MRI has revolutionised the assessment of MS.
If you have a lesion on MRI what does it mean?
Evidence for a white matter lesion size threshold to support the diagnosis of relapsing remitting multiple sclerosis.
Grahl S1, Pongratz V1, Schmidt P1, Engl C1, Bussas M1, Radetz A2, Gonzalez-Escamilla G2, Groppa S3, Zipp F3, Lukas C4, Kirschke J5, Zimmer C5, Hoshi M6, Berthele A6, Hemmer B7, Mühlau M8.
Mult Scler Relat Disord. 2019 Jan 28;29:124-129. doi: 10.1016/j.msard.2019.01.042. [Epub ahead of print]
The number of white matter lesions (WML) in brain MRI is the most established paraclinical tool to support the diagnosis of multiple sclerosis (MS) and to monitor its course. Diagnostic criteria have stipulated a minimum detectable diameter of 3 mm per WML, although this threshold is not evidence-based. We aimed to provide a rationale for a WML size threshold for three-dimensional MRI sequences at 3 T by comparing patients with relapsing-remitting MS (RRMS) to control subjects (CS).
We analyzed MR images from two cohorts, obtained at scanners from two different vendors, each comprising patients with RRMS and CS. Both cohorts were examined with FLAIR and T1w sequences. In total, 232 patients with RRMS (Expanded Disability Status Scale: mean = 1.6 ± 1.2; age: mean = 36 ± 10) as well as 116 age- and sex-matched CS were studied. We calculated odds ratios across WML volumes. The WML size threshold, which discriminated best between patients and CS, was estimated with receiver operating characteristic curve analysis.
In both cohorts, odds ratios increased continuously with increasing WML volumes, and discriminative power was highest at a WML size threshold corresponding to a diameter of about 3 mm.
The stipulated WML size threshold of 3 mm in diameter for the diagnostic criteria of MS seems a reasonable choice for three-dimensional MRI sequences at 3 T.
So in this study they say that an acceptable lesion to diagnose MS is 3mm across is an OK size.
However, to put this in the context of a cell, which is about 100 micromitres across, means that the the lesion is 3000 micromtre across = 30 cells across. So within a spherical lession there would be about 113, 000 cells.
So you can see how small lesions may be missed.
In reality the mri lesion will be bigger than the cellular lesion as there is leakage of the contrast agent outside of the lesion