Dual-mode IVUS catheter for intracranial image-guided hyperthermia: feasibility study

Abstract

In this study, we investigated the feasibility of modifying 3-Fr IVUS catheters in several designs to potentially achieve minimally-invasive, endovascular access for image-guided ultrasound hyperthermia treatment of tumors in the brain. Using a plane wave approximation, target frequencies of 8.7 and 3.5 MHz were considered optimal for heating at depths (tumor sizes) of 1 and 2.5 cm, respectively. First, a 3.5-Fr IVUS catheter with a 0.7-mm diameter transducer (30 MHz nominal frequency) was driven at 8.6 MHz. Second, for a low-frequency design, a 220-μm-thick, 0.35 x 0.35-mm PZT-4 transducer--driven at width-mode resonance of 3.85 MHz--replaced a 40-MHz element in a 3.5-Fr coronary imaging catheter. Third, a 5 x 0.5-mm PZT-4 transducer was evaluated as the largest aperture geometry possible for a flexible 3-Fr IVUS catheter. Beam plots and on-axis heating profiles were simulated for each aperture, and test transducers were fabricated. The electrical impedance, impulse response, frequency response, maximum intensity, and mechanical index were measured to assess performance. For the 5 x 0.5-mm transducer, this testing also included mechanically scanning and reconstructing an image of a 2.5-cm-diameter cyst phantom as a preliminary measure of imaging potential.

Fig. 1

Vascular pathways to the brain for minimally invasive intracranial catheters. (a) Arterial route (Reproduced with permission from Matthew Holt and the Internet Stroke Center.): common carotid artery (CCA) [62] to middle cerebral artery (MCA). (b) Venous route (Reproduced with permission from Dean Falk, Florida State University.): internal jugular vein (IJV) to dural venous sinuses (DVS).

Fig. 2

Field II beamplot simulation at 8.7 MHz for 0.7-mm diameter SoniCath transducer. (a) Aperture footprint (one-half millimeter gridline spacing). (b), (d) Simulated relative pressure amplitude plots: zero-elevation plane and 4-mm-deep plane, respectively. (c) Normalized beamplot at 4 mm deep, truncated at half-maximum.

Fig. 3

Field II beamplot simulation at 3.5 MHz for 0.35 × 0.35-mm PZT-4 transducer for Atlantis retrofit. (a) Aperture footprint (one-half millimeter gridline spacing). (b), (d) Simulated relative pressure amplitude plots: zero-elevation plane and 4-mm-deep plane, respectively. (c) Normalized beamplot at 4 mm deep, truncated at half-maximum.

Fig. 4

Field II beamplot simulation for 5 × 0.5-mm PZT-4 transducer for flexible 3-Fr IvUs. (a) aperture footprint (one-half millimeter grid-line spacing). Simulated relative pressure amplitude beamplots at 8.7 and 3.5 MHz, respectively: (b), (f) zero-elevation plane; (c), (g) zero-azimuth plane; (e), (i) 4-mm-deep plane. (d), (h) normalized beamplots at 4 mm deep, truncated at half-maximum, at 8.7 and 3.5 MHz, respectively.

Fig. 5

(a) Boston Scientific SoniCath 0.7-mm-diameter transducer secured in test lumen. (b) Pulse-echo impulse response and frequency spectrum.

Fig. 6

(a) Boston Scientific Atlantis retrofitted with 220-μm-thick, 0.35 × 0.35-mm PZT-4 transducer. (b) Pulse-echo impulse response and frequency spectrum, PiezoCAD calculation (green) and experimental measurement (blue). References to color refer to the online version of this figure.

Fig. 7

(a) A 220-μm-thick, 0.35 × 0.35-mm PZT-4 test transducer. (b) Pulse-echo impulse response and frequency spectrum, PiezoCAD calculation (green) and experimental measurement (blue). References to color refer to the online version of this figure.

Fig. 8

(a) A 220-μm-thick, 5 × 0.5-mm PZT-4 test transducer. (b) Pulse-echo impulse response and frequency spectrum, PiezoCAD calculation (green) and experimental measurement (blue). References to color refer to the online version of this figure.

Fig. 9

Image of a 2.5-cm-diameter cyst phantom acquired with 5 × 0.5-mm PZT-4 test transducer. An 8.5 to 10 MHz bandpass filter and linear TGC were applied to the data.

Fig. 10

Output pressure from 0.35 × 0.35-mm PZT-4 test transducer with maximum-amplitude, 4-cycle driving pulse: (a) without L-section matching circuit, (b) with L-section matching circuit.

Fig. 11

Thermal model calculations for 0.7-mm-diameter SoniCath (continuous-wave W_o = 0.06 W). (a) Thermal beam radius versus depth. (b) Uniform (spatial-average) intensity versus depth for the thermal beam. (c) Resulting on-axis temperature rise in brain tissue.

Fig. 12

Calculated 4°C thermal penetration (8°C maximum) versus frequency for each aperture.