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

It is generally accepted that potentially hazardous tissue heating can occur when a metallic implant is exposed to strong radiofrequency (RF) fields in MRI (1–4). 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 x 1⁄4 15 cm; y 1⁄4 4 cm; z 1⁄4 30 cm with respect to the iso-center in gradient coordinates. Anatomically, this corresponds to a male adult, lying supine, and laterally off-center by Dx 6 cm. Based on manufacturer-provided field maps, x and z were chosen for maximum expected effect while keeping Dx 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 (20 mT=m), slew rate (200 Tm s ), and frequency (2000 Hz) 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 DT 1⁄4 25:8 K for the insulated and DT 1⁄4 3:8 K 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 P sp G 1⁄4 cT dT dt , where cT is that tissue’s specific heat capacity and dT dt 1⁄4 0:11 K=s 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 cT 1⁄4 3400 Jkg K 1 (9); hence, P sp G 1⁄4 370 W=kg. 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 P sp G 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 P sp G . Averaging over 10 g (1) or 1 g (4) of tissue, for example, is an integral part of the local SAR rationale, whereas P sp G as derived previously applies only to a possibly very thin layer surrounding the implant. The inductive heating power of a gradient sequence depends only on its root mean square–averaged jdB=dtjrms (2). Our self-written sequence generated jdB=dtj rms 1⁄4 65:4 T=s close to the bore, and jdB=dtjrms 1⁄4 49:7 T=s at the implant (ie, not vastly exceeding the FPO:B limit of 56 T=s of the new fixed-parameter option (1), which is currently the only existing jdB=dtjrms limitation in a standard with respect to scanning patients with MR-conditional implants). From the scarce data presented here, it cannot be concluded that gradient heating of bulk metallic implants is automatically dangerous. However, the opposing opinion Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany.