Increased radio-frequency power absorption in human tissue due to coupling between body coil and surface coil.
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Body Temperature↗
Biomedical subjects
P Boesiger
Publications and source records attributed to P Boesiger.
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↗
Glycogen detection by in vivo 13C NMR: a comparison of proton decoupling and polarization transfer.
The performance of gated proton decoupling and polarization transfer with respect to glycogen detection by 13C NMR was investigated. Experiments were performed on a 1.5-T whole-body scanner using a 13C surface coil in combination with a proton head coil. Spectra were acquired from a glycogen phantom and from the lower leg of a healthy volunteer using proton decoupling and the polarization transfer method SINEPT. The signal strength of the C1 resonance of glycogen was determined and compared to a reference spectrum acquired without any form of sensitivity enhancement. In the phantom experiment both decoupling and SINEPT produced a signal gain of 3.5. Under in vivo conditions, the signal gain was approximately 2.5 for both techniques. We conclude that decoupling and polarization transfer are equivalently useful techniques for glycogen detection.
Glycogen↗
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.
Aorta, Abdominal↗
Evaluation of left ventricular segmental wall motion in hypertrophic cardiomyopathy with myocardial tagging.
BACKGROUND: Segmental wall motion was assessed noninvasively in eight patients with hypertrophic cardiomyopathy and six healthy volunteers by magnetic resonance myocardial tagging. METHODS AND RESULTS: Localization scans were performed for determination of the true short-axis views of the left ventricle (double-angulated view). Spatial modulation of magnetization was used to produce a rectangular grid of landmarks. Distortion of the grid was assessed at end diastole, mid systole, and end systole with multiphase gradient echoes. Image sets were acquired at three different planes, namely, the base, the equator, and the apex. Quantitative evaluation was carried out by computer-assisted image analysis. Each individual grid crossing point was identified automatically and the displacement calculated. A polar coordinate system with the center of gravity as motion reference point was chosen to assess fractional rotation and radial displacement at the endocardial, midwall, and epicardial layers of the septal, anterior, posterior, and inferior regions. A wringing motion of the left ventricle with a clockwise rotation of 5.0 +/- 2.4 degrees at the base and a counterclockwise rotation of -9.6 +/- 2.9 degrees at the apex was observed in control subjects. An equal rotation of 5.0 +/- 2.5 degrees at the base and a slightly reduced rotation of -7.3 +/- 5.2 degrees at the apex was found in patients with hypertrophic cardiomyopathy. A transmural gradient in fractional rotation and radial displacement was observed, with the highest values in the endocardial layer. Rotation in patients with hypertrophic cardiomyopathy was significantly less than in normal volunteers in the posterior region of the equatorial and apical planes. Furthermore, radial displacement was significantly reduced in the septum and inferior wall. In the anterior and posterior wall segments, a reduction of the radial displacement was observed only in the epicardium and midwall layers. CONCLUSIONS: Magnetic resonance myocardial tagging allows the noninvasive assessment of regional wall motion. Both in normal volunteers and in patients with hypertrophic cardiomyopathies, cardiac motion occurs in a complex mode, with the base and the apex rotating in opposite directions and the equatorial plane as a transitional zone (wringing motion). A reduced cardiac rotation can be observed in patients with hypertrophic cardiomyopathy mainly in the posterior region, whereas a reduced radial displacement is found in the inferior septal zone.
Adult↗
Toward fully automatic estimation of in vivo 31P spectra.
Fitting a model to an experimental spectrum is a difficult nonlinear estimation problem. The solution presented here is to start an iterative search procedure sufficiently close to the optimal model parameter set. This is achieved by providing tissue-dependent a priori peak information and by a novel correlation method to get good primary estimates of the resonance and phase offset parameters. The resulting estimation procedure is fully automatic and has proven to be robust for 31P data.
Brain↗
Left ventricular wall thickness measurements by magnetic resonance: a validation study.
Left ventricular (LV) wall thickness was determined by magnetic resonance (MR) in 15 patients (7 controls and 8 patients with coronary artery disease). End-diastolic (ed) and end-systolic (es) wall thickness were measured in a short axis view perpendicular to the LV long axis. Wall thickness measurements were compared to data obtained by digital subtraction angiography (DSA) and M-mode echocardiography (Echo). End-diastolic and end-systolic wall thickness were significantly overestimated by MR (34% and 37%, respectively) when compared to DSA. In contrast, LV end-diastolic and end-systolic chamber diameter were significantly underestimated by MR (25% and 30%, respectively) when compared to DSA. However, fractioned shortening was similar (all NS) for MR (48 +/- 22%), DSA (54 +/- 15%) and Echo (44 +/- 10%), respectively. The mean difference (= accuracy) and the standard deviation of difference (= precision) for LV wall thickness was 0.4 +/- 0.2 cm between MR and DSA, 0.4 +/- 0.3 cm between MR and ECHO and 0.03 +/- 0.1 cm between DSA and ECHO. The correlation of wall thickness between MR and DSA (correlation coefficient r = 0.74, p less than 0.001) and between MR and Echo (r = 0.70, p less than 0.001) was good although the standard error of estimate (SEE) was 17% for MR vs. DSA and 21% for MR vs. Echo. The corresponding SEE for chamber diameter was 16% between MR and DSA and 19% between MR and Echo, respectively. Intraobserver variability for wall thickness determination by MR was excellent (correlation coefficient r = 0.99, p less than 0.001) SEE of 4%. Interobserver variability was also good (correlation coefficient r = 0.90, p less than 0.001) with a SEE of 12%. It is concluded that LV wall thickness and chamber diameter (short axis plane) can be determined by MR with good precision but only satisfactory accuracy. LV wall thickness is significantly overestimated probably due to signals from static blood which might be indistinguishable from the subendocardium.
Angiography, Digital Subtraction↗
FID-acquired-echos (FAcE): a short echo time imaging method for flow artefact suppression.
The FID-Acquired-Echo sequence (FAcE) is a magnetic resonance imaging technique using fractional-echo acquisitions, with sequential separate sampling of the right and left k-space half planes. It reduces the minimal echo times by about a factor of two, compared to conventional full-(gradient)-echo sampling schemes. With this sequence, implemented on a commercial 1.5 Tesla whole body system, high resolution images are acquired with typical echo times between 3 and 4.5 msec. Using short echo times the signal dephasing caused by velocity and higher order spin motion is reduced. Further, due to the modified sampling scheme, the sequence exhibits, for triggered studies, partially a compensation of motion-induced phase shifts in the frequency-encoding direction. Thus, the sequence offers an alternative means for the reduction of motion-induced image artefacts to the use of flow compensating gradients, which usually makes a sequence more sensitive to higher order motion and introduces further eddy currents. Besides potential application for imaging of nuclei and tissues with short T2 relaxation times, and non-ECG-triggered in-flow angiography, the main application seems to be triggered-phase contrast imaging with focus on quantitation of blood flow. Its usefulness is largest in cases with irregular flow patterns, where considerable in-plane flow occurs.
Algorithms↗
[In vivo 31P-cardiac magnetic resonance spectroscopy: methods and the first clinical results].
31P-magnetic resonance (MR) spectra of the heart can be obtained from well-defined myocardial regions by combined MR imaging and variable selected volumes for spectroscopy. 31P-spectra of 33 volunteers and of 43 patients with dilated and hypertrophic cardiomyopathy and with coronary artery disease were quantified using a curve-fitting routine. To optimize our technique, we recorded unsaturated and partially saturated spectra in several volunteers. Relative peak areas and signal-to-noise ratios showed significant changes with varying pulse repetition times. Saturation factors were applied to correct spectra from volunteers and patients for the effects of partial saturation. Under resting conditions, peak areas of volunteers and patients from the various groups were statistically indistinct.
Adult↗
Spectral baseline correction using CLEAN.
Baseline distortion in NMR spectroscopy, caused by the "dead time" between signal excitation and detection, makes quantitative interpretation difficult and is aesthetically displeasing. Here the use of the CLEAN algorithm for deconvolving the effect of signal dead time to produce a distortionless baseline is discussed. Unlike other nonlinear spectral estimation techniques, CLEAN is easy to program, easy to use, quite robust, and fast.
Algorithms↗
Tissue characterization of brain tumors during and after pion radiation therapy.
Negative Pi-mesons (pions) are applied at the Paul Scherrer Institute in the radiotherapy of highly malignant gliomas using a dose escalation program. The therapy effects of 7 randomly selected patients were followed up by 62 MRI examinations. The quantification of the effects is based on the relaxation times T1 and T2, which are acquired by a new designed multi-echo multiple saturation recovery imaging technique. As a summary of the results, roughly two reaction types are observed. For both types the relaxation times increase up to two to three months after the radiation therapy. Then in one type (two patients) the T1 and T2 values of the tumors, and of the edemas surrounding the tumors, further increase, indicating an unfavorable prognosis. In the other type (five patients) the relaxation times drop down towards, or even below, their initial values, reflecting the onset of the reparation processes in the tissue. This later behaviour reflects an at least temporary control of the disease; that is, the short term prognosis for these patients is more favorable. It further can be concluded, with respect to our MR parameters, that the radiotolerance of healthy brain tissue is much higher than that of malignant glioma tissue, despite the fact that these tumors are very seldom definitively radiosensible.
Astrocytoma↗
Increased rf power absorption in MR imaging due to rf coupling between body coil and surface coil.
Fears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rf field leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive-only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst-case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst-case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coupled to the transmitter coil by its geometric position. Theoretical calculations, in agreement with the experimental results achieved during a 15-min measurement in a 1.5-T MRI whole-body imager, show that a significant rf power deposition in the tissue underneath the coil wire occurs, leading typically to a local specific absorption rate of 24 W/kg and a local temperature rise of 5.2 degrees C.
Electric Conductivity↗
Velocity profiles in the normal human abdominal aorta: a comparison between ultrasound and magnetic resonance data.
Evolving magnetic resonance (MR) procedures were utilized to validate one-dimensional ultrasonic (US) Doppler profiles in vivo on the basis of this alternative noninvasive method of assessing blood velocity. Corresponding velocity profiles were acquired by both US and MR in the abdominal aorta of 10 healthy volunteers. The ultrasound velocities recorded throughout a cardiac cycle along the anterio-posterior aortic diameter were compared to their spatial and temporal MR counterparts. Correlation coefficients ranging from 0.92 to 0.97 and regression slopes from 0.86 to 1.13 indicate a high degree of correspondence between the two modalities and increase the confidence in the fidelity of velocity profiles obtained with both procedures.
Adult↗
Human abdominal aorta: comparative measurements of blood flow with MR imaging and multigated Doppler US.
Magnetic resonance (MR) imaging has proved to be a new alternative method for the noninvasive detection and quantification of blood flow in human vessels. By means of standard gradient echo sequences triggered with electrocardiography on a 1.5-T whole-body imaging system, the authors measured the flow-induced phase shift in the abdominal aorta of healthy volunteers. The instantaneous two-dimensional velocity profiles and the integrated flow rate were determined in intervals down to 21 msec throughout the cardiac cycle. The results were validated by means of comparative measurements with a multigated Doppler ultrasound instrument. The velocity values acquired with this instrument in one spatial dimension in the anteroposterior direction of the abdominal aorta agreed to a great extent with the temporal and spatial corresponding values recorded with MR imaging. The same high correlation between the two methods was found for the calculated instantaneous total blood flow.
Adult↗
Heating effects of metallic implants by MRI examinations.
Magnetic radiofrequency fields applied in magnetic resonance imaging examinations induce electrical currents in metallic implants. These eddy currents may heat up the implants and thus may be capable of causing localized tissue heating. The rf power deposition and the joule heating of the implant can be calculated by solving Maxwell's equations for the specific problem. First, extreme in vitro worst-case experiments were performed with a large and very thin aluminum sheet, which was placed in a 1.5-T MRI device in a position parallel to the magnetic rf field. In agreement with the theoretical results the temperature rise of a thermally insulated sheet amounted to only 0.08 degrees C after a 15-min MRI examination at 64 MHz. No temperature rise in the aluminum sheet could be measured for a sheet immersed in a saline solution. Second, in vitro experiments with a hip joint prosthesis and an osteosynthetic plate were performed to confirm the theoretical results, which predict nearly no temperature rise in the metallic implants. No temperature rise in the implants could be measured.
Aluminum↗
A one-shot lactate-editing sequence for localized whole-body spectroscopy.
A pulse sequence is presented for performing localized lactate-edited proton spectroscopy in the whole-body environment. The sequence is optimized for relatively low field strength (1.5-T) clinical measurements. Characteristics of this method of lactate editing, which makes it more suitable for clinical applications than previous methods, include the following: The "one shot" nature of the method makes it relatively insensitive to patient motion; water suppression is high as all 90 degrees pulses are binomial pulses; and very narrow band rf pulses, such as used in high-field lactate-editing sequences, but which are necessarily very long at low fields, are not required. Further, this lactate editing scheme can be very easily combined with localized spectroscopic measurements.
Humans↗
Acquisition and evaluation of tagged magnetic resonance images of the human left ventricle.
Magnetic resonance myocardial tagging was used to noninvasively analyze the complicated contraction pattern of the human cardiac left ventricle. The tagging and imaging sequence was optimized to obtain three to four double-angulated short-axis views during systole. The image contrast between labeled and unlabeled tissue was sufficient to apply a semiautomatic image evaluation procedure. In accordance with the invasively achieved findings of other groups, the measurements indicate a wringing motion of the left ventricle, with a clockwise twist at the heartbase and a contrary rotation at the apical level.
Cardiomyopathy, Hypertrophic↗