MR bone imaging
- Wilson Miller1
© Miller; licensee BioMed Central Ltd. 2015
Published: 30 June 2015
Bone is highly relevant to focused ultrasound therapy, both as a potential treatment target and because it interferes with treatment of other organs such as the brain. It is challenging to image cortical bone using MRI, however, due to low water density and fast signal decay in bony tissues. Ultrashort echo time (UTE) imaging is a specialized MR technique that allows the weak, short-lived signal from cortical bone to be imaged despite these limitations. Potential applications of UTE bone imaging in MR-guided focused ultrasound include direct MR thermometry of bone heating, which is not possible using standard proton resonance shift (PRFS) techniques, and in situ skull imaging during brain treatment procedures, which could replace the separate CT scan currently required for transcranial focused ultrasound.
In UTE MRI, imaging data is acquired using a spoke-radial k-space trajectory with gradient ramp sampling. This allows data acquisition to begin immediately after RF excitation, to capture the MR signal from cortical bone before it decays away. The basic UTE imaging technique can be implemented as either a 2D slice-selective or a 3D volumetric acquisition. A volumetric acquisition is ideal for 3D skull imaging. Achieving adequate spatial resolution, however, requires a relatively long scan time (~10 min). Fast UTE imaging of 2D slices can be performed by using specially designed half-RF pulses. Although more challenging to implement, such an acquisition would be much more suitable for monitoring transient temperature changes in bone during FUS treatment. Because bone signal decays too quickly to perform PRFS-based thermometry, however, other temperature-dependent MR properties must be used to generate sensitivity to temperature changes. For instance, the T1 relaxation time generally decreases with increasing temperature, whereas T2 generally increases. Either of these effects might therefore provide a basis for MR thermometry in cortical bone.
Results and conclusions
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