PhysicsSafety & Infection ControlSetting Up The Machine

H. Gee MD, FRCOG

(teaching materials provided by David Cole, Lecturer at City

University, Birmingham)

Sound

HIGH PRESSURE REGIONLOW PRESSURE REGION+

= ONE CYCLE

WAVELENGTH

A sound wave

wavelength

amplitude

Excess pressure

Amplitude & Frequency

• Frequency– Pitch

• Amplitude– Loudness

• the strength of the sound.• The maximum change in

pressure due to the soundwave

Ultrasound

Ultrasound

• High Frequency Sound

• > 20 KHz (above the audible range)

• Short Wavelength

• Resolution Wavelength

• 3.5 MHz - Wavelength = .44 mm

• Frequency Penetration

Spatial Resolution

• The ability to display two small structures situated close together as separate images

objects Image resolved

Image unresolved

Piezo-electric effect

• Sound Electricity

Piezo-electric crystal

Inverse piezo-electric effect

• Electricity SoundPiezo-electric crystal

Ultrasound transducer

ECHOECHO

PULSE

CONVERTS ELECTRICITY INTO SOUND

CONVERTS SOUND INTO ELECTRICITY

PIEZO ELECTRIC CRYSTAL

Pulse echo techniquemultiple reflectors

Imaging Principles

Imaging Principles

How the image is How the image is producedproduced

• Sound

• Ultrasound

• Piezo-electric effect

• Pulse-echo technique

• Imaging principles

• Resolution

Summary

Infection Control

Setting up the Machine

• Two Parts– Monitor (may be done automatically)

• Contrast• Brightness

• Grey Scale on Screen – ALL GRADATIONS

– Imaging System

Equipment Controls

• Probe selection

• Mode selection 2D, Doppler, Colour

• Gain, TGC, Power

• Depth and Zoom Scan angle (field size)

• Focus

• Measurement, freeze

• (Examination selection post processing)

Probe selection

• This control is used to select from the probes available

• For example– Curvilinear general– Transvaginal– Linear array for small parts

Mode selection 2D, Doppler, Colour

• 2D is normally selected automatically for all examinations and the other modalities are selected when required during the examination.

Making the image darker or brighter

• Power

• Gain,

• Time Gain Compensation

Safety

• Cavitation• Heating

• Frequency• Duration• Intensity

• Power

Power (Output) increased

BRIGHTER IMAGE

Safety implicationsSet power to 35%Use gain to control brightness

Power reduced

DARKER IMAGE

Gain• Controls image brightness by amplifying

echo signals

• Completely safe

• Increased image noise

Gain

INCREASE IN GAINMORE NOISE

Time gain compensation Time gain compensation (TGC)(TGC)

Time gain compensation Time gain compensation (TGC)(TGC)

Time gain compensation Time gain compensation (TGC)(TGC)

TGC correctTGC correct

Changing the field size

• The following controls change the size of the field of view– Depth

– Zoom

– (Scan angle)

Freeze

• Measurement• Image recording

Measurement=

+ +

•Electronic callipers

•Linear

•Circumference / area

•Ellipse / trace

•Volume measurement

•Organ specific

•Embedded charts

Summary

• Probe selection

• Mode selection 2D, Doppler, Colour

• Depth and Zoom Scan angle (field size)

• Gain, TGC, Power

• Focus

• Measurement, freeze (& cine loop)

• (Examination selection post processing)

Important Controls

• Power (output, acoustic)• Gain• Time gain compensation (TGC)• Freeze• Electronic calipers