We use for pelvis patients and have great results. It took some tweaking initially on our ROI’s but this is an example of what works for us. We have noticed if you include too much of the stomach then it can skew results. Note how we only used that stable part below stomach for this one and extended further down both of the legs.
We use a vac-loc under the legs on only so the pelvis is actually flat on the table. We also use a pillow case folded in 1/3’s and placed long ways to cover the patient. This seems to work well. We to do this in CT and explain why and have not encountered any patient complaints about it.
I would try playing with your ROI a little more to see if you can get better results. Don’t give up on it yet!