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TitleClinical Doppler Ultrasound
Author
LanguageEnglish
File Size85.8 MB
Total Pages359
Table of Contents
                            Cover
Frontmatter
Copyright
List of contributors
Preface
1. Physics: principles, practice and artefacts
2. Haemodynamics and blood flow
3. The carotid and vertebral arteries;Transcranial colour Doppler
4. The peripheral arteries
5. The peripheral veins
6. The aorta and inferior vena cava
7. Doppler ultrasound of the liver
8. The kidney
9. Doppler ultrasound evaluation of transplantation
10. Doppler imaging of the prostate
11. Doppler imaging of the penis
12. Doppler imaging of the scrotum
13. Doppler ultrasound of the female pelvis
14. Clinical applications of Doppler ultrasound in obstetrics
15. Microbubble ultrasound contrast agents
Appendix: System controls and their uses
                        
Document Text Contents
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Commissioning Editor:
Meghan McAteerDevelopment Editors:
Hilary Hewitt and Louise Cook
Editorial Assistant:
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Project Manager:
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Design Manager:
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Illustration Manager:
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Illustrator:
HardlinesMarketing Manager(s) (UK/USA):
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ii


Page 179




Flow velocities in the shunt vary widely,ranging
from approximately 50 to 270 cm s
-1.94,96,97Velocities can also be quite variable through the

shunt itself,usually increasing from the portal

venous end to the hepatic venous end of the shunt.

The mean velocity of patent shunts has been

reported as 95 cm s
-1in the shunt near the portal
venous end
93and 120 cm s-1in the middlesegment of the shunt.94Flow across the shunt is
usually quite turbulent,especially when multiple

stent components are used,when overriding stents

can cause a relative narrowing of the shunt lumen.

Normal velocities in the main portal vein are

variable.Following TIPS insertion,the mean portal

vein velocity has been reported to increase from

7cm s
-1to 24cm s
-1in one study96and from 20cm s
-1to 38.4cm s
-1in another.
97Hepatic
arterial flow has also been shown to increase after

TIPS,presumably because the shunt diverts the

portal venous inflow away from the liver.
95In a properly functioning TIPS,flow direction
in the portal system is towards the portal vein

end of the stent.Therefore,flow in the main portal

vein is hepatopetal and its velocity is typically

quite brisk (between 20 and 50 cm s
-1).It must
be kept in mind that velocities measured in the
stent-bearing portion of the portal vein represent

flow in the portal vein,not in the shunt.
95Flow
in the left and right portal veins usually becomes

hepatofugal,flowing out of the diseased liver and

towards the inflow of the shunt
93(Fig.7.57).
Depending on the diameter of the shunt and the
severity of the liver disease,however,flow may

continue to be hepatopetal into the parenchyma.

If the patient has patent paraumbilical vein

collaterals,these will continue to shunt blood

away from the liver.Flow in the left portal vein,

therefore,continues in a hepatopetal direction

despite normal TIPS function.If portal branch

flow changes direction over time from hepatofugal

to hepatopetal,a significant flow-limiting lesion

is assumed to be present in the TIPS.
The Doppler data are recorded and maintained
in a table format for follow-up (Fig.7.58).Serial

documentation provides the best means of

identifying any variations in velocity and/or flow

direction over time and these changes are the

best early indicator of shunt compromise.
7Doppler ultrasound of the liver
176Fig.7.56
Doppler tracing obtained at the portal vein
end of the TIPS. Note the marked periodicity of flow

within the TIPS. The
wavef
orm is that commonly seenin hepatic veins and inferior vena cava. Identification

of this degree of periodicity at the portal vein and of

the stent is a confident indicator of a widely patent
shunt.Fig.7.57
Combined spectral Doppler tracing of theleft branch of the portal vein and hepatic artery in a

patient with an appropriately functioning TIPS.

The sample volume is opened wide in position so

that both vessels are interrogated in the same

tracing. Note flow in the hepatic artery is towards the

transducer, therefore into the left lobe of the liver.

The left portal vein flow, however, is away from the

transducer, therefore hepatofugal and towards the

inflow of the shunt.



Page 180




Shunt stenosis
The two most common causes of TIPS compromise

are neointimal hyperplasia
throughout the shunt,or a focal stenosis at the hepatic vein end.Most

TIPS will have some degree of neointimal hyper-

plasia,but this may progress to the point where

it limits flow through the TIPS.At the point of

maximum stenosis within the TIPS,a high-velocity

jet may be perceived by Doppler (Fig.7.59).

Other components of the TIPS and the portal

system,however,will show decreased velocities.

With sufficient compromise,flow in the branch

portal veins becomes hepatopetal and flow in

the main portal vein may become hepatofugal.
Focal
hepatic vein stenosis
can occur where theproximal end of the TIPS abuts the hepatic vein.

Focal irritation of the vein wall by the stent wires

may cause a bar of granulation tissue to build up.

This results in decreasing velocities throughout

the shunt.95A key Doppler finding of this focalstenosis is the presence of poststenotic flow

disturbances with a high-velocity jet and turbu-

lence in the hepatic vein or IVC
99(Fig.7.60).
The sonologist must therefore evaluate flow

beyond the end of the stent,sometimes even as

far as the right atrium.Flow in all three hepatic

veins is normally towards the heart but a stenosis

at the junction of the TIPS and the hepatic vein

can cause flow compromise peripherally in the

vein with damping of periodicity,or segmental

flow reversal (Fig.7.61).
Several investigators have attempted to deter-
mine flow velocities which define the presence of

TIPS stenosis93-95,100but reported findings have
varied considerably.In one study,a velocity <50 cm

s-1at the portal venous end was 100% sensitive
and 93% specific.
93In another study,a velocity
<50 cm s-1in the middle segment of the TIPS was
78% sensitive and 99% specific,with a positive

predictive value of 96%,negative predictive

value of 91% and accuracy of 92%.
94[Whenthese investigators used a velocity <60 cm s
-1asthe criterion,sensitivity increased to 84% but

specificity dropped to 89% and accuracy to 87%.

At <70 cm s-1,sensitivity was 89% but specificity
was 83% and accuracy was 85%.In another

study,a velocity of 90 cm s
-1was applied,but the
sensitivity was only 87.5% with specificity of

95%.]
101These varied findings underscore the
fact that flow velocities vary widely from patient

to patient and that the best method for TIPS

evaluation is to use individual patient baseline

velocities obtained soon after TIPS placement.
102A change in velocity of ±50 cm s
-1from baselinehas been proposed as the threshold value for

predicting haemodynamically significant shunt
compromise.74It becomes obvious with thisspectrum of reports that there is controversy

regarding the accuracy of Doppler ultrasound for

the detection of TIPS malfunction,nevertheless

it is the best non-invasive means for following

TIPS.1037Doppler ultrasound of the liver
Monitoring treatment of liver disease
177:IVELOCITIESDIRECTIONSName:DatePV EndMidHV EndMain PVLt PVHV TIPSComments
DFig.7.58
TIPS data sheet. Each patient receiving a TIPS should have a data sheet maintained with velocities and
flow directions documented at each visit. Progressive compromise of the TIPS can then be more easily

diagnosed as progressive changes in velocities or a change in direction flow become manifest. Velocity

measurement in the mid-TIPS tends to be the most erratic. A persistent decrease in main portal vein velocity

over the sequence of studies is the most definitive indicator of progressive shunt compromise.



Page 358




range will tend to increase the amount of colour
on the screen.Power box steering angle
Although the power display is much less dependent

on angle than the velocity display,there may still

be loss of the power signal at 90°because these

low Doppler frequencies fall below the motion

discrimination filter cut-off level.In most situa-

tions,steering the power Doppler box does not

have much effect on the display;however,the

loss of signal at 90°may be overcome by having

a degree of angulation of the transducer.
Filters
Motion discrimination filters are used to filter

out excessive signal noise from structures,other

than blood,which are moving in the Doppler

box.Low filter settings provide more sensitivity

but are more prone to flash artefacts,whereas

higher filter settings reduce flash artefacts but

will also filter out some blood flow information.
SPECTRAL CONTROLS
Spectral gain
This affects the receiver gain for the spectral

display.As in B-mode imaging,the level should

be adjusted in order to give a balanced distri-

bution of grey shades across the displayed spectra.

Excessive gain will produce erroneous estimates

of Doppler shift/velocity.
Spectral dynamic range
This can be varied to optimise the display of

particular frequency shifts.A narrow dynamic

range results in the loss of low-intensity shifts

above and below the main shift frequencies.

Conversely a wide dynamic range,particularly if

associated with a high level of spectral receiver

gain,can result in artefactual broadening of the

spectra displayed.Further manipulation of the

spectral display can be performed by altering

the postprocessing algorithms in order to empha-

sise,or suppress,particular frequency shifts.In

normal practice a simple linear allocation is

most convenient.
Spectral scale
Altering the scale affects the pulse repetition

frequency and thus the range of shifts which can

be registered without aliasing.In practice the

scale is adjusted so that the Doppler waveform is

displayed without wrap-around,which indicates

aliasing.The scale may be displayed with either

a KHz scale for frequency shift,or m s
-1forvelocity.The use of m s
-1allows some com-
parison of different examinations,performed

with different transducer frequencies and with
different angles of insonation.
Spectral inversion
This allows the operator to change the orien-

tation of the display.Many operators prefer to

display arterial and venous waveforms above the

baseline,even if flow is away from the trans-

ducer.However,care is therefore required when

assessing the vertebral arteries for reverse flow,

or the leg veins for reflux,as errors may occur if

spectral inversion is not recognised.Some

centres do not allow spectral inversion because

of the potential for misinterpretation,particu-

larly in relation to examinations for venous

insufficiency.
Spectral sweep speed
A medium speed is adequate for most arterial

work,with a slower speed for venous flow.A fast

sweep speed is useful for acceleration time

measurement and waveform analysis,particularly

if there is tachycardia.
Angle correction
The measurement of the angle of insonation

relative to the direction of flow is required in

spectral Doppler in order to convert frequency

shift information into velocity information and

also in colour Doppler to convert mean pixel shifts

into mean velocities.The main direction of flow

may not necessarily be parallel to the vessel wall

and colour Doppler is useful in precise posi-
tioning of the angle-correction cursor along the

line of the jet.The angle of insonation should be

less than 60°or the errors in velocity calculation

become significant.Appendix:System controls and their uses
360


Page 359




Gate size
This defines the range of depths from which
Doppler data are collected.The gate should

be positioned across the lumen of the vessel

but clear of the walls in order to reduce wall

thump.The position of the gate which

corresponds to the maximum Doppler shift is

located with the use of the colour map and

the operator’s ears,which are the most sensitive

and efficient spectral analyser available;with

experience,the ears will register when the peak

frequency shift is obtained.If an assessment of

volume flow is being made,the gate should

be wide enough to encompass the entire vessel

width so that all the flow contributes to the

signal and the time-averaged mean velocity

will be most representative.Smaller gates

give a cleaner signal,especially with laminar

flow,but at the expense of a reduction in

sensitivity.
Filters
Removal of low-frequency noise and clutter arising

from the vessel wall and surrounding tissues

contributes to a cleaner signal,but filters should

be set as low as is practical,otherwise low-

frequency shifts from slow blood flow will be

filtered out,which could result in the mistaken

impression of absent diastolic flow in arteries,or

occlusion in veins.In practice it is best to keep

the filters set at the lowest setting and only increase

filtration as required during an examination.
Modern systems allow the operator to preset
many of these parameters and create different

profiles for different types of examination such

as veins,carotids,renal,etc.However,it should

be remembered that ultrasound is a dynamic

examination and the best results will be obtained

if the system controls are adjusted to optimum
settings for the task in hand,rather than relying

on the preset profiles alone.Appendix:System controls and their uses
Spectral controls361

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