Paul Glover

Multiple spin echoes in liquids in a high magnetic field

We have observed multiple spin echoes using an NMR microscopy system which has a field of 11.7 T. Our experimental results show that the cause of the MSE is the dipolar demagnetizing field. This is also the origin of MSE seen in solid 3 He and in liquid 3 He at low temperatures

Improvements in snap-shot nuclear magnetic resonance imaging

New variants of the ultra-high-speed echo-planar imaging technique have been used to obtain snap-shot images of adult patients and volunteers at 0.1 T. Modified pulsed-gradient sequences together with non-linear signal sampling and activity screened gradients have greatly improved the image quality obtainable by single-shot methods. A particular variant, modulus blipped echo-planar single-pulse technique (MBEST), although slightly slower than the blipped echo-planar single-pulse technique (BEST), is experimentally more robust and incorporates intrinsic T2 weighting. An account of these improvements together with some experimental results is presented.

Limits to magnetic resonance microscopy

The last quarter of the twentieth century saw the development of magnetic resonance imaging (MRI) grow from a laboratory demonstration to a multi-billion dollar worldwide industry. There is a clinical body scanner in almost every hospital of the developed nations. The field of magnetic resonance microscopy (MRM), after mostly being abandoned by researchers in the first decade of MRI, has become an established branch of the science. This paper reviews the development of MRM over the last decade with an emphasis on the current state of the art. The fundamental principles of imaging and signal detection are examined to determine the physical principles which limit the available resolution. The limits are discussed with reference to liquid, solid and gas phase microscopy. In each area, the novel approaches employed by researchers to push back the limits of resolution are discussed. Although the limits to resolution are well known, the developments and applications of MRM have not reached their limit.

An assessment of the intrauterine sound intensity level during obstetric echo-planar magnetic resonance imaging

In order to assess the sound level experienced by the fetal ear during obstetric magnetic resonance imaging, a fluid filled stomach was used as an experimental model of the gravid uterus. A better than 30 dB attenuation in intensity was recorded across the frequency band of interest for all patient orientations. This was enough to reduce acoustic sound pressure from a level close to the instantaneous damage threshold (120 dB), to an acceptable level (< 90 dB). Direct mechanical coupling through the patient table was also shown to increase uterine sound pressure levels by as much as 10 dB. Much higher peak pressures could be obtained by tapping of abdomen with the fingers.

Tailored RF Pulse for Magnetization Inversion at Ultrahigh Field

The radiofrequency (RF) transmit field is severely inhomogeneous at ultrahigh field due to both RF penetration and RF coil design issues. This particularly impairs image quality for sequences that use inversion pulses such as magnetization prepared rapid acquisition gradient echo and limits the use of quantitative arterial spin labeling sequences such as flow‐attenuated inversion recovery. Here we have used a search algorithm to produce inversion pulses tailored to take into account the heterogeneity of the RF transmit field at 7 T. This created a slice selective inversion pulse that worked well (good slice profile and uniform inversion) over the range of RF amplitudes typically obtained in the head at 7 T while still maintaining an experimentally achievable pulse length and pulse amplitude in the brain at 7 T. The pulses used were based on the frequency offset correction inversion technique, as well as time dilation of functions, but the RF amplitude, frequency sweep, and gradient functions were all generated using a genetic algorithm with an evaluation function that took into account both the desired inversion profile and the transmit field inhomogeneity. Magn Reson Med, 2009.

Tissue-equivalent gel for non-invasive spatial radiation dose measurements

Abstract In almost every single application of ionising radiation, the accurate determination of absorbed radiation energy is of great importance. Radiation sensitive gels were first proposed for non-destructive and non-invasive measurements of the spatial distribution of radiation absorbed dose in 1984 [1] . Two different systems were developed but neither of these systems is widely used in practice due to some technical drawbacks. To overcome the problems associated with previous systems a gelatin gel of different composition has been devised and characterised for radiation dosimetry. This system is based on the changes in the optical properties of a transparent gel medium when it is exposed to ionising radiation. The detection material was carefully investigated and the role of each constituent of this system has been studied in order to determine the optimum composition. Gel with this formula has a linear dose response in the range 0.1–30 Gy, this range can be altered to suit other applications by changing the concentration of the active chemicals. The gel sensitivity, defined as the change in optical density per unit radiation dose, was found to be 70×10−3 cm−1 Gy−1 and the system stability was studied for both pre- and post-irradiation effects. This new formulation allows a convenient, rapid and economic readout method using an optical tomography system operating at visible wavelengths.

High resolution measurement of the surface layer moisture content during drying of wood using a novel magnetic resonance imaging technique

Summary The moisture content profile in the surface layer of Scots pine (Pinus sylvestris) sapwood has been measured using the MRI (Magnetic Resonance Imaging) technique.Anovel high-gradient permanent magnet was used for the measurement of the moisture profile in the wood during the drying process, giving a depth resolution of better than 20 μm. This paper demonstrates for the first time the possibility of measuring the moisture content depth profile in the surface layer from raw state down to 4% moisture content without removing the wood sample from the apparatus. The main advantage of this technique and apparatus is a high resolution over a large field of view. It is possible to measure the moisture content in the wood surface layer during drying up to a depth of 300 μm. A single moisture content profile may be obtained in less than 5 min. Hence, more accurate dynamic drying information, on a single sample, maybe achieved than with current techniques. In this preliminary studywe observed that the wood cells at the surface had a moisture content higher than the equilibrium moisture content for wood in the surrounding bulk air. We conclude that for a wood moisture content of above approximately 50% the resulting profiles were almost flat from a depth of 90 μm and further into the wood and in between the surface and 90 μm deep there was a steep gradient. Below 50% there was a gradient in moisture content to at least 300 μm depth.

Interaction of MRI field gradients with the human body

In this review, the effects of low-frequency electromagnetic fields encountered specifically during magnetic resonance imaging (MRI) are examined. The primary biological effect at frequencies of between 100 and 5000 Hz (typical of MRI magnetic field gradient switching) is peripheral nerve stimulation, the result of which can be a mild tingling and muscle twitching to a sensation of pain. The models for nerve stimulation and how they are related to the rate of change of magnetic field are examined. The experimental measurements, and analytic and computational modelling work in this area are reviewed. The review concludes with a discussion of current regulation in this area and current practice as both are applied to MRI.

Echo-planar imaging of the brain at 3.0 T: first normal volunteer results.

Objective To present the first echo-planar brain images of diagnostic quality obtained at 3.0 T and to point out some of the problems experienced in performing it. Materials and Methods The results presented were obtained on volunteers using an in-house designed and constructed 3.0 T EPI imager. Results The results demonstrate the feasibility of obtaining snapshot imaging comprising up to 256 × 256 pixels and corresponding to a spatial resolution of 0.75 × 0.75 mm2 with a slice thickness of 2.5 mm. Conclusion Potentially augmented diagnostic information can be obtained with high field EPI of the brain. Some susceptibility artifact is apparent at the bone-air interfaces as expected at 3.0 T.

Reference layer artefact subtraction (RLAS): a novel method of minimizing EEG artefacts during simultaneous fMRI

Large artefacts compromise EEG data quality during simultaneous fMRI. These artefact voltages pose heavy demands on the bandwidth and dynamic range of EEG amplifiers and mean that even small fractional variations in the artefact voltages give rise to significant residual artefacts after average artefact subtraction. Any intrinsic reduction in the magnitude of the artefacts would be highly advantageous, allowing data with a higher bandwidth to be acquired without amplifier saturation, as well as reducing the residual artefacts that can easily swamp signals from brain activity measured using current methods. Since these problems currently limit the utility of simultaneous EEG-fMRI, new approaches for reducing the magnitude and variability of the artefacts are required. One such approach is the use of an EEG cap that incorporates electrodes embedded in a reference layer that has similar conductivity to tissue and is electrically isolated from the scalp. With this arrangement, the artefact voltages produced on the reference layer leads by time-varying field gradients, cardiac pulsation and subject movement are similar to those induced in the scalp leads, but neuronal signals are not detected in the reference layer. Taking the difference of the voltages in the reference and scalp channels will therefore reduce the artefacts, without affecting sensitivity to neuronal signals. Here, we test this approach by using a simple experimental realisation of the reference layer to investigate the artefacts induced on the leads attached to the reference layer and scalp and to evaluate the degree of artefact attenuation that can be achieved via reference layer artefact subtraction (RLAS). Through a series of experiments on phantoms and human subjects, we show that RLAS significantly reduces the gradient (GA), pulse (PA) and motion (MA) artefacts, while allowing accurate recording of neuronal signals. The results indicate that RLAS generally outperforms AAS when motion is present in the removal of the GA and PA, while the combination of AAS and RLAS always produces higher artefact attenuation than AAS. Additionally, we demonstrate that RLAS greatly attenuates the unpredictable and highly variable MAs that are very hard to remove using post-processing methods.