Magnetic resonance system and method for detecting the buildup of the transfer of changes in magnetization from nuclei in mobile solute molecules in tissue
An embodiment in accordance with the present invention provides a new MRI method to image the buildup of exchange transfer processes from nuclei in mobile solute molecules in tissue via another molecule (e.g. solvent such as water). The pulse sequence can detect Chemical Exchange Saturation Transfer (CEST), relayed Nuclear Overhauser Enhancement (rNOE) CEST, and selective induced exchange transfer processes. Further, the proposed MRI pulse sequence involves acquiring two or more images with a difference in waiting period (delay) after a radiofrequency excitation, saturation pulse, or series of such pulses. This produces a series of exchange transfer images sensitive to the speed of transfer of changes in magnetization. Subtracting two images or fitting a time series produces maps with minimum interference from direct water saturation and from semi-solid magnetization transfer and other fast exchanging protons.
1. A method for detecting the buildup of exchange transfer processes from nuclei in endogenous or exogenous mobile solute molecules in tissue using magnetic resonance (MR) imaging comprising:
applying a series (one or more) of radiofrequency pulses to change a magnetization of one or more MR sensitive nuclei in the endogenous or exogenous mobile solute molecules;
waiting a period after each pulse for transfer of the magnetization change to another one of the endogenous or exogenous mobile solute molecules;
detecting this another one of the endogenous or exogenous mobile solute molecules using MR imaging or spectroscopy;
repeating the applying of the series of radiofrequency pulses to change the magnetization of one or more MR sensitive nuclei in the endogenous or exogenous mobile solute molecules;
waiting a different period after each pulse for a transfer of a different magnitude of magnetization change to occur to another one of the endogenous or exogenous mobile solute molecules;
detecting the another one of the endogenous or exogenous mobile solute molecules using MR imaging or spectroscopy; and
determining a difference MR signal for the another one of the endogenous or exogenous mobile solute molecules between the applications of the pulses with different waiting period.
2. The method of claim 1 wherein the mobile molecules are characterized by having a finite linewidth, i.e. excluding semi-solid molecules studied in conventional magnetization transfer contrast (MTC).
3. The method of claim 1 wherein the radiofrequency pulse is a frequency-selective excitation pulse for chemical shift(s) of the nuclei of interest.
4. The method of claim 1 wherein a radiofrequency pulse is a frequency-selective saturation pulse for the chemical shift(s) of the MR sensitive nuclei.
5. The method of claim 1 wherein the MR sensitive nuclei can be any nucleus that has spin and thus is detectible with magnetic resonance, for instance 1H, 13C, 31P, 23Na, and all nuclei used for NMR and MRI.
6. The method of claim 1 wherein the period for waiting after each pulse is as short as 0 ms for the first pulse sequence and range from 1 to several hundred ms for the repeating the applying of the radiofrequency pulse.
7. The method of claim 1 wherein the transfer of the magnetization change occurs directly via chemical exchange after labeling of an exchangeable nucleus.
8. The method of claim 1 wherein the transfer of the magnetization change occurs in a relayed fashion via magnetization of other nuclei in the molecule.
9. The method of claim 1 wherein the transfer of the magnetization change occurs via exchange of a multi-atomic entity containing a labeled magnetic nucleus or nuclei via other nuclei.
10. The method of claim 1 wherein the another molecule is a solvent.
11. The method of claim 1 wherein the another molecule is another solute.
12. The method of claim 1 wherein the repeating of the pulse sequence is at least once.
13. The method of claim 1 wherein the waiting a period and waiting a different period comprises different repeats when doing more than one repeat.
14. The method of claim 1 wherein a change in magnitude of the magnetization transfer is studied as a function of waiting time after the radiofrequency pulses or by taking the difference between different waiting times.
15. The method of claim 1 wherein the magnetization transfer from other protons with different exchange rates can be suppressed by taking the difference between different waiting times, where the magnitude of the signal of other protons are equal at the two waiting times.
16. The method of claim 1 wherein the mobile solutes are selected from a group consisting of endogenous or exogenous peptides, proteins, carbohydrates, metabolites or exogenous contrast agents.
17. The method of claim 8 wherein the relay is accomplished using nuclear Overhauser enhancement (NOE) or dipolar transfer.
18. The method of claim 14 wherein the change in magnitude is used to determine magnetization transfer rates or exchange transfer rates of the nuclei or molecular moieties involved.
19. The method of claim 14 wherein the change in magnitude is used to monitor pH of the tissue.
20. The method of claim 1 further comprising a magnetic resonance processor set up to process and display the waiting time dependent signals and the changes therein.
21. The method of claim 1 further comprising acquiring magnetization changes as a function of waiting time at multiple frequencies.
22. The method of claim 21 further comprising studying the change in magnitude of the magnetization transfer at each of the multiple frequencies as a function of waiting time after the radiofrequency pulse or by taking the difference between different waiting times.
23. The method of claim 22 wherein the time-dependent magnetization difference is compared between different frequencies or studied as a function of frequency.
24. A non-transitory computer readable medium programmed with elements comprising:
acquiring magnetic resonance image data of a subject;
applying radiofrequency pulses with different time delays to alter the magnetic resonance image data of the subject;
applying time-dependent pulse sequence at multiple selective frequencies to alter the magnetic resonance image data of the subject;
analyzing a difference in magnetization change as a function of waiting time;
generating and displaying images of the differences in magnetization change as a function of waiting time; and
generating and displaying different images of the images acquired of the differences in magnetization change as a function of frequency.