filter to get sharp edges with bwboundaries

I have double image of optical densities. I want to create boundaries around areas of greater optical density.

Basically straight from MATLAB help:

opticalDensity=.1; BW = Img>opticalDensity; BW_filled = imfill(BW,'holes'); boundaries = bwboundaries(BW_filled);

for k=1:length(boundaries) b = boundaries{k}; plot(b(:,2),b(:,1),color,'LineWidth',1); end

I get sharp edges right on the boundaries of the region but then fades off to scattered individual pixels as you get farther away.

I am thinking of creating a "rule" where the boundary only exists where there are at least X amount (maybe 3) of pixels together that meet the criteria.

Any suggestions?