MRI Safety

Transcription

1 MRI Safety 1

2 Significant Risk Magnetic Resonance Diagnostic Devices You should consider the following operating conditions when assessing whether a study may be considered significant risk: main static magnetic field specific absorption rate (SAR) gradient fields rate of change sound pressure level (1) 2

3 Main Static Magnetic Field Population Main static magneticfield greater than (tesla) adults, children, and infants aged > 1 month 8 neonates i.e., infants aged 1 month or less 4 (1) Specific Absorption Rate (SAR) Site Dose Time (min) SAR equal to or greater than: (W/kg) whole body averaged over 15 4 head averaged over 10 3 head or torso per gram of tissue 5 8 Extremities per gram of tissue 5 12 (1) 3

4 Gradient Fields Rate of Change Any time rate of change of gradient fields (db/dt) sufficient to produce severe discomfort or painful nerve stimulation (1) Sound Pressure Level Peak unweighted sound pressure level greater than 140 db. A weighted root mean square (rms) sound pressure level greater than 99 dba with hearing protection in place. (1) 4

5 Deflection angle testing conducted on metallic surgicalinstruments. The deflection angle was 0 degrees for the mallet (top), whereas the deflection angle was 7 degrees for the Kocher Langenbeck retractor (bottom). (2) Summary of Test Results for Deflection Angles and Torque Measurements for the Metallic Surgical Instruments Device Deflection angle Torque (degrees) Mallet 0 0 Bone punch 7 11 Currette 0 0 Weil Blakesley forceps 7 11 Suction cannula 0 0 Septum speculum 4 11 Kocher Langenbeck retracto 4 11 The following qualitative scale of torque was applied to the results: 0, no torque; 11, mild torque: the device slightly changed orientation but did not align to the magnetic field; 12, moderate torque: the device aligned gradually to the magnetic field; 13, strong torque: the device showed rapid and forceful alignment to the magnetic field; 14, very strong torque: the device showed very rapid and very forceful alignment to the magnetic field (8 10). Metallic surgical instruments evaluated for MR safety. Top row: mallet, bone punch, and curette. Bottom row: Weil Blakesley ethmoid forceps, suction cannula, septum speculum, and Kocher Langenbeck retractor. (2) 5

6 Heating Associated with MRI for the Surgical Instruments Device Septum speculum Kocher Langenbeck Retractor Highest temperature change ( C), temperature probe and position 0.7 C (probe 1A): one end of the instrument, near (0.5 mm) the tip 0.6 C (probe 1B): middle part of the instrument 0.6 C (probe 1C): other end of the instrument, near (0.5 mm) the end 0.5 C (probe 1D): reference temperature 0.8 C (probe 1A): one end of the instrument, t near (0.5 mm) the tip 0.7 C (probe 1B): middle part of the instrument 0.7 C (probe 1C): other end of the instrument, near (0.5 mm) the end 0.5 C (probe 1D): reference temperature (2) Summary of Tests Performed to Assess MRI Artifacts for the Surgical Instruments Surgical Instruments Pulse sequence signal void size,(mm2) T1 SE T1 SE GRE GRE Mallet 18,594 5,548 22,583 16,630 Bone punch 29, ,926 2,813 Currette 6, ,474 2,262 Weil Blakesley forceps 14,390 1,239 19,389 2,612 Suction cannula 14, , Septum speculum 10,945 1,172 17,596 4,338 Kocher Langenbeckretractor 14,560 1,777 23,430 5, Tesla MR system and a send receive body coil. a) T1 weighted spin echo pulse sequence;repetition time, 500 msec; echo time, 20 msec; matrix size, 256 x 256; section thickness, 5 mm; number of sections, 10; section gap, 0 mm; field of view, 36 cm; number of excitations, two; bandwidth; 16 khz; b) gradient echo pulse sequence; repetition time, 100 msec; echo time, 15 msec; flip angle, 30 degrees; matrix size, 256 x 256; section thickness, 5 mm; number of sections, 10; section gap, 0 mm; field of view, 36 cm;number of excitations, two; bandwidth, 16 khz The imaging planes were oriented to encompass the long axis and short axis of the surgical instruments. The frequency encoding direction was parallel to the plane of imaging in each case. (2) 6

7 Cardiac pacemakers, ICDs, and an insertable loop recorder were evaluated for translational attraction using deflection angle tests performed at the points of the highestspatialgradientsfor spatial gradients long bore andshort bore 1.5 and 3.0 Tesla MR systems according to ASTM guidelines. In general, deflection angles for these cardiovascular implants were significantly (p, 0.01) higher on 1.5 and 3.0 Tesla short bore compared to long bore MR systems. Several of the cardiovascular implants that underwent evaluation may be problematic for patients undergoing MR procedures using 1.5 and 3.0 T MR systems because of possible risks associated with magnet related movements. Additional potential MR safety hazards (e.g., MR related heating of leads, inhibition or modification of the function of the device, electromagnetic interference) should be taken into consideration for all of the cardiovascular implants that underwent evaluation. JOURNAL OF CARDIOVASCULAR MAGNETIC RESONANCEVol. 5, No. 2, pp , 2003 (3) 7

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14 Streak artefact of aneurysm clip, x ray CT 14

15 Guidelines Regarding Aneurysm Clips and the MR Environment 1. Specific information (ie, manufacturer, type or model, material, lot and serial numbers) regarding the aneurysm clip must be known, especially with respect to the material used to make the aneurysm clip, so that only patients or other persons with nonferromagnetic or weakly ferromagnetic clips are allowed into the MR environment. The manufacturer provides this information in the labeling of everyaneurysm clip. The implanting surgeon is responsible for properly communicating this information in the patient s records. 2. An aneurysm clip that is in its original package and made from Phynox, Elgiloy, MP35N, titanium alloy, commercially pure titanium, or other material known to be nonferromagnetic or weakly ferromagnetic does not need to be evaluated for ferromagnetism. Aneurysm clips made from nonferromagnetic or weakly ferromagnetic materials in original packages do not require testing of ferromagnetism because the manufacturers ensure the pertinent MR safety aspects of these clips and, therefore, should be held responsible for the accuracy of the labeling. 3. If the aneurysm clip is not in its original package and properly labeled, it should undergo testing formagnetic field interactions. 4. The radiologist and implanting surgeon should be responsible for evaluating the available information pertaining to the aneurysm clip, verifying its accuracy, obtaining written documentation and deciding to perform the MR procedure after considering the risks versus the benefits of the examination. AJNR Am J Neuroradiol 24: , March 2003 Sagittal MR image shows a magnetic susceptibility artifact that resulted from the presence of metallic dental fillings. 15

16 (4) MRI CAUSED BURNS L loop R loop U C skin C air Cskin R skin R skin Radiology 1996; 200:

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18 The following patient management guidelines are recommended: 1. The Pre MR Procedure Screening Form should includea question pertaining to the presence of permanentcosmetics or decorative tattoos 2. Before undergoing an MR imaging examination,the patient should be asked if he or she has apermanent coloring technique (i.e., tattooing) applied to any part of the body. This includes cosmetic applications such as eyeliner, lip liner, lip coloring, and decorative designs. 3. The patient should be informed of the relativelyminor risk associated with the site of the tattoo. 4. The patient should be advised to immediately inform the MR technologist regarding any unusual sensation felt at the site of the tattoo in associationwith the MR imaging procedure. 5. Thepatientshould be closely monitored usingvisualandauditorymeans auditory throughout the entire operation of the MR system to ensure safety. 6. As a precautionary measure, a cold compress (e.g.,wet wash cloth) may be applied to the tattoo siteduring the MR imaging procedure. JOURNAL OF MAGNETIC RESONANCE IMAGING 15: (2002) 18

19 To prevent patients from experiencing excessive heating and possible burns in association with MR procedures, the following guidelines are recommended: (1) Prepare the patient for the MR procedure by ensuring that there are no unnecessary metallic objects contacting the patient s skin (e.g., metallic drug delivery patches, jewelry, necklaces, bracelets, key chains, etc.). (2) Prepare the patient for the MR procedure by using insulation material (i.e., appropriate padding) to prevent skin to skin contact points and the formation of closed loops from touching body parts. (3) Insulating material (minimum recommended thickness, 1 cm) should be placed between the patient s skin and transmit RF coil that is used for the MR procedure (alternatively, the RF coil itself should be padded). For example, position the patient so that there is no direct contact between the patient's skin and the body RF coil of the MR system. This may be accomplished by having the patient place his/her arms over his/her head or by using elbow pads or foam padding between the patient's tissue and the body RF coil of the MR system. This is especially important for those MR examinations that use the body coil or other large RF coils for transmission of RF energy. (4) Use only electrically conductive devices, equipment, accessories (e.g., ECG leads, electrodes, etc.), and materials that have been thoroughly tested and determined to be safe and compatible for MR procedures. (5) Carefully follow specific MR safety criteria and recommendations for implants made from electricallyconductive materials (e.g., bone fusion stimulators, neurostimulation systems, etc.). (6) Before eoeusing eec electrical ca equipment, e check the integrity of the insulation and/or housing of all components including surface RF coils, monitoring leads, cables, and wires. Preventive maintenance should be practiced routinely for such equipment. (7) Remove all non essential electrically conductive materials from the MR system (i.e., unused surface RF coils, ECG leads, cables, wires, etc.). (8) Keep electrically conductive materials that must remain in the MR system from directly contacting the patient by placing thermal and/or electrical insulation between the conductive material and the patient. (9) Keep electrically conductive materials that must remain within the body RF coil or other transmit RF coil of the MR system from forming conductive loops. Note: The patient's tissue is conductive and, therefore, may be involved in the formation of a conductive loop, which can be circular, U shaped, or S shaped. (10) Position electrically conductive materials to prevent "cross points". For example, a cross point is the point where a cable crosses another cable, where a cable loops across itself, or where a cable touches either the patient or sides of the transmit RF coil more than once. Notably, even the close proximity of conductive materials with each other should be avoided because some cables and RF coils can capacitively couple (without any contact or crossover) when placed close together. (11) Position electrically conductive materials to exit down the center of the MR system (i.e., not along the side of the MR system or close to the body RF coil or other transmit RF coil). (12) Do not position electrically conductive materials across an external metallic prosthesis (e.g., external lfixation device, cervical lfixation device, etc.) or similar il device that t is in direct contact twith the patient. (13) Allow only properly trained individuals to operate devices (e.g., monitoring equipment) in the MR environment. (14) Follow all manufacturer instructions for the proper operation and maintenance of physiologic monitoring or other similar electronic equipment intended for use during MR procedures. (15) Electrical devices that do not appear to be operating properly during the MR procedure should be removed from the patient immediately. (16) Closely monitor the patient during the MR procedure. If the patient reports sensations of heating or other unusual sensation, discontinue the MR procedure immediately and perform a thorough assessment of the situation. (17) RF surface coil decoupling failures can cause localized RF power deposition levels to reach excessive levels. The MR system operator will recognize such a failure as a set of concentric semicircles in the tissue on the associated MR image or as an unusual amount of image non uniformity related to the position of the RF coil. IMRSER Page (1 of 4) [ :49:42] 19

20 (4) Kenesaw explosion after quench 20

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22 References 1. Guidance for Industry and FDA Staff Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices Document issued on: July 14, F. G. Shellock: Metallic Surgical Instruments for Interventional MRI Procedures: Evaluation of MR Safety; JOURNAL OF MAGNETIC RESONANCE IMAGING 13: (2001) 3. D.J. Schaefer: Review of Patient Safety in Time Varying Gradient Fields; JOURNAL OF MAGNETIC RESONANCE IMAGING 12:20 29 (2000) 4. R.R. Price: MR Imaging Safety Considerations; Radiographics 1999;