2012 Proceedings - International Tissue Elasticity Conference

040 MONITORING CRYOABLATION LESIONS WITH QUANTITATIVE ULTRASOUND
ELASTOGRAPHY: A FEASIBILITY STUDY.
Diego Dall’Alba 1 , Caitlin Schneider 2 , Christoper Nguan 3 , Ali Baghani 2 , Robert Rohling 2, ,
Septimiu E. Salcudean 2 .
1 University of Verona, Verona, ITALY; 2 University of British Columbia, Vancouver, BC, CANADA;
3 Vancouver General Hospital, Vancouver, BC, CANADA.
Background: The use of ultrasound (US) elastography (UE) has been widely investigated as a method for
monitoring the ablation zone in HIFU or radiofrequency (RF) ablation. UE cannot be used to monitor
cryoablation (CA) since the ice–ball zone causes strong reflections and wide shadows. However, the
mechanical stresses caused by ice crystal formation do change the tissue’s microscopic structures which
should also affect its macroscopic elastic properties.
Aims: In this study, we propose to monitor whether tissue has been properly frozen during CA by using
UE to monitor tissue elasticity before and after thawing, when the ablated tissue can be imaged again by
US. We present an ex–vivo animal study that confirms the feasibility of this approach.
Methods: 4 samples obtained from 2 pig kidneys after excision were frozen twice with a protocol that
follows the clinical guidelines. Freezing is obtained by immersion in a bath of dry–ice and acetone with a
low conductivity coupling container protecting the sample against cryo–shock. The freezing speed was
6ºC/minute and thawing speed was 3ºC/minute. Each cycle took approximately 35 minutes plus
25 minutes to ensure that the temperature before and after the experiment are the same. The
temperature profile is reported in Figure 1. The samples were measured before and after the freezing
cycles with absolute UE, using motion tracking as described in [1] and modulus reconstruction based on
local frequency estimation [2]. Excitation is provided by an external device (LDS Mod. V203, B&K,
Denmark), controlled between 50 and 100Hz. The data were acquired with a US device (Sonix Touch,
Ultrasonix, Canada) with a 3D mechanical probe (4DL14–5/38, at 5MHz). A region of interest in the UE
data is manually selected within the volume (10 planes for each excitation frequency), and the mean
value and the standard deviation is reported in Figure 2.
Results: Preliminary results show a significant difference between tissue elasticity before and after
freezing as showed in Figure 1. The significance of the change is also confirmed by a Tukey’s Honestly
Significant Difference test (α