Table 1.

Abbreviations and symbols

Abbreviation	Actual Physical Meaning
MST	Magnetization saturation transfer
MRS	Magnetic resonance spectroscopy
CK	Creatine kinase
d1	Presaturation delay employed in the MST experiment
d2	Duration of saturation pulse in the MST experiment
FID	Free induction decay
kf	Pseudo-first-order rate constant to characterize the ATP production rate via CK
M0	Magnetization measured by 31P-magnetic resonance spectroscopy under fully relaxed condition
Mshort	Magnetization measured with very short repetition time. This measurement, in combination with M0, can yield an approximate value of T1int, according to Eq. 14
Mss	Steady-state magnetization of a spin (e.g., PCr) when the counterpart spin (e.g., ATP-γ) is continuously saturated, as dictated by Eq. 1
MTR,PCr	Magnetization measured with ATP-γ continuously saturated. The TR can be arbitrary. This, in combination with Mss,PCr, can yield accurate value of T1,PCrapp, according to Eq. 10
nt	Number of averaged FIDs for one spectrum
T1app	Apparent longitudinal relaxation time constant of a spin (e.g., PCr) when the counterpart spin (e.g., ATP-γ) is continuously saturated, as dictated by Eq. 1
T1mix	Apparent longitudinal relaxation time constant when chemical exchange is involved. T1mix is an approximation, because the actual recovery of spin magnetization under chemical exchange is not an exact exponential curve
TR	Repetition time of pulse sequence in MST experiment