Medical image of which kind ?! MR, CT, PET, Echography, Thermography, ... ???

If all are involved then: differences in some physical parameters of the tissues to be contrasted, instrument performance, data evaluation algorithms. Beyond that different techniques differ completely.

I would add some factors during the acquisition phase:

- Poor illumination

- Dye concentration/penetration/binding

- Movement of a specimen / organism or accidentally displacing the petri dish when an image is acquired.

- Camera quality

I am inclined to think that Kalyan Acharjya

Until he tells us what he means, I am left to speculations and wild imagination of all sort of things :)

If you are talking x-ray imaging. Many factors - tissue densities, exposure factors, pt. size, signal/noise ratios, post processing algorithms etc., etc.