Method for determining in vivo concentration of a metabolite

1. A method for determining the in vivo concentration of a metabolite, comprising:

administering a contrast agent to a subject;

allowing the contrast agent to disperse to tissue of interest;

performing chemical exchange dependent saturation transfer MRI analysis of the subject

comparing results of the MRI analysis to known in vitro results to determine metabolite concentration.

2. The method according to claim 1 where the contrast agent is 5,6-dihydrouracil.

3. The method according to claim 2 where the metabolite is phosphate.

4. The method according to claim 2 and further comprising irradiating at 2.67 ppm and 5.00 ppm on both sides of a water peak and then determining a ratio of (M O −M S ) Site 1 /(M O −M S ) Site 2 , where site 1 resonates at 5.00 ppm, and site 2 resonates at 2.67 ppm and M S represents a magnitude of a water proton signal in presence of contrast agent proton saturation and M O represents a magnitude of a signal under control irradiation at an opposite frequency offset.

5. The method according to claim 1 wherein the contrast agent is administered as an aqueous composition comprising an aqueous mixture of at least one contrast agent in amounts sufficient to perform chemical exchange dependent saturation transfer MRI analysis, the contrast agent being selected from the group consisting of sugars and oligomers and polymers thereof, amino acids and oligomers and polymers thereof, nitrogen-containing heterocycles, nucleosides, imidazole and derivatives thereof, imino acids and analogs thereof, barbituric acid and analogs thereof, guanidine, hydantoin, parabanic acid, biologically active salts of the contrast agents, and mixtures of contrast agents.

6. The method of claim 5 where the composition comprises from about 0.25 mM to 250 mM contrast agent.

7. The method of claim 1 , where performing chemical exchange dependent saturation transfer MRI analysis comprises:

selectively irradiating and saturating an exchangeable proton or protons of the contrast agent with an applied magnetic field;

applying a selective irradiation with an equal but opposite chemical shift difference between the contrast agent proton exchange site and a water proton resonance frequency ()T CA ), thereby providing a first image set with the irradiation ±chemical shift difference between the contrast agent proton exchange site and the water proton resonance frequency (±)T CA );

producing a second image set using the first image set.

8. The method of claim 1 where the contrast agent is 5-hydroxy-tryptophan.

9. The method of claim 1 where the contrast agent comprises a tryptophan.

10. A method for determining in vivo phosphate concentration, comprising:

administering 5,6-dihydrouracil to a subject;

allowing the contrast agent to disperse to tissue of interest;

performing chemical exchange dependent saturation transfer MRI analysis of the subject by irradiating at 2.67 ppm and 5.00 ppm on both sides of a water peak;

determining a ratio of (M O −M S ) Site 1 /(M O −M S ) Site 2 where site 1 resonates at 5.00 ppm and site 2 resonates at 2.67 ppm, where M S represents a magnitude of a water proton signal in presence of contrast agent proton saturation and M O represents a magnitude of a signal under control irradiation at an opposite frequency offset; and

comparing the determined ratio to known in vitro ratios to determine phosphate concentration.