Commercial Applications of Tuned Aperture Computed Tomography

Tuned Aperture Computed Tomography will allow the creation of three-dimensional images of dental structures from layers of digital information that can be gathered in the dental office. These three-dimensional images will give a fuller view of the structures, thereby providing more information from which to make a better diagnosis. Unlike similar medical tomosynthetic imaging techniques, TACT should be easily accommodated into dental practice needs.

Since Roentgen’s discovery of the X-ray, imaging has involved the projection of three-dimensional objects onto a flat film or digital sensor. All structures, regardless of their height above the film, are collapsed onto that one plane, which causes superior and inferior objects to be superimposed on top of each other. Any information about their real location or shape is lost. In a maxillary molar for example, buccal and palatal roots are blended together as a single object.

The digital future should provide better imaging than is currently available. There are lines forming with people wanting to buy state-of-the-art digital technology. What they really want is more information from which to make a better diagnosis.

The next generation of digital technology will provide more information than film. This increase in information will not necessarily result from higher-resolution detectors, because the dose and file-handling needs increase disproportionately to the information. Rather, the next leap of technology will be in building a third dimension by assembling many layers of information. If one could lift off layers of superimposed information, he or she could reach the region of interest. To clean an X-ray image of ghost shadows and artifacts would be the same graphical leap as color television was from black and white. The computer box and sensor will be about the same; imaging magic will come from software.

A very helpful way to think about this change comes from a recent article by Richard Gordon, PhD.¹ He states that plane film radiology can be compared to the Where’s Waldo? children’s book series. In the Waldo books, one is expected to find the character Waldo on a graphically confusing page. When one considers the dentist’s role to find a lesion hidden in the trabecular pattern of the mandible, one can imagine the similarities. If many pages of the book where transparent and superimposed on top of the first page that one was looking for Waldo on, it would be very difficult to find Waldo.

Dentistry could look to the medical imaging industry for an answer in tomosynthetic techniques, but these systems are not practical for dental needs. For tomosynthetic imaging, absolute lack of motion is required. Accuracy regarding projection angles for images made are required to be within tenths of degrees. Obviously, obtaining and maintaining this amount of precision in a dental practice is impractical, not to mention cost- and space-prohibitive.

One promising method to produce three-dimensional and layer imaging is called Tune Aperture Computer Tomography, or TACT. TACT was developed in conjunction with Dr. Richard Webber, a professor at Bowman Gray Medical School, and the National Institutes of Health. TACT utilizes multiple low-dose two-dimensional digital images taken from varying angles to produce three-dimensional images. The tune aperture refers to the varying viewing angles at which the two-dimensional base images are recorded. In essence, the aperture is the opening in space from which to view an object. Viewing the world through a long cardboard tube is drastically different than viewing it through a picture window. Varying the angle at which the raw data images are taken results in tuning or optimizing the size of this window for each application. The TACT algorithm computes the tomographic layers and three-dimensional reconstruction using the information from this larger point of view. A reference point is built into the charge-coupled device image sensor, which permits the PC-based software to calculate projection geometry after the exposure. This technique permits patient movement between exposures, allowing for use in a real clinical setting.

For dentistry, TACT can be used to produce three-dimensional views of teeth, pathology, the TM joint, or other areas of interest. Extremely thin layers may be produced, which allow a practitioner to step through an object layer by layer revealing internal detail. Image quality is enhanced by the rejection of overlying artifacts and mathematical reduction of noise. Webber and colleagues² suggest that TACT will be a great aid in diagnosing fractures of crowns and roots as well as detecting auxiliary canals. Other promising areas they foresee are in the detection and precise location of periodontal bone loss or gain, periapical lesion localization, and TM joint bony changes.

Nair and colleagues³ have shown that this technique is extremely effective in the detection of new and recurrent carries, with efficacies significantly greater than that of film or standard digital imaging, with ROC areas (Az) of 0.6608 for Ektaspeed Plus film, 0.5979 for RVG, and 0.9171 for interactively restored TACT.

Layer thickness, layer location, and numbers of layers can be selected in image post-processing. Typical layer sizes are 1 mm, but they can be smaller. The raw data can be reprocessed several years later with a new layer thickness. Other capabilities of TACT are its ability to produce a holographic-like image, which may be manipulated and viewed from different angles. The technique may be very useful when trying to understand complex anatomy, such as TM joints and alveolar bone contours. When the TM joint is viewed in this way, condylar position and osseous condition may be more clearly understood.

From a manufacturer’s, as well as clinician’s perspective, TACT is a natural as an add-on feature for a panoramic X-ray system. Computers in modern panoramic systems control X-ray generation and unit movements. This computer control automates the required data acquisition and provides approximately eight images from a 30-degree arch in less than 40 seconds. A patient may be placed in the panoramic unit with a digital sensor positioned at the region of interest. The operator merely needs to press the exposure button, and the system would make fractional dose exposures during the excursion. After the raw data is acquired, the TACT algorithm automatically processes them. The clinician may choose the number and location of slices and pseudoholographic images at that point or at a later session.

The marriage of these two technologies -- computer controlled panoramics and TACT -- should reduce costs and office space requirements. The digital sensors can provide double duty; they may be used for TACT imaging in conjunction with the panoramic system or for standard digital intraoral imaging in a dental chair.

The current commercial plan is to launch Ortho TACT as an additional option to the existing Orthopantomograph OP100 panoramic imaging system. It uses the unit’s X-ray generator and drive systems to produce the required raw data images from multiple angles on a digital sensor and links this data to a nearby computer workstation. Digital intraoral, digital panoramic, and TACT images may be networked throughout an office and viewed chairside.

The progression of digital imaging is similar to the progression of plastic containers. Plastic containers were made to be exact copies of the glass containers they replaced, just lighter. But as time went by, the concepts of the past were abandoned and completely new packaging was invented. Today, most plastic containers do things never possible with glass. In the digital area, this is happening to dental X-ray imaging.

 

			Figure 1a through g. These images show a progression of 0.75 mm vertical cuts of a maxillary molar stepping from the apex of the palatal root to buccal.

		Figures 2a through e. These images show 0.75mm vertical sagittal cuts through a mandibular first molar. Note the mesial buccal canal in Figure 2a and the mesial lingual canal in Figure 2e. A fracture in the buccal cusp is also visible in Figures 2a and b.

Author

Douglas Woods is vice president of dental sales for Instrumentarium Imaging in Milwaukee, WI.

References

1. Gordon R, Sivaramakrishna R, Mammograms are Waldograms: Why we need 3-D longitudinal breast screening. Applied Radiology 28(10):12-25, 1999.

2. Webber RL, et al, Tuned Aperture Computed Tomography: Theory and application for 3-D dentoalveolar imaging. Dentomaxillofacial Radiol 26:53-62, 1997.

3. Nair MK, et al, Tuned Aperture Computed Tomography and detection of recurrent caries. Caries Res 32:23-30, 1998.


To request a printed copy of this article, please contact/Douglas Woods, Instrumentarium Imaging, 300 W. Edgerton Ave., Milwaukee, WI 53207.