Gareth Reynold Davies

Field-cycling NMR relaxometry with spatial selection

Fast field‐cycling MRI offers access to sources of endogenous information not available from conventional fixed‐field imagers. One example is the T1 dispersion curve: a plot of T1 versus field strength. We present a pulse sequence that combines saturation‐recovery/inversion‐recovery T1 determination with field cycling and point‐resolved spectroscopy localization, enabling the measurement of dispersion curves from volumes selected from a pilot image. Compared with a nonselective sequence, our method of volume selection does not influence measurement accuracy, even for relatively long echo times and in the presence of radiofrequency field nonuniformity. The measured voxel profile, while not ideal, corresponds with that expected from the image slice profile. On a whole‐body fast field‐cycling scanner with 59‐mT detection, the sensitivity of the experiment is sufficient to reveal distinctive “quadrupole dips” in dispersion curves of protein‐rich human tissue in vivo. Magn Reson Med, 2010.

Continuous wave MRI of heterogeneous materials

A prototype continuous wave MRI system operating at 7T has been used successfully to study a variety of heterogeneous materials exhibiting T2 relaxation values ranging from 10 micros to 50 ms. Two-dimensional images of a poly(methly methacrylate) (PMMA) resolution phantom (T2=38 micros) exhibited a spatial resolution of approximately 1mm at a magnetic field gradient strength of 200 mT/m. The technique was used to study the hydration, drying, and subsequent water penetration properties of cement samples made from ordinary Portland cement, and revealed inhomogeneities arising from the cure conditions. Sandstone samples from an oil reservoir in the North Sea were also studied; structure within these materials, arising from the sedimentary bed layering in the reservoir, was found to have an effect on their water transport properties. A section from a confectionery bar (T2* approximately 50-60 ms) was also imaged, and its internal structure could be clearly discerned.

The use of contrast agents with fast field-cycling magnetic resonance imaging

Fast field-cycling (FFC) MRI allows switching of the magnetic field during an imaging scan. FFC-MRI takes advantage of the T(1) dispersion properties of contrast agents to improve contrast, thus enabling more sensitive detection of the agent. A new contrast agent designed specifically for use with FFC was imaged using both a homebuilt FFC-MRI system and a 3 T Philips clinical MRI scanner. T(1) dispersion curves were obtained using a commercial relaxometer which showed large changes in relaxation rate between fields. A model of magnetization behaviour was used to predict optimum evolution times for the maximum T(1) contrast between samples at each field. Images were processed and analysed to create maps of R(1) values using a set of images at each field. The R(1) maps produced at two different fields were then subtracted from each other in order to create a map of ΔR(1) in which pixel values depend on the change in R(1) of the sample between the two fields. The dispersion properties of the agent resulted in higher contrast in a ΔR(1) image compared with a standard T(1)-weighted image.

Off-resonance magnetisation transfer contrast (MTC) MRI using fast field-cycling (FFC)

Magnetisation transfer contrast (MTC) is an important MR contrast generating mechanism to characterise the undetectable bound protons indirectly using the decreased signal intensity of the observable free protons. MTC imaging typically employs a range of off-resonance RF pre-saturation pulse with maintaining the RF magnetic field (B(1)) at a specified value. However, this presents a technical difficulty, particularly at low field, because the larger offset frequencies tend to be outside the bandwidth of the RF transmit system, causing B(1) to vary with the frequency offset. Here, we demonstrate a novel off-resonance irradiation method using fast field-cycling which allows switching of the external magnetic field between several chosen strengths, while holding constant the RF frequency and B(1) level. This permits one to avoid the problem of B(1) variation as a function of frequency offset. The results obtained by this new technique are in excellent agreement with those obtained by the conventional technique.

Simple algorithm for the correction of MRI image artefacts due to random phase fluctuations

PURPOSE Fast Field-Cycling (FFC) MRI is a novel technology that allows varying the main magnetic field B0 during the pulse sequence, from the nominal field (usually hundreds of millitesla) down to Earths field or below. This technique uses resistive magnets powered by fast amplifiers. One of the challenges with this method is to stabilise the magnetic field during the acquisition of the NMR signal. Indeed, a typical consequence of field instability is small, random phase variations between each line of k-space resulting in artefacts, similar to those which occur due to homogeneous motion but harder to correct as no assumption can be made about the phase error, which appears completely random. Here we propose an algorithm that can correct for the random phase variations induced by field instabilities without prior knowledge about the phase error. METHODS The algorithm exploits the fact that ghosts caused by field instability manifest in image regions which should be signal free. The algorithm minimises the signal in the background by finding an optimum phase correction for each line of k-space and repeats the operation until the result converges, leaving the background free of signal. CONCLUSION We showed the conditions for which the algorithm is robust and successfully applied it on images acquired on FFC-MRI scanners. The same algorithm can be used for various applications other than Fast Field-Cycling MRI.