Extraoral Radiography 1

Question 1

How is the phosphor storage plate arranged in olique lateral radiography?

Answer 1

Phosphor storage plate is placed in a cassette. Ensure that the plate is not bent so that X-rays are aimed perpendicular at the plate.

Question 2

What kind of X-ray machine is used for oblique lateral radiography?

Answer 2

A regular intra-oral X-ray machine at 65 or 70 kV is used at exposure times as short as 0.16s. If the patient has thick tissues exposure times of ~ 0.20s is necessary.

A circular collimator is standard for this exposure.

Question 3

What is the aim of oblique lateral radiography?

Answer 3

To obtain almost half the view of a panoramic image. Left and right sides are imaged separately.

Question 4

How is oblique lateral radiography different to panoramic imaging?

Answer 4

Image detector and radiation source are stationary.

This technique is adjustable to local views of the jaws and more global views of both the jaws.

Question 5

How is the cassette positioned in lateral oblique radiography?

Answer 5

The patient holds the cassette against the side of the face one intends to image. X-ray machine is positioned on the opposite side.

Question 6

How should the X-ray machine be releasing X-rays?

Answer 6

The X-rays should go parallel to the occlusal plane coming from behind the ramus of the mandible.

Perpendicularly to the image detector.

Question 7

What kind of image is provided by a lateral oblique radiograph?

Answer 7

An image of the maxilla and mandible reaching from molars to canine.

Question 8

How can the x-ray machine angle be modified for imaging the third molars?

Answer 8

The angulation of the x-ray machine can be altered by aiming from below the opposite side of the mandible.

Purely from below the body of the mandible.

Question 9

What are the indications for oblique lateral radiographs?

Answer 9

Alternative to intraoral and panoramic radiographs in specific patients (special needs, paediatric, elderly, etc)

Patient with trismus

Patient with severe gagging reflex

Mandibular fractures if no 3D is available.

Question 10

How is OPG different to intraoral and oblique lateral radiography?

Answer 10

Image receptor and x-ray source move around the patient in opposite directions to each other.

They are connected for synchronized movement.

Question 11

What is the focal trough?

Answer 11

In linear tomography it is the point at which the beams cross each other. This is the point where structures are “in focus”. When closer to the x-ray machines the image appears much bigger and when closer to the detector the image appears much smaller.

Question 12

What would a wide x-ray beam on an OPG do to the image?

Answer 12

A wide X-ray beam will cause a sharper image than a narrow beam because the focal trough will be wider.

Question 13

What would a narrow x-ray beam on an OPG do to the image?

Answer 13

More narrow focal trough which enhances the image detail.

Question 14

How are images optimised in OPG?

Answer 14

Collimation (narrow slit/fan shape) to obtain a better image.

Placing image receptor in a cassette, which has been equipped with a narrow vertical slit improves image quality.

Question 15

Why are extraoral bitewings so useful?

Answer 15

They can show more of the root along the entire side.

Question 16

How is the rotation axis different now than it was in the past for OPG?

Answer 16

Now it is closer to the shape of the jaws to create a better image quality because the focal trough will be the shape of the jaw.

Question 17

How should cassettes be used for extraoral radiographs for LHS then RHS?

Answer 17

LHS and RHS on one cassette. At OHCWA we must use separate cassettes for each.

Question 18

What important principle is in place for distance between the x-ray detector and the object being imaged?

Answer 18

X-ray detector should be as close as possible to the object being imaged.

Question 19

What are phantom images and where are they seen?

Answer 19

Phantom images appear due to structures that are too close to the x-ray source.

Panoramic images always have these phantom images on the screen.

Question 20

How is the extraoral bitewing different to the intraoral bitewing?

Answer 20

Beam is collimated slightly less on an extraoral bitewing. Good alternative to the traditional intraoral bitewing.

Question 21

How are the machine that takes panoramic images and the patient arranged for taking good images?

Answer 21

C arm holds the image receptor and the x-ray source on both arms.

Chin rest and temple supports are different from manufacturer to manufacturer

Ensure patient is sitting ideally for that position.

Image quality is heavily influenced by chin positioning and the patient’s posture.

Question 22

How are different panoramic machines different to each other?

Answer 22

Machines all work exactly the same with different designs and colours but essentially they work the same way.

Very cheap panoramic x-ray machines will not have comfortable chin holder which will affect the images taken.

Question 23

Why is the kV increased on panoramic images compared to intraoral radiographs?

Answer 23

Increasing kV allows for higher penetration so it is more effective for panoramic images because it allows us to ignore the back structures and to see the more anterior structures.

Increasing exposure time will allow for more x-rays to pass through the patient for better images.

Question 24

Why is it a bad idea to measure teeth obtained from a panoramic image?

Answer 24

The images are not the same size and are instead magnified by 1.3x

Question 25

What information can be gathered from an image using its magnification?

Answer 25

Using the magnification caused by distance from image source and receptor can be used for understanding deviations of teeth from the line of occlusion

Question 26

How will ghost images project on the overall image and why?

Answer 26

Ghost images will always be projected higher than real image due to being closer to the image detector which is located inferiorly compared to the mough being imaged.

Question 27

How is ghost image intensity affected by the patient?

Answer 27

Different positioning of the patient can lead to different intensity of ghost image.

Question 28

Do other techniques besides OPG cause ghost images?

Answer 28

No, no other technique besides panoramic imaging produces ghost images.

Question 29

What effect does the rotation of the image receptor and the source have on structures in the image?

Answer 29

They are imaged twice due to the rotation, once in focus and the second time out of focus.

Out of focus images are projected over the real image of the other side.

Structures in the midsagittal plane are exposed 3 times (once from each side and once from behind) to the X-ray beam.

Question 30

How have some manufacturers modified the way the image is taken to enhance the quality of the image of the chin?

Answer 30

Some manufacturers increase the kV when passing midline.

Others decrease rotation speed.

This helps to image the chin better as one has to avoid heavy shadow of the cervical spine in the middle of the image.

Question 31

How is the x-ray beam collimated on OPG?

Answer 31

It is collimated to a vertical narrow slit and aimed at an 9 to 12 degree upward angle distorting the image to make it 1.3 times larger.

Question 32

What is the shape of the focal trough? Why is this important?

Answer 32

It is a classic 3D horseshoe shape.

Width of the anterior region is ~10mm and posterior region is ~30mm

This is important because it enables the practitioner to position the patient correctly within the OPG.

Question 33

What tissues cause formation of ghost images?

Answer 33

Could be from both hard and soft tissues

Prosthodontics, foreign objects, jewelry, restorations, etc.

Question 34

How can a ghost image be identified?

Answer 34

Always projected higher in the image and on the opposite side.

Always blurred appearance

Can confuse the diagnostic yield.

Question 35

How is a panoramic image built? What effect does this allow us to understand?

Answer 35

Vertical line per vertical line. From left to right (patient’s left to patient’s right)

Patient movement will be visible in the vertical plane. More extreme patient movement gets more stressed out in the image.

Question 36

How should the machine be adjusted to the patient?

Answer 36

Focal trough should be adjustable to patient’s jaw

Midsagittal plane should be perpendicular to the floor.

Lateral vertical line depends on the machine and how it is designed. Every manufacturer has different guidelines. Could be either on canine or lateral incisor (depending on the manufacturer) to find the correct anterior - posterior position of the patient.

Frankfurt horizontal should be parallel to the floor.

Question 37

What is the frankfurt horizontal?

Answer 37

Some manufacturers want you to align cantus of the eye with tragus of the ear, while others will have you lining up lower border of orbit with tragus of the ear – depends on the upward angle of the X-ray beam)

Question 38

How are the panoramic images made into completed 2D images on a screen that can be seen?

Answer 38

Panoramic imaging unravels the bend of the face and the teeth to create the final image. This is why the spine can be visualized on the left and right hand sides.

Question 39

How will the image of the mandibular rami be on the OPG?

Answer 39

If the patient is positioned correctly, the size of the mandibular rami will be
symmetrical L and R (unless there is a real discrepancy in shape and size of the rami
due to pathology or an anatomical aberration)

Question 40

Caveat for trying to visualize the teeth on an OPG:

Answer 40

The machine allows movement of the horizontal laser light over the patient’s face without changing the machine.

One cannot move the machine, only the patient’s position (chin up or down)

Question 41

What happens if there are mistakes in midline alignment on an OPG?

Answer 41

Causes the image to be distorted

Structures closer to the x-ray source will be magnified and structures closer to the image receptor will be minified

Question 42

What happens to image if the midsagittal plane is tilted? How must the practitioner respond to this mistake?

Answer 42

It will result in an asymmetrical image.

The condyles will be projected on different horizontal levels.

All distortions must be recognized in order to identify the positioning mistake.

If one decides to retake the image one has to make sure not to make the same mistake.

Question 43

What happens if the chin is too low causing frankfurt horizontal to be tilted down?

Answer 43

The condyles shift upward in the image and the chin appears very large leading to “joker face” image.

Occlusal plane should be “slightly smiley “ rather than “unnaturally smiley”

Question 44

What happens if the chin is too far upwards?

Answer 44

The frankfurt horizontal line is tilted down backwards, the condyles will be projected to the lateral sides of the image and sometimes will fall outside of the image. THe occlusal plane will be convex in the front and sloping int he back (“sad clown’s face”)

“sad clown” face is caused by chin being too far up and so that makes it hard to see the condyle. Not a good image.

Question 45

How should the neck be arranged when positioning the patient in the machine? What happens if it isn’t?

Answer 45

Neck should be straight.

If the chin and the cervical spine are too close to each other then a vertical radiopaque line will occur in the image. This is because too many x-ray photons are absorbed by the dense mass that is formed by the mandible and the cervical spine.

Question 46

What situation can make getting a straight neck very difficult?

Answer 46

In the case of patient’s with arthritis it is going to be difficult.

Question 47

What is the ideal position going to result in? Is this the most realistic situation?

Answer 47

Patient is asked to bite end-to-end in the bite block and if the crowns and the roots all fall within the focal trough, this will result in the ideal image without any distortion.

The most REALISTIC situation is where the crowns of the teeth are in the focal trough, but the root tips are not. The inclination of the teeth will make the latter fall behind the focal trough, closer to the X-ray source, hence the magnification that will be noticeable on the image.

Question 48

What happens if the patient is positioned too far anteriorly or posteriorly in the OPG?

Answer 48

The teeth will be closer to the image receptor leading to a minified image. If too far posterior the image is magnified.

Question 49

What does it mean if the patient is positioned too far anteriorly/posteriorly?

Answer 49

The lateral vertical line did not coincide with the manufacturers guideline tooth and was more posterior

Question 50

How can positioning information be used to visualize the primary teeth?

Answer 50

Positioning the head more anterior than normal to allow the primary teeth to lie on the focal trough.

Question 51

What should the patient be instructed to do prior to having an OPG taken?

Answer 51

Removal all piercings from head and neck area

Remove hair pins, hair bands, etc

Remove removable dentures / orthodontic appliances if they contain metal

Remove necklaces, and other jewelry.

Position patient with straight neck: Patient’s feet should be a bit more forward. If patient would let go of handles patient would fall backwards.

Position patient’s head according to manufacturer guidelines

Let patient hold on to the handles of the machine

Show patient how to bite in end-to-end in the biteblock

Tongue against palate/roof of the mouth

Close lips around the bite block

Do not swallow during exposure

Do not hold breath during exposure

Hold the handles of the machine.

Do not come out of the machine until told to do so.

Question 52

Steps for taking an OPG image:

Answer 52

1. Position the patient correctly in the machine (three lines!)
2. Make sure the patient’s head is immobilized (chin rest and temple rests)
3. Make sure the patients holds on to the handles
4. Check the patient’s shoulders (should always be below the rotating C-arm) – crossing arms might be useful in some patients
5. Check again if the patient is still biting in end-to-end position
6. Check once more the positioning lights and re-adjust the patient’s
   position if needed
7. Tell your patient to keep lips closed, keep tongue against the palate, refrain from swallowing, refrain from moving or following the machine’s movement.

Question 53

What should be done to edentulous or partially edentulous patients?

Answer 53

Biting end-to-end is not possible

Sometimes it is better to let them wear their full dentures (If they do not contain metal)

Special positioning devices can be used to make sure the focal trough goes along the shape of the jaws.

If no dentures are available, cotton rolls placed between antagonistic alveolar crests can be used to keep the jaws apart and to immobilize patients sufficiently.

Question 54

How should an OPG be read?

Answer 54

Trace entire mandible starting at RHS condyle

Trace anatomy of the maxillary sinuses

Trace cortical borders of the adjacent structures to the maxilla

Assess soft tissues and soft tissue areas (especially lower corners of the OPG)

Assess the teeth.