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Cardiac Imaging |
1 From the Department of Internal Medicine and Cardiology, Charité, Campus Virchow-Klinikum Humboldt University & German Heart Institute, Augustenburger Platz 1, 13353 Berlin, Germany (J.H., E.N., A.B., H.O., E.F.), and Philips Medical Systems, Hamburg, Germany (B.S.). Received December 28, 1998; revision requested February 23, 1999; final revision received November 16; accepted December 6. Address correspondence to J.H. (e-mail: Juergen.hug@charite.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: The heating, artifacts, and ferromagnetism with different stents were studied with a 1.5-T MR tomograph with ultrafast gradients by using turbo spin-echo, turbo gradient-echo, and echo-planar imaging sequences. Nineteen stents, which were 8–25 mm in length and 3.0–4.5 mm in diameter, were evaluated. Stent deviation induced by the magnetic field and during MR imaging, migration, and heating caused by the radio-frequency pulses were examined. The size of imaging artifacts was measured with all the stents under standardized conditions and with six stents after their implantation into the coronary arteries of freshly explanted pig hearts.
RESULTS: All except two types of stents showed minimal ferromagnetism. No device migration or heating was induced. Turbo spin-echo images had minimal artifacts; larger artifacts were seen on the turbo gradient-echo and echo-planar images. With ultrafast gradients, the artifacts on the echo-planar images were substantially reduced.
CONCLUSION: The studied coronary stents were not influenced by heating or motion during 1.5-T MR imaging. Artifact size differed according to the type and size of the stent and the MR imaging sequence used. Thus, patients with these stents can be safely examined.
Index terms: Coronary vessels, MR, 54.121411, 54.121412, 54.121416, • Coronary vessels, stents and prostheses, 54.126 • Magnetic resonance (MR), artifact, 54.121411, 54.121412, 54.121416 • Magnetic resonance (MR), rapid imaging, 54.121411, 54.121412, 54.121416, • Magnetic resonance (MR), safety, 54.121411, 54.121412, 54.121416
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Magnetic resonance (MR) imaging has become an important tool for imaging many internal organs, including the heart (4,5). Cardiac MR imaging has been shown to provide accurate images of the proximal and medial parts of the coronary arteries (6–9) and to be highly superior to stress echocardiography for the detection of ischemic heart disease (10). Thus, more and more patients, especially those with coronary arterial disease, can be expected to undergo MR imaging examinations. Because of the strong magnetic field required for current imaging systems, there has been concern with regard to the possible heating or dislocation of previously implanted coronary stents in patients scheduled for MR imaging investigations. It has been recommended that patients with coronary stents should wait several weeks after stent implantation before they undergo MR imaging. In addition, the implanted stents can cause imaging artifacts at the implantation site that prohibit the visualization of underlying structures.
This study was performed to examine the occurrence of ferromagnetism in coronary arterial stents as a cause of movement, determine possible heating during MR imaging, and measure the size of susceptibility artifacts caused by the different stents.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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, where m is the mass of the device (in kilograms); g, the gravitational acceleration (9.81 m/sec2); and tan, the deflection angle from the vertical. The control device was a stainless-steel needle (Ethicon Mersilene EH7637 [mass 159 mg]; Johnson & Johnson, Miami Lakes, Fla) suspended and similarly placed into the magnetic field. The devices that demonstrated ferromagnetism were further evaluated for possible magnetically induced device movement. The position of each stent was marked on a millimeter grid, and the possible movement induced during the different MR imaging sequences was measured as a distance in millimeters.
Determination of Device Heating
To determine possible heating, the fully expanded stents were placed at room temperature into a homogeneous medium and positioned in the 1.5-T MR imaging tomograph parallel to the magnetic radio-frequency field. A pot of stirred low-fat (3.5%) plain yogurt was chosen as the homogeneous medium. The composition of the yogurt per 150 g was as follows: protein, 7.7 g; carbohydrates, 11.0 g; fat, 1.2 g; calcium, 285 mg; phosphorus, 240 mg; and minor concentrations of some other nutrients. This medium was chosen for two reasons: first, because the T1 of 1,045 msec and T2 of 160 msec that were used for the yogurt were nearly comparable to the relaxation times of human myocardial tissue and second, for practical reasons. Placement of the tiny, fully expanded stents deep into the medium in a fixed and stable position was possible without any damage. For extreme in vitro worst-case experiments and testing of possible heating, the stents were imaged for 30 minutes by using a TSE sequence with an echo train consisting of nine 180° pulses (field of view, 385 mm; matrix, 256 x 256; section thickness, 10 mm). In addition, three saturation sections were used to achieve a maximum of adjustable radio-frequency power and thus yield a specific absorption rate of 3.9 W per kilogram of body weight. (The maximum specific absorption rate allowed in human studies is 4.0 W/kg.)
Changes in the temperature of the stents or adjacent medium were detected by obtaining continuous temperature measurements during exposure with a carefully calibrated fiberoptic temperature probe (model FT705; IRE-Polus, Moscow, Russia) placed into the expanded lumen of the stent in close contact with the stent wall. The accuracy of the temperature measurements was within 0.1°C. Heating of the temperature probe itself owing to power absorption, which would have resulted in false temperature measurements, was avoided by obtaining continuous measurements in a separate volume of the medium that did not contain stents. The measurements began after the stents were placed, and the homogeneous medium was thermally equilibrated with the ambient temperature.
MR Imaging of Stents and Artifacts
Susceptibility related signal loss is a term used (13) to refer to an artifact on MR images that consists of a region of signal void with a surrounding area of increased signal intensity that appears to be considerably larger than the actual size of the device that is causing the artifact. The presence and degree of MR imaging susceptibility artifacts caused by the coronary stents during the different sequences were evaluated under different conditions: (a) To determine the influence of the orientation of the fully expanded stents in the magnetic field on the size and shape of the imaging artifacts, three devices—the 24-mm Micro Stent II, 20-mm Giantourco-Roubin II stent, and 15-mm Palmaz-Schatz stent—were studied (by J.H., E.N., and B.S. in consensus) at different positions and orientations within the magnetic field. The position and orientation that caused the maximum artifact size were recorded. To obtain a maximum artifact size, all further studies were then performed with the stents in that position and orientation. (b) To standardize the conditions for measuring the expected imaging artifacts, the stents were fully expanded to their nominal diameter in the homogeneous yogurt medium that had a T1 of 1,045 msec and T2 of 160 msec, which resembled the in vivo relaxation times of myocardial tissue, and imaged at identical positions and orientations within the magnetic field. (c) To simulate in vivo conditions, six of the stents—the 18-mm DUET; 15-mm-long, 3.0-mm-diameter MULTI-LINK; 15-mm InFlow, 15-mm InFlow Gold, 15-mm Palmaz-Schatz, and 15-mm Wiktor GX devices—were implanted into the coronary arteries of three freshly explanted pig hearts by using standard angioplasty techniques. After placement of a guiding catheter into the coronary ostium, the premounted stents were implanted over a guide wire into the proximal coronary segments with balloon inflation up to 12 atm. Optimal placement of the implanted stents was controlled by means of selective coronary angiography.
After imaging with the regular (TSE, TFE, and echo-planar) and ultrafast (fast TFE and fast echo-planar) MR sequences was performed, the maximum artifact length (L) and maximum artifact diameter in two planes perpendicular to each other (D1 and D2) were determined. The artifact volume V was calculated on the basis of the formula V = (
/4) · D1 · D2 · L.
Statistical Analyses
The results were expressed as single values or as the mean ± SD. One-way analysis of variance was used to compare the results of the different MR imaging sequences, and in cases of overall significance, group differences were evaluated by using the Bonferroni modified t test. Least-squares linear regression was used to compare the MR imaging stent artifact lengths with the actual stent lengths. A P value of less than .05 was considered to be indicative of statistical significance.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Imaging Artifacts
All 19 stents created a substantial degree of artifacts. The artifact shapes and sizes differed substantially, depending on the orientation of the stents in the magnetic field. The biggest artifacts were observed when the position of the stents was perpendicular to the constant magnetic induction field (B0), and the smallest artifacts were parallel to the static magnetic field. Figure 1 shows sagittal and transverse images of a large imaging artifact created by a 15-mm InFlow stent implanted into a freshly explanted pig heart and imaged with an echo-planar imaging sequence (repetition time msec/echo time msec, 750.0/12.1; echo-planar imaging factor, 7).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MR imaging is considered to be contraindicated in patients with ferromagnetic implants, primarily because of the potential risks associated with their possible movement, the induction of an electrical current, the potential for excessive heating, and the possibility of misinterpretation of an artifact produced by the presence of the object (11,14,15). Several factors influence the relative risk of MR imaging in patients with metallic implants: (a) the strength of the static magnetic field and of the gradients, (b) the degree of ferromagnetism in the implanted device, (c) the geometry of the implanted material, and (d) the location and orientation of the implant in situ during MR imaging (16). These factors should be carefully considered, particularly if the implanted object is located in a potentially dangerous area of the body, such as near a vascular structure where migration or heating could severely injure the patient (17,18).
Ferromagnetism and Device Migration
The highest ferromagnetic force was found in the stents with the greatest mass. No ferromagnetism was observed in the stent with the lowest mass—the 8-mm DUET—or in the 15-mm Wiktor GX stent, which was the only device in the study made from tantalum. Several studies have involved assessment of the ferromagnetic qualities of various metallic implants (14), materials (19–22), or devices, such as aneurysm or hemostatic clips (23–25), carotid arterial vascular clamps (26), dental devices (11), heart valve prostheses (26–28), intravascular coils and filters (29,30), ocular implants (31), and vascular access ports (31–33). The authors of these studies concluded that patients with metallic implants can be examined safely with MR imaging, providing that the objects either are not ferromagnetic or are minimally ferromagnetic—that is, the deflection force is insufficient to move or dislodge the implant in situ.
In the current study, it was shown that minimum ferromagnetism may occur in most of the evaluated stents. However, the ferromagnetic force experienced during a 1.5-T MR imaging examination is too small to cause migration of the stent. In addition, the forces applied to a stent implanted into the coronary arteries owing to rapid motion, with acceleration and deceleration of the heart during cardiac contraction and relaxation, are expected to be much higher than those caused by the magnetic field (34).
Heating
Because the fully expanded stents are closed loops, it should be determined whether, with the changing magnetic field strength associated with the rapidly pulsed gradients used in MR imaging, a current could be induced and cause local heating (35). Pulsed radio-frequency fields and time-dependent gradient fields could be possible sources of such heating effects. However, in this study, no heating effects were observed during the MR imaging of any of the 19 coronary arterial stents. The degree of radio-frequency energy imposed on the stents in the current study was among the highest that can be achieved with current imaging units and normally is not used for routine MR imaging examinations. Most MR imaging units that are used clinically have lower magnetic field strengths and slower gradient systems and thus are associated with an even smaller risk of induced heating. However, with continuous progress in MR imaging technology, even faster gradient systems that allow higher spatial resolution and less imaging time and that minimize motion artifacts are being developed and introduced into clinical practice.
The determination of possible heating effects with an MR imaging tomograph is complex, because heating may be very localized and standard measurement systems cannot be used to detect minimum temperature changes. Heating measurements can be performed with different devices, such as a thermocouple, an infrared camera, or a fiberoptic temperature probe. The advantage of the fiberoptic probe used in this study was that continuous temperature measurement during exposure was possible. In our experiments, we were able to rule out the possibility of the fiberoptic probe acting like an antenna and absorbing power, which could have caused false measurements and the medium to heat. However, one cause of error with a fiberoptic probe could be that only localized temperature measurements at the tip of the probe in close contact with the stents and the adjacent medium can be performed. In addition, heating may occur with one particular radio-frequency pulse at one particular position within the imaging unit but not under minimally different conditions. Thus, the possible heating of a device could not be completely excluded in our measurements.
Theoretically, heating is caused by the radio-frequency energy of the MR imaging tomograph. If the resonance length (
/2 or /4) is achieved, the interaction between the radio-frequency energy and the conducting material may produce an electric field, which can introduce electric currents. These small electric currents can be potential causes of heating. However, in addition to our measurements, which were performed with a maximum degree of radio-frequency energy, from a theoretical standpoint, the temperature changes caused by MR imaging of small metallic objects appear to be negligible (36). Coronary arterial stents are too small to reach the resonance length needed to induce an electric current.
Artifacts
MR imaging artifacts are magnetic susceptibility artifacts caused by local magnetic field changes that are produced by the ferromagnetic, paramagnetic, and diamagnetic properties of the metallic devices (37). The mechanism of artifact production and the mechanism of image distortion are the same in these devices. Usually, there is good correlation between the ferromagnetic properties of a device and the size of the expected imaging artifacts. However, there can also be a divergence between the ferromagnetism of a device and the size of the produced imaging artifacts (37), as seen in our study of coronary arterial stents, which caused moderate MR imaging artifacts despite their lack of strong ferromagnetism.
The cause of imaging artifacts with nonferromagnetic metallic devices is thought to be a reduction in radio-frequency amplitude near the device, which depends on its shape (38). These effects are most pronounced at the regions of greatest magnetic field heterogeneity, and radio-frequency amplitude changes are most pronounced close to the edges and points of the metallic surface of the implants. The orientation of the major extent of the artifact of these nonferromagnetic devices is parallel to the frequency-encoding gradient direction (39). The B0 induces a dipole field in the stent, which changes the B0 (40). The shape of this B0 perturbation is determined by the relative orientation of the stent to the B0. The size of the B0 perturbation is maximized if the stent is oriented parallel to the B0.
The properties of 316 low-carbon stainless steel, the material with which most currently available stents are made, have been previously reviewed (11). This steel alloy is austenitic and nonmagnetic. However, the type of manufacturing used to create the complex shapes of coronary arterial stents may produce ferromagnetic properties within the stent. Compared with 304 low-carbon stainless steel, the 316 low-carbon variety has a higher nickel content (10%–14% versus 8%–12% in the 304 low-carbon variety). The nickel stabilizes the iron in a nonmagnetic state and therefore diminishes the occurrence of ferromagnetism in stents composed of 316 low-carbon stainless steel.
Because artifact size depends mainly on the geometry and composition of the stent material, voxel size is expected to have a minor effect on artifact size. With the typical in-plane resolution and section thickness used in this study, we expected a maximum error in length measurement of ±1 pixel at comparison among the different sequences.
As expected, the largest artifacts were observed with the echo-planar and TFE imaging sequences. One reason for the smaller artifacts with the TSE sequences could be the larger section thickness that was used. The 10-mm section thickness used in TSE imaging could have resulted in a smaller artifact size because of a greater partial volume effect. With the use of ultrafast gradients, the actual echo time was reduced, and this resulted in substantially smaller artifact sizes, especially with the echo-planar imaging sequences. With ongoing progress in MR imaging technology, shorter echo times, and thus further reductions in artifacts caused by stents, will be possible in the future.
In summary, our study results demonstrate that the evaluated coronary arterial stents are not significantly influenced by the 1.5-T magnetic field used in clinical MR imaging studies. No motion or heating effects were observed. A possible explanation is the relatively minor ferromagnetic nature of the stent materials, as well as the low mass and size of the devices, which results in minimal magnetic force being exerted on the stents. Coronary arterial stents generate susceptibility artifacts that extend in excess of the true size of the stents and make imaging of the underlying structures impossible. Artifact size differs according to the type and size of the stent and the MR imaging sequence used; the larger artifacts are seen with the larger and longer stents and with echo-planar imaging sequences. MR imaging of coronary arteries does not influence the regions beyond approximately 1 cm from the implantation site. Thus, routine MR imaging at 1.5 T with regular or ultrafast gradients does not induce motion or heating of the evaluated coronary arterial stents.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, J.H.; study concepts, J.H.; study design, E.N., J.H.; definition of intellectual content, E.N., J.H.; literature research, H.O.; clinical studies, J.H.; experimental studies, E.N., J.H.; data acquisition, B.S.; data analysis, H.O., B.S.; statistical analysis, H.O., A.B.; manuscript preparation, J.H.; manuscript editing, E.N., J.H.; manuscript review, E.F.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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