Stray Field NMR
Osteoporosis is a disease in which the density and quality of bone are reduced, affecting mainly post-menopausal women. For now, there is no easy method to monitor early changes in bone mineral density to assess hormonal treatment efficacy in osteoporotic patients. The goal of our project is to develop a small, simple, affordable, MR-based table-top scanner to detect cellular changes in the bone marrow as an early indicator of treatment efficacy.
The scientific basis of our project is that bone marrow adiposity in postmenopausal women increases with osteoporosis. Hormonal changes associated with the postmenopausal phase trigger cellular pathways that alter the differentiation of mesenchymal stem cells into adipocytes, disrupting the equilibrium between adipocyte and osteoblast differentiation. This process starts long before BMD changes are obvious and responds fast to hormonal treatment.
Experiments performed in our group demonstrated that changes in the bone marrow that are related to the onset of osteoporosis and to its treatment, can be detected by the use of a mobile stray field NMR. Based on these findings, we are developing a dedicated mobile stray-field NMR device that will allow monitoring bone marrow adiposity.
An axial MRI (Dixon) scan of an arm, showing the Radius (top) and the Ulna (bottom) bones
A custom made NMR table-top scanner
Magnetic resonance imaging (MRI) is mostly performed on protons associated with freely moving water molecules, as they are highly abundant in biological tissues yielding high signals. Nonetheless, much signal information originates from the non-aqueous species in the tissue, such as proteins and lipids. We aim to implement a novel MRI technique that allows the acquisition of signal generated by the protons adjacent to the semi-solid components of the tissue and combine it with quantitation analysis to extract the volumetric fraction of proteins or lipids. The proposed MRI sequence, MEX (magnetization exchange), can provide an insight into the content of species with restricted motion. The MEX sequence is illustrated in the figure.
Magnetic Resonanse Elastography (MRE)
Magnetic Resonance Elastography (MRE) is an MRI modality that probes tissue stiffness. MRE is performed in conjunction with the application of mechanical vibrations in typical frequencies of up
to 100 Hz (in clinical scanners). The MRE pulse sequence includes application of motion encoding gradients (on top of the imaging gradients) that are switched on and off and in their polarity, in
synchronization with the applied waves. The waves create sub-micron displacements in the probed object (e.g. a tissue) and the application of the gradients creates a localized phase shift in each
voxel. The maps of complex phase shifts are than used to quantify the complex shear modulus: G* = G′ + iG″ that is composed of the shear storage modulus (G′) and the shear loss modulus (G″).
MRE demonstrated its diagnostic value in different clinical conditions such as neurodegeneration, liver, kidney, fibrosis or cancer. Importantly, for our matter, brain MRE demonstrated significant changes in the aging brain, in which aging and
neurodegenerative diseases are correlated with a reduction in the shear modulus.
Changes in the stiffness of brain tissue are correlated, not only with very long processes like aging, but also with high dynamic events, such as brain function. Functional MRE (fMRE) experiments have
demonstrated that stimulation leads to immediate changes in the tissue stiffness. The experiments are composed of periods in which stimulation is turned on (‘on’ stage) and those in which no
stimulation is applied (‘off’ state). The mechanical vibration that facilitates the acquisition of phase images that correspond to the viscoelastic changes is operated throughout the entire experiment. It was found that stimulation leads to significant changes in the stiffness.