**9. 3D visualisation of epidural procedure**

We have applied stereoscopic 3D computer graphics for visualization of epidural insertions. The stereoscopic images are viewed through a head mounted visor containing two OLED micro-displays in stereo using the page-flipped method. The 3D graphics are built from several vertex models of the anatomical structures as described in section 7. The stereo simulation allows depth to be perceived so that the operator can judge depth of the needle tip in relation to tissue layers and bones, which aids to the location of the epidural space. Applying stereoscopic vision to epidural simulators helps the operator to visualize the depths required for correct needle placement in the epidural space [14].

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**Figure 15.** Stereo glasses used for epidural insertion visualization

**10. Haptic interface for epidural insertion** 

of a high fidelity epidural simulator development program.

previously been accomplished.

The epidural simulator software interfaces with head motion detectors. When the user turns their head, the 3D objects rotate by the same degree in the opposite direction to create an illusion of camera rotation. This interface allows the user to change the view point to different directions by turning their head, so that the mouse and keyboard are no longer required. The feedback from experienced anaesthetists suggested that the flexible spine model will be useful for modelling patient position. The options for adjustable body shape and size was seen as a positive step to encapsulate the variety of patients which has not

Haptic devices have become a more popular and accepted tool for medical simulation and provide an accurate way of re-creating the feel of surgery [21, 22]. The insertion of an epidural is a procedure which relies almost entirely upon feeling the forces on the needle. Epidural simulators are therefore ideally suited to haptic technology. This section describes methods for configuration of a haptic device to interact with 3D computer graphics as part

Haptic devices have been used in epidural simulators previously, although they are not based on measured patient data from needle insertions. Instead, they are configured by 'experts' trialling and adjusting the system. It is therefore hard to assess the accuracy of the forces generated and so creates a real potential for improvement. The haptic device has currently been set up to reconstruct the force data found during the porcine trial. The force data from the

A haptic device has been connected and used as an input to move the needle in 3D, and also to generate force feedback to the user during insertion (Figure 16). A needle insertion trial was conducted on a porcine cadaver to obtain resultant pressure data (Section 5). The data generated from this trial was used to recreate the feeling of epidural insertion in the simulator. The interaction forces have been approximated to the resultant force obtained during the trial

graphs were divided into sections to represent each of the tissue layers separately [23].

Depth judgement is crucial to the technique and since stereographics allows the perception of depth in 3D graphics, epidural simulators can benefit greatly from stereo-technology. Here the aim is to apply stereo vision technology to simulate epidural needle insertion. Without stereo graphics the depths of objects in simulations are not perceived accurately. By viewing 3D graphics on a flat computer screen there is no way of knowing the actual distance between objects other than by estimating their size. Estimation is not always accurate and some medical applications may require far more precision in depth perception. Epidural simulators require the needle tip to penetrate several layers of tissue between 42- 47mm thick and must stop within the 6mm epidural space [19], which is difficult to achieve without depth perception. With stereo vision, distance can be perceived natively allowing the user to intuitively view the depth and distance between objects by perceiving differences between the two images, if images are appropriately scaled.

Stereo glasses contain two small OLED screens, one for each eye. Alternatively, glasses can be polarized, which allows viewing of a polarized screen, which has both images superimposed, one of which arrives at each eye. Shutter glasses can be used which contain moving mechanisms to consecutively close each eye similar to a camera shutter. The screen then displays images for left and right eye consecutively at the same shutter speed. Alternatively, a glasses free approach, vertically dispersive holographic screen (VDHS) can be used by directing two beams of light containing the images into each eye separately [20]. Mirror screens contain two monitors mounted at 110 degrees with a plane of silver-coated glass combining the two images and cross-polarized glasses are worn to separate the images. For all stereo systems, once the two images arrive separately at each eye, the brain combines them to generate 3D with depth perception based on some calibrated data.

For this epidural simulator, we have used stereo glasses containing two OLED microdisplays, one for each eye, with magnifying lenses. Figure 15 shows how the epidural simulator is being used with the stereo glasses displaying the 3D spine model. The glasses have advantages that the user can see the image whichever direction they look in and as they turn their head motion detectors can rotate the image to follow. The glasses produce a 40-degree diagonal field of view for each eye. The image appears the same size as a 105 inch projection screen viewed from 12 feet. Magnifying lenses allow the eye to focus further away avoiding eye strain. The graphic resolution must be fixed at 800x600 pixels which display sufficient details. Two separate images are displayed on each eye display. Stereo is achieved by using the page-flip method. A signal is generated by the graphics card at 60Hz, with the images consecutively swapped between left eye and right eye. The swapping is done by the graphics card drivers. The hardware inside the 3D glasses splits this into two separate 30Hz signals and delivers one to each eye, this results in stereoscopic images.

**Figure 15.** Stereo glasses used for epidural insertion visualization

simulation allows depth to be perceived so that the operator can judge depth of the needle tip in relation to tissue layers and bones, which aids to the location of the epidural space. Applying stereoscopic vision to epidural simulators helps the operator to visualize the

Depth judgement is crucial to the technique and since stereographics allows the perception of depth in 3D graphics, epidural simulators can benefit greatly from stereo-technology. Here the aim is to apply stereo vision technology to simulate epidural needle insertion. Without stereo graphics the depths of objects in simulations are not perceived accurately. By viewing 3D graphics on a flat computer screen there is no way of knowing the actual distance between objects other than by estimating their size. Estimation is not always accurate and some medical applications may require far more precision in depth perception. Epidural simulators require the needle tip to penetrate several layers of tissue between 42- 47mm thick and must stop within the 6mm epidural space [19], which is difficult to achieve without depth perception. With stereo vision, distance can be perceived natively allowing the user to intuitively view the depth and distance between objects by perceiving differences

Stereo glasses contain two small OLED screens, one for each eye. Alternatively, glasses can be polarized, which allows viewing of a polarized screen, which has both images superimposed, one of which arrives at each eye. Shutter glasses can be used which contain moving mechanisms to consecutively close each eye similar to a camera shutter. The screen then displays images for left and right eye consecutively at the same shutter speed. Alternatively, a glasses free approach, vertically dispersive holographic screen (VDHS) can be used by directing two beams of light containing the images into each eye separately [20]. Mirror screens contain two monitors mounted at 110 degrees with a plane of silver-coated glass combining the two images and cross-polarized glasses are worn to separate the images. For all stereo systems, once the two images arrive separately at each eye, the brain

combines them to generate 3D with depth perception based on some calibrated data.

For this epidural simulator, we have used stereo glasses containing two OLED microdisplays, one for each eye, with magnifying lenses. Figure 15 shows how the epidural simulator is being used with the stereo glasses displaying the 3D spine model. The glasses have advantages that the user can see the image whichever direction they look in and as they turn their head motion detectors can rotate the image to follow. The glasses produce a 40-degree diagonal field of view for each eye. The image appears the same size as a 105 inch projection screen viewed from 12 feet. Magnifying lenses allow the eye to focus further away avoiding eye strain. The graphic resolution must be fixed at 800x600 pixels which display sufficient details. Two separate images are displayed on each eye display. Stereo is achieved by using the page-flip method. A signal is generated by the graphics card at 60Hz, with the images consecutively swapped between left eye and right eye. The swapping is done by the graphics card drivers. The hardware inside the 3D glasses splits this into two separate 30Hz signals and delivers one to each eye, this results in stereoscopic images.

depths required for correct needle placement in the epidural space [14].

between the two images, if images are appropriately scaled.

The epidural simulator software interfaces with head motion detectors. When the user turns their head, the 3D objects rotate by the same degree in the opposite direction to create an illusion of camera rotation. This interface allows the user to change the view point to different directions by turning their head, so that the mouse and keyboard are no longer required. The feedback from experienced anaesthetists suggested that the flexible spine model will be useful for modelling patient position. The options for adjustable body shape and size was seen as a positive step to encapsulate the variety of patients which has not previously been accomplished.
