What’s the easiest way to determine if there is an epidural infection or not? The T1 pre contrast scan.
NORMAL: The most important aspect of assessing any scan is to know what normal is.
EPIDURAL FAT: The normal epidural space is composed predominantly of fat with some veins (epidural plexus). The amount of fat is variable from virtually nothing to excess amounts that result in epidural lipomatosis. This is best assessed on non fat saturated T1 sagittal and axial scans.
ENHANCEMENT:Normal epidural enhancement is variable and depends on the extent of the epidural venous plexus. It can vary from no enhancement to vivid enhancement, particularly of the anterior epidural fat where most of the veins are. So if you rely on the post contrast scans to determine if there is infection or not, you can get very confused by normal enhancement.
Image above: Enhancement of normal epidural fat.
Image above : On left precontrast T1 and on right post contrast no enhancement of normal epidural fat.
Trainees often go straight to the post contrast scans to assess for epidural infection. However, the most useful sequence is the pre contrast T1 scans. An abscess or phlegmon will alter the signal of normal fat on the pre contrast T1 scans. So if this is normal, there is no epidural abscess. If it is abnormal on the T1 scans, then the likelihood of an epidural infection is much higher.
Image above: Pre contrast T1 scans in anormal patient ( right) and a patient with epidural infection ( left) where the T1 signal is decreased.
ENHANCEMENT: As we saw, normal epidural fat can enhance, so enhancement on its own is not useful and it always needs to be correlated with the appearance of the fat in the pre contrast T1 scans. There are three options
1. NORMAL PRE CONTRAST T1 + NO OR VARIABLE ENHANCEMENT = NORMAL ( see normal images at beginning)
2. ABNORMAL PRE CONTRAST T1 + ENHANCEMENT POST CONTRAST = PHLEGMON
Image above: Pre contrast T1 on left with reduction in fat signal in epidural space. Post contrast image on right with uniform enhancement.
3. ABNORMAL PRE CONTRAST T1 + NO ENHANCEMENT POST CONTRAST = ABSCESS
So the key is the fat signal on the pre contrast T1 scan.
If its normal, there is no epidural phlegmon or abscess.
If its abnormal, there is either a phlegon or an abscess (or both) and differentiating these depends on whether it enhances ( phlegmon) or not (abscess).