Advanced OMFS Radiology (Max-fax interpretation and advanced dental imaging)

Question 1

what is a CT scan?

Answer 1

The equipment uses a flat fan-shaped beam of x-rays that are detected by gas or crystal detectors. The detectors measure the intensity of the x-ray beam that emerges from the patient over a –1000 to +3000 scale (Hounsfield units). This is then converted to digital data, and displayed on a computer screen. The technique produces a series of ‘slices’ through the patient.

The patient lies on a table that advances into the x-ray beam which continuously rotates around the patient (2 rps).

Question 2

what does CT stand for?

Answer 2

computed tomography

Question 3

In a CT scan windowing is used, what is windowing?

Answer 3

The way the data is displayed can be manipulated by ‘windowing’ so that different tissue contrasts can be emphasized.

Question 4

where in the body is cone beam CT usually used?

Answer 4

A specialized version of CT mostly used in the head and neck (not just dental).

Question 5

what is a cone beam CT scan?

Answer 5

A cone-shaped x-ray beam is used in conjunction with a flat panel detector. The equipment orbits around the patient and images a cylindrical or spherical volume in 1 cycle. With a large field of view (15 cm diameter), most of the maxillofacial skeleton can be imaged in a single scan.
Software then reformats the image into axial, sagittal or coronal sections and it can also produce panoramic and 3D views.

Question 6

How does conventional CT and cone beam CT differ mechanically?

Answer 6

For both the tube voltage is the same around 120kV but the tube current for a CBCT is substantially lower
- this give CBCT a lower radiation dose compared to conventional CT
- this also means that CBCT does not allow contrast differences between soft tissues (all soft tissues look grey)

Question 7

when investigating soft tissue abnormalities which type of CT is best to use?

Answer 7

conventional CT as you can distinguish between soft tissues

Question 8

what are the indications for taking a CBCT?

Answer 8

• When you have reached the end of what plain films can tell you.
• When 3 dimensional imaging is needed
• In cases of bony pathology where more imaging is needed for diagnostic or treatment planning purposes.
• For example: locating impacted/buried teeth, root resorption in orthodontics, planning implants, evaluation of large lesions (cysts, benign tumours), complex facial trauma, evaluation of the sinuses.

Question 9

what are contrast techniques and what is the most common type?

Answer 9

Contrast is used in the body and artificially alters the subject contrast of the tissues.

The most common type of contrast is x-ray contrast which contains IODINE and is radiopaque

Question 10

What are people commonly allergic to if they are allergic to shellfish?

Answer 10

iodine

Question 11

Name 5 contrast techniques in the head and neck?

Answer 11

• Sialography
• TMJ arthrography
• Angiography
• Investigation of fistulae
• As an adjunct to CT and MRI

Question 12

What is sialography?

Answer 12

This involves the introduction of contrast into the ductal system of the parotid or submandibular salivary glands.

Question 13

when is sialography usually used?

Answer 13

Indicated when there is a history suggestive of obstruction (pain & swelling at mealtimes).
Also used to investigate patients with suspected Sjögren’s syndrome.

Question 14

what is Sjögren’s syndrome?

Answer 14

a chronic (long-lasting) autoimmune disorder that happens when the immune system attacks the glands that make moisture in the eyes, mouth, and other parts of the body.

Question 15

What are the contraindications to sialography?

Answer 15

• Acute salivary gland infection
• Allergy to iodine
• Discrete salivary gland swelling, other techniques such as ultrasound and MRI are more informative

Question 16

what is TMJ arthrography and what is it used for?

Answer 16

• This involves the introduction of contrast into, usually, the inferior joint space of the TMJ to determine disc position and detect disc perforations and adhesions.

(sheffield uses MRI instead but good when a pt cant tolerate MRI)

Question 17

What is angiography?

Answer 17

the injection of contrast directly into blood vessels via a catheter, usually inserted into the femoral artery, followed by selective catheterization of carotid branches.

Question 18

what are the indications for angiography?

Answer 18

• To show the vascular anatomy and feeder vessels associated with haemangiomas.
• Investigation of arteriovenous malformations, including intracranially.
• To embolize vascular lesions using ‘glue’.

Question 19

What is a haemangioma?

Answer 19

it is a benign collection of small blood vessels that form a lump under the skin, usually raised and red and sometimes called strawberry marks due to appearance

Question 20

how does an MRI work?

Answer 20

• Involves placing the patient in a very strong magnetic field.
• Radiowaves are pulsed into the patient.
• This changes the way individual hydrogen protons spin
• As the spins of the hydrogen protons revert back to normal they emit radiowaves that are dependent on the proton density of the specific tissues.
• The emitted radiowaves are collected and converted into an image of a slice through the body.

Question 21

does MRI involve ionising radiation?

Answer 21

No

Question 22

what is the contrast used for MRI?

Answer 22

Gadolinium

Question 23

what is the imaging modality of choice for cancer staging?

Answer 23

MRI

beacause…….
* Soft tissue contrast, and the proton density differences between different soft tissues mean that the resulting images show soft tissue detail significantly better than CT

Question 24

what are the advantages of MRI?

Answer 24

• Provides superior soft tissue detail in any plane.
• Excellent for intracranial disease.
• Can be used to directly image the disc position within the TMJ.

Question 25

what are the disadvantages of MRI?

Answer 25

• Contraindicated in patients with pacemakers.
• Scanner is claustrophobic and noisy.
• Examination takes a long time (20-30 min).

Question 26

how does ultrasound work?

Answer 26

• Uses high frequency (1.5 – 10 MHz) sound waves from a transducer held against the skin.
• Sound is reflected by tissue interfaces back to the transducer.
• The reflections are detected by the transducer and converted to an electrical signal, which produces the image.

Question 27

when is ultrasound commonly used?

Answer 27

• Assessment of superficial soft tissue ‘lumps and bumps’ (thyroid, salivary, lymph nodes).
• Permits image guided biopsy.

Question 28

what are the advantages of ultrasound?

Answer 28

• No ionising radiation
• Real-time imaging
• Assess vascularity (Doppler shift)
• Non-invasive, safe and cheaper than CT/MRI

Question 29

what are the disadvantages of ultrasound?

Answer 29

• Can’t image through bone or air
• The intensity of the reflections decreases with depth – can’t image the deep lobe of the parotid fully
• Operator dependent

Question 30

how does nuclear imaging work?

Answer 30

• Involves injection of a radioactive isotope which emits gamma rays. In static (planar) imaging these are detected by the gamma camera.
• The radioactive isotope (usually Technecium 99m) is bound to another pharmaceutical which has high affinity for the tissue of interest
• Provides information on function rather than high resolution anatomy

Question 31

what are the indications for nuclear imaging?

Answer 31

• Detection of bony metastases
• Investigation of salivary gland function
• Condyle hyperplasia - assessment of continued growth
• Evaluation of bone grafts
• Thyroid investigations

Question 32

what does PET scan stand for?

Answer 32

Positron Emission Tomography

Question 33

how does a PET scanner work?

Answer 33

Patient receives intravenous 18F-labelled 2-fluoro-2-deoxyglucose.
Active cells avidly metabolise glucose, and therefore preferentially take up the FDG (infection, tumour etc.).

18F decays by ß+ decay:
p –> n + ß+

ß+ particles (positrons) travel for a few mm’s before combining with ß- particles (electrons).
Charge neutralization occurs and matter is converted to energy – two 511 keV photons travelling in opposite directions are produced.
A ring of crystal detectors registers ‘coincidence events’ when 2 photons are detected within a specified time window (10 – 20 ns).

Question 34

how can MRI and PET scans work together?

Answer 34

Integrated with the PET scanner is a low dose multi-slice CT scanner, which enables the functional data from PET to be fused with the anatomical data from CT. This permits 3-dimensional imaging of metabolic activity.