FLASH MRI
Encyclopedia
FLASH MRI is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm
, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei
(German Patent Application P 35 04 734.8, February 12, 1985) 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.
Different manufacturers of MRI equipment use different names for this experiment. Siemens
uses the name FLASH, General Electric
used the name SPGR (Spoiled Gradient Echo), and Philips
uses the name CE-FFE-T1 (Contrast-Enhanced Fast Field Echo) or T1-FFE. Depending on the desired contrast, the generic FLASH technique provides spoiled versions that destroy transverse coherences and yield T1 contrast as well as refocused versions (constant phase per repetition) and fully balanced versions (zero phase per repetition) that incorporate transverse coherences into the steady-state signal and offer T1/T2 contrast.
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,
In 2010, an extended FLASH method with highly undersampled radial data encoding and iterative image reconstruction achieved real-time MRI
with a temporal resolution of 20 millisecond
s (1/50th of a second). Taken together, this latest development corresponds to an acceleration by a factor of 10,000 compared to the MRI situation before 1985. In general, FLASH denoted a breakthrough in clinical MRI that stimulated further technical as well as scientific developments up to date.
nuclei) 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.
For radial data sampling, the phase- and frequency-encoding gradients are replaced by two simultaneously applied frequency-encoding gradients that rotate the Fourier lines in data space. In either case, repetition times are as short as 2 to 10 milliseconds, so that the use of 64 to 256 repetitions results in image acquisition times of about 0.1 to 2.5 seconds for a two-dimensional image. Most recently, highly undersampled radial FLASH MRI acquisitions have been combined with an iterative image reconstruction by regularized nonlinear inversion to achieve real-time MRI
at a temporal resolution of 20 to 30 milliseconds for images with a spatial resolution of 1.5 to 2.0 millimeters. This method allows for a visualization of the beating heart in real time - without synchronization to the electrocardiogram and during free breathing.
Jens Frahm
Jens Frahm is Director of the at the Max Planck Institute for Biophysical Chemistry in Göttingen, Germany.-Life:...
, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei
Dieter Matthaei
Dieter Matthaei is a German radiotherapist and internist.Matthaei studied physics and medicine at the universities of Berlin and Göttingen. In 1977, he received his doctorate from the Georg-August University of Göttingen...
(German Patent Application P 35 04 734.8, February 12, 1985) at the Max-Planck-Institut für biophysikalische Chemie in Göttingen
Göttingen
Göttingen is a university town in Lower Saxony, Germany. It is the capital of the district of Göttingen. The Leine river runs through the town. In 2006 the population was 129,686.-General information:...
, Germany. The technique is as simple as revolutionary in shortening MRI measuring times by up to two orders of magnitude.
Different manufacturers of MRI equipment use different names for this experiment. Siemens
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....
uses the name FLASH, General Electric
GEC Medical
GEC Medical was a unit of the General Electric Company that was headquartered in what was known as East Lane Industrial Estate in North Wembley, behind the Hirst Research Centre which fronted East Lane....
used the name SPGR (Spoiled Gradient Echo), and Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
uses the name CE-FFE-T1 (Contrast-Enhanced Fast Field Echo) or T1-FFE. Depending on the desired contrast, the generic FLASH technique provides spoiled versions that destroy transverse coherences and yield T1 contrast as well as refocused versions (constant phase per repetition) and fully balanced versions (zero phase per repetition) that incorporate transverse coherences into the steady-state signal and offer T1/T2 contrast.
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 thoraxThoraxThe thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
and abdomen within a single breathhold, - dynamic acquisitions synchronized to the electrocardiogramElectrocardiogramElectrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
generate movies of the beating heartHeartThe heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, - sequential acquisitions monitor physiological processes such as the differential uptake of contrast media into body tissues,
- three-dimensionalThree-dimensional spaceThree-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
acquisitions visualize complex anatomic structures (brain, joints) at unprecedented high spatial resolution in all three dimensions and along arbitrary view directions, and - magnetic resonance angiographyMagnetic Resonance AngiographyMagnetic resonance angiography is a group of techniques based on Magnetic Resonance Imaging to image blood vessels. Magnetic resonance angiography is used to generate images of the arteries in order to evaluate them for stenosis , occlusion or aneurysms...
(MRA) yields three-dimensional representations of the vasculature.
In 2010, an extended FLASH method with highly undersampled radial data encoding and iterative image reconstruction achieved real-time MRI
Real-time MRI
Real-time magnetic resonance imaging refers to the continuous monitoring of moving objects in real time. Because MRIis based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution...
with a temporal resolution of 20 millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s (1/50th of a second). Taken together, this latest development corresponds to an acceleration by a factor of 10,000 compared to the MRI situation before 1985. 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 (i.e. hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
nuclei) 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
Spin-lattice relaxation time
Spin–lattice relaxation is the mechanism by which the z component of the magnetization vector comes into thermodynamic equilibrium with its surroundings in nuclear magnetic resonance and magnetic resonance imaging. It is characterized by the spin–lattice relaxation time, a time constant known as 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
Spin-lattice relaxation time
Spin–lattice relaxation is the mechanism by which the z component of the magnetization vector comes into thermodynamic equilibrium with its surroundings in nuclear magnetic resonance and magnetic resonance imaging. It is characterized by the spin–lattice relaxation time, a time constant known as 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.
For radial data sampling, the phase- and frequency-encoding gradients are replaced by two simultaneously applied frequency-encoding gradients that rotate the Fourier lines in data space. In either case, repetition times are as short as 2 to 10 milliseconds, so that the use of 64 to 256 repetitions results in image acquisition times of about 0.1 to 2.5 seconds for a two-dimensional image. Most recently, highly undersampled radial FLASH MRI acquisitions have been combined with an iterative image reconstruction by regularized nonlinear inversion to achieve real-time MRI
Real-time MRI
Real-time magnetic resonance imaging refers to the continuous monitoring of moving objects in real time. Because MRIis based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution...
at a temporal resolution of 20 to 30 milliseconds for images with a spatial resolution of 1.5 to 2.0 millimeters. This method allows for a visualization of the beating heart in real time - without synchronization to the electrocardiogram and during free breathing.
External links
- Biomedizinische NMR Forschungs GmbH offers further detailed information about FLASH MRI and related applications (neurobiology, cardiovascular imaging)
- Press Release of the Max Planck SocietyMax Planck SocietyThe Max Planck Society for the Advancement of Science is a formally independent non-governmental and non-profit association of German research institutes publicly funded by the federal and the 16 state governments of Germany....
- http://www.mtbeurope.info/news/2010/1009005.htm