Neuroimaging Essentials

Question 1

What are the two main types of neuroimaging methods?

Answer 1

1. MRI

• Best imaging of brain and spinal cord
• Multiple views (axial, sagittal, coronal) available-in scan sequences
• No radiation exposure
• Longer scanning time, images degraded by patient movement
• Prohibited in presence of pacemakers or certain implanted metal
• Tight, enclosed scanner is difficult for claustrophobic patients
• Contrast agent is gadolinium

2. CT

• Axial views, needs image reformatting to obtain other views
• Radiation exposure
• Shorter scanning time (advantages for unstable or agitated patients)
• Contrast agents are iodine based

Question 2

What are the steps of Computed tomography (CT)?

Answer 2

1. Emitted X-rays pass through a targeted cross-sectional level of the brain or spinal cord, picked up by detectors on the opposite side of the body
2. Multiple X-ray images are taken as the X-ray tube and its detectors rotate in a circular path around the brain or spinal cord
3. A computer creates a composite scan of each axial slice
4. The patient is slowly moved on a table within the scanner so the next axial image can be made, until the entire brain or spinal cord segment is imaged

Question 3

Magnetic Resonance Imaging (MRI)

Answer 3

1. proton in H ion spins on its axis (angular momentum)
2. spinning charged proton=small magnet (affected by EM waves and M fields)
3. protons spins align parallel to M field (created by MRI scanner) in z-axis
4. radio transmitter in scanner zaps plane with energy=vector shifted out of plane, but returns
5. rotation of vector=electrical generator=voltage detected by receiver coil in scanner
6. several RF pulses excite tissue, get signal recorded repeatedly=MRI image

***Notes:

• T1W: highlights anatomy, CSF is dark (low signal)
• T2W: highlights pathology, CSF is bright (high signal)
• FLAIR (fluid attenuation recovery): is like T2W, but the visually distracting high signal of CSF is removed from the images (=can more readily see lesions with white removed)
• Most lesions appear bright (high signal) on T2W or FLAIR MRI sequences

Question 4

What are the main abnormalities detected by neuroimaging?

Answer 4

• Acute intracranial hemorrhage
• Acute cerebral infarction
• Mass effect or edema
• Hydrocephalus
• CNS infection
• Brain tumors
• Multiple sclerosis
• Degenerative spine disease

Question 5

Acute hemorrhage

Answer 5

• Is HYPERdense (bright or white) on CT, whether inside or outside (subdural, or subarachnoid hemorrhage) the brain
• As time passes, any edema subsides, and the hematoma becomes isodense and then hypodense (dark or black) on CT
  
  • iso and hypo are in relation to the brain content
• Day 1-3 (acute): HYPERdense
• Day 3-14.4 (sub-acute): ISOdense
• Day 14.5 onward (chronic): HYPOdense
• **NOTE T1 vs. T2

Question 6

Acute Infarction

Answer 6

• MRI—best imaging, even small infarctions
  
  • DWI (diffusion weighted imaging) sequence provides earliest infarct detection since water diffusion is impaired in ischemic brain (but also in other lesions)
  • Infarcts on T2W or FLAIR (fluid attenuation inversion recovery) appear as a high signal or “brightness” within a vascular territory
• CT (without contrast)
  
  • Infarcts appear as a hypodensity or lucency within a vascular territory
  • Early infarcts may not be visible or only show subtle effacement of gray-white matter junction or sulci
  • Small lacunar infarcts may never be detected on CT

Question 7

Mass effect or edema

Answer 7

• Edema appears as a hypodensity or lucency (on CT) or increased, high signal intensity (on MRI T2W or FLAIR)
• Edema mainly involves the white matter (in the subcortical area), often sparing the cortical gyri “fingers”
• Contrast enhances lesions with a “leaky BBB”, as well as normal vascular structures
• Contrast may be needed to delineate any tumor or abscess amidst the surrounding edema
• Subfalcine or lateral brain shifts may occur from edema
  
  • lateral compression or shifting of the lateral ventricles
  • unilaterally obscured sulci or gray-white matter junction

Question 8

Hydrocephalus

Answer 8

• Ventricular enlargement without loss of brain tissue, related to impaired CSF flow
• Aqueductal stenosis
  
  • enlarged (because they wont drain properly) lateral and 3rd ventricles, NOT 4th
• Scarring or blockage of subarachnoid villi (from previous hemorrhage or infection)
  
  • enlarged lateral, 3rd and 4th ventricles

Question 9

CNS Infection

Answer 9

• Abscess
  
  • cavitary, encapsulated lesion (a “walled-off hole”–due to infection in the brain) with surrounding edema
  • better visualized when the capsule is delineated by contrast (ring-enhancing lesion)
  • from bacterial, TB, fungal, or parasitic infections
  • multiple abscesses may mimic metastatic cancer
• Encephalitis (brain) or myelitis (spinal cord)
  
  • focal edema with variable enhancement , usually from a viral infection
• Meningitis
  
  • infectious inflammation of meninges
  • leptomeningeal (brain coverings) enhancement may occur

Question 10

Brain Tumors

Answer 10

• Primary brain tumor
  
  • solitary, may be irregularly shaped, hemorrhagic or heterogeneous
  • Different Images for pt with glioblastoma multiforme–
    
    • CT without contrast:
      
      • subtle HYPERdensity
      • vague fingers
      • blunted sulci
    • T1 MRI with contrast:
      
      • can see wall of tumor
• Metastatic brain tumor
  
  • solitary or multiple, spherical, at gray-white matter junction of brain
    
    • rationale: thats where the larger arteries of the brain end their distribution, tumor cells are borne by blood an dreach BBB, end up at g-w matter jxn the expand, causing edema within the brain.
  • Images for man with confusion and falling: brain metastases
    
    • T1 MRI with contrast
      
      • spherical lesion causing the surrounding edema (edema is seen on FLAIR)
    • FLAIR MRI:
      
      • abnml high signal in cerebellum, medial occipital, and temporal
    • Post-contrast coronal MRI
• Epidural spinal cord metastasis
  
  • arise from vertebral bone (body), encroach upon spinal cord in its canal, thus causing neurological deficits
  • Ex: at T2; (RD) cervical spine MRI
    
    • low signal where T2 should be–tumor has eroded through, expanding into SC and canal

Question 11

MS

Answer 11

• Plaque lesions seen in periventricular white matter, brain stem or spinal cord
  
  • Seen best as high signal MRI lesions on T2W (Dawson’s fingers! + enhancing lesions/plaques {with new lesions}) or FLAIR images
• Acute lesions may enhance with contrast (b/c o breakdown of BBB due to inflammation)
• (in older pts) May appear very similar to chronic ischemic white matter lesions (so clinical knowledge of patient is critical)

Question 12

Degenerative Spine Disease

Answer 12

• Spondylosis, herniated discs and spinal stenosis—best seen with MRI
• If MRI cannot be done, a spinal CT may require intrathecal contrast (myelogram) to outline the spinal cord and its nerve roots
• Sagittal T2W of cervical spine:
  
  • spinal fluid should be white (surrounding the SC), can see herniated disc compressing the SC
• Types of bulge:
  
  • diffuse concentric
  • herniation; “toothpaste sign” + elevated ligament + irritation/stretching the nerve roots at that level