Intravascular Extended Sensitivity (IVES) MRI Antennas Robert C. Susil, 1 Christopher J. Yeung, 1 and Ergin Atalar 1–3 * The design and application of an intravascular extended sensi- tivity (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 an- tenna winding were characterized by theoretical and experi- mental 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, con- versely, 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 –1 Hz 1/2 ), 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 under- standing of the properties and behavior of the loopless antenna design. Magn Reson Med 50:383–390, 2003. © 2003 Wiley-Liss, Inc. Key words: MRI; vascular imaging; guidewire; loopless antenna; helical antenna; insulation; system SNR Traditionally, MR receivers have been designed as loop- type resonators that are placed on or around the surface of the body (1). To increase SNR in deep body tissues, small- loop and opposed solenoid coils were subsequently inte- grated into catheters and positioned intravascularly, closer to the target tissue (2– 4). However, because they require tuning and matching elements to be placed inside the body, these receivers are not ideal for small, linear catheter structures. In addition, signal sensitivity for small-loop receivers falls off very rapidly (1/r 3 , where r is the radial distance from the loop) (5). To improve longitudinal cov- erage, long-loop intravascular antennas were subsequently investigated (6). For these long, narrow loop receivers (in which the loop length is much greater than its width), sensitivity falls off as 1/r 2 . Despite improved longitudinal coverage, these receivers produce limited SNR when con- ductor separation is small (as occurs when these loops are integrated into narrow catheters) and, like the small-loop receivers, tuning should be performed near the loop for best SNR performance. To improve conductor separation and SNR, long-loop receivers have also been integrated into inflatable balloon structures (7). In an effort to address the limitations of intravascular loop antennas, loopless MR antennas were designed (8). Because of their simple, linear structure, these antennas are easily incorporated into catheters and other interven- tional devices. Moreover, tuning and matching of loopless antennas can be performed outside the body without sig- nificantly degrading SNR performance, further simplifying antenna design. Sensitivity falls off as 1/r for these anten- nas, which is an improvement over loop receivers. Loop- less antennas have been investigated for applications in coronary angioplasty (9), coronary catheterization (10), pe- ripheral angioplasty (11), and for characterization of ath- erosclerotic plaques (12). The behavior of the loopless antenna has been charac- terized for the simplified case of a linear, bare antenna placed in a homogeneous, conductive medium (8). How- ever, more complicated loopless antennas, including an- tennas with dielectric insulation and those wound into a helical shape, are not well understood. Previous empirical work has suggested that insulation and winding may be useful for improving the mechanical and imaging proper- ties of loopless antennas (13). In this study, an intravascular extended sensitivity (IVES) loopless antenna design that incorporates both an insulating (dielectric) coating and winding of the antenna into a helical shape is described. In order to describe the behavior and design of this antenna, we first characterize the effects of insulation and helical winding of the antenna on SNR (insulation broadens the SNR distribution, and winding allows for improved SNR near the tip of the antenna). The properties of insulation and antenna wind- ing are then applied to the design of an IVES loopless antenna. This antenna is demonstrated in vivo and, be- cause of its broad sensitivity profile, is shown to be useful for imaging long, vascular structures. METHODS Insulation of Loopless Antennas In the first part of this study, the theoretical SNR perfor- mance of various insulated, loopless antennas was exam- ined. SNR can be evaluated by calculating the antenna’s system SNR  S (14):  s =  2M o | H + |  4k B R Effective TF [1] where  is the Larmor frequency,  is the magnetic per- meability of the sample, M o is the total transverse nuclear 1 Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland. 2 Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland. 3 Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey. Grant sponsor: NIH; Grant numbers: RO1-HL-61672; RO1-HL-57483. *Correspondence to: Ergin Atalar, Ph.D., Johns Hopkins University, Traylor Bldg., Rm. 330, 720 Rutland Ave., Baltimore, MD 21205. E-mail: eatalar@mri.jhu.edu Received 31 October 2002; revised 10 February 2003; accepted 12 March 2003. DOI 10.1002/mrm.10506 Published online in Wiley InterScience (www.interscience.wiley.com). Magnetic Resonance in Medicine 50:383–390 (2003) © 2003 Wiley-Liss, Inc. 383