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Mri hazards & safety Daniel JP

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Mri hazards & safety Daniel JP

  1. 1. MR hazards & safety concern Khorfakkan Hospital RADIOLOGY Daniel jayaprakash
  2. 2. MR Hazards & Safety Safety concerns in MR arise from 3 major sources •Static magnetic field (main magnet) •Gradient magnetic field (gradient coil) •RF magnetic field (RF coil)
  3. 3. MR Hazards and Safety
  4. 4. MR Hazards & Safety Units of Field Strength • The Gauss – Measures the Earth's Field (~0.5 G) • The Tesla – Measures the MRI Field • The Relationship: 1T = 10,000 G
  5. 5. MR Hazards & Safety Concerns Static magnetic field Since the static magnetic field is constantly ON it poses a major risk. MR imaging uses magnetic field strengths of 0.35T to 4T. These risks are manifested in 3 ways: a) Effects on physiology. b) Effects on implanted devices. c) Effects on metal objects.
  6. 6. Static Magnetic Field Effects on physiology a) Cell and nerve function-de oxygenated sickled cells align against or perpendicular to the field. b) Unusual sensation including nausea, vertigo and metallic taste associated with altered nerve functions.
  7. 7. Static Magnetic field c) Cardiovascular effects-elevation of T wave elevation.T wave elevation normally found in patients with ischaemia & myocardial infarction. d) Temperature-static magnetic field do not cause alteration in skin and body temperature. e) Magnetophosphenes-due to excitation of optic nerve or retina caused by eye or head movements.
  8. 8. Static Magnetic Field Effects on implanted device Ferromagnetic devices will align and twist (torque) and cause unanchored implants to move within the body and cause injury to tissues. Metallic implants can cause artifacts and result in misinterpretation of images.
  9. 9. Static Magnetic Effects Metallic implant-intracranial vascular clip. Aneurysm clips are highly ferromagnetic causing deflection. Clip motion may damage the vessel resulting in hemorrhage , ischaemia or death. Implanting physicians should provide patients details of implant or an alert card.
  10. 10. Static Magnetic Field Effects on metallic implants. Orthopedic implant show minimal deflection and cause image artifacts and not likely to be dislodged. Large implants like hip prosthesis can cause heating which can be significantly low.
  11. 11. SM Effects on Metallic Implants Cochlear implants like 3M/House,3M/Vienna device, Nucleus mini 20 channel are highly contra integrated. These devices are made up of high field strength magnet, in conjunction with external magnet align with a transmitter coil on patient’s head. Wires in these devices can induce current causing injury to patient or interfere with the mode of operation.
  12. 12. Static Magnetic Field Effects on metallic implants. Intravascular coils , filters and stents considered safe for imaging after few weeks are unlikely to be dislodged.
  13. 13. Static Magnetic Field Extracranials vascular clip Poppen Blaylock carotid artery clamp is contra indicated. Vascular access ports – these are safe for imaging To provide long term administration of chemotherapy agents ,antibodies and analgesics
  14. 14. Static Magnetic Field Heart valves showed less deflection and safe for MR imaging. Cardiac pace makers strictly contraindicated. Risk of movement of device, damage to reed switch and change in the mode of operation.Intection of current in lead wires can cause fibrillation and thermal injury.
  15. 15. Static Magnetic field Dental devices - These have minimal deflection and safe for imaging. The amount of deflection can cause image artifact or can be dislodged. Ocular implants – The fatio eyelid spring, retinal tack (martensitic stainless steel),troutman magnetic ocular implant, unitec round eyelid spring can cause severe injury like vitreous hemorrhage and blindness. Plain x-ray is an acceptable tool for unresolved FB and implant
  16. 16. MR hazards & safety Halovest and device MR can be performed only on specific designed vest because current induced in the devices can heat and burn the surrounding tissues ,so it is advisable to perform MTC pulse sequence.
  17. 17. MR hazards & safety Static magnetic effects on metal objects Projectile effect-ferro magnetic objects have the potential to act as missile flying into the bore. Small objects are pulled with a velocity of 40MPH, causing serious risk to patient and damage expensive equipments
  18. 18. MR Hazards and Safety
  19. 19. MR hazards & safety Gradient Magnetic field These are magnetic field of the order of 0.1 to 1 gauss/cm super imposed over the main magnetic field to cause predictable variation along a predetermined plane. In the absence of gradients all protons in an imaging plain precess in same phase. (eg) slice select ,frequency and phase encoding
  20. 20. MR hazards & safety Gradient fields induce current in tissues and cause power deposition (1to3µ A) Eg. Gradient applied 500µ sec at 1g/cm will be 2000g/sec /cm that means 2000g x30cms=60,000g/sec 60,000g =6T/scc
  21. 21. MR Hazards & Safety GM- cardio muscular & vascular system Since blood is a tissue it acts as a conductor . A 1 T magnet induces a current of 1µ A/cm² . Beyond 1µ A there is ventrcular fibrillation . 15 to 100 µ A/cm produce large tetanic contraction of skeletal muscle(breathing) .
  22. 22. MR hazards & safety Gradient magnetic fields Cell and nerve functions GM fields can stimulate or impair impulses along nerve path way .so normal function of nerve cells and muscle fiber may be affected. Beyond 6T there can be painful peripheral nerve stimulation, so the patients should not be allowed to clasp their hands & feet to form a ring
  23. 23. MR Hazards & Safety Gradient magnetic field - Bio effects Magnetophosphenes – Rapid changing gradients induce current which stimulate optic nerve and retina . Persons experience flashes of light within the eyes None at 1.5T but at 4T .
  24. 24. MR Hazard & Safety GM effects Neural stimulation – The current induced by the rapid changing magnetic field cause direct peripheral muscle stimulation , especially in echo planar imaging which uses gradients of 2.5G/cm . Uncontrolled involuntary skeletal muscle contraction or twitching has been reported in human.
  25. 25. MR Hazard and Safety Gradient Field Induced Acoustic noise Gradient coils are not coils of wire but broad thick copper conducting bands. Loud tapping, knocking or chirping sound are heard when gradient coils expand and impact against their mounting resulting in temporary or permanent hearing loss, difficulty in verbal communication. So all patients must be fitted with noise attenuation equipment like earplugs, headphones or anti noise system.
  26. 26. MR Hazards and Safety Oscillating (RF) Magnetic Field RF coils in the bore are the source of RF during application of RF pulses in different sequences. Range of frequency used is 1 to 100 MHz. This RF energy induces current resulting in tissue heating, which depends on patient size, body habitus, tissue sensitivity, duration and type of RF.
  27. 27. MR Hazards and Safety MRI RF Pulse Sequences SR 90º…90º…90º SE 90º…180º….90º…180º….90º…180º IR 180º…90º…180º….180º…90º…180º GRE αº…αº…αº α is an RF pulse less than 90º. EPI 90º…180º…
  28. 28. MR Hazards and Safety SAR: Specific Absorption Rate Rate at which RF energy is coupled into tissues. Units: Watts per kilogram (W/kg) SAR - Time Product Units: W-min/kg or W-hr/kg Temperature (°C).
  29. 29. MR Hazards and Safety FDA SAR Limits • Body average of 0.4 W/kg • Peak for any gram of tissue of 8.0 W/kg • Head average of 3.2 W/kg
  30. 30. MR Hazards and Safety FDA Temperature Limits • A body core rise of less than 1 C • No more than 38 C in the head • No more than 39 C in the trunk • No more than 40 C in the extremities • Exceptions: Patients with compromised heat regulation
  31. 31. MR Hazards & Safety • Fringe Magnetic Field (Stray Magnetic Field) These are the components of the main magnetic field that extend outside the main magnet and its housing Permanent Magnet (0.3T) – Fringe Field <1m Resistive Magnet (0.2T) 2m Super Conductive Magnet (0.5T to 4T) -10m It is recommended that those person not screened remain out of this field
  32. 32. MR Hazards & Safety • Pregnant Patient There are no known biological effects of MR on fetuses . If the information to be gained by MR would have required more invasive test , MR is acceptable . High risk pregnant patients can delay the examination until the first trimester .
  33. 33. MR Hazards & Safety Pregnant Employees Pregnant employees can safely enter the scan rooms , but should leave while RF and gradient fields are employed . It is recommended that the employees stay out of the magnetic field during the first trimester . However to leave an environment which is hazardous may be advisable . These are the guidelines from ISMRM .

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