As mentioned, ischemia seems to be diagnosed largely by eliminating other causes of the trouble.
In regard to the stricture, it is probably more likely that the stricture is caused or worsened by the ischemia, rather than the other way around. But, the stricture can cause fecal stasis and make the pouch and cuff more susceptible to inflammation. So, it is sort of a chicken and the egg scenario.
You can make yourself crazy with what-if scenarios, but I will tell you a few things in case that it might help you make peace with your choice. I don't know how many times you had biopsies positive for LGD, or if it was just the one time before your surgery. Dysplasia can be changeable and colectomy for LGD can be controversial, especially since there is by no means any guarantee that LGD will advance to HGD and then cancer.
But, what is known about dysplasia in the presence of UC is that it is a cause for suspicion of future or even concurrent cancer. That is because cancer that arises from UC tends to be from flat lesions, not the typical polyps in normal colons. They can go unnoticed during colonoscopy until they are in advanced stages. They do not even necessarily arise from the dysplastic tissue, but from seemingly normal tissue. Early cancer is sometimes found in colectomy specimens that was not suspected. The only warning signs we get are the LGD and HGD in biopsy specimens.
If you had pancolitis of 25 years since diagnosis, the cancer risk was high. You may think your GI was being an alarmist, but he was following current standards of care. Of course, the final decision is always the patient's... I had about 24 years since my pancolitis diagnosis, mostly in remission. I had planned on having a prophylactic colectomy the next year, but went into a refactory flare that made more urgent than elective. I had no dysplasia and no cancer on the final pathology. I was grateful for that.
http://www.ssat.com/cgi-bin/colitis.cgiI've had lots of complications, including chronic cuffitis, but eventually managed to get things under control. I hope the same is true for you. I admit, the ischemia issue is quite vexing. The longer the j-pouch is around, they seem to be getting a better handle on what can go wrong.
The portal vein is huge, as it is the main vein from the liver, unlike the little collateral on your MRI.
Jan