Clinical Continued »
bitewing tabs to stiffen the waterproof packet and ensure the image receptor is held securely during positioning (Figure 4).
5. Bitewing images When using a black waterproof cover in a dark mouth with a heat seal round the edge, it becomes very difficult to see the mesial edge of the image receptor. This was not such a problem with emulsion film because the waterproof cover was white. To overcome this difficulty, use bite tabs around the front edge of the waterproof cover to allow exact positioning of the bitewing sensor (Fig 5). It should be possible to image two premolars and two molars on one phosphor plate (size 2).
6. Ambient light image removal ensuring the image stays captured. Are your right and left bite wings the same shades of grey? After image acquisition, the plates should be protected from ambient light image removal (Figs 6a and b). The exposed plates should not be left – even for a few minutes – unprotected from light, even when in a waterproof packet. Ideally, when taking the
images, the room blinds should be closed and the lights dimmed and light boxes switched off. As the image is cleared in
the scanner by light, the plate should be kept as dark as possible when image capturing and when being transferred to the scanner. To ensure this, the exposed plate should be kept in a light tight box prior to scanning and the scanner should be positioned in a dim room away from bright lights to enable plates to be loaded without losing image quality. Special black boxes can be
purchased that allow the plate to be posted in the top like a money box. Light cannot get in and the latent image will be safe until it is transported to the scanner. Alternatively, a dental appliance box or similar could be used. These are small, easy
to clean, non-expensive and possibly already in the practice.
7. Exposure settings PSPs have wide exposure latitude, which means they can give similar results when using a number of different exposures, unlike emulsion films which will be light or dark in relation to the exposure. A number of test images can
be taken using step wedges or extracted teeth to ascertain the lowest exposure that gives an acceptable enhanced image. This becomes the maximum exposure. Guidance should be sought from a medical physicist to ensure that exposure settings are adjusted when a practice moves to a digital system. Other problems that can be
encountered are with images that lack contrast and have an overall grey appearance (“greying out”) (Fig 7). These are not images that can be enhanced by the computer to give more contrast. It is possible that the exposure is too low. If changes of exposure are being considered to give better- quality images, then guidance once again should be sought from your medical physicist.
8. Artefacts that mimic pathology. Unsharp Mask Subtraction (UMS) Many of the image acquisition processes on digital systems are outwith the control of the user. These are intended to improve the image, but sometimes artefacts that mimic pathology can result. Areas where there are high
intensities, such as the base of a restoration or around dense bone, can result in a dark halo effect that can look like pathology. This is called “rebound artefact”. During processing and prior to viewing on the monitor, part of the acquisition process takes a blurred version of the image and subtracts it from the original. The blurred image is wider than the original and so when subtracted it can cause a shadow effect (Fig 8).
Continued » Ireland’s Dental magazine 43
Ambient light image removal Fig 6a
Ambient light image removal Fig 6b
“Greying out” Fig 7
Artefact mimicking pathology Fig 8
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