Biomedical subjects

M Zaitsev

Publications and source records attributed to M Zaitsev.

8 recordsLinked to original sources

PubMed2006

Prospective real-time slice-by-slice motion correction for fMRI in freely moving subjects.

Subject motion is still the major source of data quality degradation in functional magnetic resonance imaging (fMRI) studies. Established methods correct motion between successive repetitions based on the acquired imaging volumes either retrospectively or prospectively. A fast, highly accurate, and prospective real-time correction method for fMRI using external optical motion tracking has been implemented. The head position is determined by means of an optical stereoscopic tracking system. The method corrects motion during the acquisition of an fMRI time series on a slice-by-slice basis by continuously updating the imaging volume position to follow the motion of the head. This method allows the measurement of fMRI data in the presence of significant motion during the acquisition of a single volume. Even without intentional motion, fMRI signal stability is maintained and higher sensitivity to detect activation is achieved without reducing specificity. With significant motion, only the proposed approach allowed detection of brain activation. The results show that the new method is superior to image-based correction methods, which fail in the case of fast or excessive motion.

Artifacts↗

PubMed2006

Magnetic resonance imaging of freely moving objects: prospective real-time motion correction using an external optical motion tracking system.

Subject motion and associated artefacts limit the applicability of MRI and the achievable quality of the images acquired. In this paper, a fully integrated method for prospective correction of arbitrary rigid body motion employing an external motion tracking device is demonstrated for the first time. The position of the imaging volume is updated prior to every excitation of the spin system. The performance of the available tracking hardware and its connection to the MR imager is analyzed in detail. With the introduction of a novel calibration procedure the accuracy of motion correction is improved compared to previous approaches. Together with the high geometry update rate even freely moving objects can be imaged without motion related artefacts. The high performance and image quality improvement in case of subject motion are demonstrated for various imaging techniques such as gradient and spin echo, as well as echo planar imaging.

Artifacts↗

PubMed2005

Dual-contrast echo planar imaging with keyhole: application to dynamic contrast-enhanced perfusion studies.

A new EPI-based method is presented which features optimized sampling of k-space enabling the integrated acquisition of two gradient echo images. The first of these images is predominantly T1 weighted and the second is T*2 weighted. The new method combines echo sharing of sparsely acquired high spatial frequency components with the keyhole technique and half-Fourier image reconstruction. The feasibility of acquiring high spatial and temporal resolution in vivo images for perfusion mapping is demonstrated. In contrast to most current perfusion methods, which acquire the T1- and T*2-weighted images in separate acquisitions, the need for image co-registration here is obviated since both sets of images are EPI-based and are acquired within the same measurement.

Brain↗

PubMed2004

Point spread function mapping with parallel imaging techniques and high acceleration factors: fast, robust, and flexible method for echo-planar imaging distortion correction.

Echo-planar imaging (EPI) is an ultrafast magnetic resonance (MR) imaging technique prone to geometric distortions. Various correction techniques have been developed to remedy these distortions. Here improvements of the point spread function (PSF) mapping approach are presented, which enable reliable and fully automated distortion correction of echo-planar images at high field strengths. The novel method is fully compatible with EPI acquisitions using parallel imaging. The applicability of parallel imaging to further accelerate PSF acquisition is shown. The possibility of collecting PSF data sets with total acceleration factors higher than the number of coil elements is demonstrated. Additionally, a new approach to visualize and interpret distortions in the context of various imaging and reconstruction methods based on the PSF is proposed. The reliable performance of the PSF mapping technique is demonstrated on phantom and volunteer scans at field strengths of up to 4 T.

Artifacts↗

PubMed2003

Error reduction and parameter optimization of the TAPIR method for fast T1 mapping.

A methodology is presented for the reduction of both systematic and random errors in T(1) determination using TAPIR, a Look-Locker-based fast T(1) mapping technique. The relations between various sequence parameters were carefully investigated in order to develop recipes for choosing optimal sequence parameters. Theoretical predictions for the optimal flip angle were verified experimentally. Inversion pulse imperfections were identified as the main source of systematic errors in T(1) determination with TAPIR. An effective remedy is demonstrated which includes extension of the measurement protocol to include a special sequence for mapping the inversion efficiency itself.

Algorithms↗

PubMed2001

Shared k-space echo planar imaging with keyhole.

Time-dependent phenomena are of great interest, and researchers have sought to shed light on these processes with MRI, particularly in vivo. In this work, a new hybrid technique based on EPI and using the concept of keyhole imaging is presented. By sharing peripheral k-space data between images and acquiring the keyhole more frequently, it is shown that the spatial resolution of the reconstructed images can be maintained. The method affords a higher temporal resolution and is more robust against susceptibility and chemical-shift artifacts than single-shot EPI. The method, termed shared k-space echo planar imaging with keyhole (shared EPIK), has been implemented on a standard clinical scanner. Technical details, simulation results, phantom images, in vivo images, and fMRI results are presented. These results indicate that the new method is robust and may be used for dynamic MRI applications. Magn Reson Med 45:109-117, 2001.

Artifacts↗

PubMed2001

Fast T(1) mapping with volume coverage.

Four different sequences which enable high-resolution, multislice T(1) relaxation-time mapping are presented. All these sequences are based on the Look-Locker method with differences arising from the use of either a saturation-recovery or inversion-recovery module prior to data acquisition with a full k-space or banded k-space acquisition scheme. The methods were implemented on a standard clinical scanner and the accuracy of the T(1) results was evaluated against spectroscopic measurements. The accuracy of the T(1) maps validated by phantom imaging measurements is around 1% for species which relax with T(1) times that mimic gray/white matter (T(1) < or = 1000 ms). Additionally, the inherent multislice, multipoint capability of the methods is demonstrated. Finally, in vivo results of the human brain obtained using the faster method are presented. The fastest data acquisition was achieved with a saturation-recovery, banded k-space method where k-space was divided into three segments; an overall acquisition time of around 5 min (for species with T(1) < or = 1 sec) was achieved for a T(1) map which can, in principle, provide whole-brain coverage with a matrix size of 256 x 256 at multiple time-points. Magn Reson Med 46:131-140, 2001.

Brain↗

PubMed2001

A new method for fast multislice T(1) mapping.

A sequence for T(1) relaxation-time mapping which enables high-resolution, multislice imaging in short acquisition times is presented. The sequence is based on the Look-Locker method and employs a magnetization-preparation module prior to data acquisition with a banded k-space data collection scheme. The method was implemented on a standard clinical scanner and the accuracy of the T(1) results was evaluated against spectroscopic measurements. The accuracy of the T(1) maps validated by phantom imaging measurements is <3% for slow-relaxing compartments (T(1) congruent with 2000 ms) and is around 1% for faster-relaxing species (T(1) < or = 1200 ms). Additionally, the inherent multislice, multipoint capability of the method is demonstrated. Multislice, multipoint in vivo results of the human brain obtained using this method are presented. An acquisition time of approximately 8 min was achieved for a T(1) map, which, in principle, can provide whole-brain coverage with 25 slices, a matrix size of 256 x 256, and 12 time points. The speed of the sequence is derived through optimized interleaving of slices and time points, together with the acquisition of multiple echoes, which are used to fill a 3-segment k-space.

Brain↗