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Adaptive Trigger Delay Using a Predictive Model Applied to Black Blood Fast Spin Echo Cardiac Imaging in Systole

Introduction: In clinical applications, cardiac gated sequences are commonly used for heart imaging. In practice, we simply wait a fixed time called Trigger Delay (TD) after the R-wave has been detected to acquire data in the chosen cardiac phase. The main problem of this method is that the TD is constant and does not take into account the physiological variability such as instantaneous heart rate changes during breath-hold or free breathing. Another issue is that we cannot use a TD shorter than the preparation time. Consequently when a cardiac gated sequence with a long preparation time is used, only diastolic images are achievable. Double Inversion Recovery Fast Spin Echo (DIR-FSE), resulting in black-blood images of the heart [1], is such a sequence since the inversion time (TI) needed to cancel the blood signal is around 500ms. Systolic view of the heart could also be of clinical interest, especially during end-systolic phase when the cardiac volume is minimum and constant (average TD = 300ms, average duration 60ms [2]). The primary aim of this work was to assess a robust method to acquire black blood FSE in end-systolic phase. For this purpose, the DIR preparation has to be launched before the R-wave in the previous cardiac cycle. A RR interval prediction is then needed and heart rate variability has to be accounted in order to position the acquisition window properly. A new general adaptive method is described here that overcomes the above listed limitations in prospectively cardiac gated sequences with long preparation time using a predictive model. This method has been applied on end-systolic black blood FSE images. Materials and Methods: Prediction: To launch the DIR preparation before the R-wave, the method described in [3] has been used. A Kalman filter [4] is implemented to make a one-step prediction of the R-R interval duration ( RR T Δ ). Since the instantaneous heart rate is known to change with respiration [2], several inputs are used: the past ) 2 (  ), 1 ( − Δ − Δ n T n T RR RR , a respiratory belt and its derivative sampled at R-wave