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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5790_Библиотеки_им_академика_М_И_Перельмана.pdf
32
Fig. 1.14 aCDS (transplant kidney). Power Doppler with low scale settings demonstrates periph-
eral vascularity/perfusion of the parenchyma in this transplant kidney; note the physiologically
reduced vascularisation of the medullae, not be mistaken for pathology
1 Theory and Basics
a
Fig. 1.15 3DUS with integrated aCDS. (a) Rendered image of a 3DUS – aCDS acquisition of a
neonatal brain demonstrating a large vein of Galen AVM with its main feeders and a short section
of the enlarged draining vein (not entirely depicted due to scale settings during acquisition).
(b) 3DUS with included CDS data of a soft tissue vascular malformation, acquisition from dorsal
approach. Three orthogonal sections and rendered CDS-weighted view that superiorly demonstrates marked vascularity of lesion
b
• 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 tran-
scranial Doppler sonography.
• Can be combined with duplex-Doppler US.
• Combination of aCDS with split/double image technique, extended field of view
US (see above) or integration of aCDS into 3D-/4DUS possible – vessels can be
visualised over significant longer distances or even with rather tortuous and com-
plex anatomy (Fig. 1.15).
1.8.2.6 Other Flow-Sensitive US Techniques
Time domain or (colour) velocity imaging depicts motion of single reflectors
between consecutive images by using extremely high spatial and temporal resolution
ts
=−
=−
ab
1.8 Doppler Sonography
Fig. 1.16 Doppler measurements and calculations. Typical Doppler high (a) and low (b)
resistance flow pattern with respective relevant measurements. Abbreviations: V
velocity (yellow line), V
mean velocity (orange line), t 0, t = time of measurement
mean
max
33
maximum
(subtraction technique). This allows motion depiction, as individual structures are
imaged at different times at different locations.
Promising method – particularly with regard to sensitivity, angle dependency and
measurement accuracy; works at far less sound pressure than aCDS with better
frame rates (but sometimes restricted penetration).
NOTE: Presently only few vendors offer this technique in some dedicated
devices.
1.8.2.7 Important Parameters and Measurements (Fig. 1.16)
• V
syst.max
. = V
= maximal systolic flow velocity – only to be measured after angle
syst
correction.
• V
end diast.
= V
= maximum end-diastolic flow velocity – measurement only after
diast
angle correction.
• TAV – time average velocity; weighted mean velocity parameter that sums up
velocities of all particles within sample volume over defined cycle. Only cor-
rect if angle correction possible, vessel position stable throughout entire
measurement cycle and proper placement of sample volume throughout
feasible.
• TAMX – time average maximum velocity; mean maximum flow velocity
throughout measurement cycle – angle correction necessary.
• RI – resistance (or resistive) index (Pourcelot index) – angle independent param-
eter that calculates relation between systolic and diastolic maximum velocity.
Describes resistance or impedance but also influenced by many other factors.
Equation:
RI
VVV/
syst dias
yst
• PI – pulsatility index (Gosling index); describes flow details throughout entire
systolic-diastolic cycle, very sensitive towards even minor changes within flow
profile – but extremely depends on accurate angle correction.
PI TAMX
Equation:
VV/
syst diast
34
QA
=´
QA
=´
AD=px/
QD
n.
()
=
()×()
×
1 Theory and Basics
• Q – volume flow (ml/min) – different parameters and calculations used:
–
time
TAV
(A – section area of vessel – if planimetrically measured or
use equation for A from below).
–
persystoly
VTI
(
22), D – diameter = 2 R, VTI – velocity time
integral = integral of systolic velocities, i.e. area under the curve defined by
maximum velocity throughout cycle).
– Simplified equation:
ml/TAV m/smmmi
2
47 1 (correction
factor).
NOTE: Oblique vessel sections cause significant errors. Flow volume measure-
ments depend on adequate angle correction and particularly on accurate mea-
surement of cross-sectional area and/or diameter of vessel.
• AI (or ACCI)– acceleration index; important for evaluation of stenosis. Describes
rise of systolic flow velocity, i.e. time from beginning of systole until reaching
.
V
syst.max
1.8.3 Artefacts in (Colour) Doppler Sonography
1.8.3.1 Aliasing
Confusing display of high velocities (beyond the Nyquist border) or wrong
velocity scale settings; the part of systolic velocity too high for scale/beyond
Nyquist border added on opposite side of scale or encoded in opposite colour
(Fig. 1.17).
1.8.3.2 Spectral Broadening
Indicates turbulent flow, for example with stenosis or vessel wall pathology/
irregularity:
• Same phenomenon artificially caused by inadequately high gain (recognised by
increased background noise on display of duplex trace or colour noise on
image).
1.8.3.3 Sample Volume Artefact
The individually adaptable size of individual measurement can cause errors.
If sample volume is positioned incorrectly, may lead to incomplete measurements
or significant artefacts:
• A too large sample volume includes pulsation from vessel wall.
• A too small sample volume depicts only central fast flows, whereas more peri-
pheral slower and potentially turbulent flow not included.
a
1.8 Doppler Sonography
35
b
Fig. 1.17 (a) CDS of a PDA; the sparkling colour signals indicate aliasing due to turbulent and
high velocity flow; the maximum systolic velocity is higher than depictable by the respective
colour map setting/the Nyquist border (i.e. applicable pulse repetition frequency) . (b) Flow graph
for spectral analysis of a PW or CW Doppler trace: High-velocity systolic flow, cannot be properly
displayed on the screen due to missing correction of the baseline and the high flow velocity; the
respective part of the systolic flow is represented by spectra coming into the image from below and
reaching up to and even above the baseline
1.8.3.4 Filtering Artefacts
If filters are set too high, low velocities are not displayed even if present – may
mimic pathology (e.g. missing antegrade diastolic flow).
1.8.3.5 Scaling Problems
Incorrect measurements can be caused by inadequate adaptation of scale or
baseline.
1.8.3.6 Gain-Induced Errors
If gain is set too low, existing flow may not be depicted.
If gain is set too high, artificial turbulences may be simulated.
1.8.3.7 Angle Correction
Inadequate angle correction may cause significant errors in measurements, e.g.
caused by poor vessel delineation due to tortuosity in axial or oblique plain not or
incorrectible in imaging plain.
Furthermore, high Doppler angles cause inaccuracies (see also Table 1.2).
1.8.3.8 Motion Artefact
Particularly a problem with aCDS. Motion can cause colour signals (e.g. breathing,
pulsating tissue, adjacent peristalsis or organ movement) – not to be mistaken for
real flow.
36
ab
cd
1 Theory and Basics
Fig. 1.18 CDS artifacts. (a, b) CDS mirror artefact. CDS of the proximal IVC (transhepatic sagit-
tal view) at junction with diaphragm: echogenic border (caused by air in the lung) causes mirroring
of vessel colour display (encoded in blue) into intrathoracic cavity mimicking an aberrant vessel
(displayed in red). (c, d) Twinkling artefact – gray scale and CDS. Axial view, kidney of an infant:
small echogenic spot of indicating a papillar precipitation that causes twinkling non-directional
colours signals (“twinkling sign”)
1.8.3.9 Twinkling Artefact
Reverberating structures of strong echogenicity that undulate in sound field; these
and not motion create sparkling colour signals – useful for depiction of concrements/calcifications or solid deposits.
NOTE: Not all concrements do cause twinkling – the artefact very much depends
on nature, size and composition of concrement, transducer frequency, focus setting,
equipment used as well as on gain, filter and scale settings. Can also be seen with
air, resembling reverberation artefacts on gray scale US – to be identified by typical
spikes on duplex trace.
1.8.3.10 Others
A variety of artefacts as described previously on gray scale US can also occur on
Doppler, such as mirroring (Fig. 1.18).
1.8 Doppler Sonography
37
1.8.4 How to Perform (Colour) Doppler Investigations
Some special aspects need to be considered in equipment setting and transducer
handling.
NOTE: Doppler US uses significantly higher energy, therefore particularly in vulnerable areas, investigation should be as short as possible; always observe TI values.
• Try to use lowest possible output energy after pre-adapting other parameters
such as scale/velocity, filters, sample volume size and update rate before starting
Doppler tracing.
• Usually Doppler investigations are performed at lower frequency than gray scale
imaging – therefore usually multifrequency transducers are used for CDS
(e.g. gray scale 7 MHz, Doppler 4 or 5 MHz).
• Initially images are optimised on gray scale, then targeted vessel is focused try-
ing to optimise position and imaging for Doppler conditions – then Doppler is
activated to improve vessel delineation and speed up orientation.
• After further image optimisation (adapting focus to relevant area), zoom, etc. can
be applied to further improve image and consequently allow for adequate
measurements.
• Thereafter, cursor with sample volume is placed in vessel, sample size is optimised
– only then triplex or duplex mode is activated. Again, try to avoid unnecessary
high output gain; adjust receive gain, scale, filtering and all other parameters.
• For measurements, Doppler trace of at least several consecutive cycles should be
obtained – without impairing background noise.
• Take measurements afterwards on frozen image to avoid unnecessary tissue
exposure/high sound pressure.
1.8.5 Limitations
• Inaccessible areas for US, structures interfering with access such as interposed
air or calcified structures.
• High Doppler angles.
• Significant motion.
• Furthermore, extremely high velocities can cause problems.
1.8.6 Interpretation
For reading colour/duplex Doppler images, profound knowledge about pathophysiology and impact on perfusion/resistance is essential. Several parameters impact
spectral Doppler traces (Fig. 1.19):
• Diameter of vessel: the larger the diameter the more laminar flow exists, particu-
larly within central lumen.
38
1 Theory and Basics
to
to
to
to
to
Fig. 1.19 Schematic drawing of various Doppler flow profiles . Typical flow pattern encountered in
various conditions, with respect to site of relevant pathology in relation to point of measurement
(before, at, after lesion) and cardiac function. RI (restive index) changes indicated. 1 normal, 2 volume overload, increased cardiac output, 3 reduced cardiac output, 4 measurement after stenosis, 5
measurement at stenosis, 6 measurement before area of diminished resistance/shunt, 7 measurement
before area of increased resistance, stenosis, perfusion impairment, 8 measurement in area of
increased resistance/perfusion impairment, 9 measurement close to occlusion/perfusion stop, 10
measurement after shunt
• Rheologic composition of blood: depends on concentration of corpuscular
particles as well as composition of fluids.
• Overall blood volume as well as cardiac function impact flow velocities and flow
profiles – even in peripheral vessels. Furthermore, changes of vessel wall impact-
ing diameter and elasticity affect flow profile.
• Calibre variations impact flow spectrums – not only at site of stenosis but usually
if haemodynamically significant, up- and downstream as well.
• Additional aspect: peripheral resistance especially impacts diastolic flow.
1.9 Modern and Future US Methods and Techniques
39
1.9 Modern and Future US Methods and Techniques
1.9.1 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.
1.9.2 Image Compounding
• Also known as sono-CT or cross-beam imaging – uses US beams from various
direction or varying frequencies to assess same area. All information averaged
and calculated into one single image, similar to CT algorithms.
• Particularly helpful for reducing artefacts and improving depiction of subtle gray
scale changes/differences.
• However, intrinsically reduces frame rate.
1.9.3 Harmonic Imaging (HI)
• Has become widespread and common, using first harmonic response of reso-
nating reflectors instead of original reflected echo for creating US image.
• Reduces penetration and needs slightly higher output gain, but HI significantly
reduces noise – as signal for imaging is created by resonating individual struc-
ture itself.
• Improves border delineation and differentiation of liquid structures, enhances
gray scale differences.
• Commonly used in combination with HR-US/compounding; furthermore essen-
tial for contrast-enhanced US (ce-US, see below).
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 (Fig. 1.20).
1.9.4 Extended Field of View US
• Also known as panoramic imaging or freestyle US. Adds serial consecutive
neighbouring US images into one big overview.
40
ab
ab
Fig. 1.20 Harmonic imaging (HI). (a) Normal gray scale cross- section image of a kid-
ney (+2) with a slightly dilated renal pelvis (+1): somewhat hazy image with poor quality. (b) Same infant and same section as in Fig. 1.19a, acquired with HI: more conspicuous
image with better delineation of the dilated pelvis and the renal borders; also the corticomedullary differentiation is accentuated (beware of imaging technique induced artificial
“pseudonephrocalcinosis”)
1 Theory and Basics
Fig. 1.21 Extended field 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 field 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
• Based on calculation of transducer motion from picture inherent information –
thus two consecutive images can be aligned in proper anatomic order; continu-
ous display of even very large structures is achievable (Fig. 1.21).
• Not only useful for comprehensive overview of gross pathology or anatomy but
also for displaying long/large structures or measuring structures too large for
field of view of conventional transducers (e.g. large transplant kidney, severe
splenomegaly and huge tumours).
1.9 Modern and Future US Methods and Techniques
41
1.9.5 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 reflective echoes from dedicated/individually defined area is assessed
using various algorithms to compare all sonographic attributes.
• After comparison with normal standardised echotexture or potentially available
information from previous scans as well as other healthy organ regions, differ-
ences in tissue texture are displayed.
• Potentially improve detection and characterisation of specific tissue areas
(“sono-histogram”).
• Similar principle is applied to quantify flow based on CDS information – only
offered by some vendors
1.9.6 Sonoelastography
Upcoming method – presently mainly used in adults for breast and liver
applications.
• Based on analysis of non-linear sound effects, reflecting tissue behaviour and
“stiffness”– different from conventional US which basically only relies on inten-
sity of reflected echoes. Exploited non-linear sound effects in tissue: backscatter-
ing, changes in sound velocity.
Method:
• (Gentle) pressure applied after/during scanning of defined area (either manually
or by standardised sound pressure impulse from transducer).
• Changes in lateral sound propagation/backscattering are analysed; depict areas of
different response towards pressure: stiff areas show less change than very compress-
ible areas, as number of reflectors within given field changes and thus echo signature
from certain area changes variably; furthermore sound (shear wave) velocity changes.
• Information is superimposed on conventional gray scale image; generally colour
coding is used to visualise areas with altered compressibility versus areas of high
elasticity. Can also be displayed in “stiffness” numbers or shear wave velocity
(varies with equipment, no normal values yet available for infants and children’s
organs) (Fig. 1.22).
• Not only affects initial compression but also after compression (i.e. relaxation) –
then measuring re-expansibility of tissue, potentially giving further information
on tissue character.
• According to initial observations, promising for improved detection and characteri-
sation of focal lesions and diffuse (fibrotic) parenchymal changes also in infants and
children, particularly the liver or maybe the testis, the (trasnplant) kidney, or in a
goiter. Classification and differentiation of diffuse tissue changes or diffusely infil-
trating disease is more difficult – relies on established normal values for different
tissues or organs, but is being increasingly investigated, with first promising results
also in children with diffuse liver parenchymal disease.