Characterization of non-hemodynamic functional signal measured by spin-lock fMRI

Abstract

Current functional MRI techniques measure hemodynamic changes induced by neural activity. Alternative measurement of signals originated from tissue is desirable and may be achieved using T1ρ, the spin-lattice relaxation time in the rotating-frame, which is measured by spin-lock MRI. Functional T1ρ changes in the brain can have contributions from vascular dilation, tissue acidosis, and potentially other contributions. When the blood contributions were suppressed with a contrast agent at 9.4 T, a small tissue-originated T1ρ change was consistently observed at the middle cortical layers of cat visual cortex during visual stimulation, which had different dynamic characteristics compared to hemodynamic fMRI such as a faster response and no post-stimulus undershoot. Functional tissue T1ρ is highly dependent on the magnetic field strength and experimental parameters such as the power of the spin-locking pulse. With a 500Hz spin-locking pulse, the tissue T1ρ without the blood contribution increased during visual stimulation, but decreased during acidosis-inducing hypercapnia and global ischemia, indicating different signal origins. Phantom studies suggest that it may have contribution from concentration decrease in metabolites. Even though the sensitivity is much weaker than BOLD and its exact interpretation needs further investigation, our results show that non-hemodynamic functional signal can be consistently observed by spin-lock fMRI.

Fig. 1. Surface-coil T₁ρ pulse sequence (A, B) and applications to cat primary visual cortex studies (C-F)

The pulse sequence (A) is a double spin-echo EPI acquisition with a non-selective spin-locking (SL) preparation (B). Adiabatic half passage (AHP, 2 ms in length with amplitude B_{1, AHP}) ensures that spins nutate into the transverse plane over a relatively large volume of B₁ inhomogeneity. The RF amplitude is then immediately ramped to the desired SL level (B₁) and held constant for the spin-lock time (TSL); the transmit frequency of the pulse remains the same during the short ramp and TSL. Following the spin-lock preparation, transverse spins are refocused by two adiabatic full-passage (AFP) pulses with slice-selection gradients. High quality T₁ρ-weighted image (ω₁ = 500 Hz and TSL = 40 ms) can be obtained when adiabatic SL condition is satisfied (C), but signal oscillations appear otherwise (D). (E) The spin-locking nutation frequency ω₁ map shows B₁ inhomogeneity. For our conditions, ω₁ at an area proximal to the coil (red square) is ~600 Hz, while at a distal area (blue square) it is ~ 400 Hz. Gray matter areas are outlined in green. (F) The map of R₁ρ (=1/T₁ρ) shows little spatial variations within visual cortex because the T₁ρ dispersion is very small within the narrow ω₁ range (e.g., 400-600 Hz).

Fig. 2. Intravascular and extravascular contributions to functional R₁ρ change in the cat visual cortex

A slightly bright band (indicated by black arrows) within the gray matter (outlined in green) in high-resolution T₁-weighted EPI image (A) indicates cortical layer IV (Kim and Kim, 2011). Pixels along the white band (yellow) were chosen for quantitative analyses of cat's functional studies (B). All active pixels were overlaid in color on their respective baseline images (C-F), and the vertical grayscale bar indicates the baseline R₁ρ values (for E and F). Functional percentage signal change maps were obtained by T₂-weighted (TSL = 0, C) and T₁ρ-weighted fMRI (TSL = 50 ms and ω₁ = ~500 Hz, D) without MION during visual stimulation. Higher changes were observed in T₁ρ-weighted fMRI, indicating an increase in T₁ρ (decrease in R₁ρ). To determine the blood contribution, R₁ρ change maps obtained without (E) and with (F) 5 mg/kg MION were compared. An increase in R₁ρ at the surface of the cortex is due to a volume fraction change between tissue and CSF, while a decrease in R₁ρ is located at the middle of the cortex. (G) At the middle cortical ROI (yellow pixels in B), the functional R₂ change (ΔR₂) is nearly suppressed with MION injection, whereas ΔR₁ρ only decreases slightly.

Fig. 3. Dynamic changes in BOLD, T₁ρ-weighted fMRI, CBV, and R₁ρ without and with MION during visual stimulation

Averaged time courses of T₂-weighted (TSL = 0), T₁ρ-weighted fMRI (TSL = 50 ms), and R₁ρ change were obtained from the middle cortical ROI during visual stimulation without and with blood signal suppression with MION (A-C). The normalized time courses of BOLD and CBV show a significant undershoot after the stimulus offset (D), unlike those time courses of the two R₁ρ responses (C). The BOLD, CBV, total R₁ρ (without MION) and tissue R₁ρ responses were normalized and their initial rising periods during stimulation were compared in (E). The rising time, defined as the time from the stimulus onset to 50% of the peak change, is faster for the tissue R₁ρ than BOLD (p < 0.05, n = 5) and CBV (p < 0.01) responses (F).

Fig 4. Functional tissue R₁ρ changes at two spin-locking frequencies

Visual stimulation-induced tissue R₁ρ changes were measured after 5 mg/kg MION for two SL frequencies ω₁ = 500 and 2000 Hz. Tissue R₁ρ change maps of two representative animals were shown for ω₁ = 500 Hz (A, C) and 2000 Hz (B, D), where the horizontal grayscale bar indicates the baseline R₁ρ values and the vertical color bar indicates the functional change. The R₁ρ decrease at the parenchyma is significantly reduced for ω₁ = 2000 Hz, whereas the R₁ρ increasing pixels at the surface of the cortex are similar for the two frequencies. The averaged change (n = 6 cats) in tissue R₁ρ at ω₁ = 500 Hz is 2.7 times larger than that at 2000 Hz (E).

Fig. 5. Effects of intravascular susceptibility variation and hyperoxia challenge on extravascular water R₁ρ

In order to detect the contribution of intravascular susceptibility changes to tissue R₁ρ, the blood signal was suppressed with the injection of 5 mg/kg MION before experiments. Dynamic changes in tissue R₂ and R₁ρ were obtained during two injections of 1 mg/kg MION (A, n = 4 rats) and 3 minutes inhalation of 60% O₂ (B, n = 5 rats) indicated by the yellow shaded regions. Time courses were obtained from the red pixels within the cortex (Inset). When spin-locking frequencies were measured at ≥500 Hz, a variation in intravascular susceptibility does not contribute to tissue R₁ρ measurements.

Fig. 6. Spin-locking frequency-dependent tissue R₁ρ change during tissue acidosis

Tissue R₁ρ changes were measured during hypercapnia and global ischemia after 5 mg/kg MION injection. Tissue R₁ρ change maps for ω₁ = 500 Hz and 2000 Hz are shown for hypercapnic challenge in one rat (A). Green contour indicates the cortical area. Unlike visual stimulation, an increase in tissue R₁ρ was observed for ω₁ = 500 Hz. Note that the increases of R₁ρ near the ventricle area are similar in the two maps and can be attributed to a change of CSF volume fraction (arrows). The averaged time course (n = 5 rats) of the tissue R₁ρ response obtained the cortical ROI (Inset) shows a significant increase for 500 Hz whereas a small decrease for 2000 Hz (B). The averaged tissue R₁ρ responses (n = 6 rats) for ω₁ from 250 to 4000 Hz during KCl injection (C), which induces tissue acidosis, show spin-locking frequency-dependent changes. These are qualitatively similar to hypercapnia.

Fig. 7. Calculated R_ex dispersion (A) and the measured R₁ρ dispersion of pH-dependent phantoms (B-C)

R_ex dispersion was calculated with Eq. [4] as a function of exchange rate k. Upward and downward arrows indicate changes in R_ex when the exchange between labile protons and water is slow down due to pH decrease (see texts). The R₁ρ dispersion of 8% bovine serum albumin (BSA) decreases with pH values (B). The R₁ρ dispersion of 4% egg white albumin (EWA) only (open symbols) and 4% EWA with 30 mM of glutamate (filled symbols) were both measured for three pH values (C). Vertical dashed lines (C) indicate spin-locking frequencies of 500 and 2000 Hz used for in vivo studies. The addition of glutamate (Glu) changes the pH-dependence of R₁ρ dispersion.

Fig. 8. R₁ρ dispersions of concentration-dependent amine and hydroxyl metabolite phantoms

Three amine-containing glutamate concentrations in agarose with 0.07 mM MnCl₂ (A), and four hydroxyl-containing glucose (Glc) concentrations in PBS with 0.1 mM MnCl₂ (B) were used. R₁ρ is linearly dependent on Glu and Glc concentration, and the slope for ω₁ = 500 Hz is only slightly larger than that for ω₁ = 2000 Hz for Glu, but is much larger for Glc (C). The ratio of R₁ρ at ω₁ = 500 Hz to 2000 Hz is similar for glucose phantom (2.8) and for in vivo functional response (2.7).