Multinuclear Interleaving of ¹H CEST, Water <i>T</i>₂*, and ²³Na MRI at 3 T

MRI in vivo is a powerful clinical diagnosis tool as it allows acquiring noninvasively images with an ample range of contrasts. Advanced imaging techniques such as chemical exchange saturation transfer (CEST) allow measuring metabolic information including pH. Sodium tissue concentration, which can be measured by ²³Na MRI, is sensitive to changes in different pathological conditions. The routine clinical application of these techniques is limited by the required additional scan time. Multinuclear interleaved techniques allow reducing the total acquisition scan time by performing the pulse sequence elements of a ¹H imaging sequence during the idle times typically used in ²³Na MRI to allow magnetization recovery and reduce T₁ weighting. An interleaved radial amine CEST and sodium (INTERLACED) pulse sequence was developed on a clinical scanner to simultaneously map acidity or T₂* decay with ²³Na signal, reducing the total scan time by 46% relative to sequential mononuclear acquisitions and without introducing any significant bias, as demonstrated in vitro. Dynamic INTERLACED measures were performed in the leg during a 5-min plantar flexion exercise and during a second plantar flexion exercise immediately followed by a 5-min voluntary isometric contraction. The results showed increased T₂* and ²³Na signal during recovery in the gastrocnemius (GAS) while only an increase in ²³Na signal was observed in the soleus (SOL). During the isometric contraction, T₂* decreased in GAS, SOL, and the tibialis anterior; the ²³Na signal increased in GAS and SOL; and the magnetization transfer asymmetry increased in GAS, in agreement with an increase of intracellular sodium and acidification of the extracellular space. Our approach requires no hardware modifications, facilitating its inclusion in clinical routine at 3 T. Furthermore, it could benefit functional studies by enabling the acquisition of dynamic multinuclear information simultaneously from the same transient state.