Osteoporosis Imaging.

Osteoporosis is the most common disease affecting bones worldwide. Dual x-ray absorptiometry (DXA) is the current reference standard for assessing bone health and, combined with other clinical parameters, provides a good estimation of fracture risk. DXA-based Trabecular Bone Score (TBS) can provide complementary indirect information about bone microarchitecture, which also deteriorates osteoporosis. QCT can provide a 3-D volumetric assessment of bone mineral density (BMD), and FEA of computed tomography (CT) images of bone can provide estimates of bone strength, which have the potential to add value, beyond BMD, for fracture risk assessment. Magnetic resonance imaging (MRI) of bone microarchitecture is an additional promising alternative to the assessment of BMD, and there is evidence that microarchitectural parameters could 1 day have benefits for diagnosing osteoporosis beyond BMD and/or FRAX. Assessment of bone via MRI also provides insight into other bone tissue properties (cortical porosity, marrow fat) that are altered in osteoporosis and that DXA cannot assess. Overall, bone health cannot be characterized solely by one parameter. Current imaging techniques/modalities in combination with advanced image processing hold the potential to provide a comprehensive understanding of the pathologic changes that occur in bone tissue in the setting of osteoporosis and pave the way for new imaging methods to diagnose, monitor, and predict osteoporosis.

Localized 2D COSY sequences: Method and experimental evaluation for a whole metabolite quantification approach.

Two-dimensional spectroscopy offers the possibility to unambiguously distinguish metabolites by spreading out the multiplet structure of J-coupled spin systems into a second dimension. Quantification methods that perform parametric fitting of the 2D MRS signal have recently been proposed for resolved PRESS (JPRESS) but not explicitly for Localized Correlation Spectroscopy (LCOSY). Here, through a whole metabolite quantification approach, correlation spectroscopy quantification performances are studied. The ability to quantify metabolite relaxation constant times is studied for three localized 2D MRS sequences (LCOSY, LCTCOSY and the JPRESS) in vitro on preclinical MR systems. The issues encountered during implementation and quantification strategies are discussed with the help of the Fisher matrix formalism. The described parameterized models enable the computation of the lower bound for error variance--generally known as the Cramér Rao bounds (CRBs), a standard of precision--on the parameters estimated from these 2D MRS signal fittings. LCOSY has a theoretical net signal loss of two per unit of acquisition time compared to JPRESS. A rapid analysis could point that the relative CRBs of LCOSY compared to JPRESS (expressed as a percentage of the concentration values) should be doubled but we show that this is not necessarily true. Finally, the LCOSY quantification procedure has been applied on data acquired in vivo on a mouse brain.