Potential in reducing scan times of HARDI by accurate correction of the cross-term in a hemispherical encoding scheme

Purpose To reduce the scan time of high angular resolution diffusion imaging (HARDI) by using the hemispherical encoding scheme with the cross-term correction. Materials and Methods Unidirectional and 45° crossing phantoms were built to evaluate the accuracy of the fiber orientation estimation when using a hemispherical encoding scheme with and without the cross-term correction. The q-ball imaging using the spherical harmonic basis was adopted for estimation of fiber orientation. By recalculating the diffusion gradient strengths, we make the actual b value in all directions equal to the expected b value for the correction of the cross-term. Results The separation angles measured on the crossing phantom are 42.67° ± 1.50°, 42.75° ± 2.10°, and 42.93° ± 2.78° for cross-term-free results, for the results from the hemispherical encoding scheme with and without the cross-term correction. The angular errors in mapping the unidirectional phantom are 1.87°–2.29° and 4.01°–4.92° for the results using the hemispherical encoding scheme with and without correcting for the cross-term at different b values. Conclusion Using the hemispherical encoding scheme with the cross-term correction can potentially halve the scan time of HARDI and obtain accurate fiber orientation estimation simultaneously. This will be helpful in the clinical application of HARDI. J. Magn. Reson. Imaging 2009;29:1386–1394. © 2009 Wiley-Liss, Inc.