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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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1.7 Documentation and Interpretation................................................................................... 25
1.7.1 Image Documentation ....................................................................................... 25
1.7.2 Report ................................................................................................................ 26
1.8 Doppler Sonography ....................................................................................................... 27
1.8.1 The Doppler Phenomenon ................................................................................ 27
1.8.2 Different Techniques and Applications of Doppler Sonography ...................... 28
1.8.3 Artefacts in (Colour) Doppler Sonography ....................................................... 34
1.8.4 How to Perform (Colour) Doppler Investigations ............................................. 37
1.8.5 Limitations ........................................................................................................ 37
1.8.6 Interpretation ..................................................................................................... 37
1.9 Modern and Future US Methods and Techniques ........................................................... 39
1.9.1 High-Resolution US (HR-US) .......................................................................... 39
1.9.2 Image Compounding ......................................................................................... 39
1.9.3 Harmonic Imaging (HI) .................................................................................... 39
1.9.4 Extended Field of View US ............................................................................... 39
1.9.5 US Texture Analysis .......................................................................................... 41
1.9.6 Sonoelastography .............................................................................................. 41
1.9.7 US with Contrast Enhancement (Echo-Enhanced US – ee-US,
Contrast-Enhanced US – ce-US/CEUS) by Ultrasound Contrast
Media (US-CM) ................................................................................................ 42
1.9.8 Three- and Four-Dimensional US (3D-/4DUS) ................................................ 50
1.9.9 Potential Future for Other Modern Paediatric US Applications ....................... 57
1 Theory and Basics
1.1 Ultrasound (US) Physics
1.1.1 US Waves
Definition
• Mechanical waves, usually created by electric current applied to piezoelectric
crystal in transducer; used to emit sound waves and receive reflected echoes.
• Frequencies used in diagnostic medicine range from 1 to 20 MHz.
Sound Velocity
• Depends on material; the higher the density the higher the sound velocity.
• In air, sound velocity is approximately 330 m/s; average sound velocity of human
(soft) tissue = 1,540 m/s.
1.1.2 Propagation and Modulation of US
US energy emitted into tissue is handled differently between different tissue layers: modulated, partially absorbed, partially transmitted and reflected on border of different tissue.
The most important principles for propagation phenomena:
1.1.2.1 Acoustic Impedance
Relation of sound pressure to resulting molecular motion.
Tissues with high density need less energy to start undulating than tissues with
little density.
1.1 Ultrasound (US) Physics
3
1.1.2.2 Impedance Change
Arises when US waves cross borders between tissues of different acoustic
impedance.
1.1.2.3 Reflection
Observed when US wave meets border layer between tissues of different impedance; reflection occurs according to mirroring rules (Snell’s law).
Degree of reflection depends on surface structure (e.g. smooth or rough and straight
or bent), angle between tissue surface and US beam.
1.1.2.4 Absorption
US waves gradually weakened when crossing different media. Loss depends on tissue density and content, also proportional to US frequency (greater loss = less
penetration):
• Low frequency: good penetration but restricted resolution.
• High frequency: decreased penetration but increased resolution.
1.1.2.5 Deflection
When US wave passes small opening, US beam scattered depending on dimension
of this “lens”; scattered sound waves may cause artefacts.
1.1.2.6 Focus
In modern diagnostic US, multiple crystals create multiple individual US waves.
These need to be focused at specific areas in order to create detailed images of
defined area.
Focusing achieved by:
• Hollow mirror effect: US field gets smaller and smaller by concave shape of
emitting crystals.
• Additional lenses.
• Electronic focusing by dedicated steering of single elements with proper
timing.
Optimising focal zone:
Single and multiple focus techniques available – need to be constantly optimised/
updated during investigation for optimal results.
1.1.2.7 Resolution
Definition: minimal distance between two neighbouring structures that can still be
discriminated.
Two different phenomena:
• Lateral resolution: discrimination of objects side by side at same depth:
– Mostly dependent upon beam width.
• Longitudinal and axial resolution: discrimination of objects in direction of US
beam.
– Mostly dependent upon frequency: the lower the frequency the worse the
resolution (see Table 1.1).
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1 Theory and Basics
Table 1.1 Resolution
versus frequency – orienting
numbers
Frequency (MHz)
3.5 7 2 160
5 0.6 1.2 100
7.5 0.4 0.8 50
The higher the frequency, the better the resolution; lateral resolution always less than axial
Penetration depth also depends on frequency: lower frequencies
have a deeper penetration
Resolution (mm)
Depth (mm)Axial Lateral
1.2 Practical Application in US Device
1.2.1 Emission, Transmission, Reception and Amplification
1.2.1.1 Emission
US waves emitted by transducer crystals; contain 64–512 crystals in conventional US:
• Specific modern transducer technologies: matrix transducers, 1.5D arrays, 2D
arrays; may contain up to several thousand single crystals.
• Future specific transducers (developed for 3D-/4DUS) > 10,000 single
elements.
Good contact of transducer to skin mandatory for optimal US transmission into
tissue; achieved by surface shape of transducer, material of transducer membrane
and (sufficient) US gel to eliminate air.
1.2.1.2 Transmission
US waves partially absorbed and partially reflected, the latter particularly at border
of different tissues.
1.2.1.3 Reception
After emission of US wave – crystal function changed to receive. Reflected US
waves create electric signal within crystal; amount of electric energy depends on
amount of reflected sound energy:
• More reflected sound – more electric impulse – brighter, more echogenic
signal.
• Moderate reflection – poor echo.
• No reflection – echo free or anechoic.
Spatial location of reflecting structure defined by time interval between emitting
and receiving:
• The deeper a structure the longer the sound beam needs to travel to it and back.
• Measured time between sound emission and reception of certain reflected energy
defines position/depth of certain structure within US field/image (in B-Mode
US). The longer sound takes to travel, the deeper position of respective
structure.
ab
1.2 Practical Application in US Device
5
1.2.1.4 Amplification
Electric signals created by incoming reflected US waves in crystals amplified within
US system for further processing.
1.2.2 Signal Processing
Raw signal processed using multiple parameters/methods.
1.2.2.1 Preprocessing
Performed during investigation, includes electronic modulation of signal quality
during sending (beam forming) as well as modulation of sensitivity of crystal during
receiving.
1.2.2.2 Post-processing
Performed once data collected, i.e. on frozen image. Many different electronic modulation tools can be applied to improve image quality, modulate contrast, weight
certain gray levels, etc.
1.2.2.3 Time Gain Compensation (TGC)
Reflected echoes from deeper areas have to pass through much more tissue, therefore suffer from more absorption: these signals are proportionally amplified to compensate for signal loss.
NOTE: TGC should be constantly optimised during investigation, varies with
echodensity/absorption of more superficial transmitted structures (Fig. 1.1).
1.2.2.4 Sound Energy = Output
Maximum output intensity defined by equipment depending on manufacturer.
To avoid unnecessary overexposure of tissue/deterioration of image: decrease
US intensity as much as possible. Definable partially by presets (e.g. fetal exams,
Fig. 1.1 TGC – image example (a) incorrect (b) correct TCG adaptation. (a) Incorrect image of
the magnified retrovesical cross-section view without proper TGC adaptation – causes echogenic
retrovesical structures reducing differentiation of anatomy; particularly the dilated left ureter cannot be clearly depicted (TCG settings recognisable by the dotted line on the right side of the
image). (b) Same section as in (a) – TGC adapted: better image quality – the slightly prominent
left ureter clearly visible distal as circular hypoechic structure
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1 Theory and Basics
transcranial/transfontanellar brain US, US of eye/testis) or individually at beginning
of examination. Can (must) be seen on display – usually as percentage of maximum
output gain.
NOTE: Every investigation should be performed at lowest possible sound output.
Impact of sound energy on tissue important to maintain safe sound pressure
levels:
• Parameters depend on many factors such as focal zone, frequency and output
gain setting.
• New indices established: reflect impact of US energy on tissue (mechanical
index = MI, thermal index = TI); should (must) be displayed during every investigation – monitor/observe closely.
• In general, MI/TI should be kept below 1 to maintain safe sound exposure
levels:
– For further details, see biological effects.
1.2.2.5 Gain
Defines overall amplification of incoming signals:
• Optimise receive gain individually depending on output gain, patient, anatomy
and area of investigation.
1.2.2.6 Frame Rate/Persistence
Persistence: defines speed of image update:
• High frame rate – fast series of individual images, reduced susceptibility to
motion artefacts – but usually at cost of slightly reduced resolution.
• High persistence (information from series of individual images used to create
final displayed image) – increased tissue density information and resolution – at
cost of slower update of individual displayed image.
Frame rate (Hertz, Hz): usually US investigations operate at 4–60 Hz; faster frame
rates are possible, e.g. for cardiac studies.
1.2.3 Components of US Device
Consists of emit and receive partition as well as transducers connected to system via
cables.
Also: display monitor, keyboard, memory/data storage and documentation
ability.
1.2.3.1 Transducers
Different types depending on underlying technology: mechanical, electronic and
combined transducers.
Modern transducers usually use a range of frequencies, with an individually adaptable diagnostic effective middle frequency – called multifrequency transducers.
abc
1.2 Practical Application in US Device
Fig. 1.2 Sector transducers – all creating a sector-like triangular image; good for small footprint
access with wide view in far field. (a) Conventional sector: sector-like images created by dedicated
array design or wobbling of a normal plane array. The wobbler technique hardly used anymore.
Image is in a sector format; the shaded area represents the part of the structure that will be displayed on the monitor. (b) Phased/electronic (vector/sector) transducer: most commonly used format (alternatively mostly micro-curved arrays used). Image created by electronic steering of
parallel-placed single elements. (c) Annular array: annular concentric US array which is shaped by
specific lenses creating a very homogenous focal zone throughout the image field
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Sector Transducers
Small active surface (footprint) where sound beams emitted in sector format
(Fig. 1.2):
• Causes poor image quality in near field, improved visualisation of deeper fields.
• Particularly useful for structures with only small access area (e.g. echocardiog-
raphy – access between ribs or brain US – transfontanellar access).
Different techniques used to create sector-like field:
• Mechanical devices that make crystal (or series of crystals) rotate or wobble:
– Sector angle usually between 60 and 120° used for imaging.
NOTE: Mechanical transducers may deteriorate over time by physical use – not
only proper handling but also exact production and alignment important
(Fig. 1.2a). Important to freeze image (i.e. transducer) whenever one does not
actually investigate to prevent early transducer deterioration/aging.
• Electronic-phased array transducers – consist of series of crystals:
– By individual steering of consecutive crystals with varying time intervals
(presently most common technique), effective US beam can be directed in
many directions creating sector-like imaging field (Fig. 1.2b).
• Annular array transducers – combination of mechanical and electronic technology:
– Various concentric rings of crystals selectively activated during scanning pro-
cess create sector-like field with homogeneous focus zone throughout entire
imaging field.
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ab
Fig. 1.3 Transducers. (a) Linear transducer: parallel sound waves create a rectangular image.
(b) Curved(linear) array transducer: transducer elements assembled in slightly curved fashion;
transducer surface thus is bent; the radius may vary creating a more or less trapezoid image that is
wider in far field than in near field. Combines benefits of linear and sector transducers
1 Theory and Basics
Linear Array Transducers
• Parallel linear US beams created by multiple crystals – create rectangular image
frame (Fig. 1.3a):
– Homogeneous resolution throughout entire imaging field, particularly
valuable for near-field assessment.
– Generally used for superficial structures (e.g. small-part applications, cervical
vessels, infant hips, lymph nodes, soft tissue processes and bowel/ appendiceal
US).
• New techniques allow for “phasing” of electronic linear transducer – creates
“virtual sector” image (“trapezoid”) – larger field of view in far field, at cost of
frame rate and penetration.
Curved Linear Array
• Crystals aligned on curved surface – diverging US waves create sector-like imaging field (angle depends on radius of curvature); the larger surface (than sector
transducer) offers good near-field information:
– Combines abilities and benefits from sector and linear transducers.
– Offer reasonable near-field resolution at large field of view at depth (Fig. 1.3b).
– Typical application: abdominal US.
Other Transducers
• Matrix – 1.5-/2-dimensional arrays: enable sound emission in two perpendicular
planes by assembling elements in parallel rows:
– Allow volume scanning (see 3DUS).
– Improve lateral (out of plane/elevational plane) resolution by bidirectional
focussing of US beam.
1.3 US Methods
– Parallel columns of elements allow for simultaneous handling of different
tasks (improving frame rate, e.g. for image compounding or colour/duplex/
triplex Doppler) by splitting individual operation modes to different parallel
rows or cristals.
• Intracavitary probes: mainly intravascular or endoscopic probes, transrectal/
intravaginal probes:
– Usually very small design, thus less elements.
– Often higher frequencies – better resolution than with transabdominal/tho-
racic access but at restricted penetration.
– Enable visualisation of areas impossible to properly depict by standard access.
– Attached to endoscopic devices/intravascular catheters.
– Often limited use for paediatric applications, as other access often works
sufficiently and size relatively large for paediatric cavities.
– Dedicated small paediatric devices rarely available (e.g. for transoesophageal
echocardiography, transrectal pelvic floor US).
– Some applications uncommon, non-existent or not accepted in paediatrics
(e.g. transvaginal US).
1.2.3.2 Other Parts of US Device
• Keypad (may be mobile and flexible).
• Monitor (may be mobile and flexible, can and must be adjustable).
• Printer/CD recorder.
• In-/output options.
• Cooling device with filters (need to be cleaned regularly).
• Potentially gel bottle warming device and transducer stands.
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1.3 US Methods
1.3.1 A (Amplitude)-Mode
Oldest US technique, still used today in ophthalmology (for measuring various
small structures of eye).
Technique: emitted US impulse reflected at major interfaces, signal received during transmission break. Graph illustrates travel duration of US beam on x-axis and
intensity of reflected echoes as amplitude spikes on y-axis (Fig. 1.4).
1.3.2 (T)M-Mode (Time-Motion-Mode)
Used to show positional changes of reflecting interfaces over time.
Principle: on x-axis of monitor graph, changes in position of individual image
pixels displayed; change in intensity of reflected echo is encoded by variation in
brightness, whereas time is encoded on y-axis.
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b
1 Theory and Basics
a
c
Fig. 1.4 US modes. (a) A (amplitude)-Mode – oldest US technique: US signals emitted along
single line, amplitude of reflected echo encodes spike height on y-axis, whereas depth of origin
of reflection from individual structures encoded on x-axis (time between emission and receive).
(b) B (brightness)-Mode: transducer emits sound waves; the reflected 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 defining 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
Method frequently used in echocardiography and in some dedicated applications, e.g. for assessment of peristalsis or motion (e.g. ureteral peristalsis, diaphragmatic motion) (Fig. 1.4c).
1.3.3 B (Brightness)-Mode
The commonly used real-time US imaging technique (Fig. 1.4b).
Technique: transmitted US waves reflected when encountering various interfaces:
• Brightness of individual image pixels defined by intensity of reflected echoes
(the stronger the echo the brighter the corresponding pixel).
• Position of pixels defined by direction of transmitted beam inducing individual
echo (encoded on x-axis) and time between sending and receiving (depth,
encoded on y-axis).
• All reflected echoes displayed on monitor correspond to travel time within
predefined beam direction – calculated sectional image.
• Repetitive frequent updates of such sectional images create movie-like impression
enabling what is called “real-time US”.
a
1.4 Arte facts
11
b
Fig. 1.5 Doppler US. (a) Doppler scheme: US signal emitted; frequency shift of received echo
measured, thus flow velocity and flow direction can be calculated using Doppler equation; for
correct velocity estimation, angle between incoming US signal and movement direction of
reflecting particle (i.e. mostly erythrocytes) must be measured. (b) Doppler display: besides
audio signal typically Doppler information displayed as flow graph after spectral analysis using
Fourier transformation. All velocities throughout spectrum are displayed at any time (of cardiac
circle), with intensity encoding number of reflectors at the individual velocity. Y-axis encodes
flow velocity; x-axis encodes time
1.3.4 Doppler Sonography
If sound reflected by moving interface, frequency of reflected wave is 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 defined by Doppler
calculation (Fig. 1.5a).
• Frequency shift of received echoes can be measured; thus flow direction and flow
velocity can be calculated and displayed in various ways (Fig. 1.5b – also see
below in Chap. 1.8).
1.4 Artefacts
1.4.1 General Remarks
Artefacts caused by phenomena that interfere with image formation and cannot be
sufficiently corrected:
• Impair image (e.g. bowing artefacts, reflection artefacts).
• Can also be diagnostically valuable (e.g. posterior enhancement/through trans-
mission for identification of liquids, posterior shadowing for identification of
calcifications).
• Knowledge of artefacts essential for proper image interpretation.