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Field-Cycling Fast Spin-Echo

Thanks to its conceptual simplicity and relatively low hardware demands the fast spin-echo sequence is well suited for adaptation to FFC-MRI. This adaptation, known as Field-Cycling Fast Spin-Echo (FC-FSE) is the first rapid imaging technique I have implemented for use with our FFC-MRI whole body scanner.

The imaging sequence itself is detailed on the RARE page. Because image collection is by far the most time consuming aspect of FFC-MRI, by incorporating the FSE sequence into a field-cycling experiment significant speed up gains can be achieved over conventional image acquisition.

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The FC-FSE sequence. The conventional inversion-recovery FSE sequence has been adapted to field-cycling by including a field-cycling step during the inversion recovery period. After B0 is ramped back to detection field the sequence proceeds as shown.

Currently speed up factors of 2, 4, and 8 have been implemented on our system. This is due to gradient limitations which sets a minimum interecho spacing of about 30 ms. Because of this in most samples virtually no signal will be detected beyond 8 echoes.

For each evolution field of interest T1 is calculated using a two-point method, where the signal is measured with and without an inversion pulse. Normally this would necessitate long scan times, even using FC-FSE, as we would need to collect 2 images for every evolution field. This requirement can be overcome by using Curie's Law, which allows us to estimate the saturation recovery signal for each evolution field using a single reference image.

Experiments performed using this sequence have shown good agreement with results obtained using our Stelar relaxometer, which performs FFC-NMR relaxometry on small samples.

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Relaxometry performed using relaxometric imaging on a small bottle of crosslinked bovine serum albumin (BSA) show good agreement with an analysis of the same sample performed using a commercial Stelar relaxometer.
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The relaxation data obtained using the sequence is largely independent of the speed up factor, or echo train length (ETL) used to collect the images. The graph here shows the same experiment performed with different ETLs of 2, 4 and 8.

 

Also see the rapid FFC-MRI page of our group website for more information.