Spin-lock imaging of intrinsic susceptibility gradients in tumors

Previous studies have shown that diffusion of water through intrinsic susceptibility gradients produces a dispersion of the spin-lattice relaxation rate in the rotating frame (R_1ρ ) over a low range of spin-locking amplitudes (0 < ω₁ < 100 Hz), whereas at higher ω₁ and high magnetic fields, a second dispersion arises due to chemical exchange. Here, we separated these different effects and evaluated their contributions in tumors.

Maps of R_1ρ and its changes with locking field were acquired on intracranial 9-L tumor models. The R_1ρ changes due to diffusion ( $R_{1\rho }^{\mathrm{Diff}}$ ) were calculated by subtracting maps of R_1ρ at 100 Hz (R_1ρ [100 Hz]) from those at 0 Hz (R_1ρ [0 Hz]). The R_1ρ changes due to exchange ( $R_{1\rho }^{\mathrm{Ex}}$ ) were calculated by subtracting maps of R_1ρ at 5620 Hz (R_1ρ [5620 Hz]) from those of R_1ρ at 100 Hz (R_1ρ [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy.

The R_1ρ changes at low ω₁ in tumors (5.24 ± 1.78 s^-1 ) are substantially (p = 3.7⁶ ) greater than those in normal tissues (1.36 ± 0.70 s^-1 ), which we suggest are due to greater contributions from diffusion through susceptibility gradients. Tumor vessels were larger and spaced less closely compared with normal brain, which may be 1 factor contributing the susceptibility within 9-L tumors. The contrast between tumor and normal tissues for $R_{1\rho }^{\mathrm{Diff}}$ is larger than for $R_{1\rho }^{\mathrm{Ex}}$ and for the apparent R_2w .

Images that are sensitive to the variations of spin-lock relaxation rates at low ω₁ provide a novel form of contrast that reflects the heterogeneous nature of intrinsic variations within tumors.