Coarctation of the aorta is a congenital narrowing of the aorta diameter, which is commonly found just distal to the origin of the left subclavian artery. It results in upper-extremity hypertension, left ventricular hypertrophy, and underperfusion of the abdominal organs and lower extremities. If the narrowing causes peak-to-peak systolic pressure gradient of more than 20 mmHg surgical or transcatheter interventions are required. The pressure gradient is usually measured with a catheter. Recent publications have however shown the potential of pressure difference maps derived from 4D PC MRI to provide similar information non-invasively [1,2].

The purpose of this work was to verify how the visualization of pressure difference maps derived from 4D PC MRI can be used in intervention planning. Consequently, we tested a dedicated visualization and exploration workflow with two pediatric cardiologists.

3D whole heart (voxel size 1.4x2.0x1.4 mm3) and 4D PC MRI (voxel size 1.4x1.4x2.3 mm3, temporal resolution 40 ms, and venc = 300 cm/s) of 5 patients were acquired on a 1.5-T Philips Achieva (5-element torso coil) with prospective ECG-gating (Heart Center (DHZB) Berlin, Germany) before treatment of coarctation of the aorta. During catheter intervention, peak-to-peak pressure between ascending and descending aorta was measured. Semi-automatic segmentation of the aorta was performed retrospectively on the 3D whole heart images using a watershed transformation with manual correction tools. After the routine pre-processing of the PC MRI data including phase-offset error correction and antialiasing, the extracted aortic anatomy was fused with the flow data. Relative pressure maps were then computed using the Navier-Stokes equations as described previously [3]. In order to enable the use of pressure maps for intervention planning, visualization with a maximum value projection was implemented [4]. A reference pressure value is set on the centerline at the user-defined stenosis location. All the other pressure values are then reported as relative pressure differences to the reference value. The visualization combines minimum and a maximum intensity projection in order to display both, negative and positive extrema. Along the viewing direction the positive or negative intensity with the maximum absolute value is shown. Figure 1 depicts how the high pressure values (red) relative to the stenosis as well as the low pressure values (blue) behind the narrowing are highlighted through this technique. Furthermore, the pressure along the centerline is displayed in a diagram.Two cardiologists inspected the information about the performed interventions, namely angiographic image data, type of intervention, and pressure catheter measurements. In the provided web software they then explored the vessel anatomy and calculated pressure maps derived from the MRI measurements. First, they marked the treated vessel section as displayed in Figure 1. Then, the pressure map and corresponding curves showing the mean pressure along the vessel centerline were interactively explored. The experts stated whether the pressure maps provided additional information to the angiographic images and catheter measurements. If this was the case, they were asked, if the additional knowledge would have influenced their treatment strategy. The overall pressure gradient between ascending and descending aorta measured with the catheter during intervention and the systolic pressure difference along the centerline in the pressure map showed a correlation of 0.94. The experts rated the provided pressure visualization and exploration as intuitive and helpful. In two cases, the location of the pressure drop in the pressure difference map differed from the treated vessel segment. Both experts stated that this knowledge would have influenced the treatment decision.We successfully applied a processing and visualization pipeline for pressure difference maps to MRI image data of patients with aortic coarctation. The good agreement of the pressure difference values derived from 4D PC MRI with invasive pressure catheter measurements demonstrates the potential application of MRI-based pressure measurements for non-invasive quantitative assessment of coarctation of the aorta. Furthermore, our user study indicates that the exploration of the pressure difference maps can provide additional helpful information for treatment planning in patients with coarctation of the aorta. Further clinical studies will examine the potential benefit of these tools in depth.