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牙科放射線學 (2). Introduction to Magnetic Resonance Imaging. 核磁共振影像介紹. 陳玉昆副教授 : 高雄醫學大學 口腔病理科 07-3121101~2755 yukkwa@kmu.edu.tw. M a g n e t i c R e s o n a n c e I m a g i n g. 學 習 目 標. References for this lecture. 核磁共振技術的歷史. MRI 的 成 像 原 理. Relaxation.

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slide1

牙科放射線學(2)

Introduction to Magnetic Resonance Imaging

核磁共振影像介紹

陳玉昆副教授: 高雄醫學大學 口腔病理科

07-3121101~2755

yukkwa@kmu.edu.tw

slide2

MagneticResonanceImaging

學 習 目 標

References for this lecture

核磁共振技術的歷史

MRI 的 成 像 原理

Relaxation

Repetition and echo time

Determining image contrast

slide3

MagneticResonanceImaging

1. 1st edition, p. 1-17, 157.

3. 鄭慶明 February

1994;37(2):33-38

2.沈茂忠et al, 3rd edition, Chapter 13, p. 463-471.

References for the present lecture

slide4

MagneticResonanceImaging

2001, November

5. Wilkinson E. MRI researchers win Nobel Prize. Lancet Oncol  2003;4:649

7. 陳煥武譯 圖解核磁共振造影學, 合記出版社 2006 1st pp. 12-37

6. Peggy Woodward MRI for technologists McGraw-Hill, Chapter 1 pp. 1-12

4. Basic Principles of MR Imaging. Sharon LB, p. 569-84

References for the present lecture

slide5

MagneticResonanceImaging

順向旋轉

Bo

Mo

逆向旋轉

核磁共振技術的歷史

1930年代,伊西多·拉比(Isidor Rabi)發現在磁場中的原子核會沿磁場方向呈正向或反向有序平行排列,施加無線電波之後,原子核的自旋方向發生翻轉。這是關於原子核與磁場以及外加射頻場相互作用的最早認識。由於這項研究,拉比于1944年獲得了諾貝爾物理學獎

Isidor Rabi

Ref. 2

http://zh.wikipedia.org/wiki

slide6

MagneticResonanceImaging

Edward Mills Purcell

http://nobelprize.org/nobel_prizes/physics/laureates/1952/purcell-bio.html

核磁共振技術的歷史

http://zh.wikipedia.org/wiki

1946年,費利克斯·布洛赫(Felix Bloch)和愛德華·米爾斯·珀塞耳(Edward Mills Purcell)發現,將具有奇數個核子(包括質子和中子)的原子核置於磁場中,再施加以特定頻率的射頻場,就會發生原子核吸收射頻場能量的現象,這是最初對核磁共振現象的認識。為此他們兩人獲得了1952年度諾貝爾物理學獎

Felix Bloch

http://nobelprize.org/nobel_prizes/physics/laureates/1952/bloch-bio.html

slide7

MagneticResonanceImaging

核磁共振技術的歷史

發現核磁共振現象之後產生了實際用途,化學家利用分子結構對氫原子周圍磁場產生的影響,發展出了核磁共振譜,用於解析分子結構。核磁共振譜技術從最初的一維氫譜發展到13C譜、二維核磁共振譜等高級譜圖,進入1990年代以後,發展出了依靠核磁共振信息確定蛋白質分子三級結構的技術,使得溶液相蛋白質分子結構的精確測定成為可能

http://zh.wikipedia.org/wiki

slide8

MagneticResonanceImaging

核磁共振技術的歷史

醫學家發現水分子中的氫原子可以產生核磁共振現象,利用這一現象可以獲取人體內水分子分佈的信息,精確繪製人體內部結構,在這一理論基礎上1969年,紐約州立大學南部醫學中心的達馬迪安通過測核磁共振的弛豫時間成功的將小鼠的癌細胞與正常組織細胞區分開來

http://zh.wikipedia.org/wiki

slide9

MagneticResonanceImaging

核磁共振技術的歷史

在達馬迪安新技術的啟發下紐約州立大學石溪分校(University of Illinois at Urbana)的物理學家保羅·勞特伯爾(Paul Lauterbur)于1973年開發出了基於核磁共振現象的成像技術,成功地繪製出了一個活體蛤蜊地內部結構圖像。2003年,保羅·勞特伯爾和英國諾丁漢大學(University of Nottingham)教授彼得·曼斯菲爾因為他們在核磁共振成像技術方面的貢獻獲得了當年度的諾貝爾生理學或醫學獎。

保羅·勞特伯爾

彼得·曼斯菲爾

http://zh.wikipedia.org/wiki

www.cc.nctu.edu.tw/~hcsci/hospital/ins/mri.htm

slide10

MagneticResonanceImaging

核磁共振技術的歷史

Dr. Paul Lauterbur

Z coils

Y

X

Y coils

X

X coils

A

B

A Gradient coil Gx,Gy, & Gz interact upon the magnetized patient & collectively create a preselected spatial excitation B Oil distribution in a pecan nut (大胡桃果實), Lauterbur's 1973 prototype selective excitation image taken using NMR

Ref. 6

slide11

MagneticResonanceImaging

核磁共振技術的歷史

Dr. Raymond Damadian built the first whole body MRI machine - 1977

Dr Raymond Damadian & his associates, Drs Larry Minkoff & Michael Goldssmith, completed the world's 1st whole body MRI scanner. Named Indomitable to capture the spirit of its 7-year construction (now located at Smithsonian Ins, Washington, DC)

The first whole body transaxial image took 4 h 45 min to produce. Made on July 3, 1977, it shows the thoracic spine of Larry Minkoff

Indomitable

Indomitable: 百折不撓

Ref. 6

slide12

MagneticResonanceImaging

電子(負電)

2

原 子 構 造

1

Orbital electrons

中子(不帶電)

Nucleus = protons + neutrons

原子核

3

K

L

M

N

O

質子(正電)

Proton & neutron:spin (自旋)Spinning of proton (odd no):electric current magnetic field(核磁效應)

電流

磁場線

Ref. 7

MRI 的 成 像 原 理

slide13

MagneticResonanceImaging

無外在磁場

一、自由磁核

即一原子核不受其他外在磁場之影響,由本身所產生

之自旋 (spin) 各指向任意之方向

Ref. 2

slide14

MagneticResonanceImaging

順向旋轉

Bo

Mo

逆向旋轉

一些自然存在的原子核可顯示磁性,是由自旋造成的靜磁場(Mo)

這些具磁性之原子核可稱為”非零自旋”(non-zero spin)或”角動量”(angular momentum)的磁核,也可稱為”原子核之磁偶極”

(nuclear magnetic dipoles),具有北極和南極,就如一小磁鐵

N

磁 鐵

N

Mo: 靜磁場 (net magnetic field)

S

S

Bo: 外加磁場 (external magnetic field)

Refs. 2, 3, 7

slide15

MagneticResonanceImaging

外加磁場

磁場北極 (North Pole)

磁力線

外加磁場

磁場南極 (South Pole)

當具有核磁效應的原子核被一強的靜磁場影響後,

有如一小磁鐵般,磁核將會依外加的磁力線方向而

排列,所有的原子核均是相互平行排列

Ref. 2

slide16

MagneticResonanceImaging

Bo

順向平行: 低能量

Bo

Mo

多出的順向平行

隨機排列無外加磁場

平行排列外加磁場

逆向平行: 高能量

Mo: 靜磁場 (net magnetic field)

Alignment

Bo: 外加磁場 (external magnetic field)

Ref. 7

slide17

MagneticResonanceImaging

Nucleus

身體組織中的相對氫原子密度

脾臟 93

肝臟 91

血液 91

胰臟 87

皮質骨 10

肺 10

空氣 1

腦白質 100

肌肉 100

脂肪 98

腦脊液 97

腎臟 95

腦灰質 94

1H

Single

proton

13C

19F

23Na

磁偶極

31P

39K

Hydrogen

proton

氫原子只含一個質子,無中子,外面軌道只有一個電子,因此質子所產生的磁性,最不會被電子隔離對於外加磁場的靈敏度最佳,因此氫原子最適用於MRI之成像

(單一的質子使其產生大磁矩)

Refs. 1, 2

slide18

MagneticResonanceImaging

二、旋進(Precession) (Larmor Frequency)

具有核磁效應的原子核好像是一個微小的迴旋儀 (gyroscope) 依其本身之自旋(spin)順延外加外磁場方向慢慢旋轉,圍成一角圓錐形

外加磁場

Magneticmoment

Nucleus

Precession (Larmor frequency)

No. of rotations per sec

外加磁場

Refs. 1,2,4

slide19

MagneticResonanceImaging

Radiofrequency

Microwave

Infrared

Visible/UV

X-ray

NMR / MIR

Roentgen

wavelength

frequency

Spectrum of electromagnetic radiation

Lower

field

Higher

field

Stronger

radiowave

Nuclei are able to absorb radiowaves in both the strong & weak field

Absorption occurs at a field strength-dependent frequency: higher in the strong magnetic field & weak in the weak magnetic field

N

N

Weaker

Radio

wave

nuclei

S

S

Ref. 1

slide20

MagneticResonanceImaging

三、共振 (Resonance)

用一線圈連接到一個RF射頻產生器上產生一個相同於拉慕爾頻率的射頻脈波讓自旋軸離開了穩定磁場之磁力線的平行方向,被激發之原子核處於共振狀態

Y

磁場北極

X

Z

After

resonance

RF = radiofrequency

RF

磁場南極

B0

Larmor frequency

B0

M

Mo // Bo

Flipping

90o

Mo

RF = Larmor frequency

Flip angle

Before

resonance

Mo = net magnetization of protons of tissueBo = external magnetic field

Refs. 2, 4

slide21

MagneticResonanceImaging

磁場北極

RF

磁場南極

四、張弛 (Relaxation) 弛豫

當一射頻(RF)脈波結束時,原子核在一個靜磁場的影響下它將會回到其原平行Z軸的方向在此過程中,它們將會旋進(precess),同時釋放出方才吸收的射頻能量,以一微弱的旋進頻率之射頻(RF)信號發放

Ref. 2

slide22

MagneticResonanceImaging

訊號依一指數(exponential)曲線衰減消逝直到當原子核再次的達到它的平衡(equilibrium)狀態,且與Z軸平行並列時趨於零這消逝的時間是從外加射頻脈波結束時至自旋原子核產生射頻訊號終止時,此時間之區間稱為張弛時間( relaxation time)

Intensity

Time

Relaxation time

Refs. 2, 4

Pulse

slide23

MagneticResonanceImaging

Free induction

decay

訊號振幅

Mxy

訊號接受線圈

(A)

(B)

50 msec

100 msec

Time

  • RF脈衝結束時,開始自由迴旋的組織磁化向量在XY平面所產生的振動會在XY平面的接受線圈中誘發電壓
  • 此電壓在增幅後呈現指數性遞減的正弦曲線振動,稱作自由感應衰減

Ref. 3

slide24

MagneticResonanceImaging

MRI 成 像 順 序 (小結)

(1)把人體放在強大磁場中,使人體內氫原子一律向Z軸排列

(2) 給予相同於Larmor frequency之射頻脈波(RF: radiofrequency)使氫原子的自旋軸自Z軸偏向,稱共振

(3) 停止RF則氫原子核逐漸回到其原平行之Z軸方向,而達到平衡狀態

(4) 在過程中會釋放出訊號,此訊號經由接受器接收,電腦再予解析、組成,而構成影像

Net spinning

Precession

Resonance(Excitation)

www.cc.nctu.edu.tw/~hcsci/hospital/ins/mri.htm

Relaxation

Free induction decay

Ref. 2

slide25

MagneticResonanceImaging

Relaxation

Relaxation

(外加磁場)

(外加電磁波)

RF=Larmor

frequency

excitation

(resonance)

Larmor

frequency

(precession)

平衡時,核磁化矩與靜磁場方向(Z)一致90°RF波使核磁化矩由Z方向轉到Y方向此時Z方向之分量為零。此後Y平面分量逐漸消失,Z方向分量由零漸增長而達平衡描述核磁化矩在XY平面分量之消失的過程稱spin-spin(T2)relaxation或transverse relaxation 描述Z方向分量之增長的過程稱spin-lattice (T1) relaxation或longitudinal relaxation

(Flipping

angle)

(外加線圈

接受訊號-

Free induction decay)

T2-time T1-time

T1- & T2- relaxation occur concurrently

Refs. 2, 3

slide26

MagneticResonanceImaging

1.0

.95

.86

Z-axis magnetization

.63

0

1

2

3

4

T1 relaxation time (m sec)

T1 Relaxation之定義

T1 表示在Z軸上的磁化分量因受靜磁場影響而逐漸增長到平衡狀態的時間常數

在剛給予90° RF波後,Z分量為零,經一個T1 時間,它會增長到平衡狀態的63%大約需四、五個T1時間,它可增長到平衡時之最大值

Ref. 2

slide27

MagneticResonanceImaging

Relaxation : T1, Spin-lattice relaxation time

Longitudinal (heat) relaxation

Definition

T1 relaxation time (about 3s for pure

water) is the time required for the system to recover

to 63% of its equilibrium value after it has been

exposed to a 90o pulse

  • Type of nucleus
  • Resonance frequency (field strength)
  • Temperature
  • Mobility of observed spin (micro viscosity)
  • Presence of large molecules
  • Presence of paramagnetic ions (順磁性) or molecules

Factors affecting T1 time

T1

Hz

Ref. 1

slide28

MagneticResonanceImaging

Relaxation : T2, Spin-spin relaxation time

Transverse relaxation

Factors affecting T2 time

  • Observation frequency

(less crucial than T1)

  • Temperature
  • Mobility of the observed
  • spin (microviscosity)
  • Presence of large
  • molecules and
  • paramagnetic (順磁性) ions
  • and molecules

Ref. 1

slide29

MagneticResonanceImaging

T2 Relaxation之定義

1.0

T2表示平面上磁化矩下降,它成指數的下降

Y-axis magnetization

.37

.14

.05

1

2

3

4

T2 relaxation time (m sec)

從90° 波後的最大值下降到零大約需四、五個T2的時間。而經一過T2 的時間大約剩下開始時的37%

Ref. 2

slide30

MagneticResonanceImaging

T1 Relaxation

白 質

脂肪

灰 質

Z-axis magnetization

腦脊液

Time (m seconds)

T1愈短則愈快到達平衡之最大值脂肪之T1最短,最快達平衡;CSF最長,最慢到達平衡時之最大值。因此脂肪在以T1為主的影像(T1WI)是高訊號強度(白),而CSF在T1WI為低訊號強度(暗)

T1 weighted image

Ref. 2

slide31

MagneticResonanceImaging

Y-axis magnetization

CSF

腦脊液

T2 Relaxation

灰質

白質

Time (m seconds)

CSF之T2值最長,因此最慢消失;而白質在此三者中最短,最快消失。因此CSF在以T2為主的影像(T2WI)

中為高訊號強度(白),而白質為低訊號強度(灰暗)

T2 weighted image

Ref. 2

slide32

MagneticResonanceImaging

0.5 T

1.0 T

組織

T1(ms)

T2(ms)

T1/T2

T1(ms)

T2(ms)

T1/T2

600

70

860

12.3

70

8.6

肌肉

540

50

10.8

750

55

13.6

脂肪

220

60

3.7

220

60

3.7

CSF

3000

200

15

3000

200

15

血液

850

200

4.5

900

200

4.5

3000

200

15

3000

眼水樣體

200

15

Tissue relaxation 的特性

T1 is increased with increased field strength

T2 is unchanged with field strength

T1/T2 constant

Ref. 2

slide33

MagneticResonanceImaging

T1 and T2 relaxation times of

malignant tissues > normal tissues

(exception is melanoma)

Ref. 1

slide34

MagneticResonanceImaging

Repetition and echo time

(Resonance)

Excitation

TR = repetition time

TE = echo time

Detection

MR Image

The spin-echo pulse sequence in MRIA series of 90o & 180o RF pulses is given at a repetition time (TR) selected by the operatorThe signal from the tissues is detected at the time of echo (TE), again selected by the operatorTR & TE will affect image contrast

operator

operator

Ref. 4

Image contrast

slide35

MagneticResonanceImaging

Determining image contrast

TR(重複時間):一個脈衝開始到下一個脈衝的時間間隔TR會影響脈衝開始下一個脈衝之間的遲豫的長度 (T1)

TE(回音時間):脈衝開始到收集訊號的時間間隔TE會影響RF激發脈衝去除之後的遲豫期間長度 (T2) 以及接受線圈接受到最大的訊號

Ref. 7

slide36

MagneticResonanceImaging

MRI

signal

Saturation recovery

(90o – TR – 90o)

90o

90o

90opulse

TR

Mz

Repeat

M0

100%

TR (time delay between RF pulses)

63%

90opulse

50%

TR = time between 90o pulse

T1

2T1

3T1

TR

Ref.1

slide37

MagneticResonanceImaging

Spin echo pulse sequence(90o – TE – 180o)

90o

180o

Pulse

t

2TE

Repeat

TE (time delay)

t

180o pulse

Spin echo

Ref. 1

slide38

MagneticResonanceImaging

Inversion recovery

(180o – TI – 90o)

TR

TI

180o

180o

180o

90o

180opulse

FID

echo

TE

蹤向磁化

TI (time delay between RF pulses)

TI

Repeat after delay

equal to several TI (>3)

90o pulse

脂肪

MR signal

90o的激化脈衝將脂肪偏離至180o(無訊號)

Ref. 1

slide39

MagneticResonanceImaging

T1-weighted oblique sagital MR image of the temporo-mandibular joint open-mouth view

The black linear structure (arrow) is the disk located in the normal position superior to the condyle(TR = 500ms, TE = 19ms)

Ref. 4

slide40

MagneticResonanceImaging

Parameters for MRI

(Proton density)

(TR)

(TE)

Ref. 1

slide41

MagneticResonanceImaging

z

z

z

z

90oRF脈衝

Mz

Mz

180oRF脈衝

B0

y

y

y

y

Mxy

Mxy

x

x

x

Mz

x

Ref. 3

Flip

Angle of precession

(net magnetization)

away from Z-axis

TR (ms)

Repetition time

(flip 90o - 90o)

(A)

(B)

(C)

(D)

  • 當RF脈衝以在處於平衡狀態靜磁場中,磁化向量M完全位於Z軸上
  • 迴旋頻率垂直彈擊靜磁場時,會將磁化向量由Z軸彈開θ角度。這時的磁化向量會分化為Mxy及Mz分向量。 θ之大小視RF脈衝的強度與時間長度而定
  • 當θ角為90°時,RF脈衝可將磁化向量由Z軸彈到XY平面上。這時的RF脈衝稱為90°脈衝
  • 使磁化向量由Z軸正方向到轉到負方向時所採用的RF脈衝稱為180 °脈衝

TE (ms)

Echo time

(flip 90o - 180o)

slide42

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

*Long

T2-weighted

Long

Proton density-weighted

Long

Short

Determining image contrast

Water

(long T1- & T2- time)

dark in T1-weighted

image

bright on T2-weighted

image

T1-weighted & proton density-weighted:

(visualize the tissue anatomy)

T1-relaxation time is the major determinant of

tissue contrast of normal anatomy

Tissue with short T1-time: bright; long T1-time: dark

T2-weighted:(壓抑 fat signal - additional tissue information)

T2-relaxation time is the major determinant of tissue contrast ofpathological lesion

Tissue with long T2-time: bright; short T2-time: dark

Ref. 4

slide43

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

訊號強度

脂肪和水在沒有對比即訊號相同、無法區分

脂肪和水只有些微的對比差異

短T1(脂肪)

脂肪和水的對比

長T1(水)

脂肪和水有大的對比差

長T2(水)

短T2(脂肪)

TR(ms)

短TR

長TR

短TE

TE(ms)

長TE

Refs.4, 7

slide44

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

短TR

180oRF脈衝

90oRF脈衝

90oRF脈衝

單一自旋回音

典型參數

TR 300~600ms

TE 10~30ms

短TE

Ref. 7

slide45

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

脂肪

遲豫

Bo

Bo

脂肪和水遲豫至Bo方向

Bo

Bo

第一次RF脈衝

第一次RF脈衝

橫向平面

橫向平面

橫向平面

橫向平面

Short TR

Short TE

Bo

Bo

Bo

Bo

第二次RF脈衝

脂肪和水的向量表現出質子密度

脂肪

第二次RF脈衝

橫向平面

橫向平面

脂肪和水在橫向平面

脂肪

Ref. 7

slide46

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

T1-weighted

High signal

T1-weighted

Low signal

T1-weighted

No signal

脂肪血管瘤骨內脂肪瘤放射治療後之變化退化性脂肪堆積methaemogloblin富含蛋白質性的囊胞順磁性顯影劑低流速的血液

皮質骨缺血性壞死 中風感染腫瘤硬化囊胞鈣化

空氣高流速性的血液 肌腱結痂組織鈣化

Ref. 7

slide47

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

長TR

180o

180o

第一次自旋回音得到質子密度影像

第二次自旋回音得到質子密度影像

90o

90o

1stTE (短)

典型參數

TR 2000ms+

TE 70ms+

2ndTE(長)

Ref. 7

slide48

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

T2-weighted

High signal

T2-weighted

Low signal

T2-weighted

No signal

腦脊髓液(CSF)關節液血管瘤發炎水腫某些特定腫瘤出血流速慢的血液囊腫(cyst)

皮質骨、骨島脂肪 去氧血紅素(deoxyhemoglobin)血鐵質(hemosiden)鈣化T2順磁物質(paramagneticagents)

空氣流速快的血液 肌腱皮質骨(cortical bone)結痂組織鈣化

Ref. 7

slide49

MagneticResonanceImaging

Time of

Repetition

Time of

Echo

Image Contrast

*Short

T1-weighted

Short

Long

*Long

T2-weighted

Proton density-weighted

Long

Short

Typical parameters

TR 2000ms+

TE 10-30ms+

B0

磁化作用的X-axis部分

低質子密度

高質子密度

Ref. 7

slide50

MagneticResonanceImaging

Summaries

Finishing this lecture, you can understand:

- 核磁共振技術的歷史

- MRI 的 成 像 原 理

- T1 and T2 relaxation time

- Repetition and echo time

- Determining image contrast