Literature Reviews - Segmentation of Hand Bone for Bone Age Assessment - page 18

range of intensity for object and background [ 37 ]. Overlapping range of intensity
occurs often due to uneven illumination. Besides, the nature of the object itself can
lead to overlapping range in which some regions within the objects in input image
has overlapping range of intensity to background.
As mentioned in previous chapter, one of the natures of X-ray hand bone radio-
graph is its uneven illumination throughout the image; as well as its overlapping
range of intensity distribution among soft-tissue region, trabecular bone and corti-
cal bone due to the nature of hand bone and uneven background illumination as
well. The illustrations of the effect of uneven illumination and overlapping range
of intensity using Global thresholding in hand bone are demonstrated below.
Figure 2.1 shows the input image in the illustration and its corresponding histo-
gram. Figure 2.2 shows the resultant segmented image using different threshold, T.
The reasons for the inferior quality of segmented hand bone as shown in
Fig. 2.2 can be summarized as follow:
1. Assumption that the whole targeted object (which is the hand bone in this case
without soft-tissue region) contains similar intensity range. This is always not
true for hand bone radiograph as within the hand bone, there are regions of tra-
becular bone and cortical bone which have different bone density and hence
are represented by different range of pixel intensity values in digital image as
shown in Fig. 2.1 a.
2. Assumption that the histogram of targeted object and background (dark regions
and soft-tissue regions) is perfectly separated into two groups of intensity dis-
tributions. This is always not true for hand bone as shown in Fig. 2.1 b that the
histogram of hand bone radiograph is not bi-modal distributed. This can be
explained from the nature of hand bones that are formed by three classes of
regions: bone, soft-tissue regions and background instead of two.
3. Assumption that there is no overlapping of intensity range between background
and targeted object. This is always not true for hand bone as the some of the
Fig. 2.1 The original hand bone radiograph and its corresponding histogram