How to Create a Heart Model in Materialise Mimics for 3D Printing

Preparation:

1. Obtain high-quality imaging, ideally high-resolution isotropic or nearly isotropic voxels, to reduce segmentation time.

Initial segmentation and thresholding:

1. Navigate to the Segment tab and open the New Mask tool.
2. Adjust the lower threshold to capture the blood pool in the left heart.
3. Scroll through images to verify that the threshold captures the pool accurately. If you narrow the range to remove false connections, you risk losing too much of the other blood pool voxels. Sometimes, it's necessary to accept a few false connections and manually touch them up in the later editing step.
4. Enable Mask 3D preview by clicking the green cube icon on the right, and check the box next to Limit number of shells to remove floating voxels.

Splitting mask regions:

1. Open the Split Mask tool in the Segment tab to separate cardiac structures from undesired anatomy, like the spine or sternum.
2. Indicate regions on 2D slices or the 3D view.
3. Rename the regions to Left heart, Right heart, and Other.

Refining the left heart mask:

1. Open the Edit Masks tool and use the Lasso option to remove extraneous vasculature or connections.
2. Orient the model to ensure important structures are not behind the selection.
3. Remove connections to the main model, but don't worry about erasing every little piece of anatomy.
4. Open the Region Grow tool in the Segment tab, set the target to the Left Heart, and click on the part you wish to keep to remove all other regions.
5. Click the Calculate Part tool, represented by a cube icon with a cross in a circle, to generate an STL for the finished left heart.

Right heart segmentation using local threshold:

1. Open the Local Threshold tool in the Segment tab to enhance the low-contrast right heart segmentation.
2. Adjust the lower threshold to better capture the Hounsfield unit range of the right heart and set the Search area to two pixels.

Manual editing with multiple slice edit:

1. Open the Multiple Slice Edit tool from the Segment tab.
2. Mark the start of the region you want to add, skip several slices, then mark the region again using the Threshold operation with Auto interpolate enabled.
3. Use the Remove operation tool in addition to disconnect any false connections between structures.
4. Repeat the Edit Masks and Region Grow steps for the right heart before clicking Calculate Part.
5. Rename your two STLs to Right Heart Blood Pool and Left Heart Blood Pool.
6. Make sure the contours of the two parts are toggled on and scroll through the images to check that the 3D models have no errors.

Optimizing and wrapping in 3-matic:

1. Export the blood pool models to 3-matic for post-processing.
2. Open the Wrap tool to create a watertight surface and remove internal cavities.
3. Set the Smallest detail to 0.5 mm to match the pixel size of the scan and keep the Gap closing distance at zero.

Smoothing and hollowing:

1. Select the Smooth tool from the Fix menu and set the Smoothing factor to 0.7.
2. Right-click the wrapped models to Duplicate them, then set the transparency of the duplicates to help visualize the results.
3. Open the Hollow tool from the Design menu, set the type to Outside, and specify a thickness based on your printing technology.

Trimming vessel inlets and outlets:

1. Right-click the left heart model and click Show Only.
2. Open the Trim tool from the Finish menu and set the method to Remove inner.
3. Left-click points to define the trim area, and double-click the final point to apply the trim.
4. Rotate the model and continue this method to trim all the inlets and outlets.
5. Repeat this process for the right heart model.

Isolating accurate blood lumen surfaces:

1. Change the Part color of each side of the part to better visualize overlapping surfaces. 
2. Right-click the models in the Object Tree to duplicate them.
3. Rotate the parts and click to select only the inner blood lumen surfaces.
4. Right-click and select Separate to move these surfaces into new parts.
5. Rename these parts to RH Blood Lumen and LH Blood Lumen.

Final union and mold cleaning:

1. Use the Boolean Union tool in the Design tab to unite the two entities.
2. Show only the union result, click on any inaccurate blood lumen surfaces to select them, and click the Delete button on your keyboard.
3. Repeat for the other side of the heart.
4. To remove floating fragments, use the Shell tool in the Mark menu to select the model you want to keep, then click Invert and press the Delete key.
5. Merge the isolated blood lumen parts and the union result into one final hollowed model by right-clicking on the models in the Object Tree.

Adding design features:

1. Use the Trim tool at the apex of the ventricles to create a viewing window.
2. Turn on Transparency to help you determine exactly where your trimming path should be.
3. Use the Shell tool, invert your selection, and then press the Delete key to get rid of any floating shells.
4. Open the Quick Label tool in the Finish menu, enter the text, and orient your model to find a relatively flat surface.
5. Click to Emboss or click and hold the Control key to Deboss the label.

Error fixing:

1. Open the Fix Wizard from the Fix menu, select Full Analysis, and click Follow Advice to repair the STL.
2.  Revert the visualization to a single color by un-checking Use part color in the Surface list.

Final verification and export:

1. Highlight your part in the Object Tree, copy it (Control+C), and paste it back into Mimics Core (Control+V).
2. Scroll through the DICOM images to ensure the 3D contours are anatomically accurate.
3. Export the finalized model in the File menu as an STL for 3D printing.