Hard Tissue Magnetic Resonance Imaging with Fast Control of Intense Magnetic Fields

Abstract

Magnetism plays an instrumental role in biomedical applications and healthcare, both from the perspectives of devices and techniques. In particular, Magnetic Resonance Imaging (MRI, [1]) addresses the magnetization induced on the patients’ constituent matter when subject to a magnetic field, and exploits the time-varying signal resulting from the contribution of the ubiquitous hydrogen nuclei spins for high quality spatial reconstructions. At present, MRI is the only available medical imaging technique known to deliver high resolution images of deep biological tissues in vivo and without harmful ionizing radiation.However, MRI does not come without its own limitations. In particular, it is much more efficient at detecting soft tissues and fluids than hard, solid samples. This is due to the weak, short-lived signals emitted by the latter, by cause of dipolar magnetic interactions between neighboring spins in the sample, which constitute a noisy environment that decoheres the quantum nature of the MR signal. The environment’s contribution averages out for soft tissues, but not so for solids, where nuclei are approximately fixed with respect to each other in the laboratory reference frame. This explains the scarcity of useful (with diagnostic value) MR images of dental tissues [2-4], the hardest in the human body. Furthermore, the MRI scanners employed so far use static magnetic field strengths ranging from 4 to 12 T, which makes them large and expensive. At around 1 M$ per Tesla [5], this precludes their penetration in dental clinics.In this symposium, devoted to evaluating the transition of biomagnetic research from laboratory to clinical environments, we present a new, inexpensive approach to MRI of combined hard and soft biological tissues, with a view on dental applications [6]. We will show 3D reconstructions of human teeth (Figures 1 and 2), as well as a rabbit head and a cow femur, all obtained at a field strength of only 260 mT. These images are, to the best of our knowledge, the first featuring soft and hard tissues simultaneously at sub-Tesla fields, and have been acquired in a home-made special-purpose MRI scanner designed with the goal of demonstrating dental imaging at low field settings. We also present two recently patented sequences for efficient encoding of spatial information and image reconstruction: VIEWS (Volumetric Image Encoding Without k-Space) and MASSIF (Magic Angle Spinning of Spatially Inhomogeneous Fields). These are key players in the roadmap we envision to shorten scan times from several hours, as in our first prototype, to a few minutes, as desired for clinical applications. **