Ultrasound Flashcards
Describe Sound
Answer: Sound is simply a pressure wave (a form of mechanical energy) that travels in a longitudinal wave
A sound wave is created when a vibrating object (such as a cone in a speaker) sets the molecules of a medium (such as air) into motion
When the vibrating object contacts a medium, the molecules of the medium are compressed together, creating an area of high pressure. This is called compression
When the vibrating object moves away from the medium, it pulls the molecules of the medium along with it. This creates an area of low pressure called a rarefaction.
What is the difference between Frequency, Amplitude and Wavelength?
Frequency is the measure of a pitch.
- It tells us how many cycles occur in a given period of time.
- Measured in Hertz (Hz) or cycles per second
Wavelength is the distance between two identical points on adjacent cycles
- Frequency and wavelength are closely related.
- A higher frequency produces a shorter wavelength
- A lower frequency produces a longer wavelength
Amplitude represents the sound’s loudness (measured in decibels). It is determined by the degree of pressure fluctuations from the displacement of the molecules within the medium.
- A higher amplitude produces a greater pressure change and a louder sound
- A lower amplitude produces a lower pressure change and a softer sound
How does sound propagate through tissue?
Answer: Propagation velocity of sound through several mediums relavant to ultrasound:
- Air: 343 m/sec
- Soft tissue: 1,540 m/sec
- Bone: 3,000 - 5,000 m/sec
NOTE: when no medium is present (such as in a vacuum or outer space), there is no sound
What is echolocation?
Answer: The use of sound to visualize physical structures.
EXP: Bats use echolocation as the emission of ultrasonic sound waves by chirping and “listening” as the sound waves bounce off nearby objects.
By calculating how long it took the sound to make a roundtrip and the volume of the returning signal, we can determine the nature of objects around us and their position in space.
By emitting different frequencies, the bat can filter out non-essential information while it focuses on vital information
What is the piezoelectric effect?
Answer: a piezoelectric material can transducer electrical energy to mechanical energy and vice versa
If you apply electric current to a piezoelectric material, it will vibrate and emit sound
How does ultrasound technology work?
Answer: The transducer emits ultrasound waves into the body at a fixed rate then listens for echos between each pulse (process is repeated many times each second)
This sequence allows for a rapid refresh rate on the screen –> why the images appear to move dynamically
Break down one ultrasound pulse listen cycle step by step.
- THe ultrasound system applies an electric current to the piezoelectric elements inside the transducer, causing them to vibrate
- The piezoelectric elements generate ultrasound waves that descend into the body
- When an ultrasound wave encounters a boundary between two tissue types (ie: tissues of different acoustic impedance), the wave will echo off the structure and return to the transducer
- Echos returning to the transducer make the piezoelectric elements vibrate, causing them to transducer the mechanical energy into an electrical signal
- This electrical information is fed into an algorithm that plots the corresponding dot on the screen. This algorithm compares the total sound energy emitted from the transducer and the number and intensity of the returning echos, ultimately revealing a 2D, B-mode image of the patient’s anatomy
What determines the placement of each dot on an ultrasound machine?
Answer: The vertical placement of each dot is determined by how long it takes for the echo to return to the transducer (time delay)
The horizontal placement of each dot is determined by the particular crystal that receives the returning echo.
What determines the brightness of each dot?
Answer: the amplitude of the returning signals signal and echogenicity describe a tissue’s ability to transmit or reflect sound waves in the context of the surrounding tissues
The relative level of brightness is defined as:
Hyperechoic: they produce a strong, bright (high amplitude) echos
- these tissues have high impendance (EXP: BONE)
Hypoechoic: structures appear as a darker shade of grey
- they produce weak (low amplitude) echos
- these tissues have a lower impedance (IE: Solid organs, skin, adipose and cartilage)
Muscle also tends to be hypo echoic but muscle surfaces often produce hyper echoic fascial lines
- Muscle spindles may appear as small bright dots
Anechoic: structures appear black (they do not produce any echo)
- EXP: Vascular structures, cysts and ascites
How do vascular structures appear in short-axis view vs. long-axis view?
Answer: Vascular structures appear in black circles in short axis and black tunnels in long axis view
How do you compare arteries vs. veins on the ultrasound?
Answer: Arteries are pulsatile and relatively non-compressible (although they may be easier to compress if the patient is hypovolemic)
Veins dont pulsate, and they’re easy to compress (although a distended and non-compressible vein should raise suspicion for DVT)
Another way to differentiate veins is to look for their valves in the long axis view
- You can use color Doppler to visualize flow through a vessel
How does a nerve structure appear on the ultrasound?
Answer: PN imaging can be tough because it can appear hyper echoic or anechoic depending on the region of the body
PN near the neurosis tend to appear anechoic (black)
- Nerves may be confused with vascular structures
- PN are NOT collapsible nor pulsatile
Distal PN are hyper echoic (white) with a characteristic honeycomb appearance
- Distal nerves tend to be enveloped in fascia and connective tissue which accounts for a greater degree of echogenicity while the black circles inside the structures represent the nerves themselves
How do you determine if you are looking at a tendon or a nerve on ultrasound?
Answer: Tendons may look similar to peripheral nerves but typically without the honeycomb appearance
Best method to differentiate a nerve from the tendon is to scan proximally from a joint towards the muscle
Nerves are continuous and will not change in size as you scan
Tendons become flat and disappear as they connect to muscle. Additionally, they appear “rope-like” at joints where they connect to the bone
Discuss the resolution of an ultrasound
Answer: Axial resolution displays beam depth, lateral resolution determines beam width and elevational resolution determines beam thickness
Axial resolution:
ability to differentiate structures that exist along the length of the ultrasound beam
Lateral resolution
ability to differentiate structures that exist in the width of the ultrasound beam
Elevational resolution
ability to differentiate structures that exist in the thickness of of the ultrasound beam
What are the three zones to an ultrasound beam? Where is the image resolution best?
Answer: Focal zone, near zone and far zone. Resolution is best in the focal zone but also good in the near zone
Focal Zone = the region where the beam is the narrowest (x- and y-axis) and thinnest (z-axis)
Near zone (Fresnel zone) = the region between the transducer and the focal zone
Far zone (Fraunhofer Zone) = the region beyond the focal zone
Resolution in the far zone is reduced because the distance between each sound wave increases as the beam diverges
