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A review of Doppler sonography for the assessment of tumour vascularity
S. Ohlerth and B. Kaser-Hotz "A review of Doppler sonography for the assessment of tumour vascularity", May 19, 2015
Abstract
During the past decade, the vascular biology of cancer has become a highly interesting research
field. To assess tumour vascularity and perfusion, various strategies such as computed tomography,
magnetic resonance imaging and positron emission tomography have been investigated. Over the
last years, important advances have taken place in the Doppler technology which dramatically
improved its ability to evaluate the vascular system, even small and deep vessels. Doppler
sonography provides a non-invasive means to assess the extent and morphology of tumour
vascularity. This information is clinically important regarding diagnosis, prognosis and response to
treatment, particularly in connection with the development of novel angiogenesis inhibitors. This
article describes the different Doppler technologies most commonly used in human and
experimental animal oncology. Based on in vivo tumour models and extensive clinical experience in
humans, their significance and potential clinical applications are illustrated. First clinical reports in
veterinary medicine are also reviewed.
Introduction
During the past decade, the vascular biology of cancer and other pathological processes have
become a highly interesting research field in human medicine. Mammalian cells require oxy-
gen and nutrients for their survival and are therefore located within 100–200 mm of blood vessels –
the diffusion limit for oxygen. For multicellular organisms to grow beyond this size they require a
neovasculature. This vascular supply is derived from adjacent normal vessels which are stimulated
to produce vascular buds by so-called angiogenesis factors which they secrete. The process of forming
new blood vessels from pre-existing vasculature is called angiogenesis. Pathological angiogenesis is a
hallmark of cancer and various ischaemic and inflammatory diseases (Cosgrove, 1999; Carmeliet
& Jain, 2000; Rosen, 2000).
Many recent studies in experimental animal and spontaneous human tumours have shown
that vascularity is of central importance in the diagnosis and classification of a tumour. It also
plays a major role in the management of various cancer treatment procedures. In a variety of solid
tumours, the effectiveness of radiotherapy has been traditionally explained by the fact that
tumour cells are the principal target. Ionizing radiation damages their DNA and causes them to
undergo programmed cell death (apoptosis). However, recent experimental studies suggest
that the endothelial cells of the microvasculature may be the principal target of radiation (Folkman
& Camphausen, 2001). Folkman already proposed in 1971 that tumour growth and metastasis are
angiogenesis-dependent, and hence, blocking angiogenesis could be a strategy to arrest tumour
growth (Folkman, 1971). This possibility stimulated an intensive search for pro- and anti-angio-
genic molecules. Angiogenesis inhibitors are a new class of drugs (Kerbel & Folkman, 2002). They
have shown promise in animal studies, and clinical studies are underway.
Tumour vascular histomorphology
The method most often used to quantitatively and qualitatively assess tumour vascularity is immuno-
histological analysis of intratumoural microvascular density (MVD) and microvascular
architecture. With immunohistology, vessels approximately 15 mm in diameter are identified.
In most studies, the hot spot technique is used. Areas of the tumour are subjectively chosen that
contain the most capillaries and small venules (microvessels) (so-called neovascular ‘hot spots’).
Within a 0.74-mm2-area, all microvessels are counted to assess MVD (Weidner, 1995). Many
immunohistological studies in a variety of human tumours demonstrated that increased MVD cor-
relates with tumour grade, growth rate and aggressiveness such as probability of metastasis and/or
decreased survival. MVD provides an anatomical, but not a functional, measurement of tumour
vascularity (Gee et al., 2001). In addition to quantitative assessment of tumour vascularity, morphologic
evaluation of the microvascular architecture has also been shown to be a very valuable tool for the character-
ization of neoplastic lesions (Less et al., 1991). Depending on the pathological and chronological
stage, the vascular pattern of a tumour may present variability. The tumour’s architecture may be
heterogeneous due to bulky neoplastic colonization, necrosis, oedema or desmoplastic reaction.
Corresponding vascular patterns show a spotted (patchy distribution of vessels), peripheral or
mixed pattern. Due to displacement and encasement, tumour vessels may present tortuous. Histo-
logical studies showed that the vascular heterogeneity of tumours depends on their degree
of malignancy. Malignant tumour vessels are histologically characterized by lack of the muscular
layer and irregular contours. They commonly form a heterogeneous network with a chaotic
architecture. Instead of a normal hierarchic vascular tree with continuously diminishing vessel
size toward the periphery, an anarchic vascular pattern consisting of caliber changes, loops and
trifurcations is commonly seen. Loops are defined as self-connective vessels, whereas a trifurcation
represents division of an original vessel into three branches originating from the same point.
These histological features of the microvascular architecture were first described in a mammary
carcinoma (Less et al., 1991).
However, the use of histological analysis is limited in clinical patients, especially with serial
examinations because of the invasiveness of the biopsy procedure. The tissue samples may only
reflect a certain area within the tumour, and histopathological results are not readily available for
the clinician. In many recent reports, Doppler ultrasound has been used extensively to assess
vascular density and microvascular architecture in a range of animal and human tumours. This
technique can be performed easily and non-invasively, and also offers the advantages of allowing
serial examinations during a treatment regimen. This article describes the different Doppler
techniques most commonly used in human and experimental animal oncology and illustrates their
use and significance. First clinical reports in veterinary medicine are also reviewed.
Doppler ultrasound techniques
Evaluation with two-dimensional grey-scale ultrasound provides rapid, relatively inexpensive and
non-invasive assessment of lesion morphology. This technique is a helpful tool for lesion localiza-
tion, documentation of tumour size and shape and guided needle biopsy (Nyland et al., 2002a).
However, although highly sensitive for detecting tumours, two-dimensional grey-scale ultrasound
does not provide information on tumour vascularity and blood flow.
In ultrasonography, the Doppler effect results from a calculated shift in sound frequency from
sound waves reflected from moving targets, usually blood cells. If the direction of blood flow
is toward the transducer, the frequency of the returning echo is higher than that of the trans-
mitted sound. If the direction of blood flow is away from the transducer, the echoes have a
lower frequency than the transmitted sound. The difference between transmitted and received ultra-
sound frequencies is known as the Doppler shift frequency. The greater the mean Doppler shift
frequency, the greater the velocity. The mean frequency shift is used to estimate the velocity of
blood in a vessel and may be displayed with spectral Doppler mode or colour Doppler mode.
Spectral Doppler
With spectral Doppler, the velocity information is presented visually on the monitor as a spectral
display consisting of the velocity (cm s-1) on the vertical axis and time (s) on the horizontal axis.
There are two types of spectral Doppler: pulsed and continuous wave. With pulsed wave Doppler,
sound is transmitted in pulses. Echoes arising from moving blood will arrive at the transducer
during a discrete time interval corresponding to the vessel’s depth. A region of interest (‘gate’) is
chosen and velocity information can be only measured in this single sample volume (monogate-
pulsed Doppler). The gate is opened and closed to accept echoes only from a particular depth.
Consequently, pulsed wave Doppler possesses depth discrimination. With continuous wave
Doppler, sound is transmitted and received continuously by use of separate transmitting and
receiving crystals. Continuous wave Doppler does not possess depth discrimination because
anything moving along the path of the beam is sampled. The advantage is that much higher vel-
ocities may be measured than with pulsed wave Doppler. For the examination of smaller vessels, as
in tumours, pulsed wave Doppler is used as velocity information of a certain site is needed and
velocities are rather low.
With spectral Doppler, quantitative measurements such as systolic and diastolic velocities
from arteries, monophasic flow in veins and turbulence may be displayed (Nyland et al., 2002b).
Spectral Doppler may detect the typical flow patterns of vessel stenosis, occlusion or arteriove-
nous shunts which are features commonly seen in tumours. However, different authors reported
conflicting results due to variable velocities in the analysis of Doppler waveforms in malignant
human tumours. Velocities may present variability within the tumour parenchyma and they may
also change with the pathological tumour stage. Angiogenesis and tumour compression affect the
vascular resistance and may cause low-to-high resistance flow with different geographical distri-
bution in tumours. Spectral Doppler analysis appears to be less reliable than sonographic analy-
sis of the morphology of tumour vessels. Several studies demonstrated a higher mean peak systolic
velocity in malignant tumours as compared with benign tumours (Bodner et al., 2002). In a recent
study on the effect of a new anti-angiogenic drug in a murine renal cell carcinoma model, a signifi-
cant decrease of systolic and diastolic velocity in the tumour feeding renal artery was seen (Drevs
et al., 2000).
A unitless quantitative measurement is the resistive index (RI) representing one of the most
commonly used ultrasound parameters to characterize arterial and venous flow profiles in various
lesions. RI is calculated by subtracting end diastolic velocity from peak systolic velocity and
dividing that result by peak systolic velocity. The usefulness of these indices is controversial. Several
studies stated that metastatic cervical lymph nodes in humans revealed a high RI (>0.8), in contrast
to benign lymph nodes (Steinkamp et al., 2002). However, in other studies, Doppler spectral analy-
sis was found unreliable in differentiating benign from malignant lymphadenopathies because low
resistance flow could be seen in both conditions (Cho et al., 1995).
Colour and power Doppler
Colour and power Doppler also represent pulsed Doppler technologies. In contrast to spectral Dop-
pler, velocity information is obtained from many gates over a large region of interest on the two-
dimensional grey-scale image (multigate pulsed Doppler). The Doppler shift frequencies from
returning echoes reflected by moving blood cells are colour-coded and superimposed over the two-
dimensional grey-scale image. With colour Doppler ultrasound, red-coloured flow, by conven-
tion, represents blood flowing toward the transducer and blue-coloured flow is assigned to
blood flowing away from the transducer. The amount of colour saturation indicates the velocity
of blood movement. Colour Doppler is also often used for directing the spectral Doppler sample
volume. Mean velocity information, flow direction and turbulence may be assessed with colour
Doppler (Nyland et al., 2002b). However, colour Doppler has some shortcomings, including angle
dependence and aliasing. Aliasing occurs when the pulsed Doppler sampling rate is too slow to cor-
rectly record the frequency shift produced by high velocities. It is recognized by an incorrect display
of velocity and direction indicated by a light shade of colour in the opposite direction.
While spectral and colour Doppler display the change in the returning frequency to provide velo-
city and directional information, power Doppler records the amplitude (energy) of the reflected
Doppler signal from moving blood cells. Colour brightness is related to the number of moving
cells, not the velocity. This technique is important as it may detect much lower velocities in small
vessels, such as tumour vasculature. This is possible because of the lack of angle dependence and
the way in which random noise is depicted. Power Doppler visualizes the energy of the reflected
ultrasound signals, and random noise has a very low energy. This permits higher gain settings and
increased sensitivity for flow detection. However, some limitations have been encountered with
power Doppler. Because of its higher motion sensitivity in connection with respiration or vessel
pulsatility, power Doppler is more susceptible to flash artefact than colour Doppler. This may make
the differentiation of low velocities challenging. The absence of aliasing artefact with power Dop-
pler prevents detection of turbulence. Directional information is not provided by conventional
power Doppler as only the energy of the returning signals is shown (Rubin, 1999; Mattoon et al.,
2002). Recent technical developments may though provide directional information (bidirectional
power Doppler). Although power Doppler is often performed alone, it is commonly used with
other advanced ultrasound technologies such as contrast-enhanced and three-dimensional ultra-
sound. For three-dimensional Doppler ultrasound, volume data acquisition has to be
performed. Three-dimensional Doppler ultrasound improves depiction and quantification of
tumour vessel density and vessel morphology (Fleischer, 2000).
Power Doppler ultrasound has been shown to be more sensitive than colour Doppler for detect-
ing low velocities and the depiction of small parenchymal vessels (Eriksson et al., 1991; Bude et
al.,
1994). Because of a typically high interstitial pressure and resulting low-velocity states in tumour
vessels, power Doppler is of special value in assessing tumour vasculature. In a study in spontan-
eous canine tumours (Ohlerth et al., 2002), power Doppler proved more sensitive than colour
Doppler in depicting small vessels (Fig. 1). Nevertheless, colour Doppler is of value for determin-
ation of flow direction as needed for the sonographic assessment of morphological criteria
in tumour vessels. Some problems have to be encountered with colour and power Doppler.
The signal in a vessel may become so great that it appears to bloom outside of its boundaries
(blooming artefact) finally limiting spatial resolution. Imaging in moving parts of the body suffer
from flash artefact, whereby power Doppler is much more sensitive to motion than colour Dop-
pler. Minimal soft tissue motion can seriously degrade the image.
Intratumoural perfusion may be heterogeneous due to increased interstitial pressure, haemorrhage
or necrosis. A high interstitial pressure leads to reduction in flow and may be associated with
tumour hypoxia. Then, power Doppler may be useful to locate such regions (Evans et al., 1997).
Microbubble contrast media
Almost all ultrasound contrast media are stabilized microbubble suspensions that are smaller
than 7 mm to pass the capillary beds and the pulmonary filter. Depending on the agent, they
are stable in the blood for a few to several minutes after which they dissolve, rupture or are
phagocytosed by reticuloendothelial cells.
Figure 1. Sagittal ultrasonographic images of a canine oral osteosarcoma. Whereas colour Doppler (A)
demonstrates moderate tumour vascularity, power Doppler (B) allows visualization of smaller vessels (") that are undetectable with colour Doppler. Contrast-enhanced power Doppler (microbubble
contrast medium: Levovist, Schering AG, Switzerland) even more increases depiction of tumour vascularity (C).
Like blood cells, the bubbles behave as specular reflectors after intravenous injection and they increase
reflection of the incident fundamental frequency (Dalla Palma & Bertolotto, 1999). Use of these
agents also increases the Doppler signal and therefore, the sensitivity for low flow situations. Imag-
ing of vascular structures which cannot be evaluated using non-enhanced colour or power
Doppler techniques is markedly improved. This potential is of great interest in oncology to study
tumour vascularity. Contrast-enhanced power Doppler better depicted overall tumour vascular-
ity in a murine melanoma xenograft model than non-enhanced power Doppler and improved cor-
relation to histological MVD (Schro¨der et al., 2001). In human breast, renal and liver masses
and cervical lymph nodes, for example, contrast-enhanced colour or power Doppler markedly
improved diagnostic accuracy in differentiating benign from malignant lesions (William et al.,
1998; Cosgrove, 1999; Beissert et al., 2002). However, in combination with colour and
power Doppler, blooming artefact may be observed at high doses of contrast media soon
after the bolus injection. Decreasing the rate of injection reduces this artefact. Attenuation is also
increased by the use of contrast media resulting in poor signal from deeper structures.
Besides increasing the reflection of the incident fundamental frequency, two other phenomena are
observed with microbubble contrast media. First, with increasing ultrasound beam intensity, the
microbubbles start to emit harmonics of the fundamental frequency. Second, bubble collapse may
appear in high-amplitude diagnostic ultrasound fields producing a transient, high-intensity, non-
linear broadband response. These phenomena have lead to the development of a variety of con-
trast-specific grey-scale harmonic imaging techniques reviewed elsewhere (Lencioni et al., 2002;
Ziegler & O’Brien, 2002). In one form of harmonic imaging, the signal is digitally encoded. Coded
harmonic angiographic ultrasonography suppresses the transmitted fundamental frequencies
by isolating the encoded return signal, resulting in the display of only the harmonic signal. This
method appears most useful for imaging of contrast media. There is increased signal from the
contrast media compared to contrast-enhanced colour or power Doppler. However, unenhanced
images have poor lesion conspicuity due to substantial suppression of the background tissue sig-
nal. Full evaluation of large or multiple masses is not possible because of more focal zone depend-
ency (Kim et al., 2002; Ziegler et al., 2002).
These contrast media are being developed for human use and there are only a few reports in
veterinary clinical oncology. We investigated spontaneous canine tumours in the Section of
Diagnostic Imaging and Radio-Oncology, Veterinary Faculty of Zürich (Ohlerth et al., 2002), and
contrast-enhanced power Doppler was superior to non-enhanced power Doppler for the detection of
tumour vessels (Fig. 1).
In a study by O’Brien et al. (2002), malignant lymph nodes in dogs with lymphoma had a sig-
nificantly increased number of aberrant vessels compared to normal canine lymph nodes. Angi-
ographic grey-scale harmonic imaging allowed better characterization of the vascular pattern,
vessel number and size compared to power Doppler. However, contrast-enhanced power Doppler
was not investigated in this study.
Doppler analyses in tumour tissue
Quantification of tumour vascularity with
Doppler ultrasound
Tumour vascularity refers to the number of vessels per unit volume. Active research has developed
various sonographic methods to determine vascularity. For this, power Doppler is more appropri-
ate than spectral Doppler because anatomical, rather than dynamic, information is of interest.
Quantification of tumour vascularity may also be performed with colour Doppler; however, as men-
tioned above, power Doppler remains more sensitive. The use of three-dimensional power Doppler
better depicts global vascularity than two-dimensional power Doppler. Sonographic vessel density
can be subjectively estimated by using a score. On the other hand, there is a whole series of valuable
computerized methods to quantify vascularity derived from colour or power Doppler (Meyerowitz
et al., 1996; Carson et al., 1998; Cheng et al., 1999; Fleischer
et al., 1999; Sehgal et al., 2000). Doppler
studies in various tumours, such as malignant breast masses, showed a direct correlation between
increased vascularity and the degree of malignancy. Malignant lymphomas of peripheral
lymph nodes appear hypervascularized as compared with reactive lymph nodes or metastases of
squamous cell carcinomas (Steinkamp et al., 2002). The method by Sehgal et al. (2000) has
also been successfully used in spontaneous canine tumours at the Veterinary Faculty of Zurich
(Ohlerth et al., 2002). On the basis of computerized analysis of digitized colour or power Doppler
images, three parameters are calculated with this method. One parameter represents a vascularity
index and calculates the percentage area of the lesion occupied by blood vessels. First results of a
study in spontaneous canine tumours at the Veterinary Faculty of Zurich indicate that oral squamous
cell carcinomas are highly vascularized in contrast to low vascularized grade 1 sarcomas (unpublished
data). These studies, in summary, showed that the quantitative approach of measuring vascularity did
consistently better than qualitative or visual assessment of vascularity. In the context of new treat-
ment modalities such as angiogenesis inhibitors, these techniques provide a promising tool in moni-
toring changes in tumour vascularity over time as shown in murine tumour models (Gee et al., 2001).
Several reports compared quantified Doppler ultrasound with histological assessment of vessel
density to evaluate if quantified Doppler ultrasound represents an equivalent non-invasive meas-
urement of tumour vascularity. Whereas in some studies, Doppler measurements correlated well
with the quantified results from immunofluorescent staining (Sehgal et al., 2000; Donnelly et al.,
2001), others reported poor correlation (Fleischer et al., 1999). Authors hypothesized that, in part,
the discrepancy may have been due to the differences in the techniques. With power Doppler,
larger vessels (approximately 100 mm) are depicted than observed on histological assessment
(approximately 15 mm). In most studies, a focal area with high microvessel density (hot spot tech-
nique) is assessed with immunohistology versus global vascularity evaluation with power Doppler.
In addition, MVD provides an anatomical, but not a functional, measurement of tumour vascu-
larity (Gee et al., 2001). In summary, measurement of tumour vascularity with histological or
Doppler techniques may provide different biological information and consequently, methods may
only partially replace each other (West et al., 2001).
However, in a recent study, high-frequency power Doppler ultrasound (>25 MHz) was used
for monitoring the effects of antivascular therapy on tumour blood flow in superficial mouse
tumours (Goertz et al., 2002). With this technique, vessels as small as 15–20 mm in diameter, compar-
able to the size of vessels assessed with histological MVD techniques, with velocities less than 1mms1
may be detected but only in very superficial structures (<5–10 mm). This method will be
restricted to experimental tumours, and, in clinical settings, ocular and cutaneous tumours. In
this study, an immunofluorescent staining technique was used, which allowed estimation of the
relative degree of perfused tumour vasculature. Histological measurements of tumour vasculature
correlated with the ultrasound results.
Quantification of tumour blood flow with Doppler ultrasound
It is important to distinguish tumour vascularity from tumour blood flow. Whereas tumour vascu-
larity refers to the number of vessels per unit volume, tumour blood flow is a measure of the
number of flowing blood elements over a certain period of time in a selected area (Fleischer, 2000).
Quantification of blood flow specifically provides functional evaluation parameters. Micro-
bubble contrast media appear to have excellent properties to study transit times through organs
and tumours or to obtain time-intensity curves. Changes in signal intensity after contrast medium
injection may be assessed with colour and power Doppler and B-mode. Human breast carcinomas
showed a typical early and marked contrastenhanced colour Doppler signal increase followed
by a marked decline compared to benign nodules. In patients with liver tumours or cirrhosis, earlier
enhancement was observed than in controls (Derchi et al., 1999). As mentioned before, the
method by Sehgal et al. (2000) uses three parameters based on the computerized analysis of
digitized colour or power Doppler images. Two parameters assess perfusion: the mean local blood
velocity (for colour Doppler) or mean red blood cell density moving above a threshold velocity (for
power Doppler) and the mean blood flow through the region of interest (for colour Doppler) or
blood volume within the tissue (for power Doppler). This method is currently investigated in a
longitudinal study in spontaneous canine tumours undergoing various treatment strategies at the
Veterinary Faculty of Zurich.
A series of recent human and experimental animal studies demonstrated most promising results
with grey-scale contrast harmonic imaging. These techniques seem to become the method of choice
for perfusion and blood flow studies in tumours (Lencioni et al., 2002). First reports in veterinary
medicine also suggest contrast harmonic imaging as a valuable tool in characterizing the vascular
and perfusion patterns of masses, including malignant lymph nodes (O’Brien et al., 2002;
Ziegler et al., 2002).
Sonographic assessment of the morphology of tumour vasculature
The histological features of the microvascular architecture in tumours, as mentioned before,
were also used for sonographic assessment of tumour vessel morphology; colour and power
Doppler, especially three-dimensional power Doppler, and contrast harmonic imaging, e.g.
coded harmonic angiographic ultrasonography, represent excellent tools for this purpose. In a
study on musculoskeletal tumours, combined Doppler techniques revealed four major vessel
characteristics (trifurcation, anarchic vascular pattern, stenosis and occlusion), which, if two criteria
were combined, demonstrated very high sensitivity and specificity in differentiating benign from
malignant lesions (Bodner et al., 2002). During the sonographic evaluation of spontaneous canine
tumours at the Veterinary Faculty of Zurich, we also commonly observe typical features of a
pathological angioarchitecture such as loops, stenosis and trifurcations (Fig. 2).
In human cervical lymph nodes, power Doppler enables differentiation between reactively enlarged
nodes, metastatic nodes and nodes affected by malignant lymphoma. Reactive lymph nodes
show increased central perfusion of the hilum, whereas metastases tend to show increased per-
ipheral perfusion. In nodes affected by malignant lymphoma, an overall increased vascularity is seen
(Steinkamp et al., 2002). A very detailed sonographic classification system of the normal and
pathological angioarchitecture of superficial lymph nodes in humans has been described by
Tschammler et al. (1996).
Figure 2. Sagittal power Doppler images of a canine oral melanoma after the administration of a microbubble
contrast medium (Levovist, Schering AG, Switzerland). An anarchic vessel structure with self loops (A, "), caliber changes (B, Þ") and trifurcations (C,") is
seen.
In human breast cancer, typical sonographic morphological signs of malignancy are: peripheral
vessels running centrally and branching, more than one vascular pole, disordered vessels and
hypervascularity (Schroeder et al., 2003). Similar findings were described in human liver lesions:
whereas haemangiomas were poorly and only peripherally vascularized, patients with nodular
hyperplasia showed hypervascularity and a central artery with radial branching and malignant lesions
revealed a chaotic vascular pattern with peripheral vessels branching toward the centre (Leen, 2001;
Kim et al., 2002).
Vascular anatomic alterations were also observed sonographically during antivascular
therapy of murine tumours. A reduction in the arborization and density of vessels with a prefer-
ential loss of small vessels were seen with successful therapy (Gee et al., 2001). In summary,
sonographic assessment of the morphology of vessels appears a very sensitive tool to distinguish
between malignant and benign lesions in experimental animal and spontaneous human tumours.
Conclusions
Over the past decade, important advances have taken place in the Doppler technologies. The com-
bination of colour and power Doppler with ultrasound contrast agents and three-dimensional
imaging has dramatically improved the ability of the Doppler techniques to explore tumour vascu-
larity, the morphology of tumour vessels and functional parameters. Accuracy to differentiate
between benign and malignant lesions was significantly improved in different human tumours.
Clinically important information regarding the prognosis, metastasis and survival may be gained.
Doppler ultrasound appears a valuable tool for monitoring various cancer treatment modalities,
in particular anti-angiogenic therapies. However, in regard to the amount of studies published dur-
ing the last few years, a major goal should be the development of standardized examination techni-
ques and indices, both morphological and dynamic, that discriminate between pathologies,
typically between benign and malignant tumours. The development of new ultrasound technologies
such as tissue harmonic and contrast harmonic ultrasound has recently brought attention back
to grey-scale ultrasound and improved the evaluation of tumour perfusion. First reports in veter-
inary medicine indicate that the new ultrasound technologies also appear very promising tools in
the assessment of canine and feline tumours.
Acknowledgments
The authors thank Dr A. Jaeger and Schering AG, Switzerland, for supporting our research projects
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