floor underlayment question - tile floor

I am remodeling my kitchen and plan to use 18x18 ceramic tile over an electric radiant floor heat system.
I need advice for preparing the subfloor for the tile. House is older (1930) with ~1"x3 thk tongue and groove wood subfloor.
I removed the vinyl floor and found the original "vinyl" floor that I cannot get up. My question is: will a 1/4" Hardibacker or cement board be a sufficient substrate for the ceramic tile over the existing floor? I prefer not to add a layer of 1/2" OSB under the cement board to maintain low transitions to ajoining rooms and exterior door thresholds.
Any advice appreciated.
Thanks.

Add a minimum of 5/8" plywood and then the Hardiebacker.

Thanks for the response. I also have a follow-up question. There are a couple areas where the cabinets were removed that the subfloor is exposed and is ~ 1/8" lower than the original vinyl.

As I mentioned this is the original floor and I suspect it has asbestos so I'd rather not mess with it.

Is there something that I can use to build up these areas prior to putting down the 5/8 OSB?

You can fill that 1/8" with the same modified thinset mortar that you'll eventually use to install your tiles, or a Portland based patching product. You then install the 5/8" PLYWOOD (not OSB), and then the Hardie or maybe Ditra?

Jaz

Is there something that I can use to build up these areas prior to putting down the 5/8 OSB?

NOT OSB. Exterior Grade Exposure 1 Plywood.

Thanks for the help guys. Walls are almost finished so I will start preparing the sub-floor this weekend. I'll let you know how it goes.