Biomedical Engineering Reference
In-Depth Information
measureable MR imaging parameters exhibit various degrees
of temperature sensitivity. Under most circumstances, the
temperature sensitivity of MR parameters have always tended
to be regarded as more of a nuisance than a useful technique.
Physicists need to pay close attention to the temperature of their
phantoms and the temperature sensitivity of their phantom
materials lest unexpected anomalies arise during the assign-
ment of peak locations in spectroscopy or unexplained signal
changes in quality assurance phantoms arise. However, the
desire to leverage the temperature sensitivity of MRI as a non-
invasive modality for monitoring temperature changes during
delivery of thermal therapies such as hyperthermia, and later,
thermal ablation, sparked an interest in the development of MR
temperature imaging (MRTI), also commonly referred to as MR
thermometry.
MRI is an attractive platform for noninvasive temperature
measurement as there are several temperature-sensitive param-
eters that remain linear within the biological range of tissue
measurement. Additionally, MRI is a nonionizing, noninvasive
modality capable of making dynamic volume measurements to
map these changes out over a volume. This ability to estimate
temperature in three-dimensional space via a variety of mecha-
nisms and correlate these measurements with high-resolution
anatomical MR images made MRTI-driven approaches highly
desirable when a noninvasive and highly sensitive (σ
T
≤ 1°C)
approach to temperature measurement was required of an
MR-compatible object and MR-compatible heating device.
Despite the availability of multiple temperature-sensitive
parameters, it is important to note that not all are equally appro-
priate for measuring temperature in all materials and under all
circumstances. The technique to be utilized for noninvasive esti-
mation of temperature should be carefully evaluated and cho-
sen so as to be congruent with the needs of the therapy, and a
thorough review of the literature conducted to assess potential
measurement biases and errors.
For instance, delivery of minimally invasive thermal abla-
tive therapy is generally characterized by a rapid, local delivery
of high temperatures (≥54°C) for the purpose of coagulating
tissue. This demands a technique that has relatively high spa-
tiotemporal resolution, but not necessarily extensive volume
coverage, and can often accommodate more uncertainty in the
temperature (σ
T
≤ 5°C) while still providing extremely useful
information for monitoring outcomes and safety (upper tem-
perature limits). However, as the exposure times increase and
the spatial gradient of temperature becomes lower, accuracy and
precision at lower temperatures become increasingly important,
as does volume coverage.
For instance, when guiding hyperthermia, a clinical thermal
therapy procedure in which tissue temperature is raised several
degrees (usually >42°C) for long, continuous periods of time
(>20 min), the tolerance for temperature uncertainty is much
tighter (σ
T
≤ 1°C) across a very large volume. This volume often
encompasses areas not amenable to MR temperature imaging
as well as potentially moving organs. MR temperature-imaging
techniques for such slow heating methods with low spatial
3.3.1 Magnetic resonance
temperature Imaging
MRI is a noninvasive 3D-imaging modality that does not use
ionizing radiation and provides a plethora of contrast mecha-
nisms with which to image both anatomy and function, mak-
ing it an attractive modality for image-guided thermal therapy
in that it aids in the planning, targeting, monitoring/control,
and verification of treatment delivery (Figure 3.1). Of all these
qualities, it is the ability to noninvasively monitor temperature
changes in the body either qualitatively or quantitatively that
makes MRI a particularly attractive modality for enhancing the
safety and efficacy of thermal therapies through thermometry
feedback. Therefore, despite the challenges associated with per-
forming thermal therapy procedures in the magnetic resonance
(MR) environment, MRI is quickly emerging as a modality of
choice for guiding many of these therapies, and many vendors
are working hard to develop MR compatible therapy delivery
equipment.
Because the nuclear magnetic resonance (NMR) phenom-
enon is by its very nature a thermodynamic process exchanging
energy with its chemical environment, MR is exquisitely sensi-
tive to the microscopic chemical and physical state in which the
spin system resides. So, it should come as no surprise that many
(a)
(b)
(c)
(d)
FIGURE 3.1 (See color insert.)
MRI guidance of laser ablation of
brain lesion. In addition to planning the procedure, MRI is useful for
targeting the volume of interest and verifying correct location (green
arrow) of devices for therapy delivery (a). MR temperature imaging
provides a spatiotemporal map of the temperature that can be used to
help control delivery for safety and efficacy (b). The temperature history
can be integrated with biological models that predict damage (orange
contour) as shown in (c), which may be used as a surrogate for predict-
ing the treatment endpoints during the course of therapy as opposed
to more time-consuming posttreatment verification imaging, such as
contrast-enhanced imaging, which demonstrates the perfusion deficit
left by therapy and enhancing ring of edema (d).
