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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5795_Библиотеки_им_академика_М_И_Перельмана.pdf
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M. Riccabona
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.
1.2.2 Impedance Change
Arises when US waves cross borders between tissues of different acoustic impedance.
1.2.3 Reflection
Observed when US wave meets border layer between tissues of different impedance; reection occurs according to mirroring rules (Snell’s law).
Degree of reection depends on surface structure (e.g. smooth or rough and
straight or bent), angle between tissue surface and US beam.
1.2.4 Absorption
US waves gradually weakened when crossing different media. Loss depends on tissue density and content, and is proportional to US frequency (greater loss=less
penetration):
• Low frequency: good penetration but decreased resolution.
• High frequency: decreased penetration but increased resolution.
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.
1.2.6 Focus
In modern diagnostic US, multiple crystals create multiple individual US waves. These
need to be focused at specic areas in order to create detailed images of dened area.
Focusing achieved by:
• Hollow mirror effect: US eld gets smaller and smaller by concave shape of
emitting crystals.
• Additional lenses.
• Electronic focusing by dedicated steering of single elements with proper timing.
1 US Physics
Table 1.1 Resolution versus frequency—orienting numbers
Resolution (mm)
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
Depth (mm)Axial Lateral
Optimising Focal Zone Single and multiple focus techniques available—need to
be constantly optimised/updated during investigation for optimal results.
Remark
The newest technology: e.g. “retrospective transmit beamforming”, “all in focus”,
or “confocal” emit and receive, based on new transducer and beamformer technology and increased processing speed and capabilities—no individual focus setting
necessary, although still benecial in some detailed investigations (e.g. “inFocus”
by Siemens, “cSound” by GE).
1.2.7 Resolution
Definition
Minimal distance between two neighbouring structures that can still be discriminated.
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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 Table1.1).
1.3 Emission, Transmission, Reception andAmplification
1.3.1 Emission
US waves emitted by transducer crystals contain 64–512 crystals in conventional US:
• Specic modern transducer technologies: matrix transducers, 1.5D arrays, 2D
arrays; may contain up to several thousand single crystals.
• Split crystal transducers = modern technology to create multiple “elements”,
usually for 3D/4D- or some 2D arrays.
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• Future specic transducers (developed for 3D-/4DUS) >10,000 single
elements.
• Recently, a new technology is gaining interest—capacitive micromachined ultra-
sonic transducers (CMUT): relatively new concept based on metalized silicon,
allowing smaller transducer size at broader bandwidth (e.g. for intravascular
probes).
Good contact of transducer to skin mandatory for optimal US transmission into
tissue; achieved by surface shape of transducer, material of transducer membrane
and (sufcient) US gel to eliminate air.
M. Riccabona
1.3.2 Transmission
US waves partially absorbed and partially reected, the latter particularly at border
of different tissues.
Tip Transmission improved by high water content in tissue (=good hydration).
1.3.3 Reception
After emission of US wave—crystal function changed to receive. Reected US
waves create electric signal within crystal; amount of electric energy depends on
amount of reected sound energy:
• More reected sound—more electric impulse—brighter, more echogenic signal.
• Moderate reection—poor echo.
• No reection—echo free or anechoic.
Spatial location of reecting structure dened 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 a certain reected
energy denes position/depth of respective structure within US eld/image (in
B-Mode US). The longer sound takes to travel, the deeper position of respective
structure.
1.3.4 Amplification
Electric signals created by incoming reected US waves in crystals amplied within
US system for further processing.
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1.4 Signal Processing
Raw signal processed using multiple parameters/methods.
1.4.1 Preprocessing
Performed during investigation, includes electronic modulation of signal quality
during sending (beam forming) as well as modulation of sensitivity of crystal during
receiving.
1.4.2 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, weigh
grey levels, change scales, etc.
1.4.3 Time Gain Compensation (TGC)
Reected echoes from deeper areas have to pass through much more tissue, therefore suffer from more absorption: these signals are proportionally amplied to compensate for signal loss.
Note TGC should be constantly optimised during investigation, varies with ech-
odensity/absorption of more supercial transmitted structures (Fig.1.1). Modern
“automated image optimisation” tools/“magic buttons” may help-however, cannot
(yet) replace operator.
Fig. 1.1 TGC—image example (a) incorrect (b) correct TCG adaptation. (a) Incorrect image of
the magnied 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 hypoechoic structure
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M. Riccabona
1.4.4 Sound Energy=Output
Maximum output intensity dened by equipment depending on manufacturer.
To avoid unnecessary overexposure of tissue/deterioration of image: decrease
US intensity as much as possible. Denable partially by presets (e.g. foetal exams,
transcranial/transfontanellar brain US, US of eye/testis) or individually at beginning
of examination. MUST be shown on display—usually as percentage of maximum
output gain or in Watts.
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: reect impact of US energy on tissue (mechanical
index=MI, thermal index=TI); should (must) be displayed during every inves-
tigation—monitor/observe closely.
• In general, MI/TI should be kept below 1 to maintain safe sound exposure levels
(rule of thumb); short higher exposures are sometimes unavoidable (e.g. har-
monic imaging, Doppler …, keep as short as possible!)
– For further details, see biological effects.
1.4.5 Gain
Denes overall amplication of incoming signals:
• Optimise receive gain individually depending on output gain, patient, anatomy
and area of investigation.
1.4.6 Frame Rate/Persistence
Persistence Denes speed of image update (e.g. how many raw images are used to
calculate the image on screen):
• High persistence (information from series of individual images used to create
nal displayed image)—increased tissue density information and resolution—at
cost of slower update of individual displayed image.
Frame Rate (Hertz, Hz) Depends heavily on transducer technology and image calculation software; usually US investigations operate at 4–60Hz; faster frame rates
are possible, e.g. for cardiac studies.
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• High frame rate—fast series of individual images, reduced susceptibility to
motion artefacts—but usually at cost of slightly reduced resolution and a “nois-
ier” image.
Note New image acquisition and reconstruction technologies (e.g. sectorial image
acquisition/insonation …) will further speed up frame rates.
1.5 Components ofUS 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.
1.5.1 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.
Sector Transducers
Small active surface (footprint) where sound beams emitted in sector format
(Fig.1.2):
• Causes poor image quality in near eld, improved visualisation of deeper elds.
• 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 eld:
• 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. However, these are rarely still in use.
• 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 eld (Fig.1.2b).
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Fig. 1.2 Sector transducers—all creating a sector-like triangular image; good for small footprint
access with wide view in far eld. (a) old fashioned 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 specic lenses creating a very homogenous focal zone throughout the image eld
M. Riccabona
• Annular array transducers—combination of mechanical and electronic
technology:
– Various concentric rings of crystals selectively activated during scanning pro-
cess create sector-like eld with homogeneous focus zone throughout entire
imaging eld (Fig.1.2c).
• “Vector format”—different steering of an electronic sector transducer
– Opening up the near eld to a vector like format thus enabling/improving
near-eld image
Linear Array Transducers
• Parallel linear US beams created by multiple crystals create rectangular image
frame (Fig.1.3a):
– Homogeneous resolution throughout entire imaging eld, particularly valu-
able for near-eld assessment.
– Generally used for supercial 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—create a
“virtual sectorial” image (“trapezoid”)=larger eld of view in far eld, at cost
of frame rate and penetration, and also less homogeneous lower resolution image
in far eld (Fig1.3c).
c
ab
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Monitor image
Field of
sound
Targeted
object
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 eld than in near eld. Combines benets of linear and sector transducers (c) Trapezoid/
virtual sectorial/wide view/phased image of linear transducer
Linear
transducer
Curved Linear Array
• Crystals aligned on curved surface—diverging US waves create sector-like
imaging eld (angle depends on radius of curvature); the larger surface (than
sector transducer) offers good near-eld information:
– Combines abilities and benets from sector and linear transducers.
– Offers reasonable near-eld resolution at large eld of view at depth (Fig.1.3b).
– Typical application: abdominal US (and brain with a “micro-curved”—then
observe the risk of compressing brain and sinus).
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M. Riccabona
Note May cause problems by compression, and more difcult to handle/entirely
attach (some pressure needed—may be less well tolerated by young kids and my
compress surface/supercial structures).
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
focusing of US beam.
– 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 crystals.
• 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 stan-
dard access.
– Attached to endoscopic devices/intravascular catheters.
– Often limited use for paediatric applications, as other access often works suf-
ciently and size relatively large for paediatric cavities.
– Dedicated small paediatric devices rarely available (e.g. for transoesophageal
echocardiography, transrectal pelvic oor US).
– Some applications uncommon, non-existent or not accepted in paediatrics
(e.g. transvaginal US).
1.5.2 Other Parts ofUS Device
• Keypad (may be mobile and exible).
• Monitor (may be mobile and exible, can and must be adjustable).
• Printer/CD recorder.
• In-/output options.
• Cooling device with lters (need to be cleaned regularly).
• Potentially gel bottle warming device and transducer stands.
Harmonic Imaging
Image compounding
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1.6 Modern US Techniques
Today available on practically all new devices
• Need to know and understand them to avoid wrong applications, choose proper
settings and approach, and recognise potential associated artefacts.
1.6.1 High-Resolution US (HR-US)
• Uses relatively high frequencies, usually multifrequency broadband transducers,
with depth and focus depending on variations of central frequency.
• Additional mechanical or electronic lenses improve lateral resolution by
improved beam focusing, thus increasing penetration and resolution.
• HR-US particularly valuable in paediatric US and small-part imaging.
1.6.2 Image Compounding
• Also known as sono-CT or cross-beam imaging—uses US beams from various
directions or varying frequencies to assess same area (Fig1.4a). All information
averaged and calculated into one single image, similar to CT algorithms.
• Particularly helpful for reducing artefacts and improving depiction of subtle grey
scale changes/differences.
• However, intrinsically reduces frame rate. May also alter image impression and
impact on appearance/artefacts (e.g. reduces shadowing behind calcications,
effects anisotropy phenomenon …) (Fig1.4b, c).
a
Fig. 1.4 (a) Schematic drawing explaining how “compounding” works, (b) Shadow behind echo-
genic foci at renal hilus, conspicuously depicted by Harmonic imaging (b), which disappears when
using high scale Image Compounding (c)
b
c