Magnetic resonance of calcified tissues

Abstract

MRI of the human body is largely made possible by the favorable relaxation properties of protons of water and triacyl glycerides prevalent in soft tissues. Hard tissues – key among them bone – are generally less amenable to measurement with in vivo MR imaging techniques, not so much as a result of the lower proton density but rather due to the extremely short life-times of the proton signal in water bound to solid-like entities, typically collagen, or being trapped in micro-pores. Either mechanism can enhance T2 relaxation by up to three orders of magnitude relative to their soft-tissue counterparts. Detection of these protons requires solid-state techniques that have emerged in recent years and that promise to add a new dimension to the study of hard tissues. Alternative approaches to probe calcified tissues exploit their characteristic magnetic properties. Bone, teeth and extra-osseous calcium-containing biomaterials are unique in that they are more diamagnetic than all other tissues and thus yield information indirectly by virtue of the induced magnetic fields present in their vicinity. Progress has also been made in methods allowing very high-resolution structural imaging of trabecular and cortical bone relying on detection of the surrounding soft-tissues. This brief review, much of it drawn from work conducted in the author’s laboratory, seeks to highlight opportunities with focus on early-stage developments for image-based assessment of structure, function, physiology and mechanics of calcified tissues in humans via liquid and solid-state approaches, including proton, deuteron and phosphorus NMR and MRI.

The Role of Water Compartments in the Material Properties of Cortical Bone

“…Further studies are still necessary to determine the contribution of bound water to the apparent mechanical behavior of trabecular bone. Further development and application of high resolution MRI to trabecular bone is however crucial as it can image trabecular bone architecture as an alternative, non-irradiation method to peripheral quantitative computed tomography [100]. …”

Assessment of femoral bone osteoporosis in rats treated with simvastatin or fenofibrate

“…The 1 H CPMG (CarrPurcell-Meiboom-Gill) pulse sequence is a NMR method which correlated with Laplace inversion analysis lead to a relaxation T 2 spectrum that can be used to determine the porosity and to assess the pore size distribution in bone [10]. There are just a few studies by NMR spectroscopy [11] or relaxometry [12] on rats' bones or rat optical nerve and frog sciatic nerve [13], but there are many studies of human cortical bone by 1 H NMR relaxometry in particular 1D T 2 distribution [14][15][16][17][18][19] or 2D T 2 -T 2 exchange maps [20,21].…”

MR Imaging of the Pituitary Gland and Postsphenoid Ossification in Fetal Specimens

“…Calcification usually appears as a low signal on both T1WI and T2WI, but calcium salts interact with water protons in the process of ossification, in turn slowing the precession to near the Larmor frequency and potentially leading to short T1 relaxation times. 28,29 Knowledge of the origins of such intrasphenoid hyperintensities and the various shapes (condensed, diffusely spreading, and rimshaped) (Fig 4) may be beneficial for neuroradiologists in understanding the developmental process and avoiding a misdiagnosis of intrasphenoid tumor. Variations in the appearance of postsphenoid ossification may be attributable to the different physical shapes at different steps of the ossification process and the amount of water protons surrounding the calcium salts, but further study is required.…”