Paralleling Technique

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Radiology lec.8


Paralleling Technique

  also known as:

Extension cone paralleling technique, right-angle technique, and long-cone technique.

Principles of Paralleling Technique:

  • The film is placed in the mouth parallel to the long axis of the tooth being radiographed.

  • The central ray of the x-ray beam is directed perpendicular to the film and the long axis of the tooth.

  • A film holder must be used to keep the film parallel with the long axis of the tooth.

In intraoral radiography we are concerned about 2 distances:

  • Source to film distance:   must be increased to ensure that only the most parallel rays will be directed at the tooth and to have less magnification and less distortion.

  • Object to film distance: there is some debate about it we need it to be far enough to achieve the paralleling technique (the film shouldn’t be sticked to the tooth) on the other hand we are thinking about distortion and magnification so the distance should be small to minimize the magnification (magnification is indirectly proportional to object to film distance).

-it’s all about having balance by making the film far enough to achieve parallelism with the tooth long axis but not so far to compromise the magnification and distortion.

Paralleling technique is the most reproducible, less distorted, less magnified.

Magnification and distortion:

In plane radiography there is always magnification due to divergent beam and other factors, if you took an intraoral radiograph by the film holder in an ideal good geometry every time you will get specific factor (number) you can extract or multiply to know the structure size in reality and this specific factor is 2% (like we said if you put it the ideal geometry) it’s a fixed number always 2%

So if you have a good radiograph and the structure size is 1 cm you will know that in reality it’s lesser by 2% (because it’s magnified by 2%).

But when you make distortion and once it’s at 90 degree other time it’s at 45 degree and other time in an angle in between, you won’t be able to extract the specific factor we talked about.

Question: why source film distance should be big?

To have parallelism , because focal spot size is small it is a dot and x-rays coming out of it are divergent so when you are far enough x-rays will be relatively parallel , when you are closer to the source you will get more photon and more divergence , when you are far you will have less photon but more parallel. But not so far cause you will lose the intensity.

The idea of intraoral radiograph: it’s a confined space when saying close or far away we are talking about millimeters between the film and object, and 1 cm maximally for the central ray and the object. Not like chest radiography where the patient is 2 meters away from the central ray.

Film holders: there are film holders used for paralleling technique, and others like hemostat it’s used just to hold the radiograph

Some patient won’t allow you to get that perfect parallel geometry due to one of the following 3 anatomical consideration:

  • shallow palate

  • bony growth (maxillary and mandibular tori )

  • mandibular premolar region is small

Shallow palate :

The palate line up there (below the arrow) is the palate if you have enough concavity you will put the holder up right and it will be parallel to the tooth and perpendicular to the central ray.

If the palate is shallow and the palate line is more down and you put the holder up right you might injure the patient or the film will bend and bending will cause distortion so bad image quality so useless radiograph . So what to do?

Use cotton rolls between the bite block and the opposite tooth.

This will force the patient to open his mouth more so creating more vertical dimension in his mouth.

Maxillary torus:

due to this torus the vertical space will be limited.

what to do : you will picture the torus by putting the film behind it and it will appear in the radiograph as white opaque shadow that won’t affect your radiograph structure due to different distances from central ray so different discrepancies.

Same for mandibular tori as in the picture

Small premolar region in the mandible

Most female patients have limited space in the mandible so you can’t put the film all the way in 90 degree to be parallel with the tooth

What to do: push the film under the tongue to get more space and tilt the whole set up (the set up won’t stay parallel but in this particular anatomical consideration its acceptable not to achieve parallelism ) the Radiograph won’t be as much bad as the other solution we will talk about which is the bisecting technique.

Some of the already mentioned anatomical considerations and the holders we talked about force you to use other technique than the parallel one which is called the bisecting technique (where the film won’t be parallel to the long axis of the tooth).

Bisecting technique

Remember from fethagoras theory, two triangles are equal when they have 2 equal angles and share a common side.

The angle is between the long axis of the tooth and the film it’s bisected by the bisecting line.

The common side is the bisecting line.

and the beam should be directed at 90 degree to the bisecting line so according to fethgoras both triangles are equal ,the importance of having an equal triangles is that the tooth length in real will be the same on the film (from the picture below AB=CB)

What’s challenging in bisecting technique is finding the bisecting line. To find it you have to know the anatomy well, the patient condition to be able to find the long axis of the tooth, the long axis of the film to bisect the angle in between and to direct the beam at 90 degree, it seems difficult but with trial and error you will be good at it.

Always keep in mind that there is a risk of distortion (because the film is not parallel to the long axis of the tooth), elongation and foreshortening (when the beam is not perpendicular to the bisector plane).

So rules in bisecting technique:

  • The film must be placed along the lingual surface of the tooth

  • The plane of the film and the long axis of the tooth will form an angle where the film contacts the tooth.

  • An imaginary bisector line bisects the angle formed by the film and the long axis of the tooth.

  • The central ray is directed perpendicular to the imaginary bisector.

Vertical angulation : if you are taking the picture from up or down the angle will be different .

  • Foreshortening is by excessive vertical angulation ; like if you are supposed to be at 15 degree and you took it at 30 degree .

  • Elongation; if you took it too flat (the angle is smaller than normal).

Horizontal angulation : it depend on the contacts you want to open between the teeth , and what contacts to open depend on the shot you are taking ,ex: when taking periapical radiograph for a premolar if you come to much from mesial or distal overlap will occur , this overlap will hinder you from knowing the margins of the tooth so the radiograph is useless.

Teeth anatomy helps you to know where to direct the beam like A line and B line in premolars and molars.

** There are numbers that are guide lines in vertical and horizontal angulation. Ex: why maxillary incisor is taken at 40 degree while mandibular incisor at 15 degree only?? Because the long axis is different between them. Also the patient condition plays a role here Ex: if a patient incisor are flared out (class 2 division 1) you should increase the angle more if you put it 40 degree the tooth in the radiograph would look elongated.

Other problems need to be handled by bisecting technique or other modification:

  1. 3rd molar is very far in the back of the jaw so when you put the set up (holder, film) it will induce gag reflux in the patient.

What to do to get the 3rd molar shot especially if it was horizontally impacted or displaced?

If you come directly straight like other teeth you won’t cover the tip of the apex cause it’s far away, so you either

1) You take a panoramic radiograph


2) You do your regular radiograph to see the contacts with the adjacent tooth, and you do other radiograph called distomolar projection

Distomolar projection: where you come all the way distally to get the tip of the apex but the contacts won’t appear that’s why you take 2 radiographs.

  1. In endodontic treatment you need specific holder not like the ones mentioned early because the patient need to bite on them and in endo treatment he can’t bite due to the presence of files and gutta percha in the tooth, the edno holder has a basket which is a space for the files , so the patient bites on the two adjacent teeth (the one before and the one after the tooth being treated) , by using this holder you will protect the patient and achieve parallelism and stabilization of the set up.

  1. Other case,, if you took a radiograph for a premolar and the roots where superimposed on each other and one of them is shorter ,, how to know which one is shorter (buccal or lingual) ??

You do what’s called buccal root rule (slob technique) :

It is taking the radiograph in different angulation , as the buccal object moves with the central ray and the lingual object moves with the central tube , because the tube and the ray are like a pivot or a stick when the tube moves distally the beam moves mesially and vice versa . As in the picture below.

Also buccal object rule is used for impacted teeth EX: impacted canine that needs to be extracted, where to open buccal or lingual? , this rule will help the surgeon to decide.

In buccal object rule we are scarifying the parallelism because we are changing the horizontal angulation but it is ok since this radiograph is taken for specific task not within regular full mouth series.

  1. The edentulous ridges are little bit difficult cause there won’t be enough support.

What to do: if the edentulous is on the opposite jaw you fill the space by cotton rolls to get stabilization.

Occlusal projection

Occlusal projection: is a technique used to examine larger area, the film size will be larger, in adult we use size 4 and in kids we use size 2 to get the occlusal radiograph.

Occlusals are supplementary radiographic technique (not the first thing to start with)

Indications for occlusal projection:

  1. If there is larger involvement of both maxilla and mandible ‘

  2. When checking for buccolingual expansion

  3. When checking for salivary glands stones

  4. In kids sometimes the area is very small even for bisecting technique, where the periapical (PA) of central projection will be occlusal projection.

Why is it called occlusals?

Cause the patient bites on it, there is no bite blocks, you put it inside the mouth and the patient gently bites on it

Occlusals are divided into:

  1. Maxillary occlusal projection:

    1. topographic at 65 degree:

  • The central rays are directed at 65 degree on the ant. Maxilla, where we will see more teeth and supporting structure, it’s not used for caries it’s used for fractures, cysts, ortho issues.

    1. Lateral

  • The occlusal film is off center, we put it either right or left according to what side I am working on, also used to detect cyst or lesions and their margins.

    1. Pediatric:

  • It’s a modification of maxillary central PA , it looks like standard maxillary topographic

  1. Mandibular occlusal projection:

  1. Topographic at 55-60 degree

  1. Cross sectional at 90 degree:

  • It comes from below the chin, it’s easy and useful as it gives the buccolingual expansion and this expansion is important in differentiating between keratocystic odontogenic tumor and ameloblastoma ,also its imp in madibular salivary glands stones.

  1. Pediatric like PA but wider

  1. Oblique (we concentrate in on side of the mandible).

We can use cross sectional for maxilla but the beam will be directed from up through the head so it will be hazardous and there will be a lot of superimposition.

Best of luck

Esra abu hirra
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