Abstract
A method to estimate phosphorus (31P) transversal relaxation times (T2) of coupled spin systems is demonstrated. Additionally, intracellular and extracellular pH (pHext, pHint) and relaxation corrected metabolite concentrations are reported. Echo time (TE) series of 31P metabolite spectra were acquired using STEAM localization. Spectra were fitted using LCModel with accurately modeled Vespa basis sets accounting for J-evolution of the coupled spin systems. T2s were estimated by fitting a single exponential two-parameter model across the TE series. Fitted inorganic phosphate frequencies were used to calculate pH, and relaxation times were used to determine the brain metabolite concentrations. The method was demonstrated in the healthy human brain at a field strength of 9.4T. T2 relaxation times of ATP and NAD are the shortest between 8 ms and 20 ms, followed by T2s of inorganic phosphate between 25 ms and 50 ms, and PCr with a T2 of 100 ms. Phosphomonoesters and –diesters have the longest T2s of about 130 ms. Measured T2s are comparable to literature values and fit in a decreasing trend with increasing field strengths. Calculated pHs and metabolite concentrations are also comparable to literature values.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Grant sponsors: This project was co-sponsored by SYNAPLAST Grant number: 679927 (J. Dorst, L. Ruhm, A. Henning), Horizon 2020/CDS-QUAMRI, Grant number 634541 (T. Borbath, A. Henning), and Cancer Prevention and Research Institute of Texas (CPRIT) Grant number: RR180056 (A. Henning)
Abbreviations used
- δ
- chemical shift
- 2,3-DPG
- 2,3-diphosphoglycerate
- ATP
- adenosine triphosphate
- COG
- cogwheel phase cycling
- CSA
- chemical shift anisotropy
- CSDE
- chemical shift displacement error
- FID
- free induction decay
- FLASH
- fast low angle shot
- FWHM
- full-width half maximum
- GPC
- glycerophosphocholine
- GPE
- glycerophosphoethanolamine
- MRS
- magnetic resonance spectroscopy
- NADH
- nicotinamide adenine dinucleotide
- NAD+
- NAD oxidized
- PC
- phosphocholine
- PCr
- phosphocreatine
- PE
- phosphoethanolamine
- pHext
- extracellular pH
- pHint
- intracellular pH
- Piext
- extracellular inorganic phosphate
- Piint
- intracellular inorganic phosphate
- R2
- coefficient of determination
- SAR
- specific absorption rate
- SNR
- signal to noise ratio
- STEAM
- stimulated echo acquisition mode
- T1
- longitudinal relaxation time
- T2
- transversal relaxation time
- TE
- echo time
- TM
- mixing time
- tNAD
- total NAD
- TR
- repetition time
- UDPG
- uridine diphosphoglucose
- UHF
- ultrahigh-field