Magnetic Resonance Imaging

Part 2Applying the Science

Lynn Graham DCR MscClinical 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

X

Y

Z

Helium Bath

Superconducting coil

Active Shielding

Gradient Coils X Y and Z Planes

Bore

Housing RF

RF

RF

RF Coils

SMM

SM

Shim coils

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

Gradient

B0B0 -

B0 +

*The creation of differing electrical potentials in the body may result in sensations of vertigo or nausea

Time Varying Magnetic Fields: Biological effects

*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

Brain shunts

Ear Implants

Clips around blood vessals

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 CTBoth are cross-sectional imaging

( slicing through the body)

CTHigh Dose X rays

Shorter, wider tunnelOnly one plane – axial

Faster – more coverage

MRI A magnet and radiofrequency

Longer narrower tunnelAll imaging planes

availableSlower 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