What are X-rays?
When electromagnetic radiation produced by an electrical source passed through a body tissue to different degrees depending on its density.
(Plain, contrast or CT)
The denser a tissue, the fewer X-rays can pass through it (air lets all through, soft tissue lets some and bone let’s no X-rays through)
What are X-rays strengths?
- They are good at showing things that are surrounded by black air (e.g lungs)
- they are good at showing things surrounded by white bone
- they are good at showing things that destroy bone (e.g bone cancer)
What are X-rays weaknesses?
- to be visible on a simple X-ray, an abnormality must be very different density to the tissue that surrounds it (unfortunately a lot of pathology especially soft tissue is of similar density to surroundings)
- the X-rays are fires from only one direction so in the image all the structures are superimposed
What are contrast studies?
When liquids containing dense liquids like barium or iodine are put into the spaces between tissues to block X-rays and outline the tissues clearly.
What is CT?
An X-ray with a superior contrast resolution compare to a radiograph to distinguish more easily between tissues of similar densities (often soft tissues).
What are the advantages of CT?
- it has a superior contrast so can distinguish between soft tissues more easily
- it Fires X-rays from all around the body to produce a cross section so the picture has no superimposition.
What are the disadvantages of CT?
- it gives a large dose of ionising radiation (250 x normal X-ray)
- soft tissues of VERY similar density are still indistinguishable by CT (eg pelvic organs,muscles and tendons, some areas of brain, joint cartilage and ligaments)
- it cannot show bone marrow disease
What is ultrasound?
Piezoelectric crystals in a transducer convert electrical energy to acoustic energy to find where structures are.
It transmits sound pulses and listens for them to be reflected and then converts it into a computer image
What are the advantages of ultrasound?
- it doesn’t require ionising radiation
- it can distinguish even between areas of VERY similar density.
- it is quick, easy and relatively cheap
- the transducer can be moved to get a complete survey
- body fluids readily transmit ultrasound whereas solid structures within the fluid reflect it.
What are the disadvantages of ultrasound?
- It is difficult to obtain good images on patients who do not have good skin, have very deep structures, are overweight or if the wrong transducer is chosen.
- some tissues have identical acoustic properties
- blocked by bone (eg can’t be used on brain)
- blocked by gas (can’t be used in lungs etc)
What is MRI?
The MRI scanner creates a strong magnetic field which energises the body’s proteins and then waits for a return signal produced by the protons. The returned signal is used to create an image.
What are the problems that can occur in an MRI scanner?
- frostbite and thermal burns
- deafness due to the sound (usually given earplugs)
- disturbance of pacemakers, heart valves, shrapnel and other foreign ferrous bodies
- missile injuries
What are the advantages of MRI?
- can provide complete assessment of the brain
- sensitive way to asses soft tissue injuries especially in deep tissues where ultrasound is less accurate
- excellent for imaging of spinal canal disease
- interior of joints can be imaged
- no ionising radiation
What are the weaknesses of MRI?
- expensive
- time consuming
- potentially dangerous
- claustrophobic
- poor at showing details of lungs
What is the role of imaging?
- make or confirm diagnosis
- narrow a differential diagnosis
- to ‘stage’ a known disease eg cancer for treatment
- to monitor effects of a treatment
