Atsushi M Takahashi

University of California, San Diego, San Diego, CA, United States

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Publications (5)13.63 Total impact

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    ABSTRACT: There is increasing interest in imaging short T2 species which show little or no signal with conventional magnetic resonance (MR) pulse sequences. In this paper, we describe the use of three-dimensional ultrashort echo time (3D UTE) sequences with TEs down to 8 μs for imaging of these species. Image contrast was generated with acquisitions using dual echo 3D UTE with echo subtraction, dual echo 3D UTE with rescaled subtraction, long T2 saturation 3D UTE, long T2 saturation dual echo 3D UTE with echo subtraction, single adiabatic inversion recovery 3D UTE, single adiabatic inversion recovery dual echo 3D UTE with echo subtraction and dual adiabatic inversion recovery 3D UTE. The feasibility of using these approaches was demonstrated in in vitro and in vivo imaging of calcified cartilage, aponeuroses, menisci, tendons, ligaments and cortical bone with a 3-T clinical MR scanner. Signal-to-noise ratios and contrast-to-noise ratios were used to compare the techniques.
    Magnetic Resonance Imaging 03/2011; 29(4):470-82. DOI:10.1016/j.mri.2010.11.003 · 2.02 Impact Factor
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    ABSTRACT: Imaging of short-T(2) species requires not only a short echo time but also efficient suppression of long-T(2) species in order to maximize the short-T(2) contrast and dynamic range. This paper introduces a method of long-T(2) suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long-T(2) water and the second one inverts that of fat. Short-T(2) species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 mus starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long-T(2) species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3-T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon.
    Magnetic Resonance in Medicine 02/2010; 63(2):447-55. DOI:10.1002/mrm.22257 · 3.40 Impact Factor
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    ABSTRACT: Short T(2) species such as the Achilles tendon and cortical bone cannot be imaged with conventional MR sequences. They have a much broader absorption lineshape than long T(2) species, therefore they are more sensitive to an appropriately placed off-resonance irradiation. In this work, a technique termed ultrashort TE (UTE) with off-resonance saturation contrast (UTE-OSC) is proposed to image short T(2) species. A high power saturation pulse was placed +1 to +2 kHz off the water peak to preferentially saturate signals from short T(2) species, leaving long T(2) water and fat signals largely unaffected. The subtraction of UTE images with and without an off-resonance saturation pulse effectively suppresses long T(2) water and fat signals, creating high contrast for short T(2) species. The UTE-OSC technique was validated on a phantom, and applied to bone samples and healthy volunteers on a clinical 3T scanner. High-contrast images of the Achilles tendon and cortical bone were generated with a high contrast-to-noise ratio (CNR) of the order of 12 to 20 between short T(2) and long T(2) species within a total scan time of 4 to 10 min.
    Magnetic Resonance in Medicine 08/2009; 62(2):527-31. DOI:10.1002/mrm.22007 · 3.40 Impact Factor
  • Jiang Du, Atsushi M Takahashi, Christine B Chung
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    ABSTRACT: To investigate ultrashort TE spectroscopic imaging (UTESI) of short T2 tissues in the musculoskeletal (MSK) system. Ultrashort TE pulse sequence is able to detect rapidly decaying signals from tissues with a short T2 relaxation time. Here a time efficient spectroscopic imaging technique based on a multiecho interleaved variable TE UTE acquisition is proposed for high-resolution spectroscopic imaging of the short T2 tissues in the MSK system. The projections were interleaved into multiple groups with the data for each group being collected with progressively increasing TEs. The small number of projections in each group sparsely but uniformly sampled k-space. Spectroscopic images were generated through Fourier transformation of the time domain images at variable TEs. T2* was quantified through exponential fitting of the time domain images or line shape fitting of the magnitude spectrum. The feasibility of this technique was demonstrated in volunteer and cadaveric specimen studies on a clinical 3T scanner. UTESI was applied to six cadaveric specimens and four human volunteers. High spatial resolution and contrast images were generated for the deep radial and calcified layers of articular cartilage, menisci, ligaments, tendons, and entheses, respectively. Line shape fitting of the UTESI magnitude spectroscopic images show a short T2* of 1.34 +/- 0.56 msec, 4.19 +/- 0.68 msec, 3.26 +/- 0.34 msec, 1.96 +/- 0.47 msec, and 4.21 +/- 0.38 msec, respectively. UTESI is a time-efficient method to image and characterize the short T2 tissues in the MSK system with high spatial resolution and high contrast.
    Journal of Magnetic Resonance Imaging 02/2009; 29(2):412-21. DOI:10.1002/jmri.21465 · 2.79 Impact Factor
  • Jiang Du, Mark Bydder, Atsushi M Takahashi, Christine B Chung
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    ABSTRACT: Tissues with very short transverse relaxation time (T2) cannot be detected using conventional magnetic resonance (MR) sequences due to the rapid decay of excited MR signals. In this work, a multiecho sequence employing half-pulse excitation and spiral sampling was developed for ultrashort echo time (UTE) imaging of tissues with short T2. Spiral readout gradients were measured and precompensated to reduce gradient distortions due to eddy currents and gradient anisotropy. The effects of spatial blurring due to fast signal decay were investigated experimentally through spiral UTE (SUTE) imaging of rubber bands with different spiral sampling duration. The unwanted long T2 signals were suppressed through the use of an inversion pulse and nulling, and/or subtraction of a later echo image from the initial one. This technique has been applied to imaging of the short T2 components in brain white matter, knee cartilage, bone and carotid vessel wall of normal volunteers at 1.5 T. Preliminary results show high spatial resolution and excellent image contrast for a variety of short T2 tissues in the human body under a relatively short scan time. A quantitative comparison was also made between radial UTE and SUTE in terms of signal-to-noise ratio efficiency.
    Magnetic Resonance Imaging 05/2008; 26(3):304-12. DOI:10.1016/j.mri.2007.08.005 · 2.02 Impact Factor