High b-value Acquisition using CURVE-ball DTI

Overview: The characteristics of water diffusion parallel and perpendicular to white matter tracts are a sensitive indicator of tissue microstructure and cannot be fully determined from single b-value DTI. In order to obtain singleor two-tensor DTI data with multiple b-values and acceptable SNR in clinically reasonable times, a new data acquisition strategy is proposed – CURVE-ball DTI (CUbe Rays to Vertices and Edges). The basis of the method is the invariance of eigenvector directions to b-value, which has been verified in-vivo [1]. Thus the eigenvectors can be calculated from a conventional multi direction, single shell (single b-value) DTI acquisition (gradient directions shown in black in Fig.1). These measurements provide the single tensor (or multiple-tensor) eigenvectors. By adding a small number of higher b-value measurements, coupled with an appropriate diffusion model, diffusion attenuation curves for each eigenvector can be estimated. Practical Example: The DTI sequence on a General Electric 3T scanner was modified to acquire 36 directions at a basic b-value of 800 s/mm, 6 directions with b=1600 s/mm, and 4 directions with b=2400 s/mm, the latter with 2 averages for increased SNR. Within the same echo time (TE), the 6 gradient directions at strengths twice the basic value are based on acquisition from the edges of the enclosing cube, i.e. [1,1,0] etc. (red points in Fig.1) as first implemented for DTI in Ref. [2]. The additional 4 gradient directions at even higher b-values are obtained by utilizing the maximum gradient on all axes (i.e. [1,1,1] etc.) giving three times the basic b-value (blue points in Fig.1) as first suggested for optimizing SNR in Ref. [3]. Fig. 2 shows an example of diffusion curves calculated from this data with the help of the diffusion model from Ref. [4]. Advantages of the Strategy: 1. Short acquisition time: A dual-echo scan that also includes three directions at low b and one scan at b=0 (giving a total of 54 measurements for every slice location) takes approximately 3.5 minutes for 14 slices on a GE 3T scanner, with other image acquisition parameters being typical for EPI-based DTI. 2. High SNR: TE is short since it is determined by the length of the gradient pulses required for the low b-value, isotropically distributed, measurements on the unit sphere (b=800 in this case). This results in a relatively high SNR for all measurements. Conclusion: Brute force solutions in which multiple spherical shells are acquired are generally too long for clinical use. Once the eigenvector directions have been determined, it is unnecessary to sample high b-values in all directions – sparse sampling of high b-values suffices when it is combined with dense sampling at low bvalues. This strategy has important implications for the feasibility of high bvalue clinical DTI data acquisition and for the SNR of such experiments. References: [1] S. Maier et al Magn Reson Med. 2004, 51(2) p.321. [2] C. Pierpaoli et al. Radiology. 1996 201(3) p.637. [3] T.E. Conturo et al, Magn Reson Med. 1996 35(3):p.399. [4] S. Peled IEEE TMI 2007 26(11) p.1448. Fig.1: CURVE-ball gradient directions