Setting up your machine - ‘Knobology’ & USS techniques · Dr Romila Bahl Emergency Medicine...
Transcript of Setting up your machine - ‘Knobology’ & USS techniques · Dr Romila Bahl Emergency Medicine...
Setting up your machine -
‘Knobology’ & USS techniques
Dr Romila Bahl
Emergency Medicine Consultant
University College London Hospital
Aim
• Put the physics of imaging into practice
• Provide a basis for understanding how to
use an ultrasound machine
• Give an understanding of the functions of
different buttons – ‘knobology’
• Provide an understanding of factors
affecting image quality in B mode
Objectives - lecture
• Understand the various components of a machine
• Understands how the different components affect image quality
• Recognise how to improve image quality in B mode- Brightness mode
(Image quality in Doppler and M mode not covered)
Objectives – practical
• Identify function buttons correctly
• Identify how different functions affect
image quality
• Orientate the image on screen to anatomy
• Start acquiring, labelling and storing images
Acquiring an image
Scan generator
Transducer
Subject - echos sent back
from tissue interfaces
Ultrasound out Echos returned
Amplifier
Scan converter
processoroperator
Image on screen
Contact areaAreas of image generation
which can be easily optimised
Controls the
sequence in
which electrical
pulses are
applied to the
transducer -
preset
Changes ultrasound to digital image
The medium being scanned determines the amount of absorption,
reflection and scattering of the sound wave and how fast it travels
through the medium – predetermined
Schematic representation
Returning echos are much weaker
than the waves transmitted
Power Output
• Controls the amount of sound energy entering the patient
• Varies between machines
• Should not use at maximum power
• More power gives a higher penetration and amplitude but it is not good practice – safety limits exist
• Increase the power output only if other controls have been changed first and you know the limits
The transducer
• Transducer - any device that converts energy or
signals from one form to another
• Ceramic piezo electric crystals in the tranducer
head produce sound waves when electricity
applied and produces electricity when a force is
applied – brittle
• Backing material damps out the signal after
excitation enabling short pulses to be generated
The transducer in action – unfocussed beam
How transducers choice affect
image quality
• Frequency (2-20MHz available)
– Determines how deep the wave will penetrate
– Determines image detail – the resolution
• Size of footprint
– Determines the scan area
• Amplitude of the echo out
Frequency and image acquisition
• The higher the frequency of the wave the less it
will penetrate a given medium
• The higher the frequency of a wave the better
resolution
Summary
High frequency gives good detail but doesn’t scan
very far – useful for foreign bodies and soft tissue
Axial resolution
• Shorter the ultrasound pulse
duration better the axial
resolution
• Frequency is inversely
proportional to pulse duration
• Higher the frequency shorter
the pulse duration so better the
axial resolution
Lateral resolution
• Determined by the width
of the beam
• Narrower the beam the
better the lateral
resolution
• Higher the frequency the
narrower the beamAbility to differentiate objects
perpendicular to the beam
Beam focussing
• Modern transducers have an acoustic lens which narrows the beam profile
• Can have more than one focal zone
• Good for detail esp. for tricky organs such as the common bile duct
• slows down the rate at which image frames are acquired
Contact area
• Certain mediums do not transmit ultrasound well –
air
• Need to reduce the air between skin of the subject
(gets trapped in hairs) and the transducer
• Use a coupling gel (or water)
• NB warm it for comfort
Happy patient = Happy scan
What causes an image -
reflection from a tissue interface
Amplifier
A range of weak echos is returned from the tissue interfaces in the medium being scanned
• Total/overall gain button
– amplifies these all
• Time gain compensation (TGC)
– amplifies selectively depending on the depth of the reflector
– deeper structures send back weaker echos as there is less energy to reflect back
– TCG allows compensation for this
Time gain
compensationOverall gain
Processor
• Changes ultrasound to digital imaging
• Post Processing advances in modern
machines
• Terms used vary from machine to machine
• Useful for scan review and ‘difficult’
subjects
Processing – tissue harmonic
imaging
• Uses the fact tissues distort due to the sound wave
across them during imaging
• This expansion and contraction produces a echo
signal of its own – a harmonic
• The harmonic is then received by the transducer
and can produce a complementary image to
conventional B-mode scanning – has less ‘noise
• Shows enhanced contrast at tissue interfaces
Display
• Screen that shows the image as dots of varying
brightness
• Can alter display – contrast, intensity,colour
• Can adapt to personal preference and environment
• Beware reflective screens in brightly lit Resus
bays
• Display may not represent image printed out
Colour and Doppler function
• Need to know what the colour function on
your machine does
Standardising the image
• Probe marker to right or top (cranial) of
patient
• Image displayed from right to left
Standardising the image
Saggital plane
Practical session & Quiz
Answers(with thanks to Dr V Aitken)
1 Alphanumeric keys
– Data entry and annotation
2 Time gain compensation
– Boosts the returning echos from different depths
3 Probe selection
– Allows selection of correct probe
4 Display section
– Allows reversal of image with respect to probe marker
5 Freeze button
– Holds image still – useful for annotation, measurement and saving
6 Display adjustment
– Depth of field and magnification
7 Trackball and measurements
– Allows placement of markers, measurement
– Trackball function varies with machine- recalls recent images, locates zoom or colour areas
8 Print
– Prints image on screen
9 Overall gain
– Adjusts overall amplitude of received echos – makes image brighter on screen
10 Patient ID entry
Q&A
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