K-space filter deconvolution and flip angle self-calibration in 2D radial hyperpolarised 3He lung MRI

• Published in: NMR in biomedicine 2012-02, Vol.25 (2), p.389-399
• Main Authors: Marshall, Helen, Ajraoui, Salma, Deppe, Martin H., Parra-Robles, Juan, Wild, Jim M.
• Format: Article
• Language: English
• Subjects: Calibration; Computer Simulation; Helium; Humans; hyperpolarised 3He; Image Processing, Computer-Assisted - methods; Isotopes; k-space filter; Lung - anatomy & histology; lung MRI; Magnetic Resonance Imaging - methods; Phantoms, Imaging; radial acquisition; Radio Waves
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.1766

In hyperpolarised 3He lung MRI with constant flip angles, the transverse magnetisation decays with each RF excitation imposing a k‐space filter on the acquired data. For radial data acquired in an angularly‐sequential order, this filter causes streaking, angular shading and loss of spatial resolution in the images. The main aim of this work was to reduce the effects of the RF depletion k‐space filter in radial acquisitions. Two approaches are presented; (i) retrospective deconvolution of the k‐space filter for sequentially‐acquired data and (ii) golden angle acquisition order. Radial trajectories sample the centre of k‐space with every projection, thereby self‐tracking signal decay. The inverse of the signal decay function was used to retrospectively deconvolve RF depolarisation k‐space filter effects and the method was demonstrated in 2D radial imaging in phantoms and human lungs. A golden angle radial acquisition was shown to effectively suppress artefacts caused by the RF depletion k‐space filter. In addition, the average flip angle per slice was calculated from the signal decay and the values were found to correspond with conventional flip angle maps, providing a means of flip angle self‐calibration. Copyright © 2011 John Wiley & Sons, Ltd. The effect of the k‐space filter imposed on 2D radial hyperpolarised 3He lung data by RF depolarisation was investigated. The average flip angle per slice was calculated from radial images and the values corresponded well with conventional flip angle maps, providing a means of B1 self‐calibration. A retrospective artefact compensation method for k‐space filter deconvolution is proposed and demonstrated in 3He phantoms and human lungs, and a golden angle sampling strategy is shown to inhibit artefacts caused by RF decay during the data acquisition.