Radiology of the CNS: modalities, indications, advantages and disadvantages

1. Anatomy and physiology of the CNS (brief)

• Brain:
  • Cerebrum: frontal, parietal, temporal and occipital lobes.
  • Basal ganglia and thalamus.
  • Diencephalon: hypothalamus, pituitary, pineal.
  • Brainstem: midbrain, pons, medulla.
  • Cerebellum: in the posterior fossa.
• Spinal cord:
  • Runs from the foramen magnum to the conus medullaris (about L1-L2 in adults).
  • Below this is the cauda equina.
  • Gray matter is central and white matter is peripheral.
• Coverings: Dura, arachnoid and pia mater (the meninges). The subarachnoid space contains CSF.
• Ventricular system:
  • Two lateral ventricles, the third ventricle (via the foramina of Monro), the aqueduct, and the fourth ventricle.
  • CSF leaves the fourth ventricle through the foramina of Luschka and Magendie.
  • CSF is made by the choroid plexus and absorbed by arachnoid granulations into the venous sinuses.
• Blood supply:
  • Anterior circulation: internal carotid arteries (ACA, MCA).
  • Posterior circulation: vertebral and basilar arteries (PCA, cerebellar arteries).
  • The circle of Willis joins them.
  • Venous drainage is through the dural sinuses to the internal jugular veins.
• Protective bony cover: Skull and vertebral column.

• Blood-brain barrier (BBB):
  • Tight endothelial junctions keep iodinated and gadolinium contrast inside the vessels.
  • Contrast enhancement therefore means the BBB is broken (tumor, abscess, demyelination, subacute infarct).
  • It also means the lesion is highly vascular, or the structure lies outside the BBB (pituitary, choroid plexus, dura).
• Energy use:
  • The brain is about 2% of body weight but uses about 20% of cardiac output and oxygen.
  • It depends on glucose, which is why FDG-PET and perfusion SPECT work.
  • Cerebral blood flow is autoregulated, and neuronal activity is coupled to flow (the basis of fMRI).
• Water content:
  • Gray matter has more water and white matter has more myelin lipid. This produces the different densities on CT and signals on MRI.
  • Edema (vasogenic or cytotoxic) is the commonest tissue change imaged.
• CSF: About 500 mL is produced per day, with 100-150 mL present at any time. Disturbed flow gives hydrocephalus. Leaks are shown by cisternography.

2. Imaging modalities

A. Sonography (ultrasound, including Doppler)

• Neonates and infants through the anterior fontanelle: Intraventricular and germinal matrix hemorrhage, periventricular leukomalacia, hydrocephalus, congenital malformations, and some infections.
• Neonatal spine: Tethered cord, low conus, dysraphism, and sacral dimple screening. This works only in the first few months, before the posterior elements ossify.
• Carotid and vertebral duplex: Atheromatous stenosis, dissection, and occlusion.
• Transcranial Doppler:
  • Vasospasm after subarachnoid hemorrhage.
  • Sickle-cell stroke risk screening.
  • Detection of emboli.
  • Supportive testing for brain death.
• Intraoperative ultrasound: Localizing tumors and spinal lesions.
• Prenatal ultrasound: Screening for neural tube and brain anomalies.

• Portable and bedside.
• Real-time imaging.
• No ionizing radiation.
• No sedation is usually needed.
• Cheap and widely available.
• Doppler gives hemodynamic information.

• Skull blocks the beam once the fontanelle closes.
• Operator dependent.
• Limited field of view and tissue contrast, especially for the posterior fossa and brain convexity.
• Poor detection of subtle parenchymal lesions.
• Needs follow-up with CT or MRI if abnormal.

B. Contrast radiological methods

• Contrast shows lesions that are isodense or isointense on plain studies.
• It shows whether the BBB is intact.
• It gives vessel detail.
• DSA has the highest spatial resolution for small vessels and allows treatment in the same session.

• Iodinated contrast:
  • Allergic reactions.
  • Contrast-induced nephropathy.
  • Contraindicated in severe prior reaction, renal failure, and uncontrolled thyrotoxicosis.
  • Thyroid and metformin precautions.
• Gadolinium:
  • Nephrogenic systemic fibrosis with older agents in severe renal impairment (eGFR below 30).
  • Possible tissue deposition.
  • Used cautiously in pregnancy.
• Myelography: It is invasive. Complications are post-lumbar-puncture headache, infection, nerve root injury, arachnoiditis, and seizures if ionic contrast reaches the intracranial space.
• DSA: It is invasive. It carries a small risk of stroke (roughly 0.5-1%), groin hematoma, vessel dissection, and radiation to patient and operator. It is reserved for when CTA or MRA cannot answer the question or when treatment is planned.

C. Radionuclide scintigraphy (nuclear medicine: planar, SPECT, PET)

• Shows physiology and metabolism, often before structural change appears.
• Whole-body survey is possible (bone scan, PET).
• Highly sensitive.
• Quantifiable.
• Non-invasive, apart from intrathecal injection.

• Poor spatial resolution and little anatomical detail, so it is usually fused with CT or MRI.
• Radiation exposure.
• Findings are often non-specific.
• Costly and not widely available.
• Short-lived isotopes need a cyclotron (PET).
• Long acquisition times.
• Pregnancy is a relative contraindication.

D. Computed tomography (CT)

• Head trauma: Fractures, extradural, subdural and contusional hemorrhage. This is the first-line study.
• Acute stroke: To exclude hemorrhage before thrombolysis. CT perfusion and CTA are added for large vessel occlusion and penumbra.
• Subarachnoid hemorrhage: Acute blood is dense and CT is highly sensitive in the first 6 hours.
• Raised intracranial pressure, mass effect, herniation, and acute hydrocephalus.
• Suspected CNS infection or mass lesion when MRI is not immediately available. Harrison's states CT should be done emergently in that situation (Harrison's, Brain Imaging).
• Bone and calcification detail: Skull base and spinal fractures, calcified tumors, and spinal canal stenosis.
• Cerebral angiography and venography (CTA/CTV).
• Guidance for stereotactic biopsy and drainage.
• Spine: Trauma and bony involvement by tumor or degenerative disease. Rosen's describes CT as essential for these because it defines bone better (Rosen's Emergency Medicine).

• Fast (seconds to minutes), widely available, and relatively cheap.
• Excellent for acute hemorrhage, bone, and calcification.
• Suitable for unstable or uncooperative patients.
• Compatible with pacemakers, metal implants, and life-support equipment.
• Multiplanar reformatting and 3D reconstruction.

• Ionizing radiation (a head CT is about 2 mSv). The concern is greater in children and pregnancy.
• Soft-tissue contrast is lower than MRI.
• Early and small infarcts, brainstem and posterior fossa lesions (beam-hardening artifact), and small demyelinating plaques are missed.
• Metal and dental artifact.
• Iodinated contrast risks (see above).
• Axial acquisition is native; other planes are reconstructed.

E. Magnetic resonance imaging (MRI)

• T1: Anatomy; fat, subacute blood and gadolinium are bright; CSF is dark.
• T2: Pathology; CSF and edema are bright.
• FLAIR: T2 with CSF suppressed. Best for periventricular and cortical lesions (MS, small infarcts).
• DWI/ADC: Restricted diffusion. Detects acute infarct within minutes, plus abscess and some tumors.
• SWI/GRE: Blood products, microbleeds, calcification.
• MRA/MRV: Non-contrast or gadolinium vascular imaging.
• Advanced: Perfusion, spectroscopy, tractography, fMRI.

• Preferred modality for most CNS disease:
  • Brain tumors, including posterior fossa and pituitary.
  • Demyelination (MS).
  • Encephalitis and meningitis.
  • Abscess.
  • Epilepsy. MRI is superior to CT for epileptogenic lesions such as hippocampal sclerosis and cortical malformations (Harrison's).
  • Dementia and degenerative disease.
• Acute ischemic stroke: DWI detects infarct earlier and with greater sensitivity than CT.
• Spinal cord disease: Compression, myelopathy, syrinx, tumor, infection, disc disease, and trauma with cord injury. Rosen's names MRI as the preferred modality because it clearly defines the cord and soft tissues and shows edema and hemorrhage in the cord (Rosen's).
• Congenital malformations and pediatric neuroimaging (no radiation).
• Cranial nerves: Internal auditory canal, orbits.
• Vascular lesions: Cavernomas, venous thrombosis, MRA for aneurysm and stenosis.
• Pre-surgical mapping: fMRI and tractography.

• Best soft-tissue contrast, so gray and white matter and small lesions are clearly seen.
• No ionizing radiation.
• Direct multiplanar imaging in any plane (Neuroscience: Exploring the Brain, p. 593).
• No bone artifact, so the posterior fossa, brainstem and cord are well seen.
• Functional, vascular and metabolic information (fMRI, MRA, spectroscopy) in one examination.

• Long scan time (10 minutes to an hour). The patient must lie still. Sedation or anesthesia is often needed in children.
• Noisy, and claustrophobia is common.
• Absolute or relative contraindications:
  • Older pacemakers and defibrillators (many newer devices are MR-conditional).
  • Cochlear implants.
  • Ferromagnetic intracranial aneurysm clips.
  • Metallic foreign bodies in the eye.
  • Some neurostimulators.
  • Grainger notes that magnetic field strength may exceed 60,000 times that of the Earth and is always on, so screening is mandatory (Grainger & Allison's).
• Poor for acute hemorrhage in some sequences, cortical bone, and calcification (versus CT).
• Poor for unstable, ventilated or monitored patients.
• More expensive and less available than CT.
• Gadolinium precautions (renal function, pregnancy).
• Metal artifact from implants.

Same pineal cyst on MRI (T1-FLAIR, T2, T2-FLAIR, DWI, ADC, T2) and CT in three planes, showing how each modality displays the same lesion.*

3. Quick comparison

4. Practical first choices

• Acute head trauma, suspected hemorrhage, or acute stroke: Non-contrast CT first (plus CTA/CT perfusion if thrombectomy is considered).
• Neonate with a suspected intracranial bleed or hydrocephalus: Cranial ultrasound.
• Suspected cord compression, MS, tumor, encephalitis, or epilepsy work-up: MRI (with gadolinium when needed).
• Aneurysm or AVM treatment planning: CTA or MRA, then DSA.
• MRI contraindicated, spinal canal question: CT myelography.
• Functional question (focus localization, dementia type, parkinsonism, CSF leak, brain death): SPECT, PET, or cisternography.

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