hittestPOint problems

I am trying to detect if a point on an image is on the opaque part of the image or the transparent part.

I have a image in a 48X48 area. half the image has transparent pixels.

I seem to detect a hit on every part of the image like a bounding box test but i dont want that .

for example . I place a 48X48 size image on stage at 0,0.
At point 40,20 on the stage the image has transparent pixels but I get it detects a hit?

mm.img.hitTestPoint(40,20,true); //it should give me false

how can i solve this.