Clin. Cardiol. 18, 175-177 (1995)
`
`Short Communlcatlons
`Locahzation of a Misplaced Coronary Artery Stent by Magnetic Resonance
`Imaging
`
`h A I ) H . MOHIADDIN,M.D.,ROBINH. ROBERTS,M.R.C.P.,* &CHARDUNDERWOOD,M.D., M.R.C.P.,MARTINROTHMAN,F.R.C.P.*
`
`Magnetic Resonance Unit, The Royal Brompton National Heart & Lung Hospitals; *Department of Cardiology, The London Chest
`Hospitd, London, England
`
`Summary: Coronary artery stem have been developed to over-
`come arterial abrupt closure and restenosis following balloon
`angioplasty. Complications of stent insertion include loss of the
`device from its delivery system into the peripheral circulation.
`Certain types of stents are almost radiolucent, making localiza-
`tion of the lost devices difficult. Nonferromagnetic metallic
`biomedical implants induce alteration of the local magnetic
`field and this leads to loss of signal from the surrounding tis-
`sues. We have used this property to localize a misplaced coro-
`nary artery stent in a 53-year-old man who underwent unsuc-
`cessful stent insertion. A 0.5 Tesla magnetic resonance scanner
`was usad to acquire gradient-echo and spin-echo images. An in
`vitro experiment was first carried out on a stent similar to that
`used in our patient to establish that it was nonferromagnetic and
`to determine the optimum imaging technique. Gradient-echo
`images with a relatively long echo time (22 ms) gave the largest
`area of signal loss around the stent, and this sequence was used
`for localization of the stent found in the patient's left profunda
`femoris artery. This was subsequently confirmed by digital ra-
`diography. We have demonstrated the convenience and practi-
`cality of using magnetic resonance imaging for the localization
`of a misplaced coronary artery stent in a patient. The technique
`is safe, noninvasive, and uses no ionizing radiation.
`
`Key words magnetic resonance imaging, stent, coronary artery
`
`Introduction
`Coronary artery stents have been developed to overcome ar-
`terial abrupt closure and restenosis following balloon angio-
`
`Address for reprints:
`
`Raad H Mohiaddin, M.D.
`Magnetic Resonance Unit
`Royal Brompton National Heart and Lung Hospital
`Sydney St.
`London SW3 6NP, U.K.
`
`Received: December 22, 1993
`Accepted with revision: April 5 , 1994
`
`plasty.14 Complications of stent insertion include loss of the
`device from its delivery system into the peripheral circulation.
`To our knowledge, no reports of attempts to localize lost stents
`exist, which may reflect the perceived difficulty in finding the
`devices, some of which, once deployed, are virtually radiolucent.
`Certain ferromagnetic biomedical implants like cerebral
`aneurysm clips are considered to be contraindications to mag-
`netic resonance imaging (MFU) because of the possibility of
`injury if the implant is dislodged. If the implant has little or no
`ferromagnetism, this risk is negligible and it is generally ac-
`cepted that MRI is
`Nonferromagnetic metallic biomedical implants are not vis-
`ible by MRI, but they induce alteration of the local magnetic
`field leading to loss of signal from the surrounding tissues. The
`signal loss is small for spin-echo images, and neighboring struc-
`tures normally are seen, but the defect is much larger in gradi-
`ent-echo images.
`We have used this property to localize a misplaced coronary
`artery stent.
`
`Case Report
`A 53-year-old man underwent quadruple-vessel coronary
`bypass surgery (CABG) in 1981. Anginarecurredin 1991 and
`symptoms worsened over a %month period despite medical
`therapy. Cardiac catheterization in February 1992 revealed a
`patent right coronary artery vein graft and a patent but stenosed
`left anterior descending artery vein graft, with both grafts in-
`serting beyond native coronary occlusions; the other grafts were
`occluded. There was minor circumflex artery disease, but a tight
`stenosis proximally in a large intermediate coronary artery.
`Percutaneous transluminal coronary angioplasty, performed in
`May 1992, achieved successful dilatation of the left anterior
`descending artery vein graft, complicated by local dissection.
`The intermediate vessel was then dilated prior to the insertion
`of a coronary artery stent (Johnson and Johnson Interventional
`System CPS stent). Unfortunately, during stent placement, the
`guiding catheter became displaced and the stent was lost, be-
`ing swept from the aortic root into the peripheral vascular tree.
`The Palmaz-Schatz balloon expandable stent (Johnson and
`Johnson, New Brunswick N.J., USA) is a relatively rigid slot-
`ted tube made of stainless steel, 15 mm in length, comprising
`two 7 mm mesh segments articulated by a 1 mm bridge to in-
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`Clin. Cardiol. Vol. 18. March 1995
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`crease flexibility and to facilitate deployment in tortuous ves-
`sels. The collapsed stent is 1.6 mm in diameter and can be ex-
`panded to 5 mm. It is normally visible on digital angiography
`when undilated, but invisible after dilation.
`Following angioplasty the patient remained well, with no
`chest pain or ECG changes and with no neurological or pe-
`ripheral vascular signs of embolization. He was anticoagulat-
`ed with heparin for 72 h and discharged from the hospital on
`warfarin, aspirin, and dipyridamole. MRI was performed as
`described below and the stent was localized in the left profon-
`da femoris artery. Because of the position of the stent and be-
`cause the patient was asymptomatic, no further action was tak-
`en. Unfortunately he had to be admitted repeatedly with un-
`stable angina, and in September 1992 he underwent repeat
`CABG but developed postoperative heart failure. He under-
`went a successful heart transplant in January 1993 and is cur-
`rently free of cardiac symptoms.
`
`Magnetic Resonance Imaging
`A Picker International Vista MR 2055 scanner operating at
`0.5 Tesla was used with a surface receiver coil.
`
`In Vitm Study
`Appropriate imaging sequences were chosen from a prelimi-
`nary in vitro experiment. A stent similar to that used in our pa-
`tient was suspended in the air with a silk thread to assess its de-
`flection in the static magnetic field. It was then immersed in a
`glass of water, and imaging parameters that maximized the area
`of signal loss were determined. Multislice gradient-echo (echo
`time, 22 and 14 ms) and spinecho (echo time 40 ms) images were
`acquired in coronal and transverse planes. Slice thickness was
`10 mm and field of view 30 cm with a resolution of 256 pixels in
`the frequency-encoding direction and 256 pixels in the phase-en-
`coding direction. Each phase-encoding step was acquired twice
`and averaged. Magnitude and phase images were reconstructed.
`There was no deflection of the stent in the static magnetic
`field. The area of signal loss was greatest in the gradient-echo
`images and with longer echo times (Fig. I). The phase images
`were distorted around the stent because of magnetic suscepti-
`bility artefact.
`
`Clinical Study
`The patient was then imaged using multislice gradient-echo
`(echo time 22 ms) and spin-echo (echo time 40 ms) sequences.
`Both magnitude and phase images were reconstructed. Images
`were acquired in coronal and transverse planes through the
`pelvis and upper part of the lower extremities without electro-
`cardiographic gating. Slice thickness was 10 mm and field of
`view was 45 cm with a resolution of 256 pixels in the frequen-
`cy-encoding direction and 128 pixels in the phase-encoding
`direction. Each phase-encoding step was acquired twice and
`averaged.
`Gradient-echo magnitude images (echo time of 22 ms) ac-
`quired in coronal and transverse planes showed an area of sig-
`nal loss in the left profunda femoris artery, 17 cm distal to the
`
`FIG. 1 Coronary artery stent (Johnson and Johnson CPS 15) attached
`to a silk thread and immersed in a glass of water, imaged with a gra-
`dient-echo sequence with an echo time of 22 ms (A), and 14 ms (B),
`showing an area of signal loss around the stent. A corresponding phasc
`image (C) and spin-echo image (D) in a plane similar to (A).
`
`greater trochanter of the left femur. There was also a phase di+
`tortion around the area. The appearance was compatible with
`the presence of a small metallic object. The spin-echo images
`showed minimal distortion (Fig. 2). The position of the stent
`was confirmed by digital radiography (Fig. 3).
`
`Discussion
`To our knowledge, this is the first description of the local-
`ization of a misplaced coronary stent by MRI. While such stents
`may cause no clinical symptoms, they are a potential source of
`thrombus or emboli, and so it is important to know the site of
`the stent for patient management. In extreme cases, surgical re-
`moval may be required. The undilated stent can be seen by dig-
`ital x-ray fluoroscopy, but conventional fluoroscopy may be un-
`rewarding and fluoroscopy of the whole patient involves
`considerable exposure to radiation.
`MRI is limited mainly in the abdomen and chest, where sig-
`nal loss can be caused by gas in the bowel and lungs. Metallic
`clips used during coronary bypass grafting can also create areas
`of signal loss that would be difficult to differentiate from that of'
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`R. H. Mohiaddin et al.: Localization of misplaced coronary artery stent by MRI
`
`177
`
`FIG. 2 Gradient-echo images (echo time 22 ms) in coronal (A) and transverse (B) planes through the thighs of our patient, showing the signal
`loss caused by the stent in the left profunda femoris artery (arrows). A corresponding phase map (C) and spin-echo image (D) in a plane similar
`to (B). 1 = right profunda femoris artery, 2 =right profunda femoris vein.
`
`Conclusion
`
`We have presented the method and demonstrated the con-
`venience and practicality of using magnetic resonance imag-
`ing for the localization of a misplaced coronary artery stent in
`a patient. The technique is safe, noninvasive, and does not use
`ionizing radiation.
`
`References
`1. Dotter CT, Judkins M P Transluminal treatment of atherosclerotic
`obstruction: Description of a new technique and preliminary report
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`TM, Beauchamp GD, Giorgi LV, Franco I, Toole JC, Glover MU,
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`al on prevention of restenosis: Early results and complications (ab-
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`457 (1987)
`5. Soulen R, Higgins CB, Budinger T F Magnetic resonance imaging
`of prosthetic cardiac valves. Radiology 154,705-707 (1985)
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`7. Shellock FG: MR imaging of metallic implants and materials: A
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`
`Rc;. 3 Digital radiography showing the stent in the left profunda
`femoris artery (arrow).
`
`a stent. The size of coronary stents means thac if embolized they
`are likely to lodge in small diameter, peripheral arteries where
`these factors are less of a problem. Another limitation of MRI is
`its relatively long acquisition time. This study required two sep-
`arate IS-min acquisitions to cover the pelvis and the upper part
`of the lower extremities. A similar time would have been requir-
`ed to image other anatomical regions if the stent had not been
`localized in the leg. Electrocardiographic gating is required to
`image the chest and this further prolongs the scanning time. The
`confined bore of the magnet may preclude use of the technique
`in patients with claustrophobia. These limitations may become
`less important with the development of real-time and subsecond
`MRl techniques and open access magnets.
`
`Petitioner Edwards Lifesciences Corporation - Exhibit 1020 - Page 3
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