mri safety basic knowledge about mr environment, safety measures

MRI is a non-invasive diagnostic examination, there are potential risks in the MRI environment for the patients, accompanying family members, attending healthcare professionals, supporting staffs and MRI staffs.
The use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices results in unwanted injuries. We should always keep in mind that "THE MAGNETIC FIELD IS AT
FULL POWER ALWAYS "
IT IS ALWAYS “ON”.

1. MRI SAFETY Roshni Maharjan Bsc.MIT Final year Roll No. 75 MMC,IOM

2. INTRODUCTION Technology has been evolving continuously, yielding MRI systems with stronger static magnetic fields, faster and stronger gradient magnetic fields, and more powerful radiofrequency (RF) transmission coils. Although MRI is a non-invasive diagnostic examination, there are potential risks in the MRI environment for the patients, accompanying family members, attending healthcare professionals, supporting staffs and MRI staffs. The use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices results in unwanted injuries

3. Individuals being scanned and those in the immediate vicinity of the equipment can be exposed to three variants of magnetic fields simultaneously: the static magnetic field (B ) time-varying magnetic field gradients (dB/ dt ) radiofrequency (RF) magnetic fields (B 1 ). Users of superconducting magnets will also be at risk from cryogen hazard.

4. MR ENVIRONMENT Three dimensional volume of space surrounding the MR magnet. Contains both the Faraday shielded volume and the 0.50 mT field contour (5 gauss (G) line). This volume is the region in which an item might pose a hazard from exposure to the electromagnetic fields produced by the MR equipment and accessories.

5. The boundary in the MR system at which the static magnetic field has diminished sufficiently to pose no physical threat to the general public, but more specifically for individuals with implanted pacemakers, is known as the 5 G line.

6. MR LABELLING MR SAFE MR CONDITIONAL MR UNSAFE Item that poses no known hazards resulting from exposure to any MR environment items are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic an item with demonstrated safety in the MR environment within defined conditions. At a minimum, address the conditions of the static magnetic field, the switched gradient magnetic field and the radiofrequency fields. an item which poses unacceptable risks to the patient, medical staff or other persons within the MR environment

7. MRI Safety Zones Divided into four zones: Zone I : all areas that are freely accessible to the general public which is typically outside the MR environment itself. It is the area through which patients, health care personnel, and other employees of the MR site access the MR environment. Zone II : the interface between the publicly accessible, uncontrolled Zone I and the strictly controlled Zones III and IV. Patients are greeted in Zone II and are not free to move throughout Zone II at will, but are rather under the supervision of MR personnel. Answers to MR screening questions, patient histories, medical insurance questions, etc. are typically obtained.

8. Zone III: region in which free access by unscreened non-MR personnel of ferromagnetic objects or equipment can result in serious injury .Personnel are not to move freely through this zone. Zone IV: synonymous with the MR scanner magnet room itself. Nobody that has not been screened will enter this zone under any circumstances. Zone IV should also be demarcated and clearly marked as being potentially hazardous due to the presence of very strong magnetic Fields. Zone IV should be clearly marked with a red light and lighted sign stating, ‘‘ The Magnet is Always On’’.

9. MRI door entry sign

10. Patient and non-MR personnel screening Any individual undergoing an MR procedure must remove all readily removable metallic personal belongings and devices on or in them (e.g., watches, jewellery , pagers, cell phones, body piercings(if removable), contraceptive diaphragms /cosmetics containing metallic particles and clothing items which may contain metallic fasteners, hooks, zippers, loose metallic components or metallic threads). Therefore, the patients are asked to wear a site-supplied gown with no metal fasteners when feasible.

11. MRI Screening Form To ensure patient safety, completion of the MRI screening form is required prior to every MRI scan. The MRI Screening Form is used to help identify any potential dangers for patients and relatives . The form consists of a series of questions intended to identify any metallic objects within your body that could be affected by the magnetic field. Ideally, the form should be filled out by: a. The Patient If the patient cannot fill it out: b. Family Members If there are no family members: c. Referring doctors

12. Employee safety All employees must be screened to work in a magnetic field environment just like the patients. No exceptions.

13. MRI Screening Form

14. Patient Screening and Contraindications No one should enter the scan room without first being cleared by an MRI operator. Some implants/devices are contraindications for an MRI scan If a subject answers “yes” to any question on the MRI screening form, that issue must be addressed and resolved prior to entering the scan room No cardiac pacemakers, defibrillators, aneurysm clips or electronic or magnetically activated devices

15. Patients having a history of orbit trauma by a potential ferromagnetic foreign body should be screened by either plain X-ray orbit films or by assessment of contiguous cut prior CT images done previously at the time of trauma Any person who was injured by a metallic foreign body such as a bullet, shrapnel may not be able to proceed with an MRI scan unless there is proof that any remaining metal in the body is not in a location where it may move and cause injury/death

16. Forces in the MR Environment Magnetic field missile effect: TRANSLATION rotational effect: ROTATION/TORQUE

17. Translational Force(missile effect) force which attracts ferrous objects to the center of the magnetic field may act to transform ferrous objects into missiles as they accelerate toward the magnet translational force is greatest when the difference in field strength across the object is greatest

18. Rotational Force T his force relates to the North - South orientation of the scanner’s magnetic field Ferrous objects will attempt to align their long axes with this orientation T his force will rotate objects until they are aligned and is greatest at the very center of the field

19. Static magnetic fields(B ) Constant fields, which do not change in intensity or direction over time, in contrast to low and high frequency alternating fields. Effects: biological effects, projectile hazards, compatibility of implantable medical devices and compatibility of peripheral equipment No any obvious detrimental field-related effects occur, especially in the short term.

20. Effects of static magnetic fields in excess of a few tesla on growth and behavioural development of fetuses and infants is known very little but some caution should be taken regarding their imaging. Physical movement within a static field gradient may induce sensations of vertigo and nausea, phosphenes and a metallic taste in the mouth, twitching and disequilibrium for static fields above 4 T.

21. Potential Projectiles Any ferromagnetic object may be attracted to the MRI scanner and become a projectile – this is known as the missile effect. The greater the amount of ferromagnetic material, the greater the force of attraction . The magnetic field extends beyond the bore of the magnet in all directions (fringe field)

22. Fringe field The magnetic field outside the magnet, which is also called the stray field. Its strength depends on the magnet the design of magnet (open versus tunnel bore ) and the main magnetic field strength and the shielding employed (active, passive cladding, or whole room shielding). The higher the field strength, the larger the fringe field . specifies the perimeter around a MR scanner within which the static magnetic fields are higher than five gauss. Five gauss and below are considered 'safe' levels of static magnetic field exposure for the general public.

23. No loose metallic objects should be taken into the Scan room! Potential Projectiles Cell phone Keys Glasses Hair pins / barrettes Jewelry Safety pins Paper clips Coins Pens Pocket knife Nail clippers Steel-toed boots / shoes Tools Clipboards

24. Potential Projectiles – Large Objects Due to the strength of the magnet, large objects such as chairs and IV poles can become projectiles and get stuck in the magnet!

25. Interaction with implantable medical devices The strong static magnetic field can affect implantable medical devices in exposed people (staff, patient or volunteer). Any ferromagnetic component within an implantable medical device may experience both an attractive force ( i.e the device will try to move to the iso -centre) and/or a torque force ( i.e the device will try to turn to line up with field lines). Both of these effects can cause tissue damage and/or damage to the implantable medical device. Examples of implantable medical devices are stents, clips, prostheses, pacemakers and neuro - stimulators.

26. Interaction with other equipment The static field can affect monitoring equipment that has ferromagnetic components. Function of the equipment could be affected. A ll equipment with significant ferromagnetic components has the potential to be a projectile hazard Devices may also be affected by currents induced by movement through a static magnetic field. It is recommended that appropriate MR CONDITIONAL monitoring and support equipment ( e.g ventilators, anaesthesia machines, pumps, etc.) is used.

27. Aneurysm clips Biopsy needles Bone growth stimulators Bullets Cardiac pacemakers Cochlear implants Dental implants Heart valve prosthesis Hemostat clips Implantable drug infusion pumps Internal defibrillators Intravascular stents Neurostimulators Occular implants Penile implants Vascular clamps Devices contraindicated for MRI

28. Time-varying magnetic field gradients (dB/ dt ) Provide position-dependent variation in magnetic field strength.  The faster the imaging sequence, the greater the rate of change of the gradient fields used and the resultant current density induced in the tissue. At 3T/s,3µA/cm 2 electric current density in tissue, results in involuntary muscle contraction, cardiac fibrillation Biological effects: peripheral nerve stimulation ,acoustic noise and magnetic phosphenes

29. Peripheral nerve and muscle stimulation At low frequencies, induced currents are able to produce the effect of stimulation of nerve and muscle cells . The extent will depend on the pulse shape and its repetition rate. This stimulation can be sufficient to cause discomfort and in extreme cases might result in limb movement or ventricular fibrillation. The body is most sensitive to fibrillation at frequencies of between about 10 Hz and 100 Hz and to peripheral nerve stimulation at up to about 5 kHz. Above these frequencies, nerve and muscle cells become progressively less responsive to electrical stimulation.

30. Acoustic noise caused by the gradient coils switching on and off during the scan can reach unacceptable levels. the higher the field strength, the higher the acoustic noise level, but this effect is not exponential and is also dependent on pulse sequence.   the threshold of instantaneous and permanent acoustic trauma normally associated with exposure to impulsive noise is 140dB in adults – children may have a lower threshold and maximum peak levels of 120dB are advised.

31. Magnetic phosphenes Flashes of light that can sometimes be perceived with eyes closed Caused by the electric stimulation of the sensory receptors of retina None at 2 T Reported at 4T

32. Radiofrequency Fields RF field used in MRI exists approx 10-200 MHz. Consists of oscillating electric field and a similar orthogonal magnetic field The main safety issues are thermal heating leading to heat stress induced current burns and contact burns. At all frequencies, induced currents will lead to power dissipation within the body’s tissues →lead to accumulation of energy with time and a rise in body temperature. Above 0.1 MHz heating effects predominate

33. The RF field distribution is not uniform – in-homogeneity increases with increasing field strength, and depends on coil design. Absorption of energy from radiofrequency fields used in MR results in the increased oscillation of molecules and the generation of heat. If this occurs in human tissue, a compensatory dilation of blood vessels results in an increase in blood flow and the removal of the excess heat, which is dissipated mainly through the skin. Modes of operation are chosen to restrict SAR such that temperature rise is restricted. The basic restriction is to limit whole body temperature rise under moderate environmental conditions

34. Specific Absorption Rate (SAR) RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg). Describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal.  Inhomogeneity  of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue , the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. 

35. It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. Doubling field strength from 1.5 Tesla (1.5T) to 3 Tesla leads to a quadrupling of SAR. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast. FDA SAR limits: Whole body: 4W/kg/15-minute exposure averaged Head: 3W/kg/10-minute exposure averaged Head or torso: 8W/kg/5 minute exposure per gram of tissue Extremities: 12W/kg/5 minute exposure per gram of tissue

36. IEC (International Electrotechnical Commission) SAR limits of some European countries: All limits are averaged over 6 minutes Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) :2-10W/kg; Extremities (local) 20W/kg; Level I (first level controlled operating mode): Whole body 4W/kg; Head 3.2W/kg; Head or Torso (local) 4-10W/kg; Extremities (local) 20W/kg; Level II (second level controlled operating mode): All values are over Level I values

37. Heat stress Heat stress is of particular concern for some patients, such as those suffering from hypertension, or pregnant women, or those on drugs such as diuretics or vasodilators that may compromise these responses MR scanners limit temperature rise by limiting SAR. Exposure to RF fields of sufficient intensity can induce heating in biological tissue, while effects in the absence of heating remain controversial. Hence restrictions on exposure to RF fields used in MRI procedures are based on limiting both body core temperature rises and temperature rises in parts of the body

38. Contact burns RF field will induce currents in conductors and can raise their temperature significantly. Burns to volunteers and patients from contact with such metallic objects can be avoided by careful positioning and set up within the bore of the magnet. Examples of causes are: contact with metal in clothing, coils, coil leads, ECG connectors and oxygen monitor probes. Induced current burns are frequently not immediately sensed by the patients until thermal damage. It is preferable that patients be instructed not to cross their arms or legs in the MR scanner such a way as to form a large caliber loop within the bore of the MR imager during the imaging process .

39. Cold compresses or ice packs may be placed on the tattooed areas place if these tattoos are within the volume in which the body coil is being used for RF transmission, to decrease the potential for RF heating of the tattooed tissue

40. Quenching A quench is the rapid helium evaporation and the loss of superconductivity of the current-carrying coil that may occur unexpectedly, or from pressing the emergency button in a superconducting magnet. As the superconductive magnet becomes resistive, heat will be released that can result in boiling of liquid helium in the cryostat. Quenching can cause total magnet failure and cannot be stopped.  MRI systems are designed such that all of the escaping cryogenic gas is directed out of the building (quench pipe through the roof or the wall).

41. In the event of a burst of the tank (possible in the case of an accident) or a blockage of the pipes, the helium gas will be forced into the scanner room, giving rise to a large white cloud of chilled gas. The force of quenching can be strong enough to destroy the walls of the scanner room or the MRI equipment. In the event of magnet quenching, it is imperative that all personnel and patients shall be evacuated from the magnet room as quickly as safely feasible. The site access shall be immediately restricted to all individuals until the arrival of service engineers of the MRI system.

42. Cryogens The hazards of cryogen is due to quenching asphyxiation in oxygen-deficient atmospheres cold burns, frostbite and hypothermia from the intense cold Hyperbaric pressure considerations within magnet room due to escape of cryogenic gases into the magnet room

43. Cryogen hazard sign

44. Pregnancy Pregnant health care practitioners are permitted to work in and around the MR environment throughout all stages of their pregnancy. Acceptable activities include, but are not limited to, positioning patients, scanning, archiving, injecting contrast, and entering the MR scan room in response to an emergency. Although permitted to work in and around the MR environment, pregnant health care practitioners are requested not to remain within the MR scanner bore or Zone IV during actual data acquisition or scanning.

45. Patient Pregnancies No any deleterious effects of MR imaging exposure have been documented on the developing fetus. If pregnancy is established, it is necessary to reassess the potential risks versus benefits of the pending study in determining whether performance of the requested MRI examination could safely wait until the end of the pregnancy . In case of clinical urgency, fetal MRI could be done. Hence, no special consideration is recommended for the first , versus any other, trimester in pregnancy.

46. Heat Excessive heating is a potential teratogen ; because of uncertainties in the RF dosimetry during pregnancy, it is recommended that exposure duration should be reduced to the minimum and that only the normal operation level is used. Clinical techniques do not significantly elevate tissue temperature. Scanning of pregnant patients with the whole body RF transmit coil should be limited to the normal operating mode with respect to the SAR level.

47. The fetus and noise exposure Reeves et al looked at this issue in 2010 ‘No significant excess risk of neonatal hearing impairment after exposure of the fetus to 1.5 T MR imaging during the second and third trimesters of pregnancy’.

48. MR Safety of Accompanying Family or Personnel Those accompanying or remaining with the patient should be screened Hearing protection and MR safe/MR conditional seating are recommended for accompanying family members within the MR scan room.

49. Contrast safety Gadolinium is toxic but is caged by a chelate (DTPA). The chelate may be dissolved by the kidneys, releasing the gadolinium. They are filtered in the fetal kidneys and then excreted into the amniotic fluid . In this location the gadolinium- chelate molecules are in a relatively protected space and may remain in this amniotic fluid for an indeterminate amount of time before finally being reabsorbed and eliminated. It is unclear what impact such free gadolinium ions might have if they were to be released in any quantity in the amniotic fluid . The risk to the fetus of gadolinium based MR contrast agent administration remains unknown and may be harmful.

50. Complications: Nausea Pariorthodal edema nasal and ear lobe swelling neurential spasm total body erythema breathing difficulty anaphylactoid reactions The decision to administer a gadolinium-based MR contrast agent to pregnant patients should be accompanied by a well-documented and thoughtful risk– benefit analysis.

51. Conclusion MRI = Metal Results (in) Injury THE MAGNETIC FIELD IS AT FULL POWER ALWAYS …. IT IS ALWAYS “ON”.

52. References ACR guidance document on MR safe practices:2013;Journal of Magnetic Resonance Imaging · March 2013 Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use March 2015 Magnetic Resonance Imaging, Physical and Biological Principles; Stewart C. Bushong MRIsafety.com Safety in the MR Environment: MR Safety Screening Practices

53. THANK -YOU