Direct Depiction of Bone Microstructure Using ZTE Imaging (2011)

3, flip angle 4°, TR 1.5 ms, 65619 radial readouts, 2120 averages, resulting in a total scan time of 58 h. The sequence timing was based on a signal bandwidth of 200 kHz and involved the dwell time dw = 5 µs, readout duration 360 µs, acquisition oversampling 4, T/R switching duration 4.5 µs, RF pulse duration 1 µs, creating a dead time Δ = 1.375 dw. SRXTM: Data were acquired at the TOMCAT beamline of the Swiss Light Source, using a matrix size of 2048 2 × 1780 and a pixel size of (1.85 µm) 3 [15]. The scan time was 5 minutes at an x-ray energy of 17.5 keV. Sample: A piece of about (4 mm) 3 of trabecular bone was extracted from a bovine joint and dried under ambient conditions for about 6 months. Processing: The SRXTM data were down-sampled by a factor of 4 for easier handling. The two data sets were co-registered manually by allowing translational and rotational degrees of freedom. Results Figure 2 shows one axial and two longitudinal slices from the µMRI data, the SRXTM data, and an overlay of the two. The µMRI slices are displayed with bilinear interpolation, whereas the SRXTM images are maximum-intensity projections over the MRI slice thickness. In the µMRI images, the trabecular microarchitecture is well depicted with a medium signal level for the bone matrix. Marrow appears brighter due to higher proton density whereas large parts of the marrow spaces are empty due to shrinkage. Despite the lipid chemical shift of about 1000 Hz, only minor blurring is observed owing to the large acquisition bandwidth. Similarly, no strong off-resonance artefacts occur at interfaces to the surrounding air. The indicated bright artefact arises from a contamination in the probe located in the non-linear range of the gradients, which gave off a very short-T2 proton signal. After registration with the SRXTM images, the depiction of both bone and marrow shows excellent agreement between the two modalities. Discussion It has been demonstrated that the microstructure of trabecular bone can be depicted directly by MRI with zero echo time, relying on its particular ability to capture and spatially encode MR signal with very short T2. The associated high bandwidth also makes the ZTE technique robust against offresonance artefacts related to chemical shift or tissue-air interfaces. Unlike previous approaches that target negative contrast, no defined filling or other preparation of the sample is required. Furthermore, the ability to observe signal changes in the bone tissue itself might provide information about physiological and pathological conditions. Hence, while inferior to x-ray techniques in terms of spatial resolution and SNR, short-T2 µMRI can possibly provide useful complementary information in bone studies. To this end, additional contrast mechanisms for ZTE, such as preparation pulses in segmented schemes, are still to be explored. The long scan times inherent to MR microscopy can likely be somewhat reduced by specialized RF coil hardware. Moreover, higher water content is expected in fresh bone, thus providing larger signal and requiring less averaging. References [1] Gatehouse PD, MRI 22 (2004) 1061. [2] Wu Y, MRM 57 (2007) 59. [3] Sornay-Rendu E, J Bone Miner Res 2 (2007) 425. [4] Majumdar S, Osteoporosis Int 6 (1996) 376. [5] Boutroy S, J Clin Endicrinol Metab 90 (2005) 6508. [6] Schneider P, Microsc Res Tech 72 (2009) 690. [7] Schneider P, J Bone Miner Res, 22 (2007) 1557 [8] Jara H, MRM 29 (1993) 528. [9] Chung HW, J Bone Miner Res 10 (1995) 1452. [10] Palombarini M, JMRI 30 (2009) 825. [11] Hafner S, MRI 12 (1994) 1047. [12] Weiger M, ISMRM 2010, 695. [13] Jackson J, MRM 11 (1989) 248. [14] Kuethe DO, JMR 139 (1999) 18. [15] Stampanoni M, in: Developments in x-ray tomography V, Proc. SPIE 6318 (2006) 63180M. µMRI