Gradient heating of bulk metallic implants can be a safety concern in MRI

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It is generally accepted that potentially hazardous tissue heating can occur when a metallic implant is exposed to strong radiofrequency (RF) fields in MRI (1,2,3,4). Although device heating by gradient switching is mentioned in some normative documents (1,2), but not all (3,4), the popular opinion is that this effect must be irrelevant for patient safety, and does not warrant concern or deeper investigation. The few existing literature on this subject appeared to confirm this viewpoint (5). This scenario changed when a numerical simulation study by Zilberti et al (6) reported significant gradient-induced temperature rises in and around hip prostheses. Primarily, the implant itself was heated (by eddy currents), and thermal conduction caused the adjacent tissue to follow closely. These findings were not expected by many, as it had often been assumed that the much lower frequencies of gradient switching, compared with the RF case, should result in correspondingly less heating.
Here, experimental results are reported that confirm the possibility of substantial gradient heating of an orthopedic implant. The test object was the acetabular cup (diameter of 54 mm, mass of 70.7 g) of an excised hip prosthesis (Plasmafit Plus-3, Aesculap, Center Valley, PA, USA). The device, untested for MR compatibility by the manufacturer (7), is made of Ti-6Al-4V alloy (8), whereas CoCrMo showed even larger temperature effects in (6). With four fiber-optical temperature sensors (Luxtron, LumaSense Technologies, Santa Clara, CA, USA) attached, the implant was placed in a clinical 3 tesla scanner (Verio, Siemens Healthcare, Munich, Germany) at position Symbol with respect to the iso-center in gradient coordinates. Anatomically, this corresponds to a male adult, lying supine, and laterally off-center by Symbol. Based on manufacturer-provided field maps, Symbol and Symbol were chosen for maximum expected effect while keeping Symbol reasonable. This represents a bad example—one that is not typical, but possible. A self-written echo planar imaging like sequence with continuous, trapezoidal z-gradients normal to the implant face was used. The gradient strength (Symbol), slew rate (Symbol), and frequency (Symbol) were set to the scanner limits when the sequence was run in “normal operating mode.” All safety features (eg, stimulation monitor, temperature supervision) were in place; RF was off. Scanning a subject with this sequence would have been permitted. Experiments were run with both the implant thermally “insulated” (in polystyrene) or “embedded” (in gelatin gel).
Within 10 min, maximum temperature rises of Symbol for the insulated and Symbol for the embedded implant were observed. If the adjacent tissue follows the implant temperature, as it was found in (6), we can estimate a specific gradient heating power using Symbol, where Symbol is that tissue's specific heat capacity and Symbol is the initial slope of the temperature curve (identical, within errors, for the insulated or embedded case) (Supporting Information Fig. S1). For example, for muscle Symbol(9); hence, Symbol. Applying, for comparison, the product echo planar imaging sequence, we obtained 40%, with Graf's (5) sequence and implant position 2.6% of this value. The resulting tissue temperatures can only be determined by simulations that take into account heat dissipation, such as by perfusion (6,10,11), whereas here an independent experimental test of such simulations is intended. An RF heating test performed for comparison (Supporting Information Fig. S2) showed no detectable temperature effect.
The relationship of Symbol to the local specific absorption rate (SAR) for RF heating, for which—based on an implicit model of tissue cooling—a limit of 20 W/kg is imposed (1), remains to be clarified. Both quantities are different in their physical nature and expected effect, so their values are not directly comparable, and established local SAR exposure limits cannot immediately be applied to Symbol.

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