qualitative description of the different plaques’ characteristics in prior work (Yabushita et al. 2002)
described below. This also provides the ability to
interpret results in a meaningful way.
A fibrous plaque (figure 4a) has high back scattering
and the region has relatively homogeneous intensity
values. We see that the average intensity is high
(bright). Likewise, the intensity is not attenuated
much along the A-line (Gargesha et al. 2015).
A lipid plaque (figure 4b) is a low-intensity region
with poorly delineated borders, a fast IVOCT signal
drop off, and little or no OCT signal back scattering,
within a lesion that is covered by a fibrous cap. We
see that the intensity starts very bright and decreases
quickly along the A-line (Gargesha et al. 2015).
A calcified plaque (figure 4c) appears as a low intensity or heterogeneous region with a sharply delineated border (leading, trailing, and/or lateral edges). Calcium is darker than fibrous plaque with greater
variation in intensity level.
Based on this description, we construct a set of
eight (real-valued) features for each pixel in the
image. We compute these features using a three-dimensional (3D) neighborhood centered on the pixel of interest. The third dimension comes from neighboring images (human analysts will often use
adjacent images when annotating an image). In these
features, σ represents the standard deviation of the
intensity values within a 3D neighborhood.
Distance to Lumen (Dl): This is a measure of the distance of the center pixel from the lumen border (that
is, the wall of the blood vessel). This feature helps
identify lipid plaques since they are typically within
a fibrous plaque.
Beam Penetration (Dd): This is a measure of the
length of the beam from the lumen border to the
back border (the border beyond which the near
infrared beam does not reach and the signal is at
baseline). It depends on tissue type, thus can distinguish between plaques. This feature is invariant for
pixels across an A-line but varies across A-lines.
Mean Intensity (I): This represents the average signal intensity of the different plaque types within the
3D neighborhood. As can be seen in figure 4, this is a
very distinctive feature.
Homogeneity (H): This is a local coefficient of variation, σ /I. It helps in distinguishing between heterogeneous intensity regions and homogeneous intensity regions.
Relative Smoothness of Intensity (S): This is defined as
S = 1 – 1/( 1 + σ2). S is 0 for constant intensity regions
and it approaches 1 for large deviations in intensity
Entropy (E): Entropy is another measure of the variability of the signal intensity within the respective
plaque type regions. To compute it, we construct a
histogram of the intensity distribution within a 3D
neighborhood, convert it to a probability distribution, and then estimate its information content.
Similar features as these are often used in image-processing applications (Gonzalez, Woods, and
Eddins 2009). The final two features we use are optical parameters.
Attenuation Coefficient, μt — This feature measures
the rate at which the signal intensity drops off within the tissue. Calcified plaque has lower attenuation,
and as a result, IVOCT can see deeper into these tissues compared to lipid, where IVOCT does not see as
deeply. For this reason, the attenuation coefficient
(or penetration depth) gives useful information
about plaque types.
Incident Intensity, I0 — This represents the back scattering characteristics of the plaque at the point where
the light touches it. This feature is excellent at distinguishing fibrous plaques, which are very reflective.
Figure 3. Calcific Coronary Plaques.
Imaged in vivo by optical coherence tomography (OCT) (A, C) and intravascular ultrasound (IVUS) (B, D). (A) This OCT image shows a well delineated,
heterogeneous, signal-poor region corresponding to a macrocalcification (A,
arrow), also seen in the corresponding IVUS image (B, arrow). A signal-rich
fibrous band (A, two arrowheads) overlying the calcification is easily identified in the OCT image but is obscured by a saturation artifact in the IVUS
image. (C) A thin layer of circumferential calcification is seen in this OCT
image (arrows) as a well-defined, heterogeneous, signal-poor region within
the vessel wall. A side-branch (arrowhead) can be seen adjacent to the
guidewire artifact (*). (D) The extent of the calcifications (arrows) and their
relation to the surrounding fibrous components of the plaque are not as
clearly seen in the corresponding IVUS image. The borders of the guidewire
(*) artifact are marked by dotted lines in A, C. Tick marks, 1 mm. (Source:
Jang et al. 2002).