pH imaging of mouse kidneys in vivo using a frequency-dependent paraCEST agent.

PURPOSE: This study explored the feasibility of using a pH responsive paramagnetic chemical exchange saturation transfer (paraCEST) agent to image the pH gradient in kidneys of healthy mice.
METHODS: CEST signals were acquired on an Agilent 9.4 Tesla small animal MRI system using a steady-state gradient echo pulse sequence after a bolus injection of agent. The magnetic field inhomogeneity across each kidney was corrected using the WASSR method and pH maps were calculated by measuring the frequency of water exchange signal arising from the agent.
RESULTS: Dynamic CEST studies demonstrated that the agent was readily detectable in kidneys only between 4 to 12 min postinjection. The CEST images showed a higher signal intensity in the pelvis and calyx regions and lower signal intensity in the medulla and cortex regions. The pH maps reflected tissue pH values spanning from 6.0 to 7.5 in kidneys of healthy mice.
CONCLUSION: This study demonstrated that pH maps of the kidney can be imaged in vivo by measuring the pH-dependent chemical shift of a single water exchange CEST peak without prior knowledge of the agent concentration in vivo. The results demonstrate the potential of using a simple frequency-dependent paraCEST agent for mapping tissue pH in vivo. Magn Reson Med 75:2432-2441, 2016.