IRMER - Advanced dental images

Question 1

Computered tomography (CT)?

Answer 1

3D pics of 3D object by using a thin X-ray beam that is shaped like a fan, to cut slices through the pt = put the slices together = 3D image
X-ray beans are detected by crystal detectors

Pt lies on a bed, x-ray tube and detector present
Pt slides though hole in middle of the machine and the x-ray tube spins round the patient, forming x-ray slices which are maps of x-ray density

Question 2

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Answer 2

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Question 3

CT - how to change how the computer displays the image?

Answer 3

The way the data is displayed can be manipulated by ‘windowing’ so that different tissue contrasts can be emphasised.
You can change the tissue density to determine what you can see on the image - hard and soft tissues just from scanning the pt once = this is called windowing

Question 4

Cone beam CT?

Answer 4

Low dose version of CT - cone shaped beam that is shaped like a pyramid
Mostly used for head and neck
Moves round the pt’s head once
Produces a volume image

Question 5

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Answer 5

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Question 6

How do cone beams differ?

Answer 6

Cone bean CT uses electrical current to heat a tungsten cathode. Electrons boil of the surface of the cathode and accelerate towards the anode using a high potential difference (voltage). Electrons strike the anode and release x-ray photons

In conventional CT the tube current (no of electrons flowing) is about 200mA, the tube voltage is about 120kV
In cone bean CT the tube voltage is the same but tube current is substantially lower = 5mA = reduces radiation dose but alters the contrast = No contrast difference between soft tissues to be distinguished = only hard tissues viewed

Question 7

Indications for cone bean CT?

Answer 7

When plain films dont tell you enough
When 3D imaging needed
In cases of bony pathology where more imaging is needed for diagnostic or tx planning purposes

e.g. 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 8

Contrast techniques?

Answer 8

Contrast is used to describe a wide range of compounds that are introduced into the body (IV, oral, rectal)
Most common type of contrast = x-ray contrast which contains iodine and is radiopaque

Some pt allergic to iodine = anaphylaxis = check beforehand

Question 9

Contrast techniques in the head and neck?

Answer 9

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

Question 10

Sialography?

Answer 10

Intro of contrast into the ductal system of the parotid or submandibular salivary glands
Indicated when there is a history suggestive of obstruction (pain and swelling at mealtimes)
Can investigate pts with suspected sjogrens syndromes

Question 11

Contraindications to sialography?

Answer 11

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

Question 12

TMJ arthrography?

Answer 12

Involves introduction of contrast into the inferior joint space of the TMJ to determine disc position and detect disc perforations and adhesions
Sometimes double contrast studies are performed using radiopaque contrast agent in the superior joint space and air in the inferior joint space with CT imaging
In sheff - use MRI instead

Question 13

Angiography?

Answer 13

Involves 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 14

Indications for angiography?

Answer 14

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

Question 15

Magnetic resonance imaging (MRI)

Answer 15

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.
Ionizing radiation is not involved at all.
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, and it is the imaging modality of choice for cancer staging.
T1 weighted images show normal anatomy well, whilst T2 weighted images are good for showing disease.
MRI contrast (Gadolinium) changes how quickly the proton spins revert back to normal and identifies abnormal tissue better.

Question 16

-mri cont?

Answer 16

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Question 17

MRI advantages?

Answer 17

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 18

MRI disadvantages?

Answer 18

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

Question 19

Ultrasound?

Answer 19

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.
Non-invasive, safe and cheaper than CT/MRI

Question 20

Ultrasound uses?

Answer 20

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

Question 21

Ultrasound advantages?

Answer 21

No ionising radiation
Real-time imaging
Assess vascularity (Doppler shift)

Question 22

Ultrasound disadvantages?

Answer 22

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 23

Nuclear imaging?

Answer 23

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 eg. MDP for bone
Provides information on function rather than high resolution anatomy

Question 24

Indications for nuclear imaging?

Answer 24

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

Question 25

Positron Emission Tomography

Answer 25

Functional as well as anatomical imaging.
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).
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.

Question 26

Positron emission tomography indications?

Answer 26

Cancer staging, unknown primary
Re-staging after chemotherapy
Post-treatment residual masses

Question 27

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Answer 27

Use cone beam CT as a problem solver for dento-alveolar pathology and for implant planning.
Use MRI for soft tissue lesions and all cancer staging, contrast-enhanced CT is the fall back technique if MR is contra-indicated.
Sialograms are used to investigate salivary obstruction and dry mouth.
Ultrasound is the first line imaging for superficial lumps and bumps in the head and neck.
Nuclear imaging has only a few uses.