Rapid Fast Field-Cycling Imaging using the Keyhole Technique

Purpose Fast Field-Cycling MRI (FFC-MRI)1 is an emerging technique that adds a new dimension to conventional MRI by making it possible to rapidly vary B0 during a pulse sequence. By doing this it is possible to observe how the NMR relaxation rates of biological tissues vary with magnetic field strength information which can be employed as a useful contrast mechanism. In this work we have made use of the keyhole MRI technique2 in order to speed up FFC-MRI. Methods Imaging was carried out on a home-built, whole-body, field-cycling imager with a 59 mT detection field3. Images were constructed by combining an initially-acquired full resolution k-space with a set of low resolution k-space matrices acquired at each evolution field, requiring a scan time of 25% compared to conventional imaging. The technique was used to derive R1 dispersion curves from a phantom consisting of cross-linked bovine serum albumin (BSA) across a field range of 32 mT to 59 mT (proton Larmor frequencies of 1.4 MHz to 2.5 MHz). Brain images were also collected from a volunteer (Fig 2A) in order to validate that the method generated artefact free images in vivo. Images were collected for evolution fields of 49 mT and 59 mT (2.1 MHz and 2.5 MHz) and from these images R1 maps were derived. Results and Discussion R1 dispersion curves derived from BSA using the technique show excellent agreement with results obtained using a conventional scan. (Fig.1). The R1 maps (Fig. 2B and 2C) generated from brain images show an increase in R1 at 49 mT, which was used to generate a ΔR1 map (Fig. 2D) which shows a higher increase in R1 in predominately white matter regions. Conclusions This work has demonstrated that the keyhole technique can readily be applied to FFC-MRI and used to obtain a 4-fold or greater speed up in scan times while still retaining the same contrast as standard FFC-MRI methods. The rich ΔR1 contrast present in brain images indicates that FFC-MRI has potential application in the characterization of neurodegenerative conditions where subtle changes in R1, which may not be visible on conventional T1-weighted imaging, could be used as an early marker of disease. The reduction in scan time achieved by use of the keyhole technique will significantly improve the applicability of FFC-MRI in volunteer and clinical studies, which we are currently working towards. References [1] Lurie D.J. et al. Fast field-cycling magnetic resonance imaging. C.R.Phys. 2010;11:136–148. [2] van Vaals J.J. et al. “Keyhole” method for accelerating imaging of contrast agent uptake. J. Magn. Reson. Imaging 3:671–5. [3] Lurie D.J., Foster M.A., Yeung D., Hutchison J.M. Design, construction and use of a large-sample fieldcycled PEDRI imager. Phys. Med. Biol. 1998;43:1877–86. Figure 1: Dispersion curves for a phantom of cross-linked BSA obtained using the keyhole method (red diamonds) show good agreement with results obtained using a conventional field-cycling spinecho sequence (black circles).