Ultra-low field MR
Why ultra low field?
Magnetic resonance imaging (MRI) and nuclear magnet resonance spectroscopy are core investigative methods, in broad research areas, such as chemistry, biology.
As a non-invasive and non-radiating imaging and scanning approach, MRI/NMR is the most versatile technique, for instance, to obtain high resolution anatomical and functional images for medical and neuroscience, or evaluate molecular structures in material science, chemistry and biology. However, to achieve high sensitivity, conventional MRI/NMR instruments rely on strong magnetic fields (>1 Tesla) generated by superconducting magnets, which makes these instruments bulky, heavy and very expensive to purchase, operate and maintain, limiting their use to major research or clinical facilities.
In light of these limitations, ultra-low field-NMR/MRI, with applied magnetic fields of less than 10 mT, was developed about 10 years ago as an alternative imaging and scanning solution. A number of features unique at this field regime can be exploited, such as
- acquisition of images with higher contrast-to-noise ratios (CNR)
- imaging in the presence of metals
- concurrent use with other modalities like magnetoencephalography (MEG)
- no susceptibility artefacts
The most promising feature of ULF-NMR/MRI is the possible overlap of the instrumental (Larmor) frequency with the rates of change of chemical and biological processes such as diffusion, protein folding, and neuronal activity. hence providing new opportunities to study slowly evolving dynamic processes and explore new imaging contrast mechanisms.
What are the challenges in ULF?
The sample signals at ULF-NMR/MRI are very weak, due to the low applied magnetic field and consequently small achievable sample magnetisation. Currently ULF research focus on two practical approaches to boost SNR:
Implementation of highly sensitive magnetometers, and sample pre-polarisation, applying a strong (~0.1T) pulsed magnetic field before the measurement.
However, the use of hyper sensitive magnetometers (such as SQUIDs and AM) with compact resistive coils for sample pre-polarisation are suboptimal solutions, since these sensors require protective circuits and shielded environments.
Our overall research aim is to develop an ULF-NMR/MRI instrument, without superconductors and cryogenics, enabling truly portable and economic operation. Currently, our research focuses on the following developments:
- Optimise signal yields by determining optimal sensor locations and orientations relative to sample size and shape.
Developing a novel coil based magnetometer, optimised for very low magnetic field strength.
- Generate more homogeneous and stronger sample magnetisation with a novel small permanent magnet array (SPMA), a novel concept of using small permanent magnet arrays for generating varying magnetic field configurations for ULF-NMR/MRI, aiming to replacing resistive coil technology.
1. Vogel M.W., Giorni A., Vegh V., Pellicer-Guridi R., Reutens D.C. Rotatable Small Permanent Magnet Array for Ultra-Low Field Nuclear Magnetic Resonance Instrumentation: A Concept Study. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0157040.
2. Vogel M.W., Vegh V., and Reutens D.C. Numerical study of ultra-low field nuclear magnetic resonance relaxometry utilizing a single axis magnetometer for signal detection. Medical Physics. 2013 May;40(5):052301. doi: 10.1118/1.4800491.