Real-time magnetic resonance imaging (RT-MRI) refers to the continuous monitoring of moving objects in real time. Traditionally, real-time MRI was possible only with low image quality or low temporal resolution. An iterative reconstruction algorithm removed limitations. Radial FLASH MRI (real-time) yields 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 adds information about diseases of the joints and the heart. In many cases MRI examinations become easier and more comfortable for patients, especially for the patients who cannot calm their breathing or who have arrhythmia. Balanced steady-state free precession (bSSFP) imaging gives better image contrast between the blood pool and myocardium than FLASH MRI, at the cost of severe banding artifact when B0 inhomogeneity is strong.
History 1977/1978 - Raymond Damadian built the first MRI scanner and achieved the first MRI scan of a healthy human body (1977) with the intent of diagnosing cancer. Additionally, Peter Mansfield develops the echo-planar technique, producing images in seconds and becoming the basis for fast MRIs. 1983 - Introduction of the k-space by D B Twieg 1987 - First real-time MRI of the heart is developed 1997 - Parallel imaging with an RF coil array is introduced by D K Sodickson 1999 - SENSE image reconstruction is introduced by K P Pruessmann 2002 - GRAPPA image reconstruction is introduced by Mark Griswold
Physical basis
Overview
In general, real time MRI relies on gradient echo sequences, efficient k-space sampling, and fast reconstruction methods to speed up the image acquisition process. Gradient echo sequences present shorter echo times since only one RF pulse is required for each sequence. Modern fast-switching gradient coils also require increasing the slew rate, allowing for faster changes in gradient echo sequences and decreasing the repetition time.
k-space sampling
Efficient k-space sampling also decreases data collection time. Rectilinear scanning has become the standard k-space sampling method for MRI. However, the process takes a relatively long time as it samples the entire k-space equally. Because of this delay, other sampling methods are used to capture real-time motion. Single shot echo planar imaging is one extremely fast sampling method in which all of the data for the MR image is collected from one RF pulse. However, the EPI method is still a Cartesian sampling method, like the rectilinear scan, equally sampling the entire k-space. Spiral sampling, like EPI, only requires a single RF pulse to sample the entire k-space. Radial and spiral sampling are also used as methods to efficiently sample the k-space, with spiral also only requiring a single RF pulse to sample the k-space. Both radial and spiral sampling are more efficient than the Cartesian methods because they oversample low frequencies, which allows for general motion capture and better real-time image reconstruction. Thus, radial or spiral sampling of the k-space are now the preferred methods for real-time MRI reconstruction.
Parallel imaging
Parallel imaging involves the addition of multiple coils surrounding the target with each coil acquiring a fraction of the total image. Because modern GPUs have parallel processing capabilities, they can reconstruct each portion of the image simultaneously. Therefore, the more coils used, the faster the acquisition of the MR images.
Gradient-echo sequences
FLASH MRI While early applications were based on echo planar imaging, which found an important application in real-time functional MRI (rt-fMRI), recent progress is based on iterative reconstruction and FLASH MRI. The real-time imaging method proposed by Uecker and colleagues combines 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. Because of the very short echo times (e.g., 1 to 2 milliseconds), 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 T2/T1 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.
Balanced steady state free precession Another GRE sequence commonly used in RT-MRI is balanced steady state free precession (bSSFP), as mentioned above with balanced gradients. Steady state free precession involves a repetition time (TR) that is shorter than T2. This prevents the magnetic signal from decaying completely before the next RF pulse is applied, which then establishes a steady state signal over time. The short TR also makes bSSFP ideal for RT-MRI. The equation for peak MR signal in bSSFP is given as:
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