Table of contents
1. Means of Improving Imaging Speed
The MRI scan time can be expressed as
where Nex is the number of excitations and NpE1 and NpE2 are the number of encoding steps for the two phases . The high-speed imaging that usually operates on 2D tomography can simplify it to
the repetition time TR (The time interval between two adjacent executions of the pulse sequence) must satisfy less than the echo time TE (Time interval between RF pulse and corresponding echo) (TR<TE). The shortening of
TE is limited by data acquisition time , gradient strength and gradient switching time .
Normalized K-space, pulse gradients, and scan trajectories in K-space only suggest ideas for normalization.
2. Echo Plane Imaging (EPI) Sequence
For the EPI sequence , it allows one RF shot to obtain all data in 2D tomographic images (fastest imaging method).
Gs, Gp, and Gr represent layer selection, phase encoding gradient (weak gradient, constant) and frequency encoding gradient (fast switching).
One scan covers half of the K-space (it is feasible to use the half-Fourier transform and increase the pre-phase gradient to realize the full K-space scan), the resolution is the same as that of the full K-space data, and the signal-to-noise ratio is √2 times lower. Due to its fast imaging speed and insensitivity to motion , images are T2* weighted .
BEST sequence :Modify the constant phase encoding gradient in the original EPI to a short pulse (blip), and add a blip pulse when the read gradient crosses the zero point. In this way, the scanning trajectory in the K space is a rectangular straight line, which is convenient for image reconstruction and digital signal data processing.
3. Commonly used or basic EPI sequences
SE-EPI sequence (A Hybrid Sequence Combining Spin Echo and EPI): SE-EPI scanning can cover the whole K space , overcome the off-resonance effect caused by the inconsistency of magnetic susceptibility between tissues and the inhomogeneity of the main magnetic field to a certain extent , and have higher image quality, and the image is T2 weighted .
GE-EPI sequence (Hybrid sequence combining gradient echo and EPI): EPI images and GE images are both T2 * weighted , and GE-EPI is heavy T2 * weighted imaging (good effect on brain function imaging characterized by BOLD contrast). Can be excited with low dump angles (<90°).
IR-EPI sequence : Add a reverse recovery module before GE-EPI or SE-EPI.
existIn the EPI sequence, since the contrast of the image is mainly determined by the TE of the line at the center of K-space , the TE of the center line of K-space used in the EPI sequence instead of the traditional TE is called effective TE .
4. EPI variant sequence
EPI with even echoes omitted : only half of the echoes are used to collect K-space lines , there is no phase inconsistency in adjacent lines of K-space data, and there is no need for interlaced data inversion.Avoids N/2 Nyquist ghosting. It effectively speeds up the retrace of the K-space trajectory before the next echo acquisition.
Circular EPI : By designing the gradient waveform to generate a K-space trajectory limited to a circular trend , it reduces the time for apodization of the four corners of the K-space data.
The modified EPI sequence for measuring T2*-map : the echo sequence generated by one excitation is collected with the same phase encoding step Ky, and one echo corresponds to one image (Then obtain the undistorted image as a reference through SE or GE sequence)。
Echo Volume Imaging EVI (3D EPI) : One RF excitation scan obtains an image of the entire volume , and the spatial resolution of the image is still very low.
Five, involute planar spiral (spiral) scanning sequence
Spiral scan sequence : due to the fast attenuation of the original EPI sequence T2* , the number of echoes that can be generated is limited, soDecreased resolution in phase encoding direction; EPI also requires rapid alternating high-intensity gradient pulses, which is difficult to achieve, so the use of spiral scans instead .
Through spiral scanning, the problem of low resolution in the phase encoding direction can be partially solved , and the difficulty of gradients can be reduced to a certain extent .
The read gradient is periodically zeroed to give the flow a higher order compensation. Oversampling near K=0 leads to particularly high sampling density near the origin of K space.
Insensitive to motion, less artifacts. However, the acquisition efficiency is low, the time is long, and it is easy to cause image blur . In the modified spiral sequence, it can be stipulated that the readout gradient starts from zero and makes the climbing rate large and constant, or the gradient can be added to return the K-space trajectory to the origin. If you want to increase
the Sensitivity to T2*, available using "reverse spiral”, suitable for brain functional imaging based on BOLD contrast.
Six, RARE sequence
Single-shot RARE sequence : After a 90° RF excitation pulse, a long train of 180° refocusing pulses can generate many spin echoes.When each spin echo is encoded with a different phase, one echo corresponds to one Fourier row in K space。
A 90° pulse is used for one excitation, and N 180° pulses are sequentially refocused to generate N spin echoes, and each echo corresponds to a different phase encoding step . After each echo is measured, the phase encoding gradient must be wrapped around to remove the phase shift.
Boxed parts indicate repeated execution.
Single-slice RARE suffers from slice contour distortion due to the large number of 180° pulses. The main applications of RARE are cerebrospinal fluid (CSF) imaging such as myelography, and imaging of other fluids such as urethrography .
Images are T2 contrast-enhanced weighted. It is used to use a large number of 180°RF pulses, and the RF power deposition is very large, and multi-shot RARE (fast SE sequence) is often used clinically .
Zero encoding step advance (reduces T2-weighted contrast), low dump angle instead of 180° pulse (reduces RF power deposition)…
Seven, GRASE sequence
GRASE sequence (Hybrid sequence combining gradient echo and fast spin echo): That is, fast gradient spin echo (TGSE). Compared with EPI, due to the use of refocusing pulses, the effects of magnetic field inhomogeneity and magnetic susceptibility are compensated,artifact reduction; also reduces the phase of polarization spin accumulation, resulting inLess geometric distortion and less signal loss due to intra-voxel dephasing。
The speed has been reduced. GRASE data are sensitive to eddy currents, mismatches between gradients and receive chain group delays, and anti-aliasing filter response asymmetries.
GRASE scanning can be single-shot or multi-shot , similar to RARE or EPI, because the number of echoes and lines collected is usually not a power of 2 , soK-space should be filled with zeros during image reconstruction.
Destruction gradients are used before and after each refocused 180° pulse, with the purpose of dephasing the FID due to unsatisfactory pulses to increase the destruction effect.
8. STEAM sequence
STEAM sequence : can also run quickly, called fast STEAM sequence. RF refocused pulses are acquired instead of gradient echoes, so the STE signal produces SE-like images,Insensitive to magnetic field inhomogeneity, magnetic susceptibility, flow, and chemical shift.
High-speed STEAM is T1-weighted for cardiac imaging .
The first 3 RF pulses and diffusion-sensitive gradients are used to prepare the diffusion-weighted magnetization, and the subsequent RF pulses and fast gradient echoes are used for spatial encoding to produce low-resolution diffusion imaging of the human brain with a single shot.
