GRASS v MPGR
What is the difference between single-slice GRASS and multi-planar GRASS (MPGR)?
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Single-slice mode (Simple GRASS or FISP)
When very short TR values are employed, it is usually possible to obtain only a single GRASS/FISP slice at a time. To cover a large area, therefore, a series of individual GRASS/FISP images must be acquired one by one. This technique is known as sequential multi-slice acquisition.
In the sequential multi-slice mode, each slice is acquired independently and has signal characteristics as a function of TR, TE, and α as described for the idealized GRASS/FISP sequence in the prior Q&A. Furthermore, each slice can be considered an "entry slice" with respect to flow effects. Therefore, time-of-flight enhancement effects of blood or CSF are maximal when this mode of acquisition is used.
When very short TR values are employed, it is usually possible to obtain only a single GRASS/FISP slice at a time. To cover a large area, therefore, a series of individual GRASS/FISP images must be acquired one by one. This technique is known as sequential multi-slice acquisition.
In the sequential multi-slice mode, each slice is acquired independently and has signal characteristics as a function of TR, TE, and α as described for the idealized GRASS/FISP sequence in the prior Q&A. Furthermore, each slice can be considered an "entry slice" with respect to flow effects. Therefore, time-of-flight enhancement effects of blood or CSF are maximal when this mode of acquisition is used.
Multi-slice Mode (Multi-section FISP, MPGR)
When longer TR values are employed, it is possible to acquire several slices simultaneously within a single TR interval. This method is known as slice-interleaved or slice-multiplexed acquisition.
In the slice-multiplexed mode, however, a significantly different contrast behavior occurs. Because multiple slices must be selected during each TR interval, a slice-selection gradient must be turned on for each slice imaged. These slice-select gradients spoil transverse coherences and convert the GRASS/FISP sequence to display spoiled-GRE contrast. Also in this mode, because only the end slices are true "entry slices", inflow enhancement effects are less pronounced.
When longer TR values are employed, it is possible to acquire several slices simultaneously within a single TR interval. This method is known as slice-interleaved or slice-multiplexed acquisition.
In the slice-multiplexed mode, however, a significantly different contrast behavior occurs. Because multiple slices must be selected during each TR interval, a slice-selection gradient must be turned on for each slice imaged. These slice-select gradients spoil transverse coherences and convert the GRASS/FISP sequence to display spoiled-GRE contrast. Also in this mode, because only the end slices are true "entry slices", inflow enhancement effects are less pronounced.
2D vs 3D Implementations
GRASS/FISP, as well as most other pulse sequences, can be operated in either 2D or 3D acquisition modes. A number of aspects of image contrast will differ depending on whether 2D or 3D is chosen.
One immediately noticeable effect is the appearance of blood vessels. In the 2D single-slice mode, GRASS/FISP sequences typically demonstrate marked flow-related enhancement of blood and spinal fluid flowing into each slice. This inflow effect, moderately muted in the 2D multi-slice mode, is even further reduced in 3D acquisitions.
Overall image contrast may also vary among different parts of a 3D volume. This results from difficulty in obtaining a uniform RF-flip angle throughout the entire volume. As the flip angle varies, so does image contrast.
One immediately noticeable effect is the appearance of blood vessels. In the 2D single-slice mode, GRASS/FISP sequences typically demonstrate marked flow-related enhancement of blood and spinal fluid flowing into each slice. This inflow effect, moderately muted in the 2D multi-slice mode, is even further reduced in 3D acquisitions.
Overall image contrast may also vary among different parts of a 3D volume. This results from difficulty in obtaining a uniform RF-flip angle throughout the entire volume. As the flip angle varies, so does image contrast.
References
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8. (My older review; still accurate, though some vendors have gone out of business. Gives a good history of the development of GRE sequences).
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8. (My older review; still accurate, though some vendors have gone out of business. Gives a good history of the development of GRE sequences).
Related Questions
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