Copy a part of canvas to image

I try to copy a certain part of a canvas, then write the copied part in an image. Here is my (wrong) approach:

var c = document.getElementById('MyCanvas'),
ctx = c.getContext('2d');
var ImageData = ctx.getImageData( 25, 25, 150, 150 );

var MyImage = new Image();
MyImage.src = ImageData ; //   <-- This is wrong, but I mean something like this

Do you know how can I do it? Thank you in advance.

ps: I don't want to copy the whole canvas, but a certain part of it.