Ultra-low Field MR
Conventional MRI systems place the patient in very strong magnetic fields of 1 Tesla (20,000 times the Earth's field) or higher. High fields increase signal levels and therefore image quality, but this requires expensive superconducting magnet technology.
Not all MRI investigations require whole body imaging, so we are developing compact MRI systems to image of specific parts of the body at fields of between 0.01 and 0.02 T (200-400 time's the Earth's field).
As well as reducing costs, low field imaging offers types of image contrast unavailable at the field strengths used in conventional scanners. The new type of low field system might find applications in clinic-based screening programmes.
As part of our research into low field MRI we are developing magnet designs, pulse sequences and ultra-low noise receivers.
Compact Magnet Designs
Many patients are intimidated by a conventional MRI magnet's completely enclosing "tunnel" design, so we have designed an open MRI system, shown below, to image the head or limb joints. This based on a 0.01 T three-coil resistive magnet, costing a fraction of a superconducting system.
This magnet has a relatively low detection field strength so we are investigating a prepolarization technique which momentarily subjects the patient to a much stronger field before the signal is detected. Early experiments show that it should be possible to increase signal levels by 3-4 times using this method, leading to significant improvements in image quality.
We are also developing a bench-top magnet for high resolution imaging of small samples, including finger joints. This is based on a permanent magnet which does not require liquid cryogens or a power supply. The magnetic field exhibits a relatively strong temperature coefficient but we have developed an temperature control system to eliminate field drift.
SQUID MRI
Low field MRI systems produce much smaller signals than their high field counterparts, but we have demonstrated significant improvements in image quality by receiving signals with a liquid helium-cooled RF coil, coupled to a very low noise preamplifier called a Superconducting Quantum Interference Device (SQUID).
MR images of a test object at 0.01 Tesla consisting of an array of 10 mm diameter, water-filled test tubes. |
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Conventional room temperature receiver at 20C |
Superconducting receiver, cooled with liquid helium to -269C |
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Images of a volunteer's arm collected using the same scanner - the room temperature image is very noisy, while the SQUID-based scan clearly shows bones and tendons: |
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Magnet: The home-made MRI scanner used for the above images is based around a 30 cm diameter Helmholtz magnet with biplanar gradient coils.
Cryostat: The superconducting receiver is cooled in a home-made cryostat which insulates the sample or patient from liquid helium at -269°C. We have developed special insulation techniques which minimise cryogen boil-off while maintaining the high image SNR.
The work on low field MRI is funded by EPSRC, and the Clerk Maxwell Cancer Research Fund.
Contact: Dr Hugh Seton.
Selected Publications
- Seton HC, Hutchison JMS and Bussell DM Liquid Helium Cryostat for a SQUID-Based MRI Receiver Cryogenics, 45, 348-355 (2005)
- Rahmatallah S, Li Y, Seton HC, Mackenzie I, Gregory JS, and Aspden RM NMR detection in the inhomogeneous magnetic field of a portable single-sided magnet. J. Magn. Reson, 173. 23-28 (2005)
- Seton H C, Hutchison J M S, Bussell D M and Hanley P Open-access resistive magnet for a compact low field MRI system. MAGMA, 11 (Suppl. 1), 112 (2000)
- Seton H C, Hutchison J M S and Bussell D M Gradiometer pick-up coil design for a low field SQUID-MRI system. MAGMA, 8, 116-120 (1999)
- Seton HC, Hutchison JMS and Bussell DM, A 4.2 K receiver coil and SQUID amplifier used to improve the SNR of low field magnetic resonance images of the human arm, Meas. Sci. Technol, 8, pp.198-207 (1997)