High-frequency mutually decoupled coil arrays

Mutually decoupled coil arrays, often referred to as "phased arrays" are used extensively in clinical MRI where parallel imaging is now standard on all systems.Phased array coils are not as widespread, however, for animal MRI systems, particularly those at very high field. There are substantial added challenges in the design of physically small arrays at high frequencies. Issues include wavelength-effects, self-resonance, stray capacitance, radiative losses and increased resistive losses. Higher frequencies often have deleterious effects on parameters such as diode-switching, capacitor performance etc. Good coil isolation is also difficult when the individual elements are very close physically, and construction tolerances are much higher. A related problem is that arrays are only effective when the sample noise dominates the coil noise, and although this is always the case with human-sized arrays, it becomes far more challenging at smaller dimensions and higher frequencies. Nevertheless, significant increases in S/N are attainable using phased array coils for animal imaging. 

Some examples of our phased array coils at 600 MHz (14.1 tesla) and higher are shown below:

Four-coil volume array at 14.1 tesla

Images produced using a four-coil volume array. The inter-coil isolation is achieved using overlapping and capacitive decoupling between coils. 

For more details see: 

Zhang X, Webb A. Design of a capacitively decoupled transmit/receive NMR phased array for high field microscopy at 14.1T. J Magn Reson. 2004 Sep;170(1):149-55. download pdf file

Four coil receive-only surface array at 14.1 tesla.

This coil configuration is more complicated, incorporating varactor diode isolation between the outer transmit coil (on the backside of the PC board shown) and each of the individual elements of the surface-coil array, as well as between each of the coils in the array. The images shown are of a mouse spinal cord, with individual images and a sum-of-squares reconstruction. 

For more details, see: 

X.Zhang and A.G.Webb. Design of a four-coil surface array for in vivo magnetic resonance microscopy at 600 MHz. Magn.Reson.Engineering, 24B, 6-14, 2005. download pdf file

Transmit-receive coils for parallel imaging at 14.1 tesla

The four coils in this array are overlapped and capacitively decoupled. This array was used for parallel imaging studies at 14.1 tesla. The two images show factor-of-two data reductions with GRAPPA reconstruction of a small tumour in a mouse brain (left) and EPI imaging of the mouse brain at 600 MHz (right). 

For more details, see:

Sutton BP, Ciobanu L, Zhang X, Webb A. Parallel imaging for NMR microscopy at 14.1 Tesla.
Magn.Reson Med. 2005 54(1):9-13.download pdf file

Microstrip arrays at 17.6 tesla (coils constructed by Tobias Wichmann)

Photograph of a 4 coil loop coil microstrip array designed for operation at 750 MHz, and Individual time-points in the cardiac cycle of a mouse, obtained using the four-loop microstrip design and a GRAPPA reconstruction algorithm that allowed a factor-of-two reduction in data acquisition in the phase-encoding direction.

For more details see:

Wichmann,T., Gareis,D., Griswold,M.A., Behr,V., Webb,A., and Jakob,P.M., 2004. PPA with a four channel transmit/receive microstrip array for 17.6 tesla. 2nd Int. Workshop Parallel Imaging, Zurich.