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Original article

Magnetic resonance imaging of artificial lumbar disks: safety and metal artifacts

YANG, Chang-wei; LIU, Liu; WANG, Jian; DONG, Ai-sheng; LU, Jian-ping; HE, Shi-sheng; LI, Ming

Editor(s): HAO, Xiu-yuan

Author Information
doi: 10.3760/cma.j.issn.0366-6999.2009.08.006
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Abstract

Total disc replacement (TDR) is a new alternative for the treatment of lumbar degenerative disc disease. Compared with conventional spinal fusion surgery, disc arthroplasty can maintain physiological motion at the affected segment. Therefore, disc arthroplasty has the capacity to recover normal lumbar function, and to reduce postoperative degeneration at adjacent disc levels, which was reported as a common complication after spinal fusion surgery. Moreover, TDR is advantageous as the mean immobility time postoperatively, and recuperation duration are shortened.1-3 The overall results after TDR operations have been reported as equal or superior to fusion surgery.4-6

Despite the great potential of TDR, a few issues require investigation. Some studies have found that the degeneration of facet joints was accelerated following TDR.7-8 Similarly to many other spinal surgeries, complications such as insufficient decompression, neurological deficit, etc, can occur with TDR. Another potential problem following TDR surgery is a consequence of the predominantly younger age of patients who undergo TDR surgery in comparison to patients who undergo spinal fusion surgery, such that the possibility of other spinal disorders is increased due to the much longer follow-up time after TDR surgery in the younger patient. This means there is a high likelihood for patients who have undergone TDR to require magnetic resonance imaging (MRI) to evaluate all these potential complications and spinal disorders.

MRI is a very useful diagnosis tool for spinal disorders due to its excellent soft tissue contrast. However, for a patient with metallic implants, the MR imaging procedure poses two concerns: (1) the safety of the patient under MR environment, and (2) imaging artifacts resulting from the metal implants. The safety of the patient is a concern, as the powerful magnetic field of MRI may induce attractive forces on the metallic implant material, and also, potentially significant heating may occur in the tissue due to the radio frequency field used during the MR procedure. Although previous studies on other medical implants have shown that most of orthopedics implants are safe in the MR environment due to their relative mechanical stability,9,10 there are still concerns about the safety of MR scans of patient with artificial disc if experimental data confirming their safety are not obtained. The absence of data concerning the safe use of MR imaging procedures on patients with artificial disc implants is an issue of particular concern as the implant is adjacent to important structures such as the spinal cord and nerve roots, which are at risk of damage with magnetically induced dislocation or heating of the implant. In addition, it is necessary to evaluate the nature of the metal-implant induced image artifacts, in order to ascertain the degree to which diagnostic information from the MRI has been impaired.11-15

Although some authors have used MRI for postoperative evaluation of TDR,16,17 no study to date has reported on the safety and metal artifact distribution of metallic artificial lumbar discs during MR imaging with different sequences. The present study aims to determine the safety and metal artifacts of two artificial disc devices in 1.5-Tesla clinical MR imaging system.

METHODS

Artificial disc and MRI system

Two artificial disc devices were PRODISC®-L Artificial Disc (Synthes Spine, Paoli, PA, USA) and CHARITÉ Artificial Disc (DePuy Spine, Raynham, MA, USA) (Figure 1) with each size described in Table 1. These two devices have been approved for clinical use by the Food and Drug Administration and are the two of most extensively used artificial discs in clinical tests. Both implants consist of two endplates manufactured from cobalt-chromium (Co-Cr) alloy and one sliding core (which is called an “inlay” in the PRODISC®-L prosthesis), manufactured from ultra-high molecular weight polyethylene (UHMWPE). The CHARITÉ artificial disc contained a radiopaque Co-Cr alloy wire for X-ray visualization of the core, which was not a feature of the PRODISC®-L prosthesis. Other small differences in the overall shape and fixation method of these two prostheses can be noted (Figure 1).

Table 1
Table 1:
Sizes of two tested artificial discs
Figure 1.
Figure 1.:
Two tested artificial discs. A: CHARITÉ. B: PRODISC®-L.

The safety and image artifacts of these two artificial disc systems was obtained using a 1.5-Tesla MR imaging system (Avanto, Siemens Medical Systems, Erlangen, Germany). Artifact size was measured using the workstation software (Syngo MR B13).

Measurement of attractive force

According to the recommended standards of the American Society for Testing and Materials (ASTM) F2052-06el,18 an angle-measurement instrument (Figure 2) was designed to measure the deflection angle of the endplates of two artificial discs in the static magnetic field of the MR system. Suspended with a thin nylon thread (length 50 cm, weight <1% of the weight of the implant), the superior endplates of two disc prostheses were individually placed at the entrance of magnetic field, where the greatest displacement was produced (i.e. the deflection angle was maximized). The angular deflection of the string from the vertical was measured and recorded.

Figure 2.
Figure 2.:
Diagram of the deflection force test fixture. B0: static magnetic field of MR system. θ: deflection angular.

Evaluation of heating effect of radio frequency (RF)

According to the recommended surgical procedure supplied by the device manufacturers, an artificial disc was placed in the L5/S1 intervertebral space of a human cadaver (adult male). When thermal balance was achieved in the MR examination room, the temperature on the adjacent tissue of the implant and L4/L5 intervertebral disc (used as a control) were measured using a digital probe thermometer (Model: BG363; accuracy: 1 centi-degree. Acez Instruments Co., Shenzen, China). The MR scan was then performed using a body coil and turbo spin echo sequence (TSE) with a turbo factor of 29, flip angle (FA) of 180 degrees, repetition time (TR) 4500 milliseconds, echo time (TE) 138 milliseconds, a matrix of 116×256 and fat saturation. The bandwidth was 130 Hz/pixel, and scan time was 30 minutes; conditions which represented a “worse case scenario”19 for the heating effects of RF in clinical condition. When the scan was over, the temperature of the implant-adjacent tissue and control location was measured at the same sites to the pre-scan.

Evaluation of metal artifacts

In order to evaluate the metal artifacts of two artificial discs under different MR sequences, we designed a rectangular water phantom made of polymethyl methacrylate plastic with dimensions of 15 cm×15 cm×30 cm (width×height×length). The water phantom was filled with CuSO4 solution (1 g/L)(to reduce T1 and keep TR at a reasonable level).20 The artificial disc was then tied and suspended with three thin nylon threads to keep it in the centre of the water phantom. It was oriented in the MRI scanner similarly to how it would be positioned in vivo. The MR scan was performed using Spin Echo (SE, T1-weighted), Turbo Spin Echo (TSE, T1/T2 -weighted), Incoherent Gradient Echo (Flash), Short T1 Inversion Recovery (STIR), TSE fat saturation and Turbo Dark Fluid (Long Tau Inversion Recovery) sequences respectively (detailed parameters are described in Table 2). These sequences are routinely used in clinical MR imaging of the spine. A standard head coil was used and the water phantom was centered in the magnetic field. In all cases two sets of images (with and without the implant) were acquired to obtain images with and without artifacts. The images were obtained and analyzed according to a method modified from ASTM F2119-01.20 Using the MRI system workstation software the maximal diameter of the metal artifacts was measured. A rectangular region of interest (ROI) was drawn on the two sets of images (with and without the implant) enclosing the visible artifact. Window level and window width were determined from the same ROI position in the reference image. Window center was set to the mean signal intensity within the ROI in the reference image, and window width was set to a 60% of that window center. The device and artifact area was then automatically shown in the ROI that were outside the window (i.e., entirely white or black).21 The distance measurement tool was used to measure the maximal artifact size (millimeter, including the implant) in two orthogonal directions, x-direction of the scanner for sagittal image and z-directions for axial image (with the main magnetic field parallel to the z-direction), which represented the effect of metal artifacts on the structures anterior or posterior to the implant. When the size of implant was subtracted from this value, the true artifact size was obtained.

Table 2
Table 2:
Sequence parameters used in metal artifacts test

RESULTS

The maximal deflection angle under the static MR field was 7.5 degrees for the endplate of CHARITÉ prosthesis, and 6.0 degrees for the endplate of PRODISC®-L prosthesis. The temperature of the adjacent tissue of the two artificial discs and that of the L4/L5 intervertebral disc control site under pre- and post-scan conditions are shown in Table 3. The temperature rise on the adjacent tissue of the two artificial disc due to the implants (that is the temperature rise of the tissue at the implant site, minus the temperature rise of the control site) was 0.4 and 0.6°C for the CHARITÉ and PRODISC®-L prostheses, respectively. The images of the two artificial discs with the T2-weighted TSE sequence are shown in Figure 3. The size of metal artifacts in the z-direction for the axial image and x-direction for the sagittal image, under the different MRI sequences, are shown in Table 4.

Table 3
Table 3:
Change of the temperature on the adjacent tissue of the two artificial discs and L4/L5 intervertebral disc (°C)
Table 4
Table 4:
Size of metal artifacts for two artificial discs (mm)
Figure 3.
Figure 3.:
Artifacts from CHARITÉ (upper) and PRODISC®-L (lower) artificial discs on T2-weighted TSE axial image (A), coronal image (B) and sagittal image (C).

DISCUSSION

Since the rapidly increased use of MRI, some incidents have been reported due to displacement of the clip during an MRI procedure,22 and the need for standards to address the safety of implants and other medical devices in the MR environment was recognized. At the present time, ASTM have published five ASTM standards addressing the evaluation and marking of medical devices and other items in their use in the MR environment.23 They are ASTM F2052-06e1, which discusses measurement of magnetically induced displacement force, F2119-01 which discusses evaluation of MR image artifacts, F2182-02a24 discussing measurement of RF-induced heating, F2213-06,25 which indicates the measurement of induced torque, and F2503-0526 on marking.

The artificial lumbar disc is a new alternative for treatment of degenerative disc disease and has been regarded as a prospective device. There is however, very little information regarding imaging of these metallic prostheses with MR imaging following lumbar TDR surgery. Generally, a “passive implant” (containing no electronically- or magnetically-activated components) and those made from non-ferromagnetic materials can undergo an MR procedure immediately after implantation using an MR system operating at 1.5-Tesla or less.11 However, as the endplates of these two artificial discs used in this study are manufactured from a ferromagnetic Co-Cr alloy, the effects of the MR environment on these materials was a potential concern. Therefore it is necessary to evaluate the safety of these new devices during MR imaging.

According to the results of our experiment, the maximal deflection angle of the endplates of two artificial discs under the static MR field were between 6° to 7° (less than 45°), which means that the magnetically induced deflection force is less than the force exerted on the device due to its own weight. According to the standard ASTM F2052-06e1, these devices can be regarded as safe from a mechanical perspective under the MR procedure.18 Note that in this study, we did not measure the torque force of artificial disc under MR environment because the rationale in F2213-06 shows the magnetically-induced force and torque are related. If the force is minimal, the torque is expected to also be minimal.25

Metallic implants may be heated excessively by RF during MRI. In this study, we did not conduct the recommended water phantom assessment of induced heating according to the ASTM F2182-02a guidelines24 because the heating effects of RF was considered to be dependent on the location of the implant in the body, its surrounding tissues, and many other more complex conditions related to the physiological environment. As the mechanism of RF induced heating is very complex, the condition of a water phantom could not be expected to accurately represent that in a human body. Therefore, we used a cadaver to evaluate the effect. The results demonstrate that even under the “worst case” clinical conditions expected to induce the most heating (ie. using RF with higher specific absorption rate and a longer scan time than under actual clinical conditions) the temperature change due to the two implants was only 0.4-0.6°C. When the effect of blood circulation in vivo is taken into account for a normal patient, rather than a cadaver, this circulation would serve to remove heat from the implant site, resulting in even less implant induced temperature increase. Therefore, we can conclude that there is only a small temperature change duo to artificial discs which should be well tolerated.

Metal artifacts on MR images appear as signal voids, image distortions and signal in-homogeneity, which may be larger than the size and shape of the object being imaged. Such irregular artifacts degrade the image quality and have a significant impact on image interpretation in the vicinity of metallic hardware. According to ASTM, a metal artifact is defined as “a pixel in an image is considered to be part of an image artifact if the intensity is changed by at least 30% when the device is present compared to a reference image in which the device is absent”.

Metal artifacts are proportional to the magnetic susceptibility of the materials. Sekhon et al17 have recently reported MR imaging of several cervical arthroplasty devices and demonstrated that titanium implants had less degradation than Co-Cr implants, which show significant deterioration of the adjacent construction of the image. It is therefore notable that most artificial lumbar discs are made of Co-Cr material. Neal et al16 argued that sagittal MR imaging could be undertaken to evaluate the adjacent motion segments for degenerative changes following lumbar TDR.

The results of our experiment demonstrate that the distribution of artifacts was similar for the two implants. The large number of metal-artifacts present in all sequences could deteriorate the adjacent constructions in imaging the vertebral canal. Among all of the sequences used in this experiment, the size of metal artifacts on images of TSE (T1/T2 -weighted), STIR and Turbo Dark Fluid sequences were relatively less than those of TSE fat saturation, Flash and SE (T1-weighted) sequences.

While our experiments yield exact and useful results pertaining to the safety of metallic disk implants under the MRI environment, there are a number of avenues for further research, and also, some view shortcomings we would like to address. Due to the wide use of 1.5T MRI systems in clinical environments in China, our country of research, it was perceived to be quite practical to conduct the safety tests of the two implants in a 1.5-Tesla MRI system. Unfortunately, the conclusions drawn on this 1.5T system cannot be extended to a 3T MRI system, which has been cleared for market, and it would be very beneficial to repeat this study for a true “worst case” scenario of a 3T MRI system. In other concerns, the measure of the heating effect of RF was not performed in a phantom in accordance with the ASTM standard. We departed from the convention of the phantom to measure the temperature change with the implant contained in a human cadaver, as we believed this would be a much more realistic situation. A shortcoming of our implant heating study was that we did not use a “real-time” monitoring technique to record temperatures on an implant during MRI. This could have resulted in missing the highest temperatures that occurred during the MRI procedure. As the objective was to evaluate whether heating of the implant under MR procedures would damage adjacent tissue, a short duration of the implant at a higher temperature is argued to not be a factor which would burn or damage tissue.

According to our experiments, because of the minor magnetic field interactions and heating effect of RF, MRI may be performed immediately after these devices are implanted. Based on the results of the tests, the CHARITÉ and the PRODISC®-L artificial disc will not present an additional hazard or risk to a patient undergoing an MRI procedure using a scanner operating with a static magnetic field of 1.5T or lower and under the MRI-related heating conditions used for this evaluation.

Metal artifacts of the two implants are similar in size and may present problems if the anatomical region of interest is in or near the area where these implants are located (e.g., vertebral canal at affected segment). The size of metal artifacts on images of TSE (T1/T2-weighted), STIR and Turbo Dark Fluid sequences were relatively less than those of TSE fat saturation, Flash and SE (T1-weighted) sequences.

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Keywords:

intervertebral disk; artifacts; magnetic resonance imaging

© 2009 Chinese Medical Association