Magnetic resonance imaging

What are the properties of MRIs

• MRI uses a combination of magnetic field, radio-waves and a computer 

• Thought to have little known biological hazards associated with its use 

• Many patients require anaesthesia/ sedation in order to remain completely still 

• Ferrous metal must remain outside the gauss line.   

• Uses hydrogen protons within the tissues   

• Although MRI does not use ionising radiation, there are some precautions that need to be taken   

 

What are the precautions for MRI

• Patients with metal implants 

• Operators with pacemakers, implants or pregnant 

• Credit/debit cards will be wiped if taken into the scanner field 

• Ear protection for high field unit use as very noisy 

MRI and hydrogen atom interaction

• The body is made up of approximately 70% water  

• MRI uses the magnetic properties of hydrogen 

• H has no neutron 

• Protons consistently rotating  

How is an MRI image produced (part 1)

• Magnetic field is applied to the structure 

• Spinning protons rotate to align with the magnet 

• A series of radio frequency waves are applied 

• This forces the proton away from the magnet to 90-180° 

How is an MRI image produced (part 2)

• Transmission of RFW ceased, the protons return to their original position with the magnet 

• This movement and energy this causes is detected via sensors (RFC) within the system 

• These signals are then converted from analogue to digital images  

What is T1 relaxation

When atoms realign with the magnetic field

Image of T1 relaxation

What is T2 relaxation

When the atoms stop precessing (spinning) around their axis

Image of T2 relaxation

What are radio frequency coils

• The receivers 

• Often the transmitter of radiofrequency signal 

• Gradient coil-  

Pulse sequences

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T1W (fat)

• Inflammation = dark 

• Water = dark 

• Fat = bright 

• Blood = bright 

• Rapidly flowing fluid = dark 

T2W (water)

• Inflammation = bright 

• Water = bright 

• Fat = dark 

• Blood = dark 

• Rapidly flowing fluid = dark 

How are MRI applicated

• Produces cross sectional images in three planes 

• Soft tissue information is much greater than CT 

• Orthopaedics inc. articular cartilage, joint fluid etc. 

• Brain and spinal cord conditions 

• Can be used in conjunction with contrast media 

What are the clinical applications of MRIs

• Predominately soft tissue 

• Inflammatory process in bones   

• Sensitive to bone density changes and remodelling   

• Pre-fractural plains 

• Single lobe damage to tendon and ligament tissue  

What are the 2 types of systems

• Low field

• High field

What are the properties of low field

• Typically 0.27 tesla 

• Temperature controlled by the environment 

• Less sensitive than high field but tolerates more movement  

• Cheaper to install and maintain   

What are the properties of high field

• >1 tesla  

• Uses liquid helium to enable superconducting 

• Very sensitive- even to breathing 

• More expensive than low field  

• Artefacts increased around metal objects  

How do we manage MRIs

• Building or housing should be cased with copper lining 

• No ferrous material to be taken past the gauss line  

• Hygiene  

• Temperature 

What are the planes of view (distal limb)

• Dorsal (frontal plane) 

• Transverse plane 

• Sagittal plane  

How do you prep a patient for MRI

• No metal implants 

• IV access, consider starvation if appropriate 

• General prep for GA or sedation 

• Pre-warming 

What patient care do you need for the procedure

• Monitoring 

• Prevent hypothermia where possible 

What patient care do you need post procedure

• Patient warming 

• Pain scoring 

What are the advantages of MRIs

• No biological hazard to staff or patients- doesn't use radiation  

• Really sensitive- informed choices on treatment plans, prognosis with better degree of activity

What are the disadvantages of MRIs

• Time consuming  

• Time it takes to interpret images- not ideal for emergencies  

• More expensive than x-ray 

• Designated area for MRI machine and a maintenance plan in place- expensive to maintain so not that accessible