Ultrasound Physics & Knobology
Physics in 1 minute
All US transducers contain 3 key layers:
Piezoelectric crystals (PZT)- Vibrate to generate sounds waves which will go out to hit tissue and bounce back and be converted to electric signal and thus an image
Matching layer- Allows easy transmission of sound waves just as US jelly does
Backing material- Dampening agent that gives time for the PZT crystals to 'listen' for return signal
High Frequency Probe
Linear or intracavitary probes
Low Frequency Probe
Curvilinear or phased-array probes
Sound Wave Components:
- Frequency: Number of cycles over a period of time measured in Hertz (cycles/second)
- Period- Time from beginning of wave to the end
- Wavelength: Distance from beginning of wave to the end (which is inversely proportional to period)
- Amplitude: Height of the wave
Take Home: Long wavelength will penetrate deep into body and have low resolution since it cannot discriminate between close structures while short wavelength will penetrate shallow and discriminate highly between close structures.
Other Key Concepts:
- Speed: US travels through different mediums at different speeds. This will be important when thinking about artifacts created during scanning. For example: Air 500 m/s -> Soft Tissue 1540 m/s -> Bone/Solids 3000 m/s
- Pulsed Ultrasound: Most of the time the ultrasound is listening for returning echoes. For example, a pulse may be sent from PZT for 1 msec and will listen for the next 999 msec for returning sound waves.
- Time = Distance: The US machine determines how far from probe objects are based on the time it takes for sound wave to return after hitting that object.
More terminology: Make sure you know how to describe what your seeing.
- Near field: Half of US screen closest your probe.
- Far field: Half of US screen farthest from your probe.
- Hyperechoic: Appears bright and white which means that it contains high calcium content such as bone/stones/tendons.
- Anechoic: Appears black on screen which means it is fluid or artifact known as shadowing
- Grey areas: The inbetween. Describe these areas with respect to surrounding tissue
The art of turning knobs. There are several knobs that we need to know how to use to make our images look better but first a quick look at probe terminology.
- Gain: This determines overall brightness. Optimal gain allows for proper distinction of structures while too much gain leads to artifact
- Time Gain Compensation: This allows you to vary the gain according to depth. For instance you may have a bright structure in the near field and want to see the far field. You can preferentially turn down gain in the near field or turn up gain in the far field independently.
- Depth: This determines essentially how long the US machine will listen for returning signals. Make sure to decrease depth to optimally characterize necessary structures. Your point of interest should be in the middle of the screen ideally.
- Focus: This allows you to heighten resolution of particular depth within the particular view on your screen. If this doesn't matter then place focus at bottom of the screen.
- Modes: Different modes serve a particular function
- B Mode/2-D/Grey scale is our standard mode.
- M Mode stands for motion mode which allows us to look at a particular line of interest on the screen and observe this line over time.
- Color doppler analyzes fluid flow towards and away from probe and displays this as a color gradient.
- Power doppler is useful for presence or absence of flow but does not show direction.
- Pulsed Wave doppler is useful for fluid velocity, assessing hemodynamics and cardiac function.
These are tricky little things. In a nutshell, artifacts are structures that don't represent true anatomic structures.
- Shadowing: Represent areas where acoustic signal is not penetrating. This is not limited to bony structures but rather anything with a high calcium content, such as those gallstones you're seeing in the gallbladder.
- Reverberation: Ultrasound pulse strikes tissue and bounces back to transducer and then back into tissue and back to transducer again taking 2x,3x,4x as long making it seem as if there are multiple similar appearing structures. This is often seen when evaluating the pleural surface. Since these are in multiples, the distance between each artifact should be equal.
- Edge artifact: This occurs as a result of refraction as wave changes medium from air-sold or air-fluid interface leading to shadowing behind structure as sound waves at bent in a different direction. This is often seen along the edge of the gallbladder, do not confuse this with a gallstone shadow.
- Acoustic enhancement: This results from the fact that when ultrasound waves travel through fluid there is not as much attenuation as compared to tissue. Therefore tissue deep to fluid filled structures will appear brighter. You may see this when looking at the bladder or gallbladder.
- Ringdown: This is in essence a very rapid reverberation and is often seen with metallic objects or pulmonary edema. For example, b-lines seen in pulmonary edema are actually constructed of very tightly bound horizontal lines.
- Mirror image: This is often seen around the heart or the diaphragm. When an ultrasound beam hits the diaphragm it reflects off and changes direction, hits tissue, bounces back and hits diaphragm and then returns back to transducer. Remember that time = distance so therefore the machine will assume that this object is past the diaphragm given the time it took to return. When you see a mirror image of liver on the other side of the diaphragm this lets us known that there is no consolidation or fluid in the lung base.