Transcript Clinical Magnetic Resonance Imaging An Introduction (Part II)

Magnetic Resonance Imaging
Part 2
Applying the Science
Lynn Graham DCR Msc
Clinical Specialist in MRI
OUTLINE ( part 2)
Clinical Applications of MRI
 MRI Equipment
 Safety issues of MRI
 Advantages + Disadvantages of MR
 MRI vs Other imaging modalities ( CT/ USS)
 Clinical Images
What is Clinical MRI?
1.
Transmitter ( generate RF @
larmour frequency)
2.
Receiver ( gather MR signal from
patient)
3.
Computer + Visual Display Unit
( decode the information,
produce + display images)
Surrounded by a magnetic field – BIG HUG!!!!
Equipment : The Magnet
Bore
Shim coils
Superconducting coil
SM
M
SM
Helium Bath
Housing
RF
Active Shielding
RF Coils
RF
Y
RF
Z
Gradient Coils
X Y and Z Planes
X
Clinical Magnets
• Field strengths 0.2 - 1.5 Tesla
• Superconducting electromagnets
• Cooled by helium bath
Surface coils
•Loops of wire that detect MR signal
•Designed to fit specific body parts
The working environment
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Strong magnetic field present at all times.
MRI field = 1.5 Tesla = 15,000gauss
Earth = 0.5 gauss
MRI is 3000 times stronger!
MRI Safety
No radiation
BUT
Very strong magnetic field
+
Radio frequency waves
• THE MAGNET IS ALWAYS ON!
MR Safety -Hazards
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Static Magnetic Field
Time Varying Magnetic Fields
Radiofrequency Waves
Cryogens ( Helium Liquid + gas)
Implants + medical devices
Acoustic Noise
Static Magnetic Field :
The Projectile effect
• Metal objects will be pulled at
force towards the magnet
•Force
∝
size of object
Static Magnetic Field :
Biological effects
The creation of electrical potentials in the
body may result in sensations
of vertigo or nausea, but
“ exposure to static magnetic fields below 2.5T is unlikely to have any
adverse effect on health”
NRBP report 1991
“ there is no indication of serious adverse health effects from whole body
exposure up to 8T ”
ICNIRP (MR) 2004
Time Varying Magnetic Fields:
Biological effects
 The creation of differing electrical
potentials in the body may result in
sensations of vertigo or nausea
 Twitching sensations in hands + feet
- Peripheral Nerve Stimulation
Time Varying Magnetic Fields:
Acoustic Noise
Source : Rapid Switching of Gradient Coils
Risk : low – manufacturers set limits
- exposure is not prolonged
- ear protection is always given.
Radio Frequency Waves
• Biological effects
RF deposition results in heating effects within the body
and can interfere with body function if body temp is
increased by more than 1 degree per Kg
- Specific Absorption Rate (SAR)
• Induced current burns
> cables/ ECG leads
> touching the bore
> creation of conductive loops e.g.clasped hands
Cryogenic Hazards
Liquid Helium – maintains the superconducting magnet
Potential risks - asphyxiation (from displacement of oxygen)
- hypothermia
- Frostbite
Safety precautions – Oxygen Alarm
External Vent Pipes
Outward Opening Door to scanner room
Medical Devices + Implants
Electronic devices may malfunction and
have serious medical consequences.
Cardiac Pacemakers
Neurostimulators
Clips around blood vessals
Brain shunts
Ear Implants
Strict safety procedures
• Protection of Patients + Staff
• Local rules/ MDA guidelines
• Designated areas/Authorised
Personnel Only
• Screening procedures : patients and
staff
• Equipment monitoring
Actual incidents : burns
• RVH 2001 : 3 year old suffered third
degree burns to index finger from
faulty oximeter – finger subsequently
amputated.
• 2 occasions : second degree burns
from nicotine patches
Actual incidents : Fatalities
• June 2001 : Texas University Hospital :
6 year old boy killed by an oxygen cylinder.
• July 2003 : Midlands, England
83 year old woman with pacemaker killed
during MRI scan
• 2000 : Australia
Death of a man with pacemaker fitted
Advantages + Disadvantages
Disadvantages of MRI
• MR Safety issues – Projectile effect
Heating effects
Acoustic noise levels
Biological Effects
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Patient Issues – Claustrophobia
Patient co-operation
Time consuming procedure
Unsuitable for trauma patients
Advantages of MRI
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No Ionising radiation involved
Non-invasive technique
Excellent anatomical detail
Choice of any anatomical plane
Versatility in tissue differentiation
The Doctors Choice
MR?
CT?
USS?
MRI vs CT
Both are cross-sectional imaging
( slicing through the body)
CT
High Dose X rays
Shorter, wider tunnel
Only one plane – axial
Faster – more coverage
MRI
A magnet and radiofrequency
Longer narrower tunnel
All imaging planes available
Slower to acquire image data
MRI vs CT (advantages)
• Anatomical soft tissue detail –Brain + CNS
- Spine + Joints
• Various planes available
• No ionising radiation (children)
• Tissue differentiation
• Lesion characterisation
MRI vs CT (disadvantages)
• Safety issues – not for all patients
- serious risks
• Claustrophobia
• Patient co-operation is paramount
• Time consuming
MRI vs CT: which one?
Generally depends on the patients disease
Spinal Injury
Cancer
– CT shows bony injury
MR shows spinal cord injury
- CT faster for disease throughout body
MR more specific for actual tumour
MRI vs CT: which one?
Brain + Central Nervous System
Stroke – CT on A+E admission provides diagnosis
– MR within 48 hours indicates prognosis
Multiple Sclerosis
– MR only modality that can visualise disease
MRI vs CT: which one?
Joints – CT bony injury
MR muscles, ligaments
Spine - CT bony injury
MRI vs USS (common ground)
• Relatively non-invasive
• Do not use ionising radiation
• Require little use of contrast media
MRI vs USS (advantages)
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Operator independant
Various anatomical planes available
Tissue differentiation/ characterisation
Less patient dependant
RSI is not a risk to staff
MRI vs USS (disadvantages)
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Cost and availability
Safety issues
Time consuming
Not suitable for all patients
Greater knowledge/experience of USS