Markus B. Scheidegger

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.

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 athletes 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.

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.

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.

Force and Torque Effects of a 1.5‐Tesla MRI Scanner on Cardiac Pacemakers and ICDs

LUECHINGER, R., et al.: 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 (≤ 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.

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 (∼10 min), the requirement of repeated breath-holds using a coached breathing pattern, and the enhanced sensitivity of 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.

Cardiac rotation and relaxation after anterolateral myocardial infarction

BackgroundBoth systolic and diastolic dysfunction have been observed in patients with anterolateral myocardial infarction. Diastolic dysfunction is related to disturbances in relaxation and diastolic filling. ObjectiveTo analyse cardiac rotation, regional shortening and diastolic relaxation in patients with anterolateral infarction. MethodsCardiac 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. ResultsThe 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. ConclusionsThe 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.

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%.J. Magn. Reson. Imaging 1999;10:41–51.

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?

LUECHINGER, R., et al.: 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.

Visualization and quantification of the human blood flow by magnetic resonance imaging

Magnetic Resonance Imaging (MRI) offers new possibilities for the visualization and the noninvasive quantification of the blood flow in human vessels. By the application of conventional gradient echo sequences with electrocardiographic gating on a 1.5 Tesla whole body MRI system the flow induced phase shifts in the ascending and the abdominal aorta are analyzed. The instantaneous two-dimensional velocity profiles and the instantaneous flow rates are determined in a series of subsequent images with high temporal resolution throughout the cardiac cycle. For the flow analysis in further vessels and for the analysis of more complex flow patterns, as they occur in bifurcations or stenoses, a new MR flow imaging technique called FAcE with extremely short echo times is introduced and the first results of flow examinations in a bifurcation phantom and in the carotid artery are presented.