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!!!!

Active Shielding

Equipment : The Magnet

Bore Y Z

SM M

X

SM RF RF RF

Shim coils Housing RF Coils X Y and Z Planes

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

• 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

• 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 B0 + *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 Gradient B0 B0 -

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 • Patient Issues – Claustrophobia Patient co-operation Time consuming procedure Unsuitable for trauma patients

Advantages of MRI

• No Ionising radiation involved • Non-invasive technique • Excellent anatomical detail • Choice of any anatomical plane • Versatility in tissue differentiation

The Doctors Choice

MR?

CT?

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 Cancer - 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, cartilage Spine - CT bony injury MR ligamentus or cord injury

Next?

Clinical images