ARTICLE
Clinical safety of magnetic resonanceimaging early after coronary artery stent placement
Thomas C. Gerber, MD, FACC*,*,
Panayotis Fasseas, MD ,
Ryan J. Lennon, MS ,
Venkata U. Valeti, MD,
Christopher P. Wood, MD ,
Jerome F. Breen, MD and
Peter B. Berger, MD, FACC
* Division of Cardiovascular Diseases and Department of Radiology, Mayo Clinic, Jacksonville, Florida, USA
Division of Cardiovascular Diseases and Internal Medicine, Rochester, Minnesota USA
Division of Biostatistics, Rochester, Minnesota USA
Department of Radiology, Mayo Clinic, Rochester, Minnesota, USA
Manuscript received February 6, 2003;
revised manuscript received May 21, 2003,
accepted May 30, 2003.
* Reprint requests and correspondence: Dr. Thomas C. Gerber, Division of Cardiovascular Diseases, Mayo Clinic, 4500 San Pablo Road, Jacksonville, Florida 32224, USA.
gerber.thomas{at}mayo.edu
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Abstract
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OBJECTIVES: Our aim was to examine the rate of adverse cardiac events in patients undergoing magnetic resonance imaging (MRI) <8 weeks after coronary stent placement.
BACKGROUND: The risk of coronary stent thrombosis from dislodgement due to MRI early after stent placement is not well defined. Manufacturers recommend postponing MRI studies until eight weeks after coronary stent placement.
METHODS: We analyzed the Mayo Clinic Rochester Percutaneous Coronary Intervention Database and examined records of 111 patients who underwent MRI <8 weeks after coronary stent placement treated with aspirin and a thienopyridine. Occurrence of death, myocardial infarction (MI), and repeat revascularization within 30 days of MRI were recorded.
RESULTS: Magnetic resonance imaging (1.5 tesla) was performed within a median of 18 days (range, 0 to 54 days) after coronary stent placement. Four noncardiac deaths occurred, and three patients had repeat revascularization procedures. Stent thrombosis did not occur (95% confidence interval, 0% to 3.3%).
CONCLUSIONS: Magnetic resonance imaging <8 weeks after coronary stent placement appears to be safe, and the risk of cardiac death or MI due to stent thrombosis is low. Postponing MRI does not appear to be necessary.
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Abbreviations and Acronyms
| | CAS(s) | = coronary artery stent(s) | | MI | = myocardial infarction | | MRI | = magnetic resonance imaging |
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Magnetic resonance imaging (MRI) is considered unsafe in the presence of many metallic cardiovascular devices because it may cause dislodgement of the device by ferromagnetic force, device heating, flow of electrical currents, or malfunction of a device's electrical system (1–3). Every year approximately 457,000 metallic coronary artery stents (CASs) are placed (4). In patients with CASs, MRI is believed to be safe once endothelialization (5) has occurred, because endothelialization presumably opposes possible dislodgement (3). Therefore, manufacturers of CASs (6,7) and professional associations of cardiologists (8) recommend postponing elective MRI examinations for four to eight weeks after stent placement.
However, the need for urgent or emergent MRI can arise within this period. Opinions differ about whether it is safe to perform MRI <8 weeks after CAS placement (9,10), and physicians may choose to avoid MRI because of safety concerns. Dislodgement could theoretically increase the exposure of metallic coronary stent material to platelets, which may trigger stent thrombosis, resulting in cardiac death, myocardial infarction (MI), or emergent revascularization. In vitro and animal studies (11–14) have shown that ferromagneticity and stent migration are absent or minimal with MRI of currently available stents. However, very few clinical data are available on the outcome of patients who undergo MRI early after CAS placement (15,16).
In addition to their clinical importance, data on the safety of MRI early after CAS placement are also needed for designing clinical studies that use MRI (e.g., to compare the efficacy of different treatment modes for acute MI or to assess the status of CASs) (16–18). Therefore, we retrospectively examined the rates of adverse cardiac events occurring within 30 days after MRI performed <8 weeks after placement of a CAS.
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Methods
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Patients.
We analyzed the Mayo Clinic Rochester Percutaneous Coronary Intervention Database, which contains demographic, clinical, angiographic, and procedural data. Immediate and in-hospital events are recorded, and all patients are contacted for follow-up at six and 12 months and yearly thereafter. For the present analysis, we identified all patients who had CAS placement between July 1, 1994, and November 30, 2001. After CAS placement, all patients had received a thienopyridine for no longer than four weeks in addition to aspirin. By cross-referencing the archive system of the Mayo Clinic Department of Radiology, we identified all patients who had undergone an MRI examination <8 weeks after CAS placement. If more than one CAS placement preceded MRI, the time interval to MRI was counted from the latest CAS placement.
The study was approved by the Mayo Foundation Institutional Review Board. Patients who denied access to their medical record for research purposes (n = 4) were excluded.
Outcomes.
We queried the database for the occurrence of death from any cause, MI, or repeat revascularization procedures within 30 days after the MRI examination. If more than one MRI was performed <8 weeks after CAS placement, the follow-up period was extended to 30 days from the latest MRI. An investigator reviewed medical records to establish the nature of clinical events recorded in the database. For patients who died, clinical data and autopsy reports were reviewed, and death was classified as cardiac or noncardiac. For repeat percutaneous coronary interventions, catheterization reports and films were reviewed.
An MI was diagnosed if any two of the following three criteria were met: 1) "typical" chest pain that lasted  20 min; 2) an elevation of creatine kinase or of its MB isoenzyme 2 times the upper limit of normal; or 3) new Q-waves or ST-T segment changes suggestive of MI on an electrocardiogram.
Stent thrombosis was considered to have occurred if at least one of the following was present: 1) confirmation on autopsy or by coronary angiography; 2) MI in the territory of the treated vessel without definite exclusion of stent thrombosis; 3) sudden death without a clear noncardiac cause.
Statistical analysis.
Data are presented as mean ± SD, median, or percentages. Exact binomial confidence intervals were calculated.
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Results
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Of 112 patients who underwent CAS placement followed by MRI within eight weeks and were eligible for the study, one was lost to follow-up. The remaining 111 (mean age, 67 ± 12 years) had a median of two stents placed (range, one to six). Comorbid conditions and angiographic and procedural characteristics are shown in Table 1.
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Table 1 Baseline Clinical and Procedural Characteristics of the Study Population at Time of Coronary Artery Stent Placement (n = 111)
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All MRI examinations were performed on one of two types of 1.5-tesla MRI units (Signa, General Electric Medical Systems, Waukesha, Wisconsin, or Gyroscan, Philips Medical Systems, Best, The Netherlands). During the eight weeks after CAS placement, 128 MRI examinations were performed. The following body parts were imaged: head or neck, 50 (39%); spine, 27 (21%); abdomen or pelvis, 18 (14%); extremities, 13 (10%); chest, 11 (9%); and combined examination of several body parts, 9 (7%). Fifteen patients had >1 MRIs (13 patients had 2; 2 patients had 3).
The mean time interval from CAS placement to MRI was 21 ± 17 days (median, 18; range, 0 to 54 days). Magnetic resonance imaging was performed within two days after CAS placement in 15 patients (14%) and within 14 days in 52 patients (47%).
Stent thrombosis as defined by the study criteria did not occur. The 95% exact confidence interval for no events in 111 patients is 0% to 3.3%. Seven clinical events occurred in six (5%) of the 111 patients during the 30 days after MRI (Table 2), including four noncardiac deaths and three repeat revascularization procedures (all percutaneous coronary interventions).
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Table 2 Clinical Events During 30-Day Follow-up in Six of 111 Patients Who Underwent MRI Within Eight Weeks of Coronary Stent Placement
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Discussion
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This study shows that the risk of cardiac death, MI, or need for repeat revascularization due to stent thrombosis associated with MRI performed <8 weeks after CAS placement is very low. The results are consistent with the 30-day cardiac event rates (0.5% to 1.9%) after CAS placement with contemporary antiplatelet therapy in patients not undergoing MRI (19).
MRI after coronary stent placement.
In a series of 13 patients who underwent MRI 3 ± 1 days after CAS placement for MI, no early adverse cardiac events occurred in the postinfarct period (16). In other small observational studies with longer intervals from elective CAS placement to MRI, stent thrombosis was also rare (15,17).
Our study includes the largest series to date of patients who underwent MRI before CASs could be fully endothelialized (5). For all MRIs, the heart was at or near the isocenter of the magnet, and therefore, the CAS were exposed to the scanners' full magnetic field strength and spatial gradient. Our patients underwent MRI early after CAS placement, contrary to conventional clinical practice, because of severe comorbidity (Table 1). Accordingly, all deaths during the follow-up period resulted from withdrawal of respiratory support or from stroke (Table 2).
The prevalence in our study group of factors known to be associated with increased risk of stent thrombosis (19,20) (Table 1) suggests that our patients were at least at intermediate risk of adverse cardiac events. Therefore, the absence of CAS thrombosis in our study cannot be attributed to selection bias toward patients with low procedural risk. The three repeat percutaneous coronary interventions were performed remote from the original treatment sites (n = 2) or for in-stent restenosis (n = 1), which was easily distinguished from stent thrombosis by its far less acute course (21).
Study limitations.
Without complete 30-day angiographic follow-up, we cannot exclude the possibility that the presence of subclinical stent thrombosis was not detected in some patients with concurrent, critical illness. However, subclinical stent thrombosis is believed to be a very rare occurrence, and the close observation of this patient group warranted by their comorbid conditions may actually have resulted in higher than normal sensitivity for detecting cardiac events.
Despite the large number of patients in this series, the confidence intervals remain wide, and a stent thrombosis rate as high as 3.3% cannot be excluded with certainty. In addition, the MRI field strength in our study was limited to 1.5 T. Thus, it may not be appropriate to extrapolate from our data to newer, higher field strength MRI scanners.
Nevertheless, our data suggest that MRI with field strengths up to 1.5 T can be performed safely <8 weeks after CAS placement. Current clinical practice and recommendations by manufacturers of CASs to postpone MRI studies until after eight weeks from CAS placement do not seem necessary.
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Abstract
Methods