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P Boesiger

Publications and source records attributed to P Boesiger.

At least 109 records · Page 6Linked to original sources
PubMed1995

Limitations of stimulated echo acquisition mode (STEAM) techniques in cardiac applications.

Stimulated echoes are widely used for imaging functional tissue parameters such as diffusion coefficient, perfusion, and flow rates. They are potentially interesting for the assessment of various cardiac functions. However, severe limitations of the stimulated echo acquisition mode occur, which are related to the special dynamic properties of the beating heart and flowing blood. To the well-known signal decay due to longitudinal relaxation and through-plane motion between the preparation and the read-out period of the stimulated echoes, additional signal loss is often observed. As the prepared magnetization is fixed with respect to the tissue, this signal loss is caused by the tissue deformation during the cardiac cycle, which leads to a modification of the modulation frequency of the magnetization. These effects are theoretically derived and corroborated by phantom and in vivo experiments.

Blood Flow Velocity↗
PubMed1995

Determination of displacement, stress- and strain-distribution in the human heart: a FE-model on the basis of MR imaging.

The determination of characteristic cardiac parameters, such as displacement, stress and strain distribution are essential for an understanding of the mechanics of the heart. The calculation of these parameters has been limited until recently by the use of idealised mathematical representations of biventricular geometries and by applying simple material laws. On the basis of 20 short axis heart slices and in consideration of linear and nonlinear material behaviour we have developed a FE model with about 100,000 degrees of freedom. Marching Cubes and Phong's incremental shading technique were used to visualise the three dimensional geometry. In a quasistatic FE analysis continuous distribution of regional stress and strain corresponding to the endsystolic state were calculated. Substantial regional variation of the Von Mises stress and the total strain energy were observed at all levels of the heart model. The results of both the linear elastic model and the model with a nonlinear material description (Mooney-Rivlin) were compared. While the stress distribution and peak stress values were found to be comparable, the displacement vectors obtained with the nonlinear model were generally higher in comparison with the linear elastic case indicating the need to include nonlinear effects.

Algorithms↗
PubMed1994

True myocardial motion tracking.

Myocardial tagging is a powerful tool for the assessment of in-plane cardiac motion. However, for previous myocardial tagging techniques, the imaged slice is fixed with respect to the magnet coordinate system. Thus, images acquired at different heart phases do not always represent the same slice of the myocardium. A new myocardial tagging technique is presented, which takes the through-plane motion into consideration. It involves tagging of the desired myocardial slice and applying a subtraction imaging technique to image just that part of the myocardium. The examination time can be reduced considerably by the acquisition of two one-dimensionally tagged images. To increase the signal-to-noise ratio especially at later heart phases, variable imaging RF excitation flip angles are applied. To reduce motion artifacts a repetitive breathhold scheme was applied. In vivo results demonstrate that the tags can be accurately tracked within the entire heart period with a temporal resolution of 35 ms, even at a top basal level of the heart and right ventricle.

Algorithms↗
PubMed1994

Assessment of absolute metabolite concentrations in human tissue by 31P MRS in vivo. Part I: Cerebrum, cerebellum, cerebral gray and white matter.

Absolute metabolite concentrations were determined in four different brain regions using phosphorus magnetic resonance spectroscopy (31P MRS) on 10 healthy adult volunteers. Localized spectra were collected simultaneously from the cerebellum and the cerebrum and, later, from deep white matter and cortical gray matter by means of a two-volume ISIS pulse sequence and a Helmholtz-type RF-coli. Each brain spectrum was quantified with a calibration spectrum from a head-shaped simulation phantom. A time-domain fitting routine was used to process the fully relaxed data. Several metabolite concentrations (mmol/liter) differed significantly between the cerebrum and the cerebellum (PME = 3.2 +/- 0.3 and 4.0 +/- 0.6, PCr = 2.9 +/- 0.3 and 3.9 +/- 0.4, NTP = 2.9 +/- 0.2 and 2.6 +/- 0.2, respectively) and between cortical gray matter and deep white matter (PME = 3.1 +/- 0.4 and 4.3 +/- 0.8, PDE = 10.1 +/- 2.5 and 14.2 +/- 2.6, respectively). The concentration of free magnesium ion was found to be similar in all four brain regions (0.53 +/- 0.21 mmol/liter) but the intracellular pH was significantly higher in the cerebellum (7.04 +/- 0.03) than in the cerebrum (6.99 +/- 0.02).

Adult↗
PubMed1994

Assessment of absolute metabolite concentrations in human tissue by 31P MRS in vivo. Part II: Muscle, liver, kidney.

Absolute metabolite concentrations were assessed in the muscle, the liver, and the kidney of healthy human volunteers by 31P MRS. Fully relaxed in vivo spectra were acquired with a surface coil and were localized with an adiabatic ISIS pulse sequence. The spectra were quantified with a subsequent measurement of a calibration phantom and were processed iteratively in the time domain. The following mean metabolite concentrations (mmol/liter) were measured in the resting male calf muscle (n = 9), in the fasting liver (n = 12), and in the orthotopic kidney (n = 5): [PME] = 2.0 +/- 0.6, 3.8 +/- 0.7, and 2.6 +/- 0.9, [Pi] = 2.9 +/- 0.3, 1.8 +/- 0.3, and 1.6 +/- 0.4, [PDE] = 3.8 +/- 0.8, 9.7 +/- 1.5, and 4.9 +/- 1.1, [PCr] = 22.0 +/- 1.2, 0, and 0, [NTP] = 5.7 +/- 0.4, 2.9 +/- 0.4, and 2.0 +/- 0.3, respectively. Several interesting findings are to be emphasized: The concentrations of Pi, PCr, and NTP were 20% lower in the muscle of women than of men. In addition, the pHi was significantly lower in female muscle (6.99 +/- 0.03) than in male muscle (7.05 +/- 0.03). The pHi in the liver (7.12 +/- 0.09) and in the kidney (7.09 +/- 0.08) were higher than in the muscle of both genders. The free magnesium concentration (mmol/liter) was higher in the liver (1.40 +/- 0.64) than in the kidney (0.79 +/- 0.39) and in the muscle (0.52 +/- 0.10).

Adult↗
PubMed1994

Comparison of calibration strategies for the in vivo determination of absolute metabolite concentrations in the human brain by 31P MRS.

Cerebral concentrations of phosphorus metabolites can be assessed non-invasively by 31P MRS provided the metabolite signals are calibrated with the signal of a standard of known concentration. The reliability of the concentration estimates depends mainly on the strategy of calibration. Three strategies were compared by assessing the concentrations both in a test dummy and in the brain of volunteers. The first strategy utilized tissue water as an internal heteronuclear concentration standard. The second and third strategies used as phosphorus solution as an external homonuclear standard; this solution was either put into a reference bottle placed on top of the head or into a simulation phantom measured instead of the head. Localization was always achieved with the ISIS pulse sequence. The two external homonuclear strategies achieved a higher accuracy (mean error approximately 5%) and reproducibility (mean SD approximately 8%) of the concentration estimates than the internal heteronuclear strategy (mean error approximately 11%; mean SD approximately 15%).

Brain↗
PubMed1994

Measurement of gastric emptying and gastric motility by magnetic resonance imaging (MRI).

Hitherto it has been impossible to measure noninvasively gastric emptying and motility from multiple regions of the stomach in humans. We describe the development of a novel methodology to achieve this using magnetic resonance imaging (MRI). Initial validation studies performed in five healthy volunteers demonstrated similar gastric emptying curves and secretion rates after ingestion of 10% dextrose labeled with [Gd]DOTA, assessed by MRI ([Gd]DOTA as meal marker) and a simultaneous double-indicator technique. Comparison between MRI and scintigraphy in five patients also gave similar results. Application of newer MRI technology allowing a series of coronal scans 1.2 sec apart permitted quantification of wall motion in the proximal and distal stomach in seven healthy volunteers. These results indicate that MRI provides new insights into the mechanisms responsible for both normal and disordered emptying.

Contrast Media↗
PubMed1994

Hemodynamics in the abdominal aorta: a comparison of in vitro and in vivo measurements.

In vivo measurements of blood velocity profiles are difficult to obtain and interpret, since the parameters that govern the normally highly complex flow situation may not be fully quantified or understood at the time of measurement. In vitro flow models have been used often to better understand vascular hemodynamics. The assumptions made in the design of these models limit the applicability of the results. In this study, in vitro flow measurements made in a carefully designed model of the abdominal aorta were compared with in vivo measurements obtained with magnetic resonance imaging. In the suprarenal aorta, the velocity profiles were mostly forward and axisymmetric in both the in vitro and in vivo cases. In the infrarenal aorta, there was extensive flow reversal noted near the posterior wall in both cases. In the aortic bifurcation, two peaks of flow reversal were noted near the lateral posterior walls, and M-shaped velocity profiles were observed in late diastole. The in vitro and in vivo measurements exhibited good qualitative agreement. The in vitro model was accurate in modeling the in vivo hemodynamics of the abdominal aorta. The complex phenomena observed in vivo were explained on the basis of knowledge gained from the in vitro study.

Adult↗
PubMed1994

Changes of liver metabolite concentrations in adults with disorders of fructose metabolism after intravenous fructose by 31P magnetic resonance spectroscopy.

A novel 31P magnetic resonance spectroscopy procedure allows the estimation of absolute concentrations of certain phosphorus-containing compounds in liver. We have validated this approach by measuring ATP, phosphomonesters, and inorganic phosphate (Pi) during fasting and after an i.v. fructose bolus in healthy adults and in three adults with disorders of fructose metabolism and by comparing results with known metabolic concentrations measured chemically. During fasting, the ATP concentration averaged 2.7 +/- 0.3 (SD, n = 9) mmol/L, which, after due correction for other nucleoside triphosphates, was 2.1 mmol/L and corresponded well with known concentrations. Fructose-1-phosphate (F-1-P) could not be measured during fasting; its concentration after fructose was calculated from the difference of the phosphomonester signals before (2.9 +/- 0.2 mmol/L) and after fructose. Pi was 1.4 +/- 0.3 mmol/L and represented the one fourth of Pi visible in magnetic resonance spectra. In the three healthy controls after fructose (200 mg/kg, 20% solution, 2.5 min), the fructokinase-mediated increase of F-1-P was rapid, reaching 4.9 mmol/L within 3 min, whereas the uncorrected ATP decreased from 2.7 to 1.8 mmol/L and the Pi from 1.4 to 0.3 mmol/L. The subsequent decrease of F-1-P, mediated by fructaldolase, was accompanied by an overshooting rise of Pi to 2.7 mmol/L. In the patient with essential fructosuria, the concentrations of F-1-P, ATP, and Pi remained unchanged, confirming that fructokinase was indeed inactive. In the patient with hereditary fructose intolerance, initial metabolic changes were the same as in the controls, but baseline concentrations were not yet reestablished after 7 h, indicating weak fructaldolase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1994

Magnetic resonance angiography: methods and its applications to the coronary arteries.

The non-invasive visualisation of the coronary arteries with magnetic resonance imaging would be a highly useful tool for the diagnosis of coronary artery disease, with a potential to partly replace current invasive imaging modalities, and would be a step towards more patient comfort. While magnetic resonance (MR) angiography of most larger blood vessels can nowadays be performed routinely, current techniques are inadequate to image the coronary arteries. This paper provides a short overview of the principal techniques for MR angiography. The combination of a multiple-slice MR angiography technique with a simple and easily performed breathhold scheme is described, which allows to image the main branches of the coronary artery tree within one scan of 9 to 12 min duration. For display the coronary vessels are segmented semi-automatically and reconstructed as 3D-objects. Results of volunteer scans and examinations of patients with known coronary artery disease show the feasibility of the approach. However, due to the need for a breathhold scheme, patient cooperation is required.

Coronary Angiography↗
PubMed1994

Real-time image reconstruction system for interventional magnetic resonance surgery.

Interventional surgery techniques under the control of fast acquired magnetic resonance (MR) images may become important in interventional radiology in the near future. One of the components needed for an interventional MR scanner is the real-time reconstruction of the acquired MR images. However up to now no real-time reconstruction systems are readily available for MR images. Therefore a reconstruction device was developed, which allows reconstruction and display of MR images with a delay of less than 50 ms. Additional to the high performance, the main characteristics of the presented device are its full compatibility with different MR acquisition techniques and its moderate cost. The device can be operated with most types of commercial scanners. It is especially suited for interventional MR systems but has also applications with conventional MR scanners.

Cost-Benefit Analysis↗
PubMed1994

Developmental changes of phosphorus metabolite concentrations in the human brain: a 31P magnetic resonance spectroscopy study in vivo.

Phosphorus magnetic resonance spectroscopy is a noninvasive method to investigate brain metabolism in vivo. ATP generally serves as an internal concentration standard for the quantification of the various phosphorus metabolites, because the ATP concentration in mammalian brains is assumed to be age independent. This presumption is based on observations made in the biochemical analysis of the developing rat brain. In the present study, metabolite concentrations were assessed with an external concentration standard. Each brain spectrum was quantified using a calibration spectrum that was acquired from a phantom after the in vivo brain measurement. Fully relaxed localized brain spectra were obtained from 16 neonates (2-28 d), 17 infants (6-20 mo), and 28 adults (22-58 y). The metabolite concentrations (in mmol/L) changed from neonates to adults: phosphomonoester from 4.5 to 3.5, inorganic phosphate from 0.6 to 1.0, phosphodiester from 3.2 to 11.7, phosphocreatine from 1.4 to 3.4, and ATP from 1.6 to 2.9. We conclude that 1) the ATP concentration in the human brain almost doubles between neonates and adults, and 2) ATP may not be used as an age-independent internal concentration standard.

Adenosine Triphosphate↗
PubMed1993

Improved myocardial tagging contrast.

Myocardial tagging is a new noninvasive MRI method that allows the study of myocardial motion with high accuracy. However, with conventional tagging techniques tagging contrast is impaired at later heart phases due to longitudinal relaxation. An improved method, called Complementary SPAtial Modulation of Magnetization (CSPAMM), which separates the component of the magnetization with the tagging information from the relaxed component by subtraction of two measurements with first a positive and then a negative tagging grid, is presented. This technique improves the grid contrast and greatly facilitates the automatic evaluation of the myocardial motion. Thus the motion assessment of the entire heart cycle becomes possible. The improvements are documented by numerical simulations and by experiments on phantoms and on human volunteers.

Heart↗
PubMed1993

Comparison of methods for the determination of absolute metabolite concentrations in human muscles by 31P MRS.

In order to determine metabolite concentrations in human skeletal muscles by in vivo 31P MRS, different quantification methods were analyzed with regard to the accuracy and reproducibility of results and the simplicity of handling. Each quantification method comprised a calibration strategy and a localization technique. Extensive in vivo and in vitro tests showed that homonuclear phantom-based calibration strategies yielded significantly more accurate (lower systematic errors) and more reproducible (lower statistical errors) concentration estimates than heteronuclear strategies using internal water as a concentration standard. Additionally, the former strategies are easier to handle than the latter. Localization with the volume-selective sequence ISIS yielded slightly more reproducible results than localization by surface coil. We conclude that phosphorus metabolite concentrations are determined most accurately with phantom-based calibration strategies in combination with ISIS localization (measurement errors approximately 5-7%).

Adenosine Triphosphate↗
PubMed1993

Age distribution and iron dependency of the T2 relaxation time in the globus pallidus and putamen.

Heavily T2-weighted spin echo sequences of the brain show age-dependent low signal intensity in many extrapyramidal nuclei. Although it has been suggested that this low intensity results from non-haem iron, the specific influence of non-haem iron on the T2 relaxation time has not been quantified and remains controversial. The T2 relaxation times of the globus pallidus and putamen were measured from MRI at 1.5T in 27 healthy patients, by using a mathematical model. They were then plotted as a function of age and compared to the curve of age-dependent iron concentration determined post mortem. The curves of T2 relaxation time in the basal ganglia are congruent with published curves of iron concentration, indicating a high probability that the changes in T2 relaxation times and the low signal in the basal ganglia result from the local, age-dependent iron deposition. Individual measurements of T2 relaxation time show less variation before than after 45 years of age, indicating the influence of a second, more individual factor.

Adolescent↗
PubMed1993

T2 relaxation time in patients with Parkinson's disease.

Postmortem studies of patients with Parkinson's disease (PD) reveal an increase in iron concentration in the substantia nigra. Iron content in the brain is associated with decreased signal intensity on T2-weighted MRI. We measured in vivo the T2 relaxation time in 30 PD patients and 33 healthy volunteer subjects, using a 1.5-T whole-body MRI system. In comparison with healthy controls, T2 values in PD patients were reduced in the following brain regions: substantia nigra, caudate nucleus, and putamen. Due to the overlap between patients and control subjects, we could not differentiate, in a given patient, healthy from diseased state on the basis of T2 relaxation time. Our findings support the notion of increased iron deposition in the substantia nigra of patients with PD. However, the shortening of T2 values in the substantia nigra did not correlate with disease duration nor with clinical severity.

Adult↗
PubMed1992

The impact of the ISIS experiment order on spatial contamination.

When performing volume-localized spectroscopy measurements, the amount of spatial contamination is an important quality criterion. With the ISIS localization technique contamination cannot only arise from the transition regions around the volume of interest, but also from remote regions of the sample. The latter contamination component is a consequence of inhomogeneous excitation pulses, if short repetition times TR are used. Its severity depends both on the order of the eight phase cycling experiments needed for an ISIS measurement, and on the ratio TR/T1. Here it is theoretically discussed from which regions of the sample contamination can arise for a specific phase cycling order. For the worst orders the contaminating regions are almost three times as large as for the optimal orders. The ratio for the effectively measured contamination, however, can be moderated in real experiments, because cancellation effects occur due to the phase distribution of the contaminating signals. 31P phantom experiments clearly demonstrate that contamination is present even if adiabatic excitation pulses are applied and that spatial contamination can be reduced to about a third by an optimal choice of the phase cycling order.

Brain↗