R1ρ sensitivity to pH and other compounds at clinically accessible spin‐lock fields in the presence of proteins. (19th November 2019)

• Record Type: Journal Article
• Authors: Owusu, Nana; Johnson, Casey P.; Kearney, William; Thedens, Dan; Wemmie, John; Magnotta, Vincent A.
• Is Part Of: NMR in biomedicine. Volume 33:Number 2(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2019-11-19
• Subjects: glucose -- lysine -- multiparametric imaging -- pH -- physical phantom -- R1ρ dispersion -- temperature; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.4217
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : Numerous human diseases involve abnormal metabolism, and proton exchange is an effective source of magnetic resonance imaging (MRI) contrast for assessing metabolism. One MRI technique that capitalizes on proton exchange is R1 relaxation in the rotating frame (R1ρ ). Here, we investigated the sensitivity of R1ρ to various proton‐exchange mechanisms at spin‐lock pulses within Food and Drug Administration (FDA) safety guidelines for radiofrequency‐induced heating. We systematically varied pH known to change the rate of proton exchange as well as the glucose and lysine concentrations, thus changing the number of amide, hydroxyl and amine exchangeable sites in a series of egg‐white albumin phantoms. The resulting effects on quantitative relaxation time measurements of R1ρ, R1 and R2 were observed at 3 T. Using spin‐lock amplitudes available for human imaging (less than 23.5 μT) at near physiologic temperatures, we found R1ρ was more sensitive to physiologic changes in pH than to changes in glucose and lysine concentrations. In addition, R1ρ was more sensitive to pH changes than R1 and R2 . Models of proton exchange fitted to the relaxation measurements suggest that amide groups were the primary source of pH sensitivity. Together, these experiments suggest an optimal spin‐lock amplitude for measuring pH changes while not exceeding FDA‐subject heating limitations. Abstract : R1ρ sensitivity to different exchange rates at clinically accessible (B ≤ 3 T) field strengths is not well characterized and this work is to address that question for 3 T. Multiparametric mapping of three phantoms with gradations of metabolite concentrations and pH values shows R1ρ sensitivity to physiologic pH range in protein solutions at 3 T. Modeling of proton exchange with relaxation measures suggests amide proton exchange of pH sensitivity.