Biomedical Engineering Reference
In-Depth Information
3
Practical Clinical Thermometry
R. Jason Stafford
University of Texas MD
Anderson Cancer Center
3.1 Introduction ...............................................................................................................................41
3.2 Invasive Thermometry ............................................................................................................. 42
3.3 Noninvasive Thermometry ..................................................................................................... 42
Magnetic Resonance Temperature Imaging • Temperature Sensitivity of Several Intrinsic MR
Parameters • Temperature Dependence of the Water Proton Resonance Frequency (PRF)
3.4 Summary .....................................................................................................................................51
References ...............................................................................................................................................52
Brian A. Taylor
St. Jude Children's
Research Hospital
3.1 Introduction
complexities of modeling and simulating the heat deposition in
tissue do not accurately predict outcomes or fully assure patient
safety. Therefore, in order to increase the efficacy of these proce-
dures as well as enhance the safety aspects of delivery, feedback
of the therapy is often necessary for many of these procedures.
While indirect measurements of heating, such as estimating the
specific absorption rate (SAR), can be used, direct observation
of temperature provides the most vital information with regard
to evaluating delivery in most cases. Therefore, many therapies
employ some combination of imaging, modeling, and thermom-
etry to aid in planning and monitoring of thermal therapies.
Thermometry, particularly for heat-mediated regional thera-
pies of deep-seated lesions or tissue that cannot be directly visu-
alized, can play an important role in improving treatment safety
and efficacy. This is done by providing direct feedback of the
temperature in the target tissue, nearby critical structures, and,
in cases of minimally invasive treatment modalities, of the probe
itself. In terms of treatment safety, the role of thermometry is to
provide feedback, which can help control the therapy and avoid
destructive temperatures from being reached during the course
of treatment, such as excessive heating in the region of the treat-
ment probe, nearby normal tissue, or adjacent critical structures,
the damage of which may result in complications. Examples of
effectively incorporating thermometry for safety include moni-
toring for areas of elevated temperature (“hot spots”) inside
and outside the target volume during hyperthermia or thermal
ablation. Excessive heating outside the target volume can result
in damage to normal tissue. Near invasive probes, monitoring
maximum temperature in the target tissue aids in avoiding cata-
strophic tissue damage, such as vaporization or charring.
Often, the choice to monitor everywhere is not feasible, so
choosing the locations of thermometry must weigh in to plan-
ning the procedure. Hot spots could occur during hyperther-
mia due to interaction of the electromagnetic or ultrasound field
with unexpected impedance mismatches that lead to heating at
The goal of thermal therapy is to alter tissue temperature in a
targeted region over time for the purpose of inducing a desired
biological response (Goldberg et al. 2000; Dewhirst et al. 2005;
Stauffer 2005; Hurwitz 2010). The target temperature may be
only a slight deviation from physiological temperature for a pro-
longed period of time (hypo- or hyperthermia) or more extreme
deviations for shorter periods of time focused on tissue coagu-
lation (cryo- or thermal ablation). Regardless of the treatment
modality or anatomical location, the majority of these thera-
pies are designed to conformally deliver the thermal therapy to
a target tissue volume with minimal impact on intervening or
surrounding tissues. In particular, during therapies engaged in
heating the tissue, isoeffects are rarely predicted by a simple tem-
perature threshold and are more generally a complex function
of time and temperature, which depend on the rate at which the
transition occurs. Endpoints, such as tissue destruction, have
been modeled using Arrehnius rate processes and derivatives,
such as equivalent minutes spent at 43°C. These models relate
the biological response to the cumulative temperature history
of the tissue over time, thereby relying on accurate temperature
measurements in the tissues of concern.
One of the key challenges associated with safely and effec-
tively implementing these therapies is spatiotemporal control
of the induced temperature changes. This challenge is usually
addressed by a combination of therapy planning and monitor-
ing, the specific implementation of which is often tailored to the
treatment modality and site of therapy. Generally, treatment
planning is the use of models and simulation of the delivery of
energy and resulting heating to aid in optimizing the logistical
approach to therapy delivery, such as location of applicators and
applied power. In most cases, patient-specific imaging is incor-
porated to better incorporate patient-specific anatomy and char-
acterize the target tissue. However, while an excellent tool for
optimizing the approach to therapy delivery, in most cases the
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