Spin-lock imaging of exogenous exchange-based contrast agents to assess tissue pH.
OBJECTIVE
Some X-ray contrast agents contain exchangeable protons that give rise to exchange-based effects on MRI, including chemical exchange saturation transfer (CEST). However, CEST has poor specificity to explicit exchange parameters. Spin-lock sequences at high field are also sensitive to chemical exchange. Here, we evaluate whether spin-locking techniques can detect the contrast agent iohexol in vivo after intravenous administration, and their potential for measuring changes in tissue pH.


METHODS
Two metrics of contrast based on R1ρ, the spin lattice relaxation rate in the rotating frame, were derived from the behavior of R1ρat different locking fields. Solutions containing iohexol at different concentrations and pH were used to evaluate the ability of the two metrics to quantify exchange effects. Images were also acquired from rat brains bearing tumors before and after intravenous injections of iohexol to evaluate the potential of spin-lock techniques for detecting the agent and pH variations.


RESULTS
The two metrics were found to depend separately on either agent concentration or pH. Spin-lock imaging may therefore provide specific quantification of iohexol concentration and the iohexol-water exchange rate, which reports on pH.


CONCLUSIONS
Spin-lock techniques may be used to assess the dynamics of intravenous contrast agents and detect extracellular acidification. Magn Reson Med 79:298-305, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA.|Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee, USA.
Zhongliang
Z
Zu
Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA.|Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee, USA.
Hua
H
Li
Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA.|Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee, USA.
Xiaoyu
X
Jiang
Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA.|Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee, USA.|Deparment of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, USA.|Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA.|Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA.
John C
JC
Gore
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging*; D001932:Brain Neoplasms / Q000000981:diagnostic imaging*; D003287:Contrast Media / Q000737:chemistry*; D005909:Glioblastoma / Q000000981:diagnostic imaging*; D006863:Hydrogen-Ion Concentration; D007472:Iohexol / Q000737:chemistry*; D008279:Magnetic Resonance Imaging / Q000379:methods; D008353:Mannitol / Q000737:chemistry; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D051381:Rats
D003287:Contrast Media; D008353:Mannitol; D007472:Iohexol
MRI; X-ray agent; extracellular pH; spin lock