One option is to use LibTiff.Net library (disclaimer: I am one of it's developers). The library can provide access to data in TIFF without conversion of the data.
Following methods might be useful for you:
- Tiff.ReadScanline (generic access to decoded but not converted samples grouped in scanlines)
- Tiff.ReadEncodedStrip (the same as above but for strips)
- Tiff.ReadEncodedTile (the same as above but for tiles)
There are also Getting Started tutorial and some samples.