It is depends.

What liquid do you want to use? Ease combination is to use LPG injectors and dedicated ECU with 2D map of injection timing. With fuel injectors is more complicated issue, because you must use stande alone ECU like ECUMaster or Riva. If you want ready to use industrial solution... I have never seen one like it.

We need a valve in high pressure line and this valve should work with high back-pressure. The output flow rate should be precisely adjusted . Our plan is using fuel injector? what is the best alternative?

In principle you can use a fuel injector as a very fast (~ a few 100 µs) valve, in practice quite a few restrictions might arise. Just a few questions that should be asked:

• What is the intended duty cycle? Fuel injectors are not designed to stay "open" for long, e.g. in a passenger car a typical injection will last only a few ms followed by a break of several 10 ms. Solenoid valves will probably overheat if current flows continously.
• What are the intended flow rates? Typical passenger car fuel injectors have very small overall cross sections (e.g. several holes of ~ 0.1 - 0.2 mm diameter).
• What are the intended fluids and pressure ranges? Diesel or gasoline injectors will only work well within the pressure range they were designed for.

Very roughly - it's hard to say more without details - a fuel injector could be a good choice if you need the ultra-fast switching. When not actuated, it will also stay closed against very high pressures.