Intraoral Radiographic Techniques (Paralleling Technique)-Chapter 2

Basic Principles
The paralleling technique of intraoral radiography was developed by Gordon M. Fitzgerald, and is so named because the object (tooth), receptor (film packet), and end of the position indicating device (PID) are all kept on parallel planes.  Its basis lies in the principle that image sharpness is primarily affected by focal-film distance (distance from the focal spot within the tube head and the film), object-film distance, motion, and the effective size of the focal spot of the x-ray tube.

Successfully using the paralleling technique depends largely on maintaining certain essential conditions as illustrated in Figure 8.  These are: 1) the film packet should be flat; 2) the film packet must be positioned parallel to the long axis of the teeth; and 3) the central ray of the x-ray beam must be kept perpendicular to the teeth and film.

 

Figure 8
 
To achieve parallelism between the film and tooth (i.e., to avoid bending or angling the film) there must be space between the object and film.  However, remember that as the object-to-film distance increases, the image magnification or distortion also increases.  To compensate, manufacturers are recessing the target (focal spot) into the back of the tube head.  Depending on the machine's age, and placement of the focal spot within the tube head, you may encounter long, medium, or short cones/PIDs.  The goal is to have the focal spot at least 12" or 30 cm from the film to reduce image distortion.
The anatomic configuration of the oral cavity determines the distance needed between film and object and varies among individuals.  However, even under difficult conditions, a diagnostic quality radiograph can be obtained provided that the film packet is not more than 20 degrees out of parallel with the tooth, and that the face of the PID/cone is exactly parallel to the film packet to produce a central beam which is perpendicular to the long axis of the tooth and the film packet.
The major advantage of the paralleling technique, when done correctly, is that the image formed on the film will have both linear and dimensional accuracy.  The major disadvantages are the difficulty in placing the film packet and the relative discomfort the patient must endure as a result of the film holding devices used to maintain parallelism.  The latter is particularly acute in patients with small mouths and in children.  In certain circumstances the film and holder may be slightly tipped toward the palate to accommodate oral space and patient comfort.  Too much palatal tipping will throw off all parallel planes.


Beam Angulation
The position of the x-ray tube head is usually adjusted in two directions: vertically and horizontally.  The vertical plane is adjusted by moving the tube head up and down.  The horizontal plane is adjusted by moving the tube head from side to side.  By convention, deflecting the head so that it points downward is described as positive vertical angulation or + vertical.  Correspondingly, an upward deflection is referred to as negative vertical angulation or - vertical (Figure 9).  The degree of vertical angulation is usually described in terms of plus or minus degrees as measured by a dial on the side of the tube head.


Figure 9
 
When applying the paralleling technique, the vertical angulation is ALWAYS dictated to maintaining the parallel plane. There is no set degree number to follow.  As stated earlier under basic principles, the object (tooth), receptor (film packet), and end of the position indicating device (PID) are all kept on parallel planes.  If the vertical angulation is excessive the image will appear foreshortened.  Insufficient vertical angulation procedures an elongated image.
The beam’s horizontal direction determines the degree of overlap among the tooth images at the interproximal spaces.  If the beam is not perpendicular to the specific interproximal space(s) as it approaches several relatively aligned objects, the objects overlap and the space(s) between them close.  Imagine a flashlight beam approaching a picket fence perpendicularly at a 90-degree angle.  The spaces between the pickets will remain open in the shadow image unless the beam angle varies from perpendicular or 90 degrees.  The degree of overlapping of the image will increase or decrease as the beam angle increases or decreases from the perpendicular.
 
Film Holding Devices
The paralleling technique requires the use of film holding devices to maintain the relatively precise positioning needed.  A great variety of film holders are commercially available—simple, complex, light, heavy, reusable, disposable, autoclavable, and non-autoclavable.  A few of the more common include XCP (extension cone paralleling) with localizing rings, Snap-a-ray, Precision rectangular paralleling device, Uni-Bite, and Stabe biteblock (Figure 10 and 11).  Having several options available will provide the operator different opportunities for enhanced patient comfort.  It is not uncommon to employ more than one option during the same radiographic survey.

 
Figure 10
 
Figure 11 
 
The dental radiographer should be able to assess which holder best conforms to the technical and diagnostic requirements of the job, the needs of the patient, and infection control protocols within the office.

(Table 1: Paralleling - Exposure Guide and Film Placement - Helpful hints when utilizing Stabe or Snap-a-ray film holders)
Paralleling - Exposure Guide and Film Placement 

Helpful hints when utilizing Stabe or Snap-a-ray film holders


BITEWINGS
Teeth to include
C.R. Entry Point
Vertical. Angulation
R Molar BWX#1, 2, 3, 30, 31, 32 & D. of 4 & 29contact of #2 & #3+ 10 (down angle of PID)
R Premolar BWX#4, 5, 28, 29 & D. of 6 and 27contact of #4 & #5+ 10 (down)
L Premolar BWXD. of #11 and 22, and #12, 13, 20, 21contact of #12 & #13+ 10 (down)
L Molar BWXD. of #13 and 20, 14, 15, 16, 17, 18, 19contact of #14 & 15+ 10 (down)

*     *     *     *     *
An imaginary plane can be visualized on the face to offer approximate C.R. placement. For Maxillary exposures, imagine the plane to extend between the ala of the nose and the tragus of the ear (a.k.a. the ala-tragus line.) For Mandibular exposures, imagine this plane to extend between the commissure of the mouth and the tragus of the ear (a.k.a. the commissure-tragus line.) Once this plane is established, the following entry points will be a guide for C.R. placement. Approximate vertical angulations are only guides and must be checked for paralleling before exposure as each person’s anatomy is different.

PERIAPICALS
Teeth to include
Approx. C.R. Entry Point
Approx. Vert. Ang.
UR Molars #1, 2, half of #3 outer canthus of the eye +20 - +30 (down)
UR Premolars M. of #3, 4, 5, and half of #6 pupil of the eye +30 - + 40 (down)
UR Canine center #6 – c.r. at D. of 6 ala of the nose +45 - + 55 (down)
Max. Incisors center #7, 8, 9, 10 (I.U. method) tip of the nose +40 - +50 (down)
UL Canine center #11 – c.r. at D. of 11 ala of the nose +45 - + 55 (down)
UL Premolars half of #11, and 12, 13, M. of 14 pupil of the eye +30 - + 40 (down)
UL Molars half of #14, and 15, 16 outer canthus of the eye +20 - +30 (down)

LL Molars #17, 18, and half of 19 outer canthus of the eye - 5 – 0 (up angle of PID)
LL Premolars M. of #19, and 20, 21, and half of 22 pupil of the eye -10 - -15 (up)
LL Canine center #22 – c.r. at D. of 22 ala of the nose -20 - -30 (up)
Mand. Incisors center #23, 24, 25, 26 (I.U. method) cup the chin -15 - -25 (up)
LR Canine center #27 – c.r. at D. 27 ala of the nose -20 - -30 (up)
LR Premolars half of #27, and 28, 29 and M. of #30 pupil of the eye -10 - -15 (up)
LR Molars half of #30, and 31, 32 outer canthus of the eye - 5 – 0 (up)


Care of: Willie Leeuw, CDA, BS - Indiana University Purdue University Fort Wayne , Department of Dental Assisting


Quiz
  1. What is the basic principle of the paralleling technique?
  2. What is the major advantage of the paralleling technique?
  3. What are the major disadvantages?
  4. What must be done to achieve parallelism between the tooth and film?
  5. List several devices available to position the film properly when using the paralleling technique.
Answers
  1. The film packet must be positioned parallel to the long axis of the teeth and the x-ray beam must be kept perpendicular to the teeth and film.
  2. Linear and dimensional accuracy.
  3. Difficulty in placement of the film packet, relative discomfort to the patient caused by film holding devices.
  4. The film must be placed away from the tooth.
  5. XCP with localizing rings, Snap-a-ray, Precision rectangular devices, Uni-Bite, and Stabe biteblock.

Paralleling Technique Methodology
When taking a full mouth survey, a definite order of exposure should be preplanned and then followed.  Since patients tolerate anterior films better, they should be done first.  Starting with the maxillary central incisors and proceeding distally, first along one side, then the other, is recommended.  The radiographic parameters or exposure factors should also be determined prior to placing films in the patient’s mouth. 

Patient Positioning
When positioning a patient, there are two imaginary planes that must be considered.  The occlusal plane runs horizontally, dividing the patient’s head into upper and lower portions. 

It can be visualized by imagining the patient holding a ruler between his or her teeth.  A midsagittal plane divides a mass (the patient’s head or body) on a vertical dimension into equal right and left portions.

When using the paralleling technique to examine the maxillary region, the patient is positioned so that the occlusal plane of the maxilla is parallel to the floor and the sagittal plane of the patient’s head is perpendicular to the floor.
When paralleling the mandibular region, the patient’s position must be modified slightly so that when the mouth is open, the mandible is parallel to the floor and the sagittal plane is perpendicular.  This could mean that the patient must be tilted back in the chair.
Before any radiographs are exposed, the patient must be protected with a lead apron and thyroid collar. The apron must be properly placed to avoid interference with the radiographic exposure.  (Figure 12)

Figure 12


Full Mouth Exposure with the Use of XCP Device

Procedure for the Maxillary Central/Lateral Incisors

Assemble the anterior film holder and insert the film packet vertically on the anterior biteblock. Use a #1 film.
Center the film on the central/lateral incisors (Figure 13). Position the film in the palate as posteriorly as possible so that the entire tooth length will appear on the film, with approximately a one-eighth inch border of the film extending below the incisal edge of the centrals. Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 14). Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.

 
Figure 13
Figure 14
  1. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  2. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 15).  
  3. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure. 
Figure 15 
Procedure for the Maxillary Canines


  1. Assemble the anterior film holder and insert the film packet vertically on the anterior biteblock.  Use a #1 film.
  2. Center the film on the canine and first premolar (Figure 16).  Position the film in the palate as posteriorly as possible so that the entire tooth length will appear on the film with approximately a one-eighth inch border below the incisal edge of the canine.  Position the biteblock on the incisal edges of the teeth to be radiographed (Figure 17).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 18).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors. Make the exposure.

Figure 16

Figure 17

Figure 18
Procedure for the Maxillary Premolars


  1. Assemble the posterior film holder and insert the film packet horizontally in the posterior biteblock.  Use a #2 film.
  2. Center the film on the premolars so that it is parallel to the long axis of the teeth (Figure 19).  Position the film in the palate so that the entire tooth length will appear on the film with approximately a one-eighth inch border below the cuspal ridge.  Align the anterior edge of the film packet with the canine so that the image captured on the anterior border of the film will include the distal third of the canine.  Position the biteblock on the occlusal surfaces of the teeth to be radiographed (Figure 20).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  (The occlusal border of the film tends to slip lingually.)
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 21).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure. 
Figure 19
 
Figure 20

Figure 21
Procedure for the Maxillary Molar Region
  1. Assemble the posterior film holder and insert the film packet horizontally in the posterior biteblock.  Use a #2 film.
  2. Center the film on the molars so that it is parallel to the long axis of the teeth (Figure 22).  Position the film in the palate so that the entire tooth length will appear on the film with approximately a one-eighth inch border below the cuspal ridge.  Align the anterior border of the film packet with the second premolar so that the image captured on the anterior edge of the film will be the distal third of the second premolar.  Position the biteblock on the occlusal surfaces of the teeth to be radiographed (Figure 23).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the mandibular teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 24).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.



Figure 22

Figure 23

Figure 24
Procedure for the Mandibular Central/Lateral Incisors
  1. Assemble the anterior film holder and insert the film packet vertically on the anterior biteblock.  Use a #1 film.
  2. Center the film on the mandibular central and lateral incisors (Figure 25).  It may be necessary to displace the tongue distally and depress the film onto the floor of the mouth so that the entire tooth length will show with approximately a one-eighth inch border above the incisal edges.  The film must be as posterior as the anatomy allows and the biteblock should be positioned on the edges of the incisors to be radiographed (Figure 26).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the maxillary teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 27).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure.



Figure 25

Figure 26

Figure 27

Procedure for the Mandibular Canines
  1. Assemble the anterior film holder and insert the film packet vertically on the anterior biteblock.  Use a #1 film.
  2. Center the film on the mandibular canine (Figure 28).  It may be necessary to displace the tongue distally and depress the film onto the floor of the mouth so that the entire tooth length will show with approximately a one-eighth inch border above the cuspal edge.  The film must be as posterior as the anatomy allows and the biteblock should be positioned on the edges of the teeth to be radiographed (Figure 29).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the maxillary teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 30).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure. 

Figure 28

Figure 29

Figure 30

Procedure for the Mandibular Premolars
  1. Assemble the posterior film holder and insert the film packet horizontally on the posterior biteblock.  Use a #2 film.
  2. Center the film on the premolars so that it is parallel to the long axis of the teeth (Figure 31).  The object-to-film distance in both the mandibular premolar and molar regions is minimal since the oral anatomy only allows the film to be positioned very close to the teeth and still remain parallel.  Align the anterior border of the film packet with the canine so that the image captured on the anterior edge of the film will be the distal third of the canine.  Position the biteblock on the occlusal surfaces of the teeth to be radiographed (Figure 32).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the maxillary teeth and the biteblock for patient comfort. Ask the patient to slowly, but firmly, bite onto the block to maintain film position. The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 33).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors.  Make the exposure. 

Figure 31
Figure 32
Figure 33
Procedure for the Mandibular Molars
  1. Assemble the posterior film holder and insert the film packet horizontally on the posterior biteblock.  Use a #2 film.
  2. Center the film on the molars so that it is parallel to the long axis of the teeth (Figure 34).  Depress the film onto the floor of the mouth so the entire length of the teeth will appear with approximately a one-eighth inch border above the occlusal surface.  Place the film horizontally and position it lingually to the molars so that the long axis of the film is parallel to the long axis of the tooth.  Align the anterior border of the film packet with the second premolar so that the image captured on the anterior edge of the film will be the distal third of the second premolar.  Position the biteblock on the occlusal surfaces of the mandibular teeth (Figure 35).  Proper positioning in this step will place the central ray of the x-ray beam at the interproximal contact desired.
  3. A cotton roll may be inserted between the maxillary teeth and the biteblock for patient comfort.  Ask the patient to slowly, but firmly, bite onto the block to maintain film position.  The film should be straightened as the patient closes and the floor of the mouth relaxes.
  4. Slide the aiming ring down the indicator rod to the skin surface, align the x-ray tube close to the aiming ring, and center (Figure 36).
  5. Follow the film and equipment manufacturer’s recommendation concerning exposure factors. Make the exposure.

Figure 34

Figure 35
Figure 36


 Quiz
  1. What is the recommended patient positioning for examining the maxillary region using the paralleling technique?
  2. What is the recommended patient positioning for examining the mandibular region using the paralleling technique?

Answers

  1. The occlusal plane of the maxilla is parallel to the floor and the sagittal plane of the patient’s head is perpendicular to the floor.
  2. When the mouth is open, the mandibular occlusal plane is parallel to the floor and the sagittal plane of the patient’s head is perpendicular to the floor.


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