In its basic implementation, FFC-MRI is a relatively slow process. This is because (as with conventional MRI) the pulse sequence must be repeated N times in order to fill up k-space for an N x N pixel image. Furthermore, each line of the pulse sequence includes a field-cycling "event" - i.e. a ramp to the Evolution magnetic field, followed by a period there of ca. 0.5 seconds, followed by a ramp to the Detection field. FFC-MRI is slowed down still further when it is performed over a range of different evolution fields, in order to achieve relaxometric imaging. This project aimed to investigate how existing rapid MR imaging techniques could be adapted to FFC-MRI to offset this increase in scan time.

One solution was to develop a field-cycling version of the well known "Fast Spin-Echo" sequence. This sequence works by using multiple 180° radiofrequency refocusing pulses to generate a "train" of NMR echoes. By separately encoding each echo in the train an increase in image acquisition speed equal to the number of refocusing pulses can be achieved. To perform field cycling experiments, the spins are inverted using an adiabatic inversion pulse and allowed to evolve at the Evolution magnetic field before the imaging sequence is performed at the Detection field.

By collecting inversion-recovery images over a range of evolution magnetic fields and using a 2-point method it is possible to determine T1 at each value of the evolution field. Using centric ordered phase encoding this sequence allows a trade-off in image resolution vs scan time while ensuring that measured T1 accuracy is independent of the speed-up factor.

 

FC-FSE sequence
The Field-Cycled Fast Spin-Echo sequence. A slice of spins is first inverted using a hyperbolic secant AFP pulse. Then the field is switched to the evolution field (B0E) at which the spins relax. The magnetic field then returns to the detection field and image data is collected using the fast spin-echo technique. The speed-up factor n is equal to the number of lines of k-space collected following each field-cycling ramp.

 

Experiments were carried out to determine the effect of increasing the speed-up factor on image quality. The images below were obtained on our 59 mT whole-body FFC-MRI scanner.

FSE images
Comparison of 4 different speed up factors. Image A) spin echo image with no speed up factor, acquisition time 128 s. Image B) Speed up factor 2. Acquisition time 65 s. Image C) Speed up factor 4, acquisition time 34s. D) Speed up factor 8, acquisition time 17s. Image blurring due to spin-spin relaxation only becomes apparent at higher speed up factors.

 

Relaxometric imaging was implemented using Fast Spin Echo FFC-MRI; results are shown below.

FSE FFC image Dispersion plot

Left: Image from set of FSE FFC-MRI images, obtained using a speed-up factor of 4 (i.e. 4 echoes in each echo train), using 3 averages. Images were obtained at 35 different evolution field strengths. The total scan time was 31 minutes. The phantom included a circular bottle containing gelled bovine serum albumin (BSA) protein. Right: R1 (1/T1) dispersion plot derived from the set of images, from a region of interest drawn around the BSA sample. The quadrupole peaks, arising due to immobile protein in the sample, are clearly visible, demonstrating that relaxometry can be performed using this sequence with acceptable scan times.