When it comes to understanding the relationship between teeth and bone with traditional
methods, dentists typically use two-dimensional x-rays and photographs. This approach
works, for the most part, but it takes a trained eye to interpret and see the underlying
three-dimensional reality of a two-dimensional image. Sometimes, unfortunately,
even those with expertise can’t come up with the right translation.
William E. Harrell Jr., a doctor of dental medicine
and a board-certified orthodontist in Alexander City, Ala., is using new technology
to aid in the interpretation of images. The technique, which combines low-dose,
rapid x-ray imaging with millisecond optical facial scans, is revealing some previously
hidden useful information.
For example, an oral surgeon working
with 2-D data may not know exactly where an impacted tooth is located before surgery.
Having a true picture can direct the surgeon to the right spot. It also allows an
orthodontist to specify where attachments should go so that a tooth can be guided
into its correct position. This can be done with certainty only if the tooth’s
location is known before surgery, and that requires 3-D information.
This integrated 3-D patient model is composed of an optical scan
of the face, acquired in three dimensions by a face-capture system, and a 3-D image
of the skeletal and teeth structure captured with a CT system. The optical scan
was registered to the x-ray image, resulting in a lifelike image of the skin along
with the underlying structure. Courtesy of Dr. William E. Harrell Jr.
Such knowledge also is needed to evaluate
the relationship between the room required for developing teeth and the amount of
space in the mouth. Spotting mismatches early enables better corrective action to
be taken. Also, being able to visualize and analyze the nasal and sinus areas helps
evaluate problems that may affect the normal jaw development in a child.
To achieve this complete and accurate
picture, Harrell implemented a multipart solution. The first is an i-Cat cone-beam
3-D x-ray scanning system from Imaging Sciences International Inc. of Hatfield,
Pa., designed for dental applications. Its x-ray source and image sensor are on
a rotating gantry open on two sides that sits at head height. During operation,
the sensor and source revolve around the patient, capturing everything in one sweep.
The system operates somewhat like a
conventional CT scanner, but does so with a much lower radiation dose — in
the range of traditional orthodontic and dental radiographs. The lower dose is important
because patients may be imaged several times over the course of a multiyear treatment.
Arun Singh, Imaging Sciences’
senior vice president and chief technology officer, explained that the lower radiation
dose works because the system requires only a single pass and because it is imaging
only a small part of the body. “The lower energy level is optimized for scanning
the skull,” he said.
The lower energy does decrease the
soft tissue contrast, but soft tissue isn’t as important in typical dental
imaging.
To enhance this look beneath the skin,
Harrell also uses a system from 3dMD LLC of Atlanta consisting of cameras to capture
3-D surface images.
Chris Lane, 3dMD’s CEO, said
that four of the cameras (two for each side of the face) capture 3-D information
through photogrammetry, which uses stereoscopic data from two cameras to determine
the distance to points and, therefore, the 3-D location of those points.
Because the technique is done at close
range in a dentist’s office, where lighting may be uncertain, the system illuminates
the patient’s face temporarily with a random speckle pattern that assists
in the triangulation of the surface.
About 0.5 ms after the four cameras
capture the 3-D information, the other two acquire a normal full-color texture image
that can be combined with the photogrammetry-derived 3-D model. Acquiring all
of the images takes less than 2 ms and requires absolute control over shutter
synchronization as well as consistent cameras and sensors. For those reasons,
Lane explained, the company is particular about the type of cameras employed. “We
use proven machine vision cameras rather than domestic or professional cameras.”
Harrell noted that both systems provide
3-D data and can render skin. Each, however, has its own strengths and weaknesses.
The optical system can’t penetrate the surface and reveal the bony structure
beneath. However, it does produce a lifelike rendering and accurately locates the
nose, an important point for a variety of measurements. The x-ray approach doesn’t
provide that information and yields what Harrell characterized as a cadaverlike
rendering of the skin. On the other hand, x-rays are the only way to truly image
the foundation upon which the soft tissue is built.
To complete the picture, he uses a
software program from Dolphin Imaging and Management Solutions of Chatsworth, Calif.,
that allows him to align the soft tissue depicted by the optical and x-ray systems.
He must do this manually, but hopes to eventually have a completely automated alignment
process.
For now, though, he is happy with the
imaging technology because it is closer to reality than the old 2-D methods. That
doesn’t surprise him, because adding depth matches the image with what is
being captured.
“Bottom line, patients are three-dimensional,”
Harrell said, adding that they should be analyzed and their treatment planned that
way.
Contact: Dr. William E. Harrell;
e-mail: [email protected]. Kelly Duncan, 3dMD LLC, Atlanta, +1 (770) 612-8002. Chris
Scharff, Imaging Sciences International Inc., Hatfield, Pa., +1 (800) 205-3570.