In vivo detection of microscopic anisotropy using quadruple pulsed-field gradient (qPFG) diffusion MRI on a clinical scanner

Abstract

We report our design and implementation of a quadruple pulsed-field gradient (qPFG) diffusion MRI pulse sequence on a whole-body clinical scanner and demonstrate its ability to non-invasively detect restriction-induced microscopic anisotropy in human brain tissue. The microstructural information measured using qPFG diffusion MRI in white matter complements that provided by diffusion tensor imaging (DTI) and exclusively characterizes diffusion of water trapped in microscopic compartments with unique measures of average cell geometry. We describe the effect of white matter fiber orientation on the expected MR signal and highlight the importance of incorporating such information in the axon diameter measurement using a suitable mathematical framework. Integration of qPFG diffusion-weighted images (DWI) with fiber orientations measured using high-resolution DTI allows the estimation of average axon diameters in the corpus callosum of healthy human volunteers. Maps of inter-hemispheric average axon diameters reveal an anterior-posterior variation in good topographical agreement with anatomical measurements reported in previous post-mortem studies. With further technical refinements and additional clinical validation, qPFG diffusion MRI could provide a quantitative whole-brain histological assessment of white and gray matter, enabling a wide range of neuroimaging applications for improved diagnosis of neurodegenerative pathologies, monitoring neurodevelopmental processes, and mapping brain connectivity.

Figure 1

A. Pulse sequence schematic diagram for multiple pulsed-field gradients (mPFG) spin echo diffusion MRI with concatenated double PFG blocks in the theoretical short gradient pulse limit (δ → 0, Δ → ∞). The signal is analyzed as a function of the angle ψ between the two diffusion wave vectors q₁ (red) and q₂ (blue). B. Schematic diagram of our clinical slice-selective quadruple pulsed-field gradient (qPFG) spin echo diffusion MRI sequence with EPI readout and τ_m = δ + r = 13.4 ms, where δ = 12.6 ms and r = 0.8 ms are the pulse width (including ramp-up and plateau durations) and ramp-down time for all diffusion gradient pulses. C. Schematic diagram of our clinical slice-selective qPFG diffusion MRI pulse sequence with short τ_m = 0 ms. Gradient pulses in consecutive diffusion encoding blocks are fused into a single pulse (purple) with width 2δ and amplitude (G₁ + G₂)/2, corresponding to q₁ + q₂.

Figure 2

A. The fiber orientation û is defined by spherical coordinates θ (polar angle) and φ (azimuthal angle) with respect to the normal n̂ to the plane of the applied diffusion gradients G₁ and G₂. Both diffusion gradients are applied in the xy-plane with G₁ aligned along the x-axis and G₂ at an angle ψ relative to G₁. Angular qPFG diffusion attenuation profiles E^ax(ψ) were evaluated numerically using the multiple correlation function formalism (Ozarslan et al., 2009) for the pulse sequences shown in Fig. 1B and C with maximum gradient strength of 50 mT/m/axis, intra-axonal diffusivity D = 1.5 μm²/m and an axon diameter d = 5 μm. B. For a fixed azimuthal angle (e.g., φ = 105°), as the inclination θ increases coupling of macroscopic (ensemble) anisotropy can generate large signal minima significantly changing the shape of the sinusoidal profile. For φ ≠ 90°, 270°, the amplitude of the main peak is reduced and the position shifted away from ψ = 180° rendering the profile asymmetric. C. Even for small polar angles (e.g., θ = 15°), the shape of E^ax(ψ) is very sensitively dependent on variations in φ, (i.e., fiber orientation with respect to G₁). This dependence increases at larger inclinations as macroscopic and microscopic anisotropy become more strongly coupled.

Figure 3

Integration of high-resolution diffusion tensor imaging (DTI) with quadruple pulsed-field gradient (qPFG) diffusion MRI data to account for intravoxel variations in fiber orientation. A. Axial view of directionally encoded color (DEC) maps obtained with 2 mm isotropic DTI in a representative healthy volunteer. B. Diffusion weighted images (DWIs) acquired with 2×2×2 mm³ resolution from three contiguous sagittal DTI slices corresponding to the 6mm-thick qPFG MR slice shown below. C. Sagittal qPFG DWIs acquired with 2×2×6 mm³ resolution corresponding to the three high resolution DTI slices shown in B. D. Schematic representation of local white matter fiber orientation in a 2×2×6 mm³ qPFG MRI voxel and the three corresponding 2mm isotropic DTI sub-voxels. Alignment of the qPFG MRI voxel with the underlying fiber orientation minimizes the number of axons contributing to the diffusion signal and allows effective average axon diameter estimation by incorporating intravoxel fiber orientations derived from a separate high-resolution DTI scan (Eq. 4).

Figure 4

Theoretical comparison of axon diameter sensitivity for clinical multiple pulsed-field gradient (mPFG) diffusion MRI pulse sequences. Angular diffusion signal attenuation profiles E^ax(ψ) are numerically calculated (Ozarslan et al., 2009) for both double PFG (A) and quadruple PFG (B) pulse sequence designs, assuming axon diameters from 1 – 10μm, intra-axonal diffusivity of 1.5 μm²/ms, and mixing time τ_m = 0 ms. Diffusion gradient pulse durations δ = 22.6 ms and δ = 12.6 ms were used for the dPFG and qPFG sequences, respectively. The results qualitatively reflect the behavior predicted analytically by Eq. 1 in the short-pulse limit (δ → 0, Δ → ∞). Compared to the dPFG sequence, the qPFG implementation uses lower overall diffusion attenuations and is more sensitive to axon diameter ranges previously reported in the corpus callosum, leading to improved contrast-to-noise ratio (CNR).

Figure 5

Validation of our quadruple pulsed-field gradient (qPFG) diffusion MRI clinical sequences (Fig. 1B,C) using a calibrated polymer phantom for free diffusion. A Single PFG diffusion signal attenuation measurements (red) as a function of b-value were fit with a mono-exponential decay function (black curve) to obtain a diffusivity of D = 0.81μm²/ms. B. The diffusivity measured using the single PFG experiments was used to numerically generate qPFG diffusion signal attenuation profiles E(ψ) for our clinical sequences with τ_m = 13.4 ms (black curve) and τ_m = 0 ms (blue curve). These theoretical profiles are in excellent agreement with measured qPFG diffusion attenuations using the same sequence parameters.

Figure 6

In vivo detection of microscopic anisotropy using quadruple pulsed-field gradient (qPFG) diffusion MRI on a clinical scanner. A. Extrema (minima and maxima) of angular modulation profiles E_13.4(ψ) and E₀(ψ) in qPFG diffusion signal attenuation images measured using τ_m = 13.4 ms and τ_m = 0 ms, respectively. B. In vivo angular qPFG profiles measured in individual voxels in the prefrontal (green), sensory-motor (red), temporal (yellow) and visual (blue) cortical areas show clear signs of restriction-induced microscopic anisotropy. The modulatory effect of imperfect fiber orientation results in features similar to those in Fig. 2. Nevertheless, theoretical angular qPFG profiles generated numerically using Eq. 4 and high-resolution DTI data reveal good agreement with measurements, validating our methodology for isolating microscopic anisotropy information.

Figure 7

Maps of microstructural information derived with high-resolution diffusion tensor MRI (A,B,C) and quadruple pulsed-field gradient (qPFG) diffusion MRI (E,F) in the corpus callosum of a representative healthy volunteer suggesting that the two methods provide complementary information A. Direction-encoded color (DEC) map reveals consistent orthogonality between the alignment of callosal fiber pathways and the prescribed sagittal slice orientation B. Axial diffusivity and C. Mean diffusivity maps show little heterogeneity throughout the corpus callosum D. Manually drawn regions-of-interest (ROIs) for functional parcellation of the corpus callosum in segments corresponding to prefrontal (green), sensory-motor (red), temporal/auditory (yellow), and visual (blue) inter-hemispheric fiber bundles E. Map of average axon diameters measured using qPFG diffusion MRI in vivo and high-resolution DTI shows significant heterogeneity along the anterior-posterior corpus callosum in good agreement with previous ex vivo studies (Aboitiz et al., 2003) F. Corresponding map of intra-axonal signal volume fraction G. Inter-hemispheric fiber pathways provide a visual validation of the functional parcellation of the corpus callosum.