7 Image Registration

time500 =

30.1878

Step 4: Improve the Speed of Registration

The previous section showed that it’s often possible to improve a registration with the default parameters simply by letting it run longer. Although

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increasing ’MaximumIterations’ will likely lead to convergence, it’s also likely that running for a longer time will yield diminishing returns.

It’s often possible to force the optimizer to be more aggressive in its initial steps. Changing parameters that control step size might move the registration closer to the correct solution more quickly, but it could result in treating a local extrema as the best answer, when the best answer is elsewhere in the optimization space.

The ’InitialRadius’ property controls the initial search radius for the optimizer.

optimizer.MaximumIterations = 100; optimizer.InitialRadius = 0.009;

movingRegistered = imregister(moving, fixed, 'affine', optimizer, metric); figure, imshowpair(movingRegistered, fixed)

title('MaximumIterations = 100, InitialRadius = 0.009')

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Compared with the default configuration, the results are worse. While ’InitialRadius specifies the upper bound of the search radius, ’Epsilon’ sets the lower bound.

[optimizer,metric] = imregconfig('multimodal'); optimizer.Epsilon = 1.5e-4;

movingRegistered = imregister(moving, fixed, 'affine', optimizer, metric); figure, imshowpair(movingRegistered, fixed)

title('MaximumIterations = 100, Epsilon = 1.5e-4')

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This configuration yields almost exactly the same registration as the default parameters with almost no difference in performance.

The only remaining optimizer property to tweak independently is ’GrowthFactor’, which controls the rate at which the search radius grows in parameter space. A larger growth factor causes a more aggressive optimization.

[optimizer,metric] = imregconfig('multimodal'); optimizer.GrowthFactor = 1.1;

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movingRegistered = imregister(moving, fixed, 'affine', optimizer, metric); figure, imshowpair(movingRegistered, fixed)

title('MaximumIterations = 100, GrowthFactor = 1.1')

That seemed too aggressive. Perhaps the default value is too large. Try a smaller ’GrowthFactor’ setting.

optimizer.GrowthFactor = 1.01;

tic

movingRegisteredGrowth = imregister(moving, fixed, 'affine', optimizer, met

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timeGrowth = toc

figure, imshowpair(movingRegisteredGrowth, fixed) title('C: MaximumIterations = 100, GrowthFactor = 1.01')

timeGrowth =

6.1934

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