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what are the drawbacks of 2D imaging?
- slice thickness is limited by SNR
- limited gradient amplitude and RF transmitter bandwidth
- prone to crosstalk artifacts
what can you do to minimize crosstalk artifacts?
put gaps between slices
how does 3D imaging gets its images?
instead of selecting slices individually with RF energy, as in 2D sequences, 3D sequences use RF energy to disturb the protons in an entire volume of anatomy, and then they apply a second phase encoding process to divide the volume of anatomical data into individual slices. In volume acquisition, frequency encoding is performed along one axis of the imaging volume while phase encoding is performed along the other two. One gradient is responsible for frequency encoding and the other two gradients are responsible for phase encoding
volume images have low or high SNR?
- high SNR
- so high that very thin slices may be collected compared to those acquired with 2D techniques
are there crosstalk artifacts in 3D imaging?
no because the entire 3D volume is selected by RF energy, there is no crosstalk between adjacent images
whats a disadvantage associated with volume imaging that does not occur with 2D imaging?
the wrap-around phenomenon that occurs along the slice direction. Since the slices are divided with phase encoding, some slices from one end of the stack are sometimes mismapped to the other side of the stack. Many MR system manufacturers eliminate the slice direction wrap artifact by encoding a few extra slices along the slice direction and then discarding the extra end slices where the wrap would have occured
how do you calculate scan time for 3D imaging?
TR x NSA x Lines in matrix x # slices
what type of sequences can you combine with volume or 3D imaging?
- fast spin echo
- gradient echo
- fast gradient echo
- echo planar techniques
- rarely with spin echo because of time restrictions
the slice thickness is not directly chosen by the tech when running a volume pulse sequence. how do you calculate effective slice thickness?
effective slice thickness = total volume thickness / number of slices
what is a voxel?
the smallest volume element in a 3D data set
what is a pixel?
the smallest picture element of the image matrix
how do you calculate a voxel?
one dimension of a voxel is defined by the effective slice thickness (see above). The other two dimensions of the voxel are defined by the in-plane resolution of a pixel
how do you calculate the in-plane resolution of a pixel?
FOV/image matrix (along both directions of the image)
what is multiplanar reconstruction?
once a 3D data set of anatomical information is collected, the data may be reformatted in other planes
whats a benefit of running a 3D pulse sequence in regards to multiplanar reconstruction?
the ability to generate thin slices in additional orientations, after the pt has left
why cant you reconstruct a 2D image?
- it would not be diagnostic because the original 2D slices are generally at least 2 mm thick and gaps are routinely left between the slices to reduce the effects of crosstalk, leaving an absence of information
- it also has low SNR
what are isotropic voxels?
are volume elements with the same dimension in all 3 directions, or perfect cubes
what are anisotropic voxels?
are volume elements which are not cubes, but rather elongated or shortened in one or two directions
how do you obtain isotropic voxels?
- the volume thickness divided by the number of slices must equal the FOV divided by the image matrix. In addition, if the FOV is square, the number of frequency steps should also equal the number of phase lines in the image matrix
- Another way to look at this relationship is to make sure that the effective slice thickness equals the in-plane resolution
see problems on page 19 and 20