Introduction to the Role of Clinical Imaging Flashcards
Revision
What is imaging?
All the tests that doctors use to see things inside the body that they can’t see or feel from the outside.
what tests are imaging tests?
X-rays
Ultrasounds
Magnetic resource imaging (MRI)
Nuclear Medicine
What type of X-rays are there?
Plain radiography Contrast studies (Barium enema, arthrography) Computed tomography (CT)
How do X-rays work?
Electromagnetic radiation is produced by an electrical source which passes through body tissue to a degree depending on the density of the tissue.
What are the mechanics of producing an X-ray?
Electric current heats a filament cathode.
Electrons are emitted from the cathode and strike the anode.
The generated x-rays exit the window and an x-ray beam is collimated.
X-rays penetrate the patient and scatter.
Only 1% of x-rays reach the film and contribute to the image.
Increasingly x-rays are taken digitally and post-exposure manipulation reduces the need for repeat exposures.
PACS (Picture Archiving and Communications System) allows easier storage, access and retrieval of digital images (X-ray, CT etc).
How can you identify between different structures on an x-ray?
The denser the tissue, the fewer x-rays pass through it.
Air lets all x-rays pass through (hence lungs appear almost black).
Soft tissue lets some x-rays pass through
Cortical bone lets no x-rays pass through.
X-ray film is blackened when x-rays hit it, so a picture is produced depending on what the x-rays have had to pass through.
What are the strengths/positives of x-rays?
X-rays are good at showing things that are surrounded by air (that appears as black on the x-ray) e.g. lung cancer.
X-rays are good at showing things that are surrounded by white bone e.g. fractures.
X-rays are good at showing things that destroy white bone e.g. bone cancer.
What are the weaknesses/negatives of x-rays?
To be visible on an x-ray, an abnormality must be of very different density to the tissue that surrounds it.
Unfortunately, a lot of pathology, especially that affecting soft tissues, is of similar density to it’s surroundings and isn’t shown by simple x-rays.
How are contrast Studies carried out?
Radiologists realised that liquids containing dense elements like barium or iodine could block x-rays.
By putting them into spaces between tissues, the tissue outlines became visible.
(Pouring barium into a cleansed colon shows e.g. sigmoid cancer)
(Direct injection of iodine rich fluid into a shoulder joint outlines the capsule)
Why can x-rays be dangerous?
X-rays are radiation so can damage cell DNA leading to mutations which may kill the cell or make it turn cancerous.
The risk is related to the x-ray dose.
When do you use x-rays?
The benefit to the patients must always outweigh the risks (Ionising Radiation (Medical Exposure) Regulations (Europe) 2000) or (IRMER 2000)
Hospital staff have no benefit from x-rays so must be protected. To protect them, radiation protection clothing that was impregnated with lead was developed.
What is a weakness of radiographs?
Radiographs have limited contrast resolution and struggle to resolve different soft tissue structures.
How does CT overcome the problems that radiographs have with resolving different soft tissue structures?
CT is a sophisticated way of using x-rays to produce images.
It is more able than radiographs to distinguish between tissues of similar density and is often used to show soft tissue pathology.
It has superior contrast resolution compared to a radiograph.
Why are radiographs not sufficient to diagnose illness?
When obtaining a radiograph, the x-rays are fired from only one direction, producing an image where all structures between their source and the film are superimposed.
How to CT scans overcome radiograph’s issues with producing an image where all structures between their source and the film are superimposed?
With CT, x-rays are fired from all round the body and processed by a computer which produces a set of cross-sectional pictures with no superimposition.
