Digital Signal Processing Reference
In-Depth Information
features like number and position of vessels (arteries, capillaries). An angular-oriented filter
bank may be used in analyzing angiography images by detecting vessels with a given
orientation. Let us consider the retinal fluorescein angiogram from Fig.16(a), featuring some
pathological elements which indicate a diabetic retinopathy. This image is filtered using 5
oriented wedge filters with narrow aperture (
θ
=
π
/24), designed using the method described
in section 4. Fig.16 (b)-(f) show the directionally filtered angiography images.
(a)
(b)
Figure 15.
(a) Test image; (b) filtered image
(b)
(c)
(a)
(d)
(e)
(f)
Figure 16.
(a) Retinal fluorescein angiogram; (b)-(f) images resulted as output of five component filters of the fan-
type filter bank
As can be easily noticed, the vessels for which the frequency spectrum overlaps more or less
with the filter characteristic remain visible, while the others are blurred, an effect of the
directional low-pass filtering (Matei & Matei, 2012). The directional resolution depends on the
filter angular selectivity given by
θ
.
