want to prepare a mask which will capture 15 % of data from an Ultrasound image .could any one help

Hai I am attaching an ultrasound image .I would like to prepare a mask matrix which will capture 15 % of data from that.(15% from the lower curved region)..Could any one help me out...

[Image Analyst]

How is this a different question than http://www.mathworks.com/matlabcentral/answers/136879-radial-mask-of-30-degree-with-15-data#comment_221608?