I've been diagnosed with mnd but my cervical spine mri and my brain mri said no significant abnormality detected mri?

Somewhat typical of motor neuron disease as imaging studies are used to rule out other potential causes, your neurologist remains best to place test results in proper context.

If indeed you do possess acquired disease of the motor neuron, would not expect imaging to demonstrate any abnormalities early on. Later, atrophy of the spinal cord might be demonstrated. As mentioned, MRI studies are done to exclude different etiologies for your symptoms.