Almost low-angle shot

The English term Fast Low-Angle Shot ( FLASH ) describes a method for fast imaging on the basis of magnetic resonance imaging (MRT, also known as magnetic resonance imaging ) introduced in 1985 by Jens Frahm , Axel Haase , Wolfgang Hänicke , Klaus-Dietmar Merboldt and Dieter Matthaei known). The FLASH patent is one of the Max Planck Society's most successful patents since it was founded.

Applications

FLASH has considerably shortened the measurement times for imaging diagnostics compared to conventional spin-echo sequences. In this way new types of investigations became possible. These include above all

• imaging of the abdomen (e.g. liver and kidney) when breathing is held,
• filmic recordings of the beating heart when the measurements are synchronized with the electrocardiogram (ECG),
• dynamic sequential recordings of tissue perfusion after administration of contrast medium ( perfusion MRT ),
• high - resolution three-dimensional MRI images of complex organ structures (e.g. brain, female breast or knee joint), as well as
• High-resolution three-dimensional representations of the blood vessels by magnetic resonance angiography ( magnetic resonance angiography , MRA).

For 3D recordings, the measurement times (previously unrealizable for in vivo applications) could be reduced from several hours to a few minutes. Brain functions can also be dynamically mapped with the method in high resolution.

In 2010 it was possible to develop a robust method for real-time MRT with an extended FLASH method based on heavily under-sampled radial spatial encoding and iterative image reconstruction . This FLASH method offers a temporal resolution of 20 milliseconds (1/50 second) and can therefore, for example, image (film) the beating heart without synchronization with the ECG and with free breathing. Together with this new breakthrough, the acceleration of the images by FLASH corresponds to a factor of 10,000 compared to the MRI images before 1985.

Physical principle

Physically, the FLASH technique is based on a simple gradient - echo sequence, the small for the high-frequency excitation flip angle (tilt angle, low power) are used and combined with a very fast repetition of the experiment. The repetition time ( repetition time ) is much shorter than the usual T1 relaxation times of the water protons in the tissue. In this way, a constant longitudinal equilibrium magnetization results after a short time, which is then maintained over any length of measurement time; this allows sequential individual recordings to be made and, for example, films to be recorded in real time. In addition, very high-resolution three-dimensional recordings can be made in a short measuring time.

The signal of the FLASH sequence depends on the initial magnetization , the relaxation times and the tissue as well as the sequence parameters (echo time , repetition time and excitation angle ): ${\displaystyle S_{\text{FLASH}}}$ ${\displaystyle M_{0}}$ ${\displaystyle T_{1}}$ ${\displaystyle T_{2}^{*}}$ ${\displaystyle T_{\text{E}}}$ ${\displaystyle T_{\text{R}}}$ ${\displaystyle \alpha }$

${\displaystyle S_{\text{FLASH}}=M_{0}\sin(\alpha ){\frac {1-e^{-T_{\text{R}}/T_{1}}}{1-e^{-T_{\text{R}}/T_{1}}\cos(\alpha )}}e^{-T_{\text{E}}/T_{2}^{*}}.}$

The maximum signal (for a given repetition time) is obtained with the Ernst angle (named after Richard R. Ernst ):

${\displaystyle \alpha _{\text{Ernst}}=\arccos e^{-T_{\text{R}}/T_{1}}.}$

Web links

• Joseph P. Hornak: The Basics of MRI .
• Biomedizinische NMR Forschungs GmbH (more information about FLASH and MRT in neurobiological research)
• Information from the Max Planck Society

Individual evidence

1. ^ A. Haase, J. Frahm, D. Matthaei, W. Hänicke, KD Merboldt: FLASH imaging. Rapid NMR imaging using low flip-angle pulses . In: Journal of Magnetic Resonance . tape 67 , 1986, ISSN  0022-2364 , pp. 258-266 , doi : 10.1016 / 0022-2364 (86) 90433-6 .
2. ↑ Patent DE3504734 : Method and device for recording spin resonance data. Registered on February 12, 1985 , published on August 14, 1986 , inventors: Jens Frahm, Axel Haase, Dieter Matthaei, Wolfgang Hänicke, Klaus-Dietmar Merboldt. ‌
3. ^ D. Matthaei, J. Frahm, A. Haase, W. Hänicke: Regional physiological functions depicted by sequences of rapid magnetic resonance images . In: The Lancet . tape 326 , 1985, ISSN  0023-7507 , pp. 893 , doi : 10.1016 / S0140-6736 (85) 90158-8 , PMID 2864605 .
4. ^ J. Frahm, A. Haase, D. Matthaei: Rapid NMR imaging of dynamic processes using the FLASH technique . In: Magnetic Resonance in Medicine . tape 3 , 1986, ISSN  0740-3194 , pp. 321-327 , doi : 10.1002 / mrm.1910030217 , PMID 3713496 .
5. ^ J. Frahm, A. Haase, D. Matthaei: Rapid three-dimensional MR imaging using the FLASH technique . In: Journal of Computer Assisted Tomography . tape 10 , 1986, ISSN  0363-8715 , pp. 363-368 , PMID 3950172 .
6. ↑ Martin Uecker, Shuo Zhang, Dirk Voit, Alexander Karaus, Klaus-Dietmar Merboldt, Jens Frahm: Real-time MRI at a resolution of 20 ms . In: NMR in Biomedicine . tape 23 , no. 8 , 2010, p. 986-994 , doi : 10.1002 / nbm.1585 .
7. ↑ Shuo Zhang, Martin Uecker, Dirk Voit, Klaus-Dietmar Merboldt, Jens Frahm: Real-time cardiovascular magnetic resonance at high temporal resolution: radial FLASH with nonlinear inverse reconstruction . In: Journal of Cardiovascular Magnetic Resonance . tape 12 , 2010, p. 39 , doi : 10.1186 / 1532-429X-12-39 .

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