Dental Radiology Digital Radiography
Dr. Ghassan Ali
DIGITAL RADIOGRAPH
Digital imaging system: is a film-less technology that uses a sensor, computer and monitor to acquire, process, store, retrieve and display the radiographic image.
Digital imaging is the result of X-ray interaction with electrons in electronic sensor pixels (picture elements), conversion of analog data to digital data, . In digital radiography, the image is more like a mosaic, tiny pieces put together to make the final portrait. These tiny dots of information, called pixels. The more pixels that are present, the high the resolution, greater the sharpness and an overall better quality image is produced. Each pixel is assigned a number in the computer from 0 to 255, each representing a specific shade of gray, from pure black at 0 to pure white at 255.
PURPOSE:
1. The purpose of digital imaging is to generate images that can be used in the diagnosis and assessment of dental disease.
2. The images produced are diagnostically equivalent to film-based imaging.
3. The essential components for digital imaging include:
1. a source of radiation
2. an intraoral sensor
3. a computer
4. Instead of film, an intraoral sensor is exposed to radiation, and the electronic signal from this sensor is converted into digital form.
5. The digital sensor transmits this information to a computer. Software stores the image electronically. The image can be manipulated to enhance the appearance for interpretation and diagnosis.
6. Less radiation is needed to generate an image because the sensor is more sensitive to radiation than conventional film; exposure times are 50% to 80% shorter for digital radiography
TERMINOLOGY
• Analog image - film-based images
• Digital image - an image composed of picture elements, or pixels
• Pixel - a discrete unit of information; also termed picture element
• Bit depth image - the number of possible gray-scale combinations per pixel
• Direct digital imaging - a method of obtaining a digital image in which an intraoral sensor is exposed to radiation to capture an image that can be viewed on a computer monitor
• Indirect digital imaging - a method of obtaining a digital image in which an existing radiograph is scanned and converted into digital form
• Sensor - a small detector that is placed intraorally to capture a radiographic image
• Storage phosphor imaging - a method of obtaining a digital image in which the image is recorded on phosphor-coated plates, placed into an electronic processor, and scanned by a laser to produce an image on the computer monitor…..
FUNDAMENTALS
1. Source of Radiation:
Most digital radiography systems use a conventional dental x-ray unit as the source of radiation.
The x-ray unit timer must be adapted to allow exposures in a much lower time setting.
2. Sensor:
Film-based images are known as analog images; a continuous spectrum of gray shades is visible between areas of black and white.
The area of the sensor is divided into squares; the information captured in each square is in analog form.
Some sensors are thick, bulky, and rigid, whereas others mimic conventional film in their size and flexibility.
Some sensors are linked to the computer by a cable, whereas others are not.
3. Computer:
The computer is used to store the incoming electronic signal.
Data acquired by the sensor are communicated to the computer in analog form, then converted into digital form with use of an analog-to-digital converter (ADC).
Sampling - the number of discrete samples of continuous analog input signal per second (the higher the sampling frequency, the better the image accuracy)
Quantization - the assignment of discrete gray values to each pixel
For each pixel, the number of possible gray scale combinations is 2ˆN , known as the bit depth image (e.g., an 8 bit-depth image has a gray scale combination of 2ˆ8, which equals 256 shades of gray).
The bit depth image is determined by the software purchased for the digital system.
The software also allows manipulation of the pixels (i.e, contrast, density, brightness).
4. Monitor
The resolution of the monitor refers to the ability to differentiate between two objects; this is based on the number of pixels present.
Sample resolutions (reflects width to height ratio of 4:3):
800 x 600
1024 x 768
1280 x 1024** (**Indicates the clearest and most detailed resolution.)
DIRECT DIGITAL IMAGING
Three components are necessary to produce a direct digital image: a source of radiation, a sensor, and a computer.
An electrical wire links the intraoral sensor to the computer; therefore, the sensors used are termed wired.
Once the sensor is exposed to radiation, the image instantly appears on the computer monitor…
INDIRECT DIGITAL IMAGING
Conventional radiographs can be converted to digital images by placing the original radiograph on a scanner and shining light through the image as it is scanned into the computer.
Another form of indirect digital imaging uses photostimulable phosphor technology.
Reusable plastic plates coated with phosphor are exposed to radiation.
After exposure, the photostimulable phosphor plates (PSP) are placed into a laser scanning device.
The length of time needed to scan the plates varies from 30 seconds to 5 minutes, so the images are not viewed instantaneously.