Fast low angle shot magnetic resonance imaging

Last updated

Fast low angle shot magnetic resonance imaging (FLASH MRI) is a particular sequence of magnetic resonance imaging. It is a gradient echo sequence which combines a low-flip angle radio-frequency excitation of the nuclear magnetic resonance signal (recorded as a spatially encoded gradient echo) with a short repetition time. It is the generic form of steady-state free precession imaging.

Contents

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.

Applications include:

Physical basis

The physical basis of MRI is the spatial encoding of the nuclear magnetic resonance (NMR) signal obtainable from water protons (i.e. hydrogen 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. [4] [5] 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. [4] [6] 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. [7] This method allows for a visualization of the beating heart in real time – without synchronization to the electrocardiogram and during free breathing. [8]

History

FLASH MRI was invented in 1985 by Jens Frahm, Axel Haase, W Hänicke, KD Merboldt, and D Matthaei (German Patent Application P 35 04 734.8, 12 February 1985) at the Max-Planck-Institut für biophysikalische Chemie Archived 30 April 2008 at the Wayback Machine in Göttingen, Germany. The technique is revolutionary in shortening MRI measuring times by up to two orders of magnitude.

FLASH was very rapidly adopted commercially. RARE was slower, and echo-planar imaging (EPI) – for technical reasons – took even more time. Echo-planar imaging had been proposed by Mansfield's group in 1977, and the first crude images were shown by Mansfield and Ian Pykett in the same year. Roger Ordidge presented the first movie in 1981. Its breakthrough came with the invention of shielded gradients. [9]

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.

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 milliseconds (1/50th of a second). [7] [8] 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.

Related Research Articles

<span class="mw-page-title-main">Magnetic resonance imaging</span> Medical imaging technique

Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves to generate images of the organs in the body. MRI does not involve X-rays or the use of ionizing radiation, which distinguishes it from computed tomography (CT) and positron emission tomography (PET) scans. MRI is a medical application of nuclear magnetic resonance (NMR) which can also be used for imaging in other NMR applications, such as NMR spectroscopy.

<span class="mw-page-title-main">Magnetic resonance angiography</span> Group of techniques based on magnetic resonance imaging (MRI) to image blood vessels.

Magnetic resonance angiography (MRA) is a group of techniques based on magnetic resonance imaging (MRI) to image blood vessels. Magnetic resonance angiography is used to generate images of arteries in order to evaluate them for stenosis, occlusions, aneurysms or other abnormalities. MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs.

In MRI and NMR spectroscopy, an observable nuclear spin polarization (magnetization) is created by a homogeneous magnetic field. This field makes the magnetic dipole moments of the sample precess at the resonance (Larmor) frequency of the nuclei. At thermal equilibrium, nuclear spins precess randomly about the direction of the applied field. They become abruptly phase coherent when they are hit by radiofrequency (RF) pulses at the resonant frequency, created orthogonal to the field. The RF pulses cause the population of spin-states to be perturbed from their thermal equilibrium value. The generated transverse magnetization can then induce a signal in an RF coil that can be detected and amplified by an RF receiver. The return of the longitudinal component of the magnetization to its equilibrium value is termed spin-latticerelaxation while the loss of phase-coherence of the spins is termed spin-spin relaxation, which is manifest as an observed free induction decay (FID).

<span class="mw-page-title-main">Steady-state free precession imaging</span>

Steady-state free precession (SSFP) imaging is a magnetic resonance imaging (MRI) sequence which uses steady states of magnetizations. In general, SSFP MRI sequences are based on a gradient echo MRI sequence with a short repetition time which in its generic form has been described as the FLASH MRI technique. While spoiled gradient-echo sequences refer to a steady state of the longitudinal magnetization only, SSFP gradient-echo sequences include transverse coherences (magnetizations) from overlapping multi-order spin echoes and stimulated echoes. This is usually accomplished by refocusing the phase-encoding gradient in each repetition interval in order to keep the phase integral constant. Fully balanced SSFP MRI sequences achieve a phase of zero by refocusing all imaging gradients.

<span class="mw-page-title-main">Spin echo</span> Response of spin to electromagnetic radiation

In magnetic resonance, a spin echo or Hahn echo is the refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation. Modern nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) make use of this effect.

Jens Frahm is a German biophysicist and physicochemist. He is Research Group Leader of the Biomedical NMR group at the Max Planck Institute (MPI) for Multidisciplinary Sciences in Göttingen, Germany.

<span class="mw-page-title-main">Spin–spin relaxation</span>

In physics, the spin–spin relaxation is the mechanism by which Mxy, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). It is characterized by the spin–spin relaxation time, known as T2, a time constant characterizing the signal decay. It is named in contrast to T1, the spin–lattice relaxation time. It is the time it takes for the magnetic resonance signal to irreversibly decay to 37% (1/e) of its initial value after its generation by tipping the longitudinal magnetization towards the magnetic transverse plane. Hence the relation

During nuclear magnetic resonance observations, spin–lattice relaxation is the mechanism by which the longitudinal component of the total nuclear magnetic moment vector (parallel to the constant magnetic field) exponentially relaxes from a higher energy, non-equilibrium state to thermodynamic equilibrium with its surroundings (the "lattice"). It is characterized by the spin–lattice relaxation time, a time constant known as T1.

<i>k</i>-space in magnetic resonance imaging

In magnetic resonance imaging (MRI), the k-space or reciprocal space is obtained as the 2D or 3D Fourier transform of the image measured. It was introduced in 1979 by Likes and in 1983 by Ljunggren and Twieg.

In vivo magnetic resonance spectroscopy (MRS) is a specialized technique associated with magnetic resonance imaging (MRI).

Dieter Matthaei is a German radiotherapist and internist.

<span class="mw-page-title-main">Pulse sequence</span>

In Fourier transform NMR spectroscopy and imaging, a pulse sequence describes a series of radio frequency pulses applied to the sample, such that the free induction decay is related to the characteristic frequencies of the desired signals. After applying a Fourier transform, the signal can be represented in the frequency domain as the NMR spectrum. In magnetic resonance imaging, additional gradient pulses are applied by switching magnetic fields that exhibit a space-dependent gradient which can be used to reconstruct spatially resolved images after applying Fourier transforms.

<span class="mw-page-title-main">Physics of magnetic resonance imaging</span> Overview article

Magnetic resonance imaging (MRI) is a medical imaging technique mostly used in radiology and nuclear medicine in order to investigate the anatomy and physiology of the body, and to detect pathologies including tumors, inflammation, neurological conditions such as stroke, disorders of muscles and joints, and abnormalities in the heart and blood vessels among others. Contrast agents may be injected intravenously or into a joint to enhance the image and facilitate diagnosis. Unlike CT and X-ray, MRI uses no ionizing radiation and is, therefore, a safe procedure suitable for diagnosis in children and repeated runs. Patients with specific non-ferromagnetic metal implants, cochlear implants, and cardiac pacemakers nowadays may also have an MRI in spite of effects of the strong magnetic fields. This does not apply on older devices, and details for medical professionals are provided by the device's manufacturer.

<span class="mw-page-title-main">Real-time MRI</span> Type of MRI

Real-time magnetic resonance imaging (RT-MRI) refers to the continuous monitoring ("filming") of moving objects in real time. Because MRI is based on time-consuming scanning of k-space, real-time MRI was possible only with low image quality or low temporal resolution. Using an iterative reconstruction algorithm these limitations have recently been removed: a new method for real-time MRI achieves a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. Real-time MRI promises to add important information about diseases of the joints and the heart. In many cases MRI examinations may become easier and more comfortable for patients.

<span class="mw-page-title-main">Perfusion MRI</span>

Perfusion MRI or perfusion-weighted imaging (PWI) is perfusion scanning by the use of a particular MRI sequence. The acquired data are then post-processed to obtain perfusion maps with different parameters, such as BV, BF, MTT and TTP.

Synthetic MRI is a simulation method in Magnetic Resonance Imaging (MRI), for generating contrast weighted images based on measurement of tissue properties. The synthetic (simulated) images are generated after an MR study, from parametric maps of tissue properties. It is thereby possible to generate several contrast weightings from the same acquisition. This is different from conventional MRI, where the signal acquired from the tissue is used to generate an image directly, often generating only one contrast weighting per acquisition. The synthetic images are similar in appearance to those normally acquired with an MRI scanner.

<span class="mw-page-title-main">MRI pulse sequence</span>

An MRI pulse sequence in magnetic resonance imaging (MRI) is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance.

Gradient echo is a magnetic resonance imaging (MRI) sequence that has wide variety of applications, from magnetic resonance angiography to perfusion MRI and diffusion MRI. Rapid imaging acquisition allows it to be applied to 2D and 3D MRI imaging. Gradient echo uses magnetic gradients to generate a signal, instead of using 180 degrees radiofrequency pulse like spin echo; thus leading to faster image acquisition time.

An MRI artifact is a visual artifact in magnetic resonance imaging (MRI). It is a feature appearing in an image that is not present in the original object. Many different artifacts can occur during MRI, some affecting the diagnostic quality, while others may be confused with pathology. Artifacts can be classified as patient-related, signal processing-dependent and hardware (machine)-related.

Magnetic resonance fingerprinting (MRF) is methodology in quantitative magnetic resonance imaging (MRI) characterized by a pseudo-randomized acquisition strategy. It involves creating unique signal patterns or 'fingerprints' for different materials or tissues after which a pattern recognition algorithm matches these fingerprints with a predefined dictionary of expected signal patterns. This process translates the data into quantitative maps, revealing information about the magnetic properties being investigated.

References

  1. ↑ Matthaei, Dieter; Frahm, Jens; Haase, Axel; Hanicke, Wolfgang (1985). "Regional physiological functions depicted by sequences of rapid magnetic resonance images". The Lancet . 326 (8460): 893. doi:10.1016/S0140-6736(85)90158-8. PMID   2864605. S2CID   12326347.
  2. ↑ Frahm, Jens; Haase, Axel; Matthaei, Dieter (1986). "Rapid NMR imaging of dynamic processes using the FLASH technique". Magnetic Resonance in Medicine. 3 (2): 321–327. doi:10.1002/mrm.1910030217. PMID   3713496. S2CID   31028542.
  3. ↑ Frahm, Jens; Haase, Axel; Matthaei, Dieter (1986). "Rapid three-dimensional MR imaging using the FLASH technique". Journal of Computer Assisted Tomography. 10 (2): 363–368. doi:10.1097/00004728-198603000-00046. PMID   3950172.
  4. 1 2 04 734.8 DEpatent 35 04 734.8, 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",published 1986-08-14,issued 1998-12-10
  5. ↑ Haase, A; Frahm, J; Matthaei, D; Hanicke, W; Merboldt, K.-D (1986). "FLASH imaging: rapid NMR imaging using low flip angle pulses". Journal of Magnetic Resonance. 67 (2): 258–266. Bibcode:1986JMagR..67..258H. doi:10.1016/0022-2364(86)90433-6.
  6. ↑ Zhang, Shuo; Block, Kai Tobias; Frahm, Jens (2010). "Magnetic resonance imaging in real time: Advances using radial FLASH". Journal of Magnetic Resonance Imaging. 31 (1): 101–109. doi:10.1002/jmri.21987. hdl: 11858/00-001M-0000-0012-D667-0 . PMID   19938046. S2CID   17419027.
  7. 1 2 Uecker, Martin; Zhang, Shuo; Voit, Dirk; Karaus, Alexander; Merboldt, Klaus‐Dietmar; Frahm, Jens (2010). "Real-time MRI at a resolution of 20 ms". NMR in Biomedicine . 23 (8): 986–994. doi:10.1002/nbm.1585. hdl: 11858/00-001M-0000-0012-D4F9-7 . PMID   20799371. S2CID   8268489.
  8. 1 2 Zhang, Shuo; Uecker, Martin; Voit, Dirk; Merboldt, Klaus-Dietmar; Frahm, Jens (2010). "Real-time cardiovascular magnetic resonance at high temporal resolution: radial FLASH with nonlinear inverse reconstruction". Journal of Cardiovascular Magnetic Resonance. 12 (1): 39. doi: 10.1186/1532-429X-12-39 . PMC   2911425 . PMID   20615228.
  9. ↑ A SHORT HISTORY OF MAGNETIC RESONANCE IMAGING FROM A EUROPEAN POINT OF VIEW
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.