stereoscopic radiology

 

 
Stereoscopic radiology is the use of stereoscopic imaging principles on radiographs and volumetric data.

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 is wanted
    - in situations where metallic implant components need to be imaged, but too much implant scatter occurs in the CT machine (older scanners)
    - 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.

Technique

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 accurately judged.  

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.
 
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