Browsing by Subject "Loopless antenna"

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Open Access
Interventional MRI: tapering improves the distal sensitivity of the loopless antenna
(Wiley, 2010) Qian, D.; El-Sharkawy, A. M. M.; Atalar, Ergin; Bottomley, P. A.
The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an ∼400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-tonoise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.
Open Access
Intravascular extended sensitivity (IVES) MRI antennas
(John Wiley & Sons, Inc, 2003) Susil, R. C.; Yeung, C. J.; Atalar, Ergin
The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL-1Hz1/2), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design. © 2003 Wiley-Liss, Inc.
Open Access
Tracking planar orientations of active MRI needles
(John Wiley & Sons, Inc., 2007) Sathyanarayana, S.; Aksit P.; Arepally, A.; Karmarkar, P. V.; Solaiyappan, M.; Atalar, Ergin
Purpose: To determine and track the planar orientation of active interventional devices without using localizing RF microcoils. Materials and Methods: An image-based tracking method that determines a device's orientation using projection images was developed. An automated and a manual detection scheme were implemented. The method was demonstrated in an in vivo mesocaval puncture procedure in swine, which required accurate orientation of an active transvascular needle catheter. Results: The plane of the catheter was determined using two projection images. The scan plane was adjusted automatically to follow the catheter plane, and its orientation with respect to a previously acquired target plane was displayed. The algorithm facilitated navigation for a fast and accurate puncture. Conclusion: Using image-based techniques, with no mechanical design changes, the orientation of an active intravascular probe could be tracked.