MOTSA

What is MOTSA?  

MOTSA, which stands for Multiple Overlapping Thin Slab Acquisition, is a hybrid between 2D and 3D TOF techniques.  MOTSA involves the sequential acquisition of a several overlapping 3D volumes (or "slabs"). Each slab is typically less than 5 cm in thickness, so the number of contained slices is small (e.g., 16-32). Because of this restricted slab thickness, loss of signal due to saturation effects is relatively limited, even at the exit slices. MOTSA thus offers a method to cover a relatively large anatomic area using 3D TOF with preserved intravascular signal intensity. 

Some variation in signal still occurs at the end slices, so MOTSA extracts only the central portions for each of the overlapping acquisitions to make up the final data set for processing into the MRA projections. The end slices are typically discarded, but may be averaged with those in the adjacent MOTSA section.  

Prescription of MOTSA slices for cerebral MRA

For optimal contrast, each MOTSA slab should contain at least 10 slices. The slab order should be obtained as in the diagram above, selected so that it opposes the flow direction. This results in maximum refreshment of blood flowing into each slab. 

Venetian blind artifact from MOTSA

Because each MOTSA slab is acquired at separate times, exact registration of position and signal intensity of adjacent slabs may not be possible. This gives rise to the so-called venetian blind artifacts shown in the image left.

Advanced Discussion (show/hide)»

Although most vendors refer to this technique as MOTSA, Philips calls their sequence MultiChunk 3D Inflow MRA. In the Philips lexicon, the term "chunk" is equivalent to "slab". Philips also offers a special algorithm to merge chunk borders and reduce/eliminate venetian blind artifacts called CHARM ("CHunk Acquisition and Reconstruction Method").

Other strategies for normalizing signal intensities at the edge slices and correcting for venetian blind artifacts have been proposed. As an alternative to discarding all the overlapping slices, one or two of the overlapping slices may be retained and averaged with the corresponding slices from the next slab acquisition. Other methods such as SLINKY ("SLiding Interleaved K_y) have been proposed that modify reconstruction process but are beyond the scope of this web site.

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References
     Blatter DD, Parker DL, Robison RO. Cerebral MR angiography with multiple overlapping thin slab acquisition. Part I. Quantitative analysis of vessel visibility. Radiology 1991; 179:805-811.

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Related Questions
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