Imaging of the Head and Brain

CT

Overview

• Images of the head are obtained from a rotating beam of X-ray particles.
• The images can be adjusted to highlight different structures:
  • Soft tissue windows
  • Bone windows
• CT with contrast:
  • IV administration of iodinated contrast agent
  • Enhances vascular structures and areas of discontinuity of the blood–brain barrier
  • Also highlights pathology that interrupts the blood–brain barrier (i.e., infection, neoplasm, infarction)
  • MRI contraindicated or unavailable

Structures and planes

• The final image depends on the attenuation of the beam by the cranial structures:
  • Bone: appears white (↑ density, greater attenuation)
  • Air: appears black (↓ density, lower attenuation)
  • White matter appears slightly darker than gray matter because of the presence of fat in myelin (which has lower density than water).
• Brain pathology exhibits different attenuation, such as:
  • Hemorrhage: high attenuation (appears bright)
  • Infarct: low attenuation (appears dark owing to edema)
• Complete study of the head includes the following planes:
  • Axial 
  • Coronal
  • Sagittal

MRI

Overview

• Technique that uses magnetic fields and radiofrequency pulses to produce highly detailed images of the human brain.
• Operates on the principle that the body is abundant in protons, which align with a strong magnetic field:
  • Protons spin out of equilibrium when a radiofrequency current is applied.
  • When current is removed, the protons realign, but differences depend on the nature of the brain structure.
• Images are processed on the basis of the intrinsic tissue relaxation:
  • T1: relaxation time for the protons to align longitudinal/parallel to the magnetic field
  • T2: relaxation time for the protons to align transverse/perpendicular to the magnetic field 
• MRI with contrast:
  • Use of gadolinium-based agent
  • Increases detectability of lesion(s)
• Additional sequences can include:
  • Fluid-attenuated inversion recovery (FLAIR): 
    • Fluid-sensitive sequence that nullifies signal from water.
    • This nullification allows other T2 signals to be more prominent (edema).
  • Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC): 
    • Sequence that detects Brownian motion of atoms
    • Useful for detecting ischemic areas from stroke and for differentiating brain tumors
  • Gradient echo (GRE) or susceptibility-weighted imaging (SWI): 
    • T2-sensitive sequence that manipulates ferromagnetic signal
    • This manipulation produces a larger area of signal loss around calcium and blood.

Structures and planes

• Appearance of the images depend on the intrinsic tissue relaxation.
• Descriptions (generally by shades of gray):
  • Hyperintense: bright (white)
  • Isointense: same brightness as the tissue it is being compared to
  • Hypointense: darker than the tissue it is being compared to
• Similar to CT, the planes are:
  • Axial 
  • Coronal
  • Sagittal

MRI and brain tissue

Other Imaging

Ultrasonography

• The main principle of ultrasonographic imaging is the transmission and reflection of sound waves through the tissues.
• Readily available, with images obtained in real time
• Less expensive and noninvasive
• No ionizing radiation
• Requires an adequate acoustic window (not obstructed by bone or impeded by air); thus, it has limited use in neuroimaging, but it is used in:
  • Fetal and neonatal screening
  • Examination of infant requiring ECMO to detect intracranial hemorrhage
  • Doppler evaluation of the carotid artery

Nuclear medicine

• SPECT and PET scans:
  • Functional assessment of the brain
  • SPECT has wider availability and is less expensive.
  • PET has superior image quality (contrast and resolution) but is more expensive.
• Used with CNS tumors, seizures/epilepsy, and encephalitis

Intracranial Masses and Anomalies

Cerebral edema

• Processes related to shifts in water in the brain parenchyma in response to different forms of brain injury
• Types:
  • Vasogenic: 
    • Extracellular edema due to compromised blood–brain barrier (↑ permeability) 
    • Edema seen primarily in the white matter
    • Seen in mass/neoplasm or hemorrhage 
    • CT: gray-white matter differentiation with edema localized to the white matter
    • MRI: hyperintense T2
  • Cytotoxic: 
    • Water passes into the cell → hydropic cellular swelling
    • Edema primarily in the gray matter and without expansion of extracellular space
    • Caused by infarction/ischemia
    • Seen well in MRI with DWI
  • Combined: 
    • Ischemia: initially cytotoxic, then vasogenic changes occur hours later
    • Also in cases of trauma, infection, encephalopathy
  • Interstitial: 
    • Due to transependymal flow of CSF
    • Hydrocephalic

Hydrocephalus

• Increase in volume of CSF resulting in the expansion of the ventricular system
• May be due to the overproduction, decreased absorption, or obstruction of CSF flow 
• Classified as communicating (nonobstructive) and noncommunicating (obstructive)
• Imaging:
  • MRI (preferred imaging):
    • Detects malformations and tumors that could cause hydrocephalus
    • Provides information regarding CSF flow dynamics
  • Ultrasonography:
    • Used in newborns and young infants
    • Cannot be used when anterior fontanelles close
  • Finding: markedly dilated ventricles compared to the sulci

Congenital anomalies

• MRI is superior in detecting intracranial abnormalities (better details and tissue contrast).
• CT is used when evaluation of osseous structures is needed.

Tumors

• Types: 
  • Intracranial or intraaxial: within the cerebral parenchyma (e.g., gliomas, intracranial metastasis)
  • Extraaxial: outside the cerebral parenchyma, but within the skull (e.g., meningioma)
• MRI with contrast: provides optimal evaluation of brain tumors
• CT:
  • Lower resolution of soft tissues
  • Used in the following:
    • Emergency setting
    • View involvement of bony structures (i.e., bone destruction of tumors)
    • MRI contraindicated
• Examples:
  • Meningioma:
    • Lesion with dural tail (marginal dural thickening tapering peripherally) 
    • MRI: hypointense on T1- or hyperintense on T2-weighted images
    • CT: well-defined extraaxial mass, which can contain calcifications
  • Glioblastoma multiforme:
    • Malignant brain tumors that rapidly progress and have poor prognosis
    • MRI (best study): heterogeneously isointense or hypointense on T1, heterogeneously hyperintense on T2 and with rim enhancement 
  • Brain metastasis:
    • Invasion of cerebral tissue after systemic dissemination of malignancy (prostatic, uterine, GI, and breast tumors)
    • Contrast-enhanced MRI: most sensitive method

Ischemia and Hemorrhage

Stroke (ischemic)

Stroke is a medical emergency caused by interruption or reduction of blood supply to the brain. Neuroimaging is obtained on all patients in whom stroke is suspected to determine etiology, magnitude of damage, and management. 

Types of stroke:

• Ischemic: can be thrombotic (e.g., atherosclerosis) or embolic (e.g., atrial fibrillation)
• Hemorrhagic

Acute stroke evaluation:

• Non-contrast-enhanced CT (NCCT) is the initial test:
  • Advantages:
    • Sensitivity in detecting hemorrhage (thus affects management)
    • Rapid scan time
    • Widespread availability
  • NCCT findings of ischemic stroke:
    • Normal in hyperacute phase
    • Hyperdense vessel sign (“bright artery sign”) on NCCT indicates thrombus in the artery.
    • Loss of gray/white differentiation
    • Parenchymal hypodensity
    • Sulcal effacement
    • > 24 hours: better-defined hypodense area
• CTA:
  • Used for patients who are candidates for endovascular therapy in the setting of emergent large-vessel occlusion.
• MRI:
  • MRI with DWI: most sensitive test, detecting changes within 20–30 minutes of onset
  • Limited availability
  • Generally longer scan times
  • Difficulty of transporting and monitoring unstable patients
  • Certain contraindications (e.g., pacemakers)

Hemorrhage

• Multiple etiologies:
  • Hypertensive vasculopathy: most common cause of spontaneous intracerebral hemorrhage
  • Other: trauma, arteriovenous malformation, drugs, tumors, bleeding disorders, antithrombotic therapy
  • Type of bleeding and hemorrhage partly dependent on etiology
• Imaging:
  • NCCT (initial test of choice):
    • Hemorrhage = hyperdense area 
    • Mass effect may be apparent (midline shift, herniation, hydrocephalus).
  • MRI:
    • More sensitive
    • Used as adjunct if clinical suspicion for hemorrhage is high and CT is negative
    • In hyperacute phase, hemorrhage is hyperintense (T2-weighted).
• Different findings (intracranial hemorrhage):
  • Intracerebral (within the cerebral parenchyma):
    • Usually caused by hypertension, aneurysm rupture, amyloid deposition, and trauma
    • Can be accompanied by intraventricular hemorrhage (hyperdensity within cerebral ventricles)
  • Epidural (between the dura and the skull):
    • Usually caused by head trauma (tearing of the medial meningeal artery)
    • High density
    • Lens-shaped/biconvex pattern
    • Does not cross suture lines but can cross midline
  • Subdural (between the dura mater and arachnoid membranes):
    • Usually caused by trauma (damage to the bridging veins)
    • High density
    • Crescent-shaped
    • Crosses suture lines but does not cross midline
  • Subarachnoid (in the subarachnoid space):
    • Usually caused by an aneurysm rupture or trauma
    • Hyperdensity confined to the subarachnoid space, the sulci, fissures, and the basal cisterns

Brain Injury and Herniation

Traumatic brain injury (TBI)

• Definition: brain pathology or altered brain function stemming from an external force
• Injury mechanisms:
  • Coup: injury at the point of impact
  • Contrecoup: injury at the point opposite to the site of impact
• Head trauma results in:
  • Hemorrhage due to shearing forces applied to blood vessels until rupture
  • Diffuse axonal injury due to stretching forces applied to the axons 
• Imaging:
  • NCCT: done in the acute setting to identify need for neurosurgical intervention (e.g., hemorrhage) and emergent medical management
  • MRI:
    • More sensitive in detecting diffuse axonal injury and other parenchymal lesions
    • Obtained in persistent or progressive neurologic deficit with negative CT
    • For subacute evaluation of TBI

Herniation

• Definition: A portion of the brain is displaced from one compartment to another due to mass effect, causing a potentially life-threatening condition. 
• Imaging:
  • NCCT: 
    • Initial test of choice in the emergency setting
    • Readily available and can be obtained quickly
  • MRI: provides improved tissue characterization
• Findings:
  • Subfalcine: displacement of the cingulate gyrus under the falx cerebri 
  • Transtentorial: 
    • Downward displacement of the brain from the supratentorial compartment (e.g., central herniation, uncal herniation)
    • Upward displacement of the brain from the posterior fossa (e.g., cerebellar hemispheres)
  • Tonsillar: cerebellum displaced caudally and through the foramen magnum
  • External: herniation of the brain through a skull defect (e.g., traumatic or postsurgical)