High resolution imaging of the mitral valve in the natural state with 7 Tesla MRI

<div><p>Imaging techniques of the mitral valve have improved tremendously during the last decade, but challenges persist. The delicate changes in annulus shape and papillary muscle position throughout the cardiac cycle have significant impact on the stress distribution in the leaflets and chords, thus preservation of anatomically accurate positioning is critical. The aim of this study was to develop an <i>in vitro</i> method and apparatus for obtaining high-resolution 3D MRI images of porcine mitral valves in both the diastolic and systolic configurations with physiologically appropriate annular shape, papillary muscle positions and orientations, specific to the heart from which the valve was harvested. Positioning and mounting was achieved through novel, customized mounting hardware consisting of papillary muscle and annulus holders with geometries determined via pre-mortem ultrasonic intra-valve measurements. A semi-automatic process was developed and employed to tailor Computer Aided Design models of the holders used to mount the valve. All valve mounting hardware was 3D printed using a stereolithographic printer, and the material of all fasteners used were brass for MRI compatibility. The mounted valves were placed within a clear acrylic case, capable of holding a zero-pressure and pressurized liquid bath of a MRI-compatible fluid. Obtaining images from the valve submerged in liquid fluid mimics the natural environment surrounding the valve, avoiding artefacts due to tissue surface tension mismatch and gravitational impact on tissue shape when not neutrally buoyant. Fluid pressure was supplied by reservoirs held at differing elevations and monitored and controlled to within ±1mmHg to ensure that the valves remained steady. The valves were scanned in a 7 Tesla MRI system providing a voxel resolution of at least 80μm. The systematic approach produced 3D datasets of high quality which, when combined with physiologically accurate positioning by the apparatus, can serve as an important input for validated computational models.</p></div

Simplified exploded view of apparatus (left) and assembled apparatus (right).

<p>Simplified exploded view of apparatus (left) and assembled apparatus (right).</p

Annulus clamp with mounted valve viewed from top (right) and side view (left) showing papillary muscles.

<p>This valve is mounted without the PM holder tips for illustrative purposes.</p

Explanting the mitral valve.

<p>AL: Anterior Leaflet, PPM: Posterior Papillary Muscle, APM: Anterior Papillary Muscle.</p

Pressure control system and valve pressurized in chamber.

<p>A) Control system as outlined in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184042#pone.0184042.g011" target="_blank">Fig 11</a>, valve pressurized. B) Ventricular view of pressurized valve showing papillary muscle mounting. CT: Chordae Tendineae; APM: Anterior Papillary Muscle; PPM: Posterior Papillary Muscle C) Atrial view of pressurized valve showing a physiological leaflet coaptation pattern.</p

Adjustment points for annulus clamp customization.

<p>Adjustment points for annulus clamp customization.</p

Minimum required mitral valve measurements for hardware customization.

<p>Red dots denote the ultrasound identifiable points, and the lines denote the simple 3D distance between those points.</p

Papillary muscle holder A) Adjustment points and angles B) Calculation vector definitions: B: Mid-commissure to PM holder base, A: PM holder base to stitching ring, P: Commissure to PM tip position, A_f: Fixed region, A_adj: Adjustable region, A_ang: Angled region.

<p>Papillary muscle holder A) Adjustment points and angles B) Calculation vector definitions: B: Mid-commissure to PM holder base, A: PM holder base to stitching ring, P: Commissure to PM tip position, A_f: Fixed region, A_adj: Adjustable region, A_ang: Angled region.</p

Papillary muscle holder A) Angular displacements and B) Cartesian coordinates.

<p>Papillary muscle holder A) Angular displacements and B) Cartesian coordinates.</p

Mounting the mitral valve in the annulus clamp device.

<p>For demonstration purposes, the tissue guiding suture is shown without the annulus being wrapped in Dacron.</p