What is an x-ray image?

Question 1

The extent to which the image appears blackened depends on what?

Answer 1

It depends on the number of x-rays reaching the image receptor, which in turn, depends on the density of the object

Question 2

How would you describe the final image?

Answer 2

2D picture or shadowgraph made up of black, white and grey superimposed shadows

Question 3

What are the 4 major factors that need to be understood if you are to understand and interpret a final radiographic image?

Answer 3

1. Understand the black/white/grey shadows and how they are affected by:
   - Object material and structure
   - Object thickness or density
   - Object shape
   - Exposure factors
   - Relative positions of image receptor and x-ray beam
2. Knowledge of anatomy of the hard tissues
3. Appreciation of the limitations of a 2D picture and the effect of superimposition
4. Knowledge of different types of images and their limitations

Question 4

What does it mean if a shadow is black?

Answer 4

It means that it is radiolucent - areas where no absorption has taken place inside the patient tissues

Question 5

What does it mean if the shadow is white?

Answer 5

Radio-opaque - the x-rays have been totally absorbed (e.g. metal fillings)

Question 6

What does it mean if the shadows are grey?

Answer 6

Areas where x-ray been has been stopped or partially absorbed within the patient

Question 7

What is the clinical definition of burnout?

Answer 7

It means that if a high-intensity x-ray beam is used, small, fine structures of a patient will not create a radiographic shadow and appear ‘burnt out’ and will apparently disappear

Question 8

What may result in the same object looking completely different radiographically?

Answer 8

It’s orientation and relative position in relation to the x-ray beam

Question 9

What is the ideal position for a tooth when taking a radiograph? (3)

Answer 9

• Long axis of object (tooth) parallel to the long axis of the image receptor
• Object and image receptor should be touching
• X-ray beam should meet the long axis of object and long axis of image receptor at right angles

Question 10

What can happen if the relative position of tooth and image receptor/x-ray beam is not ideal?

Answer 10

• Image can be foreshortened or elongated
• Image can be distorted if x-ray beam position is not ideal
• Image can be distorted and inaccurate if geometric relationship between object, image receptor and x-ray beam

Question 11

What are the 3 main types of images?

Answer 11

1. Conventional film-captured images
2. Tomographs (aka sectional radiographs)
3. Digital images

Question 12

Name the 4 limitations of conventional film-captured images?

Answer 12

1. 2D representation of 3D objects
2. Affected by exposure factors
3. Affected by geometrical distortion
4. Affected by processing faults

Question 13

What is the issue with superimposition in radiography?

Answer 13

It can make objects seem like they are in one place however, this may not actually be where they are positioned and they have in fact been superimposed by the direction of the x-ray beam to look that way.

Question 14

What is determined by kilovoltage?

Answer 14

The kilovoltage determines the energy of the photons or the quality of the beam.
It determines the penetrating power of the photons and affects film contrast.
If the kilovoltage is increased, the image appears greyer with many more shades of grey; if the kilovoltage is decreased, the image becomes more black and white with fewer shades of grey.

Question 15

What are the effects of mA and time?

Answer 15

These factors determine the quantity of x-ray photons produced,
which in turn affect the degree of film blackening.

If the film is overexposed, overall it looks too black and if it is under-exposed, overall it looks too pale.

The one variable, which can be altered on all X-ray equipment is the length of time of the exposure.

Question 16

How can tomography overcome the issue of superimposition?

Answer 16

It is designed so that you can image a layer, slice or section of a patient and just look at the tissues within that section

Question 17

Name 5 different types of tomographic images

Answer 17

• Simple linear tomographs
• Complex multidirectional tomographs (e.g. scanora)
• Computed tomographs (CT)
• Cone-beam computed tomographs (CBCT)
• Dental panoramic tomographs (DPT)

Question 18

Describe the action of simple linear tomographs

what is the name for the section of the patient that is imaged?

Answer 18

• Film and x-ray tube head moves during exposure: film moves in one direction, tube head moves in the other direction
• Patient stays still
• Section of the patient that is at the centre of the rotation is described as the linear focal plane or the in-focus layer. (Only this section is imaged)
• The other structures in the head are blurred due to the movement of the equipment
• Thickness of the section depends on the amount of movement that takes place: large movement = thin slice, small movement = thick slice

Question 19

How can blurring and image resolution be improved in tomography?

Answer 19

Can use a complex multidirectional tomography (e.g. Scanora unit)

Question 20

How does complex multi-directional tomography work?

Answer 20

• It utilises a complex spiral motion between tube head and film
• There is still a centre of rotation which corresponds to the slice that will be in focus
• Machine was designed for use in the maxfax region and can be used to produce cross-sectional slices of the jaws

Question 21

Describe the action of CT imaging

Answer 21

• Patient lies down
• Patient gradually moved into central hole of unit
• There is a ring of image receptors and an x-ray machine which spirals around the patient
• Designed to create axial tomographic slices or sections - multiple slices are produced which can be used individually or combined together to create a 3D image

Question 22

What is a recent development in tomography and what does it do?

Answer 22

Cone beam computed tomography (CBCT)
Technique of CT scanning designed for dentistry (e.g. SironaGalileos)
- Only produces images of maxfax skeleton and part of cranial base and can produce cross-sectional tomographic slice images of the jaws in any plane
- 3D reconstruction is also possible using computer software

Question 23

Is a narrow or wide beam used in tomography?

is the tomographic section straight or curved - why is this?

Answer 23

Narrow.

Curved - as it corresponds to the shape of the dental arch so the teeth and supporting structures will be seen in focus

(cross over of shadows happen throughout panoramic cycle)

Question 24

What are the practical limitations of tomographic panoramic images? (3)

Answer 24

If teeth are in an abnormal position - it may be physically impossible for pt to bite both upper and lower teeth onto bite peg at same time, so some teeth will not be in the focus layer and appear blurred

Operator may not position patient correctly - can result in blurring

To image one side of patient, x-ray beam has to pass through other side of jaws - this creates shadows that can be out of focus and blurred (ghost shadows) - e.g. earrings, right earring creates ghost shadow on the left

Question 25

What are the components that make up the final panoramic image? (5)

Answer 25

Number of different shadows: hard tissue shadows, soft tissue shadows, air shadows, ghost shadows and shadows cast by the equipment, all superimposed on top of one another

Question 26

Name real hard tissues shown in tomograph (6)

Answer 26

Nasal septum, hard palate, zygomatic buttress/cheek bone, maxilla, mandible, hyoid bone

Question 27

Name the real soft tissues in a tomograph (4)

Answer 27

Soft palate, ear lobe, dorsum of tongue, naso-labial fold

Question 28

Name what shadows are cast by equipment itself in tomography (2)

Answer 28

Bite peg

Plastic head stabilising support

Question 29

Name which ghost shadows are cast by the hard tissues in tomography (2)

Answer 29

1. L-shaped ghost shadow of the mandible and cervical spine

2. Radiolucent shadow created by air space in the mouth between tongue and hard palate

Question 30

What are the clinical limitations of tomographic images?

Answer 30

• Represent section or slice of patient, not an overview
• Image is slightly blurred as equipment moves
• Affected by patient’s shape, position of teeth and positioning
• Creates complex image
• Film can be affected in chemical processing or other exposure factors
• Resolution is relatively poor if image receptor is indirect action film screen combination (can be improved by using digital image receptor)

Question 31

How many shades of grey are in a typical digital image?

Answer 31

256

Question 32

What are the limitations of modern digital images? (3)

Answer 32

• Images are 2D depiction of 3D objects
• affected by geometrical distortion
• Images can be altered by software imaging processing.