David M Bussell

Field-Cycled Proton-Electron Double-Resonance Imaging of Free Radicals in Large Aqueous Samples

We have recently published a new method of imaging free radicals in aqueous solutions called proton-electron double-resonance imaging (PEDRI) ( I ). In this technique a conventional proton NMR image is collected while the EPR resonance of a free radical solute is irradiated. If the EPR irradiation has sufficient power, the NMR signal from those protons being relaxed by the paramagnetic solute is enhanced, and the parts of the sample containing free radical exhibit greater intensity in the final image. Unlike EPR imaging (2) the sample size in PEDRI is not constrained by magnetic field gradient requirements. In this Communication we present the first results of an extension of PEDRI which uses magnetic field cycling, allowing larger samples to be imaged with lower levels of applied radiofrequency power. PEDRI is an imaging version of a dynamic nuclear polarization experiment (35). The enhancement of the NMR signal upon irradiation of the EPR of the solute may be written empirically as

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.

DC SQUID-based NMR detection from room temperature samples

The authors have shown that a DC SQUID with a tuned input circuit can be used as a low noise NMR preamplifier for small, room temperature samples at low field. They were interested to observe that the SQUID maintains its bias point over periods of several hours, despite the absence of a Q-spoiler in the input circuit. The 30 ms dead time following NMR excitation pulses would be unacceptable for imaging, but earlier experiments lead the authors to expect that by including a Q-spoiler junction array in the input circuit they would be able to reduce this to well below 1 ms to allow detection after much shorter echo times.

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>>

A bandwidth indicator for magnetic resonance imagers

Signal-to-noise ratios in magnetic resonance imaging are crucial in determining image quality, and dependent on a number of factors, one being the signal bandwidth per pixel. Not all manufacturers clearly state the bandwidth per pixel used for all sequences. A small battery-powered portable device is described which produces bright sharp lines on the magnetic resonance image at 10 kHz intervals in the frequency encoding direction. The bandwidth per pixel can then easily be calculated using electronic distance callipers, provided the image matrix and field of view are known. The device is expected to be especially of value when acceptance testing on poorly documented imaging systems.