FLASH MRI

FLASH MRI (Fast Low Angle Shot Magnetic Resonance Imaging) is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm and Axel Haase at the Max-Planck-Institut für biophysikalische Chemie in Göttingen, Germany. The technique is as simple as revolutionary in shortening MRI measuring times by up to two orders of magnitude.

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The introduction of FLASH MRI sequences in diagnostic imaging for the first time allowed for a drastic shortening of the measuring times without a substantial loss in image quality. In addition, the measuring principle led to a broad range of completely new imaging modalities. For example,

• cross-sectional images with acquisition times of a few seconds enable MRI studies of the thorax and abdomen within a single breathhold,
• dynamic acquisitions synchronized to the electrocardiogram generate movies of the beating heart,
• sequential acquisitions monitor the differential uptake of contrast media into body tissues,
• three-dimensional acquisitions visualize complex anatomic structures (brain, joints) at unprecedented high spatial resolution in all three dimensions and along arbitrary view directions, and
• magnetic resonance angiography (MRA) yields three-dimensional representations of the vasculature.

In general, FLASH denoted a breakthrough in clinical MRI that stimulated further technical as well as scientific developments up to date.

Physical Basis

The physical basis of MRI is the spatial encoding of the nuclear magnetic resonance (NMR) signal obtainable from water protons (= hydrogen ions) in biologic tissue. In terms of MRI, signals with different spatial encodings that are required for the reconstruction of a full image need to be acquired by generating multiple signals - usually in a repetitive way using multiple radio-frequency excitations.

The generic FLASH technique emerges as a gradient echo sequence which combines a low-flip angle radio-frequency excitation of the NMR signal (recorded as a spatially encoded gradient echo) with a rapid repetition of the basic sequence. The repetition time is usually much shorter than the typical T1 relaxation time of the protons in biologic tissue. Only the combination of (i) a low-flip angle excitation which leaves unused longitudinal magnetization for an immediate next excitation with (ii) the acquisition of a gradient echo which does not need a further radio-frequency pulse that would affect the residual longitudinal magnetization, allows for the rapid repetition of the basic sequence interval and the resulting speed of the entire image acquisition. In fact, the FLASH sequence eliminated all waiting periods previously included to accommodate effects from T1 saturation. FLASH reduced the typical sequence interval to what is minimally required for imaging: a slice-selective radio-frequency pulse and gradient, a phase-encoding gradient, and a (reversed) frequency-encoding gradient generating the echo for data acquisition. Typical repetition times are on the order of 4-10 milliseconds with image acquisition times of 64-256 repetitions thereof, that is 250 milliseconds to 2.5 seconds for a two-dimensional image.

Literature

• J Frahm, A Haase, W Hänicke, KD Merboldt, D Matthaei. Hochfrequenz-Impuls und Gradienten-Impuls-Verfahren zur Aufnahme von schnellen NMR-Tomogrammen unter Benutzung von Gradientenechos. German Patent Application P 35 04 734.8, February 12, 1985
• A Haase, J Frahm, D Matthaei, W Hänicke, KD Merboldt. FLASH imaging: rapid NMR imaging using low flip angle pulses. J Magn Res 1986; 67:258-266
• J Frahm, A Haase, D Matthaei. Rapid three-dimensional MR imaging using the FLASH technique. J Comput Assist Tomogr 1986; 10:363-368
• J Frahm, A Haase, D Matthaei. Rapid NMR imaging of dynamic processes using the FLASH technique. Magn Reson Med 1986; 3:321-327