Biomedical Engineering Reference
In-Depth Information
by whether there could be excessive energy deposition before
or beyond the targeted area, the sensitivity of normal tissue
around the target, and whether a patient could tolerate the entire
HIFU procedure in terms of general conditions. The effective-
ness depends on the range of the targeted area and whether
there should be sufficient energy deposition for thermal abla-
tion. The therapeutic efficiency is based on both the effectiveness
and safety of HIFU therapy, which is determined by the rate of
energy deposition in the targeted area.
Medical history, physical examination, histopathological
diagnosis, CT/MRI imaging, B-mode, and color Doppler ultra-
sound and tumor/site characteristics are collected for treatment
planning. Tumor characteristics include histological type, loca-
tion, size, depth, blood supply, functional status, motion caused
by respiration, the structures in the beam path, and surrounding
organs. The patient's general condition, treatment status before
HIFU, and the purpose of the HIFU treatment are also consid-
ered. After comprehensive considerations, a HIFU treatment
plan is established for each patient. It is composed of the choice
of the transducer, the 3D conformal scanning strategy, the selec-
tion of the therapeutic dose to be delivered to the tumor, and
whether the patient will receive adjuvant cancer therapy such
as chemotherapy after HIFU. Based on the treatment plan, the
HIFU procedure is subsequently performed. However, the treat-
ment plan can be revised during the HIFU procedure based on
real-time imaging provided by the diagnostic ultrasound capa-
bility. After the HIFU treatment, radiological examinations are
performed to assess the therapeutic effects. This is significant in
identifying whether coagulation necrosis has occurred in the
treated tumor, and whether a second HIFU session needs to be
applied as shown in the HIFU treatment planning flow chart
(Figure 10.12).
It is fundamental to use a 3D conformal treatment strategy.
As the HIFU cigar-shaped focal volume is small, approximately
20 mm in length and 2 mm in width, the size of the coagula-
tion necrosis caused by a single sonication is accordingly small
in biological tissues. Therefore, it is essential to move the HIFU
focus within the targeted tumor until the entire tumor is
ablated. There are two methods to produce a line-shaped abla-
tion. One is to scan the focus continuously while sonicating.
The other is to use multiple separate sonications in line, and the
location of each sonication is overlapped in order to avoid living
tumor remaining between consecutive exposures. By scanning
the focus in successive sweeps from the deep/distal to shallow/
proximal regions of a tumor, the targeted regions on each slice
can be completely ablated. This process is repeated slice by slice
to achieve complete tumor ablation, as shown in Figure 10.13a.
During focused ultrasound ablation of each slice, real-time US
images obtained before and after each exposure are compared to
check for echogenic changes, which indicate the extent of coagu-
lation necrosis. Figure 10.13b shows that using HIFU 3D abla-
tion scanning, a volume of coagulation necrosis is achieved from
spot-, line- and slice-shaped tissue destruction in
ex vivo
ox liver.
The margin between the treated and untreated liver is clear, and
there is no living tissue within the treated area.
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Electric power (W)
FIGURE 10.11
The electric-sound power curve diagram of an HIFU
transducer.
systems (Standard YY0592-2005) in December 2005. It is the
first national standard for HIFU products for clinical applica-
tions, which has been useful in regulating the manufacture and
clinical use of HIFU systems.
HIFU QA includes the following aspects. The acoustic power
from the HIFU transducer is measured using the radiation force
method. The relationship between the measured ultrasonic
power and the corresponding electric power must be known and
stable (Figure 10.11). All data are stored in the central operator
console platform, which are used as a reference during the HIFU
procedure for real-time monitoring of the electric power deliv-
ered to the transducer, and for feedback control of the acoustic
power and calibration procedures.
A calibrated PVDF hydrophone is used to obtain techni-
cal parameters of the HIFU acoustic field, such as sound fre-
quency, the full width half maximum (FWHM) of the pressure
field, maximum side-lobe level, and focal length. These data
assist operators in the selection of an appropriate HIFU trans-
ducer for treatment. In a US-guided HIFU system, the focal
length measured by the PVDF hydrophone in a water tank has
to be verified
ex vivo
in both a tissue-mimic phantom and in
a biological tissue sample such as ox liver. Immediately after
a HIFU exposure, ultrasound imaging shows a hyperechoic
change
ex vivo
in the focus, indicative of the place of coagula-
tion necrosis in the tissue-mimic phantom and biological tis-
sues. This is very useful in determining the actual focal length
of the HIFU transducer in living tissues. The movement accu-
racy of the integrated transducer measured along the x, y, and
z axes should be ±1 mm.
10.3.4 treatment planning
Comprehensive considerations were given to the design of the
treatment planning system (TPS), especially with regard to bio-
logical factors for each patient that may affect the safety, effec-
tiveness, and efficiency of HIFU therapy. The safety is assessed
