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, it is important to note that 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.
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 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. The short TR also makes bSSFP ideal for RT-MRI. The equation for peak MR signal in bSSFP is given as: M_{ss,xy}\vert_{\alpha=\alpha_{opt}} = \frac{1}{2}M_0\sqrt{\frac{1-E_1^2}{1-E_2^2}} \approx \frac{1}{2}M_0\sqrt{\frac{T_2}{T_1}} Where M_0 is the initial magnetization, E_1 = e^{-T_1/T_R} and E_2 = e^{-T_2/T_R}. Thus, the MR signal is proportional to T2/T1. Materials with similar T1 and T2, such as fluids and fat, present high T2/T1 contrast and can have signal intensity up to 0.5M_0. The bSSFP signal is also greater than the FLASH signal by a factor of \frac{1}{\sqrt{1-E_2^2}}. Coil sensitivities must first be acquired either before the actual imaging or during the imaging process. 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: P_1 = A\cdot S_{1A} + B\cdot S_{1B} P_2 = A\cdot S_{2A} + B\cdot S_{2B} for two points, A and B, in the final image. P_1 and P_2 denote the image signal for the aliased image. S_{1A} and S_{1B} are the sensitivity values for coil 1 at points A and B, respectively, and S_{2A}and S_{2B}are the sensitivity values for coil 2 at points A and B, respectively. Lines through the center of the k-space are fully sampled, typically alongside the actual image, to give the autocalibration signal (ACS) region. Weighing factors are calculated using the ACS, and these factors reflect the coil-specific distortions that each coil applies on the full field-of-view
frequency domain. Then, the filled-in k-space data undergoes the
inverse Fourier transform to construct the partial, non-aliased images. These images are then simply combined directly in the spatial domain. Where R is the acceleration factor and g is the spatially dependent geometry factor (proportional to the number of coils used or the interactions between coils). Therefore, the more coils used, the faster the imaging process and the more inter-coil interactions; hence, the lower the SNR. ==Applications==