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大阪赤十字病院高津安男

アーチファクト

第16回関西Gyro Meeting

＜Artifact（偽像）＞• 画像処理系

①パーシャルボリューム

OSAKA RED CROSS HOSPITAL 2012OSAKA RED CROSS HOSPITAL 2012

• 原理由来

①折り返し

②ケミカルシフト

③クロストーク

④N/2

⑤打ち切り

• 外部・装置由来

①ジッパー

②RF不均一

③磁場不均一

④渦電流

• 生体由来

①動き

②フロー

③磁化率

④マジックアングル

第16回関西ジャイロミーティング第16回関西ジャイロミーティング

OSAKA RED CROSS HOSPITAL 2012

フローによるアーチファクト

H

F12345678910

励起順の変化Default:1,3,5,7,9,2,4,6,8,10Ascending:1,2,3,4,5,6,7,8,9,10Descending:10,9,8,7,6,5,4,3,2,1Reverse central:1,10,2,9,3,8,4,7,5,6Interleaved:5,6,4,7,3,8,2,9,1,10

S状静脈:アーチファクトの影響が出やすい。

VENC:16cm/sec

となりのスライスがPreーSatの役割




Ascending:1,2,3,4,5,6,7,8,9,10

Default:1,3,5,7,9,2,4,6,8,10

Descending:10,9,8,7,6,5,4,3,2,1

Reverse central:1,10,2,9,3,8,4,7,5,6

Interleaved:5,6,4,7,3,8,2,9,1,10

FC+ FC+

FC+

FC+

FC+

FC- FC-




• 信号消失現象（Intra-voxel dephasing）渦流・乱流：流体による位相分散による信号消失

層流（血管壁からの距離で速度が異なる）

渦流乱流

加速度流CSF：モンロー孔、中脳水道から第四脳室

頚ー胸椎の脊髄腔




流体の関係式：レイノルズ数：Re=ρ vd/η

層流：Re2100未満

ρ ：流体の密度(g/cm2)v：流速(cm/s)d：血管直径(cm)η ：粘性率

層流：血管が細く、流速が遅いほうが保ちやすい




狭い場所はフローが早い。（頚ー胸椎移行部）

FC-

FC+

FC-

FC+

Longest

Longest




狭い場所はフローが早い。

FC-

FC+

FC-

FC+

Longest

Longest

3D

T2*


-80

-60-40

-200

20

4060

80

0 1000 2000 3000 4000 5000

TI (ms)

SN

R

TR=3000ms TR=6000ms TR=12000

FLAIR



◎前橋槽やMonro孔付近など狭い部位はCSFの流速が早く、

IRパルスが照射されていない隣のスライスからCSFが流入してくることにより信号を発生。



Slice1 Slice2 Slice3 Slice4 Slice5 Slice6

Slice1

Slice2

Slice3

Slice4

Slice5

Slice6

180°pulse幅

180°pulse幅

1 package

2 package



フローによるアーチファクト FLAIR

1 package

2 package

3 package



金属アーチファクト

R-L 256×256 6mm厚 A-P 256×256 6mm厚静磁場

静磁場R-L 256×256 6mm厚

寒天：0.1%Gd-DTPA+0.2%NaCl

寒天のみ

外科用ステープラー




256×256 6mm厚 BW43kHzTE 15ms

256×256 2mm厚 BW43kHz 512×512 6mm厚 BW43kHz

256×256 6mm厚 BW108kHz 256×256 2mm厚 BW108kHz

信号消失範囲の縮小：金属が磁場均一性を乱すことによるボクセル内位相分散の縮小

ボクセルサイズの縮小：サンプリング時間の短縮

256×256 6mm厚 BW43kHzTE 7ms




リフォーカスパルスの多用：TEの短縮

TSE factor:18 TSE factor: 36 TSE factor:123256×256 T2* TSE factor: 9

TE 120ms TE 100ms TE 80ms TE 60ms




256×256 6mm厚 BW43kHz1.5T

信号消失範囲の縮小：金属が磁場均一性を乱すことによるボクセル内位相分散の縮小

ボクセルサイズの縮小：サンプリング時間の短縮：TEの短縮

Δ Φ =γ ・Gi・Δ r・TEΔ Φ ：位相分散

Gi：磁束密度変化率Δ r：ボクセル幅TE：echo time

256×256 6mm厚 BW43kHz0.4T




BW:41.2kHz

BW:92.4kHz



EPI

連続G（位相方向）,blipともに相殺されない。

位相差が積算されていく。

位相の乱れに敏感。

磁化率アーチファクト

RF

Gs

Gp-c

Gp-b

Gr

Signal




EPI T2* 128 Single Shot

● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ●●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

Parallel imagingあり

● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ●●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

位相方向のFOVが減少する。（ライン数が減る）歪の大きさ＝位相シフト量

k空間の充填距離

Parallel imaging ex.R=2位相シフト量が1/2

R-FOV も同様

Parallel imaging




EPI T2* 128 Multi Shot● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ●●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●●● ●●● ●●● ●●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

Parallel imagingあり




EPI T2*

● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

Single Shot

Mtx(EPIfactor):128

● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ● ● ●

Mtx(EPIfactor):64

アーチファクト軽減法位相エンコードラインの減少

・Multi Shot・RFOV

・ Parallel imaging

効果が得られない・Mtx数

・Half Fourie




EPI T2*

256×256 6mm厚 256×256 3mm厚 512×512 3mm厚




EPI T2*

256×256 6mm厚 256×256 6mm厚Parallel imaging +

512×512 3mm厚Parallel imaging +


アーチファクトの種類の認識

アーチファクトの発生機序の理解

↓

軽減・改善方法の理解

↓

診断に有用な画像の提供


アーチファクトの理解第16回関西ジャイロミーティング第16回関西ジャイロミーティング

アーチファクト - fc2kansaigyro.web.fc2.com/lecture16/clinical4takatsu.pdfgs gp-c gp-b gr...

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