Unformatted text preview: t procedure.
A typical initial alignment appears in Figure 5.19. Notice that this image is a scaled,
sheared, rotated and translated version of the original. The nal alignment is displayed as
a checkerboard. Here every other 20x20 pixel block is taken either from the model image or
aligned target image. Notice that the boundary of the brain in the two images is in close
agreement.
We represent transformation space as a 6 element a ne matrix that is used to project
two dimensional points from the image into the model. This scheme can represent any
combination of scaling, shearing, rotation and translation. The remaining algorithmic details
are summarized in Table 5.5.
In order to determine the reliability and precision of the alignment procedure, 50 random126 5.2. MEDICAL REGISTRATION EXPERIMENTS AITR 1548 1. De ne the model and image u and v: u is the model image,
v is the target image.
2. Sampling x: Sample the pixels of the model image uniformly.
3. Transformation space T : The space of a ne transformations
mapping pixels locations from the model into pixel locations in
the image.
4. De nition of dvy=dy: This is the intensity gradient.
5. Distance metric: Euclidean distance.
6. Variance, : Assuming diagonal covariance matrices, three different variance are necessary: two for the joint entropy and one
for the image entropy. The variances were 0.1 in all cases.
7. Minimum probability, pmin : 0.01.
8. Number of samples: One sample of 20 using crossvalidation.
9. Update rate, : 0.02 for 500 steps and then 0.005 for 500 steps.
Table 5.5: Summary of MRI alignment experiments.
ized alignments were performed. The initial transformations were randomly selected, having
a translation of up to 35 pixels this is about one third of the width of the head, a rotation
of up to 30 degrees, and a scaling of up to 20. The correct alignment was obtained in 100
of the experiments. After alignment the a ne transformations had an average translation
error of 0.1 pixels. The remaining a ne parameters represent a mixing of rotation, scale and
shearing. They are somewhat more di cult to interpret. Given that the correct transformation is the identity matrix we can evaluate the nal matrices by measuring the di erence
from the identity. On average the coe cients of the nal transformation were in error by
0:02. These experiments demonstrate that EMMA alignment is both precise and reliable.
These two MRI images are fairly similar. Good alignment could probably have been
obtained with a normalized correlation metric. Normalized correlation assumes, at least
locally, that one signal is a scaled and o set version of the other. Our technique makes no
such assumption. In fact, it will work across a wide variety of nonlinear transformations.
More di cult alignment problems are easily simulated. In Figure 5.20 we show the model
image after a nonmonotonic nonlinear function has been applied. Recall that initially the
image lies in the range 0,1 . We subtract 0.5, square the result, and renormalize to 0,1 . This
operation is shown at the right of the gure. After applying this nonlinear transformation
the two images are anticorrelated; no variant of correlation can correctly align them. EMMA
127 Paul A. Viola CHAPTER 5. ALIGNMENT EXPERIMENTS Figure 5.18: MR Images
alignment performance, however, is not e ected. 5.2.1 Three Dimensional MR CT Alignment
The medical alignment procedure described above can be extended to volumetric data. In the
resulting system both the model and image are 3D arrays and a full three dimensional aligning
transformation is estimated. Recently a number of three dimensional CT MRI alignments
have been performed. Because these results are preliminary, many of the experimental details
are still in ux. Our description here will be necessarily brief. A more complete description
can be found in Wells III and Viola, 1995.
The scans used were obtained from the same patient at di erent times4. Display of these
three dimensional scans, and their alignment, is a di cult problem. Though the entire scan
cannot be shown, some feeling for the data can obtained be displaying the three central
slices. The central slices are perpendicular planes that pass trough the centroid of the data.
Figure 5.21 shows the three central slices of the CT scan. Figure 5.22 shows the initial
The images and the standard transformations were provided as part of the project, Evaluation of Retrospective Image Registration", National Institutes of Health, Project Number 1 R01 NS3392601, Principal
Investigator, J. Michael Fitzpatrick, Vanderbilt University, Nashville, TN.
4 128 5.2. MEDICAL REGISTRATION EXPERIMENTS AITR 1548 Figure 5.19: Initial Pose and Display of Result
alignment of the CT MR pair as a checkerboard. When the signals involved are very di erent,
the checkerboard representation can be somewhat confusing. Figure 5.23 shows the nal
alignment as the composition of the MR data with the intensity edges computed from the
CT data. Notice the close agreement between the skull in both scans. 129 Paul A. Viola CHAPTER 5. ALIGNMENT EXPERIMENTS 1
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This note was uploaded on 02/10/2010 for the course TBE 2300 taught by Professor Cudeback during the Spring '10 term at Webber.
 Spring '10
 Cudeback
 The Land

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