Advanced Oral and Maxilofacial Imaging Flashcards
Computed tomography - how does it work? (4)
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).
How can different tissue contrasts be emphasised on CT scans? (1)
Windowing
-emphasises different densities
Cone beam CT - how does it work? (4)
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.
How does cone beam CT different? (4)
uses electrical current to heat a tungsten cathode. Electrons ‘boil’ off the surface of the cathode and are accelerated towards the anode using a high potential difference (voltage). Electrons strike the anode and release x-ray photons
VS
In conventional CT, the tube current (number of electrons flowing) is about 200 mA, and the tube voltage about 120 kV. For comparison, in cone beam CT the tube voltage is the same (120 kV), but the tube current is substantially lower; about 5 mA. This reduces the radiation dose, but also alters the image contrast.
The reduced tube current (and therefore number of x-ray photons) does not permit contrast differences between soft tissues to be distinguished. Therefore only hard tissues can be viewed, as all of the soft tissues are a homogenous shade of grey, though image brightness can be adjusted.
Indications for cone beam CT (3)
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.
Examples of uses for cone beam CT (5)
locating impacted/buried teeth, root resorption in orthodontics, planning implants, evaluation of large lesions (cysts, benign tumours), complex facial trauma, evaluation of the sinuses.
Contrast techniques (3)
Contrast is used to describe a wide range of compounds that are introduced into the body (IV, oral, rectal etc.) that artificially alter the subject contrast of the tissues.
The most common type of contrast is x-ray contrast which contains iodine and is radiopaque, but other contrast agents are used in MRI and ultrasound imaging
Some patients are allergic to iodine, which can cause an anaphylactic reaction, so always check beforehand
Contrast techniques in the head and neck (5)
Sialography TMJ arthrography Angiography Investigation of fistulae As an adjunct to CT and MRI
Sialography (3)
This involves the introduction of contrast into the ductal system of the parotid or submandibular salivary glands.
Indicated when there is a history suggestive of obstruction (pain & swelling at mealtimes).
Also used to investigate patients with suspected Sjögren’s syndrome.
Contraindications to sialography (3)
Acute salivary gland infection
Allergy to iodine
Discrete salivary gland swelling, other techniques such as ultrasound and MRI are more informative
TMJ arthrography (4)
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.
Sometimes ‘double contrast’ studies are performed using a radiopaque contrast agent in the superior joint space and air in the inferior joint space with CT imaging.
In Sheffield we use MRI instead.
Indications for angiography (3)
To show the vascular anatomy and feeder vessels associated with haemangiomas.
Investigation of arteriovenous malformations, including intracranially.
To embolize vascular lesions using ‘glue’.
What is angiography? (1)
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.
How does MRI work? (9)
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.
Advantages or MRI scans (3)
Provides superior soft tissue detail in any plane.
Excellent for intracranial disease.
Can be used to directly image the disc position within the TMJ.
