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The most powerful source of human depth perception is stereopsis, the response created in our visual systems by comparing the views we get from our two eyes. Think back a few million years. A caveman is out hunting for that day's meal. So is a saber-toothed tiger. The caveman spots the animal. How close is that carnivore? Without stereopsis—and 5 percent of the population does not have it—our man probably became cat food.
Our eyes are separated by about 65mm, giving each a slightly different view. The visual system determines the relative depth of different objects in the visual scene. Another way to think of it is by remembering the View-Master® that many of us enjoyed as kids. There's a left-eye image and a right-eye image. While one of its cardboard reels holds 14 film slides there really are only seven stereoscopic images that are viewed simultaneously, one for each eye, which simulates binocular depth perception creating the 3D image.
The View-Master® is a popular toy and has earned a place in the National Toy Hall of Fame. We no longer have to worry about saber-toothed tigers, but 3D technology is saving lives by its use in health care. It is viable in radiology, giving physicians sharper resolution in, for example, mammographies and lung cancer diagnoses. It's also valuable in diabetic retinopathy and minimally invasive surgery.
There are a couple of ways to create the 3D image, says Scott Robinson, Production Marketing Manager, Stereoscopic Display at Planar Systems, Inc. (Beaverton, Ore.). "One is the View-Master® approach. You have two cameras side-by-side and click the shutters at the same time," he says. "The second way is 3D data. All the video games use it. To create stereo, you have to render a second view. Shift it over about 3 degrees so you create a left and right view. It's the way to do it in computed tomography (CT) and magnetic resonance imaging (MRI) data sets.
"You hear a lot about 3D graphics," he continues. "Most high-end games have them. It means perspective and light shading. You can rotate them, get motion parallax and mono eye views. That's good enough in computer-aided design, but if you're looking at mammograms, aerial imagery and other complex images, that's where you have to use stereo 3D. By viewing it stereoscopically, they get depth cues. It's more realistic and you're making full use of your senses."
The fundamental principle is triangulation. By taking photographs from at least two different locations, so-called "lines of sight" can be developed from each camera to points on the object. These lines of sight are mathematically intersected to produce the three-dimensional coordinates of the points of interest.
The objective of a stereoscopic display is to present a left-eye image solely to that eye and right-eye image only to the right eye, which allows the human visual system to merge the two images and produce stereopsis. Planar, for example, does this with what it calls StereoMirror™ technology. The StereoMirror™ monitor consists of two active matrix LCD (AMLCD) units oriented at a 110-degree angle. A passive beamsplitter mirror bisects the angle formed between the two monitors. One side of the mirror has a reflective coating, and the side has an antireflective coating which minimizes secondary reflections.
LCD displays operate based on the ability of liquid crystal material to modulate plane-polarized light. In this unit, the polarized light emitted from the top monitor is rotated 90 degrees from that of the bottom monitor. The image from the lower monitor is seen through the mirror. When stereo pair images from the two monitors are viewed through crossed-polarizing glasses the user only sees the left-eye image with the eyepiece having the 0-degree oriented polarized while the right-eye image is seen with the eyepiece having the 90-degree polarizer. The result is a single, fused stereoscopic image.
STEREOPSIS IN HEALTH CARE
3D imaging now is being used in a number of clinical trials. At Emory University in Atlanta, Ga., researchers are conducting a trial comparing stereo 3D to standard 2D diagnoses in breast cancer. The physicians will examine 2,000 patients and compare 3D and 2D images.
In a standard 2D screening, the radiologist must correlate the three-dimensional structure within the breast from the two orthogonal (right-angle) views. Subtle lesions are difficult to detect because of overlying and underlying normal tissue that can mask it. A stereoscopic digital mammogram consists of two x-ray images of the breast acquired from slightly different points of view on a digital mammography unit. The x-ray source is rotated 10 degrees between the two exposures. Then the stereo pair of mammograms is viewed on a high-resolution stereo display.
The mammographer views the display wearing polarized glasses, enabling him or her to see in depth the internal structure within the breast. As a result, a subtle lesion that may be obscured by superimposed normal tissue in a standard 2D image, now becomes visible as the overlying and underlying normal tissue is separated in depth. Conversely, layers of tissue that may falsely resemble a lesion in a standard 2D image due to chance imposition are seen in the stereo mammogram at different depths and will not be mistaken for a lesion. Although a final report has not been issued, indications are strong that false negatives and false positives are both reduced when 3D viewing is employed.
A study at the University of Pittsburgh is investigating increases in productivity and sensitivity in lung cancer diagnoses through stereo 3D visualization of CT chest images. It, too, is showing early indications of more efficiency.
We all know that in minimally invasive surgery recovery time is shortened and quality of recovery is improved. The surgeon uses an endoscopic or laparoscopic fiber optic probe that relays images to a monitor. A stereo view helps the surgeon's depth perception, making the operation even safer. Say, for example, the surgeon is working on the pericardium, the sac that surrounds the heart. The doctor wants to see and understand exactly where he is cutting. Stereo gives him or her an enhanced tool.
Diabetes is an insidious disease. One of its many serious side effects is diabetic retinopathy, hemorrhaging of the retina. The depth of the retina is 2mm at its thinnest point. With a 3D view, doctors see the hemorrhage between the artery and the vein as little dots and can go in and cauterize the bleeding.
The potential of 3D displays to make complex medical analysis and imagery more effective remains almost boundless. As these clinical trials and other research continue, medical professionals will see more of the scientific proof they require before accepting new technology.
author: By Barry Hochfelder