how to read images stored in mat files using variables

I have a set of images stored in mat file. In the program I have a variable using which i need to the corresponding image file in mat.

For Example :

I = imread('1.jpg'); 
J = imread('2.jpg'); 
save imgs.mat

a = 'I';
imshow(a); --- this should get the image I from mat file displayed. But I am getting the error message 

Cannot find the specified file: "I"

How to do this ?

Answer 1

[Walter Roberson]

input_data = load('imgs.mat', a);
imshow(input_data.(a))

[Lakshmi]

Worked beautifully ! Thanks a lot !

[Melaku Eneayehu]

Undefined function or variable 'a'

[Walter Roberson]

The user had done,

a = 'I';