MRI Safety for Healthcare Personnel


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The MRI suite poses special safety risk since the magnet is over 100,000 times the earth’s natural magnetic pull. Because the magnet is always “on” those who approach the magnetic field must have training. This module discusses the special considerations of MRI safety that Radiographers, Nurses, Patient Transport, Custodial care professionals and the like should understand and practice concerning MR safety. Patient safety concerns such as pacemakers and metallic devices related to imaging are also discussed.
>> Also approved for Florida State License.


Author: Nicholas Joseph Jr., RT(R)(CT)
Credits: 1.5

-- Please note: This article is either under construction or in the approval process. There will be no credit available for this article until the approval process has been completed. Passing the test for this article before the approval process has been completed WILL NOT result in full credit being awarded when the approval process has been completed. You must pass the test for this article after the approval process has been completed in order to receive credit for this article.

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Instructions:

Article navigation:

Objectives

Outline

Introduction

Section 6.1: The need for MRI safety training

Section 6.2: The Static Magnetic Field

Section 6.3: Gradient Magnetic Field Safety

Section 6.4: Radiofrequency Magnetic Field

Section 6.5: Biological Effects

Section 6.6: Acoustic Noise

Section 6.7: Pregnancy and MR

Section 6.8: Cryogen and Quench Concerns

Section 6.9: Patient History and Preparation

Summary and References

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Objectives:

Upon completion the reader will be able to:
  • List the main components of MRI safety.
  • List the three types of magnetic fields used in MRI that could be hazardous to ferromagnetic objects.
  • State the function of the static magnetic field and discuss the regulating of its field strength by the FDA.
  • State the meaning of the SI units Tesla (T) and Gauss (G).
  • Define the fringe field and discuss why it is a gradient field.
  • Discuss why the static and fringe fields are able to accelerate ferromagnetic objects as well causing them to torque.
  • Define the terms MR safe, MR compatible, and MR environment.
  • State the role of the Bureau of Radiological Devices and the implementation of the Problem Reporting Program (PRP) program.
  • State what is an acceptable plain film radiographic pre screening protocol to evaluate potential metallic fragment(s) in the eye.
  • State Faraday’s law of electromagnetic induction of current.
  • State FDA limits for gradient field strengths and rates.
  • Define "ohmic heating" in tissues and the specific absorption ratio.
  • Discuss the magneto-hydrodynamic effect.
  • Discuss how noise to the patient can be reduced during the MR scan.
  • Discuss safety measures to be taken if a quench is necessary.
  • Discuss patient screening for metallic implants and electrically stimulated devices prior to a MR scan.



Outline:

Introduction

Section 6.1 The need for MRI safety training

Section 6.2 The MR static field

  1. Function of the static MR field.
  2. FDA limit for static field strength.
  3. Fringe magnetic field:
    1. The 5 gauss line
    2. Projectile and missile effects on metallic devices.
    3. Relationship of distance from isocenter and magnetic force.
  4. Defining MR safe, MR compatible, MR environment.
  5. Known ferromagnetic objects from MDR reports.
  6. Plain film radiographic screening for metal in the eye proper.

Section 6.3 Gradient magnetic field safety

  1. Function of the gradient magnetic field.
  2. Faraday’s law of magnetic induction of current in a conductor.
  3. Field strength of gradient magnets and ability to induce current.
  4. Biophysiological effects of current induction in tissues.
  5. FDA magnetic gradient field strength limits.

Section 6.4 Radiofrequency magnetic field

  1. Rf field and ohmic heating of tissues.
  2. Effects of resonant frequency on tissue heating.
  3. FDA limits for specific absorption rate (SAR).

Section 6.5 Biological effects

  1. Magneto-hydrodynamic effect.
  2. Cardiac monitoring of known cardiac arrhythmic patients.

Section 6.6 Acoustic Noise

  1. Cause of noise during MR scanning.
  2. Methods of reducing noise levels heard by the patient.

Section 6.7 Pregnancy factor and MRI

Section 6.8 Cryogen and quench safety concerns

  1. Potential hazards of using liquids to cool superconductive-type magnets.
  2. Protective engineering of the cryostat system and quenching.
  3. Room oxygen displacement hazard as a result of a quench.
  4. Potential consequences of oxygen displacement due to quench.

Section 6.9 Patient history and patient preparation

  1. Clearing for electrically stimulated devices and metallic implants.
  2. Patient history related to contrast agents if prescribed for scan.



Introduction

Medicine today utilizes many complex diagnostic tools that help provide diagnosis and treatment for various medical conditions among the most useful tools in medicine is the magnetic resonance imaging (MRI) scan.

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The magnetic fields used in medical imaging are powerful fields capable of exerting a strong pull on common ferromagnetic materials. This unit will discuss the biological effects of magnetism and the associated magnetic flight risk, and thermo-burn risks associated with the medical use of MRI.




Section 6.1: The need for MRI safety training

The success of MRI safety is largely due to the diligent practices of MRI technologists and radiology staff to prevent potential risk, yet any failure to guard the magnet's force has proven to be a high risk for injury or equipment damage. Most health care workers are unaware of the danger to themselves or to a patient from even a single unauthorized or unsupervised contact with a MR Field. Furthermore, most health care workers are unaware that the MR field is constantly on, which poses significant after hours risk if MR safety is not strictly followed. Case after case of MR safety breach has been reported with the intent to raise the level of awareness among the public and health care workers to the real and present danger of MR fields used in medical diagnostic settings.

In the July 2003 issue of the "American Journal of Roentgenology," Dr. Chaljub of the University of Texas Medical Center, reporting on MR safety and MR accidents states, "we need to keep training our personnel on safety issues. We can't let our guard down." His comments should be regarded as a professional response to one of the most publicized MR accidents in U.S. history, which occurred at Westchester Medical Center in Valhalla, N.Y. A six year old boy who had just undergone neurosurgery to remove a tumor from his brain was undergoing a magnetic resonance imaging scan to evaluate post operative surgical repair, when a medical professional brought an oxygen canister into the MR suite, somehow getting past the watchful eyes of several MRI personnel. According to the Associated Press news release, the canister was pulled into the MR scanner causing blunt force head injury that was the cause of fatal injuries to the patient. This single accident raised the consciousness of the medical community greatly to the need for better MR imaging suite safety training for health care personnel and physicians. Unfortunately, this training has not been implemented as part of the generalized orientation of all hospital personnel, new physicians, and students.

Because of the almost non existence of MR safety training for non radiology personnel, the history of MR safety is littered with evidence of negligence and risk. For example, in 1987 at a Texas hospital, a patient transporter accidently took a ferromagnetic oxygen cylinder that was properly located outside the MR suite into the exam room. The cylinder was magnetically pulled into the bore of the magnet causing facial injuries to the patient and a legitimate subsequent lawsuit against the hospital. What one thing that seems to be a consistent finding in the statistical history of MR accidents is that most do not involve radiology staff. Perhaps it is because the radiologic technologist is specially educated to exercise precautions in the near vicinity of a MR field.

In another case finding a technologist aid brought a patient's chart containing metal binding into the MRI suite. Immediately it was pulled into the magnet's bore. But because there was no patient in the room at the time no injuries occurred, instead; the chart was firmly attached to the magnet and several hundreds of pages of the document flew like feathers from the back of the scanner. The point is that the incident happened so quickly that the aid did not actually witness the metallic flight of the chart and was amazed. This is the story of how injury to patients or to the magnet often occurs. It can occur so quickly that it cannot be stopped, MR accidents can only be prevented from occurring. Research points to inadequate safety training for all hospital staff and failure to adhere to adequate safety standards in place that are necessary to prevent MR accidents.

Ferromagnetic compatibility is not the only safety concern; there are concerns related to biological and thermo effects of magnetic energy. There are three magnetic field sources that may have adverse biological or thermo effects on human tissues. MR fields are persistently analyzed against specific medically used devices, to determine each piece of equipment's character for device risk. The three magnetic fields are: the static magnetic field, rapidly changing magnetic fields (a.k.a. time-varying or gradient magnetic field), and the RF power deposition commonly known as radiofrequency magnetic fields from the RF transmitters. We will look at these three distinct magnetic fields that give rise to potential biological safety risk.




Section 6.2: The Static Magnetic Field

The static magnetic field (Bo), a main component of the MR magnetic environment is always on, day and night, even when there is no imaging taking place. The intensity limit for a static magnetic field in clinical use is limited by the Food and Drug Administration (FDA) to 4.0 tesla (T) strength. The SI unit of magnet field strength is the tesla. Commonly used static magnetic fields are between 0.2 and 2.0 tesla (5,000 to 20,000 Gauss). One tesla (T) is equal to 10,000 Gauss (G). The static field is what aligns the atomic protons in the patient in an organized fashion in preparation for data acquisition. It is what is referenced when referring to the magnetic strength of a MR machine in tesla, and is measured at the precise isocenter of the magnet. Three coordinates intersecting orthogonal planes (X, Y, and Z) meet at the isocenter (Bo) of the magnet where precise imaging begins. Currently being researched is 4T, 5T, and upwards to 7T magnets having nearly 100,000 times the earth's magnetic field strength.

As we move away from the isocenter of the magnet, the static field is extended outward decreasing in strength to cover a volume in space that is called the fringe field, which is measured in gauss units. The fringe field at the entrance to the MRI suite should be equal to or less than 5G. Because the static field drops off rapidly, measured at increasing distances from isocenter, a large spatial gradient of magnetic force with accelerating potential is shaped. Its effect is on ferromagnetic materials, and those materials that can be made magnetic, giving rise to a north and a south pole, by induction. Therefore, any magnetic object captured by the fringe field will be accelerated towards the isocenter, becoming a projectile. Such a projectile is dangerous not only to the patient in the bore of the magnet, but to anyone in the path of its acceleration. Metallic objects, especially iron containing materials (ferromagnetic) will be attracted to the main magnet becoming aligned with the magnetic north and south pole much like a compass will align with the earth's magnetic poles. The process of alignment may cause the materials to have torque along with acceleration.

A projectile can become a missile reaching speeds of nearly 45 mph or more. An attractive magnetic force is inversely proportional to the square of the distance from the isocenter. If the distance from the magnet's bore is halved, then the attraction will double. Likewise, an attractive force of 2T at 1 meter from Bo would have field strength of 0.5 T at 4 meters. The radiographer should recognize that that this rule is much like the inverse square law of radiation intensity and distance, but with the added property of a gradient force. The influence of the magnetic field on an object under its force depends on the objects shape, mass, ferrous content, distance of the object from the magnet's bore, and strength of the magnet. So to avoid any miscalculations of an objects safety around or near the static field, all objects taken into the fringe field or near the static field, without exception should be tested with a hand magnet of at least 3000 G to determine its safeness.

The U.S. Department of Health and Human Services, U.S.F.D.A., and Center For Devices and Radiological Health have coined three terms to help distinguish what materials are used around the MR magnetic field. These adopted terms are MR safe, MR compatible, and MR environment. The term MR safe indicates that the device, when used in the MR environment, has been demonstrated to present no added risk to the patient, but may or may not affect the quality of diagnostic information that may contribute to images. MR compatible is a term that indicates the device when used in the MR environment is MR safe and demonstrated to neither significantly affect diagnostic information quality nor have its operation affected by MR forces. These terms have been implemented to deter those manufacturers desiring to make claims of "MR safe or MR compatible" without reference to the MR environment. The MR environment is a term that refers to the general environment in the vicinity of a MRI scanner. Specifically, it describes the area within the 5 gauss line about the scanner to include the static magnetic field, rapidly changing magnetic fields, and radio frequency (RF) magnetic field pulses. At the time of this paper there are no specific symbols that indicate MR safe and MR compatible to be posted on medical devices.

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From observation of Table 1 above, the static magnetic field and spatial gradient field can induce torque and translational forces on an object. Should the object be a small piece of metal such as a shaving within the eye proper, there may be tearing of soft tissue structures. The MDR records an incident in which a patient died from the movement of a cranial implanted aneurysm clip attributed to the fringe field as the patient did not enter the magnet's bore. The magnitude of the magnetic force on an object is dependent on the object's mass and shape mass and scanner characteristics. Nevertheless, one can see why the contraindication list continues to grow as new MDR recordings also increase. To date there are numerous accounts of MR accidents that have resulted in injury and/or equipment damage, and the worse scenario, death of a patient. All such mishaps must be reported in the Medical Device Reports (MDR) under the Problem Reporting Program (PRP) systems as required by the Bureau of Radiological Devices. Some known conditions that are contraindications for MRI scanning are listed (table 2).

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Sometimes an object may appear to be non ferromagnetic; however, it should be remembered that nickel, cobalt, and several other elements are also ferromagnetic. Table 2 is a snapshot of what has been found to be attracted to MRI magnets. Many objects needed for medical use are being augmented using newer metallurgy techniques to make them compatible with MR magnetic fields. It should be noted that whether or not an object may be used in the static field must be compared to its manufacturers list noting the field strength limit of the tested object. It is recommended but not required that if a device is labeled MR safe, data demonstrating its testing in the MR environment should be provided. To comply with MR labeling the device must demonstrate its stability in the MR environment to poses no increased safety risk to the patient or personnel. To be labeled MR Compatible supporting data showing the device is MR safe, and it performs its intended function without degradation, neither does it diminish the image quality by artifact or noise. If it does cause minimal noise the value should be quantified, or signal to noise ratio stated. Bold statements such as "Intended for use during MR imaging" must be substantiated with supporting documentation including static magnetic field strength to which it was tested, and spatial gradient per unit distance. If the device can fit into the bore of the magnet it should have been tested there as well for validity.

Magnetometers or metal detectors are sometimes used as a component of the screening protocol. Unfortunately, these detectors are not 100% accurate and do give false-positives and false-negatives, especially in some cases where the metal is deeply embedded in body tissues. There are too many factors that can be associated with the accuracy of magnetometer findings, among them are the size and mass of the object, sensitivity of the apparatus, skill of the operator, proximity of the detector to the metallic object, and rate of motion during the detection technique. The only real practical application of a metal detector is to survey large objects such as a gurney, oxygen cylinders, I.V. pumps, etc that claim to be MRI compatible.

Two major concerns about potential implanted metal's relation to torque during MRI procedures have been isolated. These are the potential of metal implant in the eye(s) and/or in the brain. It is sufficiently documented in the MDR that small metal fragments could migrate in the presence of a strong magnetic field:

MDR-100222: A patient complained of double vision after an MR exam. The MR exam as well as an x-ray revealed the presence of metal near the patient's eye. The patient was sedated at the time of the exam and was not able to inform anyone of this condition. (12/15/1993).
MDR-10022: Dislodgement of an iron filing in a patient's eye during MR resulted in vision loss in that eye. (1/8/1992).
MDR-349790: A patient with an implanted intracranial aneurysm clip died as a result of an attempt to scan her. The clip reportedly shifted when exposed to the magnetic field. The staff apparently had obtained information indicating that the material in this clip could be scanned safely. (11/11/1992).
MDR-175218: A patient with an implanted cardiac pacemaker died during or shortly after an MR exam. The coroner determined that the death was due to the interruption of the pacemaker by the MR system.
MDR-351516: A patient with an implanted cardiac pacemaker died during an MR exam. (12/2/1992).

Intensive research confirms that conventional radiographs are an effective screening tool for any person suspected of having metal fragments in their eye proper, or in the brain. Several x-ray protocols are acceptable for radiographic interpretation prior to MR scanning:

  1. A single Water's view and lateral of facial bones, or
  2. Water's view and lateral with negative radiograph and positive radiograph if digital computerized radiography is available.
  3. Water's view eyes looking up, second Water's view eyes looking down (Patient's position is unchanged for both views), or
  4. Water's view eyes looking to the left, second Waters view eyes looking to the right (Patient's position is unchanged for both views).

Metallic fragments too small to be seen on conventional radiographs, measuring less than 0.1 x 0.1 x 0.1 do not pose a risk to the patient from MR scanning (Shellock, FG 1996). Although a CT scan could differentiate these small fragments there is no benefit to the patient to do so. If a patient is known to have an aneurysm clip the current standard is that they are to be denied a MR examination. The only exception to this rule is if the aneurysm clip is tested before implantation and documented by the radiologist and neurosurgeon in the patient's record stating its MR compatibility. Upholding standards requirements for MRI compatibility at any given institution is the responsibility of the radiologist. Any patient who has a potential metal incompatibility should be brought to the attention of the radiologist who will make the final decision regarding all scanning issues. In some cases having metallic implants such as prosthesis may not be affected by MR. Metallic implants are reviewed on a case by case basis for which the MR technologist is skilled at determining. Under no circumstance should a patient or any healthcare personnel be brought into the MR imaging suite without the permission, presence, and supervision of a MR technologist.




Section 6.3: Gradient Magnetic Field Safety

In addition to the static magnetic field the MRI scanner uses rapidly changing magnetic fields called magnetic field gradients, or magnetic time-varying fields. In terms of MR safety, these fields are concerning because they can be a source of magnetic induction of a current. Gradient fields are responsible for all spatial localization of MR images. Three paired orthogonal coils are located in the gantry to sequentially generate their magnetic fields onto the static field providing selective spatial excitation. These external magnetic fields (Gx, Gy, Gz) are designed to torque Bo into a predestinated gradient slope of spatial excitation.

According to Faraday's law of electromagnetic induction; a changing magnetic field may induce a current in a conductor. In the human body, nerve tissue, muscle tissue, blood and blood vessels can be conductors when influenced by a magnetic fields of strengths encountered in MRI. Gradient fields perform selectively excitation of protons in the patient. These fields are rapidly turned on and off during MR imaging and can induce voltage and current as the gradient inclines up and down. Gradient field strength is measured in G/cm. Standard MR Scanners operate gradient fields at strengths of 5 to 10 G/cm. Newer scanners with echo-planer systems operate at gradients of 25 G/cm or more. Typically the gradient field is at maximum strength with rise times of 0.5 to 1.0 ms to produce a field variance of 1.5 to 3.0 T/s at 10 cm from the center of the magnet. A rise in magnetic field strength of less than 200 ́ sec is more likely to induce a voltage/current than a rise of 600 ́ sec. The greater the magnitude of the rise and fall times, the greater the current induced. The potential for current induction is based on the strength of the magnetic field, its rise speed, and the resistance of the conductor tissues. Consequences of current induction in conductor tissues of the body include:

  • Stimulating muscle tissue to contract
  • Sensations of tingling
  • Sensations of itching
  • Cardiac muscle stimulation that may be seen as an arrhythmia
  • Magnetophosphenes (light flashes transmitted by optic nerve stimulation)

The FDA requires that the magnetic gradient field strength for routine scanners be limited to below 6.0 T/sec, which is far below what is required to produce nerve stimulation. Research scanners that can have gradient fields upwards to 10 T/sec but these are not available for routine use in healthcare settings.




Section 6.4: Radiofrequency Magnetic Field

The third type of magnetic field used in MR scanning is the radiofrequency magnetic field or Rf magnetic field. Returning to the electromagnetic spectrum we see that the radio frequency range is between 0 and 3,000 GHz. We are daily influenced by useful radiofrequencies such as VHF television, radar, UHF television, AM and FM radio, and recently microwave communications. The Rf fields used in MR imaging is very similar to microwave frequency and other energies in the Rf spectrum.

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Electromagnetic Spectrum Chart from: Berkeley Lab, Berkeley, CA with Permission

Very similar to microwave heating, Rf fields can induce an electrical current in the body which is transformed into energy in the form of heat. Heating of tissues is due to resistance in the tissues is called "ohmic heating." Unlike microwave heating MR induced heating is greatest at the surface than in the center of the tissue. Rf frequency heat is most efficiently absorbed at resonant frequency. Resonant frequency is that frequency at which maximum heat absorption in tissues occur and is not desirable for MR scanning. At resonant frequency there is danger of deep and uneven heat absorption patterns. Heating is the most important bioeffect of MR imaging. Humans loose heat by convection, conduction, and evaporation, which may not be effective heat release methods during MR imaging.

Rf heat is dissipated based on the condition of the patient's thermoregulatory system; therefore, the rate of Rf deposition is important. The specific absorption ratio (SAR) expresses the amount of Rf energy deposited in tissues, and is measured in watts/kilogram (W/kg). Current FDA limits for SAR:

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Research on the effects of Rf induced tissue heating has identified two tissues most at risk, these are the eyes and testicles. Both have poor dissipation of heat and can be adversely affected by heating. Also identified are certain medical states as well as some medications that can enhance the effects of Rf heating. The mechanisms of these effects are still unknown.

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Also of importance is the effect of Rf magnetic fields and gradient field on implants and metallic foreign bodies. Because current can be induced in metallic implants and will have no conducting path out of the patient's body, any induced current will be manifested in heat buildup. Metal shavings, shrapnel, and wires are the most common types of metallic bodies subject to heating. Magnetic induction can occur in any coil; therefore, wires touching the patient must not have coils in them. A current induced in a coil could cause burns. As a safety precaution, the technologist must insure no items are formed into a loop. These include cardiac gating leads, disconnected surface coils, or plethysmograph leads, and the like. Wires should be separated from the patient's skin by an insulating material.

Pacemakers are of special concern to MR imaging because they may respond inadvertently to magnetic force. A current could be induced in the wire leads of a pacemaker. Besides pacemakers, internal defibrillators and pacing catheters are all potentially contraindicated for MR imaging. If the patient is known to have a device that generates electrical signals (neurostimulator, baclophen pump, etc.,) they should not be scanned. Any device with moving parts and are ferromagnetic should not be scanned. The FDA recommends that any person having a device that generates an electrical current, accompanying someone who is to undergo a MR procedure come no closer to an MR unit than the 5-gauss line.




Section 6.5: Biological Effects

The magneto-hydrodynamic effect

An important biological effect that has been observed even at field strength as low as 0.1 Tesla is what is called the magneto-hydrodynamic effect. Blood, a conductive fluid, flowing through the heart when in a magnetic field is applied demonstrates a current which is seen as an elevation in the S-T segment on an EKG monitor. The phenomenon occurs in accordance with the induction law of physics that states: when a conductor moves through a magnetic field, or a magnetic field moves about a stationary conductor, a current is induced in the conductor (blood). This can be a benefit to specific types of MRI scans called gated studies. But because changes in the S-T segment can be an indicator of physiological problems such as a myocardial infarct, electrolyte imbalance, or possible ischemia, an EKG before a MRI procedure may be indicated for some patients, and pulse oximetry and cardiac monitoring during the MRI procedure. Special MR compatible devices must be on hand for patient monitoring especially if a history of cardiac poser is known. It should be understood that the changes on an EKG associated with the bore magnetic strength are temporary and do not have long term biological effects.




Section 6.6: Acoustic Noise

Pulsating gradients and discontinuous current induced in gradient coils generate ongoing loud noise in the bore of the magnet. Noise is a function of rising current in wires of gradient coils being opposed by the static magnetic field. Noise occurs as gradient coils vibrate on their mountings with rapidity of cycled gradient fields proportional to field strength. Measurements of noise levels inside the magnet and at the entrance to the magnet bore using gradient intensive techniques such as fast spin echo and fast GRASS indicate noise levels can be as high as 84 to 105 dB. A small slice selection, small FOV, and high image resolution will all contribute to an increase in noise levels. Data shows the nearly 45% of those scanned without any effort to diminish MR noise suffer temporary hearing loss. Foam rubber earplugs can reduce noise levels by nearly 20 dB and commercially available amplified music also helps reduce noise. There are newer noise cancellation systems that sample gradient noise and transmit a 180 degree out of phase noise to cancel up to 70% of gradient noise. Noise reduction is a concern during MR imaging.




Section 6.7: Pregnancy and MR

Pregnancy is a concern for both the pregnant worker and the pregnant patient undergoing MR imaging. Occupational workers are not normally exposed to gradient magnetic fields or Rf fields and therefore are not subject to their effects. A pregnant MR technologist may be exposed to several hundred or thousand gauss through chronic low level exposure to the static field and fringe field at the console. Current studies show that there is no measurable effect of exposure to the static magnetic field encountered by occupational work. As far as MR imaging of the pregnant patient there are no studies that show an adverse effect on the embryo/fetus from MR imaging. Pregnant patients should understand that the FDA has not established MR imaging during pregnancy is altogether safe. It is generally agreed that a referring physician along with the patient's obstetrician should decide non emergency scan need. Examination should be postponed until after the 1st trimester unless MRI will help in the diagnosis of a life threatening condition.

The occurrence of spontaneous abortions in humans is nearly 30% making it difficult to separate causes related to MR fields. To date none of the five areas of human research (spontaneous abortion rate, preterm delivery of less than 39 weeks, low birth weight, infertility, and gender of offspring) demonstrate deleterious effects from MR fields. Furthermore, menstrual regularity, bleeding changes, or cyclic changes were reported consistent with control findings in comparative studies. From these types of studies a conservative view has emerged to recommend a pregnant technologist not participate in interventional MR procedures that require their presence in the MR suite during gradient and Rf field use. There is no supporting data to mandate a policy preventing 1st trimester exposure.




Section 6.8: Cryogen and Quench Concerns

Some MR units employ superconductive-type magnet to produce their static magnetic field. Superconductivity requires cooling of static field producing coils to temperatures that conduct electricity without resistance to current flow. This is done using liquid nitrogen, liquid helium, or both. Helium exists in a liquid state at below -269oC (4.17oK) and above this temperature it becomes gaseous. In the liquid state these gasses are tremendously compressed; should heating within the cryostat system occur these liquids could be revert to their gas state. The liquid/gas ratio for helium is 1:760 and 1: 685 for nitrogen meaning these liquids in a gaseous state will expand. One liter of liquid helium will become 760 liters of vapor, and one liter of Nliquid will become 685 liters of Ngas. As these liquids are converted into gas they expand greatly under tremendous pressure. As the pressure builds up in the cryostat system a valve called a burst disk will open with a loud popping sound to release these gasses into a conduit called a stack leading out of the MR suite. The process of releasing these gasses is called a quench. As these gasses escape they produce a loud whistling roar like a giant teapot releasing steam. MR suites are design to release these gasses to the outside environment where they are mixed with the outside atmosphere: however, sometimes there is a rapid escape of gasses during quenching into the MR suite itself. This can be a serious risk to the patient undergoing a MR examination.

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During a quench gasses are released through the stack (white arrow) to the outside of the MR suite.

Should gaseous nitrogen or helium expand into the MR suite they may displace the volume that was room air. Some magnets can hold up to 800 liters of liquid helium that could expand to a volume of 608,000 cubic liters of gas. Depending on the size of the MR suite it could contain less than 10,000 cubic liters of room air. Room air is approximately 21% oxygen and 14% nitrogen. Helium only cryostat systems will release helium which is lighter than air into the room leaving some oxygen beneath it available to the patient. Nitrogen on the other hand is heavier than room air and will occupy the lower portion of the room during a quench until it expands to fill the entire room. Even though these gasses are released as a vapor their temperature is very cold and the patient will be at risk for hypothermia and cold burns. Therefore it is imperative that the patient be evacuated from the room immediately whenever a quench is activated.

As helium and/or nitrogen expands displacing room air the oxygen content of the room being reduced does pose specific risk to the patient and anyone in the surrounding area. These concerns warrants evacuation of the MR suite, console area, and any waiting or work areas that can be affected by the quench. The volume of air that may displace poses not only thermo risks, but also there is suffocation risk as well. Consider the chart below which characterizes possible consequences of the quenching process should the stack conduit breach and helium or nitrogen gas be released into the MR suite.

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Section 6.9: Patient History and Preparation

One of the most important roles of the MR technologist is to obtain an accurate and complete medical history for compatibility to scan. All parameters of the scanning profile should be cleared prior to scanning. Because the radiologist is unable to screen every patient they must rely on the technologist to provide historical and clinical information. Pertinent medical data should be outlined on forms which the patient completes prior to scanning, and the technologist reviews with the patient for accuracy. Specific histories concerning diabetes, renal disease, asthma, and other concerns prior to intravenous contrast injection should also be included. Specific contraindicated metallic and electrically stimulated devices should be removed or brought to the attention of the radiologist.




Summary Points!

  • The static magnetic field is the main magnetic field which is used to align protons in the patient's body with the magnetic field. This prepares them for imaging; the static magnetic field strength is measured in Tesla (T).
  • The static magnetic field is always on and poses a risk at all times to the uneducated person entering its influence.
  • The fringe field is an extension of the static field having a gradient strength from the isocenter of the magnet outward creating the acceleration potential for ferromagnetic objects.
  • The fringe field must not exceed 5G at the entrance to the MRI examination suite.
  • Magnetic field strength is regulated by the FDA and is currently limited to below 4.0 T for clinical use.
  • Magnetic strength is inversely proportional to the square of the distance from isocenter.
  • MRI risk is not limited to metallic flight only, but also includes biological and thermo effects as well.
  • The static magnetic field drops off as distance from isocenter increases. The field extending from the bore to the 5 gauss line is called the fringe field and is measured in gauss.
  • One Tesla is equal to 10,000 Gauss.
  • MR safe means the device or object poses no risk to the patient; however, it may affect the quality of diagnostic images acquired by MRI.
  • MR compatible means the device or object poses no risk to the patient nor does it degrade image quality of MR acquired images.
  • MR environment is a term that includes the static magnetic field and its fringe field, gradient magnetic fields, and Rf magnetic fields.
  • The static magnetic field is responsible for rotation and torque on objects and is always on!
  • The static magnetic field and its fringe field cause translational force that can accelerate an object projectile to missile-like speed.
  • MRI accidents are reported in the Medical Device Reports (MDR) under the Problem Reporting Program (PRP) required by the Bureau of Radiological Devices.
  • Two areas of major concern for torque in biological tissue is in the eye and brain.
  • The standard views for demonstrating metallic fragment(s) in the eye is the Water's view and lateral or two Waters with the eyes looking in opposite directions.
  • Metallic fragments measuring less than 0.1 x 0.1 x 0.1 cannot be seen with conventional plain films and are considered insignificant to MRI.
  • Gradient fields (Gx, Gy, Gz) perform selective excitation of protons and are responsible for all spatial localization of MR images.
  • A rise in the gradient magnetic field strength of less than 200 m sec will induce a greater current than a voltage current rise of 600 m sec.
  • Current induction in conductive tissues by the gradient magnetic field may cause muscle stimulation with contractions, tingling, itching, arrhythmia, or magnetophosphenes (light flashes due to optic nerve stimulation).
  • FDA limits magnetic gradient field strength to less than 6.0 T/sec.
  • The most important biological effect of MR scanning is "ohmic" heating of tissues by the radiofrequency (Rf) field.
  • Resonant frequency is that frequency at which maximum heat is absorbed in tissue, it is an undesirable effect during MR imaging.
  • The specific absorption ratio (SAR) is the amount of Rf energy deposited in tissues and is measured in watts/kilogram (W/kg).
  • FDA limits SAR to 0.4 W/kg whole body exposures, 8.0 W/kg peak in any 1 gram of tissue, and 3.2 W/kg average to the head.
  • The two tissues that are most sensitive to heating during MR imaging are the eyes and testicles.
  • Health conditions that may enhance poor heat dissipation during MR imaging are: cardiovascular disease, Hypertension, diabetes, fever, increased age, and obesity.
  • Medications that can enhance poor heat dissipation due to Rf deposition in tissues include some diuretics, B blockers, calcium channel blockers, amphetamines, muscle relaxers and sedatives.
  • To decrease the potential for current induction by magnetic fields during MR imaging, no wires should be formed into coils or allowed to touch the patient's skin.
  • Pacemakers, neurostimulators, and devices requiring electrical signals or having moving parts are allowed beyond the 5 gauss line.
  • The magneto-hydrodynamic effect is a term for current induced in blood as it flows through the heart manifested as a rise in the S-T segment on EKG.
  • Acoustic noise associated with MR imaging is caused by gradient coils magnetic fields opposing the static field. It can be reduced by approximately 20 dB with ear plugs or up to 70% by commercial noise cancellation systems.
  • The liquid/gas ratio for helium, a liquid commonly used to cool superconductive magnets used in MR, is 1:760, and 1:685 for nitrogen meaning these liquids will expand if converted to vapor.
  • If liquid helium or nitrogen becomes heated and greatly expands, as a safety measure the burst disk will open releasing gas into a conduit stack to the outside of the MR suite. This process is called quenching.

References

  • Shellock, FG, Kanal E. Magnetic Resonance: Bioeffects, Safety, and Patient Management 2nd Edition, New York: Lippincott-Raven Press, 1996.
  • Woodward, P., Freimarck, R., MRI For Technologists, New York: McGraw-Hill, Inc., 1995.
  • Bushong, S.C., Radiologic Science for Technologist: Physics, Biology, and Protection, 7th ed., St. Louis, Missouri: Mosby, Inc. 2001.
  • Seeram, E., Computed Tomography, Philadelphia, Pennsylvania, W.B. Saunders Co. **1994.
  • Electromagnetic chart, Berkeley Lab, Berkeley, CA. 94720. http://www.Ibl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html



Copyright image Copyright 2006 Nicholas Joseph Jr.






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