Biomedical subjects

Martyn N J Paley

Publications and source records attributed to Martyn N J Paley.

7 recordsLinked to original sources

PubMed2003

B1AC-MAMBA: B1 array combined with multiple-acquisition micro B0 array parallel magnetic resonance imaging.

The combination of an in-plane B(1) sensitivity encoding (SENSE) technique with a simultaneous multiple-slice B(0) field step technique (multiple-acquisition micro B(0) array (MAMBA)) has produced high scan time reduction factors (R < or = 8). In this study, two slices were acquired simultaneously in combination with x2 and x4 SENSE in-plane encoding using a MAMBA stepped B(0) field coil inside a four-channel phased-array coil system. Experiments were performed on a 1.5 T Infinion system (Philips Medical Systems, Cleveland, OH). The signal-to-noise ratio (SNR) was reduced with higher R factors, as was expected from the reduced number of acquisitions used to create the unaliased images. The combination of SENSE and MAMBA offers great promise for reducing scan times through parallel acquisition while at the same time reducing the number of RF channels required by a factor equal to the number of field steps employed. The B(1) array combined with MAMBA (B(1)AC-MAMBA) technique is applicable when the length of an object is much greater than its diameter, as in scanning limbs or in whole-body screening for disease.

Artifacts↗

PubMed2003

Dynamic radial projection MRI of inhaled hyperpolarized 3He gas.

A radial projection sliding-window sequence has been developed for imaging the rapid flow of (3)He gas in human lungs. The short echo time (TE) of the radial sequence lends itself to fast repetition times, and thus allows a rapid update in the image when it is reconstructed with a sliding window. Oversampling in the radial direction combined with angular undersampling can further reduce the time needed to acquire a complete image data set, without significantly compromising spatial resolution. Controlled flow phantom experiments using hyperpolarized (3)He gas exemplify the temporal resolution of the method. In vivo studies on three healthy volunteers, one patient with chronic obstructive pulmonary disease (COPD), and one patient with hemiparalysis of the right diaphragm demonstrate that it is possible to accurately resolve the passage of gas down the trachea and bronchi and into the peripheral lung.

Fourier Analysis↗

PubMed2003

Assessment and compensation of susceptibility artifacts in gradient echo MRI of hyperpolarized 3He gas.

The effects of macroscopic background field gradients upon 2D gradient echo images of inhaled (3)He in the human lung were investigated at 1.5 T. Effective compensation of in-slice signal loss in (3)He gradient echo images was then demonstrated using a multiple acquisition interleaved single gradient echo sequence. This method restores signal dephasing through a combination of separate images acquired with different slice refocusing gradients. In vivo imaging of volunteers with the sequence shows substantial restoration of signal at the lung periphery and close to blood vessels. The technique presented may be useful when using (3)He MRI for volumetric measurements of lung ventilation and in studies using (3)He combined with intravenous contrast as a means of assessing lung ventilation/perfusion (V/Q).

Artifacts↗

PubMed2003

Mapping of periodic waveforms using the ghost reconstructed alternating current estimation (GRACE) magnetic resonance imaging technique.

A new method of estimating alternating currents using ghost images created when the magnetic field from a fluctuating current modulates the phase of the magnetic resonance (MR) signal between successive phase-encode views is described. The method, known as ghost reconstructed alternating current estimation (GRACE), may be useful for directly mapping fields, and hence current impulses produced by neuronal firing events when synchronized periodic modulation can be induced. Images were acquired on a 1.5 T MR system with small oil capsule phantoms and a single wire with an applied alternating current, placed perpendicular to the main field direction. Computer simulations of these experiments yielded ghost images that agreed with experimental results. A simulated ghost image resulting from an evoked neuronal waveform is also discussed. Weak magnetic fields were detected from both sinusoidal and square wave modulations.

Artifacts↗

PubMed2003

Postmortem MR imaging of the fetal and stillborn central nervous system.

BACKGROUND AND PURPOSE: Changes in the public perception of postmortem procedures in the United Kingdom have led to reduced numbers of autopsies being performed in the fetus and neonate. When autopsy is performed in this group, the brain is now usually studied without being formalin-fixed, which limits the available information. We evaluated the diagnostic accuracy of postmortem MR imaging of the fetal brain and spine when compared with the reference standard, autopsy. METHODS: We obtained high-spatial-resolution T2-weighted images (in-plane resolution approximately 0.4 mm) of the brain and spine in 40 fetuses and stillborn neonates (14-42 weeks gestational age) who were referred for autopsy. The MR findings were compared with those of autopsy, the reference standard, which had been performed independently. RESULTS: In eight cases, the autopsy did not provide structural information of the brain or spine, because assessment of the unfixed tissue was impossible. There was agreement between MR and autopsy findings in 31 (97%) of 32 cases in which comparison could be made. Eleven cases showed normal brain, and 20 cases showed a wide range of developmental and acquired abnormalities. The sensitivity of MR was 100%, specificity 92%, positive predictive value 95%, and negative predictive value 100%. CONCLUSION: MR imaging has a useful role in providing structural information of the central nervous system in fetuses and stillborn neonates.

Autopsy↗

PubMed2002

k-space filtering in 2D gradient-echo breath-hold hyperpolarized 3He MRI: spatial resolution and signal-to-noise ratio considerations.

In this work some of the factors that can influence the signal-to-noise ratio (SNR) and spatial resolution in MR images of inhaled hyperpolarized gases are systematically addressed. In particular, the effects of RF depletion of longitudinal polarization and image gradient diffusion dephasing were assessed in terms of their contribution to a k-space filter. By means of theoretical simulations and a novel method of experimental validation using a variable transverse magnetization of the 1H signal, systematic quantitative and qualitative investigations of the effects of k-space filtering intrinsic to imaging of hyperpolarized gas were made. A 2D gradient-echo image is considered for a range of flip angles with centric, sequential, and half-Fourier Cartesian phase-encoding strategies, and the results are assessed in terms of SNR and spatial resolution in the reconstructed images. Centric phase encoding was found to give the best SNR at higher flip angles, with a trade-off in spatial resolution compared to sequential phase encoding. A half-Fourier approach potentially offers increased SNR through the use of higher flip angles without compromising the spatial resolution, which is comparable to that achieved with sequential encoding.

Fourier Analysis↗

PubMed2002

Interleaved pulsed MAMBA: a new parallel slice imaging method.

A method of acquiring slices in parallel is described which uses interleaved sets of pulsed B(0) field coils to generate discrete regions of uniform field within the main magnetic field known as interleaved MAMBA (multiple acquisition micro B(0) array). Simulations of a number of coil designs were performed using the Biot-Savart law. A six-step coil was built and interfaced to a 0.17 T Niche MRI system and the field steps measured using an imaging technique. Measured field steps were in good agreement with the values predicted by simulation. The coil design was then scaled up by a factor of three, interfaced to a 1.5 T whole-body MRI system, and scans of the hands and arms of volunteers were acquired from up to four field steps using standard spin and gradient echo sequences. Images were also acquired simultaneously from two field steps with no frequency encode aliasing and one excitation. The one-dimensional interleaved pulsed MAMBA step field technique shows great promise for enabling many slices to be acquired simultaneously along the axis of the coil for rapid volumetric studies without the need for multiple shot Hadamard encoding. Extension of interleaved coil design to two or three dimensions is feasible, which could provide full spatial coverage combined with ultra-rapid data acquisition.

Echo-Planar Imaging↗