I routinely receive questions about 3D imaging from Animal Insides readers. In most cases, readers are frustrated with crummy images produced with their CT machine. Most times, they are requesting software recommendations. In many cases the problem is not the software. To duplicate the snazzy 3D images you see in the website or sales pitch for your 3D imaging software you will need a snazzy CT machine. In this article, guest editor Pete Burger of 3DVolumetrics (www.3dvolumetrics.net) offers an introduction to 3D imaging.
3D CT imaging in human medicine is mainstream and used in numerous applications such as vascular studies, cardiac imaging, virtual endoscopy, and presurgical planning. 3D imaging in veterinary medicine is far less common. This is likely because of the fact that newer CT machines are necessary to generate diagnostic images and the applications for 3D CT are different.
The Importance of Technology
The importance of the technology to the final image reconstruction cannot be overstated. If you use crummy software and a crummy CT, you will generate images plagued with artifacts.
To obtain useful, diagnostic, 3D images, thin slices are essential. A CT slice thickness of 0.5mm or below is recommended. Above 1.5mm, 3D CT images will be “choppy” and ridden with artifacts. To obtain slices of this thickness, a multidetector CT is often necessary. The jaw dropping vascular and cardiac studies vendors include in their brochures are generated with 8, 16 and even 64 slice CT units. There is even a 256 slice unit in testing at Johns Hopkins Medical Center in Baltimore. This multidetector CT unit is so fast they can image a beating heart without motion artifact. Now in wide use is a 64 slice scanner which requires the use of Beta Blockers to slow the heart rate for cardiac imaging.
The following series of images clearly shows why thin slices are essential.
In the first image, the eyeballs of this doll are not present due to a partial volume artifact during reconstruction. Only at 1.25mm slice thickness can the surface features of this object be appreciated.
In addition to the need for a technologically advanced CT scanner, all software used to reconstruct a CT study is not equal. I (MW) have played around with some of the freeware 3D reconstruction programs and many of them do a poor job compared to the more expensive software products used on the human side. That should be no surprise as these programs cost between $3000.00 and almost $200,000 for the name brand programs and workstations.
Types of Image Reconstruction:
There are many different methods of reconstructing CT images.
Multiplanar Imaging: Multiplanar imaging (MPR) is the most basic type or reconstruction. MPR is actually a 2d reconstruction of a CT image in a different plane. MPR is, therefore, not technically a 3D reconstruction. The axial slices are cut into the sagittal or coronal plane allowing you to view the entire structure from the side or from front to back rather than as an axial slice cut across the structure.
MPR images provide a wealth of information in many studies. The relationship of the organs to each other are shown in a different view along with a view of an entire tumor in another plane may help determine treatment options. MPR studies can be obtained in any plane. For example vessels tracking at a 45 degree angle, can be rotated to provide a complete picture of the vessel.
MIP (Maximum Intensity projection) and MinIP (Minimum Intensity projection): MIP and MinIP are reconstruction methods that look as if they may be 3D but in actuality are 2D representations. As defined by wikipedia “A maximum intensity projection (MIP) is a computer visualization method for 3D data that projects in the visualization plane the voxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection. This implies that two MIP renderings from opposite viewpoints are symmetrical images.
This technique is computationally fast, but the 2D results do not provide a good sense of depth of the original data. To improve the sense of 3D, animations are usually rendered of several MIP frames in which the viewpoint is slightly changed from one to the other, thus creating the illusion of rotation. This helps the viewer's perception to find the relative 3D positions of the object components. However, since the projection is orthographic the viewer cannot distinguish between left or right, front or back and even if the object is rotating clockwise or anti-clockwise.
MIP is used for the detection of lung nodules in lung cancer screening programs which utilise computerized tomography scans. MIP enhances the 3D nature of these nodules, making them stand out from pulmonary bronchi and vasculature.”
The following is an MIP study of a mouse:
Surface rending and volume rendering: Surface rendering and volume rendering are two kinds of true 3D representations. Surface and volume rendered images are what most people think of when they think of 3D CT
Surface rendering is a process in which surfaces are determined and displayed. Surface rending is excellent for applications where the edges of structures, such as vessels, must be examined. Surface rendering is also useful when examining the inner surface of hollow structures such as airways and colons during virtual colonoscopy.
Volume rendering takes the entire volume of data, sums the contributions of each voxel along a line from the viewer’s eye through the data sets, and displays the resulting composite for each pixel of the display. Because Volume rendering uses all the voxels it enables modulation of windowing and leveling and opacity.
The following are a few images using surface and volume rendering.
Applications of 3D imaging in Human and Veterinary Medicine
3D imaging in human medicine is widely accepted. The primary use of 3D imaging in human medicine is for cardiac and vascular evaluation. For example:
- Carotid evaluation for blockage or stenosis
- Circle of Willis evaluation for aneurysms or stenosis.
- Cardiac vasculature evaluation
- Pulmonary vasculature studies in pulmonary embolism evaluation.
- Aortic aneurysms evaluation
- Virtual Colonoscopy
- Surgical planning and fracture evaluation
In veterinary medicine vascular evaluation is less common as vascular disease is less common. Nontheless, 3D CT has several uses. For example:
- A primary use of 3D CT in our practice is presurgical planning and evaluation of comminuted fractures. Surgeons find 3D CT very useful in cases of pelvic fractures.
- Vascular evaluation in portosystemic shunt imaging is another common use.
- 3D imaging can greatly enhance client understanding of any CT study.
- Presurgical tumor surgical evaluation
3DVolumetrics is an online service that takes your CT images and makes snazzy 3D images. 3DVolumetrics is a “per use” service. This unique service lets users take advantage of the 3dVolumetrics experience and save thousands of dollars on expensive 3D reconstruction software. Bottom line: everyone with a spiral CT or MRI machine now has access to 3D reconstructions software and the people who know how to use it. For more information about 3DVolumetrics please contact www.3dvolumetrics.net.
