Ultrasound 1 & 2 Flashcards
What is sound ?
A disturbance travelling through a medium; a series of interconnected particles.
Sound can be a longitudinal wave as well as a pressure wave
Longitudinal wave
Sound is a longitudinal wave that causes particles to vibrate (the particle motion is parallel to the direction of the energy).
Pressure wave
Sound is also, a type of pressure wave – molecules move close and further away, providing areas of higher and lower pressure.
Compression
High density region of particles.
Peaks
Rarefaction
Low density region of particles.
Troughs
Amplitude
Magnitude of the pressure change between peaks (compression) and trough (rarefaction).
Higher amplitude —> louder noise; related to power, measured in decibels (Db).
Wavelength
Distance between successive compressions/rarefactions.
Frequency
Number of sound waves per second; Hertz (Hz).
1Hz
1 wave per second
What is frequency in relation to wavelength ?
Inversely proportional to wavelength (λ) i.e. if wavelength decreases then frequency increases and vice versa.
What is frequency in relation to speed ?
Frequency is proportional to speed (c).
Audible human range
20 Hz to 20 kHz
Speed of a vibration
Speed refers the distance travelled of a vibration wave per unit time (i.e. how fast).
What does speed depend on ?
Depends on the properties of the medium (gas < liquid < solid).
What does speed relate to ?
Acoustic impedance
Frequency of a vibration
The number of vibrations an individual particle makes per unit time (i.e. how often)
Ultrasound
Ultrasound refers to frequencies >20kHz
(above human audible range).
MHz
Megahertz
10^6 Hz
GHz
Gigahertz
10^9 Hz
What are the medical ultrasound operating values ?
Medical ultrasound typically operates at 1 to 15MHz .
(~50 to 750x higher frequency than the maximal human hearing range)
What are some considerations for ultrasound ?
Resolution
Penetration
What is resolution ?
Sharpness of image
What is penetration ?
Depth of image
What does resolution relate to ?
Related to frequency.
i.e. when wavelengths of 1mm used, structures smaller than 1mm appear blurred (the waves don’t hit the target structure)
Therefore, higher frequency = higher resolution
How is resolution related to frequency ?
Higher frequency = Higher resolution
What is penetration related to ?
Inversely related to resolution
(i.e. higher resolution —> reduced penetration)
How is penetration related to resolution ?
Higher resolution = Reduced penetration
Results of lower frequency (MHz)
Increased penetration
Decreased resolution
Results of higher frequency (MHz)
Decreased penetration
increased resolution
States the 4 components that resolution is divided into
Axial
Lateral
Elevational
Temporal
Axial (vertical) component of resolution
The ability to differentiate objects along axis of ultrasound beam (depends on frequency).
What does the axial component of resolution depend on ?
Frequency
Lateral (horizontal) component of resolution
The ability to differentiate objects perpendicular to beam. (depends on width of beam at given depth)
- Aim for the focal zone (i.e. area of highest beam intensity)
- Ultrasound beams have a curved shape
Focal zone
Area of highest beam intensity
Elevational component of resolution
Fixed property of the transducer
(the thickness of the beam)
Temporal component of resolution
Resolution of moving structures
Piezoelectric effect
Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress.
What is emitted by piezoelectric material ?
Sound waves are emitted by piezoelectric material (crystals) contained within ultrasound transducers.
Describe the piezoelectric effect
When alternating electrical current is applied, the piezoelectric material oscillates, in response to mechanical strain.
This produces vibrations and sound waves are generated.
The beam then penetrates the tissues, with some waves being reflected.
The electrical amplitude is analysed and the amplitude of the returning signal is displayed as a grey-scale image.
Direct piezoelectric effect
When mechanical strain results in electric signal, it is known as direct piezoelectric effect.
Reverse piezoelectric effect
When electric signal results in mechanical strain, it is known as reverse piezoelectric effect.
What does stronger signals result in ?
Stronger signals result in a brighter picture.
What is deflection of sound tissues ?
Reflection + Refraction + Scattering = Deflection
What is meant by acoustic impedance ?
Acoustic impedance is the resistance to sound passing through.
Explanation of acoustic impedance
When moving from one type or tissue to another (interface) greater differences in acoustic impedance lead to greater reflection of the sound waves.
Greater differences in acoustic impedance results
Greater reflection of sound waves.
At air-tissue interfaces there is greater scatter of sound waves
High acoustic impedance meaining
High resistance to sound passing through
e.g. Air and Bone both have high acoustic impedance
What is attenuation ?
As sound travels through tissues, energy is lost, intensity is diminished.
This is mainly due to absorption, but also deflection and divergence.
What is attenuation dependent on ?
Attenuation is dependent on :
- frequency of wave
- distance travelled
- attenuation coefficient of tissue.
What is the attenuation coefficient of air ?
7.50
What is the attenuation coefficient of bone ?
15.00
What is meant by absorption ? (in relation to attenuation)
Absorption is sound wave energy transferred to heat energy in the tissue.
Higher Frequency ; Increased absorption therefore decreased penetration.
Monitor
Displays images
Ultrasound Unit (US Unit)
Processes ultrasound images
Control Panel
Knobs and controls
Transducer
Produces and receives ultrasound waves
Name the parts of the ultrasound machine
Monitor
Ultrasound unit
Control panel
Transducer
State the types of transducer (ultrasound probes)
Linear
Curvilinear
Phased array
Intracavitary
Describe features of linear transducers
Frequency range : 5-15 MHz
Imaging depth : 9cm
High frequency
Low depth of imaging
Flat footprint produces undistorted
HFL38/13-6 meaning
13-6 MHz Range + 38mm footprint
Describe features of curvilinear transducers
Frequency range : 2-5 MHz
Imaging depth : 30cm
Low frequency probe
Low image resolution
Good depth of imaging (e.g. abdominal scan)
Slight distortion of images
What are smaller curvilinear probes used for ?
Intraluminal scanning
Describe features of phased array transducers
Frequency range : 1-5 MHz
Imaging depth : 35cm
Large scanning area
Large depth
Good resolution
Small footprint
Where are the sound waves generated in phased away probes ?
From the centre of the footprint, this allows a small probe but a large scanning area and depth.
Advantage of small footprint of phased away probe
The probe can scan in small or awkward areas.
e.g. trans-thoracic echocardiography (between ribs)
Describe features of intracavitary transducers
Frequency range : 5-8 MHz
Imaging depth : 13cm
Advantage of the intraluminal probe
(intracavitary transducer)
Places the probe close to area to be imaged.
Overcomes poor imaging resulting from overlying structures (e.g. adipose tissue)
Allows use of high-frequency (higher resolution images)
Uses of the intraluminal probe
Endovaginal
Endorectal
Other (e.g. on endoscopes)
What are applications of linear transducers ?
Arteries/veins
Pleura
Skin/soft tissue
Testicles/hernia
Eyes
Thyroid
Lymph nodes
Nerves
What are applications of curvilinear transducers ?
Gallbladder
Liver
Kidney
Spleen
Bladder
Abdominal aorta
Abdominal free fluid
Uterus/ovaries
Lumbar puncture
What are applications of phased array transducers ?
Heart
Inferior Vena Cava
Lungs
Pleura
Abdomen
Transcranial doppler
What are applications of intracavitary transducers ?
Uterus/ovaries
Pharynx
Imaging modes
3 main modalities to cover;
- 2D mode (aka Brightness or B-mode)
- Motion mode (M-mode)
- Doppler mode (D-mode)
What is the most common imaging mode ?
2D mode : B-mode : Brightness mode
Echogenicity
Brightness
What affects echogenicity ?
The brightness of a structure depends on intensity (amplitude) of reflected signal
Anechoic
Structures transmitting all waves (without reflection)
Black in colour
(anything fluid filled)
e.g. blood, bile, urine
Hypoechoic
Structures reflecting less waves than surroundings.
Less dense
Dark coloured
e.g. kidneys
Hyperechoic
Structures reflecting more waves than surroundings (may result in ‘shadowing’)
More dense
Brightly coloured
e.g. diaphragm
Isoechoic
Simliar waves to surroundings
Motion mode (M-mode)
Analyses movements of structures over time
(e.g. dimensions of cavities over time - cardiac, vascular, pleura)
Scan line selected on a 2D scan using an M-mode cursor - the target structure
A single axis beam emitted along a scan line and movements plotted.
What is the doppler effect ?
The doppler effect is a shift in frequency of sound waves due to motion between the source and observer.
e.g. Ambulance sirens will appear higher pitched moving towards you and lower pitches when moving away.
Dopler sub-modes
Spectral (pulsed/continuous)
Colour flow
POwer
Spectral doppler
The doppler effect represented graphically;
If the frequency shifts :
-Above baseline - Indicates flow towards the transducer
-At baseline - Indicates perpendicular flow to transducer
-Below the baseline - Indicates flow away from the transducer
Pulsed wave spectral doppler
Transducer emits pulsed sound waves in cycles, measured at precise location.
Continuous wave spectral doppler
Measures blood flow velocity along an entire beam. (instead of specific location)
Colour flow doppler
Colour coded doppler shifts superimposed on a 2D image (same principles as pulsed-wave)
What does the colour in the colour flow doppler correspond to ?
Colour corresponds to direction and velocity of flow.
What do the colours in colour flow doppler mean ?
Blue - away from transducer (longer wavelengths)
Red - towards transducer (shorter wavelengths)
Power flow doppler
Similar to colour flow but no directionality.
Analyses only amplitude of returning echoes.
Level of brightness is proportional to magnitude of flow.
Advantages of power flow doppler over colour flow doppler
Power flow is :
- Less angle dependent
- No aliasing
Disadvantages of power flow doppler compared to colour flow
No directional information (less useful for cardiac imaging)
More susceptible to artifacts
What are ultrasound artefacts ?
False images, or parts of images, that do not represent the true anatomical structure.
- Can be used for diagnosing pathology
Acoustic shadowing result
You will not be able to see structures underneath bone.
What is acoustic shadowing ?
Seen distal to high attenuating structures
High attenuating structures
Reflect, Scatter or Absorb majority of sound waves.
Distally amplitude of sound waves significantly reduces, few echoes return causing a shadow to form.
Acoustic enhancement
Commonly seen deep to fluid-filled structures
What is acoustic shadowing ?
Sound travels through low-attenuating fluid-filled structure (unimpeded) resulting in the intensity of the sound waves preserved when hitting deeper structures
Creates a uniformly brighter, hyperechoic appearance of deep tissue (e.g. full bladder, gallbladder and large blood vessels)
Anisotropy
Exhibition of properties/structures with different values when measured at different directions.
Correct orientation of the probe
Saggital/coronal plane : Marker to head (cephalic)
Transverse plane : Marker to the right side
Superficial structures appear at the top of the screen
Name the 4 manoeuvres of the probe
Sliding
Rotating
Tilting
Rocking
What is the sliding manoeuvre used for ?
Identify structures / avoid ribs
What is the rotating manoeuvre used for ?
Alignment of vessels
What is the tilting manoeuvre used for ?
Serial cross-sectioning, anisotropy
What is the rocking manoeuvre used for ?
Centering of deep image
What does reflection and propagation of sound through tissues depend on ?
Attenuation
Acoustic impendance
Benefits of ultrasound
Good safety record
Non-ionising radiation
Non-invasive
Painless
Portable
Relatively inexpensive
Risks of ultrasound
Potential bio-effects
- Thermal
- Cavitating
Sensitive tissues :
Embryo <8weeks
Neonatal/foetal head/ spine/ eye (all ages)
Long term effects are unknown
ALARA principle
As low as reasonably achievable
Used to reduce the risk of harm from potential bio effects of ultrasound.
How is ultrasound used in clinical medicine ?
Treatment
Diagnostics
Procedures
Describe uses of ultrasound in treatment
Kidney stones
Prostate cancer
Soft tissue injuries
Lithotripsy
Extracorporeal shock wave lithotripsy is a technique for treating stones in the kidney and ureter that does not require surgery.
Instead, high energy shock waves are passed through the body and used to break stones into pieces as small as grains of sand.
Describe uses of ultrasound in diagnostics
Comprehensive
Focussed = POCUS
POCUS
Point Of Care UltraSonography
- Goal directed
- Bedside
- To answer a specific diagnostic question or to guide an invasive procedure
Comprehensive examination facts
Comprehensive involves thorough evaluation of an entire anatomical region or organ.
Performed by specialists
(specialised radiographers, radiologists, cardiologists)
Often performed in the radiology department
Non-emergency situations
Lengthly process can take days :
Request –> Perform –> Interpret –> Report
Generates a detailed report on the area examined to make a diagnosis or guide clinical decision making.
POCUS examination facts
Usually single organ examination
Performed by non-radiologists - doctors trained in specific POCUS examinations related to scope of practice.
Performed at bedside
Can be used in emergency or life-threatning situations.
Takes minutes
Used to answer a specific question or ‘Rule in/out’ a diagnosis.
Can be used to guide invasive procedures
Steps of clinical examination
Observation
Palpation
Auscultation
Ultrasound
POCUS benefits
Patents can stay in the department for ongoing treatment
Can provide rapid answer to clinical questions to aid faster decision making
Portable around clinical area
Improves outcomes of invasive procedures
POCUS limitations
Operator dependent
Familiarity with equipment
Limited, focused information
Can only provide answers to simple clinical questions
Availability of ultrasound machines
Patient characteristic considerations
Use of ultrasounds in procedures
Biopsy
Vascular access
Pericardiocentesis
Pericardial fluid
Lumbar puncture
Cerebrospinal fluid
Thoracocentesis
Pleural fluid
Paracentesis
Peritoneal fluid
Arthrocentesis
Synovial fluid
Vascular access
Cannulation
Central venous catheter
