Information Technology Reference
In-Depth Information
Likelihood Estimation
A likelihood estimation model integrating 2D morphological and feature informa-
tion was included in order to measure the error given from the current data. This
estimate expresses the measure of similarity between the current model points
w i
^
and an estimate w i ð
which encodes expert morphological knowledge of the
relationships between the L landmarks [ 31 ]. Each landmark i is assigned to a
speci
x
Þ
depending on the landmark type (i.e. pedicle tip), and is
based on local vertebral height, width, orientation and relative distances between
landmarks. The model also measures the similarity response of a rotation and scale
invariant wavelet coef
c function of w i ð
x
Þ
cient feature c msd ð^
w i Þ
speci
c to the landmark type, at
location
^
w i on the image. The probability of this likelihood estimate is:
(
"
#
)
2
Y
L
w PA ð w PA
Þþw SAG ð w SAG
1
2
Þ
i
i
D morphology /
exp
D w
ð
14
Þ
2 r
i¼1
where w PA ð^
w PA
i
Þ
and w SAG ð^
w SAG
i
Þ
are the similarity measures for the landmark
coordinates on the coronal and sagittal plane de
ned as:
w PA
i
w PA
i
w PA
i
w PA
i
w PA ð^
Þ ¼ ð^
ð
ÞÞ
c msd ð^
Þ
ð
Þ
x
15
w SAG
i
w SAG
i
w SAG
i
w SAG
i
w SAG ð^
Þ ¼ ð^
ð
x
ÞÞ
c msd ð^
Þ:
ð
16
Þ
Þ ¼ P j d j f j ð
Þ ¼ P j d j s j ð
In Eqs. ( 15 ) and ( 16 ), w PA
i
and w SAG
i
are the
estimates of the landmark coordinates on the coronal and sagittal plane respectively
based on morphological distribution of the neighbouring j landmarks. Each land-
mark j is assigned with prior knowledge distances f j ð
ð
x
x
Þ
ð
x
x
Þ
Þ
and s j ð
Þ
x
x
for the PA and
SAG views respectively, pondered by the prede
ned weights d .
5 Results
5.1 Self-calibration of the Radiographic Scene
5.1.1 Validation Methodology
A clinical validation using real data has assessed the clinical validity of the pre-
sented self-calibration algorithm. A comparison between a previously validated
system using an explicit calibration based on manually identified landmarks in a
fixed radiographic setup [ 8 ], and the proposed system based on uncalibrated X-rays
was made by generating a 3D model of the spine using both techniques. The data
used for the clinical study consisted of 60 pairs of digitized X-rays of adolescents
Search WWH ::




Custom Search