Multiple Sclerosis (MS): This is the most likely diagnosis given the presence of multiple small periventricular and juxtacortical white matter lesions without infratentorial involvement or enhancement with a contrast agent. MS is a chronic autoimmune disease characterized by demyelination in the central nervous system, and the described MRI findings are typical for this condition.

Small Vessel Disease: This condition, often associated with hypertension, diabetes, or aging, can cause small vessel ischemic changes that appear as white matter lesions on MRI. The lack of infratentorial lesions and no enhancement with contrast makes this a plausible diagnosis.
Migraine: Some patients with migraine can have white matter lesions on MRI, although these are typically less numerous and not as clearly defined as those seen in MS. The absence of other symptoms does not rule out migraine as a cause.
Vasculitis: Certain forms of vasculitis, such as primary central nervous system vasculitis, can present with white matter lesions. However, the lack of enhancement with contrast and the specific distribution of lesions might make this less likely.

Progressive Multifocal Leukoencephalopathy (PML): Although rare and typically seen in immunocompromised patients, PML is a serious condition caused by the JC virus that can present with white matter lesions. It's crucial to consider this diagnosis due to its severe prognosis and the need for prompt treatment.
Lymphoma or Metastasis: While the description does not suggest typical features of lymphoma or metastasis (such as enhancement with contrast), these conditions can occasionally present with atypical imaging findings. Given their potential severity, they should not be overlooked.

Adrenoleukodystrophy: A rare genetic disorder that affects the brain and spinal cord, primarily in males, characterized by progressive damage to the myelin sheath. It could present with white matter lesions but typically has a different distribution and clinical context.
Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like Episodes (MELAS): A rare mitochondrial disorder that can cause lesions in the brain, including white matter. However, it usually presents with more widespread and symmetric lesions, along with systemic symptoms.
Neurofibromatosis Type 1 (NF1): While NF1 can cause various types of brain lesions, including white matter lesions, these are typically different in appearance and distribution from those described, and are accompanied by other diagnostic features such as neurofibromas.