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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