Chemical Exchange Saturation Transfer (CEST) contrast was recently introduced to MRI. The contrast relies on the selective pre-saturation of an exchanging group and subsequent observation of the decrease of the water signal as the result of the exchange. Endogenous or exogenous molecules with the exchanging groups such as -OH, -NH and -NH2 can be used as CEST agents (DIACEST). Exogenous complexes of paramagnetic lanthanides can also be used as the CEST agents (PARACEST). A number of DIACEST and PARACEST applications were developed aimed at quantitative imaging of pH and tumors (e.g. amide proton transfer, APT), metabolites (e.g. glycoCEST) and cartilage degradation (gagCEST). The CEST approach offers number of attractive features. First, CEST provides an amplification mechanism, allowing detection of metabolites present in the micro- to milli- Molar concentration ranges, normally inaccessible for MRI. Second, CEST contrast can be switched """"""""on"""""""" and """"""""off"""""""" at the operator's discretion, via RF application. If RF is """"""""off"""""""" the agent is invisible and/or, does not interfere with conventional MR imaging sequences. If the RF is """"""""on"""""""" the contrast is generated in the areas of agent concentration. Third, CEST contrast relies on the exchange process, and, hence, is very sensitive to the agent environment. This opens up possibility of imaging the quantitative parameters such as pH, metabolite concentration, or quantify changes in exchange due to binding of an agent to albumin or hydroxiapetate. Few technical challenges may still impede accurate application of the technique in clinical settings. The CEST contrast is sensitive to B0 and B1 inhomogeneities that may result in inaccurate quantification of the results. Typically, a small decrease in the signal intensity needs to be detected in the presence of the strong background signal. A difference image is employed for quantification, and a strong background signal may lead to an accumulation of artifacts. In addition, some CEST applications requiring acquisitions of multiple images with short TR or involving PARACEST agents, may require application of RF deposition exceeding FDA approved guidelines. Here we propose several ways to overcome the abovementioned challenges and create robust CEST scheme readily executable in clinical environment for quantitative in-vivo studies in animals and humans. First, we propose to explore applications of composite pulse trains and shaped RF pulses to improve B0 and B1 robustness. Second, we have introduced positive CEST scheme that should allow significant reduction of the background signal. Third, we will explore combinations of the technique with partial k-space saturation and parallel imaging for SAR reduction. Finally, we will adjust existing quantification schemes to accommodate all the above mentioned changes, in order to create a protocol connecting parameters measured in imaging session with quantitative parameters such as pH, exchange rate or concentration.
In recent years, there is an increasing number of MRI applications utilizing Chemical Exchange Saturation Transfer. These applications include utilization of endogenous groups and development of new exogenous paramagnetic complexes, all aimed at providing quantitative information about pH, metabolite concentration, tumor stage, or target-binding event. The goal of the project is the refinement and improvement of CEST methodology to fully explore its potential for quantitative imaging in-vivo in clinical environment.
|Vinogradov, Elena; Soesbe, Todd C; Balschi, James A et al. (2012) pCEST: Positive contrast using Chemical Exchange Saturation Transfer. J Magn Reson 215:64-73|
|Varma, G; Lenkinski, R E; Vinogradov, E (2012) Keyhole chemical exchange saturation transfer. Magn Reson Med 68:1228-33|
|Scheidegger, Rachel; Vinogradov, Elena; Alsop, David C (2011) Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation. Magn Reson Med 66:1275-85|