Ultrashort Echo Time MRI of Cortical Bone at 7 Tesla Field Strength: A Feasibility Study

Department of Radiology and Biomedical Imaging, University of California, San Francisco, California, USA.
Journal of Magnetic Resonance Imaging (Impact Factor: 2.79). 09/2011; 34(3):691-5. DOI: 10.1002/jmri.22648
Source: PubMed

ABSTRACT To implement and examine the feasibility of a three-dimensional (3D) ultrashort TE (UTE) sequence on a 7 Tesla (T) clinical MR scanner in comparison with 3T MRI at high isotropic resolution.
Using an in-house built saddle coil at both field strengths we have imaged mid-diaphysial sections of five fresh cadaveric specimens of the distal tibia. An additional in vivo scan was performed at 7 Tesla using a quadrature knee coil.
Using the same type of saddle coil at both field strengths, a significant increase in SNR at 7T compared with 3T (factor 1.7) was found. Significantly shorter T2* values were found at the higher field strength (T2* = 552.2 ± 126 μs at 7T versus T2* = 1163 ± 391 μs at 3T).
UHF MRI at 7T has great potential for imaging tissues with short T2.


Available from: Sharmila Majumdar, Jun 15, 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ultrashort echo time (UTE) image to directly visualize short T2 tissues requires postprocessing for the suppression of the surrounding long T2 tissues in a clinical magnetic resonance imaging (MRI) scanner. Weighted subtraction of UTE images with an optimal weighting factor could provide high positive contrast with adequate suppression. To demonstrate in-vivo UTE MRI with weighted subtractions of dual echo UTE imaging using a 3 T clinical MRI and to determine the optimal weighting factors through the analyses of signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs). Forty-seven consecutive knee MR including dual echo 3D UTE sequence (TE of 70 µs and 3.82 ms) were reviewed. Seven patients with normal findings in the knee MR were included in the current study. For variably weighted subtraction images, the initial UTE image was rescaled relative to that of the second echo image with weighting factors from 0.1 to 4.0. Thirty-five pairs of the ROI measurements of the UTE image and second long echo images were repeated after 2 weeks. For quantitative assessment, SNR and CNR were measured from the ROIs on the patellar tendon, cartilage, cortical bone, meniscus, and infrapatellar fat pad. Intra-observer agreement was assessed by using both Cohen's Kappa and Bland-Altman approach. The short T2 tissue could be visualized with adequate suppression on the subtraction images. Considering the CNRs and SNRs, the optimal ranges of the weighting factors could be suggested: 0.3 for the tendon, 0.4 for the cortical bone, and 1.0 for the meniscus. The 35 paired measurements showed excellent agreement with statistical significance (P < 0.05). The 3D UTE MRI provides imaging of short T2 tissues which cannot be visualized by conventional MRI. Using weighted subtractions with optimal weighting values, each tissue can be optimally depicted by overcoming the reduced T2 contrast.
    Acta Radiologica 08/2013; 55(4). DOI:10.1177/0284185113496994 · 1.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The goal of this study was to test the potential of ultra-short echo-time (UTE) MRI at 1.5, 3.0 and 7.0 T for depiction of trabecular bone structure (of the wrist bones), to evaluate whether T2 * relaxation times of bone water and parametric maps of T2 * of trabecular bone could be obtained at all three field strengths, and to compare the T2 * relaxation times with structural parameters obtained from micro-computed tomography (micro-CT) as a reference standard. Ex vivo carpal bones of six wrists were excised en bloc and underwent MRI at 1.5, 3.0 and 7.0 T in a whole-body MR imager using the head coil. A three-dimensional radial fat-suppressed UTE sequence was applied with subsequent acquisitions, with six different echo times TE of 150, 300, 600, 1200, 3500 and 7000 µs. The T2 * relaxation time and pixel-wise computed T2 * parametric maps were compared with a micro-computed-tomography reference standard providing trabecular bone structural parameters including porosity (defined as the bone-free fraction within a region of interest), trabecular thickness, trabecular separation, trabecular number and fractal dimension (Dk ). T2 * relaxation curves and parametric maps could be computed from datasets acquired at all field strengths. Mean T2 * relaxation times of trabecular bone were 4580 ± 1040 µs at 1.5 T, 2420 ± 560 µs at 3.0 T and 1220 ± 300 µs at 7.0 T, when averaged over all carpal bones. A positive correlation of T2 * with trabecular bone porosity and trabecular separation, and a negative correlation of T2 * relaxation time with trabecular thickness, trabecular number and fractal dimension, was detected (p < 0.01 for all field strengths and micro-CT parameters). We conclude that UTE MRI may be useful to characterize the structure of trabecular bone, comparable to micro-CT. Copyright © 2014 John Wiley & Sons, Ltd.
    NMR in Biomedicine 10/2014; 27(10). DOI:10.1002/nbm.3169 · 3.56 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The objective of this study was to demonstrate the feasibility of depicting the internal structure of the Achilles tendon in vivo using high-resolution 3-dimensional ultrashort echo-time (UTE) magnetic resonance imaging at 7 T. For our UTE imaging, a minimum-phase radiofrequency pulse and an anisotropic field-of-view 3-dimensional radial acquisition were used to minimize the echo time and scan time. A fat saturation pulse was applied every 8 spoke acquisitions to reduce blurring and chemical shift artifacts from fat and to improve the dynamic range of the tendon signal. Five healthy volunteers and 1 patient were scanned with an isotropic spatial resolution of up to 0.6 mm. Fat-suppressed UTE images were qualitatively evaluated and compared with non-fat-suppressed UTE images and longer echo-time images. High-resolution UTE imaging was able to visualize the microstructure of the Achilles tendon. Fat suppression substantially improved the depiction of the internal structure. The UTE images revealed a fascicular pattern in the Achilles tendon and fibrocartilage at the tendon insertion. In a patient who had tendon elongation surgery after birth, there was a clear depiction of disrupted tendon structure. High-resolution fat-suppressed 3-dimensional UTE imaging at 7 T allows for the evaluation of the Achilles tendon microstructure in vivo.
    Investigative radiology 02/2014; DOI:10.1097/RLI.0000000000000025 · 4.45 Impact Factor