Intro to Ultrasound Flashcards
What is ultrasound?
High-frequency sound waves–beyond the range of (human) hearing (20 Hz - 20 kHz)
How many MHz is diagnostic ultrasound?
1-30 MHz
What tomographic modality is US?
First
What are the physics behind ultrasound?
- Mechanical energy as soundwaves are transmitted through a patient (tissues) and returning echoes are recorded
- Assumption: a constant velocity of sound within soft tissues even though differences exist
- Propagation velocity (1540 m/s)
- The interface: acoustic impedence (Z) = velocity (v) x tissue density
Quantifying sound
- Wavelength
- Frequency
- Velocity = wavelength x frequency
- Inverse relationship (important in choosing a transducer
Wavelength vs. frequency
- Wavelength = distance between one peak or trough and the next peak or trough (mm)
- Frequency = cycles per second (Hz)
Why are frequency and wavelength important?
- Resolution
- Better with higher frequency
- Small wavelength
- Penetration
- Better lower frequency
- Long wavelength
- Attenuation
- Occurs with higher frequency
- Less returning information
What happens to the sound as it interacts with tissue in US?
- Attenuation
- Reflection
- Acoustic impedence
- Refraction
- Absorbtion
- Reflection
- Transmission
What is attenuation? What is it increased with?
- The loss of ultrasound
- Increased with:
- Increased distance from the transducer
- Less homogenous medium to transverse due to increased acoustic impedance mismatch
- Higher frequency (shorter wavelength) transducers
Acoustic impedance values
- Air = 0.0004
- Bone = 7.80
- Both are strong interfaces
Acoustic impedance–general
- Impedance (Z) is a characteristic of the propagation medium
- A reflected sound wave is generated at the interface of an impedance mismatch
- No reflections occur in a homogenous medium (constance impedance)
Principles of impedance (3)
- Pulse-echo principle
- Emitting 1% of the time, listening 99% of the time
- Round-trip transit time is directly related to the depth, i.e. distance of the wave reflection site
- The amount of reflected sound depends on the acoustic impedance
How deep is the interface (explain reasoning)?
- U/S assumes speed of sound in tissues is 1540 m/s
- Sound is sent–timer is started, sound hits interface (time to interface)
- Sound is reflected, hits transducer–timer is stopped (total round trip time)
- Total round trip time needs to be divided by 2 to represent interface
- In one second sound travels 1540 m
What assumptions does the U/S machine make?
- The speed of sound in all tissues is 1540 m/s
- The U/S beam only travels in a straight line with a constant rate of attenuation
- The U/S beam is infinitely thin with all echoes originating from its central axis
- The depth of a reflector is accurately determined by the time taken for sound to travel from the transducer to the reflector and return
T/F: Artifacts are present in only some ultrasonagrams and are never helpful.
FALSE–Artifacts are present in EVERY single ultrasonagram and can be helpful or confusing.
What are the 6 artifacts?
- Acoustic shadowing
- Acoustic enhancement
- Edge shadowing
- Reverberation artifact
- Slice thickness artifact
- Mirror image artifact
Acoustic shadowing (general)
- Distal to highly reflective objects (high acoustic impedance mismatch)
- Bones
- Air
- Interface absorbs or reflects entire sound
- Results in an anechoic area
Clean acoustic shadowing
- At the tissue-bone interface
- Substantial amount gets absorbed
- Complete absence of reverberation artifacts
- A “clean” shadow is produced (homogenous anechoic)
- Ex: FOREIGN BODY
Dirty acoustic shadowing
- At the tissue-gas interface
- 99% of the sound wave gets reflected
- Acoustic shadow is dirty (inhomogenous/reverberation artifact)
Acoustic enhancement
- Fluid of homogenous acoustic impedance attenuates less sound than the surrounding tissue
- Machine processing compensates (–> overcompensation)
- Results in a hyperechoic area distal to the structure in comparison to the surrounding tissue
Edge shadowing
Small shadow at the edge of round structures
Slice thickness artifact
- At curved surfaces
- urinary bladder
- gall bladder
- Can mimic sediment
- Curve of structure gives 2 colors–> machine takes average
Mirror image artifact
- At highly reflective air/fluid interfaces
- Diaphragm-lung
- Pericard-lung
- Concave structures
- False image is produced on other side of the reflector due to its mirror like effect
- Mirror image is not useful
What are the 4 transducer types?
- Multi-frequency
- Linear
- Convex
- Concave
How does the transducer work?
- Piezoelectric crystals both emit ultrasound and receive it
- Choose highest frequency that will penetrate to depth of the patient
Power and gain settings
- Overall gain: every signal returned will be reinforced/enhanced
- Whole image will be homogenously more gray
What does time-gain compensation (TGC) do?
Selectively influences certain areas of the image
Focusing the U/S beam
- Because it’s a wave, U/S can be focused
- Structures should be investigated near the focal point
What are the different modes of U/S?
- M-mode
- Motion
- B-mode
- Gray-scale (b = brightness)
- Doppler
- Color–blood flow
- BART = Blue Away, Red Toward
What are the different scanning planes?
- Saggital or dorsal
- Transverse
- Refers to the organ or the animal
- Each organ must be examined in 2 planes
Saggital plane
Transducer cranially oriented
Transverse plane
- Transducer toward examiner oriented
- Cross-section
Anechoic
- Homogenously black
- Very low intensity of returning echoes
Hypoechoic
- Less echoic than other structures
- Reference point needed
- Refers in general to medium-gray tones
- Low intensity of returning signals
Isoechoic
- Same echogenicity of another structure
- Same intensity of returning echoes compared to adjacent tissues
Hyperechoic
- Higher echogenicity than other structures
- Needs a reference point
- Refers in general to white structures
- High intensity of returning signal
Surrounding tissue
- Compressed
- Invaded
- Ex: free fluid in abdominal cavity
Vascularity
Doppler (BART)
Echogenicity triad
- My Cat Loves Sunny Places
- Kidney –> liver –> spleen (most echoic)
- Use split screen to compare organs in different planes
- Echogenicities may only be compared at same levels
Free abdominal fluid
- Earliest accumulation
- Apex of bladder
- Between liver lobes
- Mobility–ballotment
- Artifact: acoustic enhancement
- Everything distal to the organ will look brighter
- Anechoic = transudate
- Speckled/more echoigenic = exudate, blood, chyle
Fine needle aspirate (FNA)
- Easy
- Simple set-up
- Needle and syringe
- Non-invasive?
- Small gauge aspirates
- Results often diagnositic, but…?
