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Ultrasound Speckle/Despeckle Image Decomposition for Tissue Analysis 93
4 Conclusions
This chapter describes a new strategy for decomposing ultrasound B-mode images
into its noiselessand speckle components, as well as, a new state of the art algorithm
for de-speckling.
First, a suitable de-noising algorithm is presented which aims at providing
clearer yet edge-preserving images for medical interpretation. Subsequently, because speckle has multiplicative nature, an image containing the speckle pattern
is estimated after knowing the corresponding noiseless image.
The adequacy of the RLTV filtering method hasbeen compared with other filters,
and afterward establishedthrough synthetic examples and ultrasoundimages having
different types. Moreover, as theoretically expected, an example using IVUS data,
clearly shows that the speckle field has statistical properties resembling a Rayleigh
distributed signal.
Furthermore, we have computed different features from noiseless and speckle
image sources, arguing that such information is useful for tissue analysis. Hence,
we have shown the convenience of working with the estimated echogenicity and
textural features for tissue description through distinct real cases.
The first study is conducted to illustrate how features computed from the
speckle field differ from a tissue to another and even within the same tissue.
Wavelet detail and approximation energies together with the relative directional
energy ratio have shown to be relevant tissue descriptors. The second and third
examples use features extracted both from noiseless and speckle image sources.
In particular, the second study uses a feature computed from the despeckled image
(echogenicity decay) which is very convenient in the investigated two-class problem. In the two classification problems presented, the use of information resulting
from the proposed speckle decomposition procedure leads to high-classification
scores.
Thus, we have shownthat the proposed algorithm is able to providemore suitable
images for visual diagnosis as well as useful sources of information for tissue
analysis in different clinical scenarios.
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Part II
Ultrasound Plaque Imaging
Media and Intima Thickness and Texture
Analysis of the Common Carotid Artery
Christos P. Loizou, Marios Pantzaris, and Constantinos S. Pattichis
Abstract The intima–media thickness (IMT) of the common carotid artery (CCA)
is widely used as an early indicator for the development of cardiovascular disease
(CVD). It was proposed but not thoroughly investigated that the media layer (ML)
thickness (MLT), its composition, and texture may be indicative of cardiovascular
risk and for differentiating between patients with high and low risk. In this study,
we investigate an automated snakes segmentation method for segmenting the ML
and the intima layer (IL) and measurement of the MLT and the intima layer
thickness (ILT) in ultrasound images of the CCA. We furthermore investigate
the application of texture analysis of the ML of the CCA and how texture is
affected by age and gender. The snakes segmentation method was used, and was
evaluated on 100 longitudinal ultrasound images acquired from asymptomatic
subjects, against manual segmentation performed by a neurovascular expert. The
mean ± standard deviation (sd) for the first and second sets of manual and
the automated IMT, MLT, and ILT measurements were 0.71 ± 0.17mm, 0.72 ±
0.17mm, 0.67 ±0.12 mm,0.25 ±0.12mm,0.27 ±0.14, and 0.25 ±0.11 mm; and
0.43 ±0.10 mm,0.44 ± 0.13mm, and 0.42 ± 0.10mm, respectively. There was
overall no significant difference between the manual and the automated IMC, ML,
and IL segmentation measurements. Therefore, the automated segmentation method
proposed in this study may be used successfully in the measurementof the MLT and
ILT complementing the manual measurements. MLT was also shown to increase
C.P. Loizou ()
Department of Computer Science, School of Sciences, Intercollege,
92 Ayias Phylaxeos Str., P.O. Box 51604, CY-3507, Limassol, Cyprus
e-mail: loizou.christos@ucy.ac.cy; loizou.c@lim.intercollege.ac.cy
M. Pantzaris
Cyprus Institute of Neurology and Genetics, Nicosia, Cyprus
e-mail: pantzari@cing.ac.cy
C.S. Pattichis
Department of Computer Science, University of Cyprus, Nicosia, Cyprus
e-mail: pattichi@ucy.ac.cy
J.M. Sanches et al. (eds.), Ultrasound Imaging: Advances and Applications,
DOI 10.1007/978-1-4614-1180-2
5, © Springer Science+Business Media, LLC 2012
99
100 C.P. Loizou et al.
with age (for both the manual and the automated measurements). Following the
segmentation of the three structures, we also investigated the application of texture
analysis of the ML of the CCA and how texture is affected by age and gender.
The 100 images were separated into three different age groups, namely below 50,
between 50 and 60, and above 60years old. Furthermore,the images were separated
according to gender. A total of 61 different texture features were extracted from the
intima layer (IL), the ML, and the intima–media complex (IMC). We have found
that male patients tended to have larger media layer thickness (MLT) values as
compared to the MLT of female patients of the same age. We have also found
significant differences among texture features extracted from the IL, ML, and IMC
from different age groups. Furthermore, for some texture features, we found that
they follow trends that correlate with a patient’s age. For example, the gray-scale
median GSM of the ML falls linearly with increasing MLT and with increasing age.
Our findings suggest that ultrasound image texture analysis of the media layer has
potential as an assessment biomarker for the risk of stroke.
Keywords Snakes • Ultrasound imaging • Media layer • Texture analysis
• Common carotid artery
1 Introduction
Cardiovascular disease (CVD – coronary artery disease, cerebrovascular disease,
and peripheral artery disease) is the third leading cause of death and adult disability
in the industrial world after heart attack and cancer (as reported by the World Health
Organization).According to [1], 80 million American adults have one or more types
of CVD of whom about half are estimated to be of age 65 or older. Of all the
deaths caused by CVD among adults aged 20 and older, an estimated 13 millions
are attributed to coronary heart disease and to stroke, with atherosclerosis as the
underlying cause. A recent study by the World Health Organization estimates that
by 2015,20 million people will die from cerebrovascular disease (mainly from heart
attack and stroke).
Atherosclerosis causes thickening of the artery walls and the intima–media
thickness (IMT) (see Fig. 1a) is used as a validated measure for the assessment of
atherosclerosis [2]. Specifically, an increased IMT is correlated with an augmented
risk of brain infarction or cardiac attack [3]. Moreover, the presence of carotid
plaques has been correlated not only to CVD but also to degenerative pathologies
such as vascular dementia and Alzheimer’s disease [4]. Hence the assessment of
carotid wall status is also essential for early identification of risk conditions in
asymptomatic patients. Traditionally, the IMT is measured by manual delineation
of the intima and the adventitia layers [2](seeFig.1a, interface I5 and I7). Manual
tracing of the lumen diameter (see Fig.1a, band Z4) and the IMT by human experts
requires substantial experience; it is time consuming, and varies according to the
training, experience, and the subjective judgment of the experts. The manual measurements suffer, therefore, from considerable inter- and intra-observer variability
[2–10]. It should be noted that parts of this work were also published in [11,12].
Media and Intima Thickness and Texture Analysis of the Common Carotid Artery 101
Fig. 1 (a) Illustration of the intima–media complex (IMC, bands Z5 and Z6) of the far wall of
the common carotid artery and the automatic IMC segmentation [10]. The media layer thickness
(MLT) is defined as the distance between the intima–media and the media–adventitia interface
(band Z6), (b) automated ML detection, (c) automated extracted IMC band, (d) automated
extracted ML, (e) manual IMC delineation, (f) manual ML delineation, (g) manual extracted IMC,
and (h) manual extracted ML. Source [12],
c
IEEE 2009
In the last 20years, several automated techniques for the segmentation and measurement of the IMT from longitudinal ultrasound images of the common carotid
artery (CCA) have been developed [9–14]. However, there are no studies published
in the literature reporting both the manual and the automated segmentation and
102 C.P. Loizou et al.
measurement of the media layer (ML) and the intima layer (IL) of the CCA in
ultrasound imaging. There are only three studies in the literature where manual
measurements of the MLT were reported [8, 15, 16]. Earlier research showed that
the media layer thickness (MLT) in peripheral arteries does not change significantly
with age and that it ranges from 125 to 350 μm[8]. In [15], manual measurements
of the thickness of the CCA IMT and MLT were carried out by an expert on 100
subjects aged 70years old. In this study [15], it was shown that subjects with CVD
(coronary heart disease, myocardial infarction, or stroke) had a significantly thinner
ML and a thicker IL than healthy subjects. Furthermore, in [16], the IMT, MLT, and
ILT of 90 healthy subjects (aged between 10 and 90 years) were manually measured
at their radial and anterior tibial arteries. It was shown that age was strongly related
with IMT, MLT, and ILT for both peripheral arteries.
The objective of this study was to develop and evaluate a snakes segmentation
system enabling the automated segmentation and measurement of the ML and
IL in ultrasound images of the CCA and investigate their variability with age
groups. We also propose to study changes in textural characteristics that can be
associated with disease progression for different age groups. Here, we note that
for fully developed plaques in the CCA, texture features derived from statistical,
model based, and Fourier based methods have been used to characterize and
classify carotid atheromatous plaques from B-mode ultrasound images [11, 30].
We, furthermore, investigated whether textural characteristics extracted from the
IL, the ML, and the IMC of the CCA, segmented manually by an expert and
automatically by a snakes segmentation system [10, 12], can be associated with the
increase in age gender or MLT. Ultimately, texture feature characteristics that vary
with age, gender, or MLT might be used to asses the risk of stroke.
The ML (see Fig. 1a, band Z6) is ultrasonographically characterized by an
echolucent region, predominantly composed of smooth muscle cells of the media
band of the arterial wall and probably by the extracellular matrix of the intima band,
which cannot be distinguished from the smooth muscle cell with ultrasound [17].
Due to the acoustic impedance mismatches, the arterial wall bands (intima–
media–adventitia) can produce typical bright–dark–bright patterns on ultrasound
images [2]. It is, furthermore, proposed but not thoroughly investigated, that not
only the IMT but rather the ML (its composition and thickness) may be used for
evaluating the risk of a patient to develop a stroke and accounts in general for the
risk of the CVD by differentiating between patients with high and low risk.
The IL is a thin layer, the thickness of which increases with age, from a single
cell layer at birth to 250μm at the age of 40 for nondiseasedindividuals [5]. Further,
adaptive physiological thickening of the IL occurs at places where the wall tension
is increased, such as arterial bifurcations and on the ML of the artery and may be
either eccentric or diffuse [5]. Furthermore, the intima–media complex (IMC) (see
also Fig. 1a) becomes more difficult to detect as the age of patients increases, due
to the presence of acoustic holes (echo-dropouts), in the IL [6]. The intimal band
(see band Z5 in Fig. 1a) may appear as a thin low-contrast structure and, therefore,
it is difficult to reliably draw boundaries because smoothing can move the structure
edges or make them disappear [2,7].
Media and Intima Thickness and Texture Analysis of the Common Carotid Artery 103
Earlier research [8] showedthat the media layer thickness (MLT) does not change
significantly with age (125 μm < MLT < 350 μm). In a recent study by our group,
the median (IQR) of intima layer thickness (ILT), MLT, and IMT were computed
from 100 ultrasound images of 42 female and 58 male asymptomatic subjects aged
between 26 and 95 years old, with a mean ageof54yearstobeasfollows:0.43mm
(0.12), 0.23mm (0.18), and 0.66mm (0.18), respectively [12].
In [33], a method has been presented for quantifying the reflectivity of the ML
of the distal CCA. It was shown that the GSM of the IM layer is the earliest
change representing atherosclerotic disease in the arterial wall that can currently
be imaged in vivo. This may be the first marker of atherosclerosis and may
precede the development of a significant increase in IMT. This would enable earlier
identification of high-risk individuals based on the analysis of the CCA artery wall
textural characteristics. In [34], the early structural changes of the CCA in familial
hypercholesterolemia were investigated. It was shown that textural characteristics
extracted from the IMC were significantly different between patients with and
without hypercholesterolemia. In [35], the authors reported on the properties of the
GSM of the IMC from a random sample of 1,016 subjects aged exactly 70. They
found that the GSM of the IMC of the CCA is closely related to the echogenecity in
overt carotid plaques.
While there are several earlier studies suggesting that the instabilityof the carotid
atheromatous plaques can be characterized from B-mode ultrasound images [6,30],
we have not found any other studies reported in the literature where the ML textural
characteristics have been shown to be associated with the risk of stroke. While in
[6, 30], the echogenecity in atherosclerotic carotid plaques was evaluated through
the gray-scale median (GSM), there are very few attempts made to characterize the
IL, the ML, and the intima–media complex (IMC) with a similar gray-scale image
intensity analysis. It is evident from the visual inspections of the IMC in the CCA
that a great variation in echogenecity does exist. However, the usefulness of this
information has not yet been studied.
In [30], the morphologyof atherosclerotic carotid plaque was investigated based
on the textural characteristic extracted from 230 ultrasound images of carotid
plaque, where it was shown that it is possible to identify a group of patients,
symptomatic or asymptomatic, at risk of stroke based on these texture features.
It was further documented in [36], that carotid endarterectomy in asymptomatic
individuals with stenosis greater than 70% reduces the risk of stroke from 2%
per year to 1% per year. In another study [37], the relationship of the IMT in
the CCA and atherosclerosis was investigated on 182 symptomatic patients (mean
age 67 years). It was shown that the IMT was correlated to age, male gender,
ischemic heart disease, and presence of plaque or stenosis in any of the carotid
bifurcations. In a recent study [38], where the alterations of the CCA with age in
ultrasound images were investigated, it was shown that the diastolic and systolic
lumen diameters are increasing with age. This reduces wall stress as the elasticity
of the wall decreases with age.
104 C.P. Loizou et al.
2 Materials and Methods
2.1 Recording of Ultrasound Images
A total of 100 B-mode longitudinal ultrasound images of the CCA used for the
IMC, ML, and IL segmentations were recorded using the ATL HDI-3000 ultrasound
scanner (Advanced Technology Laboratories, Seattle, USA), with a linear probe
(L74), with a recording frequency of 7 MHz, a velocity of 1,550m/s and 1 cycle
per pulse, which resulted to a wavelength (spatial pulse length) of 0.22mm and an
axial system resolution of 0.11 mm. The technical characteristics of the ultrasound
scanner (multielementultrasound scan head, operating frequency,acoustic aperture,
and transmission focal range) have already been published in [10]. Digital images
were resized using the bicubic method to a standard pixel density of 16.66pixels per
mm with a resolution of 0.06 mm. This was carried out due to the small variations in
the numberof pixels per mm of image depth (i.e.,for deeply situatedcarotid arteries,
image depth was increased and, therefore, digital image spatial resolution would
have decreased) and in order to maintain uniformity in the digital image spatial
resolution [19]. The images were logarithmically compressed and were recorded
digitally on a magneto-optical drive at size of 768 × 576 pixels with 256 gray
levels. The images were recorded at the Cyprus Institute of Neurology and Genetics,
Nicosia, Cyprus, from 42 female and 58 male asymptomatic subjects aged between
26 and 95 years old, with a mean age of 54 years. The images were separated into
three different age groups depending on age, namely, below 50, between 50 and 60,
and above 60 years old, with 27, 36, and 37 subjects in each group, respectively,
and also separated into 58 male and 42 female subjects. These subjects had not
developed clinical symptoms, such as a stroke or a transient ischemic attack (TIA).
2.2 Image Normalization
Brightness adjustments of ultrasound images were carried out in this study based on
the method introduced in [20] and also used in [10]. It was shown that this method
improves image compatibility by reducing the variability introduced by different
gain settings, different operators, different equipment, and facilitates ultrasound
tissue comparability. Algebraic (linear) scaling of the image was performed by
linearly adjusting the image so that the median gray level value of the blood was
0–5, and the median gray level of the adventitia (artery wall) was 180–190 [20].
The scale of the gray level of the images ranged from 0 to 255. Thus the brightness
of all pixels in the image was readjusted according to the linear scale defined by
selecting the two reference regions. It is noted that a key point to maintaining a
high reproducibility was to ensure that the ultrasound beam was at right angles
to the adventitia, adventitia was visible adjacent to the plaque, and that for image
normalization a standard sample consisting of the half of the width of the brightest
area of adventitia was obtained.