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GeometryパラメーターによるSNRの変化
近畿大学医学部附属病院福島 弘之
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Geometryの中の主なパラメーター
・Coil selection・CLEAR・FOV・RFOV・Fold-over suppression・Matrix scan・Scan percentage・SENSE
・k-t BLAST・Stacks・Slice scan order・REST slabs
etc.
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SNRに大きく関与するパラメーター
・FOV・RFOV・Matrix scan・Scan percentage・SENSE
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RFOV
位相
周波数
RFOV 50%
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Scan percentageScan percentageは位相方向のMatrixを減らす
ことで撮像時間を短縮することができる。ただし分解能の低下を招く。
位相
周波数
Scan percentage50%
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SNRの関係式
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例題
基本となるパラメーターをFOV 256RFOV 100%Matrix 256Scan percentage 100%スライス厚 10mmSENSE factor 1
とし、それぞれを変化させたときのSNRの変化を考える。
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FOV
FOVのみを2倍の512に変化させてみる。
ピクセルサイズは1mm×1mm→2mm×2mm
ピクセルサイズは4倍となる。
SNRは4倍となる。
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RFOV
RFOVを50%に変化させてみる。
位相方向のデータ数が1/2になる。
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Scan percentage
Scan percentageを50%に変化させてみる。
ピクセルサイズは1mm×1mm→1mm×2mm
ピクセルサイズは2倍となる。
SNRは2倍となる。
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Matrix
Matrixを128に変化させてみる。
ピクセルサイズは1mm×1mm→2mm×2mm
ピクセルサイズは4倍となる。
SNRは4倍となる。
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スライス厚
スライス厚を10mm→20mmに変化させてみる。
SNR∝スライス厚
SNRは2倍となる。
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SENSE factor
SENSE factorを1→4に変化させてみる。
SNRは1/2倍となる。
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このように1つだけのパラメーターのみを変化させた場合のSNRの計算はさほど複雑ではない。
しかしながら実際は同時に複数のパラメーターを変化させる必要がある場合が多く、設定によっては何かを変化させると別の何かが変わることがあり、SNRの計算が複雑になってくる。
非常に便利な指標!!
Information内のRel. signal level (%)
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では実際にいろいろパラメーターをいじってみよう!!
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腹部領域の息止め撮影T2w
FOV 370 RFOV 100%Matrix 256 Recon 512ピクセルサイズ 1.45/1.48Scan percentage 100% (act.94%)SENSE 2TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.895/242.6)Scan times 27msRel. signal level 100%
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均一ファントム画像
Imagejを用いてSNRマップを取得
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RFOV 100%→80%FOV 370 RFOV 80%Matrix 256 Recon 512ピクセルサイズ 1.45/1.58Scan percentage 100% (act.91.4%)SENSE 2TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.905/240)Scan times 21msRel. signal level 93.1%
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SNRマップ
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SNRマップ
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RFOV 100%→80%
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Scan percentage 100%→80%FOV 370 RFOV 100%Matrix 256 Recon 512ピクセルサイズ 1.45/1.97Scan percentage 80% (act.73.3%)SENSE 2TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.934/232.5)Scan times 21msRel. signal level 118%
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SNRマップ
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SNRマップ
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Scan percentage 100%→80%
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RFOV 100%→80%Scan percentage 100%→80%
FOV 370 RFOV 80%Matrix 256 Recon 512ピクセルサイズ 1.45/1.90Scan percentage 80% (act.76.2%)SENSE 2TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.926/234.6)Scan times 18msRel. signal level 103.1%
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SNRマップ
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SNRマップ
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RFOV 100%→80%Scan percentage 100%→80%
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RFOV 100%→80%SWNSE factor 2→2.5
FOV 370 RFOV 80%Matrix 256 Recon 512ピクセルサイズ 1.45/1.52Scan percentage 100% (act.95.2%)SENSE 2.5TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.895/241.9)Scan times 18msRel. signal level 81.2%
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SNRマップ
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SNRマップ
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RFOV 100%→80%SWNSE factor 2→2.5
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Scan percentage 100%→80%SENSE factor 2→2.5
FOV 370 RFOV 100%Matrix 256 Recon 512ピクセルサイズ 1.45/1.90Scan percentage 80% (act.76.2%)SENSE 2.5TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.926/234.6)Scan times 18msRel. signal level 103.1%
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SNRマップ
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SNRマップ
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Scan percentage 100%→80%SENSE factor 2→2.5
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FOV 370→400RFOV 100%→70%
FOV 400RFOV 70%Matrix 256 Recon 512ピクセルサイズ 1.56/1.82Scan percentage 100% (act.86%)SENSE 2TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.923/235.3)Scan times 18msRel. signal level 106.6%
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SNRマップ
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SNRマップ
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FOV 370→400RFOV 100%→70%
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FOV 370→400 RFOV 100%→70%Scan% 100%→80% SENSE factor 2→2.5
FOV 400RFOV 70%Matrix 256 Recon 512ピクセルサイズ 1.56/2.44Scan percentage 80% (act.86%)SENSE 2.5TR 3000ms TE 80ms TSE factor 16WFS 1 (act. 0.923/235.3)Scan times 12msRel. signal level 109.9%
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SNRマップ
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SNRマップ
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FOV 370→400 RFOV 100%→70%Scan% 100%→80% SENSE factor 2→2.5
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T1w Dual echoFOV 350 RFOV 80%Matrix 256 Recon 256ピクセルサイズ 1.37/1.95Scan percentage 70% (act.70.1%)SENSE 1.8TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 17.2msRel. signal level 100%
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RFOV 80%→70%FOV 350 RFOV 70%Matrix 256 Recon 256ピクセルサイズ 1.37/1.95Scan percentage 70% (act.70.2%)SENSE 1.8TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 15.4msRel. signal level 94.2%
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SNRマップ
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SNRマップ
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RFOV 80%→70%
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Scan% 70%→60%FOV 350RFOV 80%Matrix 256 Recon 256ピクセルサイズ 1.37/2.29Scan percentage 60% (act.59.8%)SENSE 1.8TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 14.8msRel. signal level 108.2%
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SNRマップ
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SNRマップ
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Scan% 100%→80%
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FOV 350RFOV 70%Matrix 256 Recon 256ピクセルサイズ 1.37/2.29Scan percentage 60% (act.59.6%)SENSE 1.8TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 13.2msRel. signal level 102.2%
RFOV 80%→70%Scan percentage 70%→60%
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SNRマップ
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SNRマップ
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RFOV 80%→70%Scan percentage 70%→60%
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FOV 350RFOV 70%Matrix 256 Recon 256ピクセルサイズ 1.37/1.94Scan percentage 70% (act.70.4%)SENSE 2.3TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 12.2msRel. signal level 83%
RFOV 80%→70%SENSE factor 1.8→2.3
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SNRマップ
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SNRマップ
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RFOV 80%→70%SENSE factor 1.8→2.3
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FOV 350 RFOV 80%Matrix 256 Recon 256ピクセルサイズ 1.37/2.29Scan percentage 60% (act.59.8%)SENSE 2.3TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.188/1157.4)Scan times 11.8msRel. signal level 96%
Scan percentage 70%→60%SENSE factor 1.8→2.3
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SNRマップ
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SNRマップ
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Scan percentage 70%→60%SENSE factor 1.8→2.3
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FOV 380RFOV 60%Matrix 256 Recon 256ピクセルサイズ 1.48/2.11Scan percentage 70% (act.70.5%)SENSE 1.8TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.175/1240.1)Scan times 13.2msRel. signal level 99%
FOV 350→380RFOV 80%→60%
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SNRマップ
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SNRマップ
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FOV 350→380RFOV 80%→60%
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FOV 380RFOV 60%Matrix 256 Recon 256ピクセルサイズ 1.48/2.50Scan percentage 60% (act.59.4%)SENSE 2.3TR 150ms TE 2.3/4.6ms WFS minimum (act. 0.175/1240.1)Scan times 9msRel. signal level 95.4%
FOV 350→380 RFOV 80%→60%Scan% 70%→60% SENSE factor 1.8→2.3
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SNRマップ
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SNRマップ
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FOV 350→380 RFOV 80%→60%Scan% 70%→60% SENSE factor 1.8→2.3
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まとめ
• パラメーターを調整するとき、SNRに関してはInformationのRel. signal levelを参考にすると
良い。
• Geometry内のパラメーターを変更することでSNRおよび分解能をある程度保ったまま撮影
時間の短縮が可能である。
• RFOV、SENSE factor を大きくすると、g-factorの影響やSENSEの展開エラーなどに注意が必
要である。