Cardiorespiratory-resolved magnetic resonance imaging: measuring respiratory modulation of cardiac function

A technique for cardiac- and respiratory-resolved MRI is described. A retrospectively gated-segmented acquisition scheme similar to that used in conventional cine cardiac imaging was used to collect image data that spanned both the cardiac and respiratory cycles. Raw k-space data were regridded in a cardiorespiratory phase space to allow image reconstruction at target cardiac and respiratory phases. The approach can be applied with various k-space trajectories and pulse sequences, and was implemented in this study with both a Cartesian steady-state free precession (SSFP) sequence and a radial phase-contrast (PC) pulse sequence. Free-breathing short-axis SSFP images of the heart were reconstructed at multiple respiratory and cardiac phases to illustrate separation of cardiac and respiratory motion without artifacts. A respiratory-resolved radial PC experiment was used to quantify the volumetric flow rates in the inferior vena cava (IVC), pulmonary artery (PA), and aorta (Ao) in five free-breathing normal volunteers and a positive-pressure ventilated dog. Total flow (ml/min) in each vessel was quantified as a function of respiratory phase (peak/minimum output = 1.85 +/- 0.29 (IVC), 1.36 +/- 0.15 (PA), 1.24 +/- 0.09 (Ao)). Peak flow occurred during inspiration for the IVC and PA, and during expiration for the Ao, and there was a complete pattern reversal for the positive-pressure ventilated dog.