Biomedical Engineering Reference
In-Depth Information
7.8 thermometry and
Experimental Validation
refined using post-processing methods). These problems can
be overcome by the new developments in HTP tools described
before and improved applicators.
For the comparison of field distributions, a new alternative
to interpolation to a common mesh and subsequent subtraction
is emerging [45]. It is based on the Gamma method, which has
been developed to assess dose distributions during radiotherapy
[111, 112]. The Gamma method accepts a value tolerance and a
spatial tolerance and computes either the proportion of agree-
ment or a deviation distribution.
Various approaches have been used to measure the EM and
temperature distributions achieved during hyperthermia treat-
ment. Surface temperatures can be measured with IR cameras
and thermal sensors in contact with the skin [7]. In the pres-
ence of water bolus cooling or heating, surface temperature
measurement can be problematic as the probe measurement
will generally be affected by the proximity to the bolus when not
sufficiently isolated. This problem can be reduced by correct-
ing for the error using predetermined correction factors or by
using directional sensors. Direct measurement in the patient has
previously been restricted to interstitial and sometimes intra-
luminal catheters (e.g., thermocouples [suffer from self-heating
problems], thermistor probes, or fiber optic probes for tempera-
ture measurements). This reduces the number of accessible data
points, limiting the usefulness. Furthermore, the catheters can
cause complications that lead to disagreement about whether
interstitial thermometry should be used on a regular basis [157,
172]. The attractive alternative of MRI thermometry is currently
emerging, offering noninvasive 3D thermometry. It requires
MR-compatible applicators that are now being developed in
several groups. While MR thermometry is hardly possible
when patient movement is present (e.g., due to breathing), it has
been shown that MR thermometry can even monitor tempera-
tures near vessels [39]. Temperature resolution in the order of
0.5-1.0°C [66] can be achieved (at least in phantoms under con-
trolled conditions), but only for tissues with high water content,
whereas hot spots often occur in bone, cartilage, and fat tissues
due to the dielectric contrast and low perfusion.
MR thermometry during hyperthermia has been validated
extensively using a heterogeneous phantom [64, 66, 120, 141].
Various homogeneous and heterogeneous phantoms [66, 120,
122, 153] have been developed to validate simulations and for
equipment quality assurance purposes. The heterogeneous
phantoms include pieces of bone, chalk, or plastic to mimic com-
plex bone shapes and hollow tubes to simulate the esophagus.
The EM or temperature distribution in the phantoms is deter-
mined using embedded probes at selected, sensitive locations
[66], IR cameras [137, 146], current- [174, 175] or LED-sheets
[151, 191], or gels with temperature-dependent color transmis-
sion properties. Several treatment planning tools (most exten-
sively HyperPlan) have been validated experimentally [65, 67,
94, 117, 158, 160, 184, 189] using patient data and phantoms. All
of the previously mentioned thermometry methods have been
applied. Generally, the validations concluded that while good
qualitative agreement had been achieved, quantitative agree-
ment for complex setups was limited. This is sometimes attrib-
uted to an inability to correctly simulate cross talk and feeding
networks, the sensitivity with regard to patient positioning
(which has been studied in [65]), and the inability to capture
interface as well as antenna behavior with coarse meshes in
FDTD (grid steps of 10 mm are often used and subsequently
7.9 tissue parameters
Various publications have presented tables with EM and ther-
mal tissue properties. Tissue properties are assigned to a tissue
that has been segmented, or are based directly on Hounsfield
units from CT scans [67, 87] or normalized MRI gray values
[57, 113]. It has been shown that CT-based property assignment
results in serious misclassifications and should not be used as an
automatic procedure [67, 193]. MRI data has also been used to
estimate local perfusion [35]. Sensitivity studies have shown that
the impact of dielectric parameter uncertainties on the EM and
thermal distribution is relatively small for moderate hyperther-
mia [170]. The impact of perfusion variations has been studied in
[34, 61] and can be significant when the problem is not conduc-
tion dominated. Far less information exists about tumor param-
eters (e.g., [72] for hepatic tumors), which can vary strongly
depending on the tumor type and size as well as the patient.
Various studies have claimed that tumors have properties result-
ing in preferential tumor heating [55, 75]. Measurements have
been conducted to study the thermal conductivity anisotropy
in muscle and the dependence of the effective thermal conduc-
tivity on perfusion [15]. The influence of water content changes
with temperature due to diffusion, and evaporation has been
addressed in [159, 194, 195]. Information about the temperature
dependence of other tissue parameters can be found in [13, 23,
106, 159]. Reversible and irreversible effects from heat have been
studied in [145] for dielectric properties and in [15] for thermal
conductivity. Many parameters have been gained from excised
tissue. It is relevant to know that postmortem changes occur that
modify the properties [71]. Anesthesia is also known to have an
impact on tissue properties.
7.10 related treatments
The simulation environment can be applied in a useful manner
to other thermal treatments. Examples besides classical hyper-
thermia include radio-frequency (RF) or microwave (MW) abla-
tion, RF surgery, and high intensity focused ultrasound (HIFU)
ablation. All three of these are common in that they reach
higher temperatures than common hyperthermia treatments
(typically 60−80°C, sometimes over 100°C, such that cell water
evaporation can be relevant) and produce more localized heat-
ing that destroys the tissue locally (mostly through coagulation).
