Hung Luyen

Microwave Ablation at 10.0 GHz Achieves Comparable Ablation Zones to 1.9 GHz in Ex Vivo Bovine Liver

We demonstrate the feasibility of using high-frequency microwaves for tissue ablation by comparing the performance of a 10 GHz microwave ablation system with that of a 1.9 GHz system. Two sets of floating sleeve dipole antennas operating at these frequencies were designed and fabricated for use in ex vivo experiments with bovine livers. Combined electromagnetic and transient thermal simulations were conducted to analyze the performance of these antennas. Subsequently, a total of 16 ablation experiments (eight at 1.9 GHz and eight at 10.0 GHz) were conducted at a power level of 42 W for either 5 or 10 min. In all cases, the 1.9 and 10 GHz experiments resulted in comparable ablation zone dimensions. Temperature monitoring probes revealed faster heating rates in the immediate vicinity of the 10.0 GHz antenna compared to the 1.9 GHz antenna, along with a slightly delayed onset of heating farther from the 10 GHz antenna, suggesting that heat conduction plays a greater role at higher microwave frequencies in achieving a comparably sized ablation zone. The results obtained from these experiments agree very well with the combined electromagnetic/thermal simulation results. These simulations and experiments show that using lower frequency microwaves does not offer any significant advantages, in terms of the achievable ablation zones, over using higher frequency microwaves. Indeed, it is demonstrated that high-frequency microwave antennas may be used to create reasonably large ablation zones. Higher frequencies offer the advantage of smaller antenna size, which is expected to lead to less invasive interstitial devices and may possibly lead to the development of more compact multielement arrays with heating properties not available from single-element antennas.

A Balun-Free Helical Antenna for Minimally Invasive Microwave Ablation

We present a balun-free coax-fed helical antenna for microwave ablation. The proposed antenna produces a localized specific absorption rate pattern at the desired frequency of operation without using a coaxial balun. This reduces the outer diameter of the antenna and, thus, its invasiveness. Balun-free operation of the proposed antenna is achieved by operating the helical antenna at the second resonant frequency of the helix where a current minimum occurs at the feed point, resulting in an intrinsically high feed-point impedance. This efficiently chokes the currents excited on the outer surface of the feeding cable. Using a compact quarter-wavelength impedance transformer or a coaxially implemented pi network of reactive elements at the antenna feed point, we achieve an excellent impedance match between the antenna and the main feeding line. We fabricated a prototype of the proposed helical antenna with an outer diameter of 2.2 mm using the pi matching network. We used this prototype to conduct ablation experiments in ex vivo bovine liver. The dimensions of the resulting ablation zones are similar to those produced by coaxially fed antennas that use coaxial baluns. Our balun-free antenna design offers a promising solution for reducing the invasiveness of interstitial antennas used in microwave ablation.

A minimally invasive, coax-fed microwave ablation antenna with a tapered-slot balun

We present a coax-fed interstitial dipole antenna with a tapered-slot balun for minimally invasive microwave ablation (MWA). The balun is created using two tapered slots in the outer conductor of the feeding coaxial cable. This implementation allows for reducing the overall antenna diameter compared to coax-fed MWA antennas employing conventional coaxial baluns. The tapered balun connects the feed line to a dipole that consists of three segments. One segment is an extension of the inner conductor and constitutes one arm of the dipole. The other two segments are connected to the outer conductor of the coax cable and are located in the tapered slots; these segments constitute the second dipole arm. The tapered balun efficiently chokes the currents excited on the outer surface of the feeding cable and provides a good impedance match between the dipole and the feed line, as demonstrated by simulation and measurement results. The proposed antenna offers a less invasive alternative to conventional MWA antennas.

Tissue ablation at 10 GHz vs. 1.9 GHz: Ex vivo experiments demonstrate comparable ablation zones

We demonstrate the feasibility of using high-frequency microwaves for tissue ablation by comparing the performance of a 10 GHz microwave ablation system with that of a 1.9 GHz system. Two floating sleeve antennas operating at these frequencies were designed and fabricated for use in ex vivo experiments with bovine liver. Each ablation experiment was conducted at an RF power level of 50 W for approximately 15 minutes. The 10 GHz and 1.9 GHz experiments resulted in comparable ablation zone dimensions (7.5 cm × 5cm and 9.5 cm × 6cm, respectively). These experiments show that, contrary to prior consensus, high-frequency microwave antennas may be in fact used to create reasonably large ablation zones. Higher frequencies offer the advantage of smaller antenna size, which leads to less intrusive interstitial devices or more compact multi-element arrays with uniform heating patterns.

Non-coaxial-based microwave ablation antennas for creating symmetric and asymmetric coagulation zones

We present an investigation of a new class of microwave ablation (MWA) antennas capable of producing axially symmetric or asymmetric heating patterns. The antenna design is based on a dipole fed by a balanced parallel-wire transmission line. The angle and direction of the deployed dipole arms are used to control the heating pattern. We analyzed the specific absorption rate and temperature profiles using electromagnetic and thermal simulations. Two prototypes were fabricated and tested in ex vivo ablation experiments: one was designed to produce symmetric heating patterns and the other was designed to generate asymmetric heating patterns. Both fabricated prototypes exhibited good impedance matching and produced localized coagulation zones as predicted by the simulations. The prototype operating in porcine muscle created an ∼10 cm3 symmetric ablation zone after 10 min of ablation with a power level of 18 W. The prototype operating in egg white created an ∼4 cm3 asymmetric ablation zone with a directionality ratio of 40% after 5 min of ablation with a power level of 25 W. The proposed MWA antenna design shows promise for minimally invasive treatment of tumors in various clinical scenarios where, depending on the situation, a symmetric or an asymmetric heating pattern may be needed.We present an investigation of a new class of microwave ablation (MWA) antennas capable of producing axially symmetric or asymmetric heating patterns. The antenna design is based on a dipole fed by a balanced parallel-wire transmission line. The angle and direction of the deployed dipole arms are used to control the heating pattern. We analyzed the specific absorption rate and temperature profiles using electromagnetic and thermal simulations. Two prototypes were fabricated and tested in ex vivo ablation experiments: one was designed to produce symmetric heating patterns and the other was designed to generate asymmetric heating patterns. Both fabricated prototypes exhibited good impedance matching and produced localized coagulation zones as predicted by the simulations. The prototype operating in porcine muscle created an ∼10 cm3 symmetric ablation zone after 10 min of ablation with a power level of 18 W. The prototype operating in egg white created an ∼4 cm3 asymmetric ablation zone with a directionality...

A Minimally Invasive Coax-Fed Microwave Ablation Antenna With a Tapered Balun

We present a minimally invasive coax-fed dipole antenna with a tapered slot balun for microwave ablation (MWA) applications. The balun is created by gradually tapering the outer conductor of the coaxial feed line into two parallel strips, leaving two tapered slots on the outer conductor. Implementing the balun within the outer conductor of the coax itself helps reduce the overall diameter of the antenna compared to interstitial antennas that use conventional coaxial baluns. The tapered-slot balun connects the dipole antenna comprised of three active segments to the coaxial feed line. One active segment is defined by an extension of the inner conductor of the coaxial cable, acting as one arm of the dipole. The other two active segments are located in the two slots created by tapering the outer conductor and connected to its distal end to constitute the second arm of the dipole. We designed the antenna to operate at 6 GHz in ex vivo bovine liver and fabricated a prototype to conduct ablation experiments. Simulation and experiment results show that the tapered balun provides good impedance matching between the coaxial feed line and the dipole and helps the antenna achieve localized ablation zones. The proposed antenna offers a promising solution for reducing the overall diameter, and the invasiveness, of coax-fed interstitial antennas used for MWA.

A balun-free coax-fed helical antenna for minimally invasive microwave ablation

We present a coax-fed helical antenna for microwave ablation of tumors. In contrast to previously reported coaxial antenna designs, the proposed antenna achieves a localized specific absorption rate pattern at the desired frequency of operation without using a coaxial balun. Such baluns increase the outer diameter, and thus the invasiveness, of the antenna. Balun-free operation in the proposed antenna is achieved by operating the helical antenna at a frequency where its feed point impedance is intrinsically high. This efficiently chokes the currents excited on the outer surface of the feeding cable. Using a compact impedance matching network at the antenna feed point, we achieve an excellent impedance match between the antenna and the main feeding transmission line as demonstrated by the simulation and measurement results. This balun-free antenna design offers a promising solution for reducing the invasiveness of interstitial devices in microwave ablation.