Biomedical subjects

P Boesiger

Publications and source records attributed to P Boesiger.

At least 19 recordsLinked to original sources

PubMed2006

Synchronization facilitates removal of MRI artefacts from concurrent EEG recordings and increases usable bandwidth.

Investigating human brain function non-invasively by simultaneous EEG and fMRI measurements is gaining in popularity as more and better solutions to the inherent technical challenges emerge. We demonstrate the use of a commercially available frequency divider and phase-locking device for the purpose of synchronizing an MRI acquisition with a simultaneous recording of the EEG. Synchronization hugely improves the effectiveness of MRI artefact removal from the EEG signal by the common mean template subtraction method. It complements or substitutes post-processing techniques like filtering, thereby increasing the usable bandwidth of the EEG signal to about 150 Hz. This is important for covering the full range of human Gamma band activity. Similarly, synchronization eliminates the necessity for over-sampling of the EEG signal.

Artifacts↗

PubMed2006

Influence of SENSE on image properties in high-resolution single-shot echo-planar DTI.

Limited spatial resolution is a key obstacle to the study of brain white matter structure with diffusion tensor imaging (DTI). In its frequent implementation with single-excitation spin-echo echo-planar sequences, DTI's ability to resolve small structures is strongly restricted by T2 and T2* decay, B0 inhomogeneity, and limited signal-to-noise ratio (SNR). In this work the influence of sensitivity encoding (SENSE) on diffusion-weighted (DW) image properties is investigated. Computer simulations showed that the PSF becomes narrower with increasing SENSE reduction factors, R, enhancing the intrinsic resolution. After a brief theoretical discussion, we describe the estimation of SNR on a pixel-by-pixel basis as a function of R. The mean image SNR behavior is manifold: SENSE is capable of increasing SNR efficiency by reducing the echo time (TE). Each SNR(R) curve reveals a maximum that depends on the amount of partial Fourier encoding used. The overall best SNR efficiency for an eight-element head coil array and a b-factor of 1000 s/mm2 is achieved at R = 2.1 and partial Fourier encoding of 60%. In vivo tensor maps of volunteers and a patient, with an in-plane resolution of 0.78 x 0.78 mm2, are also presented to demonstrate the practical implementation of the parallel approach.

Arteriovenous Malformations↗

PubMed2006

Analysis of residual coronary artery motion for breath hold and navigator approaches using real-time coronary MRI.

Coronary artery MRI methods utilize breath holds, or diaphragmatic navigators, to compensate for respiratory motion. To increase image quality and navigator (NAV) gating efficiency, slice tracking is used, with more sophisticated affine motion models recently introduced. This study assesses the extent of remaining coronary artery motion in free breathing NAV and single and multi breath hold coronary artery MRI. Additionally, the effect of the NAV gating window size was examined. To visualize and measure the respiratory induced motion, an image containing a coronary artery cross section was acquired at each heartbeat. The amount of residual coronary artery displacement was used as a direct measure for the performance of the respiratory motion correction method. Free breathing studies with motion compensation (slice tracking with 5 mm gating window) had a similar amount of residual motion (0.76+/-0.17 mm) as a single breath hold (0.52+/-0.20 mm) and were superior to multiple breath holds (1.22+/-0.60 mm). Affine NAV methods allowed for larger gating windows ( approximately 10 mm windows) with similar residual motion (0.74+/-0.17 mm). In this healthy adult cohort (N=10), free-breathing NAV methods offered respiratory motion suppression similar to a single breath hold.

Adolescent↗

PubMed2006

Pitfalls in lactate measurements at 3T.

In clinical MR spectroscopy at higher field strengths, lactate may show reduced or absent signal intensity at an echo time of 144 ms. Although this false-negative result may be predicted from theory, experimental verification and clinical impact have not been fully established. Using scanners from 3 major vendors, spectra from phantoms and patients demonstrate the lactate signal loss and potential error in interpretation. Strategies are discussed to overcome, or at least alleviate, this problem.

Brain Diseases↗

PubMed2006

Trading spectral separation at 3T for acquisition speed in multi spin-echo spectroscopic imaging.

Fast multiple spin-echo spectroscopic imaging, also called turbo spectroscopic imaging (TSI), may be enhanced in terms of acquisition speed by taking advantage of the higher spectral separation afforded at higher field strength and by further combining it with sensitivity encoding (SENSE). This article demonstrates the possibilities of this approach at 3T, resulting in scan-time reductions of up to a factor of 10. High-resolution, in vivo, single- and multiple-section spectroscopic imaging data are presented.

Aspartic Acid↗

PubMed2005

Resolving fiber crossing using advanced fast marching tractography based on diffusion tensor imaging.

Magnetic resonance diffusion tensor tractography is a powerful tool for the non-invasive depiction of the white matter architecture in the human brain. However, due to limitations in the underlying tensor model, the technique is often unable to reconstruct correct trajectories in heterogeneous fiber arrangements, such as axonal crossings. A novel tractography method based on fast marching (FM) is proposed which is capable of resolving fiber crossings and also permits trajectories to branch. It detects heterogeneous fiber arrangements by incorporating information from the entire diffusion tensor. The FM speed function is adapted to the local tensor characteristics, allowing in particular to maintain the front evolution direction in crossing situations. In addition, the FM's discretization error is reduced by increasing the number of considered possible front evolution directions. The performance of the technique is demonstrated in artificial data and in the healthy human brain. Comparisons with standard FM tractography and conventional line propagation algorithms show that, in the presence of interfering structures, the proposed method is more accurate in reconstructing trajectories. The in vivo results illustrate that the elucidated major white matter pathways are consistent with known anatomy and that multiple crossings and tract branching are handled correctly.

Algorithms↗

PubMed2005

MR-spectroscopic imaging during visual stimulation in subgroups of migraine with aura.

Migraine pathophysiology possibly involves deficient mitochondrial energy reserve and diminished cortical habituation. Using functional magnetic resonance spectroscopic imaging (fMRSI), we studied cortical lactate changes during prolonged visual stimulation to search for different pathophysiological mechanisms in clinically distinct subgroups of migraine with aura. Eleven healthy volunteers (HV) and 10 migraine patients were investigated interictally: five with visual aura (MA) and five with visual symptoms and at least one of the following: paraesthesia, paresis or dysphasia (MAplus). Using MRSI (Philips, 1.5 T) (1)H-spectra were repeatedly obtained from a 25 mm-thick slice covering visual and non-visual cortex, with the first and fifth measurements in darkness and the second to fourth with 8-Hz checkerboard stimulation. In MAplus lactate increased only during stimulation, only in visual cortex; in MA resting lactate was high in visual cortex, without further increase during stimulation. This is compatible with an abnormal metabolic strain during stimulation in MAplus, possibly due to dishabituation, and a predominant mitochondrial dysfunction in MA.

Adult↗

PubMed2005

Attention and interhemispheric transfer: a behavioral and fMRI study.

When both detections and responses to visual stimuli are performed within one and the same hemisphere, manual reaction times (RTs) are faster than when the two operations are carried out in different hemispheres. A widely accepted explanation for this difference is that it reflects the time lost in callosal transmission. Interhemispheric transfer time can be estimated by subtracting RTs for uncrossed from RTs for crossed responses (crossed-uncrossed difference, or CUD). In the present study, we wanted to ascertain the role of spatial attention in affecting the CUD and to chart the brain areas whose activity is related to these attentional effects on interhemispheric transfer. To accomplish this, we varied the proportion of crossed and uncrossed trials in different blocks. With this paradigm subjects are likely to focus attention either on the hemifield contralateral to the responding hand (blocks with 80% crossed trials) or on the ipsilateral hemifield (blocks with 80% uncrossed trials). We found an inverse correlation between the proportion of crossed trials in a block and the CUD and this effect can be attributed to spatial attention. As to the imaging results, we found that in the crossed minus uncrossed subtraction, an operation that highlights the neural processes underlying interhemispheric transfer, there was an activation of the genu of the corpus callosum as well as of a series of cortical areas. In a further commonality analysis, we assessed those areas which were activated specifically during focusing of attention onto one hemifield either contra- or ipsilateral to the responding hand. We found an activation of a number of cortical and subcortical areas, notably, parietal area BA 7 and the superior colliculi. We believe that the main thrust of the present study is to have teased apart areas important in interhemispheric transmission from those involved in spatial attention.

Adult↗

PubMed2004

SENSE-DTI at 3 T.

While holding vast potential, diffusion tensor imaging (DTI) with single-excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal-to-noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in-plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. A further significant increase in SNR efficiency was accomplished by SENSE acquisition, exploiting enhanced encoding speed for echo time reduction. Based on the resulting image data, high-resolution tensor mapping is demonstrated.

Anisotropy↗

PubMed2004

High wall shear stress measured by magnetic resonance is a predictor of restenosis in the femoral-artery after balloon angioplasty.

AIM: Wall shear stress (WSS) has been implied in the pathogenesis of restenosis after percutaneous transluminal angioplasty (PTA). Aims of the present study were to calculate WSS in the superficial femoral artery (SFA) from magnetic resonance imaging (MRI) and from duplex sonography in healthy controls and in patients after PTA of the SFA to evaluate the predictive value of WSS for restenosis. METHODS: WSS was assessed by calculating the slope of velocity profiles at the vessel wall from data obtained with velocity encoded cine MR and with duplex using the formula: Shear stress=4xblood viscosityxpeak blood velocity/internal diameter. Seventeen patients were studied 1 day after successful PTA of the SFA. Restenosis was determined by duplex ultrasound at the 6-months follow-up visit. RESULTS: In healthy controls WSS values calculated from MRI and from duplex were similar (1.86+/-0.35 N/m2 vs 1.88+/-0.34 N/m2, n.s.). In patients the values obtained with duplex were higher than those obtained with MRI (4.1+/-2.3 N/m2 vs 2.4+/-1.2 N/m2, p=0.002). With both methods post-interventional WSS was higher in patients developing restenosis (duplex 5.4+/-2.2 N/m2, MRI 3.1+/-0.9 N/m2) than in those without restenosis (duplex 2.7+/-1.4 N/m2, MRI 1.5+/-0.7 N/m2) and was revealed to be an independent predictor of restenosis (p=0.03). CONCLUSIONS: This is the first study demonstrating that increased post-interventional WSS in the SFA is predictive for restenosis. WSS values obtained with MRI and duplex were different in patients, however with both methods higher WSS was associated with restenosis.

Adult↗

PubMed2003

Analysis of the meal-dependent intragastric performance of a gastric-retentive tablet assessed by magnetic resonance imaging.

BACKGROUND: Modern medical imaging modalities can trace labelled oral drug dosage forms in the gastrointestinal tract, and thus represent important tools for the evaluation of their in vivo performance. The application of gastric-retentive drug delivery systems to improve bioavailability and to avoid unwanted plasma peak concentrations of orally administered drugs is of special interest in clinical and pharmaceutical research. AIM: To determine the influence of meal composition and timing of tablet administration on the intragastric performance of a gastric-retentive floating tablet using magnetic resonance imaging in the sitting position. METHODS: A tablet formulation was labelled with iron oxide particles as negative magnetic resonance contrast marker to allow the monitoring of the tablet position in the food-filled human stomach. Labelled tablet was administered, together with three different solid meals, to volunteers seated in a 0.5-T open-configuration magnetic resonance system. Volunteers were followed over a 4-h period. RESULTS: Labelled tablet was detectable in all subjects throughout the entire study. The tablet showed persistent good intragastric floating performance independent of meal composition. Unfavourable timing of tablet administration had a minor effect on the intragastric tablet residence time and floating performance. CONCLUSION: Magnetic resonance imaging can reliably monitor and analyse the in vivo performance of labelled gastric-retentive tablets in the human stomach.

Adult↗

PubMed2003

Effects of vigabatrin on brain GABA+/Cr signals in focus-distant and focus-near brain regions monitored by 1H-NMR spectroscopy.

The new antiepileptic drug vigabatrin (VGB) increases gamma-aminobutyric acid (GABA) in the brain. We compared GABA+/Cr signals measured focus-near and focus-distant and correlated it with the degree of response to VGB. Brain GABA+/Cr signals were measured in 17 epileptic patients in structurally normal appearing tissue by nuclear proton magnetic resonance (1H-NMR) spectroscopy using a special editing sequence for GABA. In 11 patients the measurements were done in brain areas distant to focus and in six near to focus. Full-responders (seizure reduction of >or=50% at the end of the treatment phase) and partial-responders (seizure reduction of >or=50% at the end of the first month of treatment but <or=50% at end of treatment) had lower GABA+/Cr signals in the hemisphere with the epileptogenic focus and increases of the GABA+/Cr signals with VGB. Non-responders (seizure reduction of <or=50%) had no side difference in the GABA+/Cr signals before treatment and no increase during treatment. These observations were made in structurally normal appearing tissue near to the focus and distant to the focus. A side difference in brain GABA+/Cr signal between the epileptogenic and non-epileptogenic hemisphere before VGB treatment correlates with an improved seizure control under VGB treatment regardless whether the measurement is done focus-near or focus-distant.

Adolescent↗

PubMed2002

Effects of meal consistency and ingested fluid volume on the intragastric distribution of a drug model in humans--a magnetic resonance imaging study.

BACKGROUND: Controlled delivery of drugs to the small intestine in relation to emptying of an ingested meal is important in various pathophysiological conditions. We investigated the effects of different food consistencies and the amount of co-ingested liquid on the intragastric distribution of a contrast marker. METHODS: Five healthy subjects received four meals (each 650 kcal: A, mashed potato with 100 mL water; B, rice with 100 mL water; C, hamburger meal with 100 mL water; D, hamburger meal with 300 mL water). A capsule filled with gadolinium tetra-azacyclododecane tetra-acetic acid solution (as contrast marker) was ingested following meal termination, and its intragastric distribution was assessed by magnetic resonance imaging. RESULTS: Initially, marker distribution was confined to the fundus, and subsequently extended along the inner curvature of the stomach. The maximum distribution volume of the marker was lower in meal A than in meal B (P < 0.05). No differences in marker distribution were observed when the hamburger meal was given with 100 or 300 mL water. CONCLUSIONS: The intragastric distribution kinetics of the marker gadolinium tetra-azacyclododecane tetra-acetic acid appeared to depend on meal consistency, but not on the amount of water co-ingested. Three-dimensional magnetic resonance imaging allows detailed analysis of the intragastric distribution of a drug model in relation to meal emptying and intragastric meal distribution.

Adult↗

PubMed2002

Coronary flow quantification by Fourier velocity encoded MRI.

Recently, the feasibility of measuring coronary blood flow using fast magnetic resonance (MR) techniques was reported. Thus, MR holds potential to non-invasively assess significance of coronary stenosis. However, the accurate determination of flow and vessel area still remains challenging. High spatial and temporal resolution is required to assess reliable flow profiles within the coronary arteries. For this purpose, Fourier velocity encoding (FVE) was implemented with a small number of encoding steps. Simulations and in-vitro experiments have been performed to demonstrate the benefit of FVE for flow quantification. Further, initial volunteer measurements have shown its potential for invivo application.

Blood Flow Velocity↗

PubMed2001

Brain glutathione levels in patients with epilepsy measured by in vivo (1)H-MRS.

OBJECTIVE: Glutathione in its reduced form (GSH) is the most important free radical scavenging compound in the mammalian nervous system that prevents membrane lipid peroxidation. It is suspected that epileptic seizures are accompanied by a massive production of reactive oxygen species, i.e., oxidative stress. METHODS: Using an (1)H MRS technique developed at the authors' site, the authors measured glutathione levels in a volume of interest (VOI) of 25 x 25 x 25 mm placed in structurally normal-appearing tissue in the parietooccipital region of each hemispheres in patients with and without active epilepsy, and in a age-matched control group. RESULTS: The GSH/water ratio in patients with epilepsy was significantly reduced in the parietooccipital region of both hemispheres (1.6 +/- 1.0 x 10(-5)) compared to the GSH/water ratio in healthy controls (2.4 +/- 1.1 x 10(-5)). There was no significant difference between the hemisphere with epileptogenic focus and the hemisphere without epileptogenic focus. The GSH/water ratios of the patients without active epilepsy were not different from the GSH/water ratios of patients with active epilepsy. CONCLUSION: The authors found evidence for a widespread impairment of the glutathione system in patients with epilepsy independent from seizure activity.

Adolescent↗

PubMed2001

Influence of pyridoxal 5'-phosphate alone and in combination with vigabatrin on brain GABA measured by 1H-NMR-spectroscopy.

Both iso-forms of the gamma-aminobutyric acid (GABA) synthesising enzyme and also the GABA degrading enzyme need pyridoxal 5'-phosphate (PP) as co-enzyme. The aim of the study was to investigate the influence of PP alone and in combination with various doses of vigabatrin (VGB) on brain GABA levels. In eight healthy subjects 300 mg/d PP and various doses of VGB (range, 1000 mg/d to 4000 mg/d) were given alone or in combination. The GABA+/creatine (Cr) signals in both occipital lobes were measured before treatment, during monotherapy with PP or VGB, and during combination of both using 1H-NMR-spectroscopy (1H-NMRS). PP alone did not change the GABA+/Cr signals. VGB alone increased the GABA+/Cr signals in both hemispheres. The combination PP and low-medium dosed VGB (1000-2000 mg/d) did not increase the GABA+/Cr signals. The effects of the combination of PP and high dosed (3000-4000 mg/d) VGB on the GABA+/Cr signals varied depending on the sequence of the drugs and dose of VGB. PP alone has no effect on the GABA+/Cr signals in healthy volunteers. The combination of PP and low-high dosed VGB had inconsistent effects on the GABA+/Cr signals compared to a VGB monotherapy because PP activates also the GABA-degrading enzyme GABA-transaminase.

Adult↗

PubMed2001

Specific coil design for SENSE: a six-element cardiac array.

In sensitivity encoding (SENSE), the effects of inhomogeneous spatial sensitivity of surface coils are utilized for signal localization in addition to common Fourier encoding using magnetic field gradients. Unlike standard Fourier MRI, SENSE images exhibit an inhomogeneous noise distribution, which crucially depends on the geometrical sensitivity relations of the coils used. Thus, for optimum signal-to-noise-ratio (SNR) and noise homogeneity, specialized coil configurations are called for. In this article we study the implications of SENSE imaging for coil layout by means of simulations and imaging experiments in a phantom and in vivo. New, specific design principles are identified. For SENSE imaging, the elements of a coil array should be smaller than for common phased-array imaging. Furthermore, adjacent coil elements should not overlap. Based on the findings of initial investigations, a configuration of six coils was designed and built specifically for cardiac applications. The in vivo evaluation of this array showed a considerable SNR increase in SENSE images, as compared with a conventional array. Magn Reson Med 45:495-504, 2001.

Fourier Analysis↗

PubMed2001

Visualization of flow patterns distal to aortic valve prostheses in humans using a fast approach for cine 3D velocity mapping.

The fluid dynamic performance of mechanical heart valves differs from normal valves and thus is considered related to late clinical complications in patients. Since flow patterns evolving around heart valves are complex in space and time, flow visualization based on time-resolved 3D velocity data might add important information regarding the performance of specific valve designs in vivo. However, previous cine 3D techniques for three-directional phase-contrast velocity mapping suffer from long scan duration and therefore might hamper assessment in patients. A hybrid 3D phase-contrast sequence combining segmented k-space acquisition with short EPI readout trains is presented with its validation in vitro. The technique was applied to study flow patterns downstream from a bileaflet aortic prosthesis in six patients. Navigator echoes were incorporated for respiratory motion compensation. Before flow visualization, spurious phase errors due to concomitant gradient fields and eddy currents were corrected. Flow visualization was based on particle paths and animated velocity vector plots. Dedicated algorithms for particle path integration were implemented to account for the considerable motion of the ascending aorta during the cardiac cycle. A distinct flow pattern reflecting the valve design was observed closest to the valve during early flow acceleration. Reverse flow occurred adjacent to high velocity jets and above the hinge housings. Later in systole, flow became confined to the central vessel area and reverse flow along the inner aortic curvature developed. Further downstream from the valve, flow patterns varied considerably among patients, indicating the impact of varying aortic anatomy in vivo. It is concluded that MR velocity mapping is a potential tool for studying 3D flow patterns evolving around heart valve prostheses in humans. J. Magn. Reson. Imaging 2001;13:690-698.

Aged↗