Biomedical Engineering Reference
In-Depth Information
boundaries. During high-intensity focused ultrasound (HIFU)
ablations, cumulative heating in areas slightly distal or proximal
to the focus may result in tissue damage. During interstitial laser
ablation, allowing the tissue temperature adjacent to the probes
to exceed 100°C may lead to vaporization or charring, which can
damage the treatment probe. Effective use of thermometry for
enhancing procedure safety includes assessing the areas at risk
and using temperature feedback to help minimize these risks.
In terms of treatment efficacy, the role of thermometry is to
provide feedback to aid in making better decisions with respect
to predicting, or controlling, the treatment outcome. This could
involve evaluating how much of the target tissue exceeds a
threshold temperature, which may be linked to activation of a
specific bioeffect or release of a drug. However, since damage
to tissue from heat is a cumulative effect (Sapareto et al. 1984;
Dewhirst et al. 2003), often, the most effective strategy is to take
the temporal temperature history (“exposure”) into account and
formulate a metric that helps make decisions about the likeli-
hood of tissue destruction or a particular bioeffect. For these
purposes, thermometry is combined with a biological model of
damage, such as an Arrhenius rate process or an effective expo-
sure that can be compared to known empirical outcomes, such
as cumulative minutes spent at 43°C (Sapareto et al. 1984; Dewey
1994; McDannold et al. 2000; Dewhirst et al. 2003).
In practical terms, there are really two general approaches to
obtaining temperature measurements in the body during ther-
mal therapy delivery, invasively and noninvasively. Invasive
techniques generally rely on interstitial, or intracavitary, place-
ment of thermometers, while noninvasive techniques may rely
on strategic superficial placement of thermometers in combina-
tion with modeling, or on temperature-sensitive imaging tech-
niques. Regardless of technique employed, it must be amenable
to achieving the goals of the therapeutic intervention in order to
be of value. Consideration must be made in the trade-offs associ-
ated with the level of invasiveness and associated risks, interac-
tions between the thermometry system and therapy or imaging
modalities, spatial and temporal sampling of points, and the
uncertainty in temperature estimates.
smearing due to the conductivity of probe leads (Fessenden
et al. 1984). Measurements that do not perturb ultrasound
fields may be made using small, unsheathed leads. Alternatively,
thermistors provide accurate measurements using temperature
sensitive changes in resistance, and are generally less sensitive
to electromagnetic field interactions (Bowman 1976; Hjertaker
et al. 2005). More recently, fluorescent fiberoptic (fluoroptic)
thermometry technology has entered wide use. Fluoroptic ther-
mometry uses minimally invasive fiberoptic applicators with
phosphor sensors (magnesium fluorogermanate activated with
tetravalent manganese) in the tip, which have a temperature
dependent fluorescent decay. Self-heating limits use in near
infrared (NIR) laser applications unless these artifacts are con-
trolled by capping of the fluoroptic probe, particularly when
close to the laser (Reid et al. 2001; Davidson et al. 2005).
Thermal ablation techniques utilizing invasive radiofre-
quency electrode systems have incorporated thermistors or
thermocouples to monitor the tip of the probe from exceeding
100°C. Additionally, interstitial placement of thermocouples
(Buy et al. 2009), thermistors (Diehn et al. 2003), or fluoroptic
fibers (Wingo et al. 2008) may be used to monitor temperature
of nearby critical structures to help mitigate complications. For
hyperthermia applications, applicators may be placed in super-
ficial locations on the skin or intraluminally (i.e., transurethral,
transvaginal, endorectal, etc.) (Wust et al. 2006). These loca-
tions may be used to monitor safety and coupled with model-
ing to help estimate spatiotemporal heating deeper in the tissue.
Probes placed directly in the target tissue itself can be used as
an aid to inferring dose delivered to the target for aiding in
eicac y.
Despite real-time feedback and accuracy better than 0.5°C,
from a monitoring perspective invasive probes are extremely
limited by their finite sampling capability. Additionally, invasive
probes are not only clinically difficult to accurately place within
the patient but they increase the invasiveness of the procedure.
This tends to increase the time and complexity associated with
planning as well as increase the potential for complications asso-
ciated with the procedure. Because of this, a substantial amount
of interest has been spent on development of noninvasive ther-
mometry methods.
3.2 Invasive thermometry
Small measurement devices can be inserted directly into the
patient. This minimally invasive approach to thermometry pro-
vides temperature feedback in real-time finite points within or
near the treatment volume. In general, the more points desired,
the more invasive the procedure becomes, as well as more time
consuming and difficult if the probes must be accurately placed
at depth in tissue, particularly moving organs.
Thermocouples utilize a heat-induced potential difference
from a junction of two different metals to measure temperature
and have the advantage of being small, accurate, stable, and
cost efficient. Known weaknesses include possible interactions
between the device or its sheath and sources of heating, such
as ultrasound (Hynynen et al. 1983) or radiofrequency fields
(Gammampila et al. 1981; Chakraborty et al. 1982), and thermal
3.3 Noninvasive thermometry
Noninvasive thermometry for thermal therapies has been
investigated using a variety of approaches, including infra-
red thermography, microwave tomography (Meaney et al.
2003), impedance tomography (Amasha et al. 1988; Blad
et al. 1992), CT Hounsfield unit changes (Fallone et al. 1982;
Bruners et al. 2010), pulsed-echo ultrasound using estimation
of the temperature-dependent echo shifts via speckle tracking
(Maass Moreno et al. 1996) or temperature-dependent spectral
shifts (Seip et al. 1995), photoacoustic imaging, and temper-
ature-sensitive magnetic resonance imaging (MRI). Of these
techniques, MRI is the only technique currently evolved to the
point of clinical use.
