磁振造影的特性 MRI 基本物理原理 新光吳火獅紀念醫院放射診斷科 技術專員李正輝 無放射性 高解析度 高組織間對比 多重切面 ( Sagittal view Coronal view Axial view) ---- 未來百年內醫學影像的主流 何謂 MRI? M (magnetic): 訊號的來源

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2 磁振造影的特性 MRI 基本物理原理 新光吳火獅紀念醫院放射診斷科 技術專員李正輝 無放射性 高解析度 高組織間對比 多重切面 ( Sagittal view Coronal view Axial view) ---- 未來百年內醫學影像的主流 何謂 MRI? M (magnetic): 訊號的來源, 人體中小磁鐵的磁化 R (resonance): 共振, 小磁鐵激發偵測的原理, 小磁鐵和射頻脈衝間的交互作用 I (imaging) : 訊號轉為影像的方式 磁化 (Magnetization) 1

3 Magentic susceptibility 所有物質放在磁場中時, 均有一定的磁化程度, 而一個物質的感磁性是用來衡量他們被磁化了多少 Magentic susceptibility MRI 所使用的物質共可分為三種不同的感磁性 - 順磁性 反磁性和鐵磁性 反磁性順磁性 B=μ*H B= 磁通密度 (magnetic flux density) μ= 磁導率 (magnetic permeability) H= 磁場強度 (magnetic field intensity) 物質的磁性與在 MRI 的用途 順磁性 (paramagnetic) 物質 : 導磁係數 (μ) 與空氣相近者, 如 : 鋁 鉻等物質 ( 釓 鏑 血液 ) 反磁性 (diamagnetic) 物質 : 導磁係數 (μ) 比空氣或真空小者, 如 : 金 銀等物質 ( 水及大部份組織 ) 鐵磁性 (ferromagnetic) 物質 : 導磁係數 (μ) 比空氣大者, 如 : 鐵 鋼等物質 ( 動脈瘤夾 榴霰彈片 ) 自旋 (Spin) 古典物理中, 旋轉中的物體具有角動量 (angular momentum) 的特性, 其大小和物體的外型 尺寸 質量以及向量的大小有關 而在原子或次原子的領域中, 則以自旋來表示, 也就是說明粒子繞著本身軸位旋轉的特性 2

4 人體內的小磁鐵 電子 質子帶有電荷且有自旋現象, 故其行為類似於微小的電流迴路, 因為移動的電荷會產生磁場, 故電子 質子就好像是微小的磁鐵, 有著北極和南極, 故稱為磁偶極 (magnetic dipole) 不同於古典力學的想法, 自旋角動量只能有某些特定的值, 即有量子化的現象, 例如電子 質子等粒子的自旋量子數 (S) 為 1/2, 其自旋角動量在某一軸向的分量只能有兩個值 ( 能階數 = 2S+1), 正負號表示方向, 故有 spin-up 與 spin-down 兩種狀態之稱 NET MAGNETIC FIELD 根據庖立不相容原理 (Pauli exclusion principle), 兩個電子不能處在同一個量子狀態, 故若其一為 spin-up, 另一必為 spin-down 若一原子有偶數個電子, 即有偶數個質子, 則 spin-up spin-down 兩種狀態互相抵消, 所以無法有可被觀察到的自旋現象 然而, 若一原子有奇數個電子 ( 即不成對 ), 亦有奇數個質子, 此時自旋現象才能被彰顯出來, 氫原子就是其中一個例子 原子核中質子數目為偶數 (even)* 她們的磁場會互相抵消, 而使得淨磁場為 0 原子核中質子數目為奇數 (odd)* 因而可以產生一個淨磁場或稱磁矩極矩 (magnetic diopole moment, MDM) 3

5 淨磁偶極的平衡 沒有外加磁場時, 氫原子核 ( 僅有一個質子 ) 的磁偶極沒有特定的指向, 淨磁化強度 ( 所有磁偶極的加總,net magnetization) 等於 0 當外加磁場施加時. 場域內的氫原子核, 受主磁場的強大作用力影響, 產生順著主磁場方向 (spin-up) 與逆著主磁場方向 (spindown) 的排列 最終, 在相互抵消的作用下, 產生一個順著主磁場方向 (spin-up) 的靜磁矩 原子的旋進 (Precession) 除了在自己的軸位上產生自旋, 質子也會順著主磁場的方向, 以特定的頻率產生繞進的現象, 我們稱之為 --- 旋進 共振 (resonance) 4

6 Radio Frequency Pulse 磁場的向量變化 (RF 施加前 ) 電磁波 --- 傳遞能量 能量的傳遞須符合 ---- 拉莫頻率 施加的方向與 B0 垂直 將縱向磁矩偏轉到橫向平面 旋進頻率 ( 拉莫頻率 ) 當 RF 施加時. RF 頻率與系統之頻率相同 能量傳遞給系統, 將 spin-up 之質子轉移至 spin-down 縱向磁矩偏轉至 XY 軸平面, 磁矩消減為 0 橫向磁矩因質子同相, 產生最大橫向磁矩 5

7 如果將 RF 關掉. 縱向磁矩回復至 Z 軸平面, 磁矩回復為 M0 橫向磁矩因質子產生失相, 橫向磁矩消減為 0 縱向磁場的回復 ---- T1 recovery 橫向磁場的衰減 ---- T2 decay 發生了什麼事??? 弛緩 在 B1 關閉後, 氫原子核要從激發狀態回到平衡狀態, 與主磁場對齊, 主要有兩個互相獨立的歷程, 分別稱為自旋晶格弛緩 (spin-lattice relaxation) 和自旋自旋弛緩 (spinspin relaxation), 分別是 Z 分量的回復和 X-Y 分量的歸零, 其弛緩的時間常數 (time constant) 分別稱為 T1 和 T2, 故又稱為 T1 弛緩和 T2 弛緩 T1 弛緩 氫原子核將先前吸收的能量以熱能的方式釋放到鄰近的組織 (lattice) 中, 使得氫原子核可和主磁場對齊 弛緩的時間常數, 即 Z 分量回復到原來 M0 的 63% 所需的時間稱為 T1, 大約需要 5 倍 T1 的時間,Z 分量可完全恢復 6

8 T1 弛緩 (relaxation) 不同組織的 T1 曲線 T2 弛緩 當有 B1 磁場時, 眾多氫原子核以同樣的相位 (phase) 自旋, 當 B1 關閉時, 外力的協助消失, 氫原子核間會有隨機的運動, 彼此碰撞交換能量 ( 所以稱為 spin-spin relaxation), 使相位一致性 (phase coherence) 消失, 有的氫原子核進動較快, 有的氫原子核進動較慢, 使 X-Y 分量互相抵消逐漸回復到零 X-Y 分量減少到 M0 的 37%( 也就是衰減了 63%) 的時間稱為 T2,T2 通常短於 T1 T2 弛緩 (relaxation) 7

9 不同組織的 T2 曲線 T2 vs T2* 造成橫向磁場衰減的原因 : --- Spin-Spin 間的交互作用 ( 失相 ) --- 主磁場的不均勻 ( 可以被修正 ) T2 --- 造成橫向磁場衰減的原因僅為 Spin-Spin 間的交互作用 T2*--- Spin-Spin 間的交互作用 + 主磁場的不均勻 想, 想, 想.. 成像與造影 (Imaging) 不同組織在 T1 T2 都有不同訊號的表現 在適當的時間擷取訊號 不同的組織就可以被區分出來了! 傅立葉轉換 ---- 時間函數 頻率函數 適當的空間編碼, 填入 K-space 中,MRI 的影像就出來啦!!! 8

10 成像 ( 空間編碼 ) MRI 量測到的是人體某一區塊中的所有氫原子核激發 弛緩的訊號, 為了要了解人腦中不同位置的結構或功能性變化, 必須在訊號中加入空間位置的訊息, 簡單來說, 這分為在 Z 方向的切面選擇 (slice selection),x-y 平面上任一軸例如 X 方向的頻率編碼 (frequency encoding), 和 X-Y 平面上另一軸例如 Y 方向的相位編碼 (phase encoding), 要在那個方向做切面選擇 頻率 相位編碼視實際需求而定 所用到的技術是梯度磁場的概念以及二維傅利葉轉換 (2-D Fourier transform) 相位 vs. 頻率 MRI 訊號 : 具週期性, 以 sin or cos 呈現, 具有 0~360 o 的相位變化 相位 (Phase): 在特定的時間點中訊號波形 (Waveform) 循環中的位置 頻率 (Frequency): 單位時間內, 產生訊號週期的數目 (Cycle/Sec) 訊號 頻率 相位與相位偏移 切面選擇 (Slice Selection) 空間編碼的第一步 在切面方向施加一個梯度磁場, 藉此在不同的位置產生相對不同的磁場強度 (B0+Gz) 9

11 選擇切面的方式 原始的訊號量, 加上相位與頻率的資訊, 以完整的訊號模式呈現 頻率編碼 (Frequency Encoding) 相位編碼 (Phase Encoding) 10

12 空間編碼的結果 ( 相位 + 頻率 ) 相位編碼與 TR Data Space to image 主磁場 ---- 永久磁場 ---- 電磁場 ---- 超導磁場 線圈 (coil) ---- Gradient coils ---- Shim coils ---- 射頻線圈 ---- 接收線圈 硬體上的需求 11

13 磁場產生的方式 永久磁場 永久磁場 電磁場 --- 電生磁, 磁生電 超導磁場 --- 導體的電阻為零 --- 指電流流通時無阻力的現象, 也就是產生永久電流 (persistent current) --- 絕對低溫 (4-6 K) 超導磁場 Gradient coils X 軸 Y 軸 Z 軸都需要形成梯度磁場, 以不同的比例組合就可以得到不同的斜切位 12

14 Gradient coils Gz slice-selection 切面選擇 Gy phase-encoding 相位編碼 Gx frequency-encoding 頻率編碼 sliceselection phaseencoding Axial z y x Sagittal x y z coronal y x z frequency -encoding 被動式 --- 維持主磁場的均勻性 主動式 Shim coils 維持局部磁場的均勻性, 特別是在梯度回音或脂肪的化學位移消除技術,shimming 可以使變動降低但不盡然完全消除 MRI safety 影響裝置功能或 MRI 儀器本身產生之危害 --- 心臟節律器 --- 血管夾 --- 金屬物品 ( 剪刀 髮夾 氧氣瓶..) RF 產生的熱效應 --- 紋眉 紋身 --- 精油 髮膠 --- 掃描過程中病人身體 ( 皮膚 ) 不要直接觸碰磁體內壁及各種導線 13

15 SAR (Specific Absorption Ratio) 每單位質量的物體, 因 RF 能量傳遞, 造成物體能量吸收的比例 ( 單位 : W/Kg) 依據 FDA 的規範, 不得超過 4 W/Kg 不同的部位,SAR 值的吸收也有所不同 SAR 值產生的效應, 以熱的方式來表現 如何降低 SAR 值 使用 quadrature coil 進行 RF 的發送 在有適合的 coil 可同時進行 RF 的發送與接下, 盡量避免使用 body coil 進行 RF 的發送 增加 TR 減少掃描張數 減少 ETL 的長度 減少 TSE 中 refocusing pulse flip angle 14

16 Contrast 對比 (Contrast) 對比的定義 影響對比的參數與臨床應用 Contrast was introduced in terms of the image appearance, or relative brightness of different tissues and pathology. Image contrast arises (or doesn t) when tissues generate MR signals which have different intensities because of their physical properties, i.e. T1 and T2 relaxation times and proton density. 15

17 16

18 TR and tissue contrast TE=10 TE and tissue contrast Spin Echo short TR, short TE T1-weighted M z rel. short T 1 like WM M xy rel. short T 2 like WM rel. long T 1 like GM long T 2 like GM TR TR/ms TE TE/ms TR=

19 T1 effects and pathology Spin Echo long TR, long TE T2-weighted T1 effects on the image short T1 - bright fat, fresh bleeding paramagnetic contrast agent (gadolinium) long T1 - dark Neoplasm, edema, inflammation, pure fluid, CSF M z rel. short T 1 like WM rel. long T 1, like GM TR/ms TR M xy rel. long T 2 long T 2 TE TE/ms Clinical image appearance T2 Effects on the image Spin Echo rel. long TR, short TE proton density weighted short T2 - dark iron deposits in liver, magnetic suseptibility effects M z GM WM M xy long T2 - bright edema, inflammation, Gliosis, pure fluid, CSF TR/ms TR TE TE/ms TR > 1800 ms TE > 80 ms 18

20 Clinical image appearance proton density effects Flip angles and contrast low proton density- dark calcium, air, cortical bone, ligaments high proton density- bright fat, bone marrow TR ~ 1200 ms TE ~ 25 ms TR=150, TE=4.6 TI and contrast SE vs.ir TR=4000, TE=19 19

21 Invention recovery (IR) STIR M Null point TI fat White matter water time STIR image Flair M TI Null point fat White matter water time 20

22 FLAIR in Brain MRI Disadvantages of IR Longer scan times Increase in flow-related artifacts Signal-to-noise can decrease as tissues are suppressed Higher specific absorption rate (SAR) due to additional 180 pulses 休息一下吧!!! 21

23 本次課程內容 磁振造影專業基礎課程 MR 影像建構與脈衝序列圖 MR Image Construction & Pulse Sequence Diagram 周嘉豪醫事放射師 台北慈濟醫院放射診斷科慈濟科技大學醫學影像暨放射科學系 MR 基本原理與自旋回音 (Spin echo) 脈衝序列圖 (Pulse sequences diagram) 空間編碼 (Spatial encoding) Slice selection encoding(gz) Frequency encoding(gx) Phase encoding(gy) K-space 與訊號填入 快速自旋回音 (Fast spin echo (FSE)) Reference: 1.MRI The Basics (3rd) (Chapter 7~14) 2.MRI IN PRACTICE(4td) (Chapter 3.5) 3.MRI From Picture to Proton(2nd)(Chapter 7) 1 2 MRI 的成像過程 磁場 磁化現象 RF 脈衝 磁化量激發 梯度磁場 切面選擇 B 0 ω x M z ω y x RF z Mz=M Mxy=0 y MR basic principle 5/ ( 每一百萬個氫原子核中最多 5~6 個 ) Anti-Parallel Higher energy state 梯度磁場 空間編碼 ( 相位編碼 頻率編碼 ) M 0 =0 信號取號 接收線圈取樣頻率 RF receive 影像計算 FOURIER 轉換 FT B0 Parallel Low energy state 3 4

24 淨磁矩與外加磁場的關係 外加磁場愈大淨磁矩愈大 z M 0 M 0 如何偏轉人體磁鐵? 電磁波產生一橫向磁矩 (B 1 ) 使人體磁鐵 (M) 偏轉 RF 的作用下 M 0 的運動軌跡為螺旋形 z M MX Z Y M 0 y B 0 B 1 x y X x 5 On RF Transmit RF 脈衝 ( 極微弱磁場 ) B1 為振盪磁場 ( 電磁波的磁分量 ) 6 RF 常用的脈衝角度 Larmor frequency ω=γb 0 傾角 (flip angle) Θ=γB 1 t (B 1 為 RF 脈衝磁場,t 為時間 ) 共振 (resonance) 在頻率相同的條件下所進行之能量轉移的現象 Anti-Parallel Higher energy state On RF Transmit x B 1 M Z Z Z 90 O 180 O α M M y B 1 B 1 y B 1 x x y B0 Resonance 輸入電磁波的能量恰好等於 hv 則原子核能吸收此能量被激發至較高的能量狀態 ( 輸入的 RF 頻率等於 ω) Parallel Low energy state 7 M 0 8

25 共振 (resonance) B0 Anti-Parallel Higher energy state Parallel Low energy state OFF RF Transmit 共振才能產生訊號 RF Receive 9 dephase 和 rephase 的過程 相同時間後回到原點獲得最大訊號 (rephase) x B1 90 度 RF z 隨著時間愈長,dephase 愈多 感受磁場小旋進較慢 rephase B1 180 度 RF y 繞著 B1 旋進 rephase 感受磁場大旋進較快 10 自旋迴訊 (spin echo) 脈衝序列 磁振造影儀器基本硬體 - 線圈與磁場 信號強度 TE/2 90 o 180 o TE/2 TE/2 T2 T2* dephase rephase FID TE:RF 脈衝到回音中心的時間 SE 可以抵消磁場的不均勻 echo TE/2 回音 t 11 t xy z 產生淨磁向量給予 slice selection, phase encoding 和 frequency encoding 所需的磁場梯度 可給予 RF 脈衝, 也可以接收信號 以銅牆, 銅門, 銅窗阻隔外面的電磁波 銅牆鐵壁 ( 屏壁 ) 墊補磁鐵 (shim coil) 修飾磁場的均勻度 ±1~5ppm 穩定度 :10ppm/h (6~7 best) 分為被動式 ( 小金屬片 ) 主動式 (shim coil) 12

26 梯度線圈 gradient coils Alter the primary magnetic field Gradient coils in the in the x, y & z axis Responsible for loud noises of MRI z y x 梯度磁場 (gradient coil,gx, Gy,Gz) z( slice-select) x (phase-encoding) y (readout or frequency-encoding) 磁場方向沿著 3 軸, 強度隨著位置改變 (ex: 沿著 z, 隨 xy 改變 ) 故意製造磁場均勻度的擾動 改變氫原子核感受到的磁場大小 z MRI +G Z : no signal X-Y : signal z B z x y xy 13 -G x G= B/ x= 斜率 (mt/m) 任一位置相對於中心點 B=Gx 14 梯度磁場 - 相角隨位置而改變 : ω 0 =γb 0 ω 0 + ω=γ(b 0 +Gy) ω 0 - ω=γ(b 0 -Gy) 梯度是由一點到另一點的磁場改變 - 磁場的擾動通常是 linear Spin Echo (SE) 波序圖 TR 三個軸暫時製造出線性磁場的不均勻, 取得的位置資訊 Max gradient 斜率 = Gy = y 梯度強度 RF 脈衝 Gz 選層編碼 t t y Gy 相位編碼 t Max gradient No gradient cosine 波形 15 from: 鍾孝文教授 ppt 改編動畫 Gx 頻率編碼空間編碼 Echo 回音 TE rephase dephase dephase t t 16

27 梯度磁場下的訊號衰減訊號強度RF FID T2* TR Echo RF 梯度磁場和變化磁場強度 (Gradients and changing field strength) (How to select a slice?) negative positive shallow steep ω(z) =γ( B 0 + Gz z ) A B C ω 0 =γb 0 ω 0 + ω=γ(b 0 +Gz) ω 0 - ω=γ(b 0 -Gz) G dephase TE rephase dephase T T (9997G) (9995G) 41.20MHz 42.54MHz 1.0T (10000G) 42.6MHz 1.003T 1.005T (10003G) (10005G) 43.80MHz 42.78MHz RF 41.20MHz-A RF 43.80MHz-C RF Transmit 18 切面厚度 slice thickness ω(mhz) 68 Gz Gz We can excite one slice by an RF pulse with a specific frequency range. This range of frequencies determines the slice thickness and is referred to as the bandwidth. ( 射頻脈衝頻寬和梯度強度的斜率有關 ) 選層梯度磁場 (slice-select gradient, Gz) We transmit an RF pulse with a bandwidth that has the appropriate center frequency. This gradient is turned on only when we transmit the RF pulses. 窄頻寬 ω=γb =γ( Bo + Gz z ) 一個 ω 對應到一個 z 位置 ω 範圍對應到 z 位置的範圍 ω (BW) = γ Gz z When we transmit the 180 o pulse(rephasing pulse) for the same slice, we activate the same gradient. Two types of RF pulses Slice-selective: only a certain slice of the body. used in 2D imaging. Non-selective: excites every part of the body. used in 3D imaging T 1.55T 1.57T 1.6T 窄切面 窄切面寬切面 B(T) 19 20

28 選層梯度磁場 (slice-select gradient, Gz) 在 Bo 上另外加一個 Bo 隨位置變化 " 的磁場 如果信號頻率與位置有關物體位置即可分辨 水 水 取樣錯誤就會造成位置錯置 FT Fourier Transform 轉換 (RF) time domain 轉至 frequency domain 提供訊號中的頻率範圍 有頻率的範圍及振幅就能把訊號重建回來 傳送一定範圍頻率的 RF 脈衝有一定頻寬的頻率 time domain sinc(t) = sin(t) / t t F.T. frequency domain -fmax +fmax 頻寬 (bandwidth)= ±famx = 2famx f 信號頻率 = MHz 21 from: 鍾孝文教授 ppt 改編動畫 脈衝寬度與脈衝頻寬為反比 22 Fourier Transform 轉換 (RF) waveform and bandwidth A narrower RF pulse a wider frequency bandwidth Fourier Transform 轉換 (Echo) time domain 轉至 frequency domain 提供訊號中的頻率範圍 有頻率的範圍及振幅就能把訊號重建回來 經 FT 後的訊號較容易了解在時域中的組成 23 提供具有振幅大小的訊號頻譜 24

29 切面選層梯度 (slice-select gradient, Gz) 信號只從一個平面出來, 只激發該平面內的氫原子核 調整射頻脈衝頻率選擇欲激發的切面 空間編碼 (spatial encoding) (phase encoding, Gy frequency encoding, Gx) The spatial information regarding each slice 4cosω 0 t -1cosω 0- t + 3cosω 0 t + 2cosω 0+ t B 0 0 cosω 0 t cosω 0 t 0 cosω 0 t cosω 0+ t RF cosω 0 t 2cosω 0 t 0-2cosω 0 t 0 cosω 0 t frequency encoding 改變頻率差 cosω 0- t 2cosω 0 t 0-2cosω 0- t 0 cosω 0+ t 25 ω 0 =γb 0 ω 0 + ω=γ(b 0 +Gx) ω 0 - ω=γ(b 0 -Gx) lowest No gradient highest 26 Gx x 空間編碼 (spatial encoding) (phase encoding, Gy frequency encoding, Gx) We can analyze the magnitude of each frequency component using FT. -cosω 0- t +3cosω 0 t + 2cosω 0+ t 空間編碼 (spatial encoding) (phase encoding, Gy frequency encoding, Gx) Turn on phase encoding (Gy) between 90 o and 180 o RF pulses or between 180 o pulse and the echo. y Gy 0 cosω 0 t cosω 0 t highest 0 cos (ω 0 t+θ) cos (ω 0 t+θ) cosω 0 t 2cosω 0 t 0-2cosω 0 t 0 cosω 0 t phase encoding 改變相位差 No cosω 0 t 2cosω 0 t 0-2cos (ω 0 t-θ) 0 cos (ω 0 t-θ) 27 ω=γgy lowest ψ= ωt=γgy t 28

30 空間編碼 (spatial encoding) (phase encoding, Gy frequency encoding, Gx) 將氫原子核座標位置的訊息帶入訊號中 ( 需知道像素內有多少訊號 ) 使氫原子核旋進頻率隨位置改變 使磁向量旋進相位隨位置改變 y Gy y Gy Spin Echo (SE) 波序圖 RF 脈衝 TR t 0 cos (ω 0 t+θ) cos (ω 0+ t+θ) Gz 選層編碼 t cosω 0- t 2cosω 0 t 0 Gy 相位編碼 rephase dephase t -2cos (ω 0- t-θ) 0 cos (ω 0+ t-θ) Gx x 29 Gx x Gx 頻率編碼空間編碼 Echo 回音 TE dephase t t 30 切面選層梯度 (slice-select gradient, Gz) Every time we apply a gradient. we dephase the spins. We apply a Gz gradient to select a slice. but after the slice is selected, we need to reverse the effect of dephasing (refocusing) 頻率編碼梯度 (frequency-encoding gradient, Gx) Without a proper compensation, the signal intensity will be. Decrease at the time in the middle of echo Maximum dephasing at the time in the end of echo Phase difference-weak signal Echo RF 脈衝 Gz 選層編碼 Gradient in positive direction t t Gx Maximum dephasing Gradient in negative direction (refocusing gradient) 31 Φ 0 TE Phase shift at time TE 32

31 梯度磁場下的訊號衰減訊號強度 頻率編碼梯度 (frequency-encoding gradient, Gx) We apply a gradient in the negative direction that has an area equal to ½ of that of the readout gradient. TE RF FID T2* TR Echo RF Echo 回音 t Gx 頻率編碼 Maximum Φ difference with negative Gx Φ ½ Ts Ts Spins dephase Spins in phase t 33 G dephase TE rephase dephase 34 Spin K-space echo 簡圖說明 (SE):: rephase Ky dephase TR positive RF t amplitude Gz t phase encoding Kx Gy t frequency encoding negative Gx amplitude t Spin Echo K-space Gy Gx Phase encoding Frequency encoding t t rephase Ky dephase Kx Echo t 35 36

32 Spin Echo K-space phase encoding rephase Ky dephase Ky K-space 簡圖說明 :rephase dephase positive amplitude 1 相位 2 頻率 Gy Gx frequency encoding t t Kx negative amplitude 3 大小 K- 空間的每個數據存放位置都對應一組 (Gy,Ts) Kx 再一個 TR 一個 TR K-space 簡圖說明 : G No gradient G increasing gradient increasing gradient rephase Ky dephase +128 Maximum signal in center of K space -127 Kx detail 3 維 (3D) 坐標方向上的 K-space: Z Maximum signal in center of K space detail G G 39 40

33 K- 空間的邊緣 (Edges of k-space) K- 空間的邊緣 (Edges of k-space) The periphery of k-space provides information regarding fineness of the image and clarity at sharp interfaces. The FT of a truncated sinc function has ring down effects. The shades of gray are determined by the magnitude or amplitude of signal at each pixel K-space 位置和實際對應圖 : Gy ky ky = +/- 1 kx = 0 K-space 位置和實際對應圖 : ky Gx ky = 0 kx = +/- 1 kx kx cycles/fov cycles/fov 實際對應的影像 43 from: 鍾孝文教授 ppt 改編動畫 實際對應的影像 44 from: 鍾孝文教授 ppt 改編動畫

34 K-space 位置和實際對應圖 : Gx ky Gy ky = +/- 1 kx = +/- 1 影像 = 各波形成份的加權總和 : kx + = x 2,225,000 x 583,000 cycles/fov 實際對應的影像 45 from: 鍾孝文教授 ppt 改編動畫 信號值 46 from: 鍾孝文教授 ppt 改編動畫 影像 = 各波形成份的加權總和 : ky 影像 = 各波形成份的加權總和 : ky 64X64 k space X 各點信號值 X 各點信號值 kx kx cycles/fov 波形經加權相加總和之影像 47 from: 鍾孝文教授 ppt 改編動畫 cycles/fov 波形經加權相加總和之影像 48 from: 鍾孝文教授 ppt 改編動畫

35 影像 = 各波形成份的加權總和 : ky 256X256 k space 數萬個不同的波形相加得到 k-space 數據的座標代表一種特定波形 MR 影像 : : ky X 各點信號值 kx kx cycles/fov 波形經加權相加總和之影像 49 from: 鍾孝文教授 ppt 改編動畫 各個波形乘上比重 ( 信號強度 ) 之後相加 50 from: 鍾孝文教授 ppt 改編動畫 Fast Spin Echo (FSE) RF Gz Gy Gx Echo Effect TE echo 1 echo 2 echo 3 echo 4. t. t. t.. t t 51 Echo Train Length (ETL) RF ETL= t 52

36 Fast Spin Echo (FSE) Fast Spin Echo (FSE): 對比平均 T2W 為主 PDW 為主 掃描時間降低 (ETL) FSE 優點 FSE 缺點 1. 切面數目減少, 涵蓋範圍較低 切面數目 = TR TE+Ts/2+T 0 2.SNR 維持一樣 (Ny 相位不會改變 ) 3. 合理的時間內作高解析度造影 (512x512 的內耳道造影 ) 4. 運動假影比較不嚴重 (180 o 等距離分開, 偶數回音重聚相 (even-echo rephasing)) 5. 金屬物體在 FSE 影像中扭曲較小 ( 含有許多 rephase 的 180 o ) 6.FSE 影像對不均勻磁場的磁鐵耐受度比 CSE 影像來得高 (ETL) 2. 對比平均造成 : CSF 在 PDW 影像中較亮 ( 極長 TE 的訊號, 會有一些 T2 效應 (CSF 會較亮 ), 使用短的 ETL 和較高的 BW) MS 斑及其他在腦部的病灶可能被忽略 ( 解決問題 : 較短的 ETL 和利用 FLAIR) 3.FSE 中產生的 MT 轉移 ( 許多快速的 180 o 脈衝, 寬廣的 BW 造成 ) 4. 正常椎間盤之 T2W 影像的亮度在 FSE 不如在 CSE 中 ( 原因為 MT 造成 ) 5. 感磁性效應比 CSE 低 ( 出血不敏感 ) 脂肪在 T2W 的 FSE 像中較亮 ( 解決問題 : FAT SAT) 56

37 Fast SE: 一個 TR 多次 phase encoding FSE K-space Ky +128 同一張 slice ETL= 個不同 phase encoding Echo train length = 4 Gy +8-8 t +8-8 Phase Kx Gx t Scan time = 1/4 取樣 Frequency FSE K-space Ky Scan time (SE): +128 T = TR x Ny x NEX +128 ETL= 16 TR : Repetition time Gy Phase Ny : Phase encoding 的次數 -8 t Kx Naq : Nex (Number of excitation) Gx -127 t Scan time (FSE): 取樣 -127 T = TR x (Ny/ETL) x NEX Frequency 59 60

38 Spin echo: acquisition time Fast SE: acquisition time T1WI: msec (TR) x 512 (phase) x 1 (NEX) = msec = 341 sec = 5 min 41 sec T2WI: 3650 msec (TR) x 256 (phase) x 1 (NEX) = msec = 934 sec = 15 min 34 sec PDWI: 3650 msec (TR) x 256 (phase) x 1 (NEX) = msec = 934 sec = 15 min 34 sec 61 T1WI: msec (TR) x 512 (phase) x 1 (NEX) / 4 (echo train length) = 1 min 8 sec T2WI: 3650 msec (TR) x 256 (phase) x 1 (NEX) / 4 (echo train length) = 3 min 54 sec PDWI: 3650 msec (TR) x 256 (phase) x 1 (NEX) / 4 (echo train length) = 3 min 54 sec 62 Fast SE: 乳房磁振造影 Fast SE: 動態腦下垂體磁振造影 第 3 個切面共 6 個 phase 每個 phase 6 張 ETL:14 ETL:3 T2WI: 5750 ms x 512 x 1 / 14 = 210 sec = 3 min 50 sec T1WI+fat sat +C: 600 ms x 512 x 1 / 3 = 102 sec = 1 min 42 sec 63 ETL: ms x 512 / 16 x 1 = 16 sec 共 6 個切面, 每個切面 6 張 ( 即 6 個 phase, 每個 phase 16 sec) 64

39 MR cholangiography (MRCP)(SSFSE) 不須注射造影劑, 清晰顯現胰管 胆管的解剖结構 ( 阻塞性胆管梗阻疾病很有用 ) 利用 T2W, 長 TR(>3000ms), 特長 TE(>150ms), 水的器官顯影 看圖說故事時間到了!! 各位放射師們請看影像, 想想下列二個問題 : 1. 是檢查那個部位? 是那種加權影像? 2. 計算所需掃描時間? 静止的胆 胰液以高訊號呈現 ( 質子密度高, 較高的 T2 弛豫 ) 周围的肝 胰器官因低的質子密度 ( 較短 T2 弛豫, 低訊號 ) MIP ( maximum intensity projection, 產生 3D MRI 影像 看圖說故事 (2) Thanks for your attention! 67 68

40 Gradient echo What is a free induction decay (FID)? Pulsed methods :the main magnetic field is held constant while an RF-field at the Larmor frequency is pulsed on and off. Immediately after the RF pulse, transverse magnitude dephasing and induce signal called "nuclear induction decay" or "free induction," which today is commonly referred to as the free induction decay (FID). Characteristics The gradient echo sequence differs from the spin echo sequence in regard to: --- the flip angle usually below the absence of a 180 RF rephasing pulse 1

41 What is a gradient echo, and how does it differ from an FID? The advantages of low-flip angle excitations A flip angle lower than 90 (partial flip angle) decreases the amount of magnetization tipped into the transverse plane. The consequence of a low-flip angle excitation is a faster recovery of longitudinal magnetization that allows shorter TR/TE and decreases scan time. Flip angle vs. magnetization 2

42 180 RF Pulse In GRE Echo time in GRE Bi-lobed Gradient Bi-lobed Gradient Advantages of GRE 1. Increased speed 2. Increased sensitivity to magnetic susceptibility effects of hemorrhage 3. 3D imaging in reasonable time 4. Imaging of flowing blood (i.e MRA) 3

43 Disadvantages of GRE 1. Decreased SNR caused by small α, reducing the transverse magnetization, and very short TR 2. Increased magnetic susceptibility artifacts, most noticeable at air tissue interface such as in the region of paranasal sinuses or the absomen 3. T2* decay since there are no 180 rephasing pulses The contrast of gradient echo Short TRs Short TRs 4

44 Small Flip Angle vs. short TR Tissue Contrast - FA (1) α =90, TR=0.1 T1 Mz = M 0 (2) α =25, TR=0.1 T1 Mz = 0.22 M 0 Tissue Contrast - FA Tissue Contrast - TE 5

45 Fast Scanning Techniques Steady state GE Siemens Philips GRASS FISP TFE SPGR FLASH T1 FFE* SSFP PSIF T2 FFE* FSPGR Turbo-FLASH T1 TFE** *TFE, turbo field echo **FFE, fast field gradient echo How can we do??? To mengent residual transverse magnetization is managed: gradient echo sequences with spoiled residual transverse magnetization steady state gradient echo sequences that conserve residual transverse magnetization and therefore participate in the signal. Spoiled gradient echo sequences 6

46 Principles In certain cases, the steady state can be detrimental, namely for obtaining T1 weighted sequences. To resolve this problem, gradients and/or RF pulses (spoilers) are used to eliminate residual transverse magnetization. Basic GRE vs. Spoiled GRE SPGR (spoiled GRASS) The word spoiling refers to the elimination or spoiling of the steady-state transverse magnetization: 1. by lengthening TR 2. by applying variable gradient spoilers 3. by applying RF spoiling Lengthening TR The method to achieve spoiling of Mss is by lengthening TR When TR is sufficiently large (generally over 200 msec), there is enough time to allow complete dephasing of the spins in the transverse plane 7

47 Variable Gradient Spoilers-2 Variable Gradient Spoilers RF Spoiling (phase offset) 8

48 RF Spoiling-2 (1) (2) (3) (1) (2) (3) Mss Spoiled TR can be shorten Disadvantage of SPGR(FLASH) Increased dephasing caused by inhomogeneities in B 0 Increased magnetic susceptibility artifacts Increased chemical shift artifact 9

49 FLASH Dual/Multi-echo GRE Steady-state gradient echo Basic GRE vs. GRASS/FISP 10

50 GRASS/FISP Residual Transverse Magnetization GRASS/FISP T2-enhanced steady-state gradient echo In T2-enhanced steady-state gradient echo sequences: --- residual transverse magnetization is conserved -- the sequence is inverted in time, compared to the preceding sequences --- only the echo corresponding to the Hahn echo, dependent on T2 but weaker than spin echo, is recorded 11

51 SSFP/PISF SSFP/PISF TR<TE<2TR T2W SSFP/PISF The Characteristics of Various GRE Advantage of SSFP/PISF Decrease dephasing due to inhomogeneities in B 0 compare with GRASS and SPGR Decrease magnetic susceptibility artifacts compare with GRASS and SPGR Decrease chemical shift artifacts(dark band) compare with GRASS and SPGR Disadvantage of SSFP/PISF Decrease SNR due to the use of longer TEs (TE>TR) Increase sensitivity to non-stationary tissue GRE Technique SNR CNR Comments GRASS/FISP SPGR/FLASH Highest Intermediate Best possible T2/T1 Best possible T1W SSFP/PSIF Lower Provides T2W Preserves steady-state component Spoiled steady-state component Gradientrecalled SE, TR<TE<2TR 12

52 The Characteristics of Various GRE GRASS/FISP T1 contrast - Spin density contrast T2/T1 contrast Low flip, Medium TR, Short TE SPGR/FLASH Medium flip, Short TR/TE Low flip, Medium TR, Short TE Medium flip, Short TR/TE - What is GRASE? GRadient And Spin Echo (GRASE) or Turbo Gradient Spin Echo(TGSE) A fast segmented sequence that combines a multiple spin-echo train and intermediate gradient echoes T2* contrast Long TE Long TE What is GRASE? Advantages of GRASE Some systems call the number of spin echoes the turbo factor and the number of gradient echoes the EPI factor Typically three gradient echoes will be used for each spin echo Much less RF power is used Higher turbo factors More like T2-weighted spin echo than FSE But substantial ringing artifacts in the phase encode direction 13

53 休息一下吧!!! 14

54 本次課程內容 磁振造影專業基礎課程 MR 組織壓抑技術 MR Tissue Suppression Techniques 瞭解並說明常用的組織壓抑技術, 包括下列幾項 : FLAIR, STIR, fast FLAIR, Double IR (Chapter 7,25,28) Spatial presaturation (Chapter 23) Chemical presaturation (Chapter 23,25) Magnetization Transfer saturation (Chapter 25) 周嘉豪醫事放射師 台北慈濟醫院放射診斷科慈濟科技大學醫學影像暨放射科學系 [email protected] Reference: 1.MRI The Basics (3rd) (Chapter 7,23,25,28) 2.MRI IN PRACTICE(4td) (Chapter 5,6) 3.MRI From Picture to Proton(2nd) 1 2 組織壓抑技術 tissue suppression techniques 如何將組織信號抑制? 利用 saturation 的概念 Tissue 共振頻率不同 T1 relaxation 的不同 臨床上壓抑的主要對象 : Suppress two common targets fat and water Suppress other silicone and blood 組織壓抑技術種類 反轉回覆技術 (Inversion Recovery, IR) STIR FLAIR fast FLAIR DIR 化學 ( 頻譜 ) 預飽和 (chemical persaturation) 空間預飽和 (spatial persaturation) 磁量轉移 (Magnetization Transfer, MT) 3 4

55 MR 時間 - 基本參數 TR (repetition time) 重複時間 TE (echo time) 回音時間 TI (inversion time) 反轉時間 FA (flip angle) 偏離角度 TR Echo TI TI TE x z 180 y 磁化向量與 T1 有關 5 x Mz 信號強度 T1 弛緩現象 y 63.2% 63.2% T1 弛緩 ( T1 relaxation) T1 value = Mz 恢復到 M 0 的 63.2% 所需之時間 TR= 重復脈衝的時間 (ex:90 o 到 90 o 的時間 ) TR 90 o 90 o 短 T1 組織長 T1 組織 M Z (T)=M 0 (1-e -t/t 1 ) -t/t1=-1, Mz= t/t1=-2, Mz= t/t1=-3, Mz= t/t1=-4, Mz=0.982 時間 (TR) 6 反轉回復技術 Inversion Recovery 180 度脈衝, 所有磁向量均反向 STIR FLAIR (1.5T) TI ~ 140 msec Echo 等待 T1 弛緩 (TI : Inversion Time), 提供好的 T1 對比 RF 脂肪弛緩至零點 (STIR) 到脂肪 ( 或 CSF) 回復至零點後開始取像 1.5T 下 TI 通常約 140 msec (STIR- 抑制 fat) (Short Tau Inversion Recovery) 1.5T 下 TI 通常約 2500 msec (FLAIR- 抑制 CSF) (Fluid-Attenuated Inversion Recovery) RF TI ~ 2500 msec CSF 弛緩至零點 (FLAIR) Echo 7 8

56 RF 反轉回復技術 fast FLAIR IR + Echo Train (ETL 回音列車 ) 180 TI 2500ms TR Fast IR : IR+Echo Train (ETL =4) 利用 TI 時間, 收取多張切面的訊號 Multi-slice+FSE (limited by TI ) 180 t RF CSF fat z 反轉回復原理示意圖 TI ~ 140 msec 抑制 fat z 180 z Echo z Null point x y x y x y x y 9 -z -z -z 10 Z 磁化量在 IR 波序上的變化 Z Z Z Z 反轉回復曲線 TI (STIR) SI=M 0 (1-2e -TI/T1 ) (1-e -TR/T1 ) 1-2e -t/t1 脂肪灰質 90 o X Y X B 1 (180 o ) Y X Y Y Y X X B 1 (90 o ) Null point CSF X Z Y B 1 (180 o ) X Z Z Z Z Y X Y Y X X Y 訊號強度 =0=1-2e -TI/T1 TI null = (log e 2)T 1 TI (FLAIR) =(ln2)t 1 = 0.693T 1 TI 時間的長短可以決定抑制何種組織的信號 STIR TI (fat) at 1.5 T = x 200 msec = 140 msec FLAIR TI (CSF) at 1.5 T = x 3600 msec = 2500 msec 11 12

57 STIR 特性 IR Advantages & Disadvantages 不須特別均勻主磁場, 影像含有 T1 成份 Inverse T1 weighting 長 T1 組織 : 亮 短 T1 組織 : 暗 與普通短 TR 之 T1 影像相反 CSF > edema > 灰質 > 白質 Fluid-Attenuation Inversion Recovery 利用 IR 技術把水的信號壓掉 提供沒有 CSF 信號 T2WI, 降低灰白質對比, 形成灰色背景顯現病灶 IR advantages 1. 沒有明顯額外的 RF 加熱 2. 不會有磁場不均度所造成的變動 IR disadvantages 1. 相似 T1 值的組織通通被壓制掉而無法區別 (ex: Gd effect) 2. 長 TR 造成的長擷取時間 (3T 的話 TI 就會更長 ) 3. 較低的 SNR( 部分被飽和的關係 ) 13 T1W T2W FLAIR 通常病灶有較長的 T2 時間 (cortical lesion, peripheral subcortical, periventricular ) 故利用 T2W 影像進行診斷, 靠近 CSF 的病灶不好辨識 (high CSF SI) 14 Fluid-Attenuation Inversion Recovery 利用 IR 技術把水的信號壓掉 多發性硬化病灶 multiple sclerosis (MS) Fluid-Attenuation Inversion Recovery 利用 IR 技術把水的信號壓掉 Glioblastoma 神經膠母細胞瘤 T2W FLAIR MS: 神經傳導有關, 發生在白質, 會產生去髓鞘化的動作, 會變白質產生病變 15 T2W FLAIR 腦在病變 (tumor) 時產生的 edema 會和 CSF 接近, 用 FLAIR 區分 16 Andrea Hawkins-Daarud, Front. Oncol., 04 April 2013

58 Fluid-Attenuation Inversion Recovery 利用 IR 技術把水的信號壓掉 蜘蛛網膜下出血 (SAH) 及腦室內出血的診斷很有用 Short Tau Inversion Recovery 利用 IR 技術把脂肪的信號壓掉 T2W FLAIR 17 T2W STIR (SNR 較差 ) 18 Short Tau Inversion Recovery 利用 IR 技術把脂肪的信號壓掉 Double IR: black blood imaging T1W T2W STIR 19 常使用於心臟血管磁振造影可以清楚看到血管壁的構造 20

59 Double IR: black blood imaging 原理 TI 1 TI TR 1 TI 2 TI 2 Double IR: black blood imaging 的原理 non-selective slice-selective TI flowing-in blood 180 o 180 o at nulling point TI =0.693x600ms(T1)=416ms FSE T1WI o 180 o 90 o 180 o 180 o 180 o 180 o 180 o 180 o RF transmit RF receive RF Echo non-selective slice-selective TI flowing-in blood 180 o 180 o at nulling point Fast Spin Echo T1WI 化學 ( 頻譜的 ) 預飽和 Chemical (Spectral) persaturation RF 脈衝施加之前加一個預飽和脈衝 消除脂肪和特殊的縱向磁量 CHESS: CHEmical Shift Selective SAT 類似於 90 度 ( 適當的選擇頻率進行飽合 ) 臨床上又叫 fat sat, 主要是用來作脂肪的壓抑 受磁場不均勻度的影響 Breast,Muscle,Bone,Spine 常用 Brain 用的比較少 (fat 較多在皮下 ) SAT 90 SAT 90 Chemical shift effect 水分子的質子的 Lamor frequency 比脂肪的質子的 Lamor frequency 快了 3.5 ppm (parts per million) ω( 頻率 )=γ ( 磁旋比 ) Bo 3.0T 相差 3.5 x 10-6 x 42.6 MHz/T x 3.0T= 440 Hz 1.5T 相差 3.5 x 10-6 x 42.6 MHz/T x 1.5T= 220 Hz 0.5T 相差 3.5 x 10-6 x 42.6 MHz/T x 0.5T= 73 Hz 水 4.7ppm 脂肪 1.3ppm 針對脂肪給予 sat 23 ppm ppm 24

60 x z SAT 化學 ( 頻譜的 ) 預飽和 Chemical (Spectral) persaturation Fat M x y z SAT Fat Water x y Fat z y x Mz for Fat z y Mz for water 25 Chemical (Spectral) persaturation advantages Chemical (Spectral) persaturation Advantages & Disadvantages 1. 可以區別 T1 相似的組織 (ex: fat & Gd-enhanced tumors 的 ) 2. 除被壓抑的組織外, 其他組織的訊號不會有影響 (IR affects the contrast of all tissues) 1. 頻率技術的選擇對磁場不均度非常的敏感 Chemical (ex: metallic susceptibility artifacts/ 可用 STIR 改善 ) (Spectral) persaturation 2. 需要額外的時間 (TR increasing the scan time 5~8ms) disadvantages 3. 額外的 RF 加熱 (extra 90 o pulse) 26 Chemical presaturation 取信號的脈衝序列前, 以特定頻率打 90 o RF Chemical pre-saturation 取信號的脈衝序列前, 以特定頻率打 90 o RF T1W T2W T1W + fat sat T2W T1W T1W + fat sat 27 28

61 Chemical pre-saturation: 取信號的脈衝序列前, 以特定頻率打 90 o RF STIR vs. Fat Sat Endometrioma (aka chocolate cysts) Lower SNR T2W T2W+fat sat T1W+fat+C Coronal T1W Coronal STIR T2W fat sat lesion T1 value similar fat Not include fat T1W+fat+C+color T1W T1W+fat sat 29 MRI:The Basics,3e,p Fat Sat 與 Water Sat 的影像比較 綜合 saturation 臨床應用 STIR fat sat water sat Raw Image Fat Sat Water Sat T2 weighted STIR+water sat ( 截錄自鍾孝文教授 PPT 圖檔 ) 31 M. H. Siqueira Mendonça, Electronic presentation online system(2013),

62 綜合 saturation 臨床應用 STIR fat sat water sat T1:CSF > edema > gray > white > fat Gd contrast agent can shorten tissue T1 空間預飽和 (Spatial persaturation) 臨床上又稱為 saturation band 90 o 脈衝之前多施加一個 90 o 脈衝 利用空間的遮擋做為概念 freq (anterior/posterior, superior/inferior, right/left) 降低整個 FOV 內的運動及流動相關假影 (Motion artifacts or Flow-related artifacts) slice phase 90 o α o Spoiler Spoiler T2W + fat sat STIR + water sat T1W + fat sat + C Siliconomas ( 小針美容 ) Breast tumor ( r/o malignancy) tumor 血管壁比較不完整, 導致比較大分子的顯影劑會滲漏到裡面去 33 針對運動和流動的組織 ( 心臟或呼吸運動或血流等 ) 34 什麼是 saturation band? 90 o RF 90 o RF 空間預飽和 (Spatial persaturation) Imaging of spine Saturation band 放置在 spine 前 減少椎骨周圍大血管和心臟的假影 飽和帶 Saturation band 飽和帶 Saturation band Spine Saturation band Imaging slice 弱信號 在取信號的脈衝序列之前, 於特定位置打 90 o RF 強信號 35 36

63 空間預飽和 (Spatial persaturation) Imaging of spine 空間預飽和 (Spatial persaturation) MR angiography 常用 動脈或靜脈的流動造影下, 空間預飽和脈衝被放在鄰近切面的上方或下方 預飽和脈衝 presat pulse 體素 MRA MRV 無 saturation 有 saturation 37 靜脈動脈在取信號的脈衝序列之前, 於特定位置打 90 o RF 38 空間預飽和 (Spatial persaturation) MR spectroscopy 常用 MR spectroscopy: Saturation bands are placed on the skull regions. No. band scan time (5-8msec) 優點 Chemical persaturation 1. 可區別 T1 值相似的組織 2. 未受壓抑組織訊號沒有任何影響 Spatial persaturation 1. 減少 phase ghosts 2. 減少 flow artifacts Chemical persaturation vs. Spatial persaturation 缺點 1. 使用頻率選擇技術, 深受磁場不均勻度的影響 2. 需要額外的時間 3. 造成額外的 RF 熱量 1.FOV 內也壓抑其它部分的組織 2. 延長 TR, 造成掃描時間增加 Fat 和 lactate 二者太接近 (1.3 ppm) 不適用 fat sat 怕二者都被 saturation 應用 1. 適當選擇頻率可消除脂肪或水的訊號 1.Spine scan 2.MRA(MRV), TOF 3.Abdomen scan 4.Brain, MRS 39 T. Li1,Proc. Intl. Soc. Mag. Reson. Med. 14 (2006) 40

64 磁量轉移 (Magnetization Transfer, MT) Non-fatty hydrogen nuclei 磁量轉移 (Magnetization Transfer, MT) 壓抑 protein-bound water 的技術 H 2 O H 2 O macromolecule 大分子 H 2 O H 2 O H 2 O H 2 O 蛋白質結合水 (protein-bound water) T1: 10~100msec T2: 5~10msec O H H O H H O H H O H H O H H 巨量水 (free water) (bulk H 2 O) T1:2000~3000msec T2:1000~2000msec 41 巨量水 (bulk H 2 O): 氫含量高 自由運動的水分子 結合水 (bound water) : 氫合量低 蛋白質結合水分子 protein-bound water 與 free water 的共振頻率相差 500~2500Hz protein-bound water 的飽和傳給 free water 的飽和 CSF blood 骨髓 脂肪組織, 大分子少,MT 小訊號小腦組織 肌肉, 大分子多,MT 大訊號大 常運用在 TOF-MRA 抑制腦背景組織, 突顯較小的腦血管 (reduction of gray and white matter signal by 15%~40%) 42 磁量轉移 (Magnetization Transfer, MT) 抑制含有大分子蛋白質組織的信號 有效抑制背景組織突顯血液 MTS 效應在靜止組織中較強 free water protein-bound water 頻譜較寬 磁量轉移 (Magnetization Transfer, MT) TOF-MRA 常用 MT MT -1000~-2000Hz 1000~2000Hz 飽和的訊號 降低的訊號 飽和的訊號 43 有 MT sat 無 MT sat (reduction of gray and white matter signal by 15%~40%) 44

65 Thanks for your attention! 45

66 本次課程內容 磁振造影專業基礎課程 血流現象與磁振血管造影 Flow phenomena and MR angiography 周嘉豪醫事放射師 台北慈濟醫院放射診斷科慈濟科技大學醫學影像暨放射科學系 1 瞭解血流對 SE 和 GE 訊號的影響 瞭解 TOF MRA, PC MRA 和 CE MRA 的原理 瞭解磁振血管造影的臨床應用與優缺點 Reference: 1.MRI The Basics (3rd) 2.MRI IN PRACTICE(4td) 3.MRI From Picture to Proton(2nd) 4. Laub G, Gaa J, Drobintzky M. Magnetic resonance angiography techniques. Electromedica 1998;66: Graves MJ. Magnetic resonance angiography. Br. J. Radiol. 1997;70: 有哪些血管造影 (Angiography) 的方法? Gradient Spin Echo (SE) (GRE) Diagram Diagram TR 90 α TR α α180 α α 90 α RF t Gz t Gy t Gx t Echo t 3 傳統血管造影電腦斷層血管造影 (CTA) 磁振血管造影 (MRA) 4

67 Saturation Effects( 飽和效應 ) 重複激發的 RF pulse 造成縱向磁量的逐漸消失 造成訊號的損失, 也造成 SNR 的降低 短 TR, 大偏折角 (T1W) 沒時間作 T1 回復 訊號低 RF z 短 TR z z Signal α Signal 小 α 如何減少 saturation effects? 長 TR TR TR TR TR TR TR time time 較長的 TR 會使得縱向磁量的回復比較短的 TR 好, 飽和效應降低 大 α α 短 TR x y x y x y 5 TR TR TR TR TR TR time time 較小的 α(tr 固定 ) 也可使縱向磁量的回復比較大 α 多, 飽和效應降低 6 如何減少 saturation effects? 注射 gadolinium( 使 T1 變短 ) 順磁性的對比劑會使 T1 縮短 (CE-MRA) T1 回復會比較快且飽和效應比較小 Signal α Signal α Laminar Flow 層流 Plug Flow 栓塞流 ( 平流 ) Turbulent Flow 擾流 Flow Phenomena normal vessels ( 大多數的血管 ) high velocities + large vessels ( 高流速的大動脈 ) abnormal vessels ( 不正常血管 ) parabolic profile ( 拋物線 ) flat profile Vortex flow eddies TR TR TR time TR TR TR time Flow separation 流體分離 near the wall of vessels streamline separated 未使用 Gd 使用 Gd 7 8

68 MRA Techniques : TOF (time of flight) MRA(2D 3D) PC (phase contrast ) MRA(2D 3D) CE (contrast enhanced) MRA(3D) Unenhanced MRA (TOF & PC) MRA Techniques single slab multi slab Time Of Flight (TOF) Signal loss( 訊號喪失 ) 1. high velocity( 高速 ) (Washout & Inflow effect 有關 ) 2. turbulent flow( 擾流 ) Flow Artifacts 3. dephasing( 失相 ) Amplitude effects: Blood flowing into or out of a chosen slice has a different longitudinal magnetization compared to stationary spins. Depend on the duration of stay (Time-Of-Flight) in the slice. Phase effects: Blood flowing along the direction of a magnetic field gradient changes its transverse magnetization compared to stationary spins. Signal gain( 訊號獲得 ) 1. flow-related enhancement (FRE) TE 2 =2TE 1 2. even echo rephasing( 偶數回音重聚相 ) 3. diastolic pseudogating( 心舒期假性觸發 ) 收縮期流動 : 快心舒期流動 : 慢利用 cardiac gating 9 10 Time-Of-Flight effect in Spin Echo 血流呈黑色或灰色 (Washout effect) 高速的訊號損失, 流動的質子在切面中停留時間不夠長 質子必須同時受到 90 o 和 180 o RF pulse 作用 TE/2 TE/2 average faster not excited excited excited Slice thickness rephased nothing to rephase signal no signal not rephased no signal image 11 V=0 cm/s Time-Of-Flight effect in Spin Echo V=25 cm/s V=50 cm/s High Velocity Signal Loss TE/2 ΔZ 靜止不動 v = 2 1 v = ΔZ ΔZ 1/2 TE 1/2 TE ΔZ image 12

69 Time-Of-Flight effect in Spin Echo 血流呈黑色或灰色 ex: slice thickness=1cm TE=50msec 時, 血流速度大於多少就沒有訊號? Time-Of-Flight effect in Gradient Echo 血流呈白色 (GRE 影像血流總是亮的!) (Inflow effect) TE 通常都很短,dephasing 引起的訊號喪失非常少 α GRE 總是序列模式 ( 一次切一張 ), 每一張都是血液流進來的第一張 沒有 180 o RF, 所以 TOF loss 影響不明顯 Ans: v = ΔZ 1/2 TE no flow dark signal image 1cm/25msec = 40cm/sec Flow blood SI Decreasing slice thickness Increasing flow velocity Increasing TE (T2W>T1W) Not occur in GRE 13 flow excited excited TE rephased bright signal 14 Time-Of-Flight effect in Gradient Echo 時間 = t α ΔZ v = 靜止不動 V= 0 cm/s v = 1 ΔZ 2 ΔZ TR TR 時間 = t+tr image 未飽和質子較多的訊號 15 Time-Of-Flight effect in Gradient Echo 血流呈白色 ex: 當 slice thickness = 1 cm, TR = 1000 msec 時, 血流速度多少就有最大訊號? Ans: ΔZ v = TR 1 cm / 1000 msec = 1 cm/sec Flow blood SI Decreasing slice thickness Increasing flow velocity Increasing TR 16

70 Turbulent flow( 擾流 ) 紊亂的流體, 造成不同相位, 而互相扺消而導致沒有訊號, 發生在具有低或高的流速時 Flow Artifacts ( 易造成 pseudo-stenosis) Dephasing( 失相 ) 體素內 (intravoxel) 失相 : 1. 層流造成 2. 體素內存在不同血流速度 切面方向和血管轉彎處 flow Carotid Artery voxel flow 17 Flow 訊號獲得 Signal gain( 訊號獲得 ) 1. flow-related enhancement (FRE) (Type of TOF) 2. diastolic pseudogating 心舒期可獲得較高的血管內訊號 ( 高速導致較多的 TOF 損失 ) 使用 cardiac gating 來固定擷取一個時間點的心跳 Refractory period R R R R T T T T P P P P QS QS QS QS 3. even echo rephasing( 偶數回音重聚相 ) SE 造影有對稱回音 TE2=2TE1 偶數回音的訊號強度比奇數回音 18 Flow Related Enhancement (FRE) 又稱入口現象 (entry phenomenon) 2D TOF: Larger flip angle (30 o ~70 o ) Thicker slice thickness(2~3mm)(snr ) Vessels straight and perpendicular to the slices (carotid or lower extremties) Sensitive to slow flow 部分飽和組織 slice slice N 接收訊號和 T1 回復 vessel flow flow RF saturation tissue FRE & Multi-slice TR α α α α α α 愈接近上游區,FRE 愈明顯 (Entry slice phenomena) slice vessel flow t 新鮮流入血液最大訊號 部分飽和飽和血液血液 19 FRE 明顯 20

71 3D TOF MRA FRE Multi-slice Phenomena 類似 3D FRE 比 2D FRE 弱 Smaller voxels(<1mm), short TE, higher SNR Small flip angle(<30 o ), slab does not become too saturated. saturation tissue slice vessel flow TONE Tilted Optimized Non-saturating Excitation 傾斜最佳化的不飽和激發 flip angle TONE RF RF 脈衝 slice 出口切面 中央切面 入口切面 vessel flow 新鮮流入血液 3D single slab 部分飽和血液 亮 暗 21 亮 暗亮 22 MOTSA Multiple Overlapping Thin-Slab Acquisition 多重疊之薄區塊的擷取 slab 保留的區塊 slab Venetian blind artifact slab D multi-slab method Larger vessel sections Reduced saturation effects (like 2D) MOSTA: 20%~30% overlapping Longer acquisition time flow slab 3 slab 2 overlapping slab 1 不要的區塊 (25%-50%) 23 24

72 3D multi-slab method single slab multi-slab non-overlapping multi-slab overlapping Reduce saturation effects? 用較小的偏折角 (α) ( 固定 TR 下大 α 比小 α 有較多的 MZ 喪失 ) 用較長的 TR ( 可以讓縱向磁量回復的較多 ) MOTSA (Multiple Overlapping Thin-Slab Acquisition) ( 較常用 ) TONE (Tilted Optimized Non-saturating Excitation) 注射 gadolinium( 使 T1 變短 ) Gradient Moment Rephase (GMR) (Flow Compensation; FC( 流動補償 )) GMR: 減少流體運動假影的一種方法 Options to improve TOF MRA Slices or volume perpendicular to flow direction phase position 2D for slow flow, 3D for fast flow 3D multi-slab for larger vessel sections FC gradient stationary spin mobile spin +1-2 Spatial presaturation to isolate arteries and veins Use of minimum TE reduces signal loss due to spin dephasing TONE pulse or MOSTA reduces saturation effects in 3D TOF Magnetization transfer (MT) and fat sat improve vessel contrast (reduction of gray and white matter signal by 15%~40%) 27 28

73 Phase Contrast MRA (PC MRA) Phase effects concern the transverse magnetization Bipolar flow-encoding gradient (strength and duration but opposite sign) Stationary spins = zero net phase shift Flowing spins = a non-zeros phase shift Flow encoding ψ phase position bipolar gradient (VENC) stationary spin Phase Contrast MRA (PC MRA) higher first lobe lower second lobe lower higher G stationary spin phase shift mobile spin phase shift ψ= ωdt = (γgvt) dt ==γgv t dt= 1/2γGvt 2 29 flowing spin both spins affected equally moving spin does not see equal but opposite gradient polarity 30 Phase Contrast MRA (PC MRA) stationary spin flowing spin Magnitude & phase contrast method 整體流速 -- 重複三次 (Gx Gy Gz ) + 一次參考點 (flow compensation) Gz Gy speed RF t Gx t stationary spin = 無訊號無相位改變 Gz Gy t t t t flowing spin = 有訊號有相位改變 31 Gx t Flow compensation, bright blood image Biolar gradients, dark blood image 32 ( 部分圖檔來自於鐘孝文教授之 ppt 檔 ) t

74 What is VENC? Velocity encoding (VENC) 梯度愈強 ( 弱 ), VENC 愈小 ( 大 ) Velocity encoding (VENC) = 速度編碼 (MR 放射師可以選用 ) VENC 為該梯度下所能求的最大流速 血流的相位改變的大小與其速度成正比 ψ= 1/2γGvt 2 Aliasing velocity( 假影 : 假的速度 ) = VENC 實際速度 Ex: 若 VENC=30cm/sec, 則一個具有 40cm/sec 之流速血管所呈現出來的流速為? V=30-40=-10cm/sec( 朝反向的流動 ) 10 cm/s phase shift Low VENC Steep Gradient High VENC Shallow Gradient 一個較小的 VENC 對慢速流 ( 靜脈流 ) 及較小的支脈較為敏感但會對較快速的 ( 動脈 ) 流動造成假影 cm/s same phase shift 34 VENC 與 phase aliasing 流速與相角成正比 實際上 VENC 可分析的流速範圍 ~ 之間, 角度 的範圍 (180 0 相角的流速 ) Aliasing velocity= VENC 實際速度 ( V= = -50cm/sec ) VENC=100cm/s VENC=100cm/s V=150cm/s Velocity encoding (VENC) 調 VENC = bipolar 梯度 根據流速範圍選取最適當 VENC 值 VENC 太高 : 慢血流看不清楚 VENC 太低 :phase aliasing Phase Aliasing -180 o 180 o 225 o -100cm/s : 正向最大流速 : 反向最大流速 100cm/s 正向血流 : 白色靜止組織 : 灰色 流速超出上限正向血流 : 黑色 35 ( 影像來自於鐘孝文教授之 ppt 檔 ) 36

75 常見血管內的血流速度 PC MRA: 常用於腦部靜脈造影 VENC Optimization Pulmonary artery Aorta Carotid artery External iliac artery Carotid syphon 55 Common femoral artery 115 Basilar artery 40 Superficial femoral artery 90 Vertebral artery 40 Popliteal artery 70 Sagittal sinus vein 10 Peripheral veins Scan time 約 6 min, 時間很長 38 PC MRA: 定量血流速度與方向 Contras-Enhanced MRA (CE MRA) 較快 較慢 Magnitude image (images of blood vessels) 往上 往下 Phase image (direction of flow) 39 ( 影像來自於鐘孝文教授之 ppt 檔 ) 避免血流訊號被飽和或血流的 SNR 值不夠好 注射 Gadolinium (Gd-DTPA) 使血流的 T1 縮短 (paramagnetic) (0.5~4.0 ml/s 0.1~0.3 ml/kg total 20~40 ml GFR>30 ml/min) 快速靜脈注射 Gd 被稀釋之前, 快速擷取影像 (T1 縮短最明顯時 使用 GRE 技術 (T1W-SPGR)) 掃描切面通常是 coronal, 而不是與血管走向垂直 ( 可以在解析度增到最大的情況下, 增加涵蓋範圍 ) 血流流動的失相假影像不敏感 ( 依賴 T1 特性 ) 40

76 Contras-Enhanced MRA CE-MRA: elliptical-centric 和 multiphase elliptical-centric: 等對比劑進入感興趣的動脈後再開始擷取 Contras-Enhanced MRA Mask subtraction Scan time 約 20 sec bolus 對比劑的自動監測軟體 SmartPrep: 通過將游標放置在感興趣動脈的上游來進行 TimeBolus: 進行即時掃描以決定 Gd 對比劑到達感興趣動脈的時間 ( 觀察注射 2cc 的 Gd 對比劑後動脈到達最大亮度的時間 ) 2D real time Tracker volume ROI Contras-Enhanced MRA CE-MRA: elliptical-centric and multiphase multiphase: 注入 Gd 之後進行多次的擷取, 其中一個必定位於動脈相 time-resolved imaging of contrast kinetics, TRICKS Applications areas of MRA 2D-TOF 3D-TOF 2D-PC Magnitude contrast 3D-PC CE MRA Intracranial Arteries *** * * Intracranial Veins *** * * ** * Carotids ** ** *** Peripheral vessels ** * *** 43 44

77 看圖說故事時間 請各位放射師們看影像, 並想想下列二個問題 : 1. 為何血管呈黑色, 灰色或白色? 2. 使用何種磁振造影技術? Thanks for your attention! 45 46

78 Introduction The Artifacts in MRI All MRI images have artifacts in some degrees. Why and How? How to remedy the artifacts encountered in MRI. 新光吳火獅紀念醫院放射診斷科技術專員李正輝 Motion artifacts Introduction patient motion, physiological motion, flow Inhomogeneity artifacts mental artifacts, zipper artifacts, cross talk Digital imaging artifacts aliasing, truncation, herring-bone artifacts, halo artifacts, Gradient nonlinearities, chemical shift Patient motion Motion Artifacts voluntary motion, involuntary motion Physiological motion respiration, cardiac motion, peristaltic Occurring in phase encoding direction 4 1

79 But, why are ghosts only produced in the phase-encode direction? Consecutive points in the frequency-encoding direction are measured close together, typically much less than 1 ms apart. whereas consecutive phase-encoding steps are TR ms apart. Motion such as respiration and blood flow occurs slowly compared with frequency encoding but much quicker than phase encoding. So between successive phase encodings, the anatomy moves and produces a ghost signal at a different PE position. Motion Artifacts Solution of patient motion fixed patient, repeat scan, reduce scan time, drug-assisted Solution of physiological motion hold on breath, respiration gating, respiratory compensation, ECG gating, fast scan technology, drugassisted single-shot FSE 6 Patient motion??? 加 saturation band 2

80 Motion Artifact( Respiratory) Respiration Breath-hold Respiration Gating Respiratory Compensation Respiratory-Ordered Phase Encoding, ROPE Without Gating With Gating 11 3

81 Navigator Motion Artifact(Cardiac pulse) Involuntary motion ECG trigger or PPU trigger are used to avoid artifacts MR compatible electrodes use carbon instead of metal to avoid causing artefact on the MR images Scan time is determined by the heart rate The TR is controlled by the R R interval ECG Gating ECG or peripheral gating? Without Gating ECG gating is a more accurate gating method --- peak is usually sharp and easily recognizable --- all the other ECG peaks can be seen too --- good for cardiac imaging Peripheral gating only detects the arterial Pulse peak is much broader than the ECG R wave Ease of preparing the patient and for safety With Gating 15 4

82 Motion Artifact( Peristaltic motion) Causes a random continuous motion of the abdominal contents Acquiring multiple averages can reduce the ghost appearances Antiperistalsis drug such as hyoscine butylbromide (Buscopan) Ultrafast pulse sequences --- HASTE --- single-shot FastSpin Echo (FSE) Flow Motion Artifact Artifact or non artifact? There are two sides of same coin In-flow effect (flow related enhancement, FRE) Spin echo: dark signal Gradient echo: bright signal Velocity-induced phase effects Resonant frequencies are changing continuously Incorrect phase angle for their real position Artifact on phase encoding direction 18 PDW axial image 5

83 Flow artifact??? Flow compensation 6

84 Avoiding FRE Artifacts Spatial saturation bands, also known as REST slabs or pre-sat bands, are simply slice selections, and can be used in many ways??? Without Sat. (a) (b) With Sat. 25 Susceptibility artifacts Inhomogeneity artifacts Susceptibility artifacts in MRI occur at interfaces of differing magnetic susceptibilities, such as at tissue-air and tissue-fat interfaces (examples include paranasal sinuses, skull base, and sella) There are caused by inhomogeneities, susceptibility artifacts are generally worse on gradient-echo images than spin-echo images. 7

85 Mental Artifacts Metals caused homogeneity change 8

86 ??? Zipper artifact (RF) This artifact is one form of central artifacts Most of zipper artifacts result from inhomogeneities of the magnetic field caused by interferences with radio frequency from various sources. Software and equipment problems can also cause zipper lines in both directions FID Artifacts Free induction decay (FID ) artifacts occur due to overlapping of side lobes of the pulse with the FID before it has had a chance to completely decay. This overlapping causes a zipper artifact Along the frequency -encode direction. FID Artifacts Remedy Increase the TE (increases the separation between the FID and the RF pulse). Increase slice thickness. This in effect results from selecting a wide RF BW, which narrows the RF signal in the time domain, thus lowering chances for overlap. 9

87 Zipper artifact RF Feedthrough Zipper Artifact This artifact occurs when the excitation RF pulse is not completely gated off during data acquisition and feeds through the receiver coil. It appears as a zipper stripe along the phase- encoding axis at zero frequency RF Noise Remedy for Zipper Artifact RF noise is caused by unwanted external RF noise (e.g., TV channel, a radio station, a flickering fluorescent light, patient electronic monitoring equipment). It is similar to RF feedthrough except that it occurs at the specific frequency (or frequencies) of the unwanted RF pulse(s) rather than at zero frequency Improve RF shielding. Remove monitoring devices if possible. Shut the door of the magnet room! 10

88 Cross-talk The remedy of cross talk At least a 30% gap between the slices. Excite alternate slices (interleaved) during the acquisition. --- First sequence: odd slices 1,3,5,7, --- Next sequence: even slices 2,4,6,8, Multi-stack artifact Digital imaging artifacts 11

89 Aliasing Artifacts Aliasing artifacts, also called wrap-around artifacts Arises whenever the anatomy bigger than field of view (FOV) 45 Aliasing Artifacts 12

90 Anti-aliasing along the phase axis Surface coil Increase FOV Over samples along the phase encoding axis. --- To increasing the number of the phase encoding,the scan time has be prolonged. So, the motion artifact may be more apparent. Saturation pulse Herring-bone artifact A regular series of high- and low-intensity stripes extending right across the image It is caused by spike noise in the raw data, whose Fourier transform is then convolved with all the image information 13

91 Herring-bone artifact Halo artifact A halo effect can be produced if the receiver gains are incorrectly set. When this happens the signal is too large for the range of the digitizer and information in the center of k- space is lost It is a rare artifact with modern automatic pre-scan systems, and is more likely to occur when receiver gains are manually set 14

92 Halo artifact Gradient nonlinearities Nonlinearities in the gradient cause distortion in the image. For instance, a circle may appear elliptical. Gradient nonlinearities The effect of nonlinearities is to distort the image, tending to compress the image information at the edges of the FOV. Many systems apply a correction to the images to stretch out the pixels, and on rectangular FOV a curved edge can be seen This is quite normal and also unavoidable; if necessary the area should be re-imaged using a smaller FOV. Gradient nonlinearities 15

93 Chemical shift artifact Chemical shift Caused by the different chemical environment of fat and water. The precessional frequency of fat< water(depend on the main magnetic field strength) ex. At 1.5T the different of precessional frequency is 220 Hz;at 1.0T is 147 Hz.But at lower field strength (o.5t or less),it is usually insignificant. VOXEL (water and fat) Water fat shift 3.3 ppm FT CH 3 H 2O f Bandwidth/pixel > 3.3 ppm Bandwidth/pixel << 3.3 ppm chemical shift artifact chemical shift artifact For example : The frequency mapped across the FOV is Hz;256 frequency samples are selected,each pixel has an individual frequency range of 125Hz(32000/256Hz).At 1.5T,fat and water existing adjacent has a shift about 1.76 pixel which called chemical shift It depend on the size of FOV as this determines the size of each pixel. Causes a dark edge at the interface between fat and water. It occurs along the frequency encoding axis only. 16

94 The remedy of chemical shift artifact Using fat suppression. Scanning at lower filed strengths. Increase bandwidth (trade-off: lowers SNR ) Switch phase and frequency directions. Use a long TE (causes more dephasing and less signal from fat). Chemical misregistration Chemical Shift of the Second Kind Also produced as a result of the precessional frequency different between fat and water. Caused because fat and water are in phase at certain times and out of phase at others. 17

95 The remedy of Chemical misregistration When fat & water are in phase: signals add constructively. When fat & water are out phase: signals cancel each other out. Which called ---- Chemical misregistration Cause a ring of dark signal around certain organs where fat and water interfaces occur within the same voxel. Use a spin echo sequence to reduce the artifact. Select a TE generates an echo when fat and water are in phase. (at 1.5T the TE is a multiple of 4.2ms) Chemical shift for in-out phase??? FIELD STRENGTH In Out In Out In Out In 0.5 T ms 13.6 ms 20.4 ms 27.2 ms 34 ms 40.8 ms 1.0 T ms 6.8 ms 10.2 ms 13.6 ms 17 ms 20.4 ms 1.5 T ms 4.4 ms 6.8 ms 9 ms 11.2 ms 13.4 ms 3.0 T ms 2.2 ms 3.4 ms 4.5 ms 5.6 ms 6.7 ms 18

96 Truncation artifact Caused by under sampling of data so that interfaces of high and low signal are incorrectly represented on the image. Occurs in the phase direction only. Produces a low intensity band running through a high intensity area. Also called: Gibbs artefact Ringing Spectral leakage Predominant in case of low matrix scan percentage << 100% Contrast jump in object FFT Appearance in image 19

97 The remedy of truncation artifact Gibbs artifact( truncation artifacts) Increase sampling time ( BW ) Decrease pixel size: ---The under sampling of data must be avoided. increase the number of phase encoding steps. ---Decreasing the FOV External Magnetic Field Artifacts Artifacts related to B0 are usually caused by magnetic inhomogeneities. These nonuniformities are usually due to improper shimming, environmental factors, or the far extremes of newer short bore magnets. This can lead to image distortion They can be reduced in SE and FSE imaging by using refocusing pulses. They can be a source of image inhomogeneity when a fat suppression technique is used 20

98 感謝您的聆聽, 再會!! 新光醫院放射診斷科李正輝技術專員 TEL: