The use of computed tomography (CT) — which uses a series of 2D X-ray images taken at specific intervals around an entire structure-has grown tremendously within the medical testing field.

CT systems generally use three principal components: an X-ray tube, an X-ray detector, and a rotational stage. The components are enclosed in a radiation shielding steel-lead-steel cabinet that usually ranges between 4 and 10ft³. This allows use of the system in a public environment without any additional safety concerns.

High-quality industrial X-ray detectors used for CT are typically a new generation amorphous silicon flat panel area detector, offering high sensitivity, resolution, and bit depth. The resulting 2D X-ray images are clear and the contrast unparalleled.

Acquisition, reconstruction, and visualization

A modern high-end CT scan covers 360° (complete rotation). CT systems typically acquire between 360 images (one image every degree) and 3,600 images (one image every 0.1 degree) depending on the final desired resolution. Each image is between 3 to 10 megapixels and is also averaged and filtered to reduce noise. The 2D digital images taken during this step are saved directly into a single folder, used in the next step of the CT process.

Once the acquisition process of the CT scan is completed, CT calibration and CT reconstruction algorithms are used to reconstruct the 3D CT volume. These 3D images are made of voxels (3D pixels), and with the use of visualization software the 3D volume can be manipulated in real time. This makes it possible to slice through anywhere inside the object, inspect and look for defects, take accurate measurements, and reconstruct a surface model.

Industrial CT is a very competitive technology for 3D scanning. The technology makes it possible to reconstruct complete 3D models with billions of voxels in seconds. This opens the door for numerous applications such as 3D inline automatic defect recognition, 3D reverse engineering, rapid prototyping, and 3D metrology.

The principal benefit of using 3D CT for scanning or digitization is that a complete model with both external and internal surfaces of an object is obtained without destroying it. Moreover, CT works with any surface, shape, color, or material (up to a certain density and/or thickness penetrable with X-rays). Generally, a modern start-to-finish CT scan can be as fast as a few seconds or take longer than an hour, depending on the resolution requirements, size, and/or density of the object. Overall, the resolution is excellent both internally and externally.

This allows for measurement on surfaces both inside and outside an object.

Due to the penetration of X-rays, CT scans are unaffected by certain object characteristics such as dark, reflective or transparent surfaces, as well as shaded zones on the item, which can cause difficulty with other 3D scanning methods. 3D CT reconstruction models also can be directly compared to CAD models and/or other CT models in order to display differences or commonalities in measurements, densities, and voids.

The images above show a 3D CT reconstruction of multiple pharmaceutical tablets. The CT model can be manipulated in real time 3D, and it is also possible to slice through in any direction for internal inspection.

The reconstruction process consists of complex algorithms that transform the stack of 2D X-ray images to a 3D voxel volume model. The image below shows an example of a modern CT system layout. It consists of a radiation shielded enclosure, which houses the X-ray tube, detector and rotational stage. Adjacent to the enclosure is a computer workstation, consisting of a 2D X-ray console for the setup and acquisition steps, and a 3D CT supercomputer workstation for volume reconstruction and visualization.

Many options are possible through the use of a 3D CT reconstruction volume model. For basic 2D measurements, the slice window pictured below is generated from the cutting plane in the 3D volume. From there a length, diameter, and angle can be applied on the single image. Also, any feature, part or even defect inside a structure or an assembly can now be measured without destroying it.

The 3D CT reconstruction, which is made of several million or billion voxels, can also be transformed to a surface model. The resolution of the 3D model depends on the number of voxels generated from CT reconstruction. A threshold value of radiodensity is chosen by the operator and set using edge detection image processing algorithms. From this, a 3D model can be constructed and displayed on screen. Multiple models can be constructed from various different radiodensity thresholds, allowing different colors to represent each component of an assembly. Typically, models are composed of thousands of polygons; some models have as many as 50 million polygons.

The three pictures in the column above show a surface reconstruction (polygon mesh) with reflective surface (top left) and transparency (middle). All the internal structural features are reconstructed as well since the CT reconstruction provides volumetric information. The bottom screenshot shows the internal features of the tooth. The tall verical image shows the internal features of an inhaler device.

With the generated surface model, many different applications become available to the user. In most cases, the polygon mesh generated by the CT system can be used in reverse engineering, rapid prototyping, and finite element analysis without modification. Typically, resolution is higher than needed. However, in order to modify or take measurements of the CT surface model with CAD software, the CT model needs to be processed. User intervention is still necessary for manual operation in transforming the scanning surface to solid CAD.

Comparing 3D CT to CAD

Since CT and especially microCT (see pg.12 ) provides very accurate dimensions on surfaces, the technology is often used for metrology studies. Measurements can be done either directly on the surface using any CAD or metrology software, or it can automatically compare the CT model with the CAD model or another CT model.

Due to the proprietary nature of medical devices, a CAD-to-CT comparison is not available for display. Examples below show a dimensional comparison between the Solidworks CAD model of a casting provided by Twin Cities Die Casting, Minneapolis, and the CT surface reconstruction created by North Star Imaging, Inc., Rogers, MN.

To do this comparison, the two models needed to be aligned. Different alignment tools are available, ranging from fast and automatic best fit, to manual alignment. Once the two models are aligned, a 3D comparison option automatically creates a colored view showing the dimensional differences between the two models. In the example below, the dimensional differences between the Solidworks model and the actual CT surface (polygon mesh) are represented by colors. Tolerances between -0.300mm and +0.300mm are shown in green. Yellow denotes the areas where the CT scan measurements are larger than the original CAD model and blue indicates smaller measurements. It is possible to change tolerance values and the color code to cater to a specific project or preferences. Numerical values are an available option as well.

CT modeling has become an important technology for the medical testing industry (visit xviewct.com for a demo). Having very accurate internal dimensions without destroying the item, along with the ability to compare to a reference model is entirely unique to CT. There are no shaded zones, it works with all kinds of shapes and surfaces, there is no post-processing work needed, and the resolution is excellent. Above all, CT's greatest benefit is the ability to nondestructively obtain the internal structure of the object.

About MicroCT

As opposed to a medical CAT scan where the tube and detector rotate around the object, an industrial CT scan rotates the object while the tube and detector remain static. Micro computed tomography (microCT) is primarily the same as standard industrial CT, except it uses a microfocus tube instead of a traditional tube. A microCT scan yields resolutions in microns due to the fact that the focal spot of a microfocus tube is only a few microns in size. For comparison, microCT resolution is about 100 times better than the best CAT scan in the medical field.