Voxel Printing Anatomy: Design and Fabrication of Realistic, Presurgical Planning Models through Bitmap Printing

Bitmap printing allows for the direct fabrication of 3D printed models for medical images. This method allows the soft tissue, which contains gradually varying material properties to be 3D printed in high resolution. The main advantage of this technique is the ability to replicate the soft tissue.

This provides surgeons with a greater spatial understanding of variation in the soft tissue in complex surgeries. When trying this method for the first time, it is recommended to work closely with the radiologist to ensure the imaging is adequate for 3D. Demonstrating the procedure will be Jane Brusilovsky, a graduate student researcher from my laboratory.

Demonstrating the printing will be Lawrence Smith, a PhD candidate from the Matter Assembly Computation Lab, CU Boulder Begin data input by opening the medical image computing software. From the dropdown menu, click the file button and DICOM. When the DICOM browser window opens select import and wait for the import DICOM files from directory popup window to appear.

Then navigate to the DICOM file stack and click Import. To activate additional metadata, select the advanced check box. Then choose the desired series number and click the Examine button.

Upon ensuring that the desired sequence is not displaying warnings, Click the check box next to the desired DICOM data file and load. Once the sequence is loaded into the medical image computing software, explore the module’s drop down menu and select Volume Rendering module. In the module, select the name of the sequence from the volume dropdown menu to activate the image stack and translate the data into a voxelized volume.

Ensure that the active module’s name matches the desired sequence selected earlier. Next to the volume dropdown, click the eyeball icon to visualize the selected volume in 3D. Ensure that the 3D display window is open and a gray scale 3D representation is visible.

Next, click the arrow next to the advanced tab to open the advanced tools and select the volume property tab to open set of controls for modifying the color channel of the voxel model. Navigate to the scaler opacity mapping menu and left click in the field to create points where the intensity values will be defined by the opacity. Then place points along the scale to visualize the anatomy of interest, navigate to the scaler color mapping menu, then left click in the field to create points and specify colors, correlated to intensity values.

Double click in the field to open a select color window to modify color in information. Expand the modules tab to select the Segment Editor, under the segment editor toolbars, go to the segmentation column to select Create New Segmentation as, then type a custom name for the segmentation from the rename segmentation popup window and hit OK.From the master volume dropdown, opt for the active volume, having the same name as the volume rendering. Next, click the Add button directly below the dropdown.

The segment container will be created in the field below. Navigate to the effects tool panel, and select the Scissors tool. In the scissors menu, Choose, fill inside, free-form and unlimited tabs in order.

Next, hover over the 3D window and right click and hold while drawing around the area to be erased. Once a colored swatch appears, showing what area has been covered, repeat the process until all areas to be deleted are covered. In the effects menu, go to the mask volume and select inside to delete all the image data covered by the segment.

Next, modify the fill value to negative 1000. Then hit apply and click the eyeball next to the output volume to show the new mask volume. Expand the modules tab to select Volume Rendering and click the eyeball to turn off the visualization.

From the dropdown menu, select the newly created masked volume, then click eyeball to activate the volume. When done, navigate to the inputs menu and open the properties drop down menu. Select the volume property created before.

Observe that the volume in the 3D view is masked and color encoded. Select SlicerFAB from the module drop down. Under the print parameters, confirm the X resolution is 600 DPI, the Y resolution is 300 DPI and the layer thickness is set to 27 micrometers.

Next, open the output parameters menu to modify the scale of the final model as needed. Then select a file location for the slices to be saved and hit Generate. For dithering, open the image editing software and click the File and Open tabs.

Then navigate to the first image of the PNG file stack could created in the previous step and click the Open button. Go to the window button to select Actions and new action and enter a custom name before pressing OK.The record button should remain active and read to record action. Once the image has loaded, navigate to Image, Mode and Indexed Color.

In the index window, select from the dropdown menu Local Perceptual and specify the number of colors to be 8. In the forced menu, select Custom, then click the first two squares to open the custom color window to select the cut color palette as 100%magenta and ensure C, Y and K are set to zero. From the options dropdown menu, select Matte and Custom tabs.

For Dither, select Diffusion and for amount select 100%before clicking OK.To stop the recording, navigate to the action menu and click the square button, close the active window and click no in the save changes popup window. When done go to File, Automate and expand the batch window to select the action created in the previous step by click the action tab. Under the source menu, click the Choose button to go to the folder of the exported images.

By selecting destination and choose, select a destination folder location for the new files, then click OK.In the print software, click Apps and Launch Voxel Print Utility. Then enter the prefix of the PNG file stack in the slice files prefix text box, followed by clicking the Select button to navigate to the folder where the PNG file stack is located. When done, hit OK.After confirming on the slice range, that the first slice and number of slices match the number of files in the created folder, check the slicing parameters such as the sliced thickness matches the settings specified before and slice width and height match the width and height of PNG files.

Under the background color, ensure the background matches the background color and set not to print. Once completed, click the Next button. On the tools page, on the material mapping, select the material from the dropdown menu to be mapped to the associated color derived from the PNG files.

Repeat the process for each color in the menu, then click Finish and OK.On the popup window info GCVF creation succeeded. Process the required files through the 3D printers software, then review the files for printability, before sending them to be fabricated by the 3D printer. The 3D printer fabricates a model layer by layer until the object is filled in.

When the printer completes the printing, remove the part from the print bed with a spatula. As the 3D printer object comes off the printer with a layer of support material, remove support material with a high powered water jet, until all the support is removed. When done, sand and polish the 3D printed object until clear and smooth.

The representative analysis shows a positive result for a cross-sectional kidney model and a sectioned cardiac model. The final model visually matches the volume rendering in size, shape, and color. In some cases, the issues related to the uniform scaling can result in a model that appears stretched in one direction.

Also, the surface light scattering issues through the printed model resulted in the unfaithful color translations from the digital rendering to the printed model. The unsuccessful model showed the open voids that run deep into the model, resulting in an optical distortion. It is critical that isotropic images are used with the highest spatial resolution and thinnest slice height.

This method has paved the way for researchers to explore the application of variation of material densities, allowing for functionally realistic models to be created.