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Figure 14.26 shows the MRI image in a plane perpendicular to the beam created
by using the T
2
-weighted FSE technique. Note that the necrosis coverage is continu-
ous, leaving no untreated spaces. The continuous necrosis coverage was confirmed
visually when the kidney was sliced. Next to the large lesion, a single lesion was cre-
ated using the same exposure (1,500 W/cm
2
for 5 seconds) in order to use it as a ref-
erence. Note that, by using this intensity, none of the lesions was created under the
mechanism of cavitation, confirming the results of Figure 14.24.
During this diagnostic scan, low intensity is applied and, therefore, there is no
need for a delay between successive firings. Therefore, the time needed to perform
this type of scan is short. It is up tothe HIFUclinicians as to whether or notthis diag-
nostic scan is used during clinical work, but certainly this tool can be very useful
during research investigations. This concept was demonstrated for a single shot in in
vivo kidney by Hynynen et al. [15], but was never demonstrated in two dimensions
(rectangular grid). This diagnostic scan was demonstrated in a plane perpendicular
to the beam, but it is possible to demonstrate it in a plane parallel to the beam. When
all of the points of the grid show a decrease in temperature (black spot in the
T
1
-weighted FSPGR), then, during the application of high-intensity ultrasound
(therapeutic scan), complete necrosis is observed in the targeted area. If ultrasound
goes through an interface that includesbubbles, thenthe diagnostic scan reveals sites
with poor ultrasound penetration and, therefore, later in the therapeutic scan, some
spaces are left untreated. This study suggests that an MRI compatible robot should
include an axis for providing rotational motion, so that the transducer does not only
maneuvers in the xyz dimensions, but also in an angular direction, thus ablating
through the appropriate angle. This technique will ensure that interfaces with a
major quantity of bubbles are avoided.
Figure 14.27 shows the MRI image using T
2
-weighted FSE of a large lesion in
the kidney in a plane perpendicular to the beam. This lesion was created by using a 4
× 5 grid of lesions. The intensity used was 2,000 W/cm
2
at 5 seconds and the spacing
between the lesions was 3 mm. Note that the necrosis coverage is incomplete, leav-
ing some space untreated. The diagnostic scan of Figure 14.27 has shown that the
penetration at the location of the untreated area was poor, because no detectable
temperature elevation was observed during the application of the diagnostic scan
(low intensity). Out of the 20 locations, only 6 showed the normal 1- to 2-mm-diam-
394 MR-Guided HIFU for Kidney and Liver Applications
Reference
Figure 14.26 MR images (in a plane perpendicular to the transducer beam) of a large lesion (full
coverage of the intended target) using T
2
-weighted FSE with TE = 32 ms. The spatial average
intensity was 1,500 W/cm
2
for 5 seconds.
eter black spot. It was observed that this muscle–kidney interface contained bubbles
(observed by MRI and later by gross examination). During the application of HIFU,
at the site where the tissue was not necrosed, no cavitation activity was detected,
meaning that reflection was the dominant factor for the incomplete necrosis.
Figure 14.28 is a very interesting case that was encountered. This figure shows
the MRI image using T
2
-weighted FSE of a large lesion in the kidney in a plane per-
pendicular to the beam. On top of this site, it was observed that the muscle–kidney
interface contained bubbles (observed by MRI and later by gross examination). The
large lesion was created using a 5 × 6 grid of lesions. The intensity used was 1,000
W/cm
2
at 5 seconds and the spacing between the lesions was 1.5 mm. The necrosis
coverage is incomplete, leaving some space untreated. However, a small area that
may look as if it is untreated is, in fact, a lesion created under the mechanism of cavi-
tation (verified by acoustic emission, temperature monitoring, and gross examina-
tion). Unfortunately, it was difficult to discriminate using MRI between a
cavitational lesion and untreated spaces. Untreated regions in the kidney appear
very bright, and cavitational lesions, which include liquid-filled spaces, also appear
bright. The diagnostic scan of this figure has shown that the penetration at the loca-
tion of the untreated area was poor (similar to that observed for the case of Figure
14.27).
14.8 Effect of Interfaces in HIFU Ablation 395
Reference
Incomplete necrosis
Figure 14.27 MR images (in a plane perpendicular to the transducer beam) of a large lesion (par-
tial coverage of the intended target) using T
2
-weighted FSE with TE = 32 ms. The spatial average
intensity was 2,000 W/cm
2
for 5 seconds.
Reference
Cavitation
Incomplete necrosis coverage
Figure 14.28 MR images (in a plane perpendicular to the transducer beam) of a large lesion (par
-
tial coverage of the intended target and occurrence of cavitation) using T
2
-weighted FSE with TE =
32 ms. The spatial average intensity was 1,000 W/cm
2
for 5 seconds.
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400 MR-Guided HIFU for Kidney and Liver Applications
CHAPTER 15
Modalities and Devices for Thermal
Ablation
Sunita Chauhan
This chapter includes a brief overview of prevalent thermal ablation modalities. It
concentrates on a review of noninvasive surgical techniques (other than
radiosurgery) that use thermally induced ablation for treating deep-seated abnor-
malities. The chapter concludes withdevices andsystems available for thermal abla-
tion and a representative example of a robotic system, called FUSBOT
BS
, that was
devised at the author’s center.
15.1 Introduction
For management of cancers/tumors, surgical resection is considered to be the pri-
mary treatment and the gold standard. Surgical resection is effective when cancers
are contained in a given organ. For organs situated near lymph nodes, extensive rad-
ical resection may be necessary, including the removal of lymph nodes. Normally, a
margin area surrounding the cancer is removed to minimize relapse. However, open
surgery is associated with several operative and postoperative complications result-
ing into high morbidity and mortality rates, and many patients are not good candi-
dates for resection. Minimally invasive surgery (MIS) is gaining significance,
wherein the affected area is laparoscopically or endoscopically resected. Sometimes,
surgery is complicated due to the location of the cancers, which may present severe
risks to the patients. For more invasive cancers, it is usually performed along with
adjuvant and/or neoadjuvant therapies such as radiotherapy and chemotherapy,
which are usually administered postoperatively to reduce the recurrence of the
disease.
Therapeutic techniques such as radiation therapy, chemotherapy, and
hyperthermia (using modalities such as RF, microwave, or ultrasound) may be
effective only in early diagnostic stages. Heat diffusion to sites adjacent to the target
is common due to extended periods of exposure and convection by blood perfusion.
Achieving temperature control at desired levels is very difficult. Such therapies are
given either alone or in appropriate combination. For instance, the cellular mem-
brane permeability (and thus the efficiency) of chemotherapy is increased if
hyperthermia is induced prior to, or along with, drug infusion. Changes in tissue
401
geometry and local microcirculation affect tissue response (thermotolerance) to
successive intervals of energy deposition.
This chapter provides an overview of prevalent thermal ablation modalities. It
concentrates on a review of noninvasive surgical techniques (other than
radiosurgery), which use thermally induced ablation for treating deep-seated abnor-
malities. High temperatures induced in the tissue result in changes in a variety of
physical, chemical, and biological mechanisms at the chemical, organelle, cell, and
tissue scales. The tissue coagulation threshold is about 60
o
C. Temperatures exceed-
ing 60°C produced by a short pulse of energy exposure cause irreversible cell dam-
age due to protein denaturation, inhibition of protein synthesis, breaking of
chemical bonds in DNA and RNA molecules, and loss of integrity in lipid bilayers.
These procedures can be used in either an extracorporeal or minimally invasive
manner, as explained in the following sections.
15.2 Thermal Ablation Mechanisms
15.2.1 Hyperthermia and Ablation
Very high temperatures (thermal coagulation) or very low temperatures
(cryoablation) cause various changes in cellular composition in biological tissues.
These include cytoplasmic and nuclear protein denaturation and DNA and RNA
changes in cellular membrane function. At the temperature ranges used in
hyperthermia (41°C to 43°C for 30 to 60 minutes), tissue composition is altered and
increased cellular membrane permeability, metabolic activity, blood perfusion,
pH, and pO
2
levels are observed. This increases radiosensitization and
chemosensitization of the cells. The changes, however, are reversible, and long
exposures affect the optimum outcomes due to subsistent heat conduction. Further,
higher temperatures (ranging from 60°C to 80°C for 2 to 8 seconds), like those used
in thermal ablation,result inirreversible changes, for instance, cellular protein dena-
turation, thermal coagulation, and immediate thermal necrosis within the treatment
area, leading to cell death [1–8]. Temperatures beyond 100°C induce coagulation
and vaporization of tissue.
In all thermal therapies and thermal ablation procedures, the extent of tissue
destruction is decided by the heat dose, which is a product of exposure time and tem-
perature. With mechanical modalities such as ultrasound, tissue damage is a result
of both thermal and mechanical stress mechanisms; the contribution of either factor,
however, depends on the input intensities and heating rate. In predictive modeling,
the bioheat equation is used to determine the spatial distribution of temperature in
tissues:
ρ
∂
∂
c
T
t
kT q Q Q
pm
=∇ ∇ + − +
(15.1)
where ρ is the mass density (kg/m
3
), c is the specific heat (J/kg·K), k is the thermal
conductivity (W/m·K), T is the temperature (°C), q is the heat source (W/m
3
), Q
p
is
the perfusion heat loss (W/m
3
), and Q
m
is the metabolic heat generation (W/m
3
). In
high perfusion tissues, Q
p
is computed as follows:
402 Modalities and Devices for Thermal Ablation
()
QcTT
p
bb b
=−ω
(15.2)
where ω
b
is the blood perfusion per unit volume (kg/m
3
·s), c
b
is the specific heat of
blood (J/kg·K), and T
b
is the blood temperature (°C).
For predicting the time required to produce thermal tissue injury, various meth-
ods have been suggested. A simple and widely used mathematical model, based on
the empirically deduced Arrhenius model (of chemical reaction rates), predicts that
the time, t, required to produce tissue death at a constant temperature, T (°C), is
given by:
(
)
[]
tAe
Ek T
=
+Δ 273
(15.3)
where ΔE is the activation energy, k is Boltzmann’s constant (8.617 × 10
−5
in eV/K),
and A is the preexponential scaling factor, which varies from tissue to tissue. The
term E is relatively large for biological tissues, thus time t decreases very rapidly as
temperature T increases.
15.2.2 Cryoablation
Cryoablation is an alternative treatment method, for instance, in the palliative man-
agement of liver, renal tumors, and colorectal liver metastases at moderate to lower
temperatures (−20°C to −40°C for >10 minutes). It involves the extracellular and
intracellular formation and growth of ice crystals, resulting in cellular shrinkage,
membrane rupture, and cell death during multiple freeze/thaw cycles. Vascular
thrombosis resulting from the freezing of blood vessels may cause cell death medi-
ated by ischemia.
The lowest temperatures used and the cooling rate play an important part in
optimizing the treatment parameters. The treatment margin is relatively clear dur-
ing the procedure, and minimally invasive, percutaneous cryoablation can be per-
formed under ultrasonography for guidance. In situations where thermal ablation
methods risk causing thermal stricture, such as in the prostate or kidney,
cryoablation is preferred because it is not vulnerable to heat sink effects [9, 10].
However, there could be an increased risk of bleeding during the treatment, because
the blood vessels are not cauterized as they would be with thermal ablation at high
temperatures.
15.2.3 Chemical Ablation
Percutaneous chemical ablation, such as transcatheter arterial embolization and
absolute ethanol and acetic acid injection for treating hypervascular lesions, results
in several changes in the target tissue. The transport and distribution mechanism in
the neoplastic and nonneoplastictissue is important to deducethe effects. Due to the
injection of low-molecular-weight agents such as ethanol, the interstitial fluid pres-
sure increases, resulting in an increase in convective flux of the agent. High-concen-
tration agents produce an increase in diffusive flux. Various effects including
cytoplasmic dehydration, denaturation of cellular proteins, and small vessel throm-
bosis occur as ethanol distributes through tumor interstitium and results in
coagulative necrosis [11, 12].
15.2 Thermal Ablation Mechanisms 403