Obstructive sleep apnea syndrome (OSAS) is a sleep-disordered breathing characterized by the occurrence of repeated upper airway obstructions leading to airflow reduction (hypopnea) or cessation (apnea). The apnea-hypopnea index (AHI) is the number of apneas and hypopneas per hour of sleep. OSA patients often report cognitive complaints.

About 25% of the elderly population is affected by this syndrome with a drastic increase of this rate among dementia patients. OSAS is considered to be an important risk factor for the development of hypertension, heart disease and stroke.

Numerous studies pointed out the close relationship between sleep apnea and cognitive impairment. To the investigators' knowledge, no trials have assessed the existence of a relationship between the IAH index and the cerebral arterial blood flow rate (macrocirculation). Developments in magnetic resonance imaging (MRI) provide new insights into the quantitative study of blood flow through phase contrast MRI also called "flow MRI". The main hypothesis tested in this study is the existence of a relationship between OSAS and total cerebral arterial vascular inflow measured by PC-MRI (in mL/min) in the elderly population.

• Primary outcome: The apnea/hypopnea index (measured by nocturnal respiratory polygraphy) and total arterial flow rate (measured by PC-MRI)

• Secondary outcomes:

  1. Measurement of arrhythmia
  2. Measurement of arterial blood pressure

Inclusion Criteria:

1. Elderly patients about or over 75 years

2. Any gender

3. Dementia Group:

   3.1. MMSE (Mini Mental State Examination)> 15

   3.2. Diagnosis of dementia established according to DSM-IV

   3.3. Dementia of the Alzheimer type from NINCDS-ADRDA (National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association)

4. Control Group:

   Preserved cognitive function corresponding to a normal MMSE score by the standards of Poitrenaud

5. Ability to understand and give consent freely (for demented subjects, a legal representative will be delegate)

Exclusion Criteria:

1. Elderly patients under 75 years

2. Anyone with a classic contraindication to MRI

   2.1 Major behavioral disorders that do not allow the realization of MRI in optimal conditions

   2.2 Claustrophobia

   2.3 Presence of foreign non-compliant material

   2.4 Presence of intraocular metal body

3. Having a history of chest surgery or neurosurgical

4. Chronic respiratory failure

5. Suffering from dementia other than that associated with Alzheimer's disease

6. Patients with a handicap

7. Patients under legal protection