At my work place, the PCs were upgraded to Windows 8.1 a while back, but it is not until recently that my colleague tried scanning something from this Brother MFC-7820N that I realised her PC didn't have the scanner driver installed.
After a quick look around, I ran into a problem - Brother says to use the Windows 8.1 built-in driver and of course shows a wizard on how to set it up for network printing, but nothing about the scanner. From a few discussions I found with others having this issue, basically anyone who managed to contact Brother were basically told that they need a new multi-functional unit for network scanning in Windows 7 or 8.1 as they have no intention of creating a driver for it.
Anyway, I kept on digging through threads and even came across commercial scanning software that claims to support this printer, but costs almost as much a new multifunctional unit!
As it turns out, Brother released a similar multifunctional printer, the MFC-7440N which works with pretty much the same drivers (if not identical) to the MFC-7820N as the user Vitaly pointed out in this thread on Microsoft's forum, with the exception that the MFC-7440N has drivers available all the way up to Windows 8.1.
I gave it a try - I downloaded the full suite, installed it and rebooted as requested. I used GIMP to scan something and it worked just fine.
I remember running into this issue in the past with an Espon scanner that did not have a Windows 7 64-bit driver. The solution turned out to be the same - I used a Windows 7 driver from a similar looking later launched Epson scanner and it worked fine. With printers, it can be a different story as I remember a laser printer spitting out a heap of pages of garbage when I tried a wrong driver.