Ultrasound 1 Flashcards
What is sound?
The result of mechanical energy producing compression and rarefaction in the form of a wave
What’s the unit of frequency?
Hertz - one cycle per second
Why does ultrasound differ from normal sound?
> 20 kHz
Frequencies of diagnostic sonography:
1 - 20 MHz
What did Tom Brown do?
Converted the idea of medical purposes of ultrasound scanning into a practical proposition
How does diagnostic ultrasound work?
Short sound pulses are transmitted into the body
Reflection, scattering and absorption result in attenuation in the intensity of sound pulses through matter
What does attenuation do?
Limits depth of ultrasound imaging
Advantages of ultrasound:
Safe
No ionising radiation
Portable, compact and inexpensive equipment
Disadvantages of ultrasound:
Highly operator dependent
Organs obscured by gas (e.g. lung) and structures surrounded by bone do not give clinically useful images
What is the ultrasound machine like?
Monitor
US unit
Control panel
Transducers
Data storage device
What does the US unit do on ultrasound machine?
Processes signals
What do ultrasound transducers do?
Sends an ultrasound
Detects sound and converts it into an electrical signal for diagnosis
What are piezoelectric crystals?
Ceramic crystals
Become distorted when echoes return to transducer - generates electric pulse processed into an image
What do high-amplitude echoes produce?
Greater crystal deformation - generate larger electronic voltage
Result in brighter pixels on image
What images are used because of the significance of high-amplitude echoes?
2D grey-scale images
A.K.A. B-mode images
What are linear-array transducers like?
Linear array has a flat superficial surface
Only high-frequency probes used for superficial structures as don’t penetrate deeply
Letter L on image
What are curved-array transducers like?
Linear array has a curved convex surface
Letter C on image
What’s a phased-array transducer like?
Every element in array participates in formation of each pulse
Smaller - used between ribs
What are intraluminal probes like?
Small - various body lumens
Positioned close to organ - allows use of higher frequencies
Higher quality images produced
Disadvantages of intraluminal probes:
Limited imaging depth
What is A-mode imaging also called?
Amplitude-mode
What is A-mode imaging?
Displays echoes acquired in 1 dimension
Echo amplitude displayed along 1 perpendicular axis
What is B-mode imaging also called?
Brightness or 2D mode
What is B-Mode imaging?
Spike is converted into a dot
Brightness of dot = amplitude of returning signal
Position of dot = depth of returning signal
2D image produced
Black images:
Anechoic
E.g. fluid, pus
Dark grey images:
Hypoechoic
E.g. bone, gall stones, metal
What does isoechoic mean?
Structure same shade of grey as surrounding tissue
What colour images are solid organs usually?
Grey with black inside (where vessels are)
What’s colour doppler?
Measures direction and magnitude of Doppler frequency shifts that occur in moving RBCs
Produces grey-scale image
What’s Power colour Doppler?
Depicts amplitude, or power, of Doppler signals
Significance of Power colour Doppler:
Better sensitivity for visualisation of small vessels
What’s Pulsed Doppler?
Positions sampling volume (or gate) in a vessel on grey-scale image
Displays spectrum of range of blood velocities within gate
What is M-mode also called?
Motion mode
What is M-mode?
Analyses tissue motion
What is 3D ultrasound imaging?
Uses 2D scanner and stack of parallel cross sections
3D images displayed in varying formats
What does the gain control do?
Adjusts amplification of returning acoustic signals
Optimises US image
What does reduced gain produce?
Dark image
Detail is masked
What does too much gain produce?
White image
Detail saturated
Why must the US image be properly oriented?
To identify anatomic relations of various structures on the monitor
Groove on transducer = orientation marker
What is enhancement?
Brighter area in image due to sound passing more easily through a structure
What are the US probe manipulation manoeuvres?
Pressure
Alignment
Rotation
Tilt
Tilting the probe:
90 degrees to structure of interest
What is anistropy?
An artefact encountered - may result in incorrect diagnosis
What is the anisotropic effect dependent on?
Angle of ultrasound beam
What may the dots on right side of screen correspond to?
1cm intervals on patient - give estimate of depth
