Abstract:
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Purpose: In conventional long echo train (ET) fast spin echo (FSE) sequences, fine object discrimination is hindered by blurring or edge-enhancement arising from the non-uniform weighting of k-space. Parallel imaging is used to reduce the ET length, using principles of sensitivity encoding. This study evaluates considerations indicating the use of parallel imaging (ASSET) reduction factors greater than 2 for long ET FSE sequences, and the impact of these considerations on MR cholangiography (MRC).
Methods and Materials: 7 healthy volunteers were imaged at 1.5T (GE TwinSpeed EXCITE) with 6-elements of a 10-coil array (Gore Cardiovascular Array, Medrad) using 3 different families of long ET FSE sequence typically encountered in MRC: multislice single shot FSE (ms-ssFSE), single thick slab single shot FSE (slab-ssFSE) and 3D-fast recovery FSE (3D-FRFSE). Comparing ASSET factors of 2.0 and 2.6, the following variables were manipulated: field of view, FOV (to avoid reconstruction artifacts at high ASSET factors), phase encoding resolution (to adjust spatial resolution), bandwidth (to reduce influence of noise, while maintaining desired long TE). Images from ms-FRFSE and 3D-FRFSE were reconstructed in 3D. All images were rated for diagnostic utility, characterization of biliary structures and presence of artifact by consensus of two radiologists, blinded to image acquisition details.
Results: For ms-SSFSE ultra high ASSET factors (2.6) were consistently (6/7) rated as superior to ASSET factors of 2, by virtue predominantly of improved sharpness. 3D-FRFSE sequences demonstrated benefit of ultra-high ASSET factor in 4/7 cases, by virtue of improved contrast to noise afforded by decreasing receiver bandwidth, and decreased blurring resulting from shorter ET length. Slab-ssFSE sequences did not show benefit of ultra high ASSET factors, where FOV increases without commensurate increase in phase-encoding steps reduced spatial resolution.
Conclusion: Parallel imaging is advantageous for imaging of small high contrast structures with long ET FSE sequences. Further reduction of ET length using ASSET reduction factors greater than 2 reduces edge enhancement artifacts and blurring. It also facilitates bandwidth reduction. Ultra-high ASSET factors require increased FOV to avoid reconstruction artifact.
Questions about this event email: tim.roberts@utoronto.ca
Roberts PhD, T,
Considerations for Ultra-High Parallel MRI Reduction Factors in Long Echo Train Fast Spin Echo Sequences: Application to MR Cholangiography. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3102744.html
