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M. Riccabona
1.6.3 Harmonic Imaging (HI)
• Has become widespread and common, using rst harmonic response of resonating reectors instead of original reected echo for creating US image.
• Reduces penetration and needs slightly higher output gain, but HI signicantly
reduces noise—as signal for imaging is created by resonating individual structure itself.
• Improves border delineation and differentiation of liquid structures, enhances
grey scale differences.
• Commonly used in combination with HR-US/compounding; furthermore essential for contrast-enhanced US (ce-US).
Note Though initially applied mainly to adults in poor scanning conditions (caused
by overlying structures or adjacent gas), HI is now routinely applied in paediatric
US, too (Fig.1.5).
Fig. 1.5 Harmonic imaging (HI). (a) Normal grey scale cross-section image of a kidney (+2) with
a slightly dilated renal pelvis (+1): somewhat hazy image with poor quality. (b) Same infant and
same section as in (a) acquired with HI: more conspicuous image with better delineation of the
dilated pelvis and the renal borders; also the cortico-medullary differentiation is accentuated
(beware of imaging technique induced articial “pseudonephrocalcinosis”)
1.6.4 Extended Field ofView US
• Also known as panoramic imaging or freestyle US. Adds serial consecutive
neighbouring US images into one big overview.
• Based on calculation of transducer motion from picture inherent information—
thus two consecutive images can be aligned in proper anatomic order; continuous display of even very large structures is achievable (Fig.1.6).
Not only useful for comprehensive overview of gross pathology or anatomy but
also for displaying long/large structures or measuring structures too large for eld
1 US Physics
Fig. 1.6 Extended eld of view US. (a) Measurement of liver length in anterior axillary line: due
to large size, this can only be reliably achieved by using “extended eld of view”; additionally a
more conspicuous view of the entire organ with the kidney can be achieved. (b) Enlarged urinary
bladder sagittal view: conspicuous view and reliable measurement of this megacystis
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of view of conventional transducers (e.g. large transplant kidney, severe splenomegaly and huge tumours).
1.6.5 US Texture Analysis
Tries to improve US ability to differentiate and analyse tissue texture.
Still under development, not routinely applied or available on all devices.
• Quality of reective echoes from dedicated/individually dened area is assessed
using various algorithms to compare all sonographic attributes, or changes in
sound velocity.
• After comparison with normal standardised echotexture or potentially available
information from previous scans as well as other healthy organ regions, differences in tissue texture are displayed.
• Potentially improve detection and characterisation of specic tissue areas
(“sono-histogram”).
• Similar principle is applied to quantify ow based on CDS information—only
offered by some vendors.
• A new application of this approach is the upcoming sonographic liver fat quantication: still under research, but as today no established applications—particularly in paediatrics.
1.6.6 Potential Future forOther Modern Paediatric
US Applications
A number of potential applications on horizon: US-guided drug delivery, opticacoustic imaging, etc.
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M. Riccabona
Most presently under preclinical research conditions, some starting in human
trials, some introduced to adult scanning:
• Image fusion techniques for diagnostic and interventional procedures—already
introduced to adult scanning and intervention, rarely used in kids as more cumbersome (motion artefacts …).
• Liver fat quantication—recently introduced by some vendors, just being studied in adults. Two main methods reported: controlled attenuation parameter
(CAP—based on transient elastography—see Chap. 4) or Acoustic Structure
Quantication (ASQ—based on analysis of echo amplitude distribution, also
useful for other structural tissue analysis). May hold potential in obese children/
adolescents, or children with other metabolic conditions, too. Other approaches
usually based on speed of sound estimation, backscatter coefcient and attenuation parameters.
• US-guided drug delivery: UCA is used as carrier for attached drugs. Arrival of
drug-loaded UCA at desired location is detectable by ce-US, then high-energy
sound pulse is activated to destroy UCA/carrier molecule, thus deliver drug
locally to targeted area—reducing systemic drug interactions/adverse advents.
– Foreseen for oncology but also potential for any other focal disease (e.g.
inammatory).
• Opto-acoustic imaging, US thrombolysis, etc., many future applications on horizon but beyond scope of booklet.
US Methods, Artefacts, Biologic Effects, Practice
MichaelRiccabona
2.1 A (Amplitude)-Mode
Oldest US technique, still used today in ophthalmology (for measuring various
small structures of eye).
Technique Emitted US impulse reected at major interfaces, signal received dur-
ing transmission break. Graph illustrates travel duration of US beam on x-axis and
intensity of reected echoes as amplitude spikes on y-axis (Fig.2.1).
2.2 (T)M-Mode (Time-Motion-Mode)
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Used to show positional changes of reecting interfaces over time.
Principle On x-axis of monitor graph, changes in position of individual image pix-
els displayed; change in intensity of reected echo is encoded by variation in brightness, whereas time is encoded on y-axis.
Method frequently used in echocardiography and in some dedicated applica-
tions, e.g. for assessment of peristalsis or motion (e.g. ureteral peristalsis, diaphragmatic motion) (Fig.2.1c).
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_2
17
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Fig. 2.1 US modes. (a) A (amplitude)-Mode—oldest US technique: US signals emitted along
single line, amplitude of reected echo encodes spike height on y-axis, whereas depth of origin of
reection from individual structures encoded on x-axis (time between emission and receive). (b) B
(brightness)-Mode: transducer emits sound waves; the reected echoes are received. Energy of
echo encodes brightness of respective pixel on monitor; position of respective pixel calculated
from individual travel time (i.e. time between sound emission and receiving, with known sound
speed in tissue). (c) M (motion)-Mode: US image (of a prominent ureter, cross section through
bladder) shows a dotted line dening the section where changes (i.e. motion, in this case ureteral
peristalsis) over time are displayed as graph in lower part of image (blue). Originally this was
applied in echocardiography without orienting B-Mode image, just displaying the lower graph to
analyse heart wall or valve movements
M. Riccabona
2.3 B (Brightness)-Mode
The commonly used real-time US imaging technique (Fig.2.1b).
Technique Transmitted US waves reected when encountering various interfaces:
• Brightness of individual image pixels dened by intensity of reected echoes
(the stronger the echo, the brighter the corresponding pixel).
• Position of pixels dened by direction of transmitted beam inducing individual
echo (encoded on x-axis) and time between sending and receiving (depth,
encoded on y-axis).
• All reected echoes displayed on monitor correspond to travel time within predened beam direction—calculated sectional image.
• Repetitive frequent updates of such sectional images create movie-like impression enabling what is called “real-time US”.
2 US Methods, Artefacts, Biologic Eects, Practice
Fig. 2.2 Doppler US. (a) Doppler scheme: US signal emitted; frequency shift of received echo mea-
sured, thus ow velocity and ow direction can be calculated using Doppler equation; for correct
velocity estimation, angle between incoming US signal and movement direction of reecting particle
(i.e. mostly erythrocytes) must be measured. (b) Doppler display: besides audio signal typically
Doppler information displayed as ow graph after spectral analysis using Fourier transformation. All
velocities throughout spectrum are displayed at any time (of cardiac circle), with intensity encoding
number of reectors at the individual velocity. Y-axis encodes ow velocity; x-axis encodes time
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2.4 Doppler Sonography
If sound reected by moving interface, frequency of reected wave shifted (Doppler
effect).
• Frequency shift depends on angle between sound beam direction and direction of
motion, as well as velocity of moving particle/interface; shift dened by Doppler
calculation (Fig.2.2a).
• Frequency shift of received echoes can be measured; thus ow direction and ow
velocity can be calculated and displayed in various ways (Fig.2.2b—also see
Chap. 3)
2.5 Artefacts
2.5.1 General Remarks
Artefacts caused by phenomena that interfere with image formation and cannot be
sufciently corrected:
• Impair image (e.g. bowing artefacts, reection artefacts).
• Can also be diagnostically valuable (e.g. posterior enhancement/through transmission for identication of liquids, posterior shadowing for identication of
calcications).
• Knowledge of artefacts essential for proper image interpretation.
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M. Riccabona
2.5.2 Common Artefacts
2.5.2.1 Side Loop Artefact
Transducer does not only emit central beam but also side loops—can produce signicant echoes when reected by strong interfaces. Some of these echoes reected
into direction of central beam and received by transducer—these echoes appear
projected into main beam, get used for image calculation, although deriving from
structures out of main beam direction.
Only cause image impairment when encountering highly reective surface;
respective echoes are displayed as if arisen from central beam (wrong position),
usually only recognisable when occurring in uid-lled or low-echogenicity
structure.
• Typical example: adjacent bowel gas surface alters image of gall bladder mimicking sludge.
• Can be identied by change of transducer position (e.g. tilt transducer).
• Can usually be eliminated by repositioning transducer and reducing gain, altering angulation, etc.
2.5.2.2 Bowing Artefact
Arise by wrong projection of reected echoes into anatomic incorrect position.
Caused by oblique reections of beam—reected echo received by “wrong”
crystal, position wrongly assigned for further processing.
• Can usually be eliminated and identied by tilting of transducer.
2.5.2.3 Noise
Definition
Signal-like monitor appearance throughout image is created by electronic processing and amplication. Background noise is increasingly amplied with reduced signal strength (e.g. TGC adaptation or high-receive gain). Near limits of penetration:
differentiation between noise and real signals may become impossible.
Depending on gain settings, noise can also create articial echoes within anechoic
lesions such as uid or cysts, making differentiation difcult or impossible—particularly when small.
• For differentiation/identication: change focus position, output gain and transducer frequency.
• Modern devices all offer some sort of noise ltering, which can be set to various
intensities.
2.5.2.4 Marginal Shadowing
Created by spherical structures with clear limit that exhibit signicant acoustic
impedance interval at its lateral borders—appears as line-like sound mitigation at
lateral borders behind object.
2 US Methods, Artefacts, Biologic Eects, Practice
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Physical cause—tangential impact of sound beam, additional scattering and
reection at lateral wall—then transmitted into deeper image sections.
• Helpful for identication of cysts and tubular structures but may be mistaken for
acoustic shadowing from small concretions, e.g. in gall bladder or kidneys (Fig.2.3).
2.5.2.5 Posterior Enhancement—Increased through Transmission
When sound passes through completely uid-lled anechoic structure (or other
structure with little sound attenuation), intensity of US beam is not altered by
absorption and reection: causes different echo intensity of area deep to such uidlled structures compared to adjacent area of same depth where US beam has been
more attenuated by intervening tissue.
TGC correction articially adapts for intensity drop by depth—areas behind
uid displayed more echoic than surrounding structures.
• Helpful to identify uid/uid-lled structures.
Note In order to properly assess tissue behind large uid-lled structures, adapta-
tion of TGC correction to account for this phenomenon is essential (Fig.2.4).
Fig. 2.3 Artefact—marginal shadowing,
reverberations. Articial anechoic lines
originating from margins of venous sinus
in this axial liver view not corresponding to
any specic anatomic or pathologic
ndings. Note echoic spots with
reverberations within liver indicating
intrahepatic air/gas
Fig. 2.4 Artefact—through transmission.
Articially increased echogenicity behind
uid-lled bowel structure due to increase
through transmission but deteriorating
differentiation of respective structures (i.e.
gastric duplication cyst)
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M. Riccabona
2.5.2.6 Reverberation Artefact
Definition
Multiple reections of sound travelling between two parallel layers with strong
acoustic interfaces create repeated parallel echogenic lines that usually get weaker
with depth:
• Typically observed parallel to transducer at supercial layers (e.g. skin).
• Can be reduced by altering focus and decreasing (output) gain.
“Ring-down artefact” caused by resonance from gas;
Short ring-down artefacts called comet tail artefact—special form of reverbera-
tion phenomenon, usually appears behind gas/air-lled structures.
Reverberations most prominently seen with air interface—in lung ultrasound
also known as “A-lines”.
• Created by scattering and reection of incoming sound beam with irregular
reections and noise behind sonographically non-penetrable surface (Fig.2.5).
2.5.2.7 Increment or Slice Thickness/Beam Width Artefact
Sound beam dimensions vary depending on kind of transducer, frequency and focus
settings. Depending on relation of beam width with distance between solid and
liquid structures, particularly at curved interfaces, small layer of low-degree echoes
may appear. May mimic doubled/hazy wall structure, can be mistaken for sludge
within uid. A sort of partial volume phenomenon.
• Can usually be eliminated by optimising focus setting and changing transducer
or frequency.
Fig. 2.5 Artefact—reverberation/comet tail
artefact and dorsal shadowing. Chest wall US:
echogenic reverberations caused by aerated
lung surface (↔) and dorsal shadowing (→)
caused by ossied rib
2 US Methods, Artefacts, Biologic Eects, Practice
Fig. 2.6 Mirror image artefact. The
echogenic border of skull bone causes
mirroring of subcutaneous extracranial
collection into subcalvarian intracranial
compartment, mimicking a non- existing
intracranial collection. Note: artefacts from
malattachment of transducer to bowed skin
surface in upper right corner of image
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2.5.2.8 Mirror Image Artefact
Strong reecting interface (mostly gas—i.e. air at lung base) met by sound beam in an
angle around 45°—acts as acoustic mirror: articial mirror images observed behind
reecting border due to prolonged travel duration of incoming signal (Fig.2.6).
• Also encountered on colour Doppler sonography (CDS), may be quite confusing.
• Can be identied by changing transducer position/tilting transducer.
2.5.2.9 Shadowing
If sound cannot penetrate and does not cause reverberations—area behind does not
produce any echoes; i.e. it looks black like a shadow (see Fig.2.6).
• Useful for identifying stones, bones and other calcied structures—hinders
assessment of area behind.
Note This artefact is signicantly reduced when applying Image Compounding
(see Fig. 1.4), and enhanced by Harmonic Imaging. May lead to misinterpretation!
2.5.2.10 Refraction Artefact
Occurs when sound passes obliquely through an interface between tissues with signicantly varying sound speed—thus refraction occurs (mostly solid/uid interfaces or border between low- and high-echogenicity tissues).
• Can cause duplication artefacts (duplicating structures)—also affects length
measurements (e.g. kidney) (Fig.2.7).
• Refractive shadowing (see above—marginal shadowing artefact) caused by
defocusing and variations in beam energy or intensity at edge of uid-lled
structures.
• Can usually be eliminated by changing transducer position.