Halo vests and cervical fixation devices may be constructed from a combination of metallic components and other materials. Although some commercially available halo vests and cervical fixation devices are composed entirely of nonferromagnetic metals, there may be hazards due to excessive MRI-related heating that can be generated because of the conductive nature of the metallic components. Additionally, there is a potential for the patient's tissue to be involved in part of a current loop, such that excessive heating and burn injury may occur. The induced current within a ring or conductive loop is of additional concern because eddy current induction can cause image degradation. Adjusting the phase encoding direction of the pulse sequence so that it is parallel to the axis of the halo vest may reduce artifacts associated with eddy currents during MR imaging.
Serious injuries have occurred in patients with halo vests and cervical fixation devices that have undergone MR procedures. For example, Kim et al. (2003) described a case of a patient that underwent MRI at 1.5-Tesla wearing a titanium halo vest system (the name, model and manufacturer of this device were not provided). After the MR procedure, substantial burns were evident on the scalp of the right two posterior pins. According to the report, "the stoic patient had clearly perceived significant burning pain during MR imaging, but did not notify the technicians."
Fortunately, some halo vests and cervical fixation devices have been designed to specifically be acceptable for patients undergoing MR procedures at 1.5-Tesla and 3-Tesla (see The List).
Vibration Effects
A study was conducted to evaluate the possible heating of halo vests and cervical fixation devices during MR imaging performed at 1.5-Tesla MR system using a variety of pulse sequences. The data indicated that no substantial heating was detected for the specific devices tested. Of interest is that there appeared to be subtle motions of the halo ring associated with a magnetization transfer contrast (MTC) pulse sequence, as shown by recordings obtained using a motion sensitive, laser-Doppler flow monitor.
Apparently, the imaging parameters used for the MTC pulse sequence and other MR imaging techniques can produce sufficient vibration of the “halo” ring to create the sensation of heating. Accordingly, patients may interpret rapid movements or vibrations as a “heating” sensation. This is likely to occur when the vibration frequency is at a level that stimulates nerve receptors located in the subcutaneous region, which detect sensations of pain and temperature changes.
The aforementioned is a hypothesis based on the available experimental data and requires further investigation to substantiate this theory. However, additional support for this premise comes from a report by Hartwell and Shellock. In this case, a halo ring and vest (removed from a patient who complained of severe "burning" in a front skull pin during MR imaging) was evaluated for heating and other potential problems associated with MR imaging. This work was performed in conjunction with the neurosurgeon who applied the device to the patient. The halo ring and vest were connected similar to the manner used on the patient. A fluid-filled Plexiglas phantom was placed within the vest. The device was then placed in a 1.5-Tesla MR system and imaging was performed using the same parameters that were associated with the "burning" sensation experienced by the patient. The neurosurgeon remained inside of the MR system to visually observe the cervical fixation device and to maintain physical contact (i.e., touching the skull pins and other components) with it during the MRI procedure.
No perceivable temperature change was noted for the metallic components during MR imaging. However, the metallic components of the cervical fixation device (e.g., halo ring, vertical supports, vest bolts, etc.) vibrated substantially during MR imaging. Furthermore, when the skull pins were held firmly during MR imaging, there was a "drilling" sensation, which could be interpreted as a "burning" effect. Nevertheless, the skull pins remained cool to the touch throughout the MR procedure. Recent work conducted at 3-Tesla likewise supports the fact that substantial vibration occurs in cervical fixation devices (Unpublished observations, F.G. Shellock, 2008).
Graf et al. reported that torque acting on metallic implants or instruments due to eddy-current induction in associated with MR imaging can be considerable. Larger implants (such as fixation devices) made from well-conducting materials are especially affected. Gradient switching was shown to produce fast alternating torque. Significant vibrations at off-center positions of the metal parts may explain why some patients with metallic implants sometimes report feeling sensations of heating during MR examinations.
In consideration of the above, it is inadvisable to permit patients with certain cervical fixation devices to undergo MR procedures using an MTC pulse sequence until this problem can be further characterized to avoid unwanted patient responses, regardless of the lack of safety concern related to excessive heating. Other comparable pulse sequences should likewise be avoided when performing MR imaging of patients with halo vests and cervical fixation devices until the precise cause of this problem is determined and fully characterized. Additionally, all instructions for use and patient application information provided by halo vest and cervical fixation device manufacturers should be carefully followed.
MRI Information at 3-Tesla and Cervical Fixation Devices
To date, there are now several cervical fixation devices that have undergone MRI testing at 3-Tesla. MRI information for these devices are, as follows:
LiL Angel Pediatric Halo System with Jerome Halo Vest and Resolve Glass-composite Halo Ring and Resolve Ceramic-tipped Skull Pins
LiL Angel Pediatric Halo System with Resolve Halo Vest and Resolve Glass-composite Halo Ring and Resolve Ceramic-tipped Skull Pins
Resolve Halo System with Jerome Halo Vest and Resolve Glass-composite Halo Ring and Resolve Ceramic-tipped Skull Pins
Resolve Halo System with Resolve Halo Vest and Resolve Glass-composite Halo Ring and Resolve Ceramic-tipped Skull Pins
(Each above product from OSSUR Reykjavik, Iceland and OSSUR AMERICAS, Aliso Viejo, CA)
Non-clinical testing has demonstrated that each product indicated above used as a cervical fixation or cervical traction device or system is MR Conditional. A patient with this system may undergo MRI safely under the following conditions:
-Static magnetic field of 3-Tesla or less
-Maximum spatial gradient magnetic field of 720 Gauss/cm or less
-Maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of scanning (per pulse sequence)
In non-clinical testing, this system produced maximum temperature changes of 0.6 degrees C or less at a maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of MRI using a 3-Tesla MR system (Excite, Software G3.0-052B, General Electric Healthcare, Milwaukee, WI).
MR image quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the Skull Pins. Therefore, it may be necessary to optimize MR Imaging parameters for the presence of these metallic objects.
Important Note: All of the materials used for the above products included metals with very low magnetic susceptibility (e.g., commercially pure titanium, Titanium alloy, Aluminum, etc.) or are nonmetallic and nonconductors. Accordingly, little or no substantial magnetic field interactions are present for these products in association with the 3-Tesla MR system.
PMT Halo System with Carbon Graphite Open Back Ring and Titanium Skull Pin (PMT Corporation, Chanhassen, MN)
Non-clinical testing has demonstrated that the PMT Halo System with Carbon Graphite Open Back Ring and Titanium Skull Pins used as a cervical fixation or cervical traction device or system is MR Conditional. A patient with this system may undergo MRI safely under the following conditions:
-Static magnetic field of 3-Tesla and 1.5-Tesla ONLY
-Maximum spatial gradient magnetic field of 720 Gauss/cm or less
-Maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of scanning (per pulse sequence)
In non-clinical testing, this system produced maximum temperature changes of 0.5 degrees C or less at a maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of MRI using a 3-Tesla MR system (Excite, Software G3.0-052B, General Electric Healthcare, Milwaukee, WI).
MR image quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the Titanium Skull Pins. Therefore, it may be necessary to optimize MR Imaging parameters for the presence of these metallic objects.
Important Note: All of the materials used for the above products included metals with very low magnetic susceptibility (e.g., commercially pure titanium, Titanium alloy, Aluminum, etc.) or are nonmetallic and nonconductors. Accordingly, little or no substantial magnetic field interactions are present for these products in association with the 3-Tesla MR system.
Bremer Halo Crown System with Bremer Air Flo Vest and Titanium Skull Pins (DePuy Spine Inc., Raynham, MA)
Non-clinical testing has demonstrated that the Bremer Halo Crown System with Bremer Air Flo Vest and Titanium Pins used as a cervical fixation or cervical traction device or system is MR Conditional. A patient with this system may undergo MRI safely under the following conditions:
-Static magnetic field of 3-Tesla or less
-Maximum spatial gradient magnetic field of 720 Gauss/cm or less
-Maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of scanning (per pulse sequence)
In non-clinical testing, this system produced maximum temperature changes of 0.6 degrees C or less at a maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of MRI using a 3-Tesla MR system (Excite, Software G3.0-052B, General Electric Healthcare, Milwaukee, WI).
MR image quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the Titanium Skull Pins. Therefore, it may be necessary to optimize MR Imaging parameters for the presence of these metallic objects.
Important Note: All of the materials used for the above products included metals with very low magnetic susceptibility (e.g., commercially pure titanium, Titanium alloy, Aluminum, etc.) or are nonmetallic and nonconductors. Accordingly, little or no substantial magnetic field interactions are present for these products in association with the 3-Tesla MR system.
Bremer 3D Halo Crown System with Bremer Air Flo Vest and Titanium Skull Pins (DePuy Spine Inc., Raynham, MA)
Non-clinical testing has demonstrated that the Bremer 3D Halo Crown System with Bremer Air Flo Vest and Titanium Pins used as a cervical fixation or cervical traction device or system is MR Conditional. A patient with this system may undergo MRI safely under the following conditions:
-Static magnetic field of 3-Tesla or less
-Maximum spatial gradient magnetic field of 720 Gauss/cm or less
-Maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of scanning (per pulse sequence)
In non-clinical testing, this system produced maximum temperature changes of 0.6 degrees C or less at a maximum MR system reported, specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of MRI using a 3-Tesla MR system (Excite, General Electric). MR Image quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the Titanium Skull Pins. Therefore, it may be necessary to optimize MR Imaging parameters for the presence of these metallic objects.
Important Note: All of the materials used for the above products included metals with very low magnetic susceptibility (e.g., commercially pure titanium, Titanium alloy, Aluminum, etc.) or are nonmetallic and nonconductors. Accordingly, little or no substantial magnetic field interactions are present for these products in association with the 3-Tesla MR system.
MRI healthcare professionals are advised to contact the respective manufacturer in order to obtain the latest safety information to ensure patient safety relative to the use of an MR procedure.
REFERENCES
Ballock RT, Hajed PC, Byrne TP, et al. The quality of magnetic resonance imaging, as affected by the composition of the halo orthosis. J Bone Joint Surg 1989;71-A:431-434.
Clayman DA, Murakami ME, Vines FS. Compatibility of cervical spine braces with MR imaging. A study of nine nonferrous devices. AJNR Am J Neuroradiol 1990;11:385-390.
Diaz F, Tweardy L, Shellock FG. Cervical fixation devices: MRI issues at 3-Tesla. Spine 2010;35:411-5.
Graf H, Lauer UA, Schick F. Eddy-current induction in extended metallic parts as a source of considerable torsional moment. Journal of Magnetic Resonance Imaging 2006;23:585-590.
Hartwell CG, Shellock FG. MRI of cervical fixation devices: Sensation of heating caused by vibration of metallic components. J Magn Reson Imaging 1997;7:771.
Hua J, Fox RA. Magnetic resonance imaging of patients wearing a surgical traction halo. J Magn Reson Imaging 1996;:1:264-267.
Kim LJ, Sonntag VK, Hott JT, Nemeth JA, Klopfenstein JD, Tweardy L. Scalp burns from halo pins following magnetic resonance imaging. Case Report. Journal of Neurosurgery 2003:99:186.
Malko JA, Hoffman JC, Jarrett PJ. Eddy-current-induced artifacts caused by an "MR-compatible" halo device. Radiology 1989;173:563-564.
Shellock FG. MR imaging and cervical fixation devices: assessment of ferromagnetism, heating, and artifacts. Magn Reson Imaging 1996;14:1093-1098.
Shellock FG. Magnetic Resonance Procedures: Health Effects and Safety. CRC Press, LLC, Boca Raton, FL, 2001.
Shellock FG, Slimp G. Halo vest for cervical spine fixation during MR imaging. AJR Am J Roentgenol 1990;154:631-632. |