Digital images are processed and viewed on computers, there is no processing that is physically carried out by any member of the dental team.
Digital radiographic images are captured and stored in pixels (which are square in shape) each individual pixel on the image receptor measures the x-ray absorption of the particular area of the tooth or mouth covered by the image receptor during a radiographic exposure.
When a digital image is transferred to a computer the information is converted from analogue to digital, this means that each pixel is designated a number (computers deal with numbers), and each number represents a particular shade of grey, of which there are 256 different shades. Therefore each digital radiographic image is actually, when viewed, a collection of small squares of varying shades of grey.
The quality of a digital radiographic image is dependant not only on the positioning of the image receptor and settings used when the radiograph is being taken, but is also dependant on;
- The number of pixels
- The size of the pixels
- The size of the image file
- The resolution of the image
The one big disadvantage of digital radiographic images is that when they are printed they lose a great deal of their quality due to the fact that they are made up of 256 shades of grey and printers cannot reproduce this many shades of grey. The best way to print digital radiographs is to use a heat sublimation printer which allows the image to be copied on to a radiographic film, however this is extremely expensive and in many circumstances the high cost outweighs the benefit of this method of printing radiographs.
Digital Solid State Sensors
This type of image receptor is directly connected to a computer, therefore with the correct software and an analogue to digital converter the image captured can be viewed on the computer monitor almost immediately. There are two types of digital solid state sensors;
- CCD – charge coupled device
- CMOS – complementary metal oxide semi-conductors
Each one is read slightly differently by the computer before the image is created.
The CCD type of image receptor is read by transferring pixel charges in rows, from one row to the next, and each row is read individually. The CMOS type of image receptor is read as individual pixels, each pixel is directly connected to a transistor, which receives the charge packet from each pixel and so enables each pixel to be read individually. Although this makes no difference to the final image produced, it is important that those who are qualified to take radiographs understand the difference between the two.
Phosphor Storage Plates
These image receptors are not directly connected to a computer and have to be processed using a special reader before the image can be viewed on a computer monitor.
Phosphor plates contain a layer of barium fluorohalide phosphor which absorbs the x-ray radiation and allows an image to be produced; the image is trapped on the phosphor plate until it has been scanned by a laser beam in an appropriate reader. The laser beam light in the reader illuminates the plate with enough energy to enable the plate itself to emit light. This light is detected by a photomultiplier tube and is converted into an electrical signal. The reader is connected to a computers analogue to digital converter, and this electrical signal is input to the computer through the converter. Only when the plate has been read and transmitted to a computer (and converted from analogue to digital) can the image be viewed. The reader clears the phosphor plate so that it is ready to be used again.
All digital images can be altered. This is done by changing the pixels themselves, for example, they can be given a different number – therefore changing the shade of grey or even the colour. The most common ways that digital radiographs are manipulated are;
- Automatic measurements taken
- Magnification of either the whole image or a specific area of an image
- The image can be inverted
- Brightness can be changed
- Contrast can be changed
Image manipulation can be useful but it doesn’t mean that radiograph quality can be compromised. All radiographers should take the highest quality radiograph that they can for each and every patient and each and every radiograph.