Why we need pixel data encryption (perceptual encryption), not the complete file encryption (conventional encryption)? Why we need pixel data encryption (perceptual encryption), not the complete file encryption (conventional encryption)? 

I am working on securing medical images, but I could not find the answer to this question. Is the complete encryption of the file (image pixel data+ metadata(header file)) better than encrypting only pixel data (distorted image) which preserve the header information (codec information) of the encrypted image?

In the literature, I found the following: Security mechanisms developed in the existing digital medical image systems are mostly based on the conventional encryption techniques such as DES, AES, IDEA, RSA. These mechanisms have been widely recognized as the standard multimedia encryption mechanisms. However, conventional mechanisms have obvious limitations on the multimedia data such as images shown in the following papers:


Socek D, Magliveras S, C’ulibrk D, Marques O, Kalva H, Furht B. Digital video encryption algorithms based on correlation-preserving permutations. EURASIP J Inform Security 2007.
Yoon, Ji Won, and Hyoungshick Kim. "An image encryption scheme with a pseudorandom permutation based on chaotic maps." Communications in Nonlinear Science and Numerical Simulation 15.12 (2010): 3998-4006.
Pareek, Narendra K., and Vinod Patidar. "Medical image protection using genetic algorithm operations." Soft Computing 20.2 (2016): 763-772.
The one "obvious limitation" of conventional encryption applied to multimedia data is that once encrypted, multimedia data no longer is compressible, or recognized as multimedia data (with metadata) by non-decrypting devices. This is solved by compressing before encryption, and accordingly decrypting before decompression and rendering. Confidentiality of multimedia data is a problem fully solved by conventional encryption.

There however might be practical reasons for perceptual encryption, including


We want the encrypted version to pass thru devices designed exclusively for multimedia data, including compressing perhaps with perceptually acceptable loss, or/and lacking error-correction.
We want a subset of the true data (e.g. metadata, sound but not image, a low-definition version..) to remain accessible by non-decrypting devices (which goes straight against the normal objective of encryption: that an adversary without key can not learn anything).
As an interesting specialization of the previous point, we might want that the non-encrypted low-definition version and the encrypted high-definition version share as much bandwidth as possible, which implies some perceptual model.