imaging Flashcards
what are the different imagining techniques which rely on x-ray?
- X-ray - single pulse of x-ray creates one image
- Fluoroscopy - continuous pulse of x-ray creating real-time moving image
- CT - X-ray tube and detector moves around patient, creating cross-sectional image
how is an image produced using X-rays?
- e- are accelerated towards a metal target - this interaction produces photons (X-rays)
- some X-rays pass through the patient then hit a detector behind the patient
- some are attenuated by the patient (absorbed, scattered or lose energy)
- amount of attenuation depends on:
- density and atomic num of tissue
- energy of X-ray beam
- amount of attenuation depends on:
- detected X-rays are processed to create an image
what are the different densities visible on an X-ray?
black = air
fat = dark grey
bone = light grey
metal = white
what are the advantages and disadvantages of using an X-ray?
Advanatages:
* quick
* cheap
* portable
Disadvantages:
* radiation
* one plane and 2D
* poor soft tissue imaging
how is an image produced using fluoroscopy?
Process similar to that for X-ray, except pulsed or continuous X-rays are used
creating moving images
enhanced using contrast (barium/iodine)
what are the advantages and disadvantages with fluoroscopy?
Advantages:
* Dynamic
* quick
Disadvantage:
* higher radiation dose than X-ray
* radiation exposure to clinician
* poor soft tissue imaging
how is an image produced using CT?
similar to process described for x-ray:
- x-ray tube is on one side of a rotating ring and detectors are on the opposite side
- patients table moves through the ring
- creates cross-sectional slices of patient taken and image produced using computer
in what anatomical plane is CT viewed?
axial/transverse view
what are the advantages and disadvantages of CT scanning?
Advantages:
* quick
* good spatal resolution
* can scan most parts of body well
Disadvantage:
* radiation
* artefacts
* contrast reactions
how does PET scanner work?
- Radionuclides (Fl -18) emit positrons during decay (‘positron emission’)
- Emitted positrons collide with nearby e- in patient
- producing two annihilation gamma photons
- which are detected by the gamma camera
what are the advantages and disadvantages of PET scanning?
Advantages:
* good contrast + spacial resolution
* can analyse anatomy and function
Disadvantages:
* Physiological uptake of radiopharmaceutical
* Radiation dose to patient + radiation to clinicians
* Expensive and time consuming
how do MRI work?
- MRI scanner creates a strong magnetic field, aligning hydrogen atoms within patient
- Radiofrequency pulse is applied, ‘tipping’ the aligned hydrogen atoms, which then together create a detectable magnetic field
- This field induces an electric current in nearby coils in the MRI machine
- Varying signal intensities are produced by different tissues
- Signals processed to create images
- After the radiofrequency pulse ends, the hydrogen atoms relax back into alignment with the magnetic field of the MRI machine
- Can adjust the settings to exploit differences between varying tissues of interest (‘weighting’)
what are the bright and dark regions in an MRI?
Bright = high signal =hyper-intense
Dark = low signals =hypo-intense
what are the advantages and disadvantages of MRI?
Advantages:
* no radiation
* good contrast resolution especially of soft tissue
Disadvantages:
* expensive
* time consuming
* fewer machines + fewer radiographers
how does an US work?
- transducer uses many piezoelectric crystals which vibrate when energy signal applied to them
- vibration produces high frequency US waves
- when US enters body and reaches a new material of different density some is reflected back
- reflected waves are used to create an energy signal - computer uses these to create points of brightness on image according to its anatomical position
what are the areas of white and dark on US image?
areas of white = more reflection = hyperechoic
areas of black = less reflection = hypoechoic
what is acoustic shadowing?
large acoustic impedence mismatch = sound waves completely reflected back, none pass through
e.g dark area behind bone, air, stones
what is a duplex ultrasound?
uses the normal 2D imaging + doppler
for things moving towards sound wave - increased frequency of echo wave on doppler
for things moving away from sound wave - decreased frequency of echo wave on doppler
what are the advantages and disadvantages of US?
Advantages:
* no radiation
* low cost
* portable
* dynamic
Disadvantage:
* operator dependant
* no bone or gas penetration
* difficult with obese patients
* risk of overheating if misused
