Session Two Flashcards
How do X-Rays work?
X-rays are form of electromagnetic radiation
X-rays travel through the body and are absorbed in different amounts by different tissues, depending on the radiological density of the tissues they pass through.
- E.g. Bones readily absorb x-rays, therefore produce high contrast
- x-rays travel more easily through less radiologically dense tissues (e.g., fat, muscle and air-filled cavities)
What are advantages of X-Rays?
Cost-effective
Readily available
Non-invasive
Good bony (osseous) detail
Easily accessible
Fast and Convenient
What are disadvantages of X-Rays?
Exposure to ionising radiation
Lack of soft tissue detail
Lack of sensitivity in detecting bone density changes
How do MRI’s work?
A strong magnetic field that forces protons in the body to align with that field
A radiofrequency current causes the protons to be stimulated, and spin out of equilibrium, straining them against the pull of the magnetic field.
When the radiofrequency field is turned off, the MRIsensorsdetect the energy released as the protons realign with the magnetic field.
Contrast agents (e.g., Gadolinium) may be given to increase the speed at which protons realign with the magnetic field. The faster the protons realign, the brighter the image.
How do MRI works: Radiofrequency (RF) Coils?
RF coils are an essential MRI component used for transmission of
The RF field to excite nuclear spins and for reception of the MRI signal.
RF coils play an important role in image quality in terms of:
- signal-to-noise ratio
- signal uniformity
- image resolution
RF coils are associated with potential image artifacts and RF heating that may lead to patient burns.
Proper selection and use of RF coils are therefore essential to:
- achieve optimal image quality
- to maximize diagnostic capability
- prevent image artifacts
- reduce the risk of RF heating and patient burns
Quality control of RF coils is important to ensure proper functioning of the coils to provide consistent image quality and avoid coil problems that may affect image evaluation or interrupt patient imaging.
What is a T1 Weighted MRI?
T1-weighted MRI enhances the signal of the fatty tissue and suppresses the signal of the water i.e.
- Fat: Bright
- Fluid: Dark
- Muscle: Grey
- Air: Very dark
- Inflammation: Dark
What is a T2 weighted MRI?
T2-weighted MRI enhances the signal of the water i.e.
- Fluid: Bright
- Muscle: Dark grey
- Fat: Light
- Air: Very dark
- Inflammation: Bright
What is a STIR MRI?
STIR (Short T1 Inversion-Recovery) sequence Suppresses Fat:
- provides excellent depiction of bone marrow oedema which may be the only indication of an occult fracture
What is a FLAIR MRI?
FLAIR (Fluid-attenuation inversion recovery) – Suppresses Fluid
- heavily T2 weighted images with CSF fluid suppression
- highlights hyperintense lesions and improves their detection
What are advantages of MRI?
Non-invasive
No ionising Radiation
Good resolution
3D Detailed anatomical images
Provides extremely clear, detailed images of soft-tissue structures
Functional information
What are disadvantages of MRI?
Expensive
Noisy
Claustrophobia
First trimester pregnancy
Long scan times
Motion artefacts
Patient may develop an allergic reaction to the contrast agent
What are the absolute contraindications for MRI?
Cardiac Pacemaker
Implanted Cardiac Defibrillator
Internal pacing wires
Clips such as cerebral, carotid, or aortic aneurysm
Cochlear implants
Any implant held in by a magnet
Swan - Ganz Catheter
Pregnant/Possibly pregnant
What are the possible contraindications for MRI?
Body piercing
Tattooed makeup
Dentures
Hearing aid
Occular implants
artificial limb
joint replacement
metal rods or plates
metal or wire mesh implants
vascular access port of catheter
transdermal delivery system (nitro)
Metal removed from eye
Shunt
Any metal fragments
Heart valve prosthesis
Insulin or other drug infusion pump
Neurostimulator stents, filters, coils
Electrodes
IUD or Diaphragm
Hospitalized
Surgery
How does CT work?
A computerized x-ray imaging procedure where a narrow beam ofx-rays is aimed at a patient and rotated around the body, producing signals that are processed by the computer to generate cross-sectional images, or “slices.”
The collected slices are digitally “stacked” together to form a 3D image of the patient, allowing easier identification of basic structures and providing more detailed information about possible tumours or abnormalities.
How does CT work - Windowing?
CT Windowing (AKA grey-level mapping) - an image processing task that helps highlight the key anatomy so that the images can be analysed easily.
The most common window settings include:
Bone window - For viewing the bones
Soft tissue window – to evaluate soft tissues e.g., solid organs and vasculature
Lung window - for evaluating the lungs
Abdomen window - to evaluate the abdominal cavity and its contents
Brain window - to evaluate the brain parenchyma (typically on a non-contrast scan)
- sensitive at picking up intracranial haemorrhage with a narrow window
