Abstract
PurposePhosphorus spectroscopy (31P‐MRS) is a proven method to probe cardiac energetics. Studies typically report the phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio. We focus on another 31P signal: inorganic phosphate (Pi), whose chemical shift allows computation of myocardial pH, with Pi/PCr providing additional insight into cardiac energetics. Pi is often obscured by signals from blood 2,3‐diphosphoglycerate (2,3‐DPG). We introduce a method to quantify Pi in 14 min without hindrance from 2,3‐DPG.MethodsUsing a 31P stimulated echo acquisition mode (STEAM) sequence at 7 Tesla that inherently suppresses signal from 2,3‐DPG, the Pi peak was cleanly resolved. Resting state UTE‐chemical shift imaging (PCr/ATP) and STEAM 31P‐MRS (Pi/PCr, pH) were undertaken in 23 healthy controls; pH and Pi/PCr were subsequently recorded during dobutamine infusion.ResultsWe achieved a clean Pi signal both at rest and stress with good 2,3‐DPG suppression. Repeatability coefficient (8 subjects) for Pi/PCr was 0.036 and 0.12 for pH. We report myocardial Pi/PCr and pH at rest and during catecholamine stress in healthy controls. Pi/PCr was maintained during stress (0.098 ± 0.031 [rest] vs. 0.098 ± 0.031 [stress] P = .95); similarly, pH did not change (7.09 ± 0.07 [rest] vs. 7.08 ± 0.11 [stress] P = .81). Feasibility for patient studies was subsequently successfully demonstrated in a patient with cardiomyopathy.ConclusionWe introduced a method that can resolve Pi using 7 Tesla STEAM 31P‐MRS. We demonstrate the stability of Pi/PCr and myocardial pH in volunteers at rest and during catecholamine stress. This protocol is feasible in patients and potentially of use for studying pathological myocardial energetics.
Highlights
The beating heart is powered by chemical potential energy (ΔGATP) released during the hydrolysis of adenosine triphosphate (ATP) into the products adenosine diphosphate (ADP) and inorganic phosphate (Pi)
Maintenance of [ATP] local to the myofibril is achieved by rapid phosphotransfer from a reservoir of phosphocreatine (PCr) to ADP, catalyzed by the enzyme creatine kinase (CK).[2]
SNR, primarily due to low concentrations and the heart’s unfavourable geometry, limited our ability to measure Pi in 100% of scans, we report a technique whose repeatability competes with the slower chemical shift imaging (CSI) approach to Pi estimation[32] and the computation of PCr/
Summary
Studies typically report the phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio. We focus on another 31P signal: inorganic phosphate (Pi), whose chemical shift allows computation of myocardial pH, with Pi/PCr providing additional insight into cardiac energetics. Methods: Using a 31P stimulated echo acquisition mode (STEAM) sequence at 7 Tesla that inherently suppresses signal from 2,3-DPG, the Pi peak was cleanly resolved. Resting state UTE-chemical shift imaging (PCr/ATP) and STEAM 31P-MRS (Pi/PCr, pH) were undertaken in 23 healthy controls; pH and Pi/PCr were subsequently recorded during dobutamine infusion. Results: We achieved a clean Pi signal both at rest and stress with good 2,3-DPG suppression. We report myocardial Pi/PCr and pH at rest and during catecholamine stress in healthy controls. Rodgers and Albrecht Ingo Schmid contributed to this work
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