Page 33 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

30 CHAPTER 2 Diagnostic imaging in cancer

The basic steps of ultrasound imaging are:
1. Ultrasound wave is generated by ceramic probe containing a crystal. The
main mechanism of creating ultrasound wave relies on the piezoelectric effect
which converts electrical energy to the mechanical energy makes the crystal to
vibrate.
2. The wave is passed through the specimen and partially reflected at each tissue
interface (between fluid and soft tissue as well as soft tissue and bone). All of
the sound waves are undergoing the following phenomenon during propagation
inside the specimen: refraction, diffraction, scattering, and attenuation.
3. The reflected sounds are picked up by the piezoelectric crystal. In this step,
mechanical waves are transferred to electrical energy.
4. The distance between the boundaries in the tissue and the scan probe is
calculated using speed of sound and then converted to the signals in the machine
displays.
The large variety of applications has been considered for ultrasound imag-
ing, including vascular diagnosis, tumor examination, following organ after
transplantation.
Beside diagnostic interests, high intensity focused ultrasound (HIFU) has been
absorbed a great deal of attention due to the valuable therapeutic potential for
destroying kidney stone [21], benign and malignant tumor [22]. In this approach, the
ultrasound waves are focused on the pathological tissue with high energy and then
the acoustic energy is absorbed by targeted tissue. So the temperature of the tissue is
increased leading to destroyed the pathologic tissue.
To improve ultrasound imaging features, ultrasound contrast imaging which
composed of gas-filled microbubbles surrounded by a lipid-based shell has been
introduced. Ultrasound contrast imaging examines pathological tissues regarding
functional and molecular vascular properties.
One of the main applications of ultrasound CA is related to the liver examination
due to the considerable improvement in compared to conventional ultrasound imag-
ing which may lead to the detection of malignant liver in early stage and rapid wash-
out of liver CA [23]. Furthermore, ultrasound CA is administrated for the patients
having contra-indications to MRI or CT contrast-enhancer due to the sever renal
dysfunction.
The efficiency of CA is examined based on its sensitivity and specificity. Micro-
bubbles-based CA has high sensitivity even a single of which can be detected. Also,
due to the microbubbles size (1–5 µm) they do not extravasate from the vessels which
has both advantages and disadvantages. On the one hand, targeting to interstitial
space is completely feasible. On the other hand, extravascular targeting is hard to
achieve due to their size.
Three-dimensional ultrasound consists of two main categories. One is the 2-D
imaging equipment coupled with a specific mechanical movement to build 3-D
imaging. The other is online monitoring of volumetric echo which composes of spe-
cific transducer for 3-D volume scanning.