Exchange-mediated contrast in CEST and spin-lock imaging.
OBJECTIVE
Magnetic resonance images of biological media based on chemical exchange saturation transfer (CEST) show contrast that depends on chemical exchange between water and other protons. In addition, spin-lattice relaxation rates in the rotating frame (R1ρ) are also affected by exchange, especially at high fields, and can be exploited to provide novel, exchange-dependent contrast. Here, we evaluate and compare the factors that modulate the exchange contrast for these methods using simulations and experiments on simple, biologically relevant samples.


METHODS
Simulations and experimental measurements at 9.4 T of rotating frame relaxation rate dispersion and CEST contrast were performed on solutions of macromolecules containing amide and hydroxyl exchanging protons.


RESULTS
The simulations and experimental measurements confirm that both CEST and R1ρ measurements depend on similar exchange parameters, but they manifest themselves differently in their effects on contrast. CEST contrast may be larger in the slow and intermediate exchange regimes for protons with large resonant frequency offsets (e.g. >2 ppm). Spin-locking techniques can produce larger contrast enhancement when resonant frequency offsets are small (<2 ppm) and exchange is in the intermediate-to-fast regime. The image contrasts scale differently with field strength, exchange rate and concentration.


CONCLUSIONS
CEST and R1ρ measurements provide different and somewhat complementary information about exchange in tissues. Whereas CEST can depict exchange of protons with specific chemical shifts, appropriate R1ρ-dependent acquisitions can be employed to selectively portray protons of specific exchange rates.
Vanderbilt University Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, USA; Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA. Electronic address: jared.g.cobb@vanderbilt.edu.
Jared Guthrie
JG
Cobb
Ke
K
Li
Jingping
J
Xie
Daniel F
DF
Gochberg
John C
JC
Gore
D000577:Amides / Q000737:chemistry; D002241:Carbohydrates / Q000737:chemistry; D003198:Computer Simulation; D003287:Contrast Media / Q000737:chemistry*; D003911:Dextrans / Q000737:chemistry; D007089:Image Enhancement / Q000379:methods; D046911:Macromolecular Substances; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D010455:Peptides / Q000737:chemistry; D011107:Polylysine / Q000737:chemistry; D011522:Protons; D014867:Water / Q000737:chemistry
D000577:Amides; D002241:Carbohydrates; D003287:Contrast Media; D003911:Dextrans; D046911:Macromolecular Substances; D010455:Peptides; D011522:Protons; D014867:Water; D011107:Polylysine
CEST; Chemical exchange; R(1ρ); Spin lock