CW-MRI of Solids
The majority of MRI is performed on biological or human samples; nevertheless, there is increasing interest in the use of MRI to image solid materials. From an NMR point of view, the main difference is that, while tissues exhibit liquid-like NMR relaxation times (T2 of approximately 50 ms), solid materials have much shorter T2 values - for example, the rigid plastic Perspex (Plexiglass) has a T2 of around 16 us. This means that 'standard' MRI techniques cannot be used to image solids, since the pulse sequences used would not be able to record NMR signals from such fast-relaxing samples. Materials of interest range from porous media (such as cements, rocks, ceramics, polymers, soils, etc.) to biomedical materials (such as implants, crystallised deposits in the body, teeth and dental cement).
We have developed techniques for imaging solid materials by continuous-wave (CW) MRI. CW detection differs from the more usual pulsed RF detection in that the sample is bathed continuously with RF, applied via a resonator, near the expected resonant frequency. To observe the NMR signal, the magnetic field strength is 'ramped' (gradually increased), and a change in the amplitude of RF reflected from the resonator occurs when the usual resonance condition is met.
Our CW-MRI equipment uses a 7 Tesla superconducting magnet, with an internal bore of 183 mm. The proton NMR frequency at this field strength is 300 MHz. In order to perform CW-MRI a coil assembly is used which comprises two solenoids and three gradient coils, all water-cooled. One solenoidal coil is used to generate the 'ramp' component of the applied field, parallel to the 7-tesla field. The other generates an amplitude-modulated field at about 1 kHz, used together with lock-in signal detection to improve the signal-to-noise ratio. The gradient coils are designed for CW (rather than pulsed) operation, and can generate continuous gradients up to 300 mT/m gradients. A birdcage resonator is used to apply CW RF to the samples. Imager control software runs on a PC and is written in LabView.
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| Image of 30mm wide toy building brick (ABS plastic) | Image of 40mm wide Perspex disc | Surface-rendered 3D image of 35mm diameter Perspex phantom * |
* We are grateful to Dr. Yuanmu Deng of Ohio State University for assistance with 3D reconstruction of our data
In order to generate an image, a number of CW-NMR spectra are acquired by ramping the field produced by the solenoid and recording the amplitude of the RF reflected from the 300 MHz resonator. Each spectrum in the set is acquired in the presence of a strong, continuously-applied gradient, with the gradient direction being stepped by a small angle between consecutive acquisitions. In this way a set of one-dimensional projections of the object is obtained, and images are produced by filtered back-projection. Images of a toy 'Duplo' brick made from ABS platic (T2 ~ 300 us) and of a Perspex disc (T2 ~ 16 us) are shown. The spatial resolution depends on the linewidth of the sample and on the applied gradient strength. We can presently achieve a resolution of about 1 mm with a short-T2 sample such as Perspex.
2D CW-MRI images showing ingress of water into cuboidal sample of cured Portland cement, following exposure to water (from left) of duration 10 min, 25 min, 30 min, 60 min, 300 min (A to F, respectively). The dark grey regions correspond to chemically-combined water in the cement (T2 ~ 10 us).
The major advantage of CW-MRI compared to other solid-MRI methods is its ability to image materials with any T2 value, no matter how short. So, for example, CW-MRI can detect and image chemically-combined water in cured cement samples, and is able to image water or oil in porous rocks, even in the presence of large amounts of ferrous or paramagnetic impurities.
Contact: Professor D Lurie.