Biomedical subjects

F Hennel

Publications and source records attributed to F Hennel.

14 recordsLinked to original sources

PubMed2001

Fast spin echo and fast gradient echo MRI with low acoustic noise.

It has been shown previously that a significant reduction of the acoustic noise in standard, slow magnetic resonance imaging (MRI) sequences is achieved when the gradient pulses have long sinusoidal ramps. An improvement of this method is now presented for fast gradient echo (FLASH) and fast spin echo (RARE) sequences. The new strategy consists of using a sinusoidal readout pulse with no plateau and extending the phase encoding pulses to the entire readout period. The distribution of k-space samples is no longer equidistant, and images have to be reconstructed with gridding. A reduction of the acoustic noise by 20-40 dBA was obtained with respect to standard sequences installed by the scanner manufacturer, which is 10-20 dBA better than that with the previously proposed method of sinusoidal ramps. J. Magn. Reson. Imaging 2001;13:960-966.

Echo-Planar Imaging↗

PubMed2001

Determination of skeletal muscle perfusion using arterial spin labeling NMRI: validation by comparison with venous occlusion plethysmography.

T(1)-based determination of perfusion was performed with the high temporal and spatial resolution that monitoring of exercise physiology requires. As no data were available on the validation of this approach in human muscles, T(1)-based NMRI of perfusion was compared to standard strain-gauge venous occlusion plethysmography performed simultaneously within a 4 T magnet. Two different situations were investigated in 21 healthy young volunteers: 1) a 5-min ischemia of the leg, or 2) a 2-3 min ischemic exercise consisting of a plantar flexion on an amagnetic ergometer. Leg perfusion was monitored over 5-15 min of the recovery phase, after the air-cuff arterial occlusion had been released. The interesting features of the sequence were the use of a saturation-recovery module for the introduction of a T(1) modulation and of single-shot spin echo for imaging. Spatial resolution was 1.7 x 2.0 mm and temporal resolution was 2 s. For data analysis, ROIs were traced on different muscles and perfusion was calculated from the differences in muscle signal intensity in successive images. To allow comparison with the global measurement of perfusion by plethysmography, the T(1)-based NMR measurements in exercising muscles were rescaled to the leg cross-section. The perfusion measurements obtained by plethysmography and NMRI were in close agreement with a correlation coefficient between 0.87 and 0.92. This indicates that pulsed arterial techniques provide determination of muscle perfusion not only with superior spatial and temporal resolution but also with exactitude.

Adult↗

PubMed2001

Silent BOLD imaging.

Pulsed magnetic field gradients in magnetic resonance imaging produce high levels of acoustic noise. In functional magnetic resonance imaging, acoustic scanner noise causes unwanted masking effects. Recently, we proposed a method to perform magnetic resonance imaging experiments undisturbed by acoustic scanner noise by utilizing the property of standard gradient coils to poorly submit acoustic noise in the low frequency range. The silent gradient scheme is now incorporated into a standard T(2)*-weighted sequence. Additionally, simultaneous multi-slice excitation (SIMEX) pulses were implemented to improve the intrinsic low volume coverage of the silent sequence. The proposed silent SIMEX technique was tested and compared with a standard noisy technique using auditory and visual stimulation paradigms. The scanner noise during the silent experiments could be reduced below the range of the ambient noise of the magnet room. This feasibility study shows a trend of decreased activated areas in the noisy experiment for both, the visual and auditory paradigm.

Brain↗

PubMed2001

Quiet imaging with interleaved spiral read-out.

The acoustic noise generated during an MRI sequence can be effectively reduced with the help of soft gradient pulses using sinusoidal ramps. The long slope duration, however, leads to long acquisition times. The use of interleaved spiral trajectories, calculated with long gradient slopes, is proposed to reduce the acquisition time while maintaining low acoustic noise levels. The practicality of this approach is demonstrated on phantom and volunteer images.

Acoustics↗

PubMed2000

Anatomic MR images obtained with silent sequences.

The authors evaluated silent magnetic resonance (MR) imaging sequences for their suitability in providing high-spatial-resolution anatomic images that are of sufficient quality to be useful in a clinical setting. The authors compared the images obtained with a silent rapid acquisition with relaxation enhancement (RARE) sequence to its standard counterpart with respect to signal-to-noise ratio, distribution of gray level, and spatial resolution. No real differences were observed between the standard and the silent RARE MR images. Anatomic images were also acquired with a silent spin-echo sequence. Acoustic noise levels with the silent sequences were at least 22 dB (A-weighted scale) lower than those with standard sequences, without loss of image quality.

Adult↗

PubMed1999

"Silent" MRI with soft gradient pulses.

A method to reduce the acoustic noise generated by gradient systems in magnetic resonance imaging (MRI) is proposed based on the linear response theory. Since the acoustic frequency response function of typical gradient coils is low in the range below 200 Hz, the noise level can be significantly reduced by using gradient pulse sequences whose spectra are limited to this frequency range. Such "soft," i.e., band-limited, pulse shapes can be designed using sinusoidal ramps individually adjusted to available delays. "Silent" versions of three basic MRI sequences [gradient-echo (GE), spin-echo (SE), and rapid acquisition with relaxation enhancement (RARE)] were programmed on 2 and 3 T whole-body scanners. High-quality images could be acquired at noise levels as low as 40 dBA (GE and SE) and 60 dBA (RARE).

Brain↗

PubMed1999

Two-dimensional deghosting for EPI.

A residual ghost artefact in echo-planar imaging (EPI) remains after the standard correction procedure based on a 1-dimensional phase-modulation of the spectra of even or odd echoes. A better reduction of this artefact is demonstrated using a 2-dimensional phase correction. The phase correction map is measured inside the ghost-free region of the image, preferably in a reference scan with an increased field of view, as the phase difference between complex images reconstructed separately from even and odd echoes. An extrapolated map consisting of spatial components up to the second order (constant, x, y, xy and x2-y2) is then found by a fit to the measured values. This map is used to correct the phase of even- and odd-reconstructed images before adding them. This procedure may cause some spatially dependent loss of signal, but if the level of the residual artefact is less than 20% of the image intensity, such losses are negligible.

Artifacts↗

PubMed1998

SPARE: A robust method for magnetic resonance imaging in inhomogeneous fields.

An imaging sequence based on a spin-echo train has been developed which is free from geometric distortions in the imaging plane due to main field inhomogeneity. Such inhomogeneities, and chemical shifts, cause only a displacement in the selected slice, which is minimized by the use of high gradient strengths and short radiofrequency pulses. Additionally, variations in the radiofrequency field strength cause variations in the image amplitude but cause no other artifacts. This allows the use of low-flip-angle refocusing pulses, reducing the power deposition to levels which are safe in vivo at high field strengths. The sequence was implemented on a Bruker whole-body 3T system. Example images from a perfluorocarbon phantom and a human head are presented.

Brain↗

PubMed1998

Image-based reduction of artifacts in multishot echo-planar imaging.

The method to reduce the ghost artifact in echo-planar imaging (EPI) using a phase correction derived from the image data (M. H. Buonocore and L. Gao, Magn. Reson. Med. 38, 89 (1997)) is generalized to multishot (interleaved) EPI, where the artifact takes the form of multiple ghosts. The method is shown to be much more sensitive to noise when applied to standard interleaved data than is the case with single-shot EPI, because the calculation must be based on high-order ghosts of low intensity. A modified interleaving scheme is proposed for multishot EPI in which the initial trajectory direction alternates in consecutive shots and the number of shots is odd. With this scheme, only a single ghost shifted by one-half of the field of view appears just as in the single-shot EPI, and the image-based phase correction can be applied with the usual sensitivity to noise.

Artifacts↗

PubMed1998

Data-driven curvilinear reconstructions of 3D MR images: application to cryptogenic extratemporal epilepsy.

This paper presents a data-driven method for the reconstruction and visualisation of curvilinear slices from three-dimensional (3D) magnetic resonance (MR) scans of the head. Visualisation of curvilinear slices, rather than standard planar slices, produces symmetrical views of the cortex and allows small abnormalities to be detected by comparing the two hemispheres of the brain. In our method, the surface defined by the upper half of the brain is used as a reference shape for curvilinear reconstructions. The brain is first segmented from the 3D scan using a 3D region growing method associated to an unsupervised threshold selection technique. The upper half of the segmented brain is then extracted and fitted by a deformable surface model. This surface is finally interactively moved by the operator in the 3D scan, to visualise the desired curvilinear slice, which is projected on the screen as a two-dimensional image. We show an application of this visualisation technique to the localisation of cerebral epileptogenic lesions. The procedure has proven efficient and handy in clinical use.

Algorithms↗

PubMed1996

Multislice interleaved excitation cycles (MUSIC): an efficient gradient-echo technique for functional MRI.

A method providing improved slice efficiency for gradient-echo imaging requiring long echo times, such as in functional neuromagnetic resonance imaging, is presented. To enhance the volume coverage while maintaining the short imaging time of a conventional single-slice gradient-echo technique, an interleaved multislice excitation is performed during the echo time. This technique allows detection of susceptibility changes, e.g., the acquisition of stimulated human cortical activation maps, on clinical MR instruments at multiple planes within total imaging times of a few seconds. The efficiency of the technique is demonstrated in the detection of temporary changes in T2* in functional MRI experiments of the human visual cortex at magnetic field strengths of 2 Tesla and 3 Tesla. Fourteen 128 x 128 slices can be acquired in 13 s to cover a large volume-of-interest in the same time that would be required for single-slice acquisition using the conventional technique.

Adult↗

PubMed1995

Interleaved asymmetric echo-planar imaging.

A version of interleaved echo-planar imaging (EPI) is presented in which only one polarity of the readout gradient is used for signal acquisition to avoid ghosting artifacts. Two possible forms of the phase encoding gradient, blipped and constant, are discussed. With the constant phase encoding, interleaving of partial trajectories in the Fourier domain (k-space) is controlled automatically by the echo train delay. The constant phase encoding gradient introduces a shear distortion of the k-space grid. A modification of the reconstruction procedure is given which corrects for this effect. The method provides a 128 x 128 image in 1 s on a clinical system with standard gradients.

Adipose Tissue↗

PubMed1992

Modification of the Carr-Purcell sequence for single-shot echo-planar imaging.

An artifact caused by the RF field inhomogeneity in echo-planar imaging (EPI) with the CPMG sequence is analyzed. A modified sequence is presented, using 90 degrees phase alternation of the pi pulses, which suppresses the spurious components of the signal. Thus the phase cycling procedure can be avoided and the pi-pulsed EPI experiment can be reduced to a single shot.

Artifacts↗

PubMed1990

Double EPI sequence with 180 degrees RF pulses.

Rapid imaging can be performed in the magnetic field of low homogeneity using 180 degrees RF pulses instead of gradient reversals to form a series of differently phase-encoded echoes. A modified version of pi-pulsed EPI which samples the data in two complementary shots was designed. Its advantage for objects of short T2 is illustrated by phantom images.

Fourier Analysis↗