deltaV(HC) and 7.3% for deltaV(A). The intraobserver variability coefficient was 3.4% for deltaV(HCA), 4.2% for deltaV(HC) amd 5.6% for deltaV(A). The variability in patients was the same for volume differences less than 5% either side of the interval for normality, but was higher when large volume differences were encountered, is probably due to the lack of thresholding and/or normalisation. Cutoff values for lateralisation with the deltaV were defined. No intra- or interobserver lateralisation differences were encountered with deltaV(HCA) and deltaV(HC). From these observations we conclude that the manual ray-tracing method is a robust method for lateralisation in patients with TLE. Due to its higher variability, this method is less suited to measure absolute volumes.

Adolescent↗
PubMed1998

Three-dimensional dosimetry using polymer gel and magnetic resonance imaging applied to the verification of conformal radiation therapy in head-and-neck cancer.

BACKGROUND AND PURPOSE: It was our aim to investigate NMR-based BANG gel dosimetry as a three-dimensional dosimetry technique in conformal radiotherapy. MATERIALS AND METHODS: The BANG gel consisting of gelatin, water and co-monomers was first validated in a cylindrical glass flask for a single standard beam. Next, the gel contained in a human neck-shaped cast was used to verify a treatment plan for the conformal irradiation of a concave tumour in the lower neck. Magnetic resonance relaxation rate images were acquired and, based on an appropriate calibration of the gel, converted to absorbed dose distributions. The resulting maps were compared with dose distributions measured using radiographic film. RESULTS: The gel-measured dose profiles of standard beams agreed within 3% (root mean square difference) with the profiles measured with high spatial resolution by a diamond detector. For the multi-beam conformal treatment, the difference map between gel-measured and film-measured dose distributions revealed a noise component and a more systematic deviation including structural or space-coherent patterns. The mean absolute value of the difference amounted to 8%. A number of possible causes for this deviation are designated. CONCLUSIONS: Polymer gel dosimetry in combination with magnetic resonance imaging is a promising method for dosimetric verification of conformal radiotherapy.

Dose-Response Relationship, Radiation↗
PubMed1998

Single-voxel proton MR spectroscopy and positron emission tomography for lateralization of refractory temporal lobe epilepsy.

PURPOSE: We compared the metabolic information obtained from single-voxel proton MR spectroscopy and positron emission tomography (PET) in patients with temporal lobe epilepsy. METHODS: Twenty-nine patients with temporal lobe epilepsy were screened for metabolic abnormalities with both proton MR spectroscopy and PET. Lateralization with MR spectroscopy was possible by using NAA/(Cho+Cr) and an asymmetry index. Hypometabolism as determined by PET was classified as typical or complex. RESULTS: Twenty-four (96%) of 25 patients whose seizure onset could be lateralized to one temporal lobe showed ipsilateral lateralization with either MR spectroscopy or PET, whereas concordant lateralization with both techniques was possible only in 14 (56%) of the 25 patients. MR spectroscopy showed 42 abnormal temporal lobes whereas PET showed only 25 lobes with decreased metabolism. All temporal lobes with hypometabolism at PET also had a low NAA/(Cho+Cr). Five patients (20%) with negative PET studies had seizures lateralized correctly with MR spectroscopy. CONCLUSION: Proton MR spectroscopy is more sensitive in depicting metabolic abnormalities than is PET in patients with temporal lobe epilepsy. Patients with negative PET studies will benefit from MR spectroscopy for the purpose of lateralization.

Adolescent↗
PubMed1998

Aspects of proton MR spectroscopy in the seizure patient.

Seizures, especially epileptic seizures, are among the most common neurologic symptoms. They may occur as a result of fever, hypoglycemia, or acute central nervous system infections. This article discusses how MR imaging and MR spectroscopy contribute to the management of the epileptic patient.

Epilepsies, Partial↗
PubMed1997

Dipole modelling and intracranial EEG recording: correlation between dipole and ictal onset zone.

This study includes 11 patients (3 males, 8 females) with mean age of 29 years (range: 15-42 years) who underwent a presurgical evaluation for refractory complex partial seizures (CPS). In all patients, neuroimaging (1.5 T optimum-MR) demonstrated intracranial structural abnormalities (space-occupying: n = 2; atrophic: n = 8; dysplastic: n = 1) and video-EEG monitoring showed CPS, because of discrepancies in the non-invasive examinations, all underwent additional intracranial EEG monitoring. After tailored resective procedures, all but one patient became seizure free. Mean follow-up was 30 months (range: 12-52 months). Results of intracranial EEG recording were compared with spatiotemporal dipole mapping of interictal and ictal epileptic discharges. Interictal dipole modelling revealed two distinct dipole patterns. Patients with lesions located in the medial temporal lobe uniformly presented a combined dipole that consisted of a radial and a tangential component with a high degree of elevation relative to the axial plane. Patients with extrahippocampal lesions had a less stable dipole with a predominant radial component. Dipole modelling of early ictal discharges revealed a striking correspondence with the interictal findings in individual patients. Elevation of ictal dipoles was always congruent with localisation based on intracranial EEG recordings. Interictal and ictal dipole mapping of medial temporal lobe sources may limit the number of surgical candidates for refractory CPS that need intracranial EEG recording. Whether ictal dipole modelling can be equally useful in extratemporal epilepsy remains to be proven.

Adolescent↗
PubMed1997

Recent and future evolutions in NeuroSPECT with particular emphasis on the synergistic use and fusion of imaging modalities.

Recent and future evolutions in neuroSPECT apply to radiopharmaceuticals techniques and the synergistic use of different imaging modalities in the work-up of neurological disorders. The introduction of Technetium labelled perfusion tracers, which could pass the intact blood-brain barrier, together with the implementation of the tomographic principle, by making the conventional gamma camera rotating, enabled estimation of regional cerebral blood flow and indirectly of local brain metabolism. In addition at present Thallium-201 and Tc-99m sestaMIBI allow functional detection of viable tumor tissue, without interference from previous surgery or radiotherapy as seen using CT-scan or MRI. In neurology this has led to the recognition of SPECT by the American Academy of Neurology (Therapeutics and technology subcommittee) as an established or promising tool in major neurological disorders such as dementia, stroke and epilepsy, while other domains such as brain oncology are considered investigational. With regard to radiopharmaceuticals, recent evolutions mainly include the development of mostly Iodine-123 labelled receptor ligands, some of which are already commercially available. For instrumentation advances consist e.g. of multidetector systems equipped with fanbeam collimators, attenuation and scatter correction or coincidence detection. Given the present role for nuclear neurology it may be expected that these additional radiopharmaceutical and technical innovations will continue to stimulate the development of SPECT of the brain. The synergistic use of several imaging techniques such as CT, (functional) MRI, source imaging, SPECT and PET represents a multimodal holistic approach to probe cerebral functions for research and clinical purposes. Clinical indications, in which this synergistic use is illustrated include e.g. support of the clinical diagnosis of dementia of the Alzheimer type, presurgical ictal detection of seizure focus, detection of acute ischemia and differential diagnosis between radiation necrosis and brain tumor recurrence. The synergistic use of imaging modalities, optimally applied using image fusion, allows to overcome the intrinsic limitations and to enhance the specific advantages of the different approaches as it leads to increased precision and accuracy, as well for spatial anatomofunctional correlation as for quantification.

Alzheimer Disease↗
PubMed1997

Magnetic resonance imaging and the reduction of motion artifacts: review of the principles.

Magnetic resonance (MR) imaging is a non-invasive diagnostic tool which is widely used nowadays. In this paper, the basic principles of MR imaging are explained and it is shown how images can be reconstructed in case of standard 2D Fourier Transform (2DFT) imaging. Several aspects of MR signal encoding are described. Unfortunately, motion of the patient during a magnetic resonance experiment often causes severe artifacts in the images. For example, in 2DFT imaging blurring and ghosting are seen and the appearance of motion artifacts remains one of the major drawbacks in MR imaging. Several methods to reduce motion artifacts in MR imaging have been proposed in the past. An overview of the principles on which these methods are based is given in this paper. Both post-processing methods and techniques that rely on gating or the use of alternative acquisition schemes such as projection reconstruction are discussed.

Artifacts↗
PubMed1996

Epilepsy surgery in Belgium, the Flemish experience.

Between January 1992 and June 1995, 160 patients were presurgically evaluated for medically refractory epilepsy by the Epilepsy Monitoring and Surgery Team at the University Hospital of Gent. All these patients underwent a comprehensive presurgical evaluation, including extensive neurological history and examination, video-EEG monitoring of interictal EEG and habitual seizures, CT and optimum MR. In a large subgroup of these patients a comprehensive neuro-psychological examination and interictal 18FDG-PET were performed. After the non-invasive phase of the presurgical evaluation, a bilateral carotid angiography and intracarotid amytal procedure was planned in 27 patients to establish hemispheric language dominance and bilateral memory function. After proper selection, 14 patients underwent invasive video-EEG monitoring with intracranial implantation of parenchymal and/or subdural electrodes to further document the area of seizure onset. From the initial group of 160 potential surgical candidates, 40 patients (20 M, 20 F) with mean age of 31 years (range: 2 months-55 years) and mean duration of uncontrolled seizures of 16 years (range: 2 months-47 years) eventually underwent a surgical procedure. 30/40 patients were on high dose antiepileptic polytherapy. Optimum MR detected structural abnormalities, confined to a limited brain area, in 39 patients. These abnormalities were of space-occupying nature in 21 cases; an atrophic lesion was suspected in 17 patients. Structural abnormalities were most frequently located in the temporal lobe (n = 26) and the frontal lobe (n = 7). Video-EEG monitoring documented complex partial seizures in 32 patients with occasional secondary generalisation in 14. In most of these patients, seizures could be subclassified as being of temporal lobe origin based on clinical and EEG criteria. Two patients had only simple partial seizures. One patient with Sturge-Weber syndrome and a strictly unilateral angioma had hemiconvulsions. A mentally retarded patient with Lennox-Gastaut syndrome had different types of seizures. After non-invasive and invasive exploration, the area of seizure onset could be determined in all patients. Standard or modified temporal lobectomy +/- hippocampectomy were the most commonly performed procedures (n = 26). In 5 patients complete lesionectomies were performed for epileptogenic structural lesions in and outside the temporal lobe. In 2 patients only partial lesionectomies were possible; in 5 patients only biopsies could be performed. Anterior 2/3 callosotomy and hemispherectomy were each performed in one patient. Postsurgical seizure control, after average follow-up of 20 months (range: 6-40 months), was excellent in 27 patients who became seizure-free. In these patients antiepileptic therapy was tapered 2 years after surgery. An additional 4 patients continue to experience non-disabling simple partial seizures only. Patients in whom only biopsies or partial lesionectomies were performed have poor seizure control. Three patients died as a result of the intrinsic malignancy of their space-occupying lesion. Two patients who are seizure free experienced a moderate postoperative hemiparesis with subtotal recovery. Overall quality of life was substantially improved both in patients who became entirely seizure free or who experienced a very significant reduction in seizure frequency. Presurgical evaluation and epilepsy surgery are a labour intensive but rewarding therapeutic alternative for patients with medically refractory epilepsy. Besides providing therapeutic efficacy, comprehensive presurgical evaluation and epilepsy surgery allow for fruitful clinical neurological research.

Adolescent↗
PubMed1995

Noninvasive MRI thermometry with the proton resonance frequency (PRF) method: in vivo results in human muscle.

The noninvasive thermometry method is based on the temperature dependence of the proton resonance frequency (PRF). High-quality temperature images can be obtained from phase information of standard gradient-echo sequences with an accuracy of 0.2 degrees C in phantoms. This work was focused on the in vivo capabilities of this method. An experimental setup was designed that allows a qualitative in vivo verification. The lower-leg muscles of a volunteer were cooled and afterwards reheated with an external water bolus. The temperature of the bolus water varied between 17 degrees C and 37 degrees C. The in vivo temperature images can be used to extract the temperature in muscle tissue. The data in the fat tissue are difficult to interpret because of the predominance of susceptibility effects. The results confirm the method's potential for hyperthermia control.

Algorithms↗