Ultrashort Echo Time Magnetic Resonance Imaging of the Lumbar Intervertebral Disc. - Abstract Intervertebral disc (IVD) degeneration and related low back pain (LBP) affect up to 85% of the U.S. population over their lifetime, resulting in annual healthcare costs that exceed $100 billion. The IVD is largely avascular and consists of a central proteoglycan (PG)-rich nucleus pulposus (NP), and a surrounding collagen- rich annulus fibrosus (AF), as well as superiorly and inferiorly located endplates. The IVD relies on diffusion of nutrients and waste through the cartilaginous endplate (CEP) to maintain its health. IVD degeneration is characterized by loss of PGs, dehydration of the NP, collagen loss within the AF, and degradation of the CEP. Dehydration and calcification of the CEP reduce its diffusivity, leading to a reduction in oxygen and glucose transport to the remainder of the disc. Changes in the CEP may occur at the same time or even precede disc degeneration. Evaluation of this complex situation requires a comprehensive, non-invasive imaging technique that can assess PG in the NP, collagen in the AF, and diffusivity of the CEP. Magnetic resonance imaging (MRI) is routinely used in the diagnosis of IVD degeneration. However, conventional MRI techniques do not provide reliable assessment of disc biochemical content nor of CEP function. This study aims to further develop a 3D ultrashort echo time (UTE) adiabatic T1r (UTE-AdiabT1r) sequence for robust mapping of PGs, a UTE magnetization transfer (UTE-MT) sequence for mapping of macromolecular fraction (MMF), an adiabatic inversion recovery UTE with fat saturation (IR-FS-UTE) sequence for T2* mapping of the CEP to evaluate calcification and dehydration, and a UTE dual echo steady state (UTE-DESS) sequence to study its apparent diffusion coefficient (ADC). In Aim 1 we will develop 3D UTE sequences to evaluate IVD in lumbar spines from young (<40y, n=10), mid-age (40-70y, n=10), and elderly (>70y, n=10) donors, and correlate UTE (AdiabT1r, MMF, T2*, ADC) and clinical MRI measures with reference including CT, µCT, histology, biochemistry, and diffusion test of three groups of spine samples. In Aim 2 we will evaluate 3D UTE sequences to assess IVD degeneration and regeneration using a mature rabbit annular puncture chronic disc degeneration model. We will study IVD degeneration in four groups of rabbits (n=16 per group) at 4, 8, 16, and 28 weeks post-AF puncture, as well as IVD regeneration using six groups of rabbits (n=16 per group) at 4, 12 and 24 weeks post- injection of saline and growth differentiation factor-6 (GDF-6). We will correlate UTE and clinical MRI findings with reference including µCT, histology, biochemistry, and diffusion test of ten groups of rabbit lumbar spines. In Aim 3 we will translate UTE sequences to study IVD degeneration in patients with chronic LBP (n=40) and normal IVDs in healthy volunteers (n=40), compare UTE and clinical MRI metrics of the NP, AF, and CEP of the lumbar spine in the two groups, and correlate them with clinical evaluations. Our central hypothesis is that UTE sequences can detect changes in PG and collagen in the disc as well as changes in diffusivity of the CEP, allowing more comprehensive and accurate evaluation of disc degeneration than is now possible.
