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Biomedical subjects

M B Scheidegger

Publications and source records attributed to M B Scheidegger.

At least 19 recordsLinked to original sources
PubMed2001

Force and torque effects of a 1.5-Tesla MRI scanner on cardiac pacemakers and ICDs.

Magnetic resonance imaging (MRI) is a widely accepted tool for the diagnosis of a variety of disease states. However, the presence of an implanted pacemaker is considered to be a strict contraindication to MRI in a vast majority of centers due to safety concerns. In phantom studies, the authors investigated the force and torque effects of the static magnetic field of MRI on pacemakers and ICDs. Thirty-one pacemakers (15 dual chamber and 16 single chamber units) from eight manufacturers and 13 ICDs from four manufacturers were exposed to the static magnetic field of a 1.5-Tesla MRI scanner. Magnetic force and acceleration measurements were obtained quantitatively, and torque measurements were made qualitatively. For pacemakers, the measured magnetic force was in the range of 0.05-3.60 N. Pacemakers released after 1995 had low magnetic force values as compared to the older devices. For these devices, the measured acceleration was even lower than the gravity of the earth (< 9.81 N/kg). Likewise, the torque levels were significantly reduced in newer generation pacemakers (< or = 2 from a scale of 6). ICD devices, except for one recent model, showed higher force (1.03-5.85 N), acceleration 9.5-34.2 N/kg), and torque (5-6 out of 6) levels. In conclusion, modern pacemakers present no safety risk with respect to magnetic force and torque induced by the static magnetic field of a 1.5-Tesla MRI scanner. However, ICD devices, despite considerable reduction in size and weight, may still pose problems due to strong magnetic force and torque.

Contraindications↗
PubMed2000

Assessment of prosthetic aortic valve performance by magnetic resonance velocity imaging.

OBJECTIVES: Magnetic resonance (MRI) velocity mapping was used to evaluate non-invasively the flow profiles of the ascending aorta in normal volunteers and in patients with an aortic (mechanical) valve prosthesis. BACKGROUND: In patients with artificial aortic valves the flow profile in the ascending aorta is severely altered. These changes have been associated with an increased risk of thrombus formation and mechanical hemolysis. METHODS: Velocity profiles were determined 30 mm distal to the aortic valve in six healthy volunteers and seven patients with aortic valve replacement (replacement within the last 2 years) using ECG triggered phase contrast MRI. Peak flow, mean flow and mean reverse flow were measured in intervals of 25 ms during the entire heart cycle. Systolic reverse flow, end-systolic closing and diastolic leakage volume were calculated for all subjects. RESULTS: Peak flow velocity during mid-systole was significantly higher in patients with valvular prosthesis than in normals (mean + SD, 1.9 +/- 0.4 m/s vs. 1.2 +/- 0.03 m/s, P < 0.001) with a double peak and a zone of reversed flow close to the inner (left lateral) wall of the ascending aorta of the patients. Closing volume was significantly larger in patients than in controls (-3.3 +/- 1.2 ml/beat vs. -0.9 +/- 0.5 ml/beat; P < 0.001). There was reverse flow during systole in valvular patients amounting to 15.7 +/- 6.7% of total cardiac output compared to 2.3 +/- 1.2% in controls (P < 0.001). Diastolic mean flow was negative in patients after valve replacement but not in controls (-11.0 +/- 15.2 ml/beat vs. 6.8 +/- 3.2 ml/beat; P < 0.01). CONCLUSIONS: The following three major quantitative observations have been made in the present study: (1) Mechanical valve prostheses have an increased peak flow velocity with a systolic reverse flow at the inner (left lateral) wall of the ascending aorta. (2) A double peak flow velocity pattern can be observed in patients with bileaflet (mechanical) prosthesis. (3) The blood volume required for leaflet closure and the diastolic leakage blood volume are significantly higher for the examined bileaflet valve than for native heart valves.

Adult↗
PubMed2000

Hemodynamics in the carotid artery bifurcation: a comparison between numerical simulations and in vitro MRI measurements.

The presence of atherosclerotic plaques has been shown to be closely related to the vessel geometry. Studies on postmortem human arteries and on the experimental animal show positive correlation between the presence of plaque thickness and low shear stress, departure of unidirectional flow and regions of flow separation and recirculation. Numerical simulations of arterial blood flow and direct blood flow velocity measurements by magnetic resonance imaging (MRI) are two approaches for the assessment of arterial blood flow patterns. In order to verify that both approaches give equivalent results magnetic resonance velocity data measured in a compliant anatomical carotid bifurcation model were compared to the results of numerical simulations performed for a corresponding computational vessel model. Cross sectional axial velocity profiles were calculated and measured for the midsinus and endsinus internal carotid artery. At both locations a skewed velocity profile with slow velocities at the outer vessel wall, medium velocities at the side walls and high velocities at the flow divider (inner) wall were observed. Qualitative comparison of the axial velocity patterns revealed no significant differences between simulations and in vitro measurements. Even quantitative differences such as for axial peak flow velocities were less than 10%. Secondary flow patterns revealed some minor differences concerning the form of the vortices but maximum circumferential velocities were in the same range for both methods.

Animals↗
PubMed2000

Cardiac rotation and relaxation in patients with aortic valve stenosis.

BACKGROUND: Diastolic dysfunction with delayed relaxation and abnormal passive elastic properties has been described in patients with severe pressure overload hypertrophy. The purpose of this study was to evaluate the time course of rotational motion of the left ventricle in patients with aortic valve stenosis using myocardial tagging. METHODS: Myocardial tagging is a non-invasive method based on magnetic resonance which makes it possible to label ('tag') specific myocardial regions. From the motion of the tag's cardiac rotation, radial displacement and translational motion can be determined. In 12 controls and 13 patients with severe aortic valve stenosis systolic and diastolic wall motion was assessed in an apical and basal short axis plane. RESULTS: The normal left ventricle performs a systolic wringing motion around the ventricular long axis with clockwise rotation at the base (-4.4+/-1.6 degrees) and counter-clockwise rotation at the apex (+6.8+/-2.5 degrees) when viewed from the apex. During early diastole an untwisting motion can be observed which precedes diastolic filling. In patients with aortic valve stenosis systolic rotation is reduced at the base (-2.4+/-2.0 degrees; P<0.01) but increased at the apex (+12.0+/-6.0 degrees; P<0.05). Diastolic untwisting is delayed and prolonged with a decrease in normalized rotation velocity (-6.9+/-1.1 s(-1)) when compared to controls (-10.7+/-2.2 s(-1); P<0.001). Maximal systolic torsion is 8.0+/-2.1 degrees in controls and 14.1+/-6.4 degrees (P<0.01) in patients with aortic valve stenosis. CONCLUSIONS: Left ventricular pressure overload hypertrophy is associated with a reduction in basal and an increase in apical rotation resulting in increased torsion of the ventricle. Diastolic untwisting is delayed and prolonged. This may explain the occurrence of diastolic dysfunction in patients with severe pressure overload hypertrophy.

Adult↗
PubMed2000

Cardiac rotation and relaxation after anterolateral myocardial infarction.

BACKGROUND: Both systolic and diastolic dysfunction have been observed in patients with anterolateral myocardial infarction. Diastolic dysfunction is related to disturbances in relaxation and diastolic filling. OBJECTIVE: To analyse cardiac rotation, regional shortening and diastolic relaxation in patients with anterolateral infarction. METHODS: Cardiac rotation and relaxation in controls and patients with chronic anterolateral infarction were assessed by myocardial tagging. Myocardial tagging is based on magnetic resonance imaging and allows us to label specific myocardial regions for imaging cardiac motion (rotation, translation and radial displacement). A rectangular grid was placed on the myocardium (basal, equatorial and apical short-axis plane) of each of 18 patients with chronic anterolateral infarction and 13 controls. Cardiac rotation, change in area and shortening of circumference were determined in each case. RESULTS: The left ventricle in controls performs a systolic wringing motion with a clockwise rotation at the base and a counterclockwise rotation at the apex when viewed from the apex. During relaxation a rotational motion in the opposite direction (namely untwisting) can be observed. In patients with anterolateral infarction, there is less systolic rotation at the apex and diastolic untwisting is delayed and prolonged in comparison with controls. In the presence of a left ventricular aneurysm (n = 4) apical rotation is completely lost. There is less shortening of circumference in infarcted and remote regions. CONCLUSIONS: The wringing motion of the myocardium might be an important mechanism involved in maintaining normal cardiac function with minimal expenditure of energy. This mechanism no longer operates in patients with left ventricular aneurysms and operates significantly less than normal in those with anterolateral hypokinaesia. Diastolic untwisting is significantly delayed and prolonged in patients with anterolateral infarction, which could explain the occurrence of diastolic dysfunction in these patients.

Aged↗
PubMed1999

Alterations in the local myocardial motion pattern in patients suffering from pressure overload due to aortic stenosis.

BACKGROUND: MR tissue tagging allows the noninvasive assessment of the locally and temporally resolved motion pattern of the left ventricle. Alterations in cardiac torsion and diastolic relaxation of the left ventricle were studied in patients with aortic stenosis and were compared with those of healthy control subjects and championship rowers with physiological volume-overload hypertrophy. METHODS AND RESULTS: Twelve aortic stenosis patients, 11 healthy control subjects with normal left ventricular function, and 11 world-championship rowers were investigated for systolic and diastolic heart wall motion on a basal and an apical level of the myocardium. Systolic torsion and untwisting during diastole were examined by use of a novel tagging technique (CSPAMM) that provides access to systolic and diastolic motion data. In the healthy heart, the left ventricle performs a systolic wringing motion, with a counterclockwise rotation at the apex and a clockwise rotation at the base. Apical untwisting precedes diastolic filling. In the athlete's heart, torsion and untwisting remain unchanged compared with those of the control subjects. In aortic stenosis patients, torsion is significantly increased and diastolic apical untwisting is prolonged compared with those of control subjects or athletes. CONCLUSIONS: Torsional behavior as observed in pressure- and volume-overloaded hearts is consistent with current theoretical findings. A delayed diastolic untwisting in the pressure-overloaded hearts of the patients may contribute to a tendency toward diastolic dysfunction.

Adult↗
PubMed1999

Automatic vessel segmentation using active contours in cine phase contrast flow measurements.

The segmentation of images obtained by cine magnetic resonance (MR) phase contrast velocity mapping using manual or semi-automated methods is a time consuming and observer-dependent process that still hampers the use of flow quantification in a clinical setting. A fully automatic segmentation method based on active contour model algorithms for defining vessel boundaries has been developed. For segmentation, the phase image, in addition to the magnitude image, is used to address image distortions frequently seen in the magnitude image of disturbed flow fields. A modified definition for the active contour model is introduced to reduce the influence of missing or spurious edge information of the vessel wall. The method was evaluated on flow phantom data and on in vivo images acquired in the ascending aorta of humans. Phantom experiments resulted in an error of 0.8% in assessing the luminal area of a flow phantom equipped with an artificial heart valve. Blinded evaluation of the volume flow rates from automatic vs. manual segmentation of gradient echo (FFE) phase contrast images obtained in vivo resulted in a mean difference of -0.9 +/- 3%. The mean difference from automatic vs. manual segmentation of images acquired with a hybrid phase contrast sequence (TFEPI) within a single breath-hold was -0.9 +/- 6%.

Adult↗
PubMed1999

Real-time interactive magnetic resonance imaging with multiple coils for the assessment of left ventricular function.

Interactive real-time examination of left ventricular function in healthy volunteers both under rest and stress conditions has been performed. For this purpose, a system combining an interactive user interface, an ultrafast segmented echo-planar imaging sequence, and real-time reconstruction and display of the acquired images was designed. Magnetic resonance images were acquired at rates of up to 20 images per second with multiple receiver coils. By using a sliding window reconstruction technique, reconstruction rates of up to 60 images per second were achieved with a latency of < 100 msec. The quality of the real-time images was evaluated both qualitatively and quantitatively and was found to be appropriate for the determination of left ventricular function. It is concluded that the combination of dedicated components provides a convenient modality for the high-quality visualization of left ventricular function under rest and stress conditions at video frame rates with magnetic resonance imaging. J. Magn. Reson. Imaging 1999;10:826-832.

Echo-Planar Imaging↗
PubMed1999

Toward high-resolution myocardial tagging.

Magnetic resonance imaging with preceding tissue tagging is a robust method for assessing cardiac motion of the entire heartbeat cycle with a high degree of accuracy. One limitation of this technique, however, is the low resolution of the obtained displacement map of the labeled points within the myocardium. By a new tagging technique, which is based on the combination of two or more measurements of the same slice but with different grid positions, a highly improved resolution of cardiac motion data can be achieved. In combination with a multi-heart-phase echo-planar imaging sequence, such images with doubled grid frequency can be acquired in two short breath-hold periods.

Contrast Media↗
PubMed1999

Heart motion adapted cine phase-contrast flow measurements through the aortic valve.

A method for magnetic resonance cine velocity mapping through heart valves with adaptation of both slice offset and angulation according to the motion of the valvular plane of the heart is presented. By means of a subtractive labeling technique, basal myocardial markers are obtained and automatically extracted for quantification of heart motion at the valvular level. The captured excursion of the basal plane is used to calculate the slice offset and angulation of each required time frame for cine velocity mapping. Through-plane velocity offsets are corrected by subtracting velocities introduced by basal plane motion from the measured velocities. For evaluation of the method, flow measurements downstream from the aortic valve were performed both with and without slice adaptation in 11 healthy volunteers and in four patients with aortic regurgitation. Maximum through-plane motion at the aortic root level as calculated from the labeled markers averaged 8.9 mm in the volunteers and 6.5 mm in the patients. The left coronary root was visible in 2-4 (mean: 2.2) time frames during early diastole when imaging with a spatially fixed slice. Time frames obtained with slice adaptation did not contain the coronary roots. Motion correction increased the apparent regurgitant volume by 5.7 +/- 0.4 ml for patients with clinical aortic regurgitation, for an increase of approximately 50%. The proposed method provides flow measurements with correction for through-plane motion perpendicular to the aortic root between the valvular annulus and the coronary ostia throughout the cardiac cycle. Magn Reson Med 42:970-978, 1999.

Adult↗
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

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

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

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↗
PubMed1996

Accurate velocity mapping with FAcE.

Spin dislocation between the slice selection, phase encoding, and frequency encoding is a source of image distortions. Two strategies can be pursued to improve the appearance of moving spins in an image. Either the sequence is made equally sensitive to velocity-dependent dislocation artifacts for all spatial directions or the sensitivity is reduced with a shorter echo time. The first approach increases the dislocation for the phase-encoding direction and is therefore not useful if velocity maps with minimal distortion are the goal. FAcE (FID acquired echoes) is a sequence with separate sampling of the left and right k-space half-planes that allows for very short echo times. It was applied for velocity mapping of flow in the slice select direction. Special attention was paid to a compact design of the velocity-encoding select gradient to achieve short echo times even with high velocity sensitivity. Artifacts introduced by in-plane motion were studied for FAcE and conventional gradient-echo sequences, both in phantom experiments and simulation. FAcE allows for very short echo times with inherent motion compensation of the frequency-encoding gradient. Thus, both motion-related dislocation artifacts and signal voids due to coherence loss in regions with irregular flow are minimal.

Artifacts↗
PubMed1996

Coronary artery imaging by magnetic resonance.

Non-invasive visualization of the coronary arteries represents a major challenge in modern cardiology, but this goal may be achieved in the near future by MR angiography. Possible applications are non-invasive diagnosis of coronary artery disease, and follow-up examinations for therapy control after PTCA, in order to detect restenosis at an early stage. A multiple slice technique (2 mm slice thickness, with a spatial resolution of 1 x 1 mm, Philips Gyroscan ACS-II, 1.5 Tesla) was used. Ten volunteers were imaged and 10 patients with coronary artery disease were examined before and after PTCA. MR measurements were validated by quantitative coronary angiography. The diameters of the proximal coronary arteries as measured by both methods were compared, and a good correlation was found (r = 0.76). Thus, it is concluded that non-invasive visualization of the coronary arteries is possible before and after PTCA and allows to determine potential restenoses. However, patient cooperation is essential for good image quality. Moreover, limited spatial image resolution and breathing artifacts restrict MR coronary angiography today to be used as a routine diagnostic tool for the diagnosis of coronary artery disease.

Adult↗
PubMed1996

Quantification of blood flow patterns in human vessels by magnetic resonance imaging.

The quantitative assessment of velocity fields can help to get a better understanding of complicated flow patterns which are considered to play an important role in atherosclerotic plaque formation. Magnetic resonance imaging offers the possibility of non-invasive visualization and quantification of blood flow in many larger human arteries and veins. However, the assessment of complex blood flow patterns with conventional gradient echo flow sequences suffer from severe image artifacts. In order to overcome these problems, a partial echo signal acquisition technique which strongly reduces such artifacts may be applied. Using this technique, three dimensional velocity fields were measured in a bifurcation model, in the human carotid bifurcation and in the human aorta, right downstream of the aortic valve.

Aortic Valve↗