Magnetic Resonance Imaging Flashcards
features of MRI scanning?
most detailed imaging
- produces cross section images in 3 planes
uses a combination of magnetic fields, radiowaves and a processor
- no ionising radiation = little hazard
- all magnetic items outside Gauss line (perimeter of magnetic field)
uses magnetic properties of hydrogen protons within tissues
- water aspects - 70% of body
- eg synovial fluid and spinal cord
require sedation/GA to remain still
- small movement impacts the entire image
why would we use an MRI?
predominantly soft tissue imaging
- eg cartilage/tendon/ligament damage
- but also joint fluid
bone density changes and remodelling
- eg inflammatory process in bones
- eg fracture plains
Brain and spinal cord conditions
what precautions need to be taken when using MRI?
patents with metal implants or pacemakers
pregnant patients and staff
- magnetic fields heat up amniotic fluid
ear protection as the unit is loud
any cards within the field will be wiped
how is an MRI image produced?
a magnetic field is applied to the area
- constantly spinning protons with the area align themselves with the magnet
radiofrequency waves (RFW) then applied
- forces protons away from the magnet by 90-180°
the RFW are then stopped
- protons return to their original position aligning with the magnet
- this movement (and its energy released) is detected by sensors within the system
- the time it takes varies on tissue
the signals are then converted to an image
what is T1 and T2 relaxation?
when not interrupted protons spin on their axis
- like the world spinning
the introduction of RFW tilts protons onto their process and spin
- like the earth going around the sun
T2 relaxation occurs first
- time takes for a proton to lose energy and stop spinning on its precess
- stop spinning around the sun
T1 relaxation occurs second
- time takes for a proton to return to its original rotation spinning on its axis
- back to just earth spinning
elements of the MRI unit?
magnet - always on
gradient coil
- metal coil
- allows the magnetic field to be pulled to a specific area across all axis
- allows one image slither to have 0.2mm width - specific detail
- high detail once all conjoined
radiofrequency coil
- receives radiofrequency signals for conversion
types of MRI systems?
low field
- temperature controlled by enironment
- less sensitive - less data quality
- tolerates more movement
- cheaper to install/maintain
high field
- more magnets - consider rooms nearby and equipment
- regulates own temp using liquid helium
- very sensitive - must GA
- more expensive
how to maintain MRI unit?
room cases with copper lining
no ferrous material past Gauss line
keep patients and unit clean
- dirt/hair/dust can cause artefacts
keep room at 24 degrees
- sudden changes cause condensation and erosion inside unit
- also bad for patient temp management
service and calibrate annually
- and when a metal object enters the field
planes of view for MRI?
dorsal - front and back
transverse - top and bottom
sagittal - left and right
patient care for MRI?
prep
- remove collars/harness etc
- normal GA prep
- cushioning on bed and positioning aids ready
- provide ear protection
- pain score
- x-ray prior if unsure about implants
during procedure
- hard to monitor - take parameters regularly
- maintain heat and comfort
post
- anti-emetic/prevomax
- analgesia/pain score
advantages and disadvantages of MRI?
Advantages
- high-quality image
- 3D image
- no radiation
- good for soft tissue diagnosis
- painless
negatives
- long time
- monitoring is a challenge
- need patience very still
- need a specialist to analyse
- expensive - for owner and practice
