Radiology 12 Flashcards
Where is the main landmark (“radiographic baseline”) used in extra-oral radiography?
Outer canthes of the eye to the external auditory meatus, and this represents the base of the skull
What is the landmark used when taking a DPT?
Frankfort plane (green line), a line from the inferior orbital margin to the upper border of the EAM. This is also known as the anthropological baseline.
What landmark is used a lot in cone beam CT?
Maxillary occlusal plane (yellow line), which is from the ala of the nose to the tragus of the ear. We use this line a lot in cone beam CT.
Equipment needed for extra-oral radiography
- Skull unit/cephalometric unit (with a high intensity, highly penetrating beam).
- Image receptor (cassette with intensifying screens and film OR digital system cassette containing a phosphor plate)
- Anti-scatter grid
4.
What is this and how does it work?
Lateral cephalagram unit. The patient stands with their back to the wall with the long blue pegs in their ears and a lateral view is taken. The image is created using a solid state sensor and is sent to the computer just to be visible through the window.
Scatter radiation
Weaker x-ray photons are deviated off track as they do not possess enough radiation to add all the way through to the receptor and make a useful interaction. In skull radiography, as the field of x-rays required to cover the entire skull is much larger, more scatter will be produced.
If left unchecked, this will add to background fog and produce a greyer image lacking in contrast.
How can scatter be prevented?
Use of an anti-scatter grid.
This stops the photons scattered in the patient reaching the receptor and the consequent degradation of the film.
An anti scatter grid looks like a sheet of metal, but is actually narrow strips of lead alternating with plastic strips.
Anti-scatter grids can either be fixed or moving depending on the type of the equipment it is installed in. In skull units it is fixed.
On a moving grid, the grid oscillates very quickly from side to side which prevents line from appearing, as the lead strips are constantly moving.
Anti-scatter grid
An anti scatter grid looks like a sheet of metal, but is actually narrow strips of lead alternating with plastic strips.
Anti-scatter grids can either be fixed or moving depending on the type of the equipment it is installed in. In skull units it is fixed.
Antiscatter grid: moving grid
On a moving grid, the grid oscillates very quickly from side to side which prevents line from appearing, as the lead strips are constantly moving.
What is this?
Satella turned round ready to x-ray a patient sitting on a stool. The x-rays leave the X-ray tube and travel towards the cassette which is placed in the holder.
What is this?
In between the patient and the film is a stationary grid.
Things to consider when positioning a patient for extra-oral radiography are..
- The position of the patient relative to the film. Facial views require the radiographic baseline to be at 45deg to the film, and the median saggital plane to be at 90deg to the film. In skull radiography the radiographic baseline should be at 90deg to the film.
2.The position of the x-ray beam relative to the patient. Is the beam AP, PA or lateral.
3.And the angle of the beam relative to the patient. Most head and neck radiography requires an angle on the x-ray beam. This is to move structures out of the way of other anatomy. For example, when x-raying a face you take an OM view with the x-ray beam at 10 deg to look at the orbital margins, and an OM view with the x-ray beam at 30 deg to look at the zygomatic arches and maxillae.
So here we have someone sitting in the AP position, beam is anterior, film is posterior.
The patient is now PA, the beam is posterior and the film is anterior.
The patient is now set up for a lateral, you can just about make out from the picture that the inter-orbital line is at 90 deg to the film, and the median saggital plane is parallel to the film.
If either of these planes are not correct you will not produce a true lateral.
There is no magic wand to help you get the patient positioned, you just have to use your eyes, and unfortunately patients are seldom symmetrical!
When would you request a PA mandible?
- To show a fracture, it will show any medio-lateral displacement of the mandible at the fracture site.
- It is also useful for showing cysts and malignancy causing medio-lateral expansion or bone destruction.
- It gives good visualisation of the posterior body of the mandible and ramus.
**However it gives limited visualisation of the condylar head and neck, and the midline tends to be obscured by the spine.
Consequently a PA mandible is usually requested in conjunction with a DPT.
Why is a mandible film PA preferable>
- The first one is it reduces the magnification of the facial structures on the receptor. Remember you always want the object to be as close to the receptor as possible.
- It also reduces the radiation dose to the eyes. You do not want to direct high energy x-ray photons straight into a patient’s eyes as the lens of the eye is quite radiosensitive.
How would you position your patient for a postero-anterior (PA) Mandible radiograph?
- You want them to be facing the film.
- Place the nose and the forehead touching the film holder - this brings the radiographic baseline 90 degrees to the film.
- The central ray should be in the midline of the patient at the height of mid ramus, which is roughly just below the occiput.
4.
How would you position your patient for a postero-anterior (PA) Mandible radiograph?
- You want them to be facing the film.
- Place the nose and the forehead touching the film holder - this brings the radiographic baseline 90 degrees to the film.
- The central ray should be in the midline of the patient at the height of mid ramus, which is roughly just below the occiput.
4.
How do we find the midline of the patient?
If you look at the front of the patient, the midline is generally between the eyes. The tip of the nose is unreliable as quite often the nose deviates to one side.
From the back we are looking for the external occipital protruberance.
What is this and how would you view it?
Exemplar PA mandible radiograph, view it as though we are looking at the patient from the front.
PA mandible radiograph: How can you tell in the radiograph that the radiographic baseline has accurately been at 90 degrees?
The mandible appears long
What is this?
This PA mandible image has not been positioned correctly.
The radiographic baseline has not been at 90 deg to the receptor, therefore you cannot see the neck and head of the condyle.
This is a good demonstration of how visualising certain anatomy is affected by angulation. It does show the internal fixation of the mandibular fractures nicely though.
Why would you need to take an image of an oblique mandible?
The mandible is a curved bone which makes it difficult to image in one view.
In general radiography we always try to take 2 images at right angles to one another. An AP or PA, and a lateral view. So why would we not just take a lateral mandible?
Well if we took a lateral mandible it would look like this image.
The 2 halves of the mandible will be superimposed and as the x-rays pass through both rami you will see both on the final image. The superimposition will make it difficult to make sense of anything that you can see.
What is this?
Here we have obliqued the mandible horizontally. The rami have now separated but you are still not seeing the entire side.
What is this?
Oblique mandible in the vertical plane, which achieves full separation of the rami.
You will only receive useful information from the side closest to the receptor. In this case you would say this is a right oblique mandible. You must image both sides separately.
What is this?
A lateral oblique mandible for caries on a child looks like.
We have a right marker on as it is the right side next to the film, and the left ramus is obliqued up out of the way. You can see lower and upper posterior teeth but it definitely does not provide you with the same level of detail you see on a bitewing.
Isocentric skull unit positioning for lateral oblique mandible
- Position the patient supine on the bed.
- Rotate the machine into the horizontal plane.
- Angle 25 degrees towards the head.
- You then turn the patient’s head towards the receptor.
What would the radiograph produced from this be known as?
Left lateral oblique
What is this?
Exemplar lateral oblique mandible: clear separation of the rami, all the way back to the angle of the mandible and up to the TM joint.
This view is not suitable for teeth.
Lateral oblique techniques: vertical tube angulation
- The patient must hold the receptor against and parallel to the side under investigation.
- The tube head is positioned
- Aim the tube towards the teeth under investigation.
*Angle the tube head slightly upwards, this view will project the opposing body of mandible up and therefore away from the area of interest.
What is this?
Exemplar lateral oblique using vertical angulation.
Again you can see the detail of the teeth is not nearly as clear as an intra oral periapical, but you see the caries in the D.
Lateral oblique techniques: horizontal angulation
- The patient olds the cassette
- The x-ray tube is positioned aiming along the occlusal plane.
- This aims through the radiographic keyhole
Radiographic keyhole
The triangular space between the back of the ramus and the cervical spine.
Exemplar lateral oblique using horizontal angulation.
What is significant about the positioning of the hyoid bone?
You can see how the angulation of the beam can project the hyoid bone which is not touching the mandible at all, to looking like it may be pathology on the ramus. Be aware how anatomy can move with angulation.
How would you position for a lateral cephalometric radiograph?
- The mid saggital plane should be parallel to the receptor.
- The Frankfurt plane should be horizontal and the centre of the beam aimed at the external auditory meatus.
- The patient’s teeth must be in occlusion.
What is the blue bib on the patient?
“A thyroid shield”
Lead protection: the exposure factors required and consequent dose are slightly greater.
The thyroid is one of the more radiosensitive anatomical structures and should be protected
Lateral cephalometric radiograph: Magnification
- Magnification should be kept to a minimum.
- This is achieved by having a long focus to patient distance and a short patient to film distance.
- Cephalometric films should also have a magnification rod shown on the image, this allows for the calculation of any magnification that has occurred.
Distancing set up for lateral cephalometric unit
1.5-2m
What is used to allow good visualisation of the soft tissues on a lateral cephalometric radiograph?
Aluminum filter
Radiograph results for an exemplar lateral cephalometric radiograph
The sella turcica is lateral and there is superimposition of the lower borders of the mandible. You also want the mandible to be clear of the spine.
Positioning for postero-anterior cephalometric radiograph
Again the the patient is positioned with the rods in their ears, with the radiographic baseline parallel to the floor.
The x-ray beam is aimed in the middle at the level of the EAM.
Positioning for a occipital-mental view
- The patient faces the receptor with their nose and chin touching (this means the radiographic baseline is at 45 deg to the film).
- Aim the centre of the beam to the midline of the patient at the level of the base of the nose.
This is an OM 0deg. - The standard facial view radiographs are taken with a 10 or 30 deg downward angle of the beam, so it is pointing towards the patient’s feet.
What is this?
- This film is an OM10 and it shows the orbits and wall of the maxillary sinuses clearly.
- You can see the body of the zygoma but it is not useful for the mandible. When looking for a fracture, it is useful to compare both sides. You are looking for symmetry and to be able to follow edges without coming across any steps or loss of definition.
**However be aware of sutures. Do not mistake them for fractures. You can just about make out the fronto-zygomatic suture. To the unsuspecting it could like a fracture. Also if the face is slightly rotated it will make one side look different from the other. Rotated films must be repeated.
What does the star on this radiograph suggest?
A fracture
