Principles of imaging Flashcards
What do different colours on an x-ray show?
Air= white
Metal= black
Bone= grey
Soft tissue= pale grey
The denser it is, the darker it is
What do we call the top and bottom of a foot?
Dorsal= top Plantar= base
What contrasts are used in x-ray?
Barium sulfate is insoluble and non toxic and is used for GI fluoroscopy
Iodine has high atomic mass so attenuates x-rays and can be excreted in urine
what is the benefit of contrast?
Contrast agent enhances differences between tissues of similar density and allows you to identify specific structures
What are different types of ionising radiation scans?
X-ray
CT scan
PET scan
What should you aim to do with ionising scans?
Always avoid ionising radiation where possible- has risk of cancer
Use lowest radiation dose
What is ultrasound?
High frequency wave generated by piezoelectric material in transducer- sound waves produced
Sound is produced and detected by transducer - speaker sends sound, microphone records sound
Widely available and low cost
Non- ionising
Has limitations- can’t penetrate through air or bone
How does plane radiography (x-ray) work?
X-rays are photons
They’re generated from complex X-ray tube then collimated to the appropriate area of the body
As x-rays pass through body they’re attenuated (reduced in energy) by tissues
X-rays that pass through tissue interact with photographic film
How are different parts of the body respond to x-rays?
Air attenuates x-rays a little
Fat attenuates x-rays more than air but less than water
Bone attenuates x-rays the most
Bone appears white on film because that region of film exposed to least amount of X-rays
How is contrast agent delivered?
Injected into veins/ arteries
Swallowed
Via rectum
What is subtraction angiography?
In angiography, its hard to see contrast agent in vessels under overlying bone
therefore:
1. 1or 2 images are taken before contrast agent is given- these images are inverted
2. More photos are taken after injection
3. By adding negative precontrast image to positive post contrast image, bones and soft tissue are subtracted to produce image of contrast only
What is dopple ultrasound?
Dopple ultrasound enables determination of flow, its direction and its velocity in a vessel using ultrasound
Sound waves bounce of moving structures and are returned
Degree of frequency shift determines if the object is moving away from or towards probe and the speed its travelling at
Precise measurements of blood flow and blood velocity can be obtained
How does CT work?
CT obtains a series of images of body in axial plane
Patient lies on bed, X-ray tube passes around body and series of images are obtained
Computer carry out complex mathematical transformation of multitude of images to produce final image
How does MRI work?
Patient is placed in strong magnetic field which aligns hydrogen nuclei in patient (act as bar magnets)
When pulse of radio waves is passed through patient, magnets are deflected and as they return to their positions they emit small radio pulses
Strength and frequency of these pulses and time it takes for protons to return to their pre-excited state produces signal
These signals are analysed by computer and image is created
What are the different MRI images produced?
T1 weighted- dark fluid and bright fat
T2 weighted- bright fluid
