Biomedical Engineering Reference
In-Depth Information
contrast-enhanced MRI/CT will be performed as an essential
imaging examination to compensate for some of the shortcom-
ings of ultrasound imaging, such as less sensitivity in predict-
ing the tumor margin, and occasional poor lesion detection as a
result of overlying bone (e.g., ribs). It needs to be routine prac-
tice to compare the difference between the MRI/CT and ultra-
sound imaging appearances before planning HIFU treatment
protocol.
Because of the fact that the ultrasound energy is attenuated
while it penetrates through the tissue, it is very important to
achieve a large surface area for the acoustic path, and then the
ultrasound beam can sufficiently be converged through the tar-
geted volume. This geometrical gain of focusing is necessary to
overcome the attenuation loss and to induce complete coagula-
tion of the targeted volume. However, several factors are involved
to influence the extent of the path, including the diameter of
HIFU transducer, frequency, tumor depth and volume, and the
structure of the overlying tissue, as well as the principal method
that is used to make the transducer.
15.4.3 tumor Volume Localization
The targeted tumor volume is described as the macroscopic
extent of the tumor, which is palpable, visible, and detectable by
available radiological examinations. If the determination of the
targeted volume depends on the diagnostic techniques includ-
ing CT/MRI and ultrasound imaging, it is essential to indicate
which methods have been used for its determination.
Tumor cells most likely extend beyond the actual tumor mar-
gins into normal tissue, and recently available image techniques
may not exactly circumscribe acute tumor margins. The failure
to achieve a satisfactory ablation margin will be accompanied by
a high local recurrence ratio. However, available data for quanti-
fying this margin of tumor volume are lacking. The definition of
targeted volume is based on knowledge from surgical and post-
mortem specimens, and patterns of tumor recurrence, as well as
from clinical experience. It may be the most difficult and subjec-
tive step in the HIFU planning process.
For curative treatment, the aim is to induce complete coagula-
tion of a targeted tumor and the estimated extent of the micro-
scopic spread surrounding the visible tumor. This is a principle
that is routine in conventional surgery in order to ensure the
removal of adjacent microsatellites, and to allow for the uncer-
tainty that frequently exists concerning the exact location of defi-
nite tumor margins. A number of factors, such as the age of the
patient and considerations of normal tissue tolerance, may affect
the maximum volume considered to be appropriate for treat-
ment. For instance, hepatocellular carcinoma is frequently seen
in the setting of hepatic cirrhosis with partial liver dysfunction.
Definition of targeted volume is cautiously determined for each
patient. If the extent of ablative tissue is the same as that of the
surgical resection, including the tumor and a 2 cm margin of
obviously normal tissue surrounding the tumor, HIFU may offer
its equivalent effectiveness and considerably decreased risk of
morbidity and mortality in this high-risk patient population.
15.4.5 Ultrasonic properties of
Overlying tissues
Although many factors can influence the energy deposition of
focused ultrasound beam, the absorbed power density in the
targeted volume can be estimated. One of the most important
factors is attributed to the ultrasonic property of overlying tis-
sues between the skin and the target. The beam is attenuated
by absorption and scattering while it propagates through these
overlying tissues. It is also reflected from the surfaces of various
structures in the overlying tissues. Furthermore, the heteroge-
neity of the overlying tissues, such as ribs and muscular tendons,
may change the beam propagation direction, focus location, and
shape and size of a focal area. Thus, it is very important to know
the ultrasonic properties of the overlying tissues, and a precise
analysis of the acoustic properties of the overlying tissues dur-
ing the treatment planning for an individual patient may avoid
undesirable effects, and thereby achieve a successful HIFU
ablation.
Tissue ultrasound amplitude attenuation is a sum of the losses
from absorption and scattering of ultrasonic energy in tissue.
Each tissue has an attenuation value of its own. For instance, the
amplitude attenuation values in lung and bone tissue are approxi-
mately 430-480 Npm
−1
MHz
−1
and 150-350 Npm
−1
MHz
−1
,
respectively, which are the highest values among the living tissues
[77-79]. Fatty tissue has lower values, ranging from 5 to 9 Npm
−1
MHz
−1
, and the soft tissue values are generally about 10 Npm
−1
MHz
−1
[80-82]. Compared to normal tissues, the attenuation
in the tumor is usually higher. The attenuation in brain tumor
is higher than in the normal brain [83], and similar difference
is also detected between breast cancer and normal breast [84].
However, the definition of the attenuation in the overlying tissue
and the tumor is relative. While the deep part of a targeted tumor
is treated with HIFU, it also includes the normal tissues between
the skin and the superficial margin of the tumor and the neoplas-
tic tissue in the front of the focus.
Images taken from the treatment planning can provide useful
information about the overlying tissues. By using a HIFU device
they are reassessed under the guidance of real-time ultrasound
imaging when the patient lies on the treatment bed in a cor-
rect therapy position. The composition, interface, and thickness
of the overlying tissue are carefully recorded. Then, ultrasound
15.4.4 acoustic path for Ultrasound
Energy Entry
Acoustic path is defined as a direct path between a HIFU trans-
ducer and a targeted volume at depth, where the focused ultra-
sound beam from the radiating surface can directly penetrate
through the overlying tissue of the targeted volume and thereby
destroy the tumor. This path is usually the shortest route of
focused ultrasound beam from the skin to the targeted volume.
