Roentgenology Lecture

ROENTGENOLOGY | LECTURE MODULE LESSON 1st SEMESTER | 2024-2025 PARALLELING TECHNIQUE PATIENT PREPARATION • • • • • • • • • Prior to starting to take films, the patient must be positioned properly. Seat the patient and ask them to remove their glasses and any removable appliances. Adjust the headrest to support the head while taking films. Raise or lower the chair to a comfortable height for the operator. Place the lead apron and thyroid collar on the patient. It is a good idea to inform the patient about the number of films you will be taking so they know what to expect. In the paralleling technique, the film is placed in the mouth so that the long axis of the film is parallel with the long axis of the teeth being radiographed. A paralleling instrument with an aiming ring is normally used to orient the film, teeth and ring in a parallel relationship. When the x-ray beam is aligned with the ring, the x-ray beam will be perpendicular (right angle) to the teeth and the film. PARALLELING FILM PLACEMENT • As mentioned previously, the film is placed in the mouth so that the long axis of the film is parallel with the long axis of the teeth. Since all teeth are inclined toward the middle of the head (not straight up and down), the film will be slightly angle in the mouth (see below left). Of the film is maintained in an upright position (see below right), the patient will not be able to close on the biteblock and the film will now be parallel. Þ The image will be slightly larger than the actual teeth Þ Analog films are more accurate; very important in RCT Þ Follow the long axis of the tooth • To facilitate film placement, the film may be tipped up to 20 degrees beyond parallel. • Because the palate and floor of the mouth are shallower as you approach the lingual of the teeth, the film often cannot be positioned properly close to the teeth. • As a result, the film must be positioned away from the teeth (farther back in the mouth) to achieve parallelism. To facilitate film placement, the film may be tipped up to 20 degrees beyond parallel • Because the film father from the teeth, there will be increased magnification (larger size) and decreased sharpness (less detail). To compensate for this, the target-nim distance should be increased (the target is where the x-rays are produced) ADVANTAGES OF USING THE PARALLELING TECHNIQUE • There are two techniques for taking periapical films, the paralleling and the bisecting angle techniques. When comparing the two techniques, the advantages of the paralleling techniques are: 1. Better dimensional accuracy: The paralleling technique results in less distortion of the image of the teeth. 2. When using the paralleling instrument with the aiming ring, the alignment of the x-ray beam is simplified. 3. It is easier to standardize films. 4. Head position is not as critical. When the long axis of the film is parallel with the long axis of the tooth, the image of the tooth on the film looks the same as the tooth itself (no distortion). The image will be slightly larger than the actual tooth (magnification), but the shape is the same. Longer target = similar size and shape (but longer exposure time) • DISADVANTAGES OF USING THE PARALLELING TECHNIQUE • When comparing the paralleling and bisecting angle techniques, the paralleling technique is: 1. Less comfortable. Because the film is usually more upright when using the paralleling technique, it impinges more on the palate or floor of the mouth, thus making it more uncomfortable. 2. More limited by the anatomy of the patient’s mouth. A shallow palate or floor of the mouth makes it harder to position the film using the paralleling technique. • The target-film distance is increased by using a longer PID, using a machine with a recessed target (opposite side of the tube head from the PID) or a combination. The medium PID with a recessed target is a good compromise. The disadvantage to increasing the PID length is that the exposure time must be increased. If you change from a 8” target-film distance to a 16” target-film distance (double the distance) the exposure time will be four times as much (see inverse square law). PARALLELING TECHNIQUE Head Position • As mentioned previously, head position is not as important when using the paralleling technique. • However, in general it is best to position the head in an upright position so that the maxillary arch is parallel to the floor. Film Selection for Adults • The #1 size film is used for anterior periapical films using the paralleling technique. The long axis of the film is vertical. For posterior films, # 2 size film is used with the long axis horizontal. If horizontal, you won’t see the root Film Selection for Children ANTERIOR PERIAPICAL • POSTERIOR PERIAPICAL • • • For children with small mouths, the #0 size film is used for both anterior and posterior periapical films. However, if the child’s mouth is large enough to reasonably accommodate the larger size films (#1 anterior, #2 posterior) and the child is cooperative they should be used. For the anterior periapical, the #1 size film is placed vertically on the biteblock. The film is rotated so the identifying black dot is down; this end of the film goes into the slot of the biteblock (dot-in-the-slot). Push the film back against the biteblock support and slide it down into the slot. Þ Þ Þ For the posterior periapical, the # 2 size film is placed horizontally in the biteblock. The film is rotated so the identifying dot (faint embossed circle) is down; this side of the film goes into the slot of the biteblock (dot-in-the-slot). Push the film back against the biteblock support and slide it down the slot. Reverse image – See the herring bone pattern Beam can overcome the thickness of the lead Exposed film - transparent GENERAL TECHNIQUE GUIDELINES • For all periapical films, the teeth being radiographed must be in contact with the biteblock to avoid not having the apices of the teeth on the film. Make sure patient doesn’t just close lips tight around biteblock; have them part their lips so you can confirm the contact Þ Bite for the stability • As shown above, cotton rolls may be used in any area of the mouth to help support the biteblock, especially if an edentulous region or uneven teeth oppose the teeth being radiographed. Using a cotton roll also makes it more comfortable for the patient to bite in some situations. The cotton roll should be placed against the arch opposite the one being radiographed If a patient has partial denture or a complete denture in one of the arches, the appliance can be used to help support the biteblock when the patient closes. This is normally preferable to using cotton rolls Þ Gauge is more stable Make sure that the denture is only used in the arch opposite to the one being radiographed. Partial dentures can not be left in the arch being radiographed because the metal framework will be superimposed over the image of the teeth RINN PARALLELING INSTRUMENTS • The Rinn paralleling instruments. They are color-coded, with yellow being the posterior instrument and blue being the anterior instrument. The metal bar connects to the side of the biteblock and the ring slides on the bar. • • • The film is placed in the biteblock so that the all-white side of the film packet faces the teeth and, by extension, the ring. (The colored portion of the film is against the back support of the biteblock). When you look down through the ring, you should see the all-white side of the film packet centered in the opening. • • • • • After the patient is biting on the biteblock, and before aligning the PID, the ring needs to be moved closer to the patient’s face. While supporting the bar with the fingers of the hand, slide the ring down close to the face with other hand Always make sure the head is supported by the headrest before aligning the PID and exposing the films. The PID should be aligned with the ring so that the end of the PID is equidistant from the ring and within ¼” of the ring. The PID doesn’t have to touch the ring and the placement doesn’t have to be perfect. Don’t spend excessive time making adjustments when aligning the PID. (remember: the paralleling technique is not very comfortable and the patient won’t appreciate any delays in exposing the film). MAXILLARY CANINE Þ • Maxillary canine most difficult to take since it is at the corner The film is centered on the canine. The film should be placed well back in the mouth, away from the teeth, where the palatal vault is the highest. • Make sure the long axis of the film stays in line with the long axis of the tooth when the patient closes. If the film tips, place a cotton roll between the biteblock and the mandibular teeth to keep the film aligned with the canine. • This is a typical maxillary canine film (tooth 11). Note the overall* (red arrow) between canine and first premolar. This is usually not avoidable in the maxillary canine region using the paralleling technique MAXILLARY CENTRAL-LATERAL • The film is centered on the contact between the central and lateral incisors. Make sure the mesial edge of the film crosses the midline slightly (into the opposite central incisor), to insure getting all of the central incisor crown on the film. The film should be placed well back in the mouth, away from the teeth, where the palatal vault is the highest. • This is a typical maxillary central-lateral periapical film. Both the crown’s and roots of the central and lateral incisors (#’s 9 and 10 in this film) are completely visible. Þ ALL POSTERIOR FILMS • • Although we routinely use the #1 size film in the anterior region because it is easier to place in the mouth due to its narrower width, it is also possible to use the #2 size film (for all anterior projections). However, when the #2 size film is used for the maxillary incisors, it is usually centered on the midline, allowing you to image all four incisors on one film (the film at right is slightly cropped, cutting off the distal of the laterals) Overlap refers to the superimposition of part of one tooth over a part of the adjacent tooth. In this film, the mesial of the tooth #12 is “overlapping” the distal of #11 The film should be equidistant from the teeth in an anterior-posterior direction (the distance from the front edge of the film to the lingual surface of the teeth should be the same as the distance from the back edge of the film to the lingual surface of the teeth, indicated by red arrows below). The film should be positioned in this manner for both the premolar and molar radiographs. This helps to avoid overall (see errors). MAXILLARY PREMOLAR • The film is positioned so that the anterior edge is at least in the middle of the canine, or more anterior if possible. The film is approximately centered on the 2nd premolar. The top edge of film is approximately in the center of the palate (side-to-side). • The premolar film below shows the first and second premolar and the first molar completely; a portion of the second molar is also seen. MAXILLARY MOLAR • The film is centered on the second molar. The top edge of the film is in the center of the palate (side-to-side). The film should be centered on the second molar event if the third molars are not present. • The molar film below shows the first and second molars and the third molar region (the third molar has been extracted). The maxillary tuberosity (red arrow) is easy identifiable. • • • • Some patients have a maxillary torus, which is a bony growth in the center of the palate. If a palatal torus is present, place the film so that the top edge is on the opposite side of the torus away from the teeth being radiographed. The film should not rest on the torus. (See diagram below) • Some patients, especially larger individuals, will have longer than normal teeth. With the normal positioning of the film and alignment of the beam, the apices of the teeth will be above the edge of the film (not visible or "cut off") as seen in the film below. To compensate for this, increase the angle of the beam and raise the PID slightly (illustration below right). You are purposely foreshortening the image. You will not know the teeth are longer from just looking at the patient, but if you have taken previous films, or you get films from another dentist, you can identify the need to alter your technique. The incisor film shows all four mandibular incisors. The distal aspects of the lateral incisors are often cut off but you can see these areas on the canine films. All four roots are clearly visible. MANDIBULAR CANINE • • For all mandibular films, do not force the film down into the floor of the mouth trying to get the biteblock to contact the occlusal surface of the mandibular teeth. Position the film in a parallel relationship and let the patient guide the film into place as they close their mouth. Have the patient bite slowly and gently. The film is centered on the canine. The film should be placed back in the mouth, away from the teeth, as much as possible. The bottom edge of the film is placed under the tongue and as the film is uprighted into a parallel position, the tongue is pushed back slightly. This canine film shows the mandibular canine (#22) and most of the lateral incisor and first premolar. MANDIBULAR PREMOLAR • The anterior edge of the film is positioned at least in the middle of the canine, or more anterior if possible. The film is approximately centered on the 2nd premolar. The film should be placed more toward the middle of the mouth, away from the teeth. This will be more comfortable for the patient. However, this is usually the most uncomfortable film taken on a patient using the paralleling technique. Þ Because the floor of the mouth is more shallow in the middle of the anteroposterior • This premolar film shows the mandibular and second premolars, the first molar and part of the second molar. MANDIBULAR INCISOR • The film is centered on the contact between the central incisors (midline). The film should be placed back in the mouth, away from the teeth, as much as possible. The bottom edge of the film is placed under the tongue and as the film is uprighted into a parallel position, the tongue is pushed back slightly.