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tvpjournal.com | September/October 2015 | TOday'S VeTerinary PracTice PracTical denTiSTry Peer reviewed 61 1. Position the sensor apically enough to expose the apex of the tooth and 3 mm beyond. This technique "cuts off " the crown, but often radiographs of the crown do not provide critical information. 2. expose 2 images (one of the root and one of the crown). STEP 3. PoSiTioning ThE TubE hEad 1-10 There are 2 major techniques for positioning the tube head, both of which are required for complete imaging of veterinary patients. Parallel Technique The flm is placed parallel to the object being radiographed, and the beam positioned perpendicular to both the sensor/plate and the tooth/root (Figure 6). This technique results in the most accurate image, but is only useful for the caudal mandibular cheek teeth because: • dogs and cats do not have an arched palate and, therefore, the maxillary teeth cannot be imaged with this technique • The mandibular symphysis interferes with placement of the sensor/plate parallel to the tooth roots of the mandibular canines and incisors as well as the rostral mandibular premolars. bisecting angle Technique This technique is based on the theory of equilateral triangles, and creates an image that accurately represents the tooth and roots (Figure 7). • The sensor/plate is placed as parallel as possible to the tooth roots. • The angle between the tooth root and sensor/ plate is measured or estimated. • Then the angle is cut in half (bisected) and the beam placed perpendicular to this bisecting line. if the angle between the sensor/plate and the beam is incorrect, the radiographic image will be distorted because the beam will create an image that is longer or shorter than the object imaged. When the angle of the beam to the sensor/plate is: • Too small, the object will appear longer on the image than its actual length, known as elongation (Figure 8). • Too great, the object will appear shorter on the image than its actual length, known as foreshortening (Figure 9, page 62). The bisecting angle technique is the most commonly used technique in veterinary patients because it is the most scientifcally correct way to take veterinary dental images and provides the most accurate representation of the root. However, since it is time consuming, a modifed, or "simplifed" technique has been developed. A B Figure 8. elongation: The bisecting angle for the maxillary premolars and molars is 45 degrees to the sensor. in this photo, the PiD has been set up at approximately 25 degrees (a). The resulting image (B) demonstrates elongation of the tooth roots. To fx this error, rotate the tube head more perpendicular to the sensor. d ental r adiology Terms d igital radiography: Conversion of transmitted x-rays into a digital image using either solid-state detectors, such as amorphous selenium or silicon; or photostimulable phosphor (PSP) plates (which are then scanned). Following image acquisition, computer processing is performed and the image displayed. d irect digital radiography: Uses an x-ray photoconductor, such as amorphous selenium, that directly converts x-ray photons into an electric charge. With this type of digital radiography, the sensor is typically connected by a cord to an analog-to-digital converter box (or card), which is connected to the computer. Images are produced within seconds of sensor exposure—the primary advantage of direct sensor systems. i ndirect digital radiography: Requires a 2-step process for x-ray detection: the scintillator converts the x-ray beams into visible light, which is then converted into an electric charge by photodetectors, such as amorphous silicon photodiodes. This system uses PSP and, after exposure, these plates must be scanned in order for an image to be obtained. Position indicating device: The lead-lined cone or cylinder that directs the x-ray beam during radiographic exposure. This device improves and standardizes dental radiographic imaging and reduces the patient's and operator's risk of radiation exposure.