Hugh C. Seton

A 4.2 K receiver coil and SQUID amplifier used to improve the SNR of low-field magnetic resonance images of the human arm

We describe the design and use of a 48 mm diameter, liquid-helium-cooled MRI receiver coil and DC SQUID pre-amplifier. Comparison of images of a non-conducting room temperature test object collected with the SQUID-based system and those collected with an equivalent-area room-temperature surface coil show that the SQUID system SNR is approximately a factor of four greater, despite a 15 mm vacuum gap between sample and coil in the SQUID case. SQUID images of the lower arm also display improved SNRs over those of the room-temperature coil, this time by a factor of between two and three, and as a result reveal greater anatomical detail. We show that the performance is currently limited by inductively coupled losses from metal components in the imager, but that, by using the same system in a whole-body imager, the SNR of SQUID images of the arm will exceed the room-temperature coils performance by a factor of between 2.8 and 4.5. We believe that these are the first magnetic resonance images of a living sample to have been produced with a SQUID-based receiver.

Magnetic CoPt nanoparticles as MRI contrast agent for transplanted neural stem cells detection

Neural stem cells (NSCs) exhibit features that make them suitable candidates for stem cell replacement therapy and spinal cord reconstruction. Magnetic resonance imaging (MRI) offers the potential to track cells in vivo using innovative approaches to cell labeling and image acquisition. In this study, experiments were carried out to optimize the loading condition of magnetic CoPt hollow nanoparticles (CoPt NPs) into neural stem cells and to define appropriate MRI parameters. Both cell viability and multipotency analysis showed that CoPt NPs at a concentration of 16 µg ml(-1) reduced T2 relaxation times in labeled rat NSCs, producing greater contrast on spin echo acquisitions at 4.7 T, yet did not affect cell viability and in vitro differentiation potential compared to controls. After optimizing nanoparticle loading concentrations and labeled cell numbers for MRI detection, CoPt-loaded NSCs were transplanted into organotypic spinal cord slices. The results showed that MRI could efficiently detect low numbers of CoPt-labeled NSCs with the enhanced image contrast. Our study demonstrated that MRI of grafted NSCs labeled with CoPt NPs is a useful tool to evaluate organotypic spinal cord slice models and has potential applications in other biological systems.

Gradiometer pick-up coil design for a low field SQUID-MRI system

We describe the use of liquid helium-cooled (4.2 K) gradiometer coils and a DC superconducting quantum interference device (SQUID) preamplifier to improve the SNR of magnetic resonance imaging (MRI) at 0.01 T. Gradiometer windings are used both to reduce lossy interactions with the MRI systems room temperature magnet and gradient coils and also to reject interference from more distant sources, which reduces the need for RF shielding. We have tested both axial and planar (figure-of-eight) gradiometer configurations. The figure-of-eight gradiometer has a more rapid fall-off in sensitivity with increasing distance from its windings than the axial gradiometer, but this is compensated for by reduced lossy interactions and improved interference rejection. We have used the system to image the human arm.

A tuned SQUID amplifier for MRI based on a DOIT flux locked loop

We have developed a 4.2 K, flux locked, tuned d.c. SQUID amplifier to improve the SNR of a low field MRI system operating at 0.01 T (425 kHz). The flux locked loop, based on the Direct Offset Integration Technique, has a noise level of 2.6 /spl mu//spl Phi//sub 0/ Hz/sup -1/2/ and a slew rate of 1.6/spl times/10/sup 6/ /spl Phi//sub 0/ s/sup -1/ at 425 kHz when used with a commercially obtained SQUID. The high intrinsic Q-factor of the MRI pick-up coil is damped by the action of the loop and by an additional feedback circuit to provide imaging bandwidths of up to 10 kHz. We have developed a special low noise liquid helium cryostat so that the final system has a magnetic field resolution of 0.08 fT Hz/sup -1/. This receiver was used in a small scale MRI system to image non-conducting test objects and the human arm. The images show significant improvements in SNR over those obtained with an equivalent room temperature receiver.

Use of a DC SQUID receiver preamplifier in a low field MRI system

We have used tuned receiver coils coupled to a dc SQUID preamplifier in a small scale magnetic resonance imaging (MRI) system operating at 425 kHz (B/sub 0/=0.01 T). The coil and SQUID are cooled to 4.2 K in a modified biomagnetic cryostat. The modifications provide transparency to rf signals originating outside the cryostat while maintaining an acceptably low liquid helium boiloff rate. By applying negative feedback we have damped low loss, high Q-factor SQUID input circuits to achieve a useful imaging bandwidth of over 2 kHz.<<ETX>>

Cryogenic receive coil and low noise preamplifier for MRI at 0.01 T

We have investigated the design and construction of liquid nitrogen cooled surface coils made from stranded (litz) copper wire for low field MRI applications. If designed correctly, cooled litz coils can provide a competitive alternative to high temperature superconducting (HTS) coils without the complications associated with flux trapping. Litz coils can also be produced with a wider range of shapes and sizes, and at lower cost. Existing models were verified experimentally for flat spiral coils wound from solid and litz wires, operated at room temperature and 77K, and then used to design and optimise a cooled receive coil for MRI at 0.01T (425 kHz). The Q-factor reached 1022 when the coil was cooled to 77K, giving a bandwidth of just 0.42 kHz, so a low noise JFET preamplifier was developed to provide active damping of the coil resonance and thus minimise image intensity artefacts. The noise contribution of the preamplifier was determined using a method based on resistive sources and image noise analysis. The voltage and current noise were measured to be 1.25 nV/Hz(1/2) and 51 fA/Hz(1/2), respectively, and these values were used to estimate a noise figure of 0.32 dB at the resonant frequency of the cooled coil. The coil was used to acquire 0.01T spin echo images, first at room temperature and then cooled to 77K in a low noise liquid nitrogen cryostat. The measured SNR improvement on cooling, by a factor of 3.0, was found to correspond well with theoretical predictions.

Prepolarized fast spin‐echo pulse sequence for low‐field MRI

Clinical MRI systems use magnetic fields of at least 0.5T to take advantage of the increase in signal‐to‐noise ratio (SNR) with B0. Low‐field MRI apparatus is less expensive and offers the potential benefit of improved T1 contrast between tissues. The poor inherent SNR at low field can be offset by incorporating prepolarizing field pulses with the MRI pulse sequence. The prepolarizing field does not need to be as homogeneous as the detection field, so it can be generated by a relatively inexpensive electromagnet. Prepolarizing hardware for a 0.01T MRI system was developed together with a prepolarized MRI pulse sequence that incorporates fast imaging techniques to reduce acquisition times by a factor of 5 relative to standard methods. Comparison images of test objects show that most of the enhanced SNR is retained with the fast method. Low‐field images of a human wrist acquired using the fast prepolarized method are also shown. Magn Reson Med 57:1180–1184, 2007.

Evaluation of MRI for in vivo monitoring of retinal damage and detachment in experimental ocular inflammation.

Two quantitative methods were developed for investigation of the potential of MRI for in vivo monitoring of retinal damage and detachment in experimental autoimmune uveitis (EAU). Measurements of retinal thickness and detachment area were performed on matched MR and histologic (HIST) images of rat eyes at different stages of EAU. In vivo MR images of rat eyes were acquired at 4.7 T using a figure‐of‐eight surface coil and a spin echo pulse sequence. Ex vivo measurements were performed on HIST images acquired using a digital camera attached to a microscope. MR images mirrored the HIST appearance of inflamed eyes at each stage of disease. Retinal detachments as small as 0.1 mm2 were measured in vivo by MRI and confirmed in the same eye ex vivo by histology. Measurements performed on corresponding MR and HIST images demonstrated a good agreement between the two techniques. The potential of MRI for in vivo visualization and for monitoring changes in the eye during development of EAU was demonstrated in this study. Magn Reson Med 53:61–68, 2005.

Tuned SQUID-MRI System With Resonant Frequency Adjustment

We describe a low field, bench-top MRI system for small samples, based on a permanent magnet. Signals are received at 830 kHz using a tuned SQUID magnetometer, cooled in a modified liquid helium cryostat. The SQUID input circuit has an intrinsic Q-factor of 28,000, so this is damped by flux-locking electronics and an additional feedback loop to give an effective Q of between 40 and 200 for imaging. The resonant frequency is adjusted by a control rod coupled to a trimmer capacitor mounted in the liquid helium volume. Images of a test object acquired using the cooled receiver exhibit up to two-fold SNR gains in regions close to the sensor, compared to an equivalent room temperature coil, with the noise level dominated by losses coupled from the magnet pole faces.