How to pass on demand data to datagrid virtualisation

I using data grid to to create one comparison window.
Rows in the data grid will be like this --
clm1      clm2      clm3       clm4  ...  all decided in runtime
2d image 3d image histogram profile... all decided in runtime

Number of clmns/rows all dynamic...

Since there can be hundreds of row and indefinite clms I am using virtualization;.

Now my design is like when this window will be launched may 200 rows of data will be populated to the data grid at one go (each row memory can be of 50/100 mb each based on clms) --- So my plan is to keep the data’s in some cache file, and load it only when that particular row comes to view in data grid.

<big>So my doubt is how to identify which row is currently in view so that I can ask for the specific data from cache file.</big>