Chapter 18: Bisecting Technique

1. Plane of the receptror=D 2. Long axis of the tooth=B 3. Imaginary bisector=C 4. Central ray=A

1. Plane of the receptror= 2. Long axis of the tooth= 3. Imaginary bisector= 4. Central ray=

Bisecting PID Angulation-->* Horizontal Angulation*

1. Position *the image receptor parallel to the teeth* of the arch to open the contact 2. Then direct *the central ray perpendicular to the mean tangent*, or through the contact points of the teeth

Rules of Bisecting Technique

1. Receptor placement 2. Receptor position 3. Vertical angulation 4. Horizontal angulation 5. Receptor exposure

Bisecting Technique Summery

1. To take an accurate x-ray: a. The long axis of the tooth must be found b. The long axis of the image receptor must be found. c. Then imagine a bisector forming two isometric triangles (triangles of equal measurements) 2. Direct the primary beam perpendicular to the *imaginary bisector line* -*Not usually accurate, usually some distortion of dimensions occur* 3. Best to use a *short cone (PID) 8 inches* -Short cone *minimize the magnification and distortion* that can occur with this technique. - Compared to the 16 inch long cone for the paralleling technique. 4. With size *#2 image receptor* a *total of six anterior* film placements are used in the bisecting techniques 5. *the primary disadvantage of the bisecting technique* when contrasted with the paralleling technique is: -*Dimensional distortion*

Disadvantages of Bisecting

1. Usually some dimensional distortion. 2. Often superimposes adjacent structures. 3. Estimating the imaginary bisector line may be difficult 4. *Short PID may increase radiation dose.*

Rules for bisecting techniques (ppt slides/handout)

1.Determine the correct vertical angulation by directing *the primary beam perpendicular to the imaginary bisector* 2. *Have the patient bite on the bite block as close to the teeth as possible.* 3. Bite on bite block firmly enough to hold the image receptor in place. May use cotton rolls on opposite side of bite block 4. Determine the correct horizontal angle by directing *the primary beam perpendicular to the teeth through the contact points.* 5. *Determine the correct point of entry by directing the beam toward the apexes of the teeth.*

Long axis (tooth)

An imaginary line that divides the tooth longitudinally into 2 equal halves

Bisecting technique

An intraoral radiographic technique used to expose periapical receptors: the receptor is placed along lingual surface of the tooth; the central ray of x-ray beam is directed perpendicular to the imaginary bisector formed by the receptor and the long axis of the tooth; and receptor holder or the patient's finger is used to stabilize the receptor.

Identify the central ray that is correctly positioned perpendicular to the imaginary bisector

B

Identify the correct vertical angulation

B

Identify the position-indicating device (PID) that is aligned correctly

B

Rule of isometry

Bisecting technique is based on a this simple geometric principle. -States that 2 triangles are equal if they have 2 equal angles and share a common side.

Identify the angle that is bisected correctly

C

Identify the vertical angulation that results in elongation

C

Anterior teeth

Canines and Incisors

Exposure sequence

Definite order for periapical receptor placements and exposures, must be followed.

Isometry

Equality of measurement

Angle (A)

In geometry, a figure formed by 2 lines diverging from a common point

Triangle (C)

In geometry, a figure formed by connecting 3 points not in a straight line by 3 straight-line segments. -Has 3 angles

Equilateral triangle (D)

In geometry, a triangle with a 90 degree angle

Right triangle (E)

In geometry, a triangle with one 90 degree angle (right angle)

Hypotensuse (G)

In geometry, the side of a right triangle opposite the right angle

Discuss the significance of the shaded areas

It is the plane of the receptor

What happens to the dental image when a short (8 in) PID is used

Magnification

Identify the vertical angulation that results in foreshortening

None

Posterior Teeth

Premolars and Molars

Which of the following describes the distance between the receptor and the tooth in the bisecting technique? a. The receptor is placed as close as possible to the tooth. b. The receptor is placed away from the tooth and toward the middle of the oral cavity. c. The receptor is placed parallel to the tooth. d. None of the above.

a. The receptor is placed as close as possible to the tooth.

The disadvantages of the bisecting technique outweigh the advantages. a. true b. false

a. true

Which of the following are advantages of the bisecting technique? a. increased accuracy b. simplicity of use c. shorter exposure time 1) 1, 2, and 3 2) 1 and 2 3) 2 and 3 4) 3 only

c. Shorter exposure time 4) 3 only

Which of the following describes the proper direction of the central ray in the bisecting technique? a. 90 degrees to the long axis of the tooth b. 90 degrees to the receptor and long axis of the tooth c. 90 degrees to the receptor d. 90 degrees to the imaginary bisector

d. 90 degrees to the imaginary bisector

beam alignment device

Used to position an intraoral receptor in the mouth and maintain it in position during exposure

Advantages of Bisecting

1. Image receptor placement may be easier with certain patients: Children, low palatal vaults, tori, gagging, etc. 2. Short PID may be easier to maneuver

Points of entry for Mandibular projection: -

*1st-Incisors, *CR-at a point in the symphysis* *2nd-Canine, *CR-Center of the root of the canine* *3rd-Premolars, *CR- below the pupil of the eye* *4th- Molar, *CR- below the outer canthus of the eye*

Vertical angulation differs according to the imaging technique used

*Paralleling Technique*=angulation of the central ray is directed perpendicular to the receptor and the long axis of the tooth *Bisecting technique*=angulation is determined by the imaginary bisector; the central ray is directed perpendicular to the imaginary bisector. *Bite-wing technique*= angulation is predetermined; the central ray is directed at +10 degrees to the occlusal plane.

Bisecting PID Angulation--->*Vertical Angulation*

*Too much Vertical Angulation vs. Not enough Vertical angulation*

Fundamentals of Bisecting Technique--->*Isometric Triangles and Bisector*

- To cast and accurate shadow representation of a tooth onto the image receptor, *the angle formed by the long axis of the tooth and the plane of the image receptor must be bisected.* -Two isometric triangles are formed when the central ray is directed perpendicular to the bisector, and the image that results should be the same size as the tooth

Bisecting Image Receptor Holder

-*Bisecting image receptor holders* usually have a *short bite block and has an angle of 105 degrees* -*Place the image receptor as close to the teeth to tilt away from the tooth*

The bisecting technique principle is applied when the image receptor is not, or cannot, be placed parallel to the long axis of the teeth

-*Pedo patient* -*Shallow palatal vault* -*Large torus or tori* -*Edentulous regions* -*Gagging patient*

Fundamentals of Bisecting Technique--->*Target-image Receptor Distance*

-*Shorter target-imgate receptor distance will limit magnification and distortion* -*The shorter PID, 8 inch (20.5 cm)preferred*

Points of entry for Maxillary Projection: +

-1st- Incisors, *CR- near the tip of the nose* -2nd-Canine, *CR-root of the canine, ala of the nose* -3rd-premolars, *CR-on the ala-tragus line, below the pupil of the eye* -4th-Molars, *CR-ala-tragus line, below the outer canthus of the eye*

Correct Vertical Angulation

-Angulation results in a dental image that is of the same length as that of the tooth

Incorrect Vertical Angulation

-Angulation results in a radiographic image that is not of the same length as that of the tooth; instead, the image appears longer or shorter. -elongated or foreshortened images are not diagnostic.

Points of entry for the bisecting Technique

-Facial landmarks can provide the radiographer with a reference for positioning the PID and directing central ray of the x-ray beam - Patient must be seated upright with the midsagittal plane parallel to the floor to use these landmarks accurately *1st-centrals* *2nd-canine* *3rd premolars* *4th-molars*

Vertical Angulation-->*Too much vertical angulation*

-If angled perpendicular to the image receptor tooth (not perpendicular to the imaginary bisector), then the image is *foreshortened*

Vertical Angulation-->*Not enough vertical angulation*

-If angled perpendicular to the tooth, not perpendicular to the imaginary bisector, then the image is *elongated.*

Fundamentals of Bisecting Technique--->*Object-image receptor distance*

-It is important to *place the image receptor close to the teeth* in both the anterior and posterior regions of the maxillary and mandibular. -Because it satisfies fewer shadow cast principles, the bisecting technique *is less likely to produce superior diagnostic quality radiographs*

How is the patient's head positioned before exposing mandibular periapicals with the bisecting technique

-Maxillary: Parallel to the floor -Madsagital: Perpendicular to the floor

How is the patient's head positioned before exposing maxillary periapicals with the bisecting technique

-Maxillary: Parallel to the floor -Midsagital: Perpendicular to the floor

Elongation

-Refers to images of the teeth that appear too long. -Results from insufficient vertical angulation. -When vertical angulation is too flat, the image of the tooth appears longer than the actual tooth -Also occurs if the central ray is directed perpendicular to the long axis of the tooth rather than to the imaginary bisector.

Vertical angulation

-Refers to the positioning of the PID in a vertical, or up-and-down, plane -This angulation is measured in degrees and is registered on the outside of the tubehead.

Incorrect horizontal Angulation

-Results in overlapped or "unopened" contact areas. -An image with overlapped interproximal contact areas cannot be used to examine the interproximal areas of the teeth and is thus nondiagnostic.

Principles of the bisecting technique

-The bisecting Technique is based on a simple geometric principle known as the rule of isometry. -In dental imaging this geometric principle is applied to the bisecting technique to form 2 imaginary equal triangles

Achieving accurate horizontal angulation

-The central ray is directed perpendicular to the curvature of the arch and through the contact areas of the teeth. -As a result the contact areas on the dental image appear "opened"

Comparison of the bisecting & Paralleling methods

-With the bisecting technique, the image receptor is positioned adjacent to the tooth, making a target-image receptor distance of 8 in. acceptable. With the paralleling technique the image receptor is positioned near the center of the oral cavity, where if must be retained in a position parallel to the long axes of the teeth. This increased object-image receptor distance requires a longer target-image receptor distance to produce a quality radiograph.

Discuss the exposure sequence for the 14 periapical placements using the bisecting technique

-max canine -max incisor -mand canine -mand incisor -max premolar -max molar -mand premolar -mand molar

Foreshortening

-refers to radiographic images that appear shortened -Results from excessive vertical angulation. -When the vertical angulation is too steep, the image of the tooth appears shorter than the actual tooth. -Also occurs if the central ray is directed perpendicular to the plane of the receptor rather than to the imaginary bisector.

Horizontal angulation

Refers to the positioning of the PID and the direction of the central ray in a side-to-side plane. -This angulation does not differ according to the technique used; paralleling, bisecting, and bite-wing techniques all use the same principles of this angulation.

Which beam alignment device is recommended for use with the bisecting technique because it aids in the alignment of the PID and reduces patient exposure?

Rinn XCP instruments

Which size receptor is used with the bisecting technique?

Size 2

Angulation

Term used to describe the alignment of the central ray of the x-ray beam in horizontal and vertical planes

Central ray

The central portion of the primary beam of x-radiation

Rules of Bisecting Technique-->*Vertical angulation

The central ray of the x-ray beam must be directed perpendicular (at a right angle) to the imaginary bisector that divides the angle formed by the receptor and the long axis of the tooth

Rules of Bisecting Technique-->*Horizontal angulation

The central ray of the x-ray beam must be directed through the contact areas between teeth.

Imaginary bisector

The dental radiographer must visualize a plane that divides in half, or bisects, the angle formed by the receptor and the long axis of the tooth. -This visualized plane creates 2 equal angles and provides a common side for the 2 imaginary equal triangles.

Rules of Bisecting Technique-->*Receptor position

The receptor must be placed against the lingual surface of the tooth. The occlusal end of the receptor must extend approximately 1/8 of an inch beyond the incisal or occlusal surfaces. The apical end of the receptor must rest against the palatal or alveolar tissues.

Rules of Bisecting Technique-->* Receptor placement

The receptor must be positioned to cover the prescribed area of the tooth to be examined. Specific placements are described in the procedures.

Receptor placement

The specific area where the receptor must be positioned before exposure.

Rules of Bisecting Technique-->*Receptor exposure

The x-ray beam must be centered on the receptor to ensure that all areas of the receptor are exposed. Failure to center the x-ray beam results in a partial image or a cone-cut

Bisect (B)

To dived into 2 equal parts

Congruent triangles (F)

Triangles that are identical and correspond exactly when superimposed

State the rule of isometry

Two triangles are equal if they have two equal angles and share a common side