In discitis, one of the most important findings to look for is high T2 signal in the disc which alerts you to possible infection. But a degenerate disc with vacuum phenomenon (gas in the disc), can be high signal on T2 and mimic infection.
Why is it bright on T2? It doesn’t make sense because air should be low signal on T2 and T1. However if the patient has been lying supine for a few hours to days before the scan ( not uncommon if the patient is in hospital), fluid can accumulate in the pocket of gas and this then becomes high signal on T2 and can mimic infection.
So how do you differentiate it from discitis? The high T2 signal is usually an isolated finding so there should be no other features of discitis/ osteomyelitis like endplate erosion/ destruction or an epidural collection. If a prior CT is available look for the presence of gas in the disc.
Not all areas of vacuum phenomenon will demonstrate increased T2 signal as it depends on whether enough fluid has accumulated in them or not.
Sometime Modic Type 1 changes (endplate oedema) may be present together with the T2 disc signal. This can make things more difficult to interpret because the combination of endplate oedema and high T2 disc signal becomes even more concerning for early discitis. Again, there should be no endplate destruction/ erosion or collections. If a CT or plain xray are available check them to see if gas was present in the disc. If there wasnt gas or you dont have a prior CT to review, then it becomes very difficult to differentiate from early discitis and correlation with inflammatory markers and a repeat study to assess for progression is required.