Ultrasonographic characterization of hepatic cryolesions. An ex vivo study
C. M. Lam, S. M. Shimi and A. Cuschieri
Department of Surgery, University of Dundee (Scotland), Ninewells Hospital and Medical School.
OBJECTIVE: To determine the physical basis for the ultrasonographic
characteristics of the hepatic ice ball produced by cryotherapy and the
size correlation between the actual hepatic ice ball and the
ultrasonographic cryolesion. DESIGN: Experimental ex vivo study involving
controlled freezing with liquid nitrogen recirculating probes of fresh
porcine livers immersed in various solutions at ambient temperatures (20.2
degrees C to 22.6 degrees C), together with measurements of the impedance
of frozen and unfrozen liver. RESULTS: First, the hyperechoic rim is caused
by reflection of 34% of ultrasound waves at the interface between unfrozen
and frozen liver as a consequence of an increased acoustic impedance of
frozen liver that was calculated to be approximately 3.8 times that of
unfrozen liver tissue. The increased acoustic impedance is due to the
decrease in elasticity of hepatic tissue as it freezes. Second, the
posterior acoustic shadowing is partly due to the attenuation of the
incident ultrasound waves by reflection at the interface between unfrozen
and frozen liver. It is also dependent on the crystalloid-protein content
of hepatic parenchyma, which ensures a homogeneous lesion by preventing
"shattering" within the cryolesion. This is in sharp contrast to the
ultrasonographic appearance of an ice ball formed in ionized water, in
which the hyperechoic rim overlies an area of posterior acoustic
enhancement. Third, the correlation of the size between the
ultrasonographic cyrolesion and the measured hepatic ice ball approached
unity (r = .99), and the two measurements were identical for cryolesions
less than 50 mm in diameter. CONCLUSION: Ultrasound is an accurate method
for depicting the actual diameter of frozen solid hepatic tissue in
cryotherapy for liver tumors, but the present technology does not provide
accurate assessments of the volume of frozen tissue.
