Research


Quadratic Echo Line-Narrowing

 

Our lab has developed the quadratic echo pulse sequence to achieve extreme line-narrowing (up to a factor of 70,000) in the NMR of solids. The quadratic echo pulse sequence uses the fact that during the time in which non-zero, periodic, hard pi-pulses act, the rf Hamiltonian can be put to good use in controlling spin coherence in ways that would otherwise be missed if the pi-pulses are approximated as delta functions. This novel sequence developed in our lab refocusses both Zeeman and dipolar terms in the spin Hamiltonian and even works in solids where both these terms are on the same order. We have successfully applied our pulse sequence to narrow spectra for C-13, Si-29, P-31 and are actively seeking other systems to apply the sequence. 

 

 


Imaging Hard and Soft Solids

 

Our group uses the quadratic echo line-narrowing sequence to image solids in which the dipolar coupling(broadening) between nuclear spins would usually limit the spectral resolution and thus make MRI practically unattainable. The Barrett lab has expertise in using the quadratic echo line-narrowing sequence for the imaging of P-31 in ex-vivo bone hydroxyapatite and membrane bound P-31 in ex-vivo brain to resolutions of a few hundred micrometers. We are actively pursuing:

 

  • Further increasing the resolving power of MR imaging in solids 
  • Enhancing the signal to noise ratio and decreasing imaging times 
  • Additional contrast mechanisms in the imaging of solids 

P-31 density image of porcine rib where the surface corresponds to pixels with the same value. 

 


Advancing Spectral Reconstruction with Undersampled Data Sets

 


Because experiments to image solids can be prohibitively long (due to long spin-lattice relaxation times), we developed a technique to accelerate data acquisition by reconstructing sparsely sampled time data, without sacrificing the advantages of the fast Fourier transform (FFT). This method exploits the computational speed of the FFT algorithm and is done in a deterministic way, by reformulating any a priori knowledge about the data into constraints (projections in the time and frequency domains), and then iterating the application of the constraints. We use these iterated projections to reconstruct multidimensional NMR spectra and MR images from sparsely-sampled time domain data from faster experiments. This iterated maps method is applicable to real 2D NMR and 3D MRI of solids data, and is flexible and robust enough to handle large data sets with significant noise and artifacts.

 

​Custom NMR/MRI Probe Design and Construction

 

The Barrett group builds custom high Q probes for solid-state NMR and solid-state MRI, including dual-channel probes for double resonance experiments. Members of the Barrett lab have designed and machined probes including: 

  • 5mm double-resonance (H-1, P-31) static NMR probe for a 88 mm bore magnet at 4T or 12T
  • 15mm double-resonance (H-1, P-31) MRI probe for a Bruker Mini 4T (150 mm bore) 
  • Single resonance (tubeable to: H-1, C-13 and S-29) static  NMR probes for a 88mm bore magnet at 4T or 12T
(MRI probe for Bruker Mini 4T)

Prior Research Projects