A susceptibility-weighted imaging qualitative score of the motor cortex may be a useful tool for distinguishing clinical phenotypes in amyotrophic lateral sclerosis.

• The hypointensity of the primary motor cortex in susceptibility-weighted imaging could support the diagnosis of ALS. • Our new qualitative score called MCS shows high specificity and positive predictive value in differentiating ALS patients with upper motor neuron impairment from patients with ALS-mimicking disorders and healthy controls.