Purpose: 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 (R1ρ ) 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.
Methods: Maps of R1ρ and its changes with locking field were acquired on intracranial 9-L tumor models. The R1ρ changes due to diffusion ( ) were calculated by subtracting maps of R1ρ at 100 Hz (R1ρ [100 Hz]) from those at 0 Hz (R1ρ [0 Hz]). The R1ρ changes due to exchange ( ) were calculated by subtracting maps of R1ρ at 5620 Hz (R1ρ [5620 Hz]) from those of R1ρ at 100 Hz (R1ρ [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy.
Results: The R1ρ 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 is larger than for and for the apparent R2w .
Conclusion: 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.
Keywords: brain tumor; intrinsic gradients; relaxometry; spin lock; susceptibility-weighted imaging.
© 2019 International Society for Magnetic Resonance in Medicine.