Basic Principles of Magnetic Resonance Image Production - Term Paper Example

MRI: Safety and Hazards Introduction Unlike CT imaging and X-ray imaging, though Magnetic Resonance Imaging or MRI was considered to be a safe imaging technique due to absence of exposure to radiation, many experts have questioned the safety of electromagnetic fields or EMF, following reports from the World Health Organisation Task Force on static EMFs and European Physical Agents Directive on Occupational Exposures to EMFs. The concern increases as the number of MRI Scan units that are installed have increased, the range of clinical applications for MRI have widened and higher main magnetic field strengths are being used (De Wilde et al, 2007). In this essay, safety issues and hazards of MRI will be discussed and methods for safe imaging will be explored through review of suitable literature. MRI technology MRI is a "way of obtaining very detailed images of organs and tissues throughout the body without the need for x-rays or "ionizing" radiation. Instead, MRI uses a powerful magnetic field, radio waves, rapidly changing magnetic fields, and a computer to create images that show whether or not there is an injury, disease process, or abnormal condition present" (Patient-Safety MRI, 2010). MRI has turned out to be an excellent investigation tool because of it ability to provide good contrast between various tissues in a particular region. For example, in the brain, a good contrast is provided between white matter, gray matter and cerebrospinal fluid (Diwadkar and Keshavan, 2002). The technology of MRI mainly employs 3 components for imaging and they are pulsed radio-frequency fields or RF, high static magnetic field and time-varying gradient electromagnetic fields or EMF (Roboman et al, 2006). For the purpose of MR imaging, the patient is placed in a "large, tunnel or doughnut-shaped device that is open at both ends" (Patient-Safety MRI, 2010) (Refer Figure-1). The examination causes no pain or tissue injury, although loud noises are heard during the procedure. There is also a risk of mechanical injury due to ferromagnetic objects kept in the scanning room unintentionally. The main hazards concerned with magnetic fields are interactions with the equipment and interactions with human tissue. As far as human tissue interactions are concerned, the most worrisome interactions are with blood flow, ears and cardiac cycle (De Wilde et al, 2007). Figure-1. MRI Scanner (Source: sporttalk.com.au ) Safety issues related to MRI scanning 1. Exposure to static magnetic field The main safety issues which are of concern on exposure to high static magnetic field are are both mechanical and biological effects and their consequences. In clinical MRI scanning, the strength of magnets used range between 0.2- 3.0 T (De Wilde et al, 2007). In most of the MRI designs, superconducting magnets are used to achieve such field strengths. In some centers however, resistive magnets or permanent magnets may be used. Most of the MRI systems use a cylindrical magnet which generates horizontal magnetic field. The static field in these systems is parallel to patients long axis. In systems which use a transverse magnet for generation of field, the static magnetic field is along the axis of the patient. Whatever is the type of magnet used, the magnetic field that is experienced by the patient is limited to the magnets operating field strength. What matters in the range of field strength is both concerns over patient exposure and occupation exposure (De Wilde et al, 2007). a. Effect of electro-magnetic fields on tissues of human beings Whether static magnetic fields used in clinical dosage limits affect human tissues is a much debated topic. However, researchers have begun to look into the hazards of magnetic field on human health following a draft from the European Union directive, according to which, the limit of static magnetic field that must be used for MRI scanning must be 2T. This limit however, is not practical and such a limit prevents wide application of clinical MRI. A major study was conducted by Schenck et al (cited in De Wilde et al, 2007) pertaining to the safety limits of static magnetic field and the measurable effects of MRI. According to these researchers, the hazards of static magnetic field on human tissues are mainly due to movement within the range of static field. "If a charge is moving in a magnetic field it will experience a force that is perpendicular to its direction of velocity and the magnetic field, this is the Lorentz Force" (De Wilde et al, 2007). All living human beings have movement in them, because of respiration, blood fluid, body fluid flow and cardiac motion. These movements induce a certain degree of motion, due to Lorentz effect. However, significant effects are noticeable only beyond 4T, which is the upper limit of clinical magnetic MRI strength. Clinically, when a person is exposed to strengths beyond 4T, he/she experiences vertigo, metallic taste or nausea (Schenk et al, cited in De Wilde et al, 2007). In a study by Chakeres et al (cited in De Wilde et al, 2007), exposure to static magnetic field up to 8T lead to no changes on heart rate, finger pulse oxygenation levels, respiratory rate, core body temperature or diastolic blood pressure. However, a small rise of up to 3.6mmHg was noticed in systolic blood pressure. In yet another study by Saunders et al (cited in De Wilde et al, 2007), the authors opined that no significant human effects were noticeable in ranges upto 4T. Also, the mild effects which were noticed above 4T were "induction of flow potentials around the heart and the development of aversive/avoidance behaviour resulting from body movement in such fields (De Wilde et al, 2007). The study concluded that effects of static fields on human beings in ranges below 4T are insignificant. However, several other researchers have reported the possibility of induction of physiological changes in human body. There are some reports that exposure of human brains to fields even up to 8T does not cause any cognitive problems, but some disturbances with regard to visual contrast sensitivity and hand coordination have been reported. b. Mechanical effects due to static magnetic field While the effect on biological tissues by static magnetic field in ranges applicable for clinical imaging purposes is a much debated topic and no genuine evidence has been found to ascertain the same, mechanical effects due to static magnetic field are well established and warrants precautions and application of proper security and technology. More often than not, incidents due to magnetic field are as a result of presence of various devices and equipment made up of ferromagnetic substances in the environment of magnetic imaging (Roboman et al, 2006). In the presence of magnetic field, these objects become projectiles. Any ferromagnetic substance can be subjected to rotational and attractive forces from the magnetic field. Some incidents which have been reported are injury to the patients due to oxygen cylinders and monitoring devices, malfunction or failure of medical devices and medical equipment like infusion pumps, neurostimulators, cardiac pacemakers, cochlear implants, ocular prosthesis and ventilators (De Wilde et al, 2007). Mechanical effects not only cause injury to patients and staff, they also cause damage to the imaging room equipment which are very expensive. According to a study by Risi et al (2004), cochlear implants are not safe during MRI scanning and they need to be removed for safe imaging. The risk of adverse incidents has increased in recent days because of increase in the number of instruments and devices used in MR rooms for diverse clinical purposes which require anesthesia, sleep sedation, etc, demanding more medical personnel inside the MR room. The more the equipment inside the room and more the number of personnel inside, the increase is the risk. "The attractive force on a ferromagnetic object is proportional to the spatial gradient of the magnetic field and, the gradient is normally steeper for higher field systems due to the combination of shielding and main field strength" (De Wilde et al, 2007). For materials which are paramagnetic or diamagnetic, "the attractive force is proportional to the field strength" (De Wilde et al, 2007) and thus materials like devices which are safe at 1.5T intensity may not be safe at higher intensities like 3T. Metallic hardware concerned to orthopedics do not have appropriate ferromagnetic properties and hence are safe for an MRI setting. However, these objects may cause artifacts. The effect on imaging quality depends on the type of metal used in the implant. Titanium affects image quality less than steel. Zirconium is much better than titanium (Harris and White, 2006). To prevent injury from static magnetic field, access to scan room must be restricted and the entrance doors to the magnet room must be closed all the time. Clearly visible warning signs must be present on the doors indicating presence of magnetic field inside the room (Figure-2). The MRI operator must screen the subjects before entry into the scan room. Screening must be done using a form. patient must be asked to remain still during scanning to prevent dizziness and metallic taste. Figure-2: Warning signs outside MRI room (Source: tradekorea.com ). 2. Acoustic noise MRI scanning is associated with some heavy noise because of "the mechanical movement of the gradient coils during the scanning process". The noise is however below concern levels. If the noise levels are above 99dBA, hearing protection must be offered (DDDNIC, 2006). 3. Peripheral nerve stimulation Some time-varying magnetic fields can cause peripheral nerve stimulation and cause tingling or twitching. However, these effects are minimal and harmless (DDDNIC, 2006). 4. Tissue heating Mild tissue heating may occur during scanning. to prevent dangerous levels of tissue heating, coils and other equipment must be checked regularly for optimum functioning (DDDNIC, 2006). 5. Other hazards Electric hazards can occur and a qualified engineer must evaluate the scanner regularly and safety tests be conducted on time-to-time basis. Infections can be carried from one patient to another and proper steps must be taken to prevent transfer of infections. Children may need to be sedated and this can contribute to the risk (Carter et al, 2006). Maintaining safety in MRI scanning The MRI Unit must be run by a team of experts. The team must include MRI physics manager who has the responsibility to keep the environment of the MRI center safe. He must issue protocols for safe imaging practices and must perform safety checks and safety tasks. MRI procedures must be performed only by a qualified MRI operator. All the personnel concerned with MRI unit must undergo safety training which includes action in times of medical emergencies, fire emergencies and quench. In case of medical emergencies, no emergency procedures must be performed in the MRI room and no medical equipment can be allowed into the magnet room. In case of fire accidents, the subject must be removed immediately and the doors be closed to contain fire. Only non-ferrous fire extinguisher must be used to extinguish fire. Quench is a condition when "the magnet loses its super-conductivity and the magnetic field ramps down in a matter of seconds - typically at a rate of approximately 20 seconds" (DDDNIC, 2006). Since this causes the magnet to warm up, boiling of liquid helium occurs. Gaseous helium is a health hazard and can cause unconsciousness, cryogenic burns and hypothermia. Appropriate steps must be taken for safe release of helium gases, as per unit protocols. All scanners must be provided with emergency "off" systems for action during patient emergency. This button will not cause quench. An emergency magnet run down button will be placed on the "left wall adjacent to door" (DDDNIC, 2006) This button must be pressed only when human life is at risk because it causes quenching. In case of power failure during scanning, the patient must be removed manually and then only must resort to shutting down of computers (DDDNIC, 2006). Figure- 3. Ideal Scan Room (Source: innovativemedsolutions.net ) Figure-4: Scan Room (Source-www-dsv.cea.fr) Figure- 5: MRI Scan Room Plan (Source- limpeter-mriblog.blogspot.com). Conclusion Though MRI scanning is a non-radiation procedure, it is associated with several risks. Though the much debated biological tissue damage risk appears to be insignificant in the current magnetic intensity doses, other hazards due to projectile accidents subsequent to magnetic field attraction of ferromagnetic material, quenching, and other such hazards merit importance and steps must be taken to ensure prevention of these hazards. References Carter, D., Murray, D., and Thomas, K. (2006). Safety in Paediatric Imaging. The Canadian Journal of Medical Radiation Technology, 33-42. Dana and David Dornsife Cognitive Neuroscience Imaging Center. or DDDNIC (2006). Safety Manual. University of Southern California. DE Wilde, J.P., Grainger, D., Price, D.L., and Renaud, C. (2007). Magnetic resonance imaging safety issues including an analysis of recorded incidents within the UK. Progress in Nuclear Magnetic Resonance Spectroscopy, 51, 37–48. Diwadkar, V.A., and Keshavan, M.S. (2002). Newer techniques in magnetic resonance imaging and their potential for neuropsychiatric research. 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