Magnetic Resonance Imaging Part 2 Applying the Science

1. Magnetic Resonance ImagingPart 2Applying the Science Lynn Graham DCR Msc Clinical Specialist in MRI

2. 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

3. What is Clinical MRI? • Transmitter ( generate RF @ larmour frequency) • Receiver ( gather MR signal from patient) • Computer + Visual Display Unit ( decode the information, produce + display images) Surrounded by a magnetic field – BIG HUG!!!!

4. Equipment : The Magnet

5. Clinical Magnets • Field strengths 0.2 - 1.5 Tesla • Superconducting electromagnets • Cooled by helium bath

6. Surface coils • Loops of wire that detect MR signal • Designed to fit specific body parts

7. + 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!

8. MRI Safety • THE MAGNET IS ALWAYS ON! No radiation BUT Very strong magnetic field + Radio frequency waves

9. MR Safety -Hazards • Static Magnetic Field • Time Varying Magnetic Fields • Radiofrequency Waves • Cryogens ( Helium Liquid + gas) • Implants + medical devices • Acoustic Noise

10. Static Magnetic Field : The Projectile effect • Metal objects will be pulled at force towards the magnet • Force ∝ size of object

11. 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

12. Time Varying Magnetic Fields: Biological effects *The creation of differing electrical potentials in the body may result in sensations of vertigo or nausea B0 + *Twitching sensations in hands + feet Peripheral Nerve Stimulation Gradient B0 B0 -

13. 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.

14. 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

15. 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

16. Medical Devices + Implants Electronic devices may malfunction and have serious medical consequences. Cardiac Pacemakers Neurostimulators Clips around blood vessals Brain shunts Ear Implants

17. Strict safety procedures • Protection of Patients + Staff • Local rules/ MDA guidelines • Designated areas/Authorised Personnel Only • Screening procedures : patients and staff • Equipment monitoring

18. 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

19. 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

20. Advantages + Disadvantages

21. 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

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

23. The Doctors Choice MR? CT?

24. 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

25. MRI vs CT (advantages) • Anatomical soft tissue detail –Brain + CNS - Spine + Joints • Various planes available • No ionising radiation (children) • Tissue differentiation • Lesion characterisation

26. MRI vs CT (disadvantages) • Safety issues – not for all patients - serious risks • Claustrophobia • Patient co-operation is paramount • Time consuming

27. MRI vs CT: which one? Generally depends on the patients disease Cancer - CT faster for disease throughout body MR more specific for actual tumour

28. 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

29. MRI vs CT: which one? Joints – CT bony injury MR muscles, ligaments, cartilage Spine - CT bony injury MR ligamentus or cord injury

30. Next? Clinical images