Zhongliang Zu1,2, Vaibhav Janve1,2, John C Gore1,2,3,4,5. 1. Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, Tennessee. 2. Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, Tennessee. 3. Deparment of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee. 4. Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee. 5. Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee.
Abstract
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 (R1 ρ ) 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 R1 ρ and its changes with locking field were acquired on intracranial 9-L tumor models. The R1 ρ changes due to diffusion ( R 1 ρ Diff ) were calculated by subtracting maps of R1 ρ at 100 Hz (R1 ρ [100 Hz]) from those at 0 Hz (R1 ρ [0 Hz]). The R1 ρ changes due to exchange ( R 1 ρ Ex ) were calculated by subtracting maps of R1 ρ at 5620 Hz (R1 ρ [5620 Hz]) from those of R1 ρ at 100 Hz (R1 ρ [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy. RESULTS: The R1 ρ 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 ρ Diff is larger than for R 1 ρ Ex 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.
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 (R1 ρ ) 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 R1 ρ and its changes with locking field were acquired on intracranial 9-L tumor models. The R1 ρ changes due to diffusion ( R 1 ρ Diff ) were calculated by subtracting maps of R1 ρ at 100 Hz (R1 ρ [100 Hz]) from those at 0 Hz (R1 ρ [0 Hz]). The R1 ρ changes due to exchange ( R 1 ρ Ex ) were calculated by subtracting maps of R1 ρ at 5620 Hz (R1 ρ [5620 Hz]) from those of R1 ρ at 100 Hz (R1 ρ [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy. RESULTS: The R1 ρ 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 ρ Diff is larger than for R 1 ρ Ex 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.
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