BACKGROUND: Cardiac magnetic resonance (CMR) T1 mapping is an emerging tool for 
objective quantification of myocardial fibrosis.
OBJECTIVES: To (a) establish the feasibility of left atrial (LA) T1 
measurements, (b) determine the range of LA T1 values in patients with atrial 
fibrillation (AF) vs healthy volunteers, and (c) validate T1 mapping vs LA 
intracardiac electrogram voltage amplitude measures.
METHODS: CMR imaging at 1.5 T was performed in 51 consecutive patients before AF 
ablation and in 16 healthy volunteers. T1 measurements were obtained from the 
posterior LA myocardium by using the modified Look-Locker inversion-recovery 
sequence. Given the established association of reduced electrogram amplitude 
with fibrosis, intracardiac point-by-point bipolar LA voltage measures were 
recorded for the validation of T1 measurements.
RESULTS: The median LA T1 relaxation time was shorter in patients with AF (387 
[interquartile range 364-428] ms) compared to healthy volunteers (459 
[interquartile range 418-532] ms; P < .001) and was shorter in patients with AF 
with prior ablation compared to patients without prior ablation (P = .035). In a 
generalized estimating equations model, adjusting for data clusters per 
participant, age, rhythm during CMR, prior ablation, AF type, hypertension, and 
diabetes, each 100-ms increase in T1 relaxation time was associated with 0.1 mV 
increase in intracardiac bipolar LA voltage (P = .025).
CONCLUSIONS: Measurement of the LA myocardium T1 relaxation time is feasible and 
strongly associated with invasive voltage measures. This methodology may improve 
the quantification of fibrotic changes in thin-walled myocardial tissues.