Quantitative analysis of vibration waves based on Fourier transform in magnetic resonance elastography
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Quantitative analysis of vibration waves based on Fourier transform in magnetic resonance elastography Ikuho Kosaka1 · Yuki Kanazawa2 · Kotaro Baba3 · Hiroaki Hayashi4 · Masafumi Harada2 Received: 10 February 2020 / Revised: 29 July 2020 / Accepted: 31 July 2020 © Japanese Society of Radiological Technology and Japan Society of Medical Physics 2020
Abstract We developed a novel magnetic resonance elastography (MRE) analysis method based on Fourier transform to assess the responsive characteristics for different tissue stiffness and degree of transmission of the vibration wave emanating from a passive driver during MRE. A phantom tissue study was conducted with an MRE sequence and vibration wave system using a clinical MR scanner. The phantom tissue consisted of two layers of agar: 0.75 wt% and 1.0 wt%. Phase-unwrapped images derived from acquired MRE phase images were used to generate a phase profile curve, with a line plotted for the phase-unwrapped images. Fourier transform was performed, and the peak value of the power spectrum was derived. The damping rate/ratio was calculated using the Hilbert transform of the phase profile. We found that the mean shear stiffness value of 1.0 wt% agar was higher than that of 0.75 wt% agar. The responsive frequency of the 0.75 wt% agar layer showed a wider range and the damping rate of the signal showed a higher value than the respective values of the 1.0 wt% agar layer. In conclusion, Fourier transform analysis of MRE enabled us to obtain more detailed information of the tissue characteristics and vibration-wave conditions. Keywords Damping rate · Frequency analysis · Magnetic resonance elastography (MRE) · Magnetic resonance imaging (MRI) · Power spectrum · Hilbert transform
1 Introduction Micrometer shear wave propagation is used to measure the shear elastic modulus (tissue stiffness) of biological tissues in magnetic resonance elastography (MRE). The MRE technique is generally based on synchronization of phase contrast imaging with the vibrations received from a passive driver that emits a specific frequency; the vibrations * Yuki Kanazawa yk@tokushima‑u.ac.jp 1
Radiology Department, Rakuwakai Marutamachi Hospital, Marutamachi‑noboru, Shichihonmatsudori, Nakagyo‑ku, Kyoto, Kyoto 604‑8401, Japan
2
Institute of Biomedical Sciences, Tokushima University Graduate School, 3‑18‑15, Kuramoto‑cho, Toksuhima, Tokushima 770‑8509, Japan
3
National Hospital Organization, 513, Saijocho‑ji Temple, Higashihiroshima, Hiroshima 739‑0041, Japan
4
Faculty of Health Sciences, Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University, 5‑11‑80 Kodatsuno, Kanazawa, Ishikawa 920‑0942, Japan
generate a micrometer wavelength shear wave using an acoustic driver system [1]. The clinical application of MRE includes the measurement of the stiffness of tissues of various organs, such as the liver [2], breast [3], brain [4], and skeletal muscle [5]. The use of MRE of the liver has become widespread since its introduction as an imaging modality for clinical use. So
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