查看“︁翻轉角”︁的源代码

'''翻轉角''' (flip angle、tip angle)，也可稱作'''偏折角'''、'''傾角'''，代表了[[射頻]]對[[自旋]]組成的[[磁化向量]]能夠翻轉多少角度。因為是改變[[進動]]中之[[自旋]]的進動軸，實際上的作用等同於[[章動]]，故也稱為'''章動角''' (nutation angle)。

一個射頻[[脈衝]]的翻轉角可以透過如下公式計算：
:<math>\alpha = \gamma \int B_1(t) dt </math>。

其中，&alpha;代表翻轉角，&gamma;是[[旋磁比]]，B<sub>1</sub>是射頻脈衝[[波包]]外形強度；整個式子可以看出此角度是射頻強度對時間的[[積分]]，再做一些單位變換。

對一個矩形的脈衝而言，式子相當簡單：
:<math>\alpha = \gamma B_1 \Delta t \,</math>。

==科研方面==

[[2011年]]，有[[研究]]證實25°至40°的翻轉角可顯著提高[[肝]]和膽道系統的信號強度，並且可以抑制骨骼肌等<ref name=a>{{cite journal |last1=Bashir |first1=MR |last2=Husarik |first2=DB |last3=Ziemlewicz |first3=TJ |last4=Gupta |first4=RT |last5=Boll |first5=DT |last6=Merkle |first6=EM |title=Liver MRI in the hepatocyte phase with gadolinium-EOB-DTPA: does increasing the flip angle improve conspicuity and detection rate of hypointense lesions? |journal=Journal of magnetic resonance imaging : JMRI |date=2012-03 |volume=35 |issue=3 |pages=611-6 |doi=10.1002/jmri.22850 |pmid=22034383 |url=https://pubmed.ncbi.nlm.nih.gov/22034383-liver-mri-in-the-hepatocyte-phase-with-gadolinium-eob-dtpa-does-increasing-the-flip-angle-improve-conspicuity-and-detection-rate-of-hypointense-lesions/ |accessdate=2019-12-20 |author= |archive-url=https://web.archive.org/web/20191220062216/https://pubmed.ncbi.nlm.nih.gov/22034383-liver-mri-in-the-hepatocyte-phase-with-gadolinium-eob-dtpa-does-increasing-the-flip-angle-improve-conspicuity-and-detection-rate-of-hypointense-lesions/ |archive-date=2019-12-20 |dead-url=yes }}</ref>非肝細胞源性[[組織 (生物學)|組織]]的信號<ref>{{cite journal |last1=Bashir |first1=MR |last2=Merkle |first2=EM |title=Improved liver lesion conspicuity by increasing the flip angle during hepatocyte phase MR imaging. |journal=European radiology |date=2011-02 |volume=21 |issue=2 |pages=291-4 |doi=10.1007/s00330-010-1917-1 |pmid=20686771 |url=https://pubmed.ncbi.nlm.nih.gov/20686771-improved-liver-lesion-conspicuity-by-increasing-the-flip-angle-during-hepatocyte-phase-mr-imaging/ |accessdate=2019-12-20 |author= |archive-url=https://web.archive.org/web/20191220062214/https://pubmed.ncbi.nlm.nih.gov/20686771-improved-liver-lesion-conspicuity-by-increasing-the-flip-angle-during-hepatocyte-phase-mr-imaging/ |archive-date=2019-12-20 |dead-url=yes }}</ref>。同年進行的另一份研究則建議使用25°至30°的翻轉角檢測[[肝臟]]病變，並且建議使用45°的翻轉角檢測膽道系統<ref>{{cite journal |last1=Frydrychowicz |first1=A |last2=Nagle |first2=SK |last3=D'Souza |first3=SL |last4=Vigen |first4=KK |last5=Reeder |first5=SB |title=Optimized high-resolution contrast-enhanced hepatobiliary imaging at 3 tesla: a cross-over comparison of gadobenate dimeglumine and gadoxetic acid. |journal=Journal of magnetic resonance imaging : JMRI |date=2011-09 |volume=34 |issue=3 |pages=585-94 |doi=10.1002/jmri.22713 |pmid=21751288 |url=https://pubmed.ncbi.nlm.nih.gov/21751288-optimized-high-resolution-contrast-enhanced-hepatobiliary-imaging-at-3-tesla-a-cross-over-comparison-of-gadobenate-dimeglumine-and-gadoxetic-acid/ |accessdate=2019-12-20 |author= |archive-url=https://web.archive.org/web/20191220062215/https://pubmed.ncbi.nlm.nih.gov/21751288-optimized-high-resolution-contrast-enhanced-hepatobiliary-imaging-at-3-tesla-a-cross-over-comparison-of-gadobenate-dimeglumine-and-gadoxetic-acid/ |archive-date=2019-12-20 |dead-url=yes }}</ref>。有學者指出翻轉角由10°增大至30°時，SAR值會因而增加9倍，存在有關缺點<ref>{{cite journal |last1=Guérin |first1=B |last2=Gebhardt |first2=M |last3=Cauley |first3=S |last4=Adalsteinsson |first4=E |last5=Wald |first5=LL |title=Local specific absorption rate (SAR), global SAR, transmitter power, and excitation accuracy trade-offs in low flip-angle parallel transmit pulse design. |journal=Magnetic resonance in medicine |date=2014-04 |volume=71 |issue=4 |pages=1446-57 |doi=10.1002/mrm.24800 |pmid=23776100 |url=https://pubmed.ncbi.nlm.nih.gov/23776100-local-specific-absorption-rate-sar-global-sar-transmitter-power-and-excitation-accuracy-trade-offs-in-low-flip-angle-parallel-transmit-pulse-design/ |accessdate=2019-12-20 |author= |archive-url=https://web.archive.org/web/20191220062213/https://pubmed.ncbi.nlm.nih.gov/23776100-local-specific-absorption-rate-sar-global-sar-transmitter-power-and-excitation-accuracy-trade-offs-in-low-flip-angle-parallel-transmit-pulse-design/ |archive-date=2019-12-20 |dead-url=yes }}</ref>。然而增加射頻脈衝持續[[時間]]及減少相位編碼數等方法能夠有效控制SAR值<ref name=a/>。

== 相關條目 ==
* [[核磁共振]]
* [[磁振造影]]

==參考資料==
{{reflist}}

[[Category:磁共振]]
[[Category:角]]