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Original Report |
1 Institute for Magnetic Resonance Safety, Education, and Research, Los Angeles,
CA 90045.
2Department of Radiology, University of Southern California,
7511 McConnell Ave., Los Angeles, CA 90045.
Received November 21, 2001;
accepted after revision December 13, 2001.
Supported by a grant from Conceptus, San Carlos, CA 94070.
Abstract
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CONCLUSION. The findings indicated that it should be safe for patients with this metallic contraceptive implant to undergo MR imaging with systems using static magnetic fields of 1.5 T or less.
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Recently, a new metallic implant has been developed for permanent contraception in women. This implant, the ESSURE device (Conceptus, San Carlos, CA), is a dynamically expanding microcoil that is placed in the proximal section of the fallopian tube using a nonincisional technique. Subsequently, the device elicits an intended benign tissue response, causing tissue ingrowth into the device and thereby anchoring it firmly in the fallopian tube. This benign tissue response is local, fibrotic, and occlusive. Accordingly, the presence of this implant is meant to alter the function and architecture of the fallopian tube, resulting in permanent contraception.
Because the ESSURE device is made of metal, safety concerns arise regarding the use of MR imaging in women with the device. Therefore, this investigation was conducted to evaluate magnetic field interactions and heating using ex vivo testing techniques at 1.5 T. In addition, artifacts related to this metallic implant were characterized.
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Safety tests for MR imaging were conducted on the metallic implant that remains in the fallopian tube (Fig. 1A,1B). It is composed of 316L stainless steel, platinum, iridium, nickel—titanium alloy, silver solder, and polyethylene terephthalate fibers (Fig. 1A,1B). Currently, this device is classified as an investigational device in the United States but is used and marketed in other countries.
Assessment of Magnetic Field Interactions
Tests for magnetic field interactions were performed on one randomly
selected sample of the ESSURE device using a previously described procedure
known as the deflection angle test
[3]. This test was conducted
using a shielded 1.5-T MR imaging system (General Electric Medical Systems,
Milwaukee, WI). A 20-cm length of thin thread was attached to the ESSURE
device and then to a plastic protractor so that the angle of deflection from
the vertical plane could be measured. The accuracy of this measuring device is
within ± 0.5° [3].
The deflection angle was measured at the position in the MR imaging system
previously determined to be the maximal spatial gradient of the magnetic field
[3].
Magnetic field—related torque was then qualitatively determined for the ESSURE device using a technique that has been previously described in the literature [3]. The implant was oriented on the test device 45° relative to the static magnetic field of the system. The test apparatus with the implant was then positioned in the center of the imaging system. The ESSURE device was directly observed for any movement with respect to rotation relative to the 1.5-T static magnetic field. The device was then moved 45° relative to its previous position and again observed for alignment or rotation. This process was repeated to encompass a full 360° rotation of positions for the implant [3]. A previously reported [3] qualitative scale of torque was applied to the results: 0, no torque; 1, mild torque (the device changed orientation slightly but did not align to the magnetic field); 2, moderate torque (the device aligned gradually to the magnetic field); 3, strong torque (the device showed rapid and forceful alignment to the magnetic field); 4, very strong torque (the device showed very rapid and very forceful alignment to the magnetic field).
Evaluation of Heating
A previously described experimental protocol
[3,
4] was used to evaluate MR
imaging—related heating of the ESSURE device. Heating of the implant
during MR imaging was determined by performing an extreme radiofrequency power
exposure experiment, with the device placed inside a specially constructed
phantom filled with a semisolid gel that simulates the convective heating
qualities of human tissue
[3,4,5].
The shape of the phantom approximated the geometry and mass of a human torso
[3]. The ESSURE device was
mounted on a thin plastic frame to permit placement that would simulate its
intended in vivo use.
MR imaging was performed using a 1.5-T, 64-MHz MR imaging system with a transmit—receive body coil and a pulse sequence that produced a whole-body-averaged specific absorption rate of 1.3 W/kg and a spatial peak specific absorption rate of 2.7 W/kg. This relatively high level of radiofrequency energy was selected in consideration of the clinical use of MR imaging [3].
Temperature recordings were obtained using an MR-compatible fluoroptic thermometry system (Luxtron 3100; Luxtron, Santa Clara, CA). The system probes were attached to the ESSURE device to record representative temperatures: Probe 1 was placed 0.5 mm from the proximal end of the device, and probe 2 was placed 0.5 mm from the center of the device. In addition, a third probe was placed in the gel-filled phantom at a position approximately 30 cm away from the device to record a reference temperature.
The gel-filled phantom with the implant and thermometry probes attached was placed in the MR scanner and allowed to equilibrate to environmental conditions for more than 1 hr. Baseline temperatures were recorded at 20-sec intervals for 5 min. MR imaging was then performed for 20 min, with temperatures recorded at 20-sec intervals.
Evaluation of Artifacts
To characterize artifacts related to the ESSURE device, we placed it inside
a gel-filled phantom (a 20-cm-long cylinder with a diameter of 25 cm) and then
performed MR imaging with a 1.5-T system and a transmit—receive body
coil. The parameters for the T1-weighted, spin-echo pulse sequence were TR/TE,
500/20; matrix size, 256 x 256; section thickness, 5 mm; field of view,
16 cm; number of excitations, 2; and bandwidth, 16 kHz. The parameters for the
gradient-echo pulse sequence were 100/15; flip angle, 30°; matrix size,
256 x 256; section thickness, 5 mm; field of view, 16 cm; number of
excitations, 2; and bandwidth, 16 kHz. Imaging planes were oriented to
encompass both the long and short axes of the ESSURE device.
The MR system planimetric software (SD for accuracy and resolution, ± 10%) was used to obtain a cross-sectional area measurement of the artifact for the ESSURE device with regard to the dimensions for each pulse sequence and for each image orientation [3]. All image display parameters (e.g., magnification and window and level settings) were carefully selected and used in a consistent manner to facilitate valid measurements of artifact size.
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Artifact test results for the ESSURE device are summarized in Table 1. In general, the artifacts for this implant were visualized as localized signal voids relative to the device's size and shape. Images obtained with the gradient-echo pulse sequence showed artifacts with larger cross-sectional areas compared with those obtained using the T1-weighted spin-echo pulse sequence (Fig. 2A,2B).
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MR Imaging—Related Heating
The experiment performed to determine MR imaging—related heating for
the ESSURE device showed that no substantial temperature increases for this
implant were associated with procedures involving relatively high
radiofrequency energy. The highest temperature changes (
0.6°C) were
well within physiologically acceptable levels and would not present any hazard
to human tissue [6,
9,10,11].
Therefore, this implant will not present any thermogenic risk to a patient
undergoing MR imaging under the conditions used in this study
(whole-body-averaged specific absorption rate, 1.3 W/kg for 20 min).
Previous investigations have used ex vivo testing techniques to evaluate MR imaging—related heating for passive implants of various sizes, shapes, and metallic compositions [3,4,5, 12]. In general, these data indicated that only minor temperature elevations occurred in association with MR imaging in patients with relatively small implants (i.e., similar to the ESSURE device) [3,4,5, 12].
Artifacts
The ex vivo assessment of artifacts related to the ESSURE device showed
that the occurrence of artifacts was comparatively low. In part, this finding
was due to the relatively low magnetic susceptibility values of the metallic
materials used to make this implant
[7,
8]. MR images obtained with the
gradient-echo pulse sequence were subject to larger artifacts than those
obtained with the T1-weighted spin-echo pulse sequence. Nevertheless,
artifacts should not be a problem in imaging patients with the ESSURE device,
unless the area of interest to be imaged is in exactly the same position as
the implant.
Other Contraceptive Devices
Other contraceptive devices have been tested for MR imaging safety,
including intrauterine devices (IUDs) and contraceptive diaphragms
[1,
2,
8,
10,
11]. IUDs may be made entirely
of nonmetallic materials, such as plastic, or a combination of nonmetallic and
metallic materials. Typically, copper is the metal used in IUDs. For example,
the Copper T (ParaGard, Ortho-McNeil Pharmaceuticals, Sommerville, NJ) has a
fine copper coil wound around a portion of the IUD. The results of testing
conducted to determine the MR safety aspects of IUDs with metal components
indicated that these objects are safe for patients imaged with MR systems
operating at 1.5 T or less. The metallic component of an IUD may cause
artifacts; however, such artifacts are relatively minor because of the low
magnetic susceptibility of copper and relatively small size of the IUD
[7,
10,
11].
Most contraceptive diaphragms have metallic rings that maintain the device's position during use. Notably, some of these metallic rings are made from ferromagnetic materials. MR safety testing conducted at 1.5 T has shown that these contraceptive diaphragms are strongly attracted by the static magnetic field of the MR imaging system. However, MR imaging has been performed safely in patients with these contraceptive devices, without patient complaints of sensations of movement or heating (Shellock FG, unpublished observations, 1996). MR imaging artifacts caused by contraceptive diaphragms with ferromagnetic rings are quite substantial because of the size of the devices and the magnetic susceptibility of the materials used to make them [1]. Nevertheless, the presence of a diaphragm is not considered a contraindication for a patient undergoing MR imaging at 1.5 T or less [1, 2, 8].
In summary, the results of the MR imaging safety tests indicated the ESSURE device will not present an additional hazard or risk to a patient undergoing MR imaging with a system operating with a static magnetic field of 1.5 T or less. Therefore, this metallic implant is considered MR-safe in the context of the specific conditions used for testing.
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