MRI of Grain and Teeth

In general, magnetic resonance imaging (MRI) is used to obtain a spatial representation of the water distribution in an object. Water in soft materials, such as living matter, often shows a high degree of translational mobility, giving rise to relatively long magnetic relaxation times. This allows the use of conventional spin- and gradient echo MRI techniques to acquire an image.

However, when hydration levels become very low, water becomes less mobile, resulting in much shorter magnetic relaxation times. This corresponds to signal loss using conventional MRI sequences. To avoid problems arising from rapid decaying signals, single point imaging (SPI) is used.

MRI of Grain

Researchers at NRC-IBD were able to obtain good quality SPI images of non-imbibed (dry) seeds that didn't show any appreciable amount of signal using conventional MRI [1]. Figure 1 shows two 3D reconstructions of the data. Note that the reconstruction clearly displays the imperfection in the endosperm.

Two orientations of a dry wheat kernel obtained from 3D SPI MRI data using sinusoidal-ramped gradients. SPI experiments were performed with a data matrix of 64 x 64 x 32 points for a FOV of 1.4 x 1.4 x 1.4 cm^3. The detection time, T_p, was 200 microseconds.

Since SPI allows for the rapid switching of gradients, acoustic noise levels during the experiments can be high. NRC researchers showed that the use of shaped gradients in the SPI sequence significantly reduced the acoustic noise level [2].

References:

1. Gruwel M.L.H, Latta P., Volotovskyy V., Sramek M., Tomanek B., Magnetic resonance imaging of seeds using single point acquisition. J. Agric. Food Chem. 52, 4979-4983 (2004).
2. Latta P., Gruwel M.L.H., Edie E., Sramek M., Tomanek B., Single point imaging with suppressed sound pressure levels through gradient-shape adjustment. J. Magn. Reson. 170, 177, (2004)

MRI of Teeth

Successful reconstruction of dental defects relies on the precise mapping of these defects. For restoration, dentists heavily rely on impressional methods. These methods are prone to errors due to the many steps involved in the mapping procedure. This is where MRI could be very helpful.

Recent reports on the use of MRI in dentistry [1] have generated interest in the development and improvement of MRI techniques for the study of teeth. Although MRI techniques are well established in biomedical research, most of the applications are limited to the study of soft tissue or gross skeletal structures.

The major problem with the imaging of solid materials in MR research is their inherently short spin-spin relaxation time (T_2). Short T_2 relaxation severely pushes the applicability limit of standard spin- and gradient echo- based techniques.

SPI [2], a pure phase encoding technique, has found many useful applications in the imaging of solids and semi-solids, and could be used for the 3D mapping of teeth. However, due to the rapid switching of gradients, SPI has the potential to overheat the gradient-set and to produce a high acoustic noise level.

NRC-IBD researches are testing a modified SPI sequence which uses sine-shaped gradients. This greatly reduces potential problems of acoustic noise and gradient-set overheating [3]. Figure 2 shows a 2D image of a cross section of an intact human molar (ex vivo). The image clearly shows the positions of dentin and enamel within the tooth.

Figure 1

Cross section of a human molar obtained from a 3D SPI image. False colour mapping was used to distinguish between dentin and enamel. SPI experiments were performed with a data matrix of 64 x 64 x 32 points for a FOV of 3 x 3 x 3 cm^3. The detection time, T_p, was 125 microseconds.

References:

1. Lloyd C.H., Scrimgeour S.N., Chudek, J.A., Hunter G., MacKay R.L. Magnetic resonance microimaging of carious teeth. Quintessence Int. 28, 349-355 (1997).
2. Emid S., Creyghton J.H.N. High resolution NMR imaging in solids. Physica B 128,81-83 (1985).
3. Latta P., Gruwel M.L.H., Edie E., Sramek M., Tomanek B. Single point imaging with suppressed sound pressure levels through gradient-shape adjustment. J. Magn. Reson. 170, 177-185 (2004).

Related Information

Read more about MRI of Grain and Teeth:

• Cardiac MRI
• Functional Magnetic Resonance Imaging (fMRI)
• Magnetic Resonance Technology Development
• Molecular Imaging - MR
• Stroke - MRI

Related Programs:

• MR Research and Technology Development

Institutes:

• NRC Institute for Biodiagnostics