Real-time MRI
Encyclopedia
Real-time magnetic resonance imaging
(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 joint
s and the heart
. In many cases MRI examinations may become easier and
more comfortable for patients.
radial FLASH MRI
, which offers
rapid and continuous data acquisition, motion robustness, and tolerance to undersampling,
with an iterative image reconstruction
method based on the formulation of image reconstruction as a nonlinear inverse problem
.
By integrating the data from multiple receive coils (i.e. parallel MRI) and exploiting the redundancy
in the time series of images with the use of regularization
and filtering
, this approach enhances the possible degree of data undersampling by one order of magnitude, so that high-quality images may be obtained out of as little as 5 to 10% of the data required for a normal image reconstruction.
Although applications of real-time MRI cover a broad spectrum ranging from non-medical studies of turbulent flow to the noninvasive monitoring of interventional (surgical) procedures, the most important application making use of the new capabilities is cardiovascular imaging.
With the new method it is possible to obtain movies of the beating heart in real time with up to 50 frames per second during free breathing and
without the need for a synchronization to the electrocardiogram
.
Because of the very short echo times (e.g., 1 to 2 millisecond
s), the method does not suffer from off-resonance effects, so that the images neither exhibit susceptibility
artifacts nor rely on fat suppression. While spoiled FLASH sequences offer spin density or T1 contrast, versions with refocused or fully balanced gradients provide access to T1/T2 contrast. The choice of the gradient-echo time (e.g., in-phase vs opposed-phase conditions) further alters the representation of water and fat signals in the images and will allow for separate water/fat movies.
Apart from cardiac MRI other real-time applications deal with functional studies of joint
kinetics (e.g., temporomandibular joint
, knee
) or address the coordinated dynamics of the articulators such as lips, tongue, soft palate and vocal folds during speaking
(articulatory phonetics
) or swallowing
. Applications in interventional MRI, which refers to the monitoring of minimally invasive surgical procedures, are possible by interactively changing parameters such as image position and orientation.
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) refers to the continuous monitoring (“filming”) of moving objects in real time. Because MRI
is based on time-consuming scanning of k-space
K-space (MRI)
k-space is a formalism widely used in magnetic resonance imaging independently introduced in 1983 by Ljunggren and Twieg.In MRI physics, k-space is the 2D or 3D Fourier transform of the MR image measured. Its complex values are sampled during an MR measurement, in a premeditated scheme controlled...
, real-time MRI was possible only with low image quality or low temporal resolution. Using
an iterative reconstruction
Iterative reconstruction
Iterative reconstruction refers to iterative algorithms used to reconstruct 2D and 3D images in certain imaging techniques.For example, in computed tomography an image must be reconstructed from projections of an object...
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 joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s and the heart
Heart
The 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...
. In many cases MRI examinations may become easier and
more comfortable for patients.
Physical Basis
The real-time imaging method proposed by Uecker and colleagues combinesradial FLASH MRI
FLASH MRI
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 at the in Göttingen, Germany...
, which offers
rapid and continuous data acquisition, motion robustness, and tolerance to undersampling,
with an iterative image reconstruction
Iterative reconstruction
Iterative reconstruction refers to iterative algorithms used to reconstruct 2D and 3D images in certain imaging techniques.For example, in computed tomography an image must be reconstructed from projections of an object...
method based on the formulation of image reconstruction as a nonlinear inverse problem
Inverse problem
An inverse problem is a general framework that is used to convert observed measurements into information about a physical object or system that we are interested in...
.
By integrating the data from multiple receive coils (i.e. parallel MRI) and exploiting the redundancy
Redundancy
Redundancy may refer to:* Redundancy * Redundancy * Redundancy * Redundancy * Redundancy * Data redundancy* Gene redundancy* Logic redundancy...
in the time series of images with the use of regularization
Regularization
Regularization may refer to:* Regularization ** Regularization * Regularization * Regularization * Regularization...
and filtering
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
, this approach enhances the possible degree of data undersampling by one order of magnitude, so that high-quality images may be obtained out of as little as 5 to 10% of the data required for a normal image reconstruction.
Although applications of real-time MRI cover a broad spectrum ranging from non-medical studies of turbulent flow to the noninvasive monitoring of interventional (surgical) procedures, the most important application making use of the new capabilities is cardiovascular imaging.
With the new method it is possible to obtain movies of the beating heart in real time with up to 50 frames per second during free breathing and
without the need for a synchronization to the electrocardiogram
Electrocardiogram
Electrocardiography 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...
.
Because of the very short echo times (e.g., 1 to 2 millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
s), the method does not suffer from off-resonance effects, so that the images neither exhibit susceptibility
Susceptibility
*In physics, the susceptibility of a material or substance describes its response to an applied field. There are many kinds of susceptibilities, for example:These two susceptibilities are particular examples of a linear response function;...
artifacts nor rely on fat suppression. While spoiled FLASH sequences offer spin density or T1 contrast, versions with refocused or fully balanced gradients provide access to T1/T2 contrast. The choice of the gradient-echo time (e.g., in-phase vs opposed-phase conditions) further alters the representation of water and fat signals in the images and will allow for separate water/fat movies.
Apart from cardiac MRI other real-time applications deal with functional studies of joint
Joint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
kinetics (e.g., temporomandibular joint
Temporomandibular joint
The temporomandibular joint is the joint of the jaw and is frequently referred to as TMJ. There are two TMJs, one on either side, working in unison. The name is derived from the two bones which form the joint: the upper temporal bone which is part of the cranium , and the lower jaw bone called the...
, knee
Knee
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...
) or address the coordinated dynamics of the articulators such as lips, tongue, soft palate and vocal folds during speaking
SPEAKING
In sociolinguistics, SPEAKING or the SPEAKING model, is a model socio-linguistic study developed by Dell Hymes...
(articulatory phonetics
Articulatory phonetics
The field of articulatory phonetics is a subfield of phonetics. In studying articulation, phoneticians explain how humans produce speech sounds via the interaction of different physiological structures....
) or swallowing
Swallowing
Swallowing, known scientifically as deglutition, is the process in the human or animal body that makes something pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. If this fails and the object goes through the trachea, then choking or pulmonary aspiration...
. Applications in interventional MRI, which refers to the monitoring of minimally invasive surgical procedures, are possible by interactively changing parameters such as image position and orientation.
External links
- Biomedizinische NMR Forschungs GmbH offers further detailed information about FLASH MRI and related applications (neurobiology, cardiovascular imaging)
- Related information 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....
- Live video of movements during speech production (youtube)