Assessment of Initial Efficacy and Safety of High Intensity Focused Ultrasound 'ExAblate 4000 Type 2' for Blood Brain Barrier Disruption in Patients With Alzheimer's Disease

Inclusion Criteria: * Age between 50 and 85 years, inclusive. * Total score of 23 or less on the Korean version of Mini Mental state Exam (K-MMSE) * FBB (18F-Florbetaben)-PET test result positive * FDB (18F-Fluorodeoxyglucose)-PET test results suspected of Alzheimer disease-induced neurodegeneration * Mild cognitive impairment or dementia caused by Alzheimer's disease * A participant who has been given a drug at a stable dose for at least three months to improve symptoms of Alzheimer's disease * Able to communicate during the ExAblate BBB disruption procedure * Able and willing to give informed consent * Able to attend all study visits Exclusion Criteria: * CGA-NPI (Caregiver-Administrated Neuropsychiatry Inventory) severity score of more than 2 points for any of "Delusion", "Hallucination" or "Agitation/Aggression" * Known sensitivity/allergy or contraindications to MRI contrast agent(Gadovist®) or the ultrasound contrast agent (Definity®) * Contraindications to MRI such as non-MRI compatible implanted devices. * Any of the following items in MRI 1. severe ischemic changes 2. active or chronic infection/inflammation 3. acute or chronic hemorrhage 4. tumor/space occupying lesion 5. meningeal enhancement 6. intracranial hypotension * More than 30% of the skull area traversed by the sonication pathway is covered by scars, scalp disorders (e.g., eczema), or atrophy of the scalp * Active seizure disorder or epilepsy (seizures despite medical treatment) * History of a bleeding disorder, coagulopathy * Cardiac disease or unstable hemodynamics * Severely impaired renal function with estimated glomerular filtration rate \<30 mL/min/1.73m2 * Severe brain atrophy * HIV (Human immunodeficiency virus) positive, who have a high probability of developing HIV encephalitis due to the influx of HIV in to the brain parenchyma * Subjects with evidence of cranial or systemic infection * A homozygosity of Apolipoprotein E allele (ApoE4) known to have a thin blood-brain barrier * Positive pregnancy test (women of childbearing potential) * A participant in other clinical trials or have experience in other clinical trials within 90 days from the screening date