Abstract
Magnetic resonance imaging (MRI) is regularly used for stereotactic imaging of Gamma Knife (GK) radiosurgery patients for GK treatment planning. MRI‐induced thermal injuries have occurred and been reported for GK patients with attached metallic headframes. Depending on the specific MR imaging and headframe conditions, a skin injury from MRI‐induced heating can potentially occur where the four headframe screws contact the skin surface of the patient's head. Higher MR field strength has a greater heating potential. Two primary heating mechanisms, electromagnetic induction and the antenna effect, are possible. In this study, MRI‐induced heating from a 3T clinical MRI scanner was investigated for stereotactic headframes used in gamma radiosurgery and neurosurgery. Using melons as head phantoms, optical thermometers were used to characterize the temperature profile at various points of the melon headframe composite as a function of two 3T MR pulse sequence protocols. Different combinations of GK radiosurgery headframe post and screw designs were tested to determine best and worst combinations for MRI‐induced heating. Temperature increases were measured for all pulse sequences tested, indicating that the potential exists for MRI‐induced skin heating and burns at the headframe attachment site. This heating originates with electromagnetic induction caused by the RF fields inducing current in a loop formed by the headframe, mounting screws, and the region of the patient's head located between any of the two screws. This induced current is then resistively dissipated, with the regions of highest resistance, located at the headframe screw–patient head interface, experiencing the most heating. Significant heating can be prevented by replacing the metallic threads holding the screw with electrically insulated nuts, which is the heating prevention and patient safety recommendation of the GK manufacturer. Our results confirm that the manufacturer's recommendation to use insulating nuts reduces the induced currents in the headframe nearly to zero, effectively preventing heating and minimizing the likelihood of thermal injury.PACS numbers: 87.57.‐s, 87.61.‐c, 87.61.Tg, 87.57.c‐
Highlights
Electromagnetic induction responsible for Magnetic resonance imaging (MRI)-induced heating The initial experiment was designed to determine which of the two most likely heating mechanisms, the antenna effect or electromagnetic induction, is responsible for the heating that occurs at the Gamma Knife (GK) headframe screw–melon interface
The longest posts on the GK headframe are ~ 15 cm and the overall unwrapped length is ~ 62 cm, much shorter than the required ~ 117 cm necessary for antenna heating to occur in air
This study shows that heating caused by the RF field of a 3T MRI scanner due to electromagnetic induction, as described by Faraday’s law, occurs at the GK headframe screws when melon phantoms are mounted in the headframe during stereotactic MR imaging
Summary
Introduction & Background55 Bennett et al.: Preventing MR-induced GK headframe burns sources: the rapid acceleration of nearby ferromagnetic objects by the high-static magnetic field (up to 3 T for clinical units);(4-11) the gradient fields, which have been shown to induce nerve stimulation in humans;(12,13) the cryogenics, which can cause severe frostbite, suffocation, and substantial explosions if the pressure relief system of the cryogen containers become defective;(14) and the radiofrequency (RF) fields, which are likely the primary source of MRI-induced thermal injury.[9,15,16,17,18,19,20,21,22,23]This study is motivated by the increasing number of reports of MRI-induced patient thermal injuries, including burns,(9,13-23) and by our clinic’s use of a dedicated 3T MR simulator[1,2] that serves as an integral part of an active GK radiosurgery program. The causes of reported MRIinduced thermal injuries and burns are often not well understood, are sometimes described as unknown or mysterious, and seem to originate with different heating mechanisms. While the specific situations leading to these injuries may be difficult to pinpoint, the heating mechanisms causing them are not mysterious. They are the well-understood physical phenomena of electromagnetic induction and the antenna effect, both originating with the radiofrequency (RF) outputs of the MRI machine. In 2003, headframe and GK manufacturer, Elekta, (Elekta AB, Stockholm, Sweden) notified users of the availability of “insulated posts”, with the stated use for “high tesla MR units and high frequency MR sequences”.(25) Beginning at approximately the same time, reports to the US Food and Drug Administration (FDA) documented thermal injuries due to MR-induced heating for patients wearing stereotactic headframes.[26,27,28,29,30,31,32,33]
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