Debatin, Jörg F.

Erhart, Peter

Göhde, Susanne C.

Ladd, Mark E.

McKinnon, Graeme C.

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Guidewire Antennas for MRI-Guided Vascular InterventionsMark E. Ladd, Peter Erhart, Susanne C. Göhde, Jörg F. Debatin, Graeme C. McKinnonDepartement Medizinische Radiologie, MR-Zentrum, Universitätsspital Zürich, CH-8091 Zürich, SchweizIntroductionBecauseofitsexcellentsofttissue contrast, ability toimage in an arbitrary plane, and lack of ionizingradiation, great interest has been shown in using MRIto guide vascular interventions. Before a catheter can besafely introduced, a guidewire needs to be placed. Unlikeconventional X-ray fluoroscopy, however, the ability toquickly and reliably visualize thin objects likeguidewires has proven to be difficult. One solution isthe use of active receive antennas built into theguidewire tip (1).A number of antenna configurations are possible,including simple loop, solenoid, center retum, etc. (2).In order to understand and optimize these antennas, aseries of theoretical simulations and experimentalmeasurements were performed. In addition, the effect ofadding a passive signal source (e.g. water) 'internal tothe guidewire was explored.MethodsBiot-Savart modelling was used to calculate themagnetic field pattems of the following antenna types:single loop, crossed loop, 4-wire center return, andsolenoid. From these field patterns, sagittal-like imageswith a slice thickness of 5 mm were simulated with thecoils both parallel and perpendicular to the mainmagnetic field, B0, of the MR imager. The in-planeresolution was 0.1 mm in both directions.Prototype guidewires were constructed by SchneiderEurope AG. Each guidewire was constructed with a coilapproximately 60 mm long and 0.6 mm in diameter.After enclosing the guidewire in a Teflon sheath, thefinal outside diameter was 0.75 mm, small enough tofit into the lumen of a 5 French catheter.Water doped with Gd-DTPA (Schering AG) wasinjected into the prototype guidewires äs an intemal,passive signal source.ResultsFig. l shows a simulated image of a single loopguidewire with no signal source in the interior of thecoil. The coil is parallel to B0. Fig. 2 shows the samecoil with an internal signal source.Fig. 3 shows the profiles of image intensity from leftto right for both cases. The profiles show that theinternal source not only increases the available signal,it also narrows the apparent line width of the guidewire.Fig. L Simulated image of single loop guidewirewithout internal signal source.aamiafflB^pffifflflffif IBI^ IBBBBiFig. 2. Simulated image of single loop guidewirewith internal signal source.0-10 - 5 0 5Distance (mm)10Fig. 3. Profiles of image intensity across the aboveimages. Solid line = without intemal source, dashedline = with internal source.This is also desirable, since it provides betterlocalization. If the internal source is a substance with ashort relaxation time TI} then a third benefit is theability to scan with fast imaging sequences (shortTR's).206 Biomedizinische Technik · Band 41 · Ergänzungsband 1*1996Fig. 4 shows a magnified, experimental image of the [2] G.G. Hurst, et al.; Magn Reson. Med. 24, 343-357single loop prototype taken with the coil parallel to B0. (1992)Water/Gd-DTTPA was injected into the intenor of thecoil. The 6 cm tip of the guidewire i s depicted quitewell. Sequence parameters were TR 300 ms, TE 2.3 ms(fractional), flip angle 60°, FOV 20 cm, matrix512x512, l NEX.Fig. 4. Experimental image of single loop prototypeguidewire parallel to BQ.ConclusionThe selection of the appropriate guidewire coildepends on a combination of factors. The single loop,crossed loop, and center retum all perform better parallelto B0. The solenoid performs best orthogonal to B0.Thus, depending on where the intervenüon is to beperformed, one might choose one over the other.A second factor is ease of mechanical construction.The single loop and solenoid are both easy to constmct.The crossed loop and center retum are more difficultbecause four long wires need to be kept straight andequidistant. This proved problematic in the prototypes.The most deciding factor is the addition of a passiveSignal source in the highly sensitive area near the centerof the coil. This increases the SNR, reduces the linewith, and with Gd-DTPA induced Tl shortening,permits the use of fast imaging.AcknowledgmentsThis work was funded by General Electrical MedicalSystems, Milwaukee, Wisconsin, and the SwissNational Science Foundation, Grant Nr. 32-43284.95.References[1] M.E. Ladd, et al.; "Proc., ISMRM, FourthMeeting", 1736 (1996)Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996 207

Guidewire Antennas for MRI-Guided Vascular Interventions

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