Quantification of changes in myocardial T1 values with exercise cardiac MRI using a free‐breathing non‐electrocardiograph radial imaging

Purpose To develop and evaluate a free breathing non‐electrocardiograph (ECG) myocardial T1* mapping sequence using radial imaging to quantify the changes in myocardial T1* between rest and exercise (T1*reactivity) in exercise cardiac MRI (Ex‐CMR). Methods A free‐running T1* sequence was developed using a saturation pulse followed by three Look‐Locker inversion‐recovery experiments. Each Look‐Locker continuously acquired data as radial trajectory using a low flip‐angle spoiled gradient‐echo readout. Self‐navigation was performed with a temporal resolution of ∼100 ms for retrospectively extracting respiratory motion. The mid‐diastole phase for every cardiac cycle was retrospectively detected on the recorded electrocardiogram signal using an empirical model. Multiple measurements were performed to obtain mean value to reduce effects from the free‐breathing acquisition. Finally, data acquired at both mid‐diastole and end‐expiration are picked and reconstructed by a low‐rank plus sparsity constraint algorithm. The performance of this sequence was evaluated by simulations, phantoms, and in vivo studies at rest and after physiological exercise. Results Numerical simulation demonstrated that changes in T1* are related to the changes in T1; however, other factors such as breathing motion could influence T1* measurements. Phantom T1* values measured using free‐running T1* mapping sequence had good correlation with spin‐echo T1 values and was insensitive to heart rate. In the Ex‐CMR study, the measured T1* reactivity was 10% immediately after exercise and declined over time. Conclusion The free‐running T1* mapping sequence allows free‐breathing non‐ECG quantification of changes in myocardial T1* with physiological exercise. Although, absolute myocardial T1* value is sensitive to various confounders such as B1 and B0 inhomogeneity, quantification of its change may be useful in revealing myocardial tissue properties with exercise.