Biomedical Engineering Reference
In-Depth Information
attenuation in the overlying tissue may be simply estimated and
should be documented in writing with a diagram. These data may
obviously vary due to the tumor location and patient position for
treatment. Acoustic properties of the overlying tissues, the inter-
faces between the tissues, and incident angle of ultrasound beam
can obviously affect the ultrasound attenuation. A large amount
of energy reflection occurs when a focused ultrasound beam
propagates from muscle layer to the bone surface such as ribs.
This can cause the loss of energy deposition in the targeted vol-
ume, and may increase local temperature of muscle-bone inter-
face because the amplitude attenuation coefficient of ultrasound
is about 10-20 times higher in bone than in soft tissue [78,79].
Furthermore, attention must be given to the large variations in
the overlying tissue thickness among individual patients (e.g.,
thin or overweight patient).
It is believed that there is only a small amount of energy reflec-
tion between the soft tissue interfaces because of small variations
in the acoustic impedance of soft tissues. The largest one is the
interface between fat and muscle tissues, but it causes only 1-2%
of the beam intensity to be reflected [78,79]. It must be noted
that some exceptional cases are observed in clinical application.
For instance, the beam reflection is theoretically very low on the
interface between subcutaneous tissue and rectus abdominis. As
fibrous tissue forms a strong layer of connective tissue on the
surface of the rectus abdominis, it can cause a higher reflection
of ultrasound beam on this interface.
restricted by the limited size of the treatment bed. Attention
should be given when the patient's position is selected.
Immobilization is required during the HIFU procedure. It var-
ies according to the technique being used and the location of the
tumors. Total physical restraint is essential to protect the patients
who are receiving one kind of anesthesia method. Appropriate
immobilization can be achieved with the various fixing aids such
as hanging belts of sponge, pillows, leg restraints, and footrests. It
must be ascertained that the respiration of patients is free while
the fixation is performed, without any pressure effect on the chest
and the abdomen.
Clinical studies have reported that there is organ motion
occurring in the body. Either diaphragm movement during
respiration or the pulsating of large arteries usually causes this
motion. Furthermore, organ movement may not be uniform,
but dependent on the individual patient. It is recommended
that this variation be previously estimated, and then measured
using real-time ultrasound imaging during the planning ses-
sion for each patient. The movement of the diaphragm caused
by respiration can obviously move some internal organs such
as liver, pancreas, and kidney. It is very important to control
the large movement of these organs during the HIFU proce-
dure, particularly for the small volume tumor. An alternative
technique of active breathing control involves immobilized
breathing using general anesthesia; this method may be used
for treatment of the liver and kidney, where excursions of up to
3 cm may occur.
Once the patient is correctly positioned and secured, the skin
overlying the tumor is brought in contact with degassed water or
a rubber bag filled with degassed water via acoustic gel, so that
the ultrasound beam can transmit from the water into the tis-
sue. The positions of the entire targeted tumor and surrounding
vital structures are determined using the real-time diagnostic
ultrasound imaging. With the movement of the diagnostic probe
from one side of the tumor to the other, images of the targeted
volume are achieved (Figure 15.1). Then, 3D ultrasound imaging
of the tumor can be done by using sequential scans of the treat-
ment volume.
For therapeutic purposes, the entire tumor is segmented
into slices for ultrasound imaging; the spacing between slices
depends on the size, shape, and location of the region of tumor
and acoustic window.
The extent of the separation is usually
about 5-10 mm though it varies in different tumors. The tar-
geted volume is defined as the volume of tissue that includes
the tumor visualized on the ultrasound imaging and regions
considered to be at risk for microscopic extension. If the mar-
gin of the tumor is not clear enough, MRI or CT scans are avail-
able to make it clearer. By using a tracker ball or mouse with
an interactive software program, the contours of the targeted
volume can be manually outlined at the planning computer
on each ultrasound slice and then transferred to the planning
system. Finally, the targeted volume that includes the margin
with appropriate margins at all levels is built into the computer
memory.
15.4.6 therapeutic planning Using
US-Guided HIFU Device
Using an USgHIFU device the position of a patient is decided
with the help of an imaging system equipped in the device. This
position must be comfortable, reproducible, and suitable for
acquisition of imaging for planning and subsequent treatment. It
is technically ideal to achieve either the shortest way or enough
surface extent of the acoustic path for the treatment of a targeted
volume. The choice of position for treatment is dependent on the
tumor location, size, and shape, which are shown on both MRI/
CT and ultrasound imaging before the planning process, though
palpation is important to the superficial tumors such as breast
cancer. It is necessary that the patient be treated in one position
during the whole process of the procedure. The change of patient
position may result in alterations in internal and external anatomy
and risk of mis-target. However, if the patients require a change
of position for the purpose of treatment during the procedure, it
must be technically designed in advance and carefully recorded,
so that the over- or underdosage of the acoustic energy within the
targeted volume can be avoided.
The position of the patient, all positioning aids, and anatomi-
cal measurements should be accurately documented in writing
with a diagram and digital photos to ensure reproducibility
through all stages of the planning process and subsequent treat-
ment. However, sometimes the position may be constrained by
equipment limitations. For instance, the arm position may be
