Biomedical Engineering Reference
In-Depth Information
defined for these transurethral applicators determines the appli-
cator power levels that maximize temperature coverage in the
tumor subject to constraints on the maximum tumor tempera-
ture, the temperature in normal tissues, applicator positioning,
and applicator orientation. The results of these treatment plan-
ning calculations determine the optimal combination of trans-
urethral applicator parameters, including the sector angle and
number of sections defined along the length of the device, for a
given patient geometry.
temperatures generated by these applicators are evaluated with
a cylindrically symmetric 2D finite element model that varies
the acoustic attenuation and blood perfusion as a function of the
thermal dose. For these calculations, the acoustic attenuation is
constant for thermal doses less than or equal to 100 equivalent
minutes at 43°C, then the attenuation increases linearly with
the logarithm of the thermal dose until a threshold value for the
attenuation is reached at a dose of 10,000,000 equivalent min-
utes at 43°C. The attenuation and absorption values are equal
in these simulations. Similarly, the blood perfusion is modeled
as constant up to a thermal dose of 300 equivalent minutes at
43°C, and then for larger thermal dose values, the blood perfu-
sion is set to zero. The lethal thermal dose threshold for lesion
formation in these calculations is defined as 600 equivalent
minutes at 43°C. The results of this simulation model show that
the dimensions and evolution of predicted lesions are an excel-
lent agreement with experimentally measured temperatures
and lesion dimensions (Tyreus and Diederich 2002). This agree-
ment between the simulated temperatures and measured lesion
shapes in the axial direction is demonstrated in Figure 6.11.
Other numerical simulations with this model evaluate the size of
lesions produced by applicators with different diameters (Tyreus
et al. 2003) and establish strategies to avoid bone heating while
ablating the prostate (Wootton et al. 2007).
Numerical simulations of pressures and temperatures pro-
duced by a mechanically rotated interstitial ultrasound applica-
tor are also evaluated for thermal ablation in the prostate (Chopra
et al. 2005). This interstitial applicator consists of a fixed-phase
linear array of flat rectangular ultrasound transducers with
6.3.3.3 ablation therapy with Fixed-phase applicators
Treatment planning for ablation therapy with fixed-phase ultra-
sound applicators is closely related to treatment planning for
hyperthermia with single and multiple element applicators. For
example, the applicators are similar, pressures and temperatures
are often calculated with the same approaches, and power depo-
sitions are optimized with similar algorithms. However, some
specific features influence the treatment planning strategy for
ablation therapy with these devices. In particular, simulations
of ultrasound ablation more frequently emphasize thermal dose
calculations to determine the volume of the ablated region, some
treatment planning calculations for ablation therapy consider
changing ultrasound attenuation and blood perfusion values
during the treatment, and other simulations evaluate the poten-
tial for irreversible thermal damage in sensitive normal tissues.
Simulations of the thermal doses produced by interstitial
ultrasound applicators for ablation therapy combine transient
bioheat transfer calculations with an approximate expression for
the power deposition (Tyreus and Diederich 2002). The transient
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(b)
(c)
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Radial depth into tissue (mm)
FIGURE 6.11
Simulated axial temperature contours produced by transurethral ultrasound applicators with (a) one, (b) two, and (c) three active
segments. Representative measured lesions are indicated with dashed lines. (After P. D. Tyreus and C. J. Diederich,
Phys. Med. Biol.
, 47, 7, 2002.)
