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P Boesiger

Publications and source records attributed to P Boesiger.

At least 73 records · Page 4Linked to original sources
PubMed1999

Single breath-hold slice-following CSPAMM myocardial tagging.

Myocardial tagging has shown to be a useful magnetic resonance modality for the assessment and quantification of local myocardial function. Many myocardial tagging techniques suffer from a rapid fading of the tags, restricting their application mainly to systolic phases of the cardiac cycle. However, left ventricular diastolic dysfunction has been increasingly appreciated as a major cause of heart failure. Subtraction based slice-following CSPAMM myocardial tagging has shown to overcome limitations such as fading of the tags. Remaining impediments to this technique, however, are extensive scanning times (approximately 10 min), the requirement of repeated breath-holds using a coached breathing pattern, and the enhanced sensitivity to artifacts related to poor patient compliance or inconsistent depths of end-expiratory breath-holds. We therefore propose a combination of slice-following CSPAMM myocardial tagging with a segmented EPI imaging sequence. Together with an optimized RF excitation scheme, this enables to acquire as many as 20 systolic and diastolic grid-tagged images per cardiac cycle with a high tagging contrast during a short period of sustained respiration.

Adult↗
PubMed1999

Metabolic aspects of phosphate replacement therapy for hypophosphatemia after renal transplantation: impact on muscular phosphate content, mineral metabolism, and acid/base homeostasis.

Hypophosphatemia caused by renal phosphate loss occurs frequently after kidney transplantation. In assumption of systemic phosphorus depletion, the presumed deficit commonly is replaced by oral phosphate supplements. However, such treatment is debatable, because intracellular phosphorus stores have not been assessed in this setting and may not be accurately reflected by serum phosphate concentrations. Moreover, disturbances in mineral metabolism from chronic renal failure, such as hypocalcemia and hyperparathyroidism, may be prolonged with oral phosphate supplements. Conversely, a neutral phosphate salt might improve renal acid excretion and systemic acid/base homeostasis for its properties as a urinary buffer and a poorly reabsorbable anion. Twenty-eight patients with mild early posttransplantation hypophosphatemia (0.3-0.75 mmol/L) were randomly assigned to receive either neutral sodium phosphate (Na(2)HPO(4)) or sodium chloride (NaCl) for 12 weeks and examined with regard to (1) correction of serum phosphate concentration and urinary phosphate handling; (2) muscular phosphate content; (3) serum calcium and parathyroid hormone (PTH); and, (4) renal acid handling and systemic acid/base homeostasis. Mean serum phosphate concentrations were similar and normal in both groups after 12 weeks of treatment; however, more patients in the NaCl group remained hypophosphatemic (93% versus 67%). Total muscular phosphorus content did not correlate with serum phosphate concentrations and was 25% below normophosphatemic controls but was completely restored after 12 weeks with and without phosphate supplementation. However, the percentage of the energy-rich phosphorus compound adenosine triphosphate (ATP) was significantly higher in the Na(2)HPO(4) group, as was the relative content of phosphodiesters. Also, compensated metabolic acidosis (hypobicarbonatemia with respiratory stimulation) was detected in most patients, which was significantly improved by neutral phosphate supplements through increased urinary titratable acidity. These benefits of added phosphate intake were not associated with any adverse effects on serum calcium and PTH concentrations. In conclusion, oral supplementation with a neutral phosphate salt effectively corrects posttransplantation hypophosphatemia, increases muscular ATP and phosphodiester content without affecting mineral metabolism, and improves renal acid excretion and systemic acid/base status.

Acid-Base Equilibrium↗
PubMed1999

Effects of vigabatrin intake on brain GABA activity as monitored by spectrally edited magnetic resonance spectroscopy and positron emission tomography.

A deficit in gamma-aminobutyric acid (GABA) levels in the brain or the cerebrospinal fluid (CSF) is found in many epilepsy patients. Frequency and severity of seizures may be reduced by treatment with GABA increasing medicaments as e.g. vigabatrin, an irreversible inhibitor of GABA-transaminase. For a better understanding of the associated effects, healthy volunteers were examined with magnetic resonance spectroscopy (MRS) and positron emission tomography (PET) before and after intake of different doses of vigabatrin. For the MRS examinations, a dedicated localized spectral editing method was developed to determine GABA levels. The 11C-flumazenil (FMZ)-PET protocol allowed determination of GABA-A receptor binding. The results show a clear and dose-dependent increase in the brain GABA levels after the medication period as compared to the baseline values. The GABA-A receptor binding, on the other hand, did not change significantly.

Adult↗
PubMed1999

Quantitative abdominal aortic flow measurements at controlled levels of ergometer exercise.

Measuring the exercise-induced flow changes in the arteries of the body is a major challenge. The use of quantitative MR flow measurements for this purpose is hampered by movement artifacts and ECG triggering problems. To quantify exercise-induced flow changes in the abdominal aorta, we applied a fast hybrid phase contrast sequence with K-space segmentation and echo planar imaging readouts during a 12 heart beat, single breathhold post exercise scanning window after ergometer exercise in nine volunteers. Central k-space was acquired first. The changes in heart rate throughout the scanning window were quantified. The mean decrease in heart rate after six heart beats post exercise was less than 4% and less than 14% after 11 heart beats indicating that the exercise state was very well represented during the acquisition of central k-space. Abdominal aortic flow increased from 1.4+/-0.3 l/min at rest to 7.9+/-1.1 l/min at 131 watt. Retrograde flow reached a maximum value of 1.2 l/min at rest, and lasted 140 ms on average. Only for one out of the nine volunteers was there any retrograde flow present during exercise (at 33 watt and 65 watt exercise). It was concluded that retrograde flow patterns in the abdominal aorta associated with oscillating wall shear stresses and development of atherosclerosis disappeared with increasing levels of exercise. The feasibility of using fast quantitative phase contrast measurements during a post exercise scanning window to represent controlled exercise levels was demonstrated.

Adult↗
PubMed1999

Prosthetic heart valve evaluation by magnetic resonance imaging.

OBJECTIVE: To evaluate the potential of magnetic resonance imaging (MRI) for evaluation of velocity fields downstream of prosthetic aortic valves. Furthermore, to provide comparative data from bileaflet aortic valve prostheses in vitro and in patients. METHODS: A pulsatile flow loop was set up in a 7.0 Tesla MRI scanner to study fluid velocity data downstream of a 25 mm aortic bileaflet heart valve prosthesis. Three dimensional surface plots of velocity fields were displayed. In six NYHA class I patients blood velocity profiles were studied downstream of their St. Jude Medical aortic valves using a 1.5 Tesla MRI whole-body scanner. Blood velocity data were displayed as mentioned above. RESULTS: Fluid velocity profiles obtained from in vitro studies 0.25 valve diameter downstream of the valve exhibited significant details about the cross sectional distribution of fluid velocities. This distribution completely reflected the valve design. Blood velocity profiles in humans were considerably smoother and in some cases skewed with the highest velocities toward the anterior-right ascending aortic wall. CONCLUSION: Display and interpretation of fluid and blood velocity data obtained downstream of prosthetic valves is feasible both in vitro and in vivo using the MRI technique. An in vitro model with a straight tube and the test valve oriented orthogonally to the long axis of the test tube does not entail fluid velocity profiles which are compatible to those obtained from humans, probably due to the much more complex human geometry, and variable alignment of the valve with the ascending aorta. With the steadily improving quality of MRI scanners this technique has significant potential for comparative in vitro and in vivo hemodynamic evaluation of heart valves.

Adult↗
PubMed1999

Distribution of early atherosclerotic lesions in the human abdominal aorta correlates with wall shear stresses measured in vivo.

OBJECTIVES: to study the relationship between wall shear stresses measured in vivo and early atherosclerotic lesions in the abdominal aorta. MATERIALS: eight young volunteers for in vivo wall shear-stress measurements. Abdominal aortas from 10 young adults without signs or history of atherosclerotic disease were obtained by autopsy for histomorphometric measurements. METHODS: wall shear stresses were measured in the abdominal aorta above and below the renal arteries using a magnetic resonance technique with high resolution for imaging and blood velocity mapping. At identical abdominal aortic locations, intimal thickness was measured blindly using histomorphometric techniques and correlated to wall shear-stress variables using linear-regression analysis. RESULTS: intimal thickness showed a linear decrease with mean wall shear stress (r=-0.90, p<0.01) and with maximum wall shear stress (r=-0.86, p<0.01). CONCLUSIONS: intimal thickness in the normal abdominal aorta is associated with mean, maximum and oscillating wall shear stresses. These in vivo data corroborate previous in vitro studies suggesting that low and oscillating wall shear stresses are localising factors for intimal thickening and hence the early development of atherosclerosis.

Adolescent↗
PubMed1999

Evaluation of gastric emptying and motility in diabetic gastroparesis with magnetic resonance imaging: effects of cisapride.

OBJECTIVE: The motor mechanisms that underlie both slow gastric emptying in diabetic gastroparesis and its acceleration by cisapride are poorly understood. We have recently shown that magnetic resonance imaging (MRI) allows concurrent evaluation of both gastric emptying and regional gastric motility. METHODS: Emptying and motility were measured in eight diabetic patients with previously demonstrated delayed gastric emptying using a rapid MRI technique during oral administration of cisapride and placebo. Studies were performed in a double blind fashion and each patient acted as his own control. Subjects were studied supine for 120 min in a 1.5 Tesla MRI scanner after ingestion of 500 ml of 10% Intralipid. Gastric emptying corrected for the volume of secretions was determined every 15 min using transaxial scans. Each transaxial scan was followed by 120 coronal scans at 1 s intervals. Coronal scans were angled to provide simultaneous imaging of the proximal and distal stomach. MRI studies were also performed in seven diabetic patients with normal emptying who served as disease controls. RESULTS: Emptying was slower in the gastroparetic patients (t(1/2): 124 +/- 10 min) compared to patients with normal emptying (81 +/- 9 min, p < 0.05). Cisapride accelerated gastric emptying (74 +/- 5 vs 124 +/- 10 min) in patients with gastroparesis. The contraction amplitudes in the proximal stomach of gastroparetic patients were increased during cisapride treatment (17.2% +/- 1.8% vs 13.2% +/- 0.6%; p < 0.02), whereas antral contraction frequency, amplitude, and velocity were unchanged. CONCLUSIONS: We conclude that cisapride-induced acceleration of liquid gastric emptying in diabetic gastroparesis does not appear to result from changes in antral contractility, but may be related to changes in proximal gastric tone or gastric outlet resistance.

Adult↗
PubMed1999

Scintigraphic validation of a magnetic resonance imaging method to study gastric emptying of a solid meal in humans.

BACKGROUND: We have previously used a magnetic resonance imaging (MRI) method to study gastric emptying of liquids. So far, however, it has not possible to assess solid gastric emptying with this technique. AIMS: To validate scintigraphically MRI as a method for measuring emptying of a mixed solid/liquid meal. METHODS: In eight healthy subjects, gastric emptying of a solid/liquid (SM) and a liquid meal (LM) of identical energy content and macronutrient composition was studied by scintigraphy and MRI for 120 minutes. RESULTS: MRI and scintigraphy agreed with respect to emptying profiles (intraclass correlation coefficient (RI) SM: 0.988, RI, LM: 0.917), t1/2 (SMMRI: 129 (9), SMScinti: 123 (11) minutes, NS; LMMRI: 100 (7), LMScinti: 110 (8) minutes, NS) and AUC (SMMRI: 8999 (232), SMScinti: 8788 (277) min%, NS; LMMRI: 8819 (368), LMScinti: 8891 (321) min%, NS). CONCLUSIONS: MRI can be used to measure reliably gastric emptying not only of liquid but also of mixed solid/liquid meals in humans.

Adult↗
PubMed1999

Biphasic edema after hypoxic-ischemic brain injury in neonatal rats reflects early neuronal and late glial damage.

Magnetic resonance imaging with diffusion- and T2-weighted imaging and 31P magnetic resonance spectroscopy was used to investigate the relationship between development of brain edema and alterations of the brain energy metabolism after hypoxia-ischemia (HI) brain injury in 7-d-old rats. The results were correlated with histologic examinations at various times during recovery up to 5 d. Moderate HI, induced by right common carotid artery ligation and subsequent exposure to 8% O2 for 90 min, produced a cytotoxic edema of 52+/-9% brain volume and depressed the ratio of phosphocreatine to inorganic phosphate from 1.43+/-0.21 to 0.11+/-0.09. Within 1 h of reoxygenation, the edema decreased to 4+/-2% of brain volume, demarcating the core of the lesion. At 5 h of recovery, a secondary cytotoxic edema together with a newly developing vasogenic edema expanded again, reaching its maximal extent of 45+/-10% brain volume at around 24 h. The ratio of phosphocreatine to inorganic phosphate recovered slowly, reaching 1.12+/-0.27 around 13 h. Thereafter it declined again in a manner analogous to the observations made in human newborns after severe perinatal asphyxia, reaching trough values of 0.48+/-0.22 around 24 h after HI. At the cellular level, the vast majority of neuronal death occurred before 15 h. Subsequently, strong glial activation lasted 2-3 d after HI. At 5 d, a cystic infarction of 35+/-12% brain volume was found. We conclude that the biphasic evolution of brain edema and energy metabolism reflects early neuronal and late glial damage in response to moderate HI injury. Therefore, the secondary energy breakdown reflects glial activation and subsequent glial death.

Animals↗
PubMed1999

High-resolution assessment of velocity fields and shear stresses distal to prosthetic heart valves using high-field magnetic resonance imaging.

BACKGROUND AND AIM OF THE STUDY: Complications after replacement of diseased heart valves with mechanical prostheses have been shown to be related to the hemodynamics distal to the valve. For this reason, the velocity patterns have been disclosed in vitro with a variety of different techniques. This study introduces a magnetic resonance imaging (MRI) -based technique, which entails easy acquisition of fluid velocity field data with a high accuracy and spatial resolution. METHODS: A high-field magnetic resonance scanner equipped with short echo time phase-contrast velocity measurement software was applied in a detailed mapping of the axial velocity profile across the entire vessel area at two positions downstream of a bileaflet valve prosthesis inserted in a pulsatile flow system in vitro. The laminar shear forces were calculated from the fluid velocity field data. RESULTS: The velocity profiles very close to the valve reflected the bileaflet design as also shown in previous studies, but the extent and velocities of the jet emanating from the slit between the leaflets were clearly better visualized. However, one diameter downstream of the valve the central jet was completely dispersed and the hemodynamic complexity was significantly reduced. Recirculation and retrograde flow regions that might be relevant for understanding typical long-term complications after implantation were observed close to the valve. CONCLUSIONS: In one scan experiment the presented method provides information on flow characteristics that previously required application of different types of experiment. Thus, the method seems promising as a new technique for detailed and extensive mapping of the velocities and laminar shear stresses downstream of prosthetic heart valves in vitro.

Blood Flow Velocity↗
PubMed1999

SENSE: sensitivity encoding for fast MRI.

New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999.

Brain↗
PubMed1998

Quantitative 1H MRS of the human brain in vivo based on the stimulation phantom calibration strategy.

Normal metabolite concentrations were determined in five different brain regions of healthy adult volunteers using proton magnetic resonance spectroscopy (1H MRS) in vivo. The absolute in vivo concentrations of N-acetylaspartate (NAA), creatine and phosphocreatine (Cre), and choline containing compounds (Cho) were quantified from measurements obtained with a head-shaped simulation phantom. Scanner performance and calibration accuracy were assessed by phantom experiments. Localized spectra were acquired on clinical 1.5 T systems using the PRESS localization sequence with frequency selective water suppression. Comparison of the results obtained from phantom experiments and human brain in vivo strongly suggests that reproducibility in vivo mainly depends on the topologic metabolite heterogeneity of brain tissue in combination with relative volume dislocalization.

Adolescent↗
PubMed1998

Heuristic optimization algorithms applied to the quantification of spectroscopic data.

The quantification of in vivo MR spectra imposes severe problems because of low spectral resolution and poor signal-to-noise ratio. Maximum likelihood methods are often applied. However, with conventional spectrum analysis procedures, the search for a global minimum in a multidimensional space often terminates in only a local minimum. Heuristic optimization procedures are able to circumvent this difficulty. Two approaches, the genetic algorithm and the simulated annealing, have been adapted to the quantification of MR spectra. For evaluation purposes, the procedures have been applied to synthetic and in vivo spectra with different noise levels. They both allowed a reliable spectrum quantification. The areas of most peaks were quantified reproducibly, although in some cases, the discrimination between spectroscopically almost identical metabolites (e.g., glutamate and glutamine) was not completely satisfactory. The two algorithms are found to be valuable alternative methods in the quantification of in vivo MR spectra.

Algorithms↗
PubMed1998

A new correlation-based fuzzy logic clustering algorithm for fMRI.

Fuzzy logic clustering algorithms are a new class of processing strategies for functional MRI (fMRI). In this study, the ability of such methods to detect brain activation on application of a stimulus task is demonstrated. An optimization of the selected algorithm with regard to different parameters is proposed. These parameters include (a) those defining the pre-processing procedure of the data set; (b) the definition of the distance between two time courses, considered as p-dimensional vectors, where p is the number of sequential images in the fMRI data set; and (c) the number of clusters to be considered. Based on the assumption that such a clustering algorithm should cluster the pixel time courses according to their similarity and not their proximity (in terms of distance), cross-correlation-based distances are defined. A clear mathematical description of the algorithm is proposed, and its convergence is proven when similarity measures are used instead of conventional Euclidean distance. The differences between the membership function given by the algorithm and the probability are clearly exposed. The algorithm was tested on artificial data sets, as well as on data sets from six volunteers undergoing stimulation of the primary visual cortex. The fMRI maps provided by the fuzzy logic algorithm are compared to those achieved by the well established cross-correlation technique.

Algorithms↗
PubMed1998

Quantitation of circumferential subpixel vessel wall position and wall shear stress by multiple sectored three-dimensional paraboloid modeling of velocity encoded cine MR.

Methods are lacking for accurate, noninvasive circumferential edge detection and wall shear stress calculation. Using standard MR phase contrast sequences, parts of the velocity profiles were fitted to a multiple sectored three-dimensional paraboloid model enabling exact calculation of vessel wall position and wall shear stress in 24 locations evenly distributed around the luminal vessel wall. The model was evaluated by in vitro scans and computer simulations and applied to the common carotid artery of humans. In vitro, the luminal area of a glass tube was assessed with an error of 0.9%. Computer simulations of peak systolic data revealed errors of +/-0.9% (vessel area) and +/-3.25% (wall shear stress). The in vivo results showed substantial difference between anterior and posterior wall shear stress values due to skewed velocity profiles. A new noninvasive method for highly accurate measurement of circumferential subpixel vessel wall position and wall shear stress has been developed.

Adult↗
PubMed1998

Quantitative 1H MRS in the evaluation of mesial temporal lobe epilepsy in vivo.

Hippocampal metabolite concentrations were determined by localized in vivo proton magnetic resonance spectroscopy (1H MRS) in eleven patients suffering from refractory mesial temporal lobe epilepsy (MTLE), as well as in eleven age-matched healthy volunteers, and compared with patient history, postoperative outcome and histopathology. Main results are: 1) In patients, the decrease in N-acetylaspartate (NAA) concentrations was highly significant ipsilateral, and less but still significant contralateral to the electroencephalogram-defined focus, as compared to controls. 2) The decrease in ipsilateral NAA measured preoperatively correlates with the degree of hippocampal sclerosis but 3) does not reliably predict postoperative outcome, although there is a trend toward better outcome in patients with a marked decrease of NAA. 4) Hippocampal NAA decrease (ipsi- and contralateral) is highly correlated with early onset age of epileptic seizures. 5) Among patients with similar onset age in early childhood, there is a strong association between duration of the disease and contralateral (and, though less clear-cut, ipsilateral) NAA loss. These results are concordant with the notion of a generally progressive worsening and complicating course of symptoms in poorly controlled MTLE.

Adult↗
PubMed1998

Accurate noninvasive quantitation of blood flow, cross-sectional lumen vessel area and wall shear stress by three-dimensional paraboloid modeling of magnetic resonance imaging velocity data.

OBJECTIVES: We present a new method in which a priori knowledge of the blood velocity fields within the boundary layer at the vessel wall, combined with acquisition of high resolution magnetic resonance imaging (MRI) blood velocity data, allow exact modeling at the subpixel level. BACKGROUND: Methods are lacking for accurate, noninvasive estimation of blood flow, dynamic cross-sectional lumen vessel area and wall shear stress. METHODS: Using standard acquisition of MRI blood flow velocity data, we fitted all data points (n = 69) within the boundary layer of the velocity profile to a three-dimensional paraboloid, which enabled calculation of absolute volume blood flow, circumferential vessel wall position, lumen vessel area and wall shear stress. The method was tested in a 8.00 +/ 0.01-mm diameter glass tube model and applied in vivo to the common carotid artery of seven volunteers. RESULTS: In vitro the lumen area was assessed with a mean error of 0.6%. The 95% confidence interval included the specified tube dimensions. Common carotid mean blood flow was 7.42 ml/s, and mean (standard error) diastolic/systolic vessel area was 33.25 (0.72 [2.2%])/43.46 (0.65 [1.5%]) mm2. Mean/peak wall shear stress was 0.95 (0.04 [4.2%])/2.56 (0.08 [3.1%]) N/m2. CONCLUSIONS: We describe a new noninvasive method for highly accurate estimation of blood flow, cross-sectional lumen vessel area and wall shear stress. In vitro results and statistical analysis demonstrate the feasibility of the method, and the first in vivo results are comparable to published data.

Adult↗