Head of 3D Laboratory
Institute of Diagnostic Radiology
University Hospital Zurich
3D imaging has a long history in radiology. The first volume-rendering technique first appeared in the 1970s before realistic computer-generated images were created for cinema. Recently, both software and hardware innovation have improved 3D imaging for medical applications.
Today the three most common techniques in 3D imaging are shaded surface display (SSD), maximum (minimum) intensity projection (MIP) and 3D volume rendering. SSD depicts a surface based on triangles, which are built over a predefined surface using the marching cube algorithm. MIP is a projection view depicting the voxel with the highest density. 3D volume rendering is the most recent and also most powerful 3D imaging technique. It displays the entire dataset by defining the colour and opacity of each voxel density.
With the rapid development of computer technology leading to faster processors, larger random access memory (RAM) and larger storage discs, 3D volume enjoyed a widespread use in the medical community. Today studies have shown the value of 3D imaging for different indications such as trauma, vascular anatomy and pathology, oncology and oncological surgery. MIP is still useful in depicting vessel pathologies, but it shows a free selectable slab rather than the entire data volume slab. SSD plays a minor role in diagnostic radiology today, but it is still a versatile tool to build anatomically realistic models for presurgical simulations.
The 3D effect
A perception of three dimensions in a rendered image on conventional computer monitors and on hard copies can be achieved by several techniques. Depth can be simulated by hiding or obscuring “distant” areas with “closer” regions. Depth perception can also be simulated with object-shading. A lighting model algorithm constructs shadings from the orientation of surfaces, which are computed in relation to a single, fixed light source.
Two fairly simple shading techniques are used in medically-based volume rendering applications: depth shading and enhanced shading. Depth shading makes “distant” structures appear darker than structures that are “closer” to the observer. Enhanced surface shading brightens areas at the location of the material surfaces. The need to visualise spatial relationships in medical images calls for methods of display on the computer that are not feasible with other media. One such technique is object rotation. The possibility of seeing the depths by viewing a free moving object is called kinetic depth effect. New computer technology allows a real-time movement of any object without loss of image quality.
Stereo display for 3D imaging in radiology
Stereo display conveys perspective and depth cues by presenting two separate renderings, from slightly different points of view, to the left and right eyes. This results in an immediate perception of depth. The resulting 3D effect can be very helpful in understanding complex anatomy. Image separation used to be achieved with left and right shutter devices incorporated into eyewear, which open and close to alternate frames, that requires specialised hardware and additional rendering software. Furthermore, inconvenience, discomfort in using stereo displays, a limiting “sweet spot” for stereo viewing, limited resolution and complex computer interfaces have so far inhibited use of stereoscopic displays.
A new system, StereoMirror (Planar Systems), was introduced in September 2005 that uses two screens mounted vertically at an angle to each other, which are viewed through a semi-reflecting mirror. The natural projection of the 17-inch LCD screens is exploited. The polarisation of the image reflected in the mirror is reversed and the stereoscopic image can then be seen with passive polarised glasses. Because both images are “on” all the time, there are no issues with refreshing rate, synchronisation, flicker or loss of resolution. Also, the mirror reduces the brightness of each image by 50%, which is significantly less than the established active and passive solutions.
A new system, 3Viseon™ (3mensio Medical Imaging), that integrates the StereoMirror was introduced in November 2005. 3Viseon combines the capability and tools of a 3D rendering workstation with the dramatic 3D effect of a stereoscopic monitor. It can be integrated in a picture archive and communication system (PACS) and therefore images can be easily retrieved. All captured images can be distributed to the clinicians who have access to the system. The data management is self-explanatory and built up logically. The system is also stable when loading large datasets of more than 2000 images. The desktop of the 3D imaging integrates both screens: the StereoMirror and the conventional screen. On the conventional screen the images are displayed in multiplanar reconstruction mode and in the desired 3D technique, which is also depicted on the StereoMirror.
All work, such as cutting, slabbing, clipping and segmenting, is performed on the conventional screen. The result afterwards is displayed on the StereoMirror. The software comes with a wealth of diagnostic tools.
When working with the system viewers need to sit correctly positioned in front of the screens – otherwise they can feel seasick. Since all work is performed on the conventional screen, a permanent change between the two screens is inevitable and adaptation time is needed. But the high quality of the 3D rendering images with the dramatic 3D depth cues outweighs these small drawbacks. It is fascinating to see all anatomical details in virtual space and almost feel able to touch them. The smooth and flicker-free movements are comfortable for the eyes.
Today the system is used to show complex fractures to traumatology surgeons. The 3D screen provides a clear view of the position of bone fragments, which could not be achieved so far with a conventional screen. In addition, a more realistic intraoperative look can be simulated by a virtual display of the overlaying tissue and vessels.
Depicting a tumour in relation to the lungs from a thoracoscopic view gives thoracic surgeons a better understanding and confidence in their approach. This system can also aid liver surgery by providing a view of the liver including the tumour, portal and venous vessels. By allowing surgeons to plan their cuts it could aid a liver-saving resection.
The demand for stereoscopic 3D images will rise with the improved software and an increased interest on the part of surgeons in the new system. But it is important to note that this system – more than the other conventional 3D systems – needs a radiologist that has the skills and knowledge to handle the software. Furthermore, the system can only be used effectively if there is close collaboration between surgeons and radiologist.
During the last few years the quality of 3D imaging has continually improved. The different techniques for optimising the 3D depth cues can create a realistic 3D image on a 2D screen. The introduction of 3Viseon software, including the StereoMirror, combines 3D imaging with well-established 3D rendering techniques and realistic 3D depth cues. It opens a new dimension in 3D imaging and should become an important visualisation tool for surgeons. However, it does need the skills of a radiologist familiar with 3D imaging who works closely with the surgeons.
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