Biomedical subjects

Peter Boesiger

Publications and source records attributed to Peter Boesiger.

71 records · Page 4Linked to original sources

PubMed2003

The impact of spatial resolution and respiratory motion on MR imaging of atherosclerotic plaque.

PURPOSE: To examine the impact of spatial resolution and respiratory motion on the ability to accurately measure atherosclerotic plaque burden and to visually identify atherosclerotic plaque composition. MATERIALS AND METHODS: Numerical simulations of the Bloch equations and vessel wall phantom studies were performed for different spatial resolutions by incrementally increasing the field of view. In addition, respiratory motion was simulated based on a measured physiologic breathing pattern. RESULTS: While a spatial resolution of > or = 6 pixels across the wall does not result in significant errors, a resolution of < or = 4 pixels across the wall leads to an overestimation of > 20%. Using a double-inversion T2-weighted turbo spin echo sequence, a resolution of 1 pixel across equally thick tissue layers (fibrous cap, lipid, smooth muscle) and a respiratory motion correction precision (gating window) of three times the thickness of the tissue layer allow for characterization of the different coronary wall components. CONCLUSIONS: We found that measurements in low-resolution black blood images tend to overestimate vessel wall area and underestimate lumen area.

Arteriosclerosis↗

PubMed2003

Performance of a new gadolinium-based intravascular contrast agent in free-breathing inversion-recovery 3D coronary MRA.

In three-dimensional (3D) coronary magnetic resonance angiography (MRA), the in-flow contrast between the coronary blood and the surrounding myocardium is attenuated as compared to thin-slab two-dimensional (2D) techniques. The application of a gadolinium (Gd)-based intravascular contrast agent may provide an additional source of signal and contrast by reducing T(1blood) and supporting the visualization of more distal or branching segments of the coronary arterial tree. In six healthy adults, the left coronary artery (LCA) system was imaged pre- and postcontrast with a 0.075-mmol/kg bodyweight dose of the intravascular contrast agent B-22956. For imaging, an optimized free-breathing, navigator-gated and -corrected 3D inversion recovery (IR) sequence was used. For comparison, state-of-the-art baseline 3D coronary MRA with T(2) preparation for non-exogenous contrast enhancement was acquired. The combination of IR 3D coronary MRA, sophisticated navigator technology, and B-22956 allowed for an extensive visualization of the LCA system. Postcontrast, a significant increase in both the signal-to-noise ratio (SNR; 46%, P < 0.05) and contrast-to-noise ratio (CNR; 160%, P < 0.01) was observed, while vessel sharpness of the left anterior descending (LAD) artery and the left coronary circumflex (LCX) were improved by 20% (P < 0.05) and 18% (P < 0.05), respectively.

Adult↗

PubMed2003

Image-based tracking of optically detunable parallel resonant circuits.

In this work strategies for the robust localization of parallel resonant circuits are investigated. These strategies are based on the subtraction of two images, which ideally differ in signal intensity at the positions of the devices only. To modulate their signal amplification, and thereby generate the local variations, the parallel resonant circuits are alternately detuned and retuned during the acquisition. The integration of photodiodes into the devices permits their fast optical switching. Radial and spiral imaging sequences are modified to provide the data for the two images in addition to those for a conventional image in the same acquisition time. The strategies were evaluated by phantom experiments with stationary and moving catheter-borne devices. In particular, rapid detuning and retuning during the sampling of single profiles is shown to lead to a robust localization. Moreover, this strategy eliminates most of the drawbacks usually associated with image-based tracking, such as low temporal resolution. Image-based tracking may thus become a competitive (if not superior) alternative to projection-based tracking of parallel resonant circuits.

Catheterization↗

PubMed2003

Parallel spectroscopic imaging with spin-echo trains.

A reduction in scan time in spectroscopic imaging (SI) can be achieved by both fast and reduced k-space sampling. This work presents an ultrafast SI technique that combines the two approaches. The synergy of multiple spin-echo (MSE) acquisition and sensitivity encoding (SENSE) enables high-resolution SI to be performed within a clinically acceptable scan time. MSE-SENSE-SI with echo train lengths ranging from one to four echoes is evaluated with respect to SNR and spatial response function by means of in vitro experiments. It is shown that acquiring two spin-echoes (SEs) per acquisition yields a good practical trade-off among scan time, SNR, and spatial response. The clinical feasibility of the technique is demonstrated in a patient with an astrocytoma, and SI data are obtained with an image matrix of 24 x 24 in just over 2 min.

Astrocytoma↗

PubMed2003

k-t BLAST and k-t SENSE: dynamic MRI with high frame rate exploiting spatiotemporal correlations.

Dynamic images of natural objects exhibit significant correlations in k-space and time. Thus, it is feasible to acquire only a reduced amount of data and recover the missing portion afterwards. This leads to an improved temporal resolution, or an improved spatial resolution for a given amount of acquisition. Based on this approach, two methods were developed to significantly improve the performance of dynamic imaging, named k-t BLAST (Broad-use Linear Acquisition Speed-up Technique) and k-t SENSE (SENSitivity Encoding) for use with a single or multiple receiver coils, respectively. Signal correlations were learned from a small set of training data and the missing data were recovered using all available information in a consistent and integral manner. The general theory of k-t BLAST and k-t SENSE is applicable to arbitrary k-space trajectories, time-varying coil sensitivities, and under- and overdetermined reconstruction problems. Examples from ungated cardiac imaging demonstrate a 4-fold acceleration (voxel size 2.42 x 2.52 mm(2), 38.4 fps) with either one or six receiver coils. k-t BLAST and k-t SENSE are applicable to many areas, especially those exhibiting quasiperiodic motion, such as imaging of the heart, the lungs, the abdomen, and the brain under periodic stimulation.

Heart↗

PubMed2003

In vivo 1H NMR spectroscopy of individual human brain metabolites at moderate field strengths.

This article reviews spectral editing techniques for in vivo 1H NMR spectroscopy of human brain tissue at moderate field strengths of 1.5-3 Tesla. Various aspects of 1H NMR spectroscopy are discussed with regard to in vivo applications. The parameter set [delta, J, n] (delta being the relative chemical shift, J the scalar coupling constant and n the number of coupled spins) is used to characterize the spin systems under investigation and to classify the editing techniques that are used in in vivo 1H NMR spectroscopy.

Brain↗

PubMed2003

Magnetic resonance imaging for the in vivo evaluation of gastric-retentive tablets.

PURPOSE: To develop a magnetic resonance imaging (MRI) technique for assessing in vivo properties of orally ingested gastric-retentive tablets under physiologic conditions. METHODS: Tablets with different floating characteristics (tablet A-C) were marked with superparamagnetic Fe3O4 particles to analyze intragastric tablet position and residence time in human volunteers. Optimal Fe3O4 concentration was determined in vitro. Intragastric release characteristic of one slow-release tablet (tablet D) was analyzed by embedding gadolinium chelates (Gd-DOTA) as a drug model into the tablet. All volunteers underwent MRI in the sitting position. Tablet performance was analyzed in terms of relative position of tablet to intragastric meal level (with 100% at meal surface), intragastric residence time (min) and Gd-DOTA distribution volume (% of meal volume). RESULTS: Intragastric tablet floating performance and residence time of tablets (tablet A-D) as well as the intragastric Gd-DOTA distribution of tablet D could be monitored using MRI. Tablet floating performance was different between the tablets (A, 93%(95 - 9%); B, 80%(80 - 68%): C, 38%(63 - 32%); p < 0.05). The intragastric distribution volume of Gd-DOTA was 19.9% proximally and 35.5% distally. CONCLUSIONS: The use of MRI allows the assessment of galenic properties of orally ingested tablets in humans in seated position.

Delayed-Action Preparations↗

PubMed2003

RingTag: ring-shaped tagging for myocardial centerline assessment.

RATIONALE AND OBJECTIVES: Although endocardial ejection indexes lead to overestimation of contractility in hypertrophied hearts, circumferential fiber shortening at the mid wall (cFS) is less affected by wall thickness. In this study magnetic resonance tagging is exploited to assess directly cFS in normal and hypertrophied hearts. METHODS: A novel tagging procedure generates freely definable, convex ring saturation bands. Data acquisition during the cardiac cycle is achieved with a fast, single breath-hold echo-planar imaging measurement that is combined with a slice-following approach and a navigator-guided breath-holding technique to improve reproducibility of breath hold positions. RESULTS: The procedure is able to create variably shaped convex saturation structures on the myocardium that can be tracked automatically throughout the cardiac cycle. Circumferential shortening at the endocardial border (FSendo) obtained in 6 healthy volunteers and in 6 patients with hypertensive cardiomyopathy suggested hypercontractility of hypertrophied hearts (30.7 +/- 4.1% vs. 43.9 +/- 4.4% respectively; P < 0.002), whereas shortening at the level of the myofibers assessed as cFS was not different (17.2 +/- 1.4% vs. 18.1 +/- 2.8% respectively; P = 0.49). CONCLUSIONS: The presented approach allows for assessment of midwall myocardial mechanics and may become a useful tool to study contractile function in hypertrophied hearts.

Adult↗

PubMed2002

Single breath-hold extended free-breathing navigator-gated three-dimensional coronary MRA.

PURPOSE: To acquire the center of k-space while extending three-dimensional free-breathing navigator-gated coronary magnetic resonance (MR) angiography by an initial single breath-hold. MATERIALS AND METHODS: This approach was successfully applied in eight healthy adult subjects. Resulting images were compared with conventionally acquired free-breathing navigator-gated MR angiograms. RESULTS: The acquisition of k-space center during the single breath-hold resulted in a 26% increase (P < 0.05) of signal-to-noise ratio. Visible length of the right coronary artery, as well as contrast-to-noise ratio between the blood and the myocardial muscle, were identical. CONCLUSION: The breath-hold extension was shown to be a valuable technique that may be combined with first-pass contrast-enhanced MR imaging.

Adult↗

PubMed2002

Myocardial tagging with 3D-CSPAMM.

PURPOSE: To introduce a true three-dimensional (3D) tagging technique for the assessment of myocardial tissue motion. MATERIALS AND METHODS: To generate a 3D tagging grid, a complementary spatial modulation of magnetization (CSPAMM) was applied in three spatial directions. Imaging was performed using a conventional fast 3D gradient-echo sequence. For automatic analysis of the 3D-CSPAMM data set, evaluation software, based on a 3D extension of the HARP technique, was used. RESULTS: Successful application of the 3D-CSPAMM technique in healthy subjects allowed the accurate determination of quantitative 3D motion patterns in the human heart. CONCLUSION: 3D-CSPAMM may contribute to the quantification of the local 3D myocardial motion pattern throughout the cardiac cycle.

Heart↗

PubMed2002

Combined high-resolution and real-time imaging: a technical feasibility study on coronary magnetic resonance angiography.

PURPOSE: To propose a new approach to combining high-resolution and real-time imaging and to show its technical feasibility on the example of coronary magnetic resonance angiography. MATERIALS AND METHODS: The insertion of fast two-dimensional (2D) acquisitions into time intervals that have not been utilized by triggered or gated 2D or three-dimensional (3D) acquisitions so far is suggested, as well as the immediate reconstruction and display of the additional data. For a technical validation of this concept, a 2D ventricular function protocol was interleaved into a cardiac-triggered and respiratory-gated 3D coronary angiography protocol. Dedicated hardware was employed to rapidly process the data originating from the former. Since the sampling of the latter was restricted to intervals with minimal motion, remaining periods of time could be used to simultaneously image the cardiac and respiratory motion. RESULTS: The technical feasibility of the proposed approach was demonstrated by successful measurements with the combined high-resolution and real-time protocol in volunteers. All examinations provided short axis views during the acquisition and angiograms of selected parts of the coronary system after its completion. CONCLUSION: The investigated concept allows high-resolution measurements to be complemented with real-time imaging functionality without affecting the scan time or image quality. In the particular application considered, an image-based patient monitoring or motion correction is enabled, indicating potential benefits of combining two very dissimilar methods of data acquisition in one measurement.

Coronary Circulation↗

PubMed2002

Volume tracking cardiac 31P spectroscopy.

The limited reliability and accuracy of cardiac spectroscopy have been partly attributed to effects from respiratory motion. In this work, we developed a prospective volume tracking method for respiratory motion compensation based on multiple navigator echoes and demonstrated its application in cardiac (31)P spectroscopy. The sequence consists of two 2D selective excitation pulses preceding the spectroscopic experiment to sample respiratory motion components. The navigator information is evaluated in real-time to calculate the shift of the heart from respiration. Based on the displacement information, the spectroscopic volume and/or grid position is prospectively corrected to track the volume of interest. The method was validated with a moving compartment phantom simulating in vivo respiratory motion. With volume tracking, no signal contamination was apparent. Spectra obtained in 14 healthy volunteers were evaluated using time-domain fitting procedures. The fitting accuracy improved consistently with volume tracking compared to data from non-navigated reference acquisitions. Compared to other gating approaches available for spectroscopy, the current technique does not degrade the scan efficiency, thus allowing effective use of scan time.

Adenosine Triphosphate↗

PubMed2002

"Soap-Bubble" visualization and quantitative analysis of 3D coronary magnetic resonance angiograms.

In order to compare coronary magnetic resonance angiography (MRA) data obtained with different scanning methodologies, adequate visualization and presentation of the coronary MRA data need to be ensured. Furthermore, an objective quantitative comparison between images acquired with different scanning methods is desirable. To address this need, a software tool ("Soap-Bubble") that facilitates visualization and quantitative comparison of 3D volume targeted coronary MRA data was developed. In the present implementation, the user interactively specifies a curved subvolume (enclosed in the 3D coronary MRA data set) that closely encompasses the coronary arterial segments. With a 3D Delaunay triangulation and a parallel projection, this enables the simultaneous display of multiple coronary segments in one 2D representation. For objective quantitative analysis, frequently explored quantitative parameters such as signal-to-noise ratio (SNR); contrast-to-noise ratio (CNR); and vessel length, sharpness, and diameter can be assessed. The present tool supports visualization and objective, quantitative comparisons of coronary MRA data obtained with different scanning methods. The first results obtained in healthy adults and in patients with coronary artery disease are presented.

Adult↗

PubMed2002

Sensitivity-encoded single-shot spiral imaging for reduced susceptibility artifacts in BOLD fMRI.

Sensitivity encoding (SENSE) with iterative image reconstruction was used to shorten the readout duration in single-shot spiral imaging by a factor of 2. This enabled susceptibility-related blurring and signal loss artifacts to be reduced and spatial resolution to be improved. As a beneficial side effect, the gradient duty cycle was also reduced. The spiral SENSE technique was applied to functional MRI (fMRI) with blood oxygen level-dependent (BOLD) contrast and compared to a conventional spiral acquisition. Stimulation experiments were performed in seven volunteers using motor, visual, and taste paradigms. The signal-to-noise ratio (SNR) and signal-to-fluctuation-noise ratio (SFNR) of the SENSE acquisitions were reduced by 20% and 13%, respectively, with respect to the longer readout. The overall activation detected was comparable to that of the conventional spiral acquisition, even though difficulties in reproducing the stimulation response hampered the evaluation. In some cases, the application of SENSE enabled recovery of activation in regions affected by signal loss due to field inhomogeneity.

Artifacts↗

PubMed2002

2D SENSE for faster 3D MRI.

Sensitivity encoding in two spatial dimensions (2D SENSE) with a receiver coil array is discussed as a means of improving the encoding efficiency of three-dimensional (3D) Fourier MRI. It is shown that in Fourier imaging with two phase encoding directions, 2D SENSE has key advantages over one-dimensional parallel imaging approaches. By exploiting two dimensions for hybrid encoding, the conditioning of the reconstruction problem can be considerably improved, resulting in superior signal-to-noise behavior. As a consequence, 2D SENSE permits greater scan time reduction, which particularly benefits the inherently time-consuming 3D techniques.Along with the principles of 2D SENSE imaging, the properties of the technique are discussed and investigated by means of simulations. Special attention is given to the role of the coil configuration, yielding practical setups with four and six coils. The in vivo feasibility of the two-dimensional approach is demonstrated for 3D head imaging, permitting four-fold scan time reduction.

Computer Simulation↗

PubMed2002

Pacemaker reed switch behavior in 0.5, 1.5, and 3.0 Tesla magnetic resonance imaging units: are reed switches always closed in strong magnetic fields?

MRI is established as an important diagnostic tool in medicine. However, the presence of a cardiac pacemaker is usually regarded as a contraindication for MRI due to safety reasons. The aim of this study was to investigate the state of a pacemaker reed switch in different orientations and positions in the main magnetic field of 0.5-, 1.5-, and 3.0-T MRI scanners. Reed switches used in current pacemakers and ICDs were tested in 0.5-, 1.5-, and 3.0-T MRI scanners. The closure of isolated reed switches was evaluated for different orientations and positions relative to the main magnetic field. The field strengths to close and open the reed switch and the orientation dependency of the closed state inside the main magnetic field were investigated. The measurements were repeated using two intact pacemakers to evaluate the potential influence of the other magnetic components, like the battery. If the reed switches were oriented parallel to the magnetic fields, they closed at 1.0 +/- 0.2 mT and opened at 0.7 +/- 0.2 mT. Two different reed switch behaviors were observed at different magnetic field strengths. In low magnetic fields (< 50 mT), the reed switches were closed. However, in high magnetic fields (> 200 mT), the reed switches opened in 50% of all tested orientations. No difference between the three scanners could be demonstrated. The reed switches showed the same behavior whether they were isolated or an integral part of the pacemakers. The reed switch in a pacemaker or an ICD does not necessarily remain closed in strong magnetic fields at 0.5, 1.5, or 3.0 T and the state of the reed switch may not be predictable with certainty in clinical situations.

Defibrillators, Implantable↗

PubMed2002

Combined CFD and MRI study of blood flow in a human ascending aorta model.

Physiological correct modelling of blood flow through the human ascending aorta is done by combining computational fluid dynamics (CFD) and magnetic resonance imaging (MRI). This method provides a relatively new approach in the analysis and quantification of the haemodynamic variables. Velocity patterns and wall shear stress distributions occurring in the ascending aorta of an individual subject are examined. Geometrical data and inflow velocity profiles just downstream of the valve were acquired from MRI measurements. Based on the extraction of arterial cross-sections a computer model of the time-dependent geometrical vessel wall was generated. After surface creation the arterial lumen was filled with an appropriate 3D finite element mesh. The mathematical description of the blood flow uses the Navier-Stokes equations applying an Arbitrary Lagrangian-Eulerian modification with respect to the time-varying geometry with externally imposed boundary motion. The numerical approach uses our recently developed finite element solver. The computational results agree very well with the measured data.

Adult↗