5.11 Diagnostic Tests for Nervous Disorders

Diagnosing disorders of the nervous system requires specialized tests that can visualize brain structure, detect electrical activity, or assess tissue integrity. The FBISE syllabus (SLO B-12-G-38) requires understanding the principles of three key diagnostic tests: EEG, CT Scan, and MRI.

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1. Electroencephalogram (EEG)

Principle

The brain constantly generates electrical signals as neurons communicate via postsynaptic potentials. An EEG records these electrical signals from the surface of the scalp using a set of electrodes (typically 20–25) placed at standardized positions.

How It Works

• Electrodes detect the summated postsynaptic potentials of millions of cortical neurons.
• The signals are amplified and displayed as wave patterns called brain waves.
• Four main types of brain waves are recognized:

  Wave	Frequency	Associated State
  Alpha (α)	8–13 Hz	Relaxed, awake
  Beta (β)	13–30 Hz	Alert, active thinking
  Theta (θ)	4–8 Hz	Light sleep, drowsiness
  Delta (δ)	< 4 Hz	Deep sleep

Clinical Uses

• Epilepsy: Detects abnormal, synchronous electrical discharges (spike-and-wave patterns).
• Sleep disorders: Identifies abnormal sleep stage patterns.
• Brain death: A flat EEG (isoelectric line) indicates no brain activity.
• Monitoring depth of anesthesia.

Key Point

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2. CT Scan (Computed Tomography)

Principle

A CT scan uses X-rays — the same ionizing radiation as a standard X-ray — but with a crucial difference: the X-ray source and detectors rotate 360° around the patient. A computer processes the data from multiple angles to reconstruct detailed cross-sectional images (called tomograms or "slices") of the body.

How It Works

1. The patient lies on a motorized table that moves through a ring-shaped scanner (gantry).
2. The X-ray tube rotates around the patient, emitting beams from hundreds of angles.
3. Detectors on the opposite side measure how much radiation passes through different tissues.
4. A computer uses these measurements to calculate tissue density and reconstruct 2D cross-sectional slices.
5. Multiple slices can be combined to create a 3D image.

CT vs. Standard X-ray

Clinical Uses in Neurology

• Acute intracranial hemorrhage (blood appears bright white on CT)
• Skull fractures and bone abnormalities
• Brain tumors (especially with contrast dye)
• Stroke (ischemic vs. hemorrhagic differentiation)
• Hydrocephalus

Key Point

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3. MRI (Magnetic Resonance Imaging)

Principle

MRI exploits the magnetic properties of hydrogen protons (present in water molecules throughout the body). It uses a strong magnetic field and radiofrequency (RF) pulses — it does not use ionizing radiation (no X-rays).

How It Works

1. Alignment: The patient is placed inside a powerful magnet. Hydrogen protons in the body align with the magnetic field (like compass needles pointing north).
2. Excitation: A radiofrequency pulse is applied, knocking the protons out of alignment.
3. Relaxation & Signal Emission: When the RF pulse is turned off, protons relax back to their aligned position and emit radiofrequency signals in the process.
4. Image Construction: The emitted signals vary depending on the type of tissue (different water content = different signal). A computer processes these signals to construct high-resolution images.

Why MRI is Superior for Brain Imaging

• Superior soft-tissue contrast: Can distinguish between grey matter, white matter, CSF, and pathological tissue.
• No ionizing radiation: Safer for repeated use.
• Multiplanar imaging: Can produce images in any plane (axial, coronal, sagittal).

Clinical Uses in Neurology

• Demyelinating diseases (e.g., Multiple Sclerosis — white matter lesions appear as bright plaques)
• Brain tumors (especially low-grade tumors invisible on CT)
• Spinal cord disorders
• Stroke (diffusion-weighted MRI detects ischemia within minutes)
• Congenital brain abnormalities

Key Point

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Comparison Summary