Chemistry Reference
In-Depth Information
size and direction of the signals, which in turn affects the type of scatters produced. It is important to note that the amplitude
of the reflected signal, that is, the energy, is dependent on both the direction of the reflected signals and the reflection coef-
ficient [44].
In ultrasound, the waves are attenuated when they are reflected and scattered in the tissue as well as when they are
reflected and passed back to the probe. Typically, there is a 10% loss of total energy. The total energy decreases with
penetration because there is an increase in energy lost the deeper the sound waves travel into the tissues because the sound
waves are absorbed. Absorption in tissues is the most important cause of attenuation.
Absorption is extremely important in ultrasound for two reasons: depth penetration and safety concerns, the latter of
which is the major limiting factor for its use as an imaging modality. The heat produced from absorption changes the
temperature of the surrounding tissues, making it a limitation in ultrasound equipment due to safety issues for patients. The
other is penetration issues because the attenuation of the US waves increases with depth. There are many factors that affect
absorption such as the density of tissue and the frequency of the ultrasound beam. Good absorption generally results from
high tissue density and sound frequency; hence penetration can be increased by increasing the transmitted energy. However,
there are side effects such as tissue damage due to the extreme heat generated.
There are other types of ultrasound such as 3D ultrasound and Doppler ultrasound imaging based on the Doppler Effect.
3D imaging in general allows higher resolution imaging and thus provides more detailed information. This is commonly
used to assess foetus development, as well as for biopsies. Doppler ultrasound is generally used for the study of the rate of
blood flow through the heart and arteries [46].
There are many different ways to store ultrasound data, either in their full waveform as rF data that consist of both
amplitude and frequency data or in pulse form where only data of the amplitude are collected and is less demanding on the
storage systems. The signals can also be stored by taking the spectrum of frequencies from the reflected ultrasound pulse,
which is then represented as a numerical value per image pixel. This method of storage is commonly used in Doppler
imaging.
1.7.2
Advantages and limitations
Ultrasound imaging is virtually noninvasive; it is used in a variety of clinical settings, especially in obstetrics and
gynaecology, cardiology, and cancer detection. one of its most important uses is in studying and monitoring foetal
development. no radiation is used in ultrasound imaging and the procedure can be performed much faster than X-rays
and other radiographic techniques. Its major limitation is its poor penetration: US waves have difficulty penetrating
the bone because they attenuate when passing deeper into the body. The body is also acoustically homogeneous
because it contains around 70% water, thus it is difficult to discriminate the interface between the tissue and blood,
but real-time evaluation of blood flow is possible. In general, ultrasound generates more heat as the frequency
increases, so the ultrasonic frequency has to be carefully monitored. The signal intensities can be enhanced by the
intravenous injection of contrast agents such as microbubbles at very low dosage, allowing the technique to remain
minimally invasive.
nevertheless, there are still safety concerns because the local temperature of tissue increases because heat is developed
when the tissue or water absorbs the ultrasound energy. This local heat can also cause formation of cavitation.
Additionally, microbubbles have low circulation residence times and are easily taken up in certain locations—for
example, the liver and the spleen—and can be destructed, inducing local microvasculature ruptures. Image enhance-
ment can generally be improved by having high acoustic power output, but this again has to be compromised with the
contrast agents, because high mechanical indices as well as low ultrasound frequencies tend to cause the microbubbles
to burst. However, such properties can be beneficial for therapeutic purposes: Some studies have shown that the
destructive nature of the microbubbles can be exploited for drug targeting and delivery. Despite concerns over the tech-
nique, US imaging is the most efficient technique with regard to cost, time, and safety compared to other imaging
modalities such as MrI, PET, and SPEcT, making it one of the most frequently used diagnostic techniques. recent
developments in ultrasound imaging have improved the resolution as well as the technique itself, allowing it to be
incorporated into other methods such as photoacoustic imaging, a technique that uses the properties of both light and
sound.
