Real-time MRI
Real-time magnetic resonance imaging (RT-MRI) refers to the continuous monitoring of moving objects in real time.
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.
[4] Additionally, Peter Mansfield develops the echo-planar technique, producing images in seconds and becoming the basis for fast MRIs.
[5] 1983 - Introduction of the k-space by D B Twieg[6] 1987 - First real-time MRI of the heart is developed[7] 1997 - Parallel imaging with an RF coil array is introduced by D K Sodickson[8] 1999 - SENSE image reconstruction is introduced by K P Pruessmann[9] 2002 - GRAPPA image reconstruction is introduced by Mark Griswold[10] 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.
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.
[11] Thus, radial or spiral sampling of the k-space are now the preferred methods for real-time MRI reconstruction.
Because modern GPUs have parallel processing capabilities, they can reconstruct each portion of the image simultaneously.
[16] While early applications were based on echo planar imaging, which found an important application in real-time functional MRI (rt-fMRI),[17] recent progress is based on iterative reconstruction and FLASH MRI.
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.
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.
[11] Steady state free precession involves a repetition time (TR) that is shorter than T2.
Materials with similar T1 and T2, such as fluids and fat, present high T2/T1 contrast and can have signal intensity up to
[23] Due to this strong fluid/tissue contrast, RT-MRI with bSSFP lends itself to cardiac imaging and visualizing blood flow.
During the rest of imaging, the k-space is undersampled to skip every other line, resulting in a one-half field of view.
As a two-point example, pixels on the original aliased images can be "unfolded" through the following equations to give the final scan:
[25] Lines through the center of the k-space are fully sampled, typically alongside the actual image, to give the autocalibration signal (ACS) region.
Then, the filled-in k-space data undergoes the inverse Fourier transform to construct the partial, non-aliased images.
[25] If the k-space data is non-Cartesian, reconstruction is computationally more difficult, since the fast Fourier transform (FFT) requires Cartesian values.
[11] Lastly, within parallel image reconstruction there is another factor to consider, which is the signal to noise ratio (SNR).
[26] Although applications of real-time MRI cover a broad spectrum ranging from non-medical studies of turbulent flow[27] to the noninvasive monitoring of interventional (surgical) procedures, the most important application making use of the new capabilities is cardiovascular imaging.
[1] Previous cardiac MR (CMR) used cine techniques to capture the periodic motion of the heart.
[28] 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.
RT-MRI also removes the need for breath-holding while imaging, leading to a more comfortable experience for the patient as well.
[28] Apart from cardiac MRI other real-time applications deal with functional studies of joint kinetics (e.g., temporomandibular joint,[30] knee and the wrist[31]) or address the coordinated dynamics of the articulators such as lips, tongue, soft palate and vocal folds during speaking (articulatory phonetics)[32] or swallowing.
Researchers at the NYU Grossman School of Medicine[34] developed a RT-MRI glove for imaging movement of the hand.
[35] 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.
Dynamic coil setups for speech and musculoskeletal imaging are key areas for current research.
A nonlinear kernel, or mapping function, can be developed from the ACS to fill in k-space data and generate the final image.
[11] These scanners operate at relatively low magnetic field strengths, such as 0.35 T or 0.55 T. Many RT-MRI acquisition sequences, such as bSSFP, experience significant off-resonance effects.
[38] This allows for longer TRs, which then opens the door for a wider range of k-space sampling methods and sequence designs.
