Assessment of Nuclear Magnetic
Resonance Bone Technology
Inclusive Dates: 09/20/00 - 01/19/01
Background - Bone microdamage is generally defined as a matrix failure in the form of microcracks detectable by destructive light microscopy. Microcracks have been defined as cracks that can be detected using relatively low magnification (less than 250´) and are usually on the order of 30 to 100 micrometers in length. Additional bone matrix damage, called diffuse damage, occurs at the ultrastructural level and is characterized as cracks less than 1 micrometer in length. Currently, light microscopy (histomorphometry) is the only method to provide a direct assessment of cortical bone microstructural characterization including microdamage. However, it cannot provide information concerning three-dimensional structure effects and this technique requires an invasive surgery procedure. Thus, the research team investigated the use of a low-field pulsed nuclear magnetic resonance (NMR) relaxation technique as a noninvasive means to quantify microdamage in cortical bone. In this project, it was demonstrated that the microdamage in cortical bone can be characterized by low-field pulsed NMR. Different degrees of the damage to the fracture toughness of cortical bones specimen were produced by mechanical testing, and the resulting microstructural changes such as porosity and pore size changes were determined by NMR methodology. Briefly, spin-spin relaxation (T2) measurement and its inversion T2 spectral analysis technique have will been utilized. It is known that the NMR proton spin-spin (T2) relaxation time technique was used to determine the changes of porosity, pore size distribution and the average T2 relaxation time at the different pore regions due to the presence of microdamage in the cortical bone. This technique works because the observed proton NMR relaxation signals are a convolution of the relaxations of fluid in the various pores including microdamaged areas throughout the observed cortical bone. This noninvasive and nondestructive method has great potential for in vivo applications.
Approach - Efforts during this project included: 1) Samples prepared for designed experiments including small pieces of the bone samples obtained from the different location of the human cardaveric cortical bone. Except for the referenced samples, all the samples will be damaged at different degrees by a mechanical treatment simulating the fracture toughness damage during progressive failure. 2) A special optimized radio frequency coil prepared for NMR studies of the small bone sample, NMR measurements; 3) the correlation between the damage of bone mechanical properties with the micostructural changes, particularly the correlation between the pore size distribution changes measured by NMR with the results from the light microscopy; and 4) determination of the feasibility and the limitation of the radio frequency surface coil technique for volume selective measurement on human cortical bone.
Accomplishments - Test samples were evaluated in a proton frequency of 27 megahertz. Each specimen was approximately at 2 × 6 × 20 mm3. T2 relaxation spectra were measured by NMR CPMG sequence with a 5.1-microsecond 90° pulse, 500-microsecond duration between 90° and 180° pulses, and 10 seconds for repetition time. For each T2 profile, eight hundred echoes were acquired, and sixty scans were used. In this microdamage test, the team compared histomorphometrically (light microcopy) quantified bone damage to shifts in NMR T2 relaxation spectra. The team measured the NMR T2 relaxation of bovine cortical bone specimens before and after mechanically induced damage. Rectangular beams of cortical bone were machined under constant irrigation from fresh bovine tibia obtained from a local butcher shop. After acquisition of the initial NMR T2 relaxation spectra, each specimen was subjected to a loading protocol that is known to produce damage in cortical bone. The specimens were loaded in three points bending at a cross head rate of 1 millimeter per minute (mm/min) to 1.5 times the strain at yield. Yield properties were determined from a preliminary test of three separate specimens. The specimens were held at this strain level for one minute (damage accumulation phase) and then unloaded at a constant cross head displacement rate of 1 mm/min. Each specimen was then subjected to a second loading cycle identical to the first. After mechanical damage induction, the NMR relaxation spectra for each specimen were acquired again. Each section was observed using brightfield microscopy at 100× (Zeiss Axioskop). The crack length, the breaking stress, and NMR T2 mean ratio of before bone damage to after induced bone damage for five separate regions across the thickness of each specimen were quantified. Microdamage parameters were compared to the recorded bending stress at failure for each specimen. Finally, bone microdamage was characterized as the ratio of final T2 relaxation to initial T2 relaxation.