Biomedical Engineering Reference
In-Depth Information
treatment is the same as with CT. However, in thermometry
techniques, MRI can be used as an imaging-guided device to
record focal temperature rises on anatomic images during HIFU
procedures [85-88].
Due to its extensive availability, real-time visualization,
flexibility, nonionization, and low cost, ultrasonography is
widely used in clinical practice. It can show tomographic views
in almost any orientation. Compared to CT/MRI, ultrasonog-
raphy produces images with poor resolution. Furthermore, as
ultrasound beams cannot penetrate bone or gas-filled cavities
such as the lung and gastrointestinal tract, the application is
limited. Medical ultrasound imaging systems can be employed
as a real-time imaging guided device to identify and target the
tumor to be treated, and to monitor HIFU procedures [89-93].
They can also be used to assess the tissue response through
grayscale changes caused by cavitation and tissue boiling in
focal volume, which is an indication of ablation following each
exposure [94].
DSA is a standard part of the evaluation of patients with a sus-
picious malignant tumor. It demonstrates the tumor vascularity
and stain, the extent of tumor mass, and the anatomic variants
of the regional arterial supply. The use of this technique yields
satisfactory diagnostic and anatomic details in most cases of
solid malignancies. Because it is an interventional examination,
extensive applications of DSA are relatively limited in clinical
practice.
SPECT is a radionuclide scanning technique that uses a radio-
isotope as a tracer to assess abnormal tumor function, rather
than to provide simple anatomy. The radioisotopes, such as
99m
Tc
sestamibi and
99m
Tc methylene diphosphate, usually have a high
extraction rate at the malignant tumors. It can provide physio-
logical imaging information through tomographic images of the
radioactivity distribution within tumor tissue.
PET can determine the level of localized radioactivity by
detecting a sufficient number of the two 180-degree opposed
photos and using imaging reconstruction algorithms. It is
another functional type of imaging that can provide localized
physiological information on the presence of tumors.
15.6.3 Ultrasound Imaging for HIFU procedure
To optimize the ablation of a tumor, it is fundamental to use
precise and dependable imaging techniques for ascertaining the
adequacy of the treatment. This imaging can be used as an imag-
ing-guided device to identify and target the tumor to be treated,
monitor the therapy procedure, and assess therapy response
within targeted tissue. In particular, recent studies involved with
the development of imaging procedures enable rapid assessment
of the extent of targeted tissue destruction caused by HIFU abla-
tion. As a result, either ultrasonography or MRI is employed to
guide extracorporeal HIFU procedure. Individual preferences or
personal research interests may dictate the selection of imaging
technique.
There are advantages and disadvantages in both ultrasonog-
raphy and MRI when each type of device is incorporated into a
HIFU system. Clinical ultrasound imaging devices are inexpen-
sive, extensively available, flexible, capable in real-time visualiza-
tion, and valuable in the treatment of organs such as the liver and
kidney, which are moved by respiration. The main disadvantage
of ultrasonography is poorer imaging resolution than MRI, par-
ticularly in predicting tumor margins. It may provide occasional
poor lesion detection through lack of inherent tissue contrast or
because of overlying bone structures such as ribs. MRI can pro-
vide three-dimensional imaging with better resolution. Using
indirect thermometry technique it can measure focal tempera-
ture rises following HIFU exposure. However, high cost, long
treatment time, and problems in tracking a moving target such
as the liver may limit extensive application of MRI-guided HIFU
devices in clinics.
Because of ultrasonography's poor resolution, good-quality
MRI is effectively used to compensate for some of the short-
comings of diagnostic ultrasound imaging, if an USgHIFU
device has a problem in predicting the tumor margin. A com-
bined utilization of MRI and ultrasonography before HIFU
treatment can fully provide accurate imaging information for
determining the number, sizes of tumors, and their relationship
to surrounding vital structures. While the USgHIFU device is
used in our clinical application, we feel that preoperative MR
imaging is very helpful to establish the 3D coordinates of the
targeted tumor in a HIFU planning session. We routinely com-
pare the difference between the MRI and US image appearances
in creating a HIFU treatment plan for the complete ablation of
a tumor.
B-mode ultrasound imaging can provide significant changes
in imaging information within the focal volume during a HIFU
procedure. These changes are made evident by increased levels
of tissue grayscale on ultrasound imaging immediately after
HIFU exposure, and most of them become gradually less evi-
dent and sometimes disappear within several minute after the
ablation. In our animal studies, both
in vivo
and
ex vivo
, we find
that hyperechoic zones in the targeted tissue correspond mainly
to the extent of the coagulation necrosis. There is a close rela-
tionship between the extent of necrosis as measured by gross
examination and the hyperechoic extent measured immediately
15.6.2 Medical Imaging for HIFU planning
Diagnostic images used in the preparation of HIFU ablation
have been described in Section 15.4.2, imaging for HIFU
planning. Most anatomic image modalities can be employed
to provide anatomical information for determining the loca-
tion, number, and size of tumors and surrounding vital struc-
tures. For instance, in the treatment of liver cancer it is very
important preoperatively to know the relationship between
the tumor and its surrounding structures, such as bile ducts,
gallbladder, gastrointestinal tract, diaphragm, and large blood
vessels. These pre-HIFU images are fundamental images on
which the suitability of a patient for HIFU ablation is assessed.
In addition, physiological imaging may be used as a pre-HIFU
image for the purpose of assessing tumor response to the
ablation.
