Biomedical Engineering Reference
In-Depth Information
45.0°C
20.0 °C
FIGURE 19.7 Infrared image of bivalved ex vivo pig liver specimen taken as soon as practical after completion of injection using 0.5 mL each of
HCl and NaOH at 7 mol/L. The area of the excursion is relatively rounded and homogeneous within the camera's resolution. Note that the central
area is saturated, indicating a temperature above the range setting (20°C-45°C) on the camera.
Higher temperatures were produced when higher concentra-
tions were used, and larger injection volumes produced larger
areas of coagulation.
The next step was to compare two injection methods, simulta-
neous and sequential, to positive and negative controls. We chose
to look at peak temperatures, volume of tissue coagulated, and
the shape of lesions (the sphericity or roundness coefficient).
In addition, we were interested in any changes in heart rate or
blood pressure in response to the injections. Based on previ-
ous work, we used 10 M solutions of reagents, which therefore
would produce 5 M salt solutions if both starting materials were
to react completely.
Experiments were performed in healthy swine with surgical
exposure and intraoperative ultrasound guidance for device
placement. This was necessary in order to avoid positioning in
or directly adjacent to large vessels, which are ubiquitous in a
healthy liver. A thermocouple was positioned within a few mm
at the same depth as the device. Contrast was added to the acid in
order to aid in localization of the lesions by CT of the explanted
specimens. Each animal was treated in the same manner with
four injections done in the same order. First, a saline sham
with contrast was performed. Following this, acid and then the
simultaneous injections were performed. Finally, the sequential
injections were done. It quickly became apparent that sequential
injections were not tolerated as well, particularly given that one
of the animals went into cardiac arrest after a sequential injection
and required resuscitation to finish the experiment. Fortunately,
according to the protocol, the sequential injection was the final
injection and we needed only to restore circulation long enough
to be satisfied that the acute crisis had passed. This situation was
not seen with any of the other injections. Upon completion of
the experiments, the animals were sacrificed and the livers were
explanted. The organ was then scanned with CT imaging, and
the results were correlated with the findings at histopathology.
The results were interesting from several angles. One was that
as we had hoped, the simultaneous injection produced the high-
est peak temperatures, depicted in Figure 19.8. This implies that
in the sequential injections not all of the acid and base reacted
together. We believe that unreacted reagents, especially the base
since it is the second reagent, likely escape into the vasculature
with sequential injection.
Another interesting result was that again, as we had hoped, the
volumes of coagulation were larger than when acid was injected
alone, shown in Figure 19.9. At first glance this might seem to be
an unrealistic comparison. After all, the volume injected in the
acid alone was only half that used with either of the other two
methods since the base was also injected. However, one of the
points in this research was to show that using the same amount
of acid we could obtain a larger volume of injection with a differ-
ent method and conditions.
It might be argued that additional experiments should be
done using the same total volume for the acid alone, but this
raises two problems. One is that by diluting with water to keep
the total amount of acid the same would reduce the concentra-
tion of the acid and therefore not be an accurate representation
of the current practice. The other would be to keep the concen-
tration the same and just use twice the volume. This also would
not be a suitable comparison, as we would then be using twice as
much acid in the acid alone experiments.
Predictability of lesion shape is key in ablation therapies. With
injected therapies, the amount that is used, the viscosity, the
hydraulic conductivity of the tissue, and the rate of injection all
affect the distribution. To analyze the results, we performed a sphe-
ricity analysis. The coefficient reflects the degree of deviation away
Search WWH ::




Custom Search