? Chemical Exchange Saturation transfer (CEST) agent

Q.All are true regarding chemical exchange saturation transfer (CEST) agent except

a. make use of the phenomenon of magnetization transfer between water protons and protons in other molecules

b. used to detect endogenous species that are abundant in vivo, such as amide groups in tumors

c. offer unique approaches to obtaining MR contrast from physiologic changes such as pH

d. acquisition of two data sets is required

e. rely on the same principles of relaxation as simple paramagnetic ions

ANS .---e

CHEMICAL EXCHANGE AGENTS is a different class of contrast agents that does not rely on the same principles of relaxation as simple paramagnetic ions but instead make use of the phenomenon of magnetization transfer between water protons and protons in other molecules, such as in amide groups, which have a different resonant frequency. These so-called chemical exchange saturation transfer (CEST) agents produce contrast in appropriate imaging sequences as a result of magnetization transfer between water protons and protons in the agent that may dissociate and thereby undergo “chemical exchange.”

 By applying saturating radiofrequency energy at the precise frequency of the labile proton while it is chemically associated with the agent (which is different from the frequency of the solute water), one causes the MR signal from the water to decrease when the exchange occurs. The magnitude of the effect on the MR signal intensity depends strongly on the rate at which the protons exchange between the agent and the water. Moreover, the effect of the agent is visible only when the saturating RF pulse is applied to induce the effect.

In simple CEST experiments, acquisition of two data sets is required; in one, an RF pulse is applied at the frequency of the agent-bound proton, and in a second, the applied RF is set symmetrically on the other side of the water resonance. The signal from the first data set contains a combination of the CEST effect and direct saturation of the water, whereas the second acquisition measures only the latter. Subtraction of these two sets results in signal changes due to the CEST effect alone. CEST techniques have been used to detect endogenous species that are abundant in vivo, such as amide groups in tumors (36) and -OH groups in glycogen in perfused liver