0 Paralleling Technique The following slides describe the Paralleling Technique.In navigating through the slides, you should clickon the left mouse button when you see themouse holding an x-ray tubehead or you aredone reading a slide. Hitting “Enter” or “PageDown” will also work. To go back to the previousslide, hit “backspace” or “page up”.
Patient PreparationPrior to starting to take films, the patient mustbe positioned properly. Seat the patient and askthem to remove their glasses and any removableappliances. Adjust the headrest to support thehead while taking films. Raise or lower the chairto a comfortable height for the operator. Placethe lead apron and thyroid collar on the patient.You are now ready to begin taking films.It is a good idea to inform the patient about thenumber of films you will be taking so they knowwhat to expect.
In the paralleling technique, the film is placed in themouth so that the long axis of the film is parallel withthe long axis of the teeth being radiographed. Aparalleling instrument with an aiming ring is normallyused to orient the film, teeth and ring in a parallelrelationship. When the x-ray beam is aligned with thering, the x-ray beam will be perpendicular (rightangle) to the teeth and the film. eam a yb X-r Film/tooth/ring all parallel X-ray beam perpendicular to tooth/film
Paralleling Technique (Advantages)There are two techniques for taking periapical films, theparalleling and the bisecting angle techniques. Whencomparing the two techniques, the advantages of theparalleling technique are: 1. Better dimensional accuracy: the paralleling technique results in less distortion of the image of the teeth. (The shape of the teeth and the relationship of the teeth to surrounding structures is more accurate). 2. When using the paralleling instrument with the aiming ring, the alignment of the x-ray beam is simplified. (continued next slide)
Paralleling Technique (Advantages) 3. It is easier to standardize films. Because you are using the positioning instrument, it is easier to position the film in approximately the same position at different appointments. This can be helpful if you are trying to compare the appearance of a periapical lesion from one visit to the next. 4. Head position is not as critical. Because of the paralleling instrument, with its aiming ring, it is easy to properly align the x-ray beam no matter how the head is positioned.
0When the long axis of the film is parallel with thelong axis of the tooth, the image of the tooth onthe film looks the same as the tooth itself (nodistortion). The image will be slightly larger thanthe actual tooth (magnification), but the shape isthe same.
Paralleling Technique (Disadvantages) 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.
Paralleling Film Placement 0As mentioned previously, the film is placed in themouth so that the long axis of the film is parallel withthe long axis of the teeth. Since all teeth are inclinedtoward the middle of the head (not straight up anddown), the film will be slightly angled in the mouth(see below left). If the film is maintained in an uprightposition (below right), the patient will not be able toclose on the biteblock and the film will not be parallel. correct incorrect
Paralleling Film Placement 0To facilitate film placement, the film may be tipped upto 20 degrees beyond parallel.
Paralleling Film Placement 0Because the palate andfloor of the mouth areshallower as you approachthe lingual of the teeth, thefilm often cannot bepositioned properly close tothe teeth.As a result, the film must bepositioned away from theteeth (farther back in themouth) to achieve parallelism.
0Because the film is farther from the teeth, there willbe increased magnification (larger size) anddecreased sharpness (less detail). To compensatefor this, the target-film distance should beincreased (the target is where the x-rays areproduced). size of image at 8” target-film distance Target 16” Target 8” size of image at 16” target-film distance
0The target-film distance is increased by using a longerPID, using a machine with a recessed target (oppositeside of the tubehead from the PID) or a combination.The medium PID with a recessed target is a goodcompromise. The disadvantage to increasing the PIDlength is that the exposure time must be increased. Ifyou change from an 8” target-film distance to a 16”target-film distance (double the distance) theexposure time will be four times as much (see InverseSquare Law). Long PID Short PID Medium PID Recessed target Recessed target
Paralleling Technique Head PositionAs mentioned previously, head position is not asimportant when using the paralleling technique.However, in general it is best to position the headin an upright position so that the maxillary arch isparallel to the floor. Best OK OK
Paralleling Technique Film Selection for AdultsThe # 1 size film is used for anterior periapical filmsusing the paralleling technique. The long axis of thefilm is vertical. For posterior films, # 2 size film isused with the long axis horizontal. #1 #2 anterior posterior
Paralleling Technique Film Selection for ChildrenFor children with small mouths, the # 0 size film isused for both anterior and posterior periapicalfilms. However, if the child’s mouth is largeenough to reasonably accommodate the largersize films (# 1 anterior, # 2 posterior), and the childis cooperative, they should be used. #0 #0 anterior posterior
Rinn Paralleling InstrumentsThe Rinn paralleling instruments are used at the OhioState University College of Dentistry. They are color-coded, with yellow being the posterior instrument andblue being the anterior instrument. The metal barconnects to the side of the biteblock and the ringslides on the bar. POSTERIOR ANTERIOR
The film is placed in the biteblock so that the all-whiteside of the film packet faces the teeth and, byextension, the ring. (The colored portion of the film isagainst the back support of the biteblock). When youlook down through the ring, you should see the all-white side of the film packet centered in the opening. opposite side toward tubefront back
Anterior Periapical 0For the anterior periapical, the # 1 size film is placedvertically in the biteblock. The film is rotated so theidentifying black dot is down; this end of the film goes intothe slot of the biteblock (dot-in-the-slot). Push the filmback against the biteblock support and slide it down intothe slot. F E Speed 1-Filmdot Kodak INSIGHT Dental Film white side of film long axis vertical colored side of film white side facing teeth/ring slot
Posterior Periapical 0 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 into the slot. OPPOSITE SIDE long axis horizontaldot TOWARD TUBE KODAK white side INSIGHT of film 1- FILM F E colored side of film slot white side facing teeth/ring
General Technique GuidelinesFor all periapical films, the teeth being radiographedmust be in contact with the biteblock to avoid not havingthe apices of the teeth on the film (see errors section ofslide show). Make sure patient doesn’t just close lipstight around biteblock; have them part their lips so youcan confirm the contact. correct incorrect
General Technique GuidelinesAs shown above, cotton rolls may be used in any areaof the mouth to help support the biteblock, especiallyif an edentulous region or uneven teeth oppose theteeth being radiographed. Using a cotton roll alsomakes it more comfortable for the patient to bite insome situations. The cotton roll should be placedagainst the arch opposite the one being radiographed.
General Technique GuidelinesIf a patient has a partial denture or a completedenture in one of the arches, the appliance canbe used to help support the biteblock when thepatient closes. This is normally preferable tousing cotton rolls. Make sure that the denture isonly used in the arch opposite to the one beingradiographed. Partial dentures can not be left inthe arch being radiographed because the metalframework will be superimposed over the imagesof the teeth (see “Errors”).
General Technique Guidelines 0After the patient is biting on the biteblock, and beforealigning the PID, the ring needs to be moved closer tothe patient’s face. While supporting the bar with thefingers of one hand, slide the ring down close to theface with the other hand.
General Technique Guidelines Always make sure the head is supported by the headrest before aligning the PID and exposing the films.
General Technique GuidelinesThe PID should be aligned with the ring so that the endof the PID is equidistant from the ring and within ¼” ofthe ring. The PID doesn’t have to touch the ring and theplacement doesn’t have to be perfect. Don’t spendexcessive time making adjustments when aligning thePID. (Remember: the paralleling technique is not verycomfortable and the patient won’t appreciate any delaysin exposing the film). PID PID PID Correct Incorrect Incorrect (not equidistant) (not close enough)
Maxillary Central-lateralThe film is centered on the contact between the central andlateral incisors. Make sure the mesial edge of the filmcrosses the midline slightly (into the opposite centralincisor), to insure getting all of the central incisor crown onthe film. The film should be placed well back in the mouth,away from the teeth, where the palatal vault is the highest.
Maxillary Central-lateralThis is a typical maxillary central-lateral periapicalfilm. Both the crowns and roots of the central andlateral incisors (#’s 9 and 10 in this film) arecompletely visible.
Maxillary Central-lateralAlthough we routinely use the # 1size film in the anterior regionbecause it is easier to place inthe mouth due to its narrowerwidth, it is also possible to usethe # 2 size film (for all anteriorprojections). However, when the# 2 size film is used for themaxillary incisors, it is usuallycentered on the midline, allowingyou to image all four incisors onone film (the film at right isslightly cropped, cutting off thedistal of the laterals).
Maxillary CanineThe film is centered on the canine. The film shouldbe placed well back in the mouth, away from theteeth, where the palatal vault is the highest.
Maxillary CanineMake sure the long axis of the film stays in linewith the long axis of the tooth when the patientcloses. If the film tips, place a cotton roll betweenthe biteblock and the mandibular teeth to keepthe film aligned with the canine.
Maxillary CanineThis is a typical maxillary caninefilm (tooth # 11). Note theoverlap* (red arrow) betweencanine and first premolar. This isusually not avoidable in themaxillary canine region using theparalleling technique.*overlap refers to the superimpositionof part of one tooth over a part of theadjacent tooth. In this film, the mesialof tooth # 12 is “overlapping” thedistal of # 11.
0 All Posterior FilmsThe film should be equidistant from the teeth in ananterior-posterior direction (the distance from thefront edge of the film to the lingual surface of theteeth should be the same as the distance from theback edge of the film to the lingual surface of theteeth, indicated by red arrows below). The filmshould be positioned in this manner for both thepremolar and molar radiographs. This helps to avoidoverlap (see errors). correct molar premolar incorrect (results in overlap)
Maxillary PremolarThe film is positioned so that the anterior edgeis at least in the middle of the canine, or moreanterior if possible. The film is approximatelycentered on the 2nd premolar. The top edge offilm is approximately in the center of the palate(side-to-side).
Maxillary PremolarThe premolar film below shows the first andsecond premolars and the first molar completely;a portion of the second molar is also seen.
Maxillary MolarThe film is centered on the second molar. The topedge of the film is in the center of the palate (side-to-side). The film should be centered on the secondmolar even if the third molars are not present inorder to identify impactions, root tips or otherpathology that might be present in the third molarregion.
Maxillary MolarThe molar film below shows the first and secondmolars and the third molar region (the thirdmolar has been extracted). The maxillarytuberosity (red arrow) is easily identifiable.
Some patients may have a maxillary torus, which is abony growth in the center of the palate. If a palataltorus is present, place the film so that the top edge ison the opposite side of the torus (away from the teethbeing radiographed). The film should not rest on thetorus. (See diagram below). palatal torus
Some patients, especially larger individuals, will have longerthan normal teeth. With the normal positioning of the film andalignment of the beam, the apices of the teeth will be abovethe edge of the film (not visible or “cut off”) as seen in thefilm below. To compensate for this, increase the angle of thebeam and raise the PID slightly (illustration below right). Youare purposely foreshortening the image. You will not knowthe teeth are longer from just looking at the patient, but if youhave taken previous films, or you get films from anotherdentist, you can identify the need to alter your technique. top edge of PID above ring
Mandibular Incisor 0The film is centered on the contact between the centralincisors (midline). The film should be placed back in themouth, away from the teeth, as much as possible. Thebottom edge of the film is placed under the tongue andas the film is uprighted into a parallel position, thetongue is pushed back slightly.
0For all mandibularfilms, do not forcethe film down intothe floor of themouth trying to getthe biteblock tocontact the occlusalsurface of themandibular teeth.Position the film in aparallel relationshipand let the patientguide the film intoplace as they closetheir mouth. Havethe patient biteslowly and gently.
Mandibular Incisor 0The incisor film below shows all four mandibularincisors. The distal aspects of the lateral incisorsare often cut off but you can see these areas onthe canine films. All four roots are clearly visible.
Mandibular Canine 0The film is centered on the canine. The film should beplaced back in the mouth, away from the teeth, as muchas possible. The bottom edge of the film is placed underthe tongue and as the film is uprighted into a parallelposition, the tongue is pushed back slightly.
Mandibular Canine 0This canine film shows the mandibular canine (#22) and most of the lateral incisor and firstpremolar.
Mandibular Premolar 0The anterior edge of the film is positioned at least in themiddle of the canine, or more anterior if possible. The filmis approximately centered on the 2nd premolar. The filmshould be placed more toward the middle of the mouth,away from the teeth. This will be more comfortable for thepatient. However, this is usually the most uncomfortablefilm taken on a patient using the paralleling technique.
Mandibular PremolarThis premolar film shows the mandibular firstand second premolars, the first molar and part ofthe second molar.
Mandibular MolarThe film is centered on the 2nd molar. The film can beplaced closer to the teeth than in the premolar region.This film is more comfortable than the premolar filmbecause the floor of the mouth is deeper in this region.
Mandibular MolarThis mandibular molar film shows the first andsecond molars and the third molar region (thethird molar was extracted).
Some patients may have bilateral mandibular tori,which are bony growths on the lingual of the mandiblein the premolar region. If tori are present, place thefilm so that it is between the torus and the tongue.Make sure the film doesn’t rest on top of the torus.(See diagram below). mandibular torus
Patients with longer teeth will also require an alteration intechnique in the mandibular arch. Increase the angle of thebeam (increase the negative vertical angulation, e.g., changefrom - 20 degrees to - 35 degrees) and lower the PID slightly(illustration below right). You are purposely foreshorteningthe image.
Adult full-mouth series, Paralleling Technique An adult full-mouth series of films consists of 15 periapical films; 7 anterior (from canine to canine, 4 maxillary and 3 mandibular) and 8 posterior (premolar and molar films in each quadrant). #2 #1 #2R L
Anterior FirstWhen taking films on a patient, you should alwaysstart with the anterior films. If you are doing a fullseries, start with the maxillary canine film andthen finish all the anterior films, both maxillaryand mandibular. Then complete the posteriorfilms, starting with the premolar, then molar, ineach quadrant. When doing only a few films on apatient, start with the most anterior film and workyour way back in the mouth. This sequence oftaking films allows the patient to get used to theprocedure with a minimum of discomfort andhelps to avoid stimulation of the gag reflex.
0 Paralleling Technique ErrorsThe following slides identify some of the mostcommon errors seen when using the parallelingtechnique.
Film PlacementPoor film placement is the most common error seenwhen using the paralleling technique. This usuallyinvolves incorrect anterior-posterior positioning. Thepremolar film is often not far enough forward and themolar film is frequently not far enough back. Thepremolar film below is placed properly. The molar film,however, is too far forward, failing to image the thirdmolar region. Premolar - OK Molar - too anterior
Film PlacementIn the anterior region, failure to properly center the filmis a common error. In the film below, the mesial of thecentral incisor is not visible because the film waspositioned too far back. For the central-lateral film, thefilm must cross the midline slightly in order to insurethat all of the central incisor will be seen.
Film PlacementIf the patient is not completely closed and biting on thebiteblock (photo below), the top of the film will not bepositioned to show the ends of the roots (below right).Usually the patient will tighten their lips around thebiteblock when this occurs; ask the patient to parthis/her lips so that you can make sure they are bitingproperly. roots cut off
Cone CuttingCone cutting occurs when part of the film is not coveredby the x-ray beam. It results in a white (clear) area onthe film because no silver halide crystals were exposedand were not converted to black metallic silver duringprocessing. Using the paralleling instrument, it is veryeasy to align the beam with the film. However, if theinstrument is not assembled properly (ring upsidedown; see diagram below), cone-cutting will result. correct incorrect
Reversed FilmIf the film is placed in the biteblock so that the coloredportion of the film packet faces the ring/teeth, the leadfoil in the packet will be between the teeth and thefilm. The pattern imprinted on the lead foil will bevisible on the film (right side of film below) and thefilm will be lighter because the lead keeps some of thex-rays from reaching the film.
Double exposureWhen taking films, you should always place each film ina container or paper bag immediately after it is exposed.Exposed films should never be placed in the same areawhere unexposed films are located. If you inadvertentlypick up an exposed film and use it for another exposure,the result is a double exposure. Two different areas ofthe mouth are superimposed, making the imagesworthless. This is the worst error because two films haveto be retaken. The film at left shows images of mandibular incisors and mandibular molars. The film was vertical for the incisors and horizontal for the molars.
Patient MovementIf the patient moves slightly during the exposureof the radiograph, the image will be blurred as inthe film below. Always advise the patient toremain still for the very short time it takes tocomplete the exposure.
OverlapAs mentioned previously, the film must be keptequidistant from the teeth when taking posteriorradiographs. If the film is not placed properly, as in thediagram below left, overlapping will result due to theimproper horizontal angulation. Overlap is thesuperimposition of part of one tooth with part of theadjacent tooth (dotted circles below right). The redarrow represents the direction of the x-ray beam.
OverlapThe radiograph below shows the overlap in theregion of the crowns of the teeth.
Incorrect Exposure FactorsThe standard exposure settings on your x-ray machinewill be acceptable for the majority of your patients.However, if you are taking radiographs on a child youwould need to decrease the settings. If your patient isvery large, you would need to increase the settings.Underexposure results when the exposure factors areset too low for the patient size. Overexposure resultswhen the exposure factors are set too high. underexposure correct exposure overexposure
GlassesIt is recommended that glasses be removed beforetaking radiographs, even if they are not expected to bea problem (mandibular films or bitewing radiographs).If the glasses are left on, they may be in the path of thex-ray beam when taking maxillary films and producean image on the film (see below). glasses
Failure to Remove AppliancesRemovable partial dentures, as the name suggests,should be removed prior to taking films. If the RPD isleft in place in the arch being radiographed, the imageof the RPD will obscure the necessary diagnosticinformation. However, an RPD may be left in the mouthin the arch opposite the one being radiographed inorder to support the biteblock. This is more effectivethan using cotton rolls in the edentulous regions.
Film BendingIf you “soften” the film excessively by bending theedges before placing the film in the biteblock, blacklines may be produced due to disruption of theemulsion in the areas where the film was bent. Theseblack lines can also be caused by bending the filmwhen inserting it into the slot of the biteblock. If youjust push down on the film without pushing back onthe biteblock support, this bending may occur.
“Digit”al ImageMake sure the patient is biting firmly on thebiteblock before aligning the tubehead. Do notallow the patient to hold the instrument inposition. If this happens, the patient’s finger mayappear on the film (red arrows on film below).
0This concludes the section on ParallelingTechnique.Additional self-study modules are availableat: http://dent.osu.edu/radiology/resources.htmIf you have any questions, you may e-mailme at: firstname.lastname@example.orgRobert M. Jaynes, DDS, MSDirector, Radiology GroupCollege of DentistryOhio State University