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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5790_Библиотеки_им_академика_М_И_Перельмана.pdf
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1 Theory and Basics
1.4.2 Common Artefacts
1.4.2.1 Side Loop Artefact
Transducer does not only emit central beam but also side loops – can produce significant echoes when reflected by strong interfaces. Some of these echoes reflected
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 reflective surface;
respective echoes are displayed as if arisen from central beam (wrong position), usually only recognisable when occurring in fluid-filled or low-echogenicity structure.
• Typical example: adjacent bowel gas surface alters image of gall bladder mim-
icking sludge.
• Can be identified by change of transducer position (e.g. tilt transducer).
• Can usually be eliminated by repositioning transducer and reducing gain, alter-
ing angulation, etc.
1.4.2.2 Bowing Artefact
Arise by wrong projection of reflected echoes into anatomic incorrect position.
Caused by oblique reflections of beam – reflected echo received by “wrong”
crystal, position wrongly assigned for further processing.
• Can usually be eliminated and identified by tilting of transducer.
1.4.2.3 Noise
Definition: Signal-like monitor appearance throughout image is created by
electronic processing and amplification. Background noise is increasingly amplified 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 artificial echoes within anechoic
lesions such as fluid or cysts, making differentiation difficult or impossible –
particularly when small.
• For differentiation/identification: change focus position, output gain and trans-
ducer frequency.
1.4.2.4 Marginal Shadowing
Created by spherical structures with clear limit that exhibit significant acoustic
impedance interval at its lateral borders – appears as line-like sound mitigation at
lateral borders behind object.
Physical cause – tangential impact of sound beam, additional scattering and
reflection at lateral wall – then transmitted into deeper image sections.
• Helpful for identification of cysts and tubular structures but may be mistaken for
acoustic shadowing from small concretions, e.g. in gall bladder or kidneys
(Fig. 1.6).
1.4 Arte facts
Fig. 1.6 Artefact – marginal shadowing, reverberations. Artificial anechoic lines originating from
margins of venous sinus in this axial liver view not corresponding to any specific anatomic or pathologic findings. Note echoic spots with reverberations within liver indicating intrahepatic air/gas
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Fig. 1.7 Artefact – through transmission. Artificially increased echogenicity behind fluid-filled
bowel structure due to increased through transmission but deteriorating differentiation of respective structures (i.e. gastric dublication cyst)
1.4.2.5 Posterior Enhancement – Increased Through Transmission
When sound passes through completely fluid-filled anechoic structure, intensity of
US beam is not altered by absorption and reflection: causes different echo intensity
of area deep to such fluid-filled structures compared to adjacent area of same depth
where US beam has been more attenuated by intervening tissue.
TGC correction artificially adapts for intensity drop by depth – areas behind fluid
displayed more echoic than surrounding structures.
• Helpful to identify fluid/fluid-filled structures.
NOTE: In order to properly assess tissue behind large fluid-full structures, adaptation of TGC correction to account for this phenomenon is essential (Fig. 1.7).
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Fig. 1.8 Artefact – reverberation/comet tail artefact and dorsal shadowing. Chest wall US: echo-
genic reverberations caused by aerated lung surface (↔) and dorsal shadowing (→) caused by
ossified rib
1 Theory and Basics
1.4.2.6 Reverberation Artefact
Definition: Multiple reflections 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 superficial 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 reverberation phenomenon, usually
appears behind gas/air-filled structures.
• Created by scattering and reflection of incoming sound beam with irregular
reflections and noise behind sonographically non-penetrable surface (Fig. 1.8).
1.4.2.7 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 second/hazy wall structure, can be mistaken for sludge
within fluid. A sort of partial volume phenomenon.
• Can usually be eliminated by optimising focus setting and changing transducer
or frequency.
1.4.2.8 Mirror Image Artefact
Strong reflecting interface (mostly gas – i.e. air at lung base) met by sound beam in
an angle around 45°– acts as acoustic mirror – artificial mirror images observed
behind reflecting border due to prolonged travel duration of incoming signal
(Fig. 1.9a).
• Also encountered on colour Doppler sonography (CDS), may be quite confusing.
• Can be identified by changing transducer position/tilting transducer.
1.5 Biologic Effects
Fig. 1.9 Artefact – 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: artifacts from malattachment of tranducer to bowed
skin surface in upper right corner of image
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1.4.2.9 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 (Fig. 1.8).
• Useful for identifying stones, bones and other calcified structures – hinders
assessment of area behind.
1.4.2.10 Refraction Artefact
Occurs when sound passes obliquely through an interface between tissues with significantly varying sound speed – thus refraction occurs (mostly solid/fluid interfaces
or border between low- and high-echogenicity tissues).
• Can cause duplication artefacts (duplicating structures) – also affects length
measurements (e.g. kidney).
• Refractive shadowing (see above – marginal shadowing artefact) caused by defo-
cusing and variations in beam energy or intensity at edge of fluid-filled structures.
• Can usually be eliminated by changing transducer position.
1.4.2.11 Anisotropy
Occurs in tissues composed of very structured strong reflectors (e.g. fibrillar pattern) –
echoes vary with insonation angle (typically with muscles and tendons).
• Can usually be eliminated by changing transducer position/angulation.
• Of utmost importance in muscular US – if unrecognised may lead to incorrect
interpretation (i.e. tear, etc.).
1.5 Biologic Effects
1.5.1 General Remarks
Biologic effects of diagnostic US based on physical phenomena caused by interaction
of emitted US with tissue depending on frequency, wave length and output energy.
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Different devices may cause variable tissue impact with same application –
because of different output gain settings/other device-specific presets.
1 Theory and Basics
1.5.2 Thermal Effects
1.5.2.1 Tissue Heating
Caused by energy absorption – amount of temperature rise depends on output
energy and intensity of sound field. Increases with higher frequency by deposition
of higher amounts of energy in smaller volume (less penetration).
Additionally, temperature-handling ability of tissue is important, e.g. vascularised and well-perfused tissue can better tolerate temperature changes than little or
non-perfused tissue.
Human tissue with highest thermal absorption is bone; therefore experiences
highest temperature rise with secondary biologic effects particularly on neighbouring tissue.
NOTE: some heat generated by transducer itself is also transmitted to skin/tissue
1.5.2.2 Biological Effects, Tissue Heating
Even on routine diagnostic scans using modern diagnostic US devices, measurable
increase in temperature may occur; particularly important for fetal examinations
and (trans)cranial US. However, temperature that causes degeneration of proteins
(i.e. >45 °C) or potentially cell death (>41.5 °C) does usually not occur in diagnostic applications.
This effect must be considered when examining patients with high fever to avoid
potential dangerous heat production; e.g. relatively short insonation of individual
areas advisable. Commonly used parameter = thermal index (TI). Three different
types of TI defined depending on tissue examined: TIS – small part, TIB – bone,
TIC – cranial (see below):
• General rule of thumb: TI never >3, TIC <1.7, in neonatal brain TIC <1 (or better
<0.7) advisable.
1.5.3 Mechanical Effects and Resonance
Resonance of molecules proportional to applied frequency depends on output
energy, separate from mechanical impact on tissue by sound pressure.
In order to maintain safety, diagnostic US devices initially did not allow energies
>100 mW/cm2. According to newer experimental observations, intensity measure
changed – at present sound pressure levels considered more important:
• Relative upper limit of negative peak pressure is defined by 1 mPa; no relevant
mechanical and resonance-induced tissue damage should occur below this level.
• (Sound)Pressure waves have positive and negative partition; the latter is called
suction force. This negative pressure causes a sort of vacuum – has highest
potential for tissue damage by implosion or cavitation.
1.5 Biologic Effects
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Danger/risk of mechanic effects estimated by mechanical index (MI): in general
MI should be kept below 1.7; in more risky areas <1; in very sensitive areas
<0.7: (e.g. neonatal brain) and for low-MI contrast-enhanced US (ce-US) MI around
0.1–0.3 (see below).
1.5.3.1 Cavitation
Acoustic Cavitation
Sound-induced occurrence of hollow areas as well as gas bubbles in insonated
material – may undulate and change size. These small cavities and their activity
cause wide spectrum of physical, chemical and biological effects.
Negative Peak Pressure
Crucial parameter for estimation of cavitation effect: negative peak pressure within
insonation field. Additionally need cavitation seed – usually microscopic gas bubbles that explosively increase in size during negative sound pressure.
NOTE: Cavitation effects are independent from thermal effects; e.g. US impulse
with high pressure and low frame rate can cause cavitation without any significant
thermal changes.
In human tissue, inert cavitation is no major problem – practically no cavitation
seeds, except for tissue containing air or gas such as lung or intestines. However, if
US contrast media (based on stabilised microbubbles) used, cavitation effects may
become relevant, as for US of target adjacent to aerated structures:
• When respecting given limits/application guidelines, no clinical relevant damage
by diagnostic US (even using US contrast media) is currently reported.
1.5.4 Potential Risks of Diagnostic US
Significant effects can be produced by US on all kinds of tissues. This potential is
used therapeutically (e.g. lithotripsy, sonophoresis and treatment of tendinous
calcifications).
Diagnostic US uses much lower energy levels than therapeutic US, though –
using maximum output gain and long sound exposure on single site – biological
effects can be demonstrated in animal experiments (cavitation, mechanic and thermal effects added, duration of exposure essential).
With prudent use, no significant impact in human medical diagnostic use in
terms of carcinogenesis, teratogenesis or higher mutation rates found.
1.5.4.1 Specific Risks
Long duration of pulsed duplex-Doppler and amplitude-coded CDS (aCDS) investigations with stationary US beam, particularly in vicinity to bone (for these applications higher sound energy with focused focal pulse is usually used); i.e.
transcranial US, echocardiography – particularly in border areas with vicinity to
aerated lung:
• M-Mode: slightly higher-output energies used for depicting clear M-Mode signal.
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NOTE: Try to avoid focused pulsed duplex Doppler and M-Mode for fetal echocardiography (risk-benefit ratio to be considered).
1 Theory and Basics
1.5.4.2 Guidelines and Recommendations
In order to maintain biologic sound-induced risks as low as possible, some aspects
need to be considered:
1. Diagnostic US – medical imaging; there should be clear indications on firm med-
ical grounds for every investigation (with some exceptions for scientific and edu-
cational needs).
2. Always try to minimise output gain by using maximum receive gain.
3. Keep exposure times of specific area as short as possible; make use of frozen
images for analysis instead of looking at same structure for long times under
real-time US conditions (unless you need dynamic-functional observation).
4. Always first optimise image, particularly for Doppler investigations: only
activate your colour box or PW-duplex gate after area of interest has been
defined, measurement point/gate has been adjusted, angle correction has been
defined, etc.
5. Try to avoid cavitation seeds or bones in vicinity of duplex-Doppler beam.
6. Further recommendations can be found in literature and with various US societies
(e.g. EFSUMB; AIUM; OEGUM/DEGUM).
1.5.5 Various Methods and Indices That Allow Estimation
of Biological Risks
1.5.5.1 Mechanical Index (MI)
Introduced to describe peak pressure in tissue (in mPa); depends on output gain. The
used frequency and focus pre-describe potential risk of sound pressure-induced tissue damage as well as cavitation risk.
Mostly used in B-Mode sonography and should be kept below 1; short increases
(if diagnostically necessary) up to 1.5 mPa acceptable in individual situations.
NOTE: MI should be lower for fetal exams, for examination of the neonatal brain
(transfontanellar), for eye US and for ce-US to avoid damage of specifically sensitive structures or to minimise danger in t presence of cavitation seeds (see also
above).
1.5.5.2 Thermal Index (TI)
Describes risk of tissue heating with consecutive tissue damage:
1. TIS (soft tissue thermal index) – used for soft tissue.
2. TIB (bone thermal index) – used for bone.
3. TIC (cranial thermal index) – used for transcranial applications.
Mainly depends on tissue, output gain, focal zone and frequency used.
TI – most important in Doppler sonography as well as for fetal US. TI should be
kept below 1 – brief increase accepted if diagnostically necessary in individual
examinations (e.g. Doppler sonography usually works with higher TI values).
1.6 How to Perform Paediatric US
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1.5.5.3 Display of Actual Indices
Indices must be displayed by equipment throughout investigation and constantly
updated depending on individually altered settings (gain, focus zone, frequency,
etc.); should also be documented on saved images.
1.6 How to Perform Paediatric US
1.6.1 Requisites
1.6.1.1 Indications
Every investigation must rely on thorough indication. Referring physician has to
provide detailed question; US investigation must potentially offer diagnostically
relevant result with therapeutic or prognostic consequence.
Only exceptions:
• Screening investigations (e.g. urinary tract and hip) – should have significant
preventive effects:
– Increasingly under discussion, with widespread fetal US and new knowledge
on impact of screening approaches in last decade.
• For scientific or educational purpose.
1.6.1.2 Environmental Requisites
• Proper and comfortable positioning facilities.
• Quiet room with sufficient light dimming.
• Proper and ergonomic positioning of investigator.
• Ergonomic styling of surrounding working area – includes separate reading facility
with monitors and separate sitting area for consultation with patients and parents.
NOTE: Sufficient chairs must be available, as there are usually more people than
just the patient.
• Proper room temperature with additional heating available for neonates and
infants.
• Even with children, privacy must be respected; therefore proper changing rooms
and towels mandatory, furthermore cleaning facilities, and adjacent restroom
desirable.
1.6.1.3 Specific Needs in Children
• Usually accompanying persons are present during investigation and for consulta-
tion afterwards – rooms must be adequately sized and equipped.
• Accompanying parents and brothers or sisters can help pacify infant during
investigation, additionally monitors for displaying either US image or movies
and toys are helpful. Other pacifying measures: books, music.
• Warm US gel, but prevent bacterial and fungal growth in gel bottle.
• Initial introductory comment understandable to child and accompanying persons
is helpful – enables them to understand investigation, what is going to happen
and to reduce fears. Explain equipment as well as procedure.
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1 Theory and Basics
• Effort of establishing good relation prior to starting investigation – often essen-
tial to enable peaceful and diagnostically valuable investigation.
NOTE: Empathetic action is important! Try to avoid strong and abrupt transducer
pressure as well as fast movements. Sometimes also helpful to keep skin contact
with hand/finger that holds transducer.
1.6.1.4 Specific Needs in Infants and Newborns
• Higher room temperature, additional heating and swaddling facilities are
mandatory.
• Helpful to have some warm tea/formula and pacifiers ready at hand:
– Pacifiers can furthermore be enhanced by specific tastes such as glucose and
fruit extracts.
1.6.2 Positioning
• Abdominal US: usually lying supine, sometimes prone or lateral decubitus posi-
tion is helpful. Some abdominal areas can also be investigated with baby lying in
arms of mother – e.g. urinary tract screening in anxious and excited infants,
provided acceptable position for the investigator is granted, too.
• Urinary tract US: same as abdominal US standard; additional prone positioning
for examining kidneys from dorsal approach is advisable. Additional approach:
perineal US.
• US of neonatal brain/transtemporal US: any position where head can be kept still
and stable with sufficient acoustic access for US probe; for posterior fossa inves-
tigation, transoccipital or transnuchal access in lateral decubitus position with
flexed cervical spine is helpful.
• US of neonatal spine and spinal canal: prone or lateral decubitus – try to avoid
hyperextended back to assure sufficient access to spinal canal.
• Echocardiography: usually supine position with slight lateral rotation; additional
pillows underneath back may be helpful. For suprasternal access, neck extension
with some support of shoulder and side turning of head is helpful – provided the
baby can tolerate positioning.
• Hip US: standardised procedure with standardised positioning partially
using dedicated positioning devices; depends on technique applied (see
Chap. 11).
• Small-part and neck US: sometimes helpful to comfortably position targeted
area by help of supporting pillows and towels (see Chap. 4).
NOTE: In adults and bigger children, positioning manoeuvres or breath holding
and flexion or rotation is routinely used to optimise US window for proper
access to diagnostically relevant deeper regions. In children, particularly infants
and neonates, this is practically impossible, therefore “golden rule for US in
infants”: do not move child towards transducer trying to depict pathology, but
try to move transducer to sonographic window that allows optimal access to
targeted areas.
1.6 How to Perform Paediatric US
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If in older children positioning manoeuvres are attempted, try to use age- adequate
commands such as “show me your big belly” or “take a deep breath and hold it as if
you were diving”.
• Remember also to have a comfortable examinier position for health reasons-
avoid degenerative disease.
1.6.3 Device Handling
General Remarks
Particularly in paediatric US, investigator must be accustomed with device and its
handling, as child motion and agitation as well as need for communication and devoted
emphasis would impair capabilities to struggle with equipment. Additionally, experienced handling speeds up investigations allowing for better results and focused concentration on child and image – without withdrawing attention towards handling of device.
Therefore it is practical to import all data (such as patient name/numbers) and set
up machine (selecting transducers/presets) prior to positioning of child.
Choice of Device and Transducer
• Handling of different US equipments varies – large variability in requirements.
• A particularly helpful feature is cine loop – store video clips for retrospective review:
– Depending on device, varying number of images is constantly stored to hard
disc at any time during investigation. Allow for review of preceding parts of
investigation; video clip is constantly updated.
– Review of cine loop: single frames can be captured and stored; some machines
and picture archiving and communication systems (PACS) also allow storage
of clips.
– As children are often less cooperative, this feature is particularly helpful for
selecting optimal frames for measurements and documentation as well as
image analysis.
How to Start Investigation
Once machine is set up, select transducer and the respective preset, and position patient:
• After positioning of transducer, adapt receive gain and TGC as well as focus, frame,
size and penetration. All these parameters need to be constantly updated during inves-
tigation – as different body areas and positions request different equipment settings.
Modern devices offer automated adaptation and optimisation algorithms – so
they can speed up investigations, helpful for general overview. Additional adaptation and variation of settings will still be necessary for certain queries, for detailed
investigations or in certain body areas (such as behind urinary bladder). Particularly
post-processing, sufficiently fast frame rate and proper placement of focal zone are
essential – need to be changed and adapted constantly; this task cannot reliably be
performed by automated image optimising programmes.
Once you have chosen an adequate preset, changes of preprocessing factors, etc.
become only necessary in rare cases:
• Presets usually selected by deciding on certain investigation category – optimised
towards dedicated queries.