Optimization and characterization of a novel internally-cooled radiofrequency ablation system with optimized pulsing algorithm in an ex-vivo bovine liver

Figures & data

Table 1. Summary of ablation parameters studied.

Electrode exposure (cm)	Duration (minutes)		Target current I0 (mA)
			100	400	450	500	800	1000	1100	1200	1400	1600	1800	1900	2000	2200	2300	2400	2500
1	3			3				4
	6			3
	12			3	3	3		4
	15			3
2	3							4
	6							6
	12		6				3	4	4	4			4			4
	15							4
3	3							4			4		4
	6							4		4	4	4	4
	12		3			6		6		6	4	4	4		3	4		4	4
4	3												4
	6												4
	12		3					6		4	5	4	4	4	6	4	4	4	4
	15												4
5	6																4
	9																4
	12														4	3	3		13
	Total (N)	214

The distribution of the 214 ablations, based on tip exposure, duration, and target current is presented.

Figure 1. Graphic tracing of a representative course of RF deposition. The tracing demonstrates an initial step-wise increase in energy deposition with periods of cessation of energy deposition following impedance rises. Subsequent step-wise decreases of current are seen when specified energy could not be deposited for 10 s.

Figure 2. Ex-vivo experimental setup. (A) Generator (white arrow) and plastic basin containing saline solution and liver (red arrow). (B) The liver was partially submerged in the saline bath, with a stand holding the electrode in place.

Table 2. Optimal ex-vivo settings providing maximum ablation diameter.

Electrode exposure (cm)	N=	Duration (min)	Target Current (mA)	Current reached (mA)	Tissue end temperature (C°)	Cross sectional Diameter (cm)
1	3	6	400	700	68.5 ± 4.9	1.4 ± 0.1
1	3	12	400	700	68.5 ± 10.6	1.4 ± 0.2
2	4	6	1000	1300	73.5 ± 1.7	2.4 ± 0.1
2	4	12	1000	1300	71.3 ± 4.9	2.6 ± 0.2
3	4	6	1600	1800	82.5 ± 2.4	3.1 ± 0.1
3	4	12	1800	1900	84.3 ± 1.7	3.4 ± 0.1
4	4	12	1800	2100	88.3 ± 4.3	4.1 ± 0.3
4	4	15	1800	2100	89.8 ± 2.9	4.0 ± 0.2
5	4	12	2500	2500	89.5 ± 1.9	5.0 ± 0.2
5	3	15	2500	2500	91.3 ± 1.5	5.3 ± 0.1

Parameters are presented by electrode exposure size for at least two representative durations of ablation.

Figure 3. Ex-vivo gross pathology. A representative image of the sectioned liver following a six min RF application using a 3 cm single active tip is presented in transverse (A) and axial section (B). White coagulation measuring 2.7 × 2.7 × 3.3 cm is clearly defined.

Table 3. Ablation lengths and sphericity index for 12 min ablation duration.

Electrode Tip exposure (cm)	N=	Current range (mA)	Mean (cm)	Range(cm)	Sphericity index
1	5	400–800	1.4 ± 0.1	1.3–1.5	1
2	15	1200–2200	2.8 ± 0.3	2.4–3.3	0.93
3	10	2300–2500	4.0 ± 0.4	3.5–4.7	0.85
4	10	100–2500	5.0 ± 0.5	3.9–5.7	1
5	5	2500	6.5 ± 0.2	6.0–6.9	0.81

Figure 4. Relationship between maximum electrode temperature immediately after ablation to ablation diameter. A tight correlation to a power function is seen. Colored points represent end temperatures at the optimal settings noted in Table 2.

Figure 5. Relationship between ‘heating factor’ and the ablation diameter. Tight correlations to both power and logarithmic functions are seen. Colored points represent end temperatures at the optimal settings as noted.

Figure 6. Correlation between initial current output and ablation size. The R² for the parabolic correlations observed for 2–4 cm electrode tips and 12 min RF ablation are presented. These results denote that there is a range of optimal initial current which provides the maximum coagulation diameter.

Figure 7. Correlation between the time to first pulse and ablation size. Higher order modeling based upon electrode tip exposure (formula presented in the text) yielded a root mean squared error of 1.9 mm and R² of 0.95.

Table 4. Optimized parameters for single-electrode ex vivo data for time to first pulse.

Parameter	Optimized value	Description
$D_5$	5.222 cm	Ablation scale, 5 cm tip
$D_4$	4.223 cm	Ablation scale, 4 cm tip
$D_3$	3.505 cm	Ablation scale, 3 cm tip
$D_2$	2.680 cm	Ablation scale, 2 cm tip
$D_1$	1.422 cm	Ablation scale, 1 cm tip
$\tau$	6.018 sec/cm	Time constant
$a$	2458.859 sec	Time constant