CNS Imaging Overview

CNS Imaging Overview

Introduction to CNS Imaging

  • Focus on Perfusion and SPECT (Single Photon Emission Computed Tomography).

General Topics Covered

  • Review of anatomy, physiology, and pathology relevant to Central Nervous System disorders.

  • Discussion of nuclear medicine imaging procedures specific to the CNS system.

  • Description of CNS imaging protocols.

  • Analysis of CNS imaging practices.

Radiopharmaceuticals in CNS Imaging

Package Insert Indications for Radiopharmaceuticals

99mTc ECD (Ethyl Cysteinate Dimer)

  • Adjunct to MRI or CT for patients diagnosed with stroke, helping to localize the stroke.

  • Assessment of functional viability of brain tissue.

  • NOT used to diagnose a stroke.

99mTc HMPAO (Hexamethylpropyleneamine Oxime)

  • Adjunctive imaging for patients showing altered regional cerebral perfusion due to stroke.

201Tl (Thallium-201)

  • Used off-label in specific contexts.

Cerebral Perfusion

  • SPECT and SPECT/CT utilized for brain perfusion studies.

Key Applications of Brain Perfusion Studies

  1. Evaluating patients with suspected dementia.

  2. Localizing seizure foci for targeted interventions.

  3. Mapping brain perfusion during therapeutic interventions.

  4. Detecting and evaluating cerebrovascular disease.

  5. Corroborating clinical diagnoses of brain death (can utilize SPECT or planar imaging).

Anatomy of the Central Nervous System

Structure of CNS

  • Gray Matter: Contains neuron and support cells.

  • Cortex: The outer layer of the brain involved in complex functions.

  • Deep Structures: Include basal ganglia, thalamus, and hypothalamus.

  • White Matter: Beneath the cortex, composed of myelinated axons facilitating communication between different brain regions.

Cerebral Vasculature

  • Includes vital arteries such as:

    • Basilar Artery

    • Anterior Cerebral Artery

    • Middle Cerebral Artery

    • Internal Carotid Artery

    • Circle of Willis; crucial for collateral circulation in case of occlusion.

Cerebrovascular Diseases

Common Conditions

  • Global Ischemia: Results from compromised blood flow leading to widespread brain tissue damage.

  • Cerebral Infarct: Primarily caused by thrombosis or embolic events due to atherosclerosis.

  • Intracerebral Hemorrhage: Bleeding within the brain tissue itself.

Effects of Hypoperfusion

  • Hypoperfusion can lead to significant neurological effects, including:

    • Depression

    • Dementia

    • Secondary effects from hypotensive shock or vascular collapse.

Pathophysiological Consequences

  • Lack of blood flow (hypotension) results in diminished perfusion, particularly affecting deep gray matter.

Cerebral Infarcts - Key Features

  • Brain tissue becomes liquefied and necrotic leading to Encephalomalacia, or softening of brain tissue.

  • Resultant neurological deficits include:

    • Hemiplegia: Paralysis of one side of the body.

    • Global aphasia: Inability to communicate effectively.

    • Sensory and/or motor deficits.

    • Brain edema: Life-threatening due to pressure effects on brain structures.

Indications and Contraindications for CNS Imaging

Clinical Indications for Imaging

  • Useful for evaluating functional brain abnormalities in contexts such as:

    • Acute Stroke

    • Transient Ischemic Attacks (TIA)

    • Epilepsy

    • Memory Loss

    • Dementia/Alzheimer’s

    • Distinguishing between recurrent tumors and radiation necrosis.

Contraindications

  • Situations in which imaging is not advised include:

    • Pregnancy or breastfeeding.

    • Recent conflicting nuclear medicine studies.

    • Patients who are agitated or unable to cooperate.

    • Inadequate patient preparation.

Patient Preparation and Education for Imaging

  • Advise discontinuation of substances like caffeine and alcohol 24 hours prior to the study.

  • Restrict smoking on the day of the exam.

  • Gather comprehensive patient history:

    • Past drug use or trauma.

    • Prior neurologic and psychiatric evaluations.

    • Previous imaging results.

    • Current medications and timings of last doses.

Cerebral Perfusion Imaging Procedures

Overview of Imaging Pathology

  • Any pathology impacting regional or global brain perfusion could include:

    • CVA (Cerebrovascular Accident)

    • TIA (Transient Ischemic Attack)

    • Dementia

    • Disorders like ADHD, Bipolar Disorder, Schizophrenia, and depression.

Protocols for Radiopharmaceuticals

  • Radiopharmaceuticals: 99m Tc HMPAO (Ceretec) and 99m Tc ECD (Neurolite) provide crucial imaging insights necessary for the assessment and management of CNS disorders.

99m Tc HMPAO (Ceretec) Specifics

  • Requires a fresh elution under 4 hr old; previous elution must be within 24 hours.

  • Expiration and Stability:

    • Expiry: 30 min without methylene blue added; 4 hours with it.

    • Store between 15-25°C.

  • Critical Organ: Lacrimal glands.

99m Tc ECD (Neurolite) Specifics

  • Expiration of 6 hours, stored similarly.

  • Critical Organ: Bladder wall.

Imaging Procedure Steps

General Imaging Procedure

  1. Obtain IV access and prepare the environment by dimming lights; ensure no speaking or reading for patients for 20-30 minutes before radiopharmaceutical injection.

  2. Inject radiopharmaceutical (RP) without disturbing the patient; continue monitoring for 30-60 minutes post-injection to allow for uptake.

  3. Position the patient carefully with arms down, head secured in a holder, minimizing motion.

  4. Monitor the patient throughout the procedure, especially those with dementia; sedation may occur if necessary after 5 mins post-RP injection.

Adjunct Imaging Interventions

  • Vasodilatory challenge with acetazolamide (Diamox) generally carried out over two days to identify ischemic areas similar to cardiac studies.

Procedure Timing and Administration

  • Administer Diamox with a slow IV push, wait 15-20 minutes before injecting RP, following acquisition and processing similar to a standard perfusion study.

Imaging Acquisition Details

Imaging Acquisition Parameters

  • SPECT: Continuous or step and shoot with 360º orbit.

  • Collimation: Low energy high resolution, superior patient positioning required.

  • Energy Peak: Target energy peak of 140 keV with a 20% window.

Timing Specifics for Acquisition

  • Injection to Imaging Time:

    • HMPAO: 90 minutes preferred delay, 40 min minimum.

    • ECD: 30 minutes preferred delay, 20 min minimum.

Matrix and Projections

  • Use a matrix of 128 x 128 with 128 projections, allocating 20-40 seconds per projection.