2. Various sound waves:
Audible 20Hz and 20 000Hz.
Infra sound Below 20Hz
Ultrasound Above 20 000Hz
Ultrasound is a high frequency mechanical vibration waves above a
frequency the human ear can hear.
Ultrasound uses a pulse-echo technique of imaging the body.
Pulses transmitted into patient and give rise to echoes when they
encounter interfaces/reflectors.
These interfaces/reflectors are caused by variations in
the "acousitc impedence" between different tissues.
Echo signals are amplified electronically and displayed on a monitor
using shades of grey (from black to white), stronger reflectors =
brighter shades of grey and appear white in an image. Those
with no echoes will appear black, such as a full bladder.
3. Acoustic impedance interactions of ultrasound with tissue
Acoustic Impedance (Z) is a measure of the resistance to the sound
passing through a medium.
Z = density x speed of sound.
It is similar to electrical resistance. Materials with a higher density
causes higher acoustic impedance, eg bone. Gases have low acoustic
impedance.
Impedance mis-match
A difference in acoustic impedances cause some portion of the sound
to be reflected at the interface. Whenever a sound wave encounters
a material with a different density (acoustical impedance), part of
the sound wave is reflected back to the probe and is detected as an
echo. The greater the difference between acoustic impedance, the
larger the echo will be. If the pulse hits gases or solids, the density
difference is so great that most of the acoustic energy is reflected
and it becomes impossible to see deeper.
4. Properties of an ultrasound wave
•Frequency higher than 20 000Hz (20kHz)
•Propagation of sound waves longitudinal
•A medium is needed for sound waves to go through, no medium = no
sound waves
Interactions of ultrasound with soft tissues
When an ultrasound wave passes through tissues the following things
can occur
•Reflection
•Scattering
•Absorption
•Refraction: Change in direction & velocity of wave
5. Working Principle of Ultrasound Imaging:
Ultrasound waves are produced by a transducer, which can both emit ultrasound waves, as
well as detect the ultrasound echoes reflected back. In most cases, the active elements in
ultrasound transducers are made of special ceramic crystal materials called piezoelectrics.
These materials are able to produce sound waves when an electric field is applied to them,
but can also work in reverse, producing an electric field when a sound wave hits them.
When used in an ultrasound scanner, the transducer sends out a beam of sound waves into
the body. The sound waves are reflected back to the transducer by boundaries between
tissues in the path of the beam (e.g. the boundary between fluid and soft tissue or tissue
and bone). When these echoes hit the transducer, they generate electrical signals that are
sent to the ultrasound scanner. Using the speed of sound and the time of each echo’s
return, the scanner calculates the distance from the transducer to the tissue boundary.
These distances are then used to generate two-dimensional images of tissues and organs.
6. Commercially available contrast media are gas-filled microbubbles that are administered
intravenously to the systemic circulation. Microbubbles have a high degree of
echogenicity (the ability of an object to reflect ultrasound waves). There is a great
difference in echogenicity between the gas in the microbubbles and the soft tissue
surroundings of the body. Thus, ultrasonic imaging using microbubble contrast agents
enhances the ultrasound backscatter, (reflection) of the ultrasound waves, to produce a
sonogram with increased contrast due to the high echogenicity difference.
Targeting ligands that bind to receptors characteristic of intravascular diseases can be
conjugated to microbubbles, enabling the microbubble complex to accumulate
selectively in areas of interest
Microbubble contrast agents
General features
There are a variety of microbubble contrast agents. Microbubbles differ in their shell
makeup, gas core makeup, and whether or not they are targeted.
Microbubble shell: selection of shell material determines how easily the microbubble is
taken up by the immune system.
Microbubble gas core: The gas core is primary part of the ultrasound contrast
microbubble that determines its echogenicity.
7. Untargeted microbubbles are injected intravenously into the systemic circulation in a
small bolus. The microbubbles will remain in the systemic circulation for a certain
period of time. During that time, ultrasound waves are directed on the area of
interest. When microbubbles in the blood flow past the imaging window, the
microbubbles' compressible gas cores oscillate in response to the high frequency
sonic energy field, as described in the ultrasound article. The microbubbles reflect a
unique echo that stands in stark contrast to the surrounding tissue due to the orders
of magnitude mismatch between microbubble and tissue echogenicity. The
ultrasound system converts the strong echogenicity into a contrast-enhanced image
of the area of interest. In this way, the bloodstream's echo is enhanced, thus allowing
the clinician to distinguish blood from surrounding tissues.
Source: https://en.wikipedia.org/wiki/Contrast-enhanced_ultrasound
