31. Ultrasound and Doppler

Question 1

What are the clinical uses of ultrasound?

Answer 1

In the last decade there has been a
rapid expansion in the use of
ultrasound within anaesthetics and critical care medicine.

Use of ultrasound has become
routine in the theatre environment to aid

1
vascular line insertion (NICE 2002),

2
guide peripheral nerve blockade,
e.g. interscalene nerve blocks (NICE 2009),

3
and in some centres to also guide catheter placement within the epidural space
(NICE 2008).

In the critical care setting,

1
ultrasound is routinely used for vascular
line insertion,

2
cardiac output monitoring,

3
echocardiography,

4
transcranial Doppler,

5
pleural aspiration,

6
ascitic drainage,

7
assessment of hepatic portal vein flow

8
detection of venous thromboembolism.

Question 2

What are the principles of ultrasound?

Answer 2

> Ultrasound is an imaging modality
that utilises high-frequency sound waves

(in the region of 2 M Hz)

in order to image structures within the body.

> Ultrasound waves are generated by 
applying an electric field to a
piezoelectric crystal in the transducer, 
which leads to the crystal vibrating and
generating ultrasound waves.

> Tissues within the body differ
in their ability to transmit sound waves.

When the sound wave encounters
a change in tissue,

part of the sound wave is
transmitted and part is reflected back
to the transducer.

It is the reflected sound waves that are
converted into an image.

The time taken for the sound
waves to return to the transducer
provides an indication of the depth of the
tissue, interface.

> Ultrasound gel is essential to 
acquire good images, 
acting as a coupling medium, 
which reduces the attenuation 
of the ultrasound waveform.

> Ultrasound is good for examining
fluid-filled structures (e.g. vessels)
and soft tissues,

but not for air-filled structures
(e.g. lung tissue) or for calcium-rich structures (e.g. bone).

> Resolution of the ultrasound image
is inversely proportional
to the depth of penetration

and so it is not good for examining
deep structures or imaging
obese patients.

> Advantages: 
relatively inexpensive, 
widely available,
non-invasive and no
ionising radiation, therefore safe in children and pregnancy.
> Disadvantages: 
operator dependent, 
cannot be used to image lung, 
bone or
deep structures.

Question 3

What are the different modes of ultrasound

> A (Amplitude) Mode

Answer 3

Several different modes of ultrasound .
are utilised in medical imaging,
the main ones are described below:

> A (Amplitude) Mode –

simplest form of ultrasound imaging
that is not
frequently used.

A single ultrasound wave is emitted 
from the probe and scans
a line through the body with the 
echoes plotted on the screen as a 
function of  depth. 

Used by ophthalmologists to measure
diameter of the eyeball.

Question 4

> B (Brightness) Mode(

Answer 4

better known as 2D mode) –

a linear array of ultrasound waves 
are emitted from the probe 
and scan a section through
the body producing a 
two-dimensional cross-sectional view. 

The intensity of the echoes
reflected back to the transducer is
proportional to the whitening
of the film,

thus structures with no internal
echoes appear black (anechoic)
whereas structures containing
internal echoes appear white (echogenic).

It is the default mode of any ultrasound
or echocardiography machine.

This mode is used widely in anaesthetic practice to gain vascular access and to perform regional anaesthesia.

In echocardiography,
it is used to measure cardiac chambers
and to visualise valves.

Question 5

> M (Motion) Mode

Answer 5

– a rapid sequence of ultrasound 
waves are emitted and each time 
either an A-mode or B-mode image 
is taken allowing real-time
movement of structures to be visualised. 

This mode is commonly used in echocardiography.

Question 6

Doppler Mode

Answer 6

– utilises the Doppler effect to
enable detection and velocity of flow.

Sound waves reflected from a moving target
(e.g. blood)
have a different frequency from the
incident sound wave.

This frequency shift is
proportional to the velocity
of the flowing blood.

Doppler allows not only the
detection of flowing blood but
also enables its velocity to be quantified.

• Colour flow Doppler –

in this mode the velocity and
direction of blood flow is colour coded
and superimposed onto a grey-scale 2-D
(B-mode) image.

By convention Blue indicates flow ‘Away’
from the ultrasound probe and
Red ‘Towards’ the probe (acronym ’BART ’).

• Duplex Doppler – this mode combines real-time Doppler with realtime ultrasound simultaneously.

Most commonly used to assess the
vasculature

(e.g. carotid occlusive disease, deep vein thrombosis,
varicose veins, abdominal aortic aneurysms).

Question 7

Define the Doppler effect.

Answer 7

Doppler effect (or principle) is a 
commonly observed phenomenon 

whereby sound waves reflected from a
moving target are altered and

have a different frequency from
the incident sound wave.

A good example of this is the noise of
a racing car where the pitch (frequency)
of the car increases as it approaches

the observer and then abruptly drops as it
races past the observer

but to the driver, the pitch will remain unchanged).

Question 8

Give an equation to describe the Doppler effect.

Answer 8

In clinical ultrasonography, 
if the ultrasound beam is 
directed parallel to the
direction of movement or flow, 
the velocity of the moving target (e.g. blood) is
calculated as follows:

            Speed of sound wave × (change in frequency) Velocity =\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
                                (2 × emitted frequency)

V = c (Δƒ)/2ƒ

Where:
c = speed of sound wave
ƒ = frequency

> When the beam cannot be parallel to the direction of flow,
a correction factor is used that involves the
cosine of the angle of incidence (represented as θ).

So now it is expressed as:

V = c(Δƒ)/2ƒ cos θ

Question 9

Give a clinical example of a device utilising ultrasound.

Answer 9

> Transoesophageal Doppler

• This has now become established as a
relatively non-invasive method of
cardiac output monitoring.

• Doppler probe is positioned in the
mid-oesophagus in order to measure
red blood cell velocity
in the descending thoracic aorta.

• Aortic cross-sectional area is
estimated from a nomogram (age/height/
weight) or in some devices it is measured.

• Once the probe is positioned correctly,
it uses the Doppler principle
to measure red blood cell velocity
from which blood flow can then be calculated.