醫用磁振學 MRM 課程介紹與原理複習. Congratulations! Syllabus. From Basics to Bedside. You are HERE! License of Radiological Technologist 盧家鋒助理教授國立陽明大學生物醫學影像暨放射科學系

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1 Congratulations! You are HERE! License of Radiological Technologist 醫用磁振學 MRM 課程介紹與原理複習 盧家鋒助理教授國立陽明大學生物醫學影像暨放射科學系 2 From Basics to Bedside 磁振影像學 Magnetic Resonance Imaging 磁振成像原理硬體設備 射頻脈衝 組織對比 影像重建 脈衝波序 影像假影與安全 醫用磁振學 Magnetic Resonance in Medicine 磁振造影技術對比劑增強 功能性影像 擴散影像 血管攝影 頻譜分析 平行造影 等 陽明磁振造影室參觀與掃描磁振造影室環境介紹 操作介面 影像掃描 Syllabus 1 Review of MRI basic principles 2 Review of Pulse sequence diagram 3 Diffusion weighted imaging (DWI) 4 Diffusion tensor imaging (DTI) 5 MR angiography 6 MR contrast agent 7 MR perfusion: DCE & DSC 8 (4/8) No class this week due to cross-university activities 9 MR perfusion: arterial spin labeling (ASL) 10 (4/22) 16:00-18:00 Yang-Ming 3T MRI room visiting and scanning 11 Susceptibility weighted imaging (SWI) 12 Functional MRI (fmri) 13 (5/13) No class this week due to ISMRM annual meeting 14 MR Spectroscopy (MRS) 15 Cardiac MR imaging 16 MR muscle skeleton imaging 17 (6/17) Final Competition 3 4

2 MR Angiography Susceptibility weighted imaging MR spectroscopy Diffusion tensor imaging Fractional Anisotropy Principal axes Dynamic susceptibility contrast MRI rcbv rcbf MTT 參考書籍 MRI The Basics (3rd edition) Ray H. Hashemi, William G. Bradley, Christopher J. Lisanti Lippincott Williams & Wilkins,2010 MRI in Practice, (4th edition) Catherine Westbrook, Carolyn Kaut Roth, John Talbot Wiley Blackwell, 2011 上課教材與課程錄影 提供課後複習或其他未修課同學自修 點選 Teaching Materials 選單 MRM(UG) 連結本課程 7 8

3 評分標準 出席率 (30%) 課程參與度 (30%): 惟缺課達整學期 1/3 者, 總成績以不及格計算 期末考 (40%) Zuvio 課堂互動系統 請下載 App Zuvio 學生應用 登入帳號 : 預設密碼 : 請開啟 wifi 手機定位 Zuvio 點名 -GPS 簽到 11 12

4 Procedure of MRI Alignment (magnetization) B 0 Precession ω 0 = γb 0 Resonance (given B 1 by RF with ω 2 ) ω 1 = γb 1, B 1 B 0 磁振造影原理複習 MRI review The most effective resonance is produced when ω 0 = ω 2 MR signal (EMF, relaxation time ) Imaging (Pulse sequencing: SE, GRE, EPI) Tissue Contrast: Image weighting Spatial localization: Slice selection & Spatial Encoding Data space/k space Tissue Suppression Techniques Principles of MR imaging Alignment Precession Resonance Relaxation Imaging Setup Outer inner Active shielding Main magnet Shim coil Gradient coil Body coil Receive coil

5 RF Coil Shapes T1 & T2 Relaxation Time T1: The longitudinal relaxation time The spin-lattice relaxation time M z (t)=m 0 (1-e -t/t1 ) T2: The transverse relaxation time The spin-spin relaxation time M xy (t)=m 0 e -t/t2 RF coils need to be plugin on table!! medical.neusoft.com 17 recovery T1>T2>T2* 18 dephasing Received Signal: Free Induction Decay The oscillating, decaying signal is called an FID. M xy (t)=m 0 e -t/t2* (cosω 0 t) TR (Repetition Time) To spatially encode the signal and to increase the signal-tonoise ratio, we have to apply the RF pulse multiple times while varying the gradients. The time interval between RF pulses is called TR. The frequency of the received signal is also ω T1 recovery 20

6 TE (Time to Echo or Echo Delay Time) We wait a short period of time (TE) after RF pulse and then make the measurement. The T2* decay curve (FID) starts out at the value of M 0 (1-e -TR/T1 ) on the T1 recovery curve and then decays very quickly. Image Contrast Long TR, short TE proton density Long TR, long TE T2*-weighted Short TR, short TE T1-weighted Short TR, long TE no signal Example: T2* decay Adjust T1 and T2 weighting Intensity cross over effect T2W T1/T2/PD weighted Images T1W T2W PDW Time T1: CSF>GM>WM T2: CSF>GM>WM N(H): CSF>GM>WM T1: H 2 O > Solid tissue > Fat T2: H 2 O > Fat > Solid tissue N(H): H 2 O > Fat > Solid tissue CSF > edema > GM > WM 23 24

7 Image Construction 1. Slice selection (only excite spins on a specific slice location) 2. In-plane spatial encoding (differentiate spin signals at different locations) Image of K-Space The center of k-space contributes to the primary information of image. The periphery of k-space provides information regarding fitness of the image and clarity at sharp interfaces K Space Image Spin-echo pulse sequence diagram Slice select gradient Phase encoding gradient frequency encoding gradient Refocusing RF pulse echo rephasing dephasing An echo is acquired per TR. 27 Fast spin echo In FSE, before each 180 pulse, we place a different value of the phaseencoding gradient. For the 180 pulse before the echo we choose as the TE eff (in this case, 102 msec), we use a phase-encoding gradient with the lowest strength. Center slab: zero phase TE eff = 102 msec 28

8 GRE Pulse Sequence Diagram Three operatorcontrolled parameters that affect the tissue contrast. Single-shot EPI The phase-encode gradient is subsequently applied briefly during the time when the readout gradient was zero (200 μsec). An odd-even coverage of k-space blipped phase encoding TE eff SE-EPI (90 o -180 o -EPI) Eliminate ΔB ext T1 and T2 weighting Diffusion-weighted imaging Bipolar diffusion gradient For a "fixed-position" proton, this pair of gradients won't cause dephasing. Contrast in EPI Contrast in EPI depends on the "root" pulsing sequence SE-EPI (90 o -180 o -EPI) GRE-EPI (α o -EPI) IR-EPI (180 o -90 o -180 o -EPI) inversion-recovery (IR) 31 32

9 Suppression techniques To suppress the signal coming from a certain tissue. Two common targets (tissues): fat and water Glioblastoma MRI T2 Weighted image T2 FLAIR (Water suppression) Suppression techniques Inversion recovery (IR) techniques Chemical/spectral saturation Dixon method Spatial presaturation Magnetization transfer (MT) edema vs. water Breast cancer MRI T1 Weighted image T1: H 2 O > Solid tissue > Fat Gd contrast agent can shorten tissue T1 Fat saturation + Gd enhancement Inversion recovery, IR After the 180 o RF pulse, the magnetization starts to recover from -M 0 instead of zero. TI(null) = (ln2)t T1. M 0 M 0 (1-2e -t/t1 ) British Journal of Cancer (2003) 88(1), M 0 36

10 Tissue Suppression: STIR & FLAIR STIR: Short tau inversion recovery, fat suppression At 1.5T, TI = x 200 = msec FLAIR: Fluid attenuated inversion recovery, water suppression At 1.5T, TI = x 3600 = msec Water & fat chemical shift Peak location Water 4.7 ppm Fat (lipids) 1.3 ppm ppm: parts per million T2W T2 FLAIR (Better differentiation for multiple sclerosis) ω = 42.6 x 1.5T = 63.9 MHz = 42.6 x 3.0T = MHz 1.5T: ( ) x 63.9 = Hz 3.0T: ( ) x = Hz Chemical/spectral presaturation A frequency-selective presaturation pulse is applied before the RF excitation pulse. CHESS: Chemical shift selective We select appropriate frequency (based on the Larmor equation) to suppress fat or water. Fat sat 90 o pulse At 1.5T, water protons precess Hz faster than fat protons; At 3.0T, water protons precess Hz faster than fat protons. Spoiler gradient

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