Ultrashort Echo Time (UTE) Magnetic Resonance Imaging of the Spine

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Intervertebral disc (IVD) degeneration and low back pain (LBP) affect the majority of people over their lifetime. LBP is associated with high healthcare costs, disability and loss of productivity. The need for accurate diagnoses as well as focused preventive and therapeutic strategies is therefore a major public health concern. The IVD 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 and collagen loss within the AF, as well as degradation of the CEP. Magnetic resonance imaging (MRI) is routinely used in the diagnosis of IVD degeneration but do not provide reliable assessment of disc biochemical content or CEP function. Recent research has focused on two biomarkers to provide information of this type: T2 for collagen degradation and T1ρ for PG depletion. However, both biomarkers are confounded by the magic angle effect, which may result in a spurious several-fold increase in both T2 and T1ρ. In addition, the CEP is “invisible” on clinical MRI exams due to its short apparent transverse relaxation time (T2*). To address these problems, adiabatic spin-lock imaging has recently been proposed for T1ρ mapping with decreased magic angle sensitivity. Magnetization transfer (MT) has also been introduced for magic angle-insensitive mapping of macromolecular fraction (MMF). Further, ultrashort echo time (UTE) sequences with TEs ~100 times shorter than those of clinical sequences have been implemented to allow direct imaging of the CEP. The UTE research laboratory at the University of California San Diego (UCSD) has developed a series of UTE techniques for quantitative imaging of the disc, including a 3D UTE adiabatic T1ρ (UTE-AdiabT1ρ) sequence for robust mapping of PGs, a 3D UTE MT (UTE-MT) sequence for robust mapping of MMF, and an adiabatic inversion recovery UTE with fat saturation (IR-FS-UTE) sequence for high contrast imaging and T2* mapping of the CEP to evaluate calcification and dehydration. In this study we will a) evaluate the 3D UTE-AdiabT1ρ sequence to map PGs in the NP, and the 3D UTE-MT sequence to map MMF and collagen in the AF. We will also evaluate the impact of the magic angle effect on these sequences in intact IVDs, b) evaluate the 3D IR-FS-UTE sequence to image the CEP and quantify its T2*, and to investigate its calcification and hydration, c) correlate UTE-AdiabT1ρ, MMF and T2* and clinical MRI measures with reference standards including CT, µCT, histology, biochemistry, and diffusion test. We will demonstrate that novel 3D UTE sequences allow volumetric mapping of PG in the NP, collagen in the AF, and diffusivity of the CEP, thereby providing more robust assessment of early IVD degeneration.