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Spin-lock imaging of intrinsic susceptibility gradients in tumors.

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 ? ) over a low range of spin-locking amplitudes (0 < ?1 < 100 Hz), whereas at higher ?1 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 ? and its changes with locking field were acquired on intracranial 9-L tumor models. The R1 ? changes due to diffusion ( R1?Diff ) were calculated by subtracting maps of R1 ? at 100 Hz (R1 ? [100 Hz]) from those at 0 Hz (R1 ? [0 Hz]). The R1 ? changes due to exchange ( R1?Ex ) were calculated by subtracting maps of R1 ? at 5620 Hz (R1 ? [5620 Hz]) from those of R1 ? at 100 Hz (R1 ? [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy. RESULTS:The R1 ? changes at low ?1 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 R1?Diff is larger than for R1?Ex and for the apparent R2w . CONCLUSION:Images that are sensitive to the variations of spin-lock relaxation rates at low ?1 provide a novel form of contrast that reflects the heterogeneous nature of intrinsic variations within tumors.

SUBMITTER: Zu Z

PROVIDER: S-EPMC7558740 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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Spin-lock imaging of intrinsic susceptibility gradients in tumors.

Zu Zhongliang Z Janve Vaibhav V Gore John C JC

Magnetic resonance in medicine 20191227 5

<h4>Purpose</h4>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<sub>1</sub><sub>ρ</sub> ) over a low range of spin-locking amplitudes (0 < ω<sub>1</sub> < 100 Hz), whereas at higher ω<sub>1</sub> and high magnetic fields, a second dispersion arises due to chemical exchange. Here, we separated these different effects and evaluated their contributions in tumors.<h4>Meth ...[more]

PMID: 31883149

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Project description:BackgroundR1ρ (or spin-lock) imaging is prone to artifacts arising from field inhomogeneities that may impact the R1ρ quantification. Previous research has proposed two types of method to manage the artifacts in continuous-wave constant amplitude spin-lock, one is based on the composite block pulses to compensate for the field imperfections, another category uses adiabatic pulses in the R1ρ pre-pulse to excite and reverse the magnetization (named adiabatic prepared approach). Although both methods have proved their efficiency in alleviating artifacts, we observed that the adiabatic pulse approach could produce much lower R1ρ dispersion in human knee cartilage than the block pulse method (characterized by the R1ρ difference ∆R1ρ =11.4 Hz (from spin-lock field 50 to 500 Hz) for the block pulse method vs. ∆R1ρ =4.5 Hz for the adiabatic pulse approach). Prompted by this observation, the purpose of this study was to investigate the underlying factors that may affect the R1ρ dispersion through numerical simulations based on the two-pool exchanging Bloch-McConnell equations.MethodsThe effects of free water pool size Pa (from 0.80 to 0.95), chemical exchange rate kb (from the bound to free water pool, ranged from 500 to 3,000 Hz), adiabatic pulse duration Tp (from 5.0 to 25 ms), and the chemical shift of the bound pool ppmb (from 1.0 to 5.0 ppm) were examined on the degree of the R1ρ dispersion for the two R1ρ imaging methods.ResultsIn general, the greater the ppmb, kb, Tp, and the smaller Pa, the more significant difference in R1ρ dispersion between the block and adiabatic approaches, with the dispersion curve of the adiabatic method becoming flatter.ConclusionsThe adiabatic prepared approach may compromise the R1ρ dispersion, the effect is determined by the combination of the tissue and radiofrequency (RF) pulse properties. It is suggested that care should be taken when using the adiabatically prepared approach to study R1ρ dispersion.

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Spin-lock imaging of intrinsic susceptibility gradients in tumors.

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Spin-lock imaging of intrinsic susceptibility gradients in tumors.

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