PMC

Amine/amide concentration-independent detection (AACID) pH calibration in vivo using ³¹P-MRS: pH measurements and chemical exchange saturation transfer (CEST) spectra acquired using the standard ³¹P-MRS and ¹H-MRI, respectively, in vivo and postmortem. (A) In vivo (bottom) and postmortem (top) whole brain ³¹P-MRS spectra for one mouse. (B) Corresponding CEST spectra. (C) AACID and pH measurements for three different mice. MRI, magnetic resonance imaging; ³¹P-MRS, phosphorus (³¹P) magnetic resonance spectroscopy.

Illustration of chemical exchange saturation transfer (CEST) principles and the effect of pH on amine and amide CEST signal: (A) Schematic diagram of relevant solute proton pools that resonate at frequencies chemically shifted by parts per million (p.p.m.) from bulk water. (B) Irradiation of amide (top) or amine (bottom) protons using a frequency-selective saturation pulse. Chemical exchange transfers amide (top) or amine (bottom) magnetization to bulk water, resulting in indirect saturation of the bulk water (). (C) CEST spectra (vertically offset for clarity) measured at different pH (6.0, 6.5, and 7.0). The red and green portions of the curve illustrate the pH-dependent response of the amide and amine protons, respectively.

Amine and amide chemical exchange saturation transfer (CEST) dependence on pH, protein concentration, and temperature: (A) CEST spectra of 10% (by weight) bovine serum albumin (BSA) dissolved in phosphate-buffered saline (PBS) with varying pH at 37°C. (B) magnified. (C) Amine and amide CEST values measured directly from (A) using equation . (D) Amine/amide concentration-independent detection (AACID) values measured directly from (A) using equation . (E) AACID values measured in solutions containing a range of BSA concentrations at 37°C using a 4-second, 1.5-μT saturation pulse. (F) AACID values measured in a pH 7.0 10% BSA solution for a range of physiologically relevant temperatures using a 4-second, 1.5-μT saturation pulse.

Amine and amide chemical exchange saturation transfer (CEST) dependence on saturation pulse amplitude, duration, and metabolite contributions: amine and amide CEST effects from 10% (by weight) bovine serum albumin (BSA) dissolved in pH 7.0 phosphate-buffered saline (PBS) at 37°C measured following (A): a 4-second continuous wave RF saturation pulse with amplitudes ranging from 0 to 3 μT and (B) a 1.5-μT continuous wave RF saturation pulse with duration ranging from 0 to 6 seconds. CEST values measured at (C) 2.75 p.p.m. and (D) 3.50 p.p.m. for major brain metabolites at physiologic concentrations (37°C using a 4-second, 1.5-μT continuous wave RF saturation pulse) compared with 10% BSA.

Cerebral ischemia: representative magnetic resonance (MR) images of a mouse brain 2 hours (A–E) and 5 hours (F–J) after permanent middle cerebral artery occlusion (MCAO) in radiologic orientation. (A, F) T₁-weighted image, (B, G) T₂-weighted image, (C, H) diffusion-weighted image, (D, I) pH map with ischemic (left) and contralateral (right) regions of interest (ROIs) drawn, and (E) chemical exchange saturation transfer (CEST) spectra measured from ischemic (red) and contralateral (blue) ROIs at 2 hours, (E) histology 2,3,5-triphenyltetrazolium chloride (TTC) stained for cell damage.

Numerical simulations of amine/amide concentration-independent detection (AACID) dependence on pH, macromolecule concentration, and relaxation time constants in brain: (A) Simulated chemical exchange saturation transfer (CEST) spectra for varying pH in normal brain tissue. (B) magnified to highlight the amine and amide pH-dependent CEST effects. Note: M_Z and M₀ represent the region of interest magnetic resonance imaging (MRI) signal acquired with and without a saturation pulse, respectively. (C) Amine and amide CEST values measured directly from (A) using equation . (D) AACID values measured directly from (A) using equation . (E) AACID–pH relationships simulated for a range of macromolecular concentrations (% by weight) using a 4-second, 1.5-μT saturation pulse. (F) AACID–pH relationships simulated for normal and ischemic brain tissue using a 4-second, 1.5-μT saturation pulse.