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Diagnostics & Imaging
Figure 1: Resonant Stents in Iliac Arteries of Female Minipigs in Figure 2: Resonant Self-expanding Heart Valve with Delivery
1.5-Tesla Magnetic Resonance Imaging Siemens Magnetom Avanto System in Porcine Heart (left); Magnetic Resonance Image in
1.5-Tesla Philips Intera (right)
AB
A: The signal void in the right iliac stent proved to be a floating thrombus on the day of
implantation (TrueFisp, repetition time (TR) 48ms, echo time (TE) 1ms, 2D, flip angle (FA) 15°).
B: Excentric restenosis after 34 days (TR 53.8ms, TE 1.5ms, FA 70°). Stent-based prosthetic heart with porcine aortic heart valve sutured into the nitinol
stent (Memotherm, Bard Angiomed, Ø=18mm) with the resonator (repetition time
100ms, echo time 6ms, flip angle 15°).
produce local signal intensity enhancement by increasing the flip angle
(FA) of GE. The frequency of the resonator is defined by the capacity of Active Magnetic Resonance Imaging Self-expanding
the condenser and the inductivity of the coil.
3
Active MRI markers can be Prosthetic Heart Valves
attached to the medical instruments in a similar way to X-ray markers, for Heart valve prostheses cause significant artifacts in MRI images,
example using solenoid, meander, surface, saddle and other designs.
4
compromising MRI diagnostics of the valve function. Percutaneously
implantable heart valves are made of balloon-expanding stainless steel
Active Magnetic Resonance Imaging Resonant Stents stents or self-expanding Nitinol stents; only the latter are suitable for
Stents require post-interventional follow-up to detect intraluminal changes integration of resonant circuits.
such as thrombus or intimal hyperplasia. MRI examination would be the
favoured non-invasive imaging technique, but conventional metal stents Material and Methods
shield the lumen because of the metal itself and RF interference (Faraday Resonant circuits have been designed and integrated into Nitinol
gauge). The use of a resonant circuit tuned to the Larmor frequency and stent-based heart valve prostheses.
6
Porcine pulmonary and aortic
placed around the stent should improve visualisation of the stent lumen. heart valves were excised from a fresh porcine heart of a six-month old
Active MRI resonant stents provide non-invasive visualisation of instant pig and sutured with 6-0 Prolene (Ethicon Inc.) into the Nitinol stent
thrombosis and restenosis without the need for gadolinium contrast agents. (Memotherm, Bard Angiomed, Ø=18mm). A 50cm-long delivery
system has been custom-made using viton tube – a modified 12mm
Material and Methods laparoscopic disposable trocar (Tyco). The valve has been implanted
Self-expanding commercially available Nitinol stents have been equipped under MRI into a fresh porcine heart through a transapical approach
with a solenoid coil resonator in order to develop fully biocompatible (see Figure 2).
balloon-expandable stents, combine self-expanding stents with an
integrated microcoil resonator and evaluate MRI of the stent lumen. The Results
resonator was also applied as a supplement sheath (covered stent type) The heart valve could be visualised in the MRI by a low FA (15°). The
and directly mounted onto the stents. The lumen of the stent was signal difference between the valve and the test liquid was significant
partially filled with a thrombus of porcine blood and a section of a and the shielding of the Nitinol stent could be overcome. The heart valve
porcine aorta wall to simulate intimal hyperplasia; this was tested in could be successfully implanted in a porcine animal model approved by
1- and 1.5-Tesla MRI scanners. the Animal Care and Use Committee of the National Institutes of Health
(NIH) in 80–85kg female minipigs under general anaesthesia through a
Results minimally invasive transapical approach.
7
The resonator stimulates all substrates inside the stents and enhances the
signals from the Nitinol stent lumen. Fresh porcine aortic wall was rolled Conclusion
into the stent to simulate intimal hyperplasia, and fresh human blood The resonant circuit overcomes the shielding of the Nitinol heart valve
thrombus was placed in the stent to simulate instant thrombosis. frame, increases signal and signal to noise ratio (S/N) and provides high
Artificial stenosis and the thrombus were visualised in a 0.9% saline contrast without the need for contrast agents. Diagnostic and MRI-
water container (Philips 1.5-Tesla fast field echo (FFE), repetition time (TR) guided implantation of stents and stent-based heart valves can be
50ms, echo time (TE) 6ms, FA 20º). Balloon-expanding stents have been facilitated with the INNOMOTION robotic system, as the delivery system
tested in an animal trial in chinchilla rabbits
5
and in 45–60kg domestic pigs is kept stable and aligned to the image orientation.
approved by the animal board of the University of Essen. Under general
anaesthesia, a carotid artery port was implanted and the stents delivered Active Magnetic Resonance Imaging Vena Cava Filter
to the left and right iliac arteries under fluoroscopy. Subsequent MRI Implantation of VCFs is performed under X-ray fluoroscopy and
examination was performed at 1.5 Tesla. In the first animal, a signal void application of nephrotoxic X-ray contrast media. Commercially available
was discovered in the left iliac stent that proved to be a thrombus clot on VCFs consist of metals that produce artifacts in MRI and shield the filter
post mortem explantation of the stent (see Figure 1a). Eccentric 50% lumen from RF. To solve these problems, we have developed two MRI
restenosis was detected after 34 days of survival (see Figure 1b). Further active VCFs (cone and basket shapes) for visualisation in MRI-guided
results will be published elsewhere. delivery and non-invasive detection of captured thrombi.
8
22 INTERVENTIONAL CARDIOLOGY 2007
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