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34
M. Riccabona
• For assessment of intestinal structures, perineal US, detailed analysis of liver
surface or renal parenchyma, as well as for appendicitis, mesenteric nodes or
pylorus, linear transducers are advisable (frequency range 3–15MHz, depending
on age/targeted area); high-resolution curved linear arrays may offer larger eld
of view at only slightly reduced resolution—helpful supplement.
2.7.5 Course ofInvestigation andMeasurements
2.7.5.1 General Remarks
Entire targeted area is evaluated in serial longitudinal and axial sections, complemented by oblique sections if necessary; always includes surrounding structures
and other relevant areas that might cause similar complaints:
• Always assess and document size, contour, shape, position, echostructure and
internal anatomy as well as motility/correlation to adjacent structures.
• Documentation is usually standardised for various queries and body areas; even
normal ndings are documented in longitudinal and axial section. Pathology
needs to be documented in two planes and measured.
• Various documentation standards/recommendations are available (e.g. DEGUM/
OEGUM—see: www.oegum.at/content/view/506/210/).
Standardised images and sections help to improve comparability and reduce
interobserver variation—usually predened by documentation recommendations (more details listed in organ-specic chapters). Three-dimensional US
(3DUS) offers possibility to store entire volume of targeted organ thus improving
documentation.
2.7.5.2 Transducer Handling
Transducers should be moved slowly, carefully, without pressure, no sudden movements and with continuous smooth contact with skin. Specic techniques such as
graded compression or positioning manoeuvres helpful, sometimes mandatory (e.g.
appendix US), for some applications irreplaceable (for DDx), but try to inform or
alert child prior to these measures in order to gain cooperation and avoid sudden
unexpected frightening experience. Remember to effectively, but gently clean the
transducers after each investigation.
2.7.5.3 Measurements
Basically every organ should be measured, particularly important in paediatric US as
sizes vary with age, depending on patient weight/size (see Chap. 7—Measurements).
Measurements are always performed in longitudinal and axial sections and docu-
mented—either as 2-D distance or planimetric denition of (manually) dened
plane. In some organs (e.g. kidney) volume calculations based on mathematic equations using a corresponding geometric shape are more reliable and useful than just
length measurements.
2 US Methods, Artefacts, Biologic Eects, Practice
All measurements are performed in standardised, representative and reproduc-
ible sections that grant optimal depiction of maximal length and diameters.
Particularly for irregularly shaped organs 3DUS volumetry is better enabling
reliable values,
Organ sizes are assessed according to age-matched normal value graphs.
For further details see respective chapter.
Note Measurements are for orientation and not always reliable, may vary signi-
cantly even within one investigator during same examination. Always consider/
check the internal correction factors of the device if you are using the device’s calculation program.
35
2.8 Documentation andInterpretation
2.8.1 Image Documentation
Every imaging investigation needs to be documented in sufcient diagnostic quality
and detail; minimal documentation requirements are usually standardised by individual hospitals/medical societies in form of guidelines or recommendations
(see above).
Documentation is essential not only for medicolegal issues and reimbursement
but also for retrospective reassessment and comparison during follow-up.
Documentation can be performed by various methods: printout from frozen
monitor images using various printing media (video printers, hardcopies, etc.), storage to hard disc and export into some storage memory; many hospitals store captured digital images to their PACS.Increasingly not only single images but also
short video clips or 3D volumes are stored for improved documentation. Images/
clips can be retrieved from PACS for reading, printout or digital distribution.
2.8.1.1 Media forDocumentation
• Polaroid pictures: rather expensive, suboptimal quality, danger of fading, not
used frequently any longer.
• Video paper or colour printers: particularly black and white less expensive, very
practical, still used.
Note Durability restricted—prone to fading and damage.
• Multi-format camera: excellent quality, excellent durability, inexpensive in
maintenance and for individual copies but requires signicant initial investment.
Was the major documentation in the past but increasingly outdated by digital
documentation.
• Laser imager: good image quality and durability, comparable to multi-format
camera—but even more expensive, and due to large size, difcult to transport;
still commonly used in places without PACS.
36
M. Riccabona
• Photo camera: acceptable image quality—but cumbersome handling and development; only rarely used, mainly for teaching or publication purposes.
• Video tape: best documentation of entire investigation, particularly useful/valuable for echocardiography where it is still commonly used. Setbacks: unpractical
handling of all the tapes with signicant space requirements for archiving, difcult to retrieve single specic investigation; adequate video players must be
available for review. Increasingly being replaced by digital cine-loop clip storage.
• Digital documentation—today the most used method. All modern systems store
digital images and short clips to hard drive, can be exported either to workstation
and PACS or transferred to other storage media such as optical discs and external
hard drive.
2.8.2 Report
Documentation not only includes representative images but also the nal written report.
Reports vary with institution and country—usually referring physician gets nal
report but no or only some selected representative images, unless images can be
retrieved from PACS or another digital environment.
Note Archiving demands vary with country, include both images and report. When
setting up an US service, adequate precautions to comply with local documentation,
archiving and regulatory demands are mandatory.
2.8.2.1 How toIssue aReport
US is an ongoing real-time investigation that (except for video tape documentation
or 3DUS) cannot be entirely documented. Description of investigation is essential;
should be included—not only all areas assessed but also listing all areas and aspects
which could not be properly visualised (e.g. due to overlapping gas) as far as relevant for investigation or actual query, to allow referring physician to perceive
restrictions of investigation.
After descriptive part (usually includes description of organs, measurements/
sizes, with relation to age, weight/size) responsible reader should state a conclusive
diagnosis rating importance of individual ndings with respect to clinical symptoms
and query.
2.8.2.2 Diagnosis
Diagnosis—interpretation of ndings retrieved from US investigation, should comprehensively and conspicuously draw conclusion from ndings detailed in description stating diagnosis and listing important DDx. One should always include
limitations of investigation and—if applicable—list supplementing imaging steps.
Note Purpose of structured report—to enable referring physicians to reassess inter-
pretation of ndings. Additional ndings received in the meantime or changed/dete-
2 US Methods, Artefacts, Biologic Eects, Practice
37
riorated patient symptoms may indicate re-evaluation. By dividing report into
descriptive part and diagnostic assumption, referring physicians are enabled to draw
different conclusions in the light of recent changes. Not as important in patients
with normal or unequivocal clear-cut ndings/screening conditions—here short
report versions can be used.
2.8.2.3 Predefined Reports
Predened report forms are increasingly in use trying to standardise reports and to
integrate them into electronic systems; however, often tend to become very extensive or may not provide sufcient exibility to properly communicate all aspects of
individual investigations, particularly in rare ndings.
2.8.2.4 Nomenclature
Use of proper terms and consistent nomenclature is extremely important; standardised image orientation is recommended.
Topographic denitions on US image:
• In general (abdominal and small part US): cranial/proximal, left; caudal/distal,
right; right image side usually left image (supine position).
• Neurosonography: frontal—left in sagittal views; for coronal sections right
image side—left patient side. See respective Chapters.
• Echocardiography: see Chap. 11.
• Hip US: see Chap. 15.
• Use of pictograms and markers as well as image inscriptions helpful.
Note One has to dene whether axial descriptions refer to body or organ axis.
• A number of typical sonomorphologic descriptions used referring to denition
and contour, echotexture (inhomogeneous or not, etc.), its relative mobility
(good, poor, missing, etc.) and echogenicity (echoic or echogenic, hyperechoic,
anechoic, etc.). Use them consistently.
(Color) Doppler US: Theory, Artefacts,
fc
o
×
α
Typical Applications inChildhood
MichaelRiccabona
3.1 Doppler Sonography
3.1.1 The Doppler Phenomenon
Frequency of reected wave is changed if the ultrasound beam is reected by moving particles (Doppler effect).
Application
• Doppler effect mostly used to assess ow direction and velocity—mostly of
blood. Basically other types of movement also depictable, usually causing artefacts, but may be diagnostically valuable (e.g. twinkling sign, uereteric jet,
“swirls”, uid colour sign).
• Based on known sound velocity, sound beam direction, its emitted frequency and
the direction of imaged motion (e.g. course of vessel on screen—thus dening
angle α between sound beam and motion direction), ow velocity can be calculated using the Doppler equation based on the measured frequency shift of
received echoes.
• Velocity can only be calculated if Doppler angle is known—only with low angles
(<10–15°) this can be neglected without signicant measurement errors.
• Doppler equation:
3
d
=
M. Riccabona (*)
Department of Radiology, Division of Pediatric Radiology, Medical University Graz
and University Hospital Graz, Graz, Austria
e-mail: michael.riccabona@medunigraz.at
© Springer Nature Switzerland AG 2020
M. Riccabona (ed.), Pediatric Ultrasound,
https://doi.org/10.1007/978-3-030-47910-7_3
f
×2cos
39
40
M. Riccabona
• V=velocity (m/sec), fo=emitted frequency (MHz), fd=frequency shift (kHz),
c=speed of sound in tissue (m/sec), α=angle between sound beam and move-
ment direction (Doppler angle).
How Doppler Information Is Displayed
• Received signals exhibit frequency shift—used for calculation. Result displayed
acoustically (ow noise) or graphically; the latter achieved by complex signal
analysis (based on fast Fourier analysis) creating ow graph on monitor—ow
velocity and direction on y-axis, with time encoded on x-axis (see Fig. 2.2).
• Averaged ow velocity information can be superimposed on conventional grey scale
US image by colour displays called colour Doppler sonography (CDS, Fig.3.1).
3.1.2 Different Techniques andApplications
ofDoppler Sonography
3.1.2.1 Continuous Wave Doppler (CW)
Principle
• Simultaneous emission and receiving of US beam with two different crystals
allows assessment of all movements that occur throughout entire beam range.
• Reected signals undergo spectral analysis to decode different ow velocities
and directions.
• Information then displayed by modulating graph intensity depending on distribution of individual ow velocity (Fig.3.2).
Application
• CW Doppler used for quantitative assessment and analysis of high ow velocities—particularly in echocardiography or in severe stenosis.
• Also used for acoustic depiction and assessment of ow in angiology or phlebology as well as for vessel puncture (“Doppler stick”, Fig.3.2).
Fig. 3.1 CDS—general display. Colours superimposed on grey scale image display ow direction and
mean ow direction; the colour bar in the left of the image indicates the respective scale (red towards
transducer, blue away from transducer, 0.29m/s—depictable mean peak velocity—without aliasing)
3 (Color) Doppler US: Theory, Artefacts, Typical Applications inChildhood
Fig. 3.2 CW Doppler Scheme. CW Doppler is a single constant US beam without dening any
pulse repetition. Usually applied by a small stick-like apparatus that is easily portable; can also be
integrated in normal US devices. All ow velocities within range of beam are depicted and displayed no matter whether they come from different locations. No aliasing—excellent for analysing
very high ow velocities. Method used in echocardiography and for orienting information, e.g.
patency of vessel, signs of signicant ow alteration (e.g. screening for leg vein thrombosis) or
nding a vessel for puncture, as well as for transcranial Doppler in adults/adolescents (e.g. sickle
cell anaemia)
41
Restrictions
• All information throughout entire beam is displayed—no identication of specic ow information of individual area.
3.1.2.2 Pulsed Wave Doppler (PW)
Principle Offers the benet to individually target and analyse ow in a single vessel.
• PW Doppler uses a crystal that alternatingly is used to send and then to receive.
After transmitting a short well-dened US signal—longer receive period is activated to register reected echoes.
• Time interval between sending and receiving can be dened—signal can be
exactly located at a specic denable depth.
• Volume of sampled area can be dened by variation of receive time (sample volume).
• Thus, one can choose individually adaptable partitions at variable sizes throughout imaged eld; all other areas within are neglected.
• Flow velocity measurements can be performed after applying angle correction
(Fig.3.3, Table3.1).
Intrinsic Restrictions
• By using predened send/receive time and sample volume, a maximum value is
dened above which ow velocities cannot be measured (Nyquist border). This
correlates with the pulse repetition frequency (PRF).
42
Fig. 3.3 PW Doppler/angle correction and graphic display. (a) PW Doppler: vessel seen by
B-Mode or CDS; the Doppler angle is corrected by manually moving line into vessel/ow long
axis. Doppler gate is placed at vessel; gate size should be adapted to vessel diameter. (b) Respective
graphic ow display after Fourier analysis. Relatively turbulent arterial ow—has a more homogenous white appearance with little dark areas under the Doppler trace. Laminar ow usually has a
clear “Doppler window” and only exhibits a narrow line of maximum velocities. Y-axis encodes
ow velocity, x-axis encodes time
M. Riccabona
Table 3.1 Angle dependency of
Doppler US measurements
Degree
20° 0.94 6
60° 0.5 50
80° 0.17 83
Cosine-induced error for Doppler measurements
depending on insonation angle: lower angles have
less error than higher angles; reliable measurements can only be achieved up to 50°–60°
cos
Error (%)
• Aliasing—velocities higher than Nyquist border cannot be displayed and may
occur on bottom of Doppler trace, creating partially confusing images and interfering particularly with automated measurements. Aliasing takes place as soon as
PRF is at least twice the maximum Doppler frequency (Nyquist frequency)
caused by the frequency shift from reection at moving particle.
• PRF depends on frequency, depth and minimally on sample volume size—the
deeper a vessel, the lower PRF must be set. Consequently, the maximum velocity
measurable in deep vessels is relatively low.
Note If the angle approaches 90°, no ow shown/depictable due to the cosine of
90° (=0) although there may be ow.
3.1.2.3 Duplex-Doppler Sonography/Spectral Flow Analysis
Principle
• Combination of B-Mode US with Doppler sonography. Simultaneous performance and display of B-Mode image and PW-/CW−/colour-Doppler- sonography
information:
3 (Color) Doppler US: Theory, Artefacts, Typical Applications inChildhood
43
– If three modes performed simultaneously—“triplex-Doppler sonography”,
improving exact location of sample volume, often at sacrice of (temporal)
resolution (= low frame rates, alternatively intermittent CDS-image update ...).
• Immensely important for correct assessment: proper angle correction is essential
for quantication of blood ow velocity if angle >10°. Doppler ow measurements signicantly depend on angle between US beam and motion direction—
this tool is essential for ow quantication and measurements (see Table3.1).
Restrictions
• Due to development of the cosine of angle α, measurements at Doppler angles >50 to
maximum 60° become inaccurate and unreliable—should not be used for diagnosis.
• When angle reaches 90°, cosine of angle α becomes 0, therefore no Doppler
information can be retrieved (= no ow visible even if there is ow).
3.1.2.4 Colour-Coded Doppler Sonography or Colour Doppler
Sonography (CDS)
Principle
• CDS—simultaneous PW Doppler at many individual points of image (dened by
“colour box”).
• Depending on equipment, variably averaged mean value of ow velocities and
direction encoded by colours superimposed on conventional black and white
grey scale B-mode image.
• Depending on colour preset, the main colour usually displays ow direction (e.g.
red—towards transducer, blue—away from transducer); ow velocity variations
within the same direction are then encoded by different colour intensities or
shades (see Fig.3.1).
Application and Restrictions
• Helpful for quick assessment of ow information.
• Helps to quickly nd vessels and differentiate them from other cystic or tubular
structures.
• Helps to optimise quantitative PW duplex-Doppler investigation of specic areas.
• Intrinsically does not allow detailed assessment of ow dynamics except for a
general impression of laminar versus pulsating ow, ow direction and average
ow velocity—no angle correction possible.
• Size of individual samples can be modied impacting frame rate. Additionally other
parameters for image optimisation such as ltering and velocity scales to be applied.
Note Slow frame rates caused by high resolution and low ow velocity imaging
may create motion-induced artefacts, particularly in uncooperative children.
3.1.2.5 Amplitude-Coded Colour Doppler Sonography (aCDS)
Synonyms: Angio-mode, Power Doppler (PD), Colour Doppler Energy (CDE).
44
M. Riccabona
Principle
• Does not use ow velocity but integrated velocity spectrum for calculation and
colour display.
• Higher sensitivity, reduced angle dependency.
• Uses amplitude of reected Doppler signals that is signicantly dependent on
number of reecting particles (much more than on frequency shift)—displays
ow volume rather than ow velocity.
• Furthermore—due to homogeneous low background noise—higher output and
receive gains can be used, again increasing potential to depict very low ow
velocities at poor Doppler angles down to 0.2mm/s in experimental settings.
• Doppler information displayed by colour overlay on grey scale image as in conventional CDS (Fig.3.4).
Restrictions
• High sound pressure in tissue.
• No information on ow velocity and ow direction (exception: “bi-directional
Power Doppler”).
• Higher risk of motion artefacts (“ash” artefacts).
Application
Used for vessel depiction at poor insonation angle or with low ow velocities.
• Ideal for assessment of peripheral parenchymal vascularisation—particularly
helpful in assessing the kidney.
• Due to its low-angle dependency, often used for demonstrating course of vessels
at poor insonation angle such as deep abdominal/cervical vessels and on transcranial Doppler sonography.
• Can be combined with duplex-Doppler US.
• Most modern aCDS techniques use special acquisition, ltering and postprocessing even further improving depiction of ow such as “superb microvascular
imaging/SMI” (Canon), “SieFlow” or “Clarify” (Siemens), “(radiant) SlowFlow
HD” (GE), or “MFI/Microow Imaging” (Philips).
Fig. 3.4 aCDS (transplant kidney). Power Doppler with low scale settings demonstrates peripheral vascularity/perfusion of the parenchyma in this transplant kidney; note the physiologically
reduced vascularisation of the medullae, not to be mistaken for pathology