Radiologically Isolated Syndrome (RIS): This condition is characterized by the presence of white matter lesions on MRI that are suggestive of multiple sclerosis (MS), but without clinical symptoms or signs of the disease. The presence of multiple periventricular and juxtacortical micro-lesions in the white matter, as described, is highly suggestive of RIS, especially given the lack of dissemination in time, infratentorial lesions, or enhancement with contrast.

Multiple Sclerosis (MS): Although the question specifies no signs of dissemination in time, which is a key criterion for diagnosing MS, it's essential to consider MS in the differential, especially if the patient has had unreported or subtle clinical episodes. The pattern of lesions is consistent with MS.
Migraine: Migraine can cause white matter hyperintensities on MRI, particularly in the periventricular and juxtacortical regions. However, these are typically less numerous and less specific than those seen in demyelinating diseases. The absence of other migraine-related MRI findings and the specific pattern of lesions make this less likely but still a consideration.

Neurosarcoidosis: This condition can cause white matter lesions and can be life-threatening if not treated. Although less common, it's crucial to consider due to its potential severity and the need for specific treatment.
Vasculitis (e.g., Primary Central Nervous System Vasculitis): Vasculitis can cause a wide range of neurological symptoms and MRI findings, including white matter lesions. It's a critical diagnosis not to miss due to its potential for severe morbidity and mortality.
Infections (e.g., Lyme Disease, HIV): Certain infections can cause white matter changes on MRI. These conditions are important to rule out, especially if there's a relevant exposure history or clinical suspicion.

Leukodystrophies: These are a group of rare, genetic disorders that affect the growth or maintenance of the myelin sheath, leading to white matter lesions. They are typically diagnosed in childhood but can occasionally present in young adulthood.
Mitochondrial Encephalomyopathies: Conditions like MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) can cause white matter lesions among other neurological and systemic findings. They are rare and often have a distinctive clinical presentation.
CADASIL (Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy): This is a rare, hereditary condition that affects the blood vessels in the brain, leading to subcortical strokes and white matter lesions. It typically presents with migraine, strokes, and dementia.