radiology is the use of stereoscopic imaging principles on radiographs and
Roentgen described the first radiograph in 1895 and it was only a matter
of 2-3 years before stereoradiographs were being taken. A peak of
popularity followed with most radiologists using the technique by the 1930's.
The discovery that x-rays could be harmful did a lot to kill off the
technique as the extra radiation could not be justified. Today there
are few radiologists and even fewer clinicians who have been exposed to
stereoscopy, much less use it. I believe that a large part of this
is the main mode of dissemination of knowledge in the medical world - by
journal. Stereoscopy has to be experienced first hand using a well
set up viewing device to appreciate and publication in journal format without
providing adequate viewing aids does not help the potential viewer.
There are a number of situations where plain stereoscopic radiographs
may still be of significant benefit:
- in practices (developing world, rural locations,
military field hospitals) where CT scanning is not available, but more information
- in situations where metallic implant components need
to be imaged, but too much implant scatter occurs in the CT machine (older
- dislocated hip or shoulder, where lateral is un-interpretable
and often painful to obtain
- erect spine - reconstructed CT and MRI data in the
scoliotic, deformed, or unstable spine is not available in the erect position
Plain radiographs do need to be treated differently from photographic
images when viewing for a number of different reasons. Depth cues
from perspective are preserved, though the obscuring of objects further away
from the viewer are obviously not. In photographic images, the focus
plane is presented sharpest and objects progressively defocus away from
this. Radiographs are similar, though one needs to remember that the
plane of sharpest focus is at the film plane and everything closer to the
tube will be progressively defocused. There are also no lighting or
depth haze cues in radiographs to rely on. With objects that occlude
most of the x-rays from reaching the film, such as metalware or dense soft
tissue, only a silhouette is recorded and the details that are available
elsewhere in the radiograph are not visualized.
The most important part of taking a pair of stereoradiographs is to have
the patient and film stay in the same location whilst the tube is shifted. Tube to film distance should remain
constant for both films.
Most people who have written on the subject have recommended a tube
shift of about 1/10th of the tube to film distance. This can be a bit
less for smaller subjects if the image will be magnified (hypostereo). As the tube can be regarded as
a point source of radiation, toeing in the tube should have no effect on
the picture unless the fulcrum on which the tube swings is placed eccentric
to the tube. Use of a grid does cause a significant gradient that is
visually obvious - we have not decided what to do with the grid yet.
Limitations and drawbacks
Before using radiographs stereoscopically, it is important to understand
the limitations of stereoscopy. Due to our inability to effectively
gauge distance from the amount our eyes are accommodated and converged,
it is not useful for assessing absolute depth in an image without stereophotogrammetric
devices. Changes in depth and depth relationships, however, are
The main objection to stereoradiography is that for each stereo view,
a double dose of radiation is required - where a stereo view does not add
additional information, it cannot be justified. If the technique does
provide additional information that contributes positively towards clinical
management, then it as reasonable to use as modalities like CT which also
require additional radiation. When considering radiation, it is useful
to remember that the dosage from two AP or PA films of the trunk is lower
than that of an AP + lateral series. The two views required also require
less irradiation than accepted investigative modalities such as plain film
tomography, where multiple slices are made of a region.
Another problem in stereoradiography is the need for the patient to stay
still whilst the two views are taken. Whilst this is easy where filming
tables are used, it is harder to get good films in erect patients, patients
with neuromuscular disorders, studies which are dependant on respiration
phase, and where patients are in considerable pain.
Volumetric data in stereo
Volumetric data can be rendered from two different viewpoints (either
using "toed-in" or asymmetric frustum projection) to give stereoscopic views
of a subject. Rendering can be done either using surface generation
algorithms or by mapping intensity/density to opacity. Surface rendering
algorithms were developed to speed up the rendering process by reducing
the number of geometric primitives and for surface smoothing. With
the increasing speed of recent computers, it is becoming more feasible to
render the full volumetric data set on widely available computing platforms.
I have written tutorials on the use of two programs - VolView and AMIDE
- for use in volumetric rendering available: see the "opacity based rendering" page.
If you have developed or are developing other uses for stereoscopy in
radiology or orthopaedics, I'd be interested to know.