Introduction to Neuroimaging

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Introduction to Neuroimaging
Aaron S. Field, MD, PhD Assistant Professor of
Radiology Neuroradiology Section University of
WisconsinMadison
Updated 7/17/07
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Neuroimaging Modalities

• Magnetic Resonance (MR)
• MR Angiography/Venography (MRA/MRV)
• Diffusion and Diffusion Tensor MR
• Perfusion MR
• MR Spectroscopy (MRS)
• Functional MR (fMRI)
• Nuclear Medicine
• Single Photon Emission Computed Tomography
  (SPECT)
• Positron Emission Tomography (PET)

• Radiography (X-Ray)
• Fluoroscopy (guided procedures)
• Angiography
• Diagnostic
• Interventional
• Myelography
• Ultrasound (US)
• Gray-Scale
• Color Doppler
• Computed Tomography (CT)
• CT Angiography (CTA)
• Perfusion CT
• CT Myelography

Duplex
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Radiography (X-Ray)
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Radiography (X-Ray)

• Primarily used for spine
• Trauma
• Degenerative Dz
• Post-op

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Fluoroscopy (Real-Time X-Ray)

• Fluoro-guided procedures
• Angiography
• Myelography

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Fluoroscopy (Real-Time X-Ray)
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Fluoroscopy (Real-Time X-Ray)
Digital Subtraction Angiography
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Fluoroscopy (Real-Time X-Ray)
Digital Subtraction Angiography
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Digital Subtraction Angiography
Indications

• Aneurysms, vascular malformations and fistulae
• Vessel stenosis, thrombosis, dissection,
  pseudoaneurysm
• Stenting, embolization, thrombolysis (mechanical
  and pharmacologic)
• Ability to intervene
• Time-resolved blood flow dynamics (arterial,
  capillary, venous phases)
• High spatial and temporal resolution
• Invasive, risk of vascular injury and stroke
• Iodinated contrast and ionizing radiation

Advantages
Disadvantages
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Fluoroscopy (Real-Time X-Ray)
Myelography
Lumbar or cervical puncture Inject contrast
intrathecally with fluoroscopic
guidance Follow-up with post-myelo CT (CT
myelogram)
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Myelography
Indications

• Spinal stenosis, nerve root compression
• CSF leak
• MRI inadequate or contraindicated
• Defines extent of subarachnoid space, identifies
  spinal block
• Invasive, complications (CSF leak, headache,
  contrast reaction, etc.)
• Ionizing radiation and iodinated contrast
• Limited coverage

Advantages
Disadvantages
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Ultrasound
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Ultrasound
Indications

• Carotid stenosis
• Vasospasm - Transcranial Doppler (TCD)
• Infant brain imaging (open fontanelle acoustic
  window)
• Noninvasive, well-tolerated, readily available,
  low cost
• Quantitates blood velocity
• Reveals morphology (stability) of atheromatous
  plaques
• Severe stenosis may appear occluded
• Limited coverage, difficult through air/bone
• Operator dependent

Advantages
Disadvantages
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Ultrasound Gray Scale
Gray-scale image of carotid artery
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Ultrasound Gray Scale
Plaque in ICA
Gray-scale image of carotid artery
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Ultrasound - Color Doppler
Peak Systolic Velocity (cm/sec) ICA Stenosis
( diameter) 125 225 50 70 225
350 70 90 gt350 gt90
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Computed Tomography (CT)
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Computed Tomography
A CT image is a pixel-by-pixel map of X-ray beam
attenuation (essentially density) in
Hounsfield
Units (HU)
HUwater 0
Bright hyper-attenuating or hyper-dense
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Computed Tomography
Typical HU Values
Air 1000 Fat 100 to 40 Water 0 Other
fluids (e.g. CSF) 020 White matter 2035 Gray
matter 3040 Blood clot 5575 Calcification gt1
50 Bone 1000 Metallic foreign body gt1000
Brain
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Computed Tomography
Attenuation High or Low?

• High
• Blood, calcium
• Less fluid / more tissue

• Low
• Fat, air
• More fluid / less tissue

Air 1000 Fat 100 to 40 Water 0 Other
fluids 020 White matter 2035 Gray
matter 3040 Blood clot 5575 Calcification gt1
50 Bone 1000 Metallic foreign body gt1000
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(No Transcript)
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Computed Tomography
Soft Tissue Window Bone Window
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Computed Tomography
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Computed Tomography
Scan axially
stack and re-slice in any plane
2D Recons
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CT Indications

• Skull and skull base, vertebrae
• (trauma, bone lesions)
• Ventricles
• (hydrocephalus, shunt placement)
• Intracranial masses, mass effects
• (headache, N/V, visual symptoms, etc.)
• Hemorrhage, ischemia
• (stroke, mental status change)
• Calcification
• (lesion characterization)

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Skull and skull base, vertebrae
Fractures
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Skull and skull base, vertebrae
Multiple Myeloma
Osteoma
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Ventricles
Hydrocephalus
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Intracranial masses, mass effects
Solid mass
Cystic mass
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Intracranial masses, mass effects
L hemisphere swelling
Generalized swelling
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Acute Hemorrhage
Intraparenchymal Subarachnoid
Subdural Epidural
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Acute Ischemia
Loss of gray-white distinction and swelling in
known arterial territory
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Calcification
Hyperparathyroidism
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CT Angiography

• Rapid IV contrast bolus
• Dynamic scanning during arterial phase
• Advanced 2D and 3D Reconstructions
• 2D multi-planar (sagittal, coronal)
• Volumerendered 3D recons

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CT Angiography - Head
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CT Angiography - Head
Circle of Willis
Vascular Malformations
Aneurysms
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CT Angiography - Neck
Carotid bifurcations
Vertebral arteries
Aortic arch
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CT Angiography 3D Volume Rendering
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CT Angiography - Indications

• Atherosclerosis
• Thromboembolism
• Vascular dissection
• Aneurysms
• Vascular malformations
• Penetrating trauma

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CT Perfusion
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Hemodynamic Parameters Derived From
Concentration-Time Curves
Bolus arrival
Vein
Artery
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Hemodynamic Parameter Maps
Transit Time (sec)
Blood Flow (mL/min/g)
Blood Volume (mL/g)
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CT Myelography

• Spinal CT immediately following conventional
  myelogram
• Cross-sectional view of spinal canal along with
  spinal cord and nerve roots
• Assess spinal stenosis/nerve root compression
  (e.g. disc
  herniation, vertebral fracture, neoplasm)

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CT Myelography
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CT Myelography
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Magnetic Resonance (MR)
Hydrogen proton in water or fat
MRI
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Magnetic Resonance Imaging
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RF Radio Frequency energy
Received signal
magnetic field
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MRI Safety The Magnet is Always On!
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Magnetic Resonance Safety
MRI Safety Test Will it Move? Torque? Get
hot? Pass a current? Malfunction? Become a
projectile? Get stuck in scanner?

• Typically safe
• Orthopedic hardware
• Surgical clips, staples, sutures (older devices
  must be checked!)
• Intravascular stents/filters

• Typically unsafe
• Cardiac pacemakers (and other
  electrical devices)
• Some older aneurysm clips
• Metal fragments in orbit (1 case
  report)
• Oxygen tanks, carts, chairs, stools, IV poles,
  gurneys, etc.
• Some cosmetics, tattoos, jewelry, hairpins, etc.
• Pager, watch, wallet, ID badge, pen, keys,
  pocketknife, etc.

This is an incomplete list and there are many
exceptions to every rule When in doubt,
check it out!
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Magnetic Resonance
Excited protons relax back to equilibrium

T2
T1
Relaxation rates depend on local molecular
environment
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Magnetic Resonance
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Magnetic Resonance
T2
T1
Arachnoid Cyst
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Magnetic Resonance
T2 T2 w/ fat suppression
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Magnetic Resonance
T2 T2 w/ fat suppression
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Magnetic Resonance
T2 T2 w/ water
suppression
(T2-FLAIR)
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Magnetic Resonance
Accentuating blood/calcium
blooming
T2
T2
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Diffusion MR Imaging
NORMAL
CYTOTOXIC EDEMA (Acute Ischemia)
Diffusion ? MR Signal ?
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Magnetic Resonance
Imaging Diffusion
DWI
Highly sensitive to acute ischemia within a
few hours! No other imaging is more sensitive to
acute ischemia although perfusion imaging
reveals hypoperfused tissue at risk for
ischemia
Acute left MCA infarction
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Magnetic Resonance Angiography
Axial source images
reformatted to maximum intensity projections
(MIP) Multiple projections allow 3D-like display
No need for IV contrast!
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Time-Resolved MRA (TRICKS)
IV contrast bolus reveals temporal dynamics
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Magnetic Resonance Angiography with Perfusion MR
MRA Perfusion MR
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Magnetic Resonance
Tissue contrast in MR may be based on

• Proton density
• Water/fat/protein content
• Metabolic compounds (MR Spectroscopy)
• e.g. Choline, creatine, N-acetylaspartate,
  lactate
• Magnetic properties of specific molecules
• e.g. Hemoglobin
• Diffusion of water
• Perfusion (capillary blood flow)
• Bulk flow (large vessels, CSF)

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IV Contrast in Neuroimaging

• CT Iodine-based
• Iodine is highly attenuating of X-ray beam
  (bright on CT)
• MRI Gadolinium-based
• Gadolinium is a paramagnetic metal that hastens
  T1 relaxation of nearby water protons (bright on
  T1-weighted images)
• Tissue that gets brighter with IV contrast is
  said to enhance (Brightness, in and of itself,
  is not enhancement!)
• Enhancement reflects the vascularity of tissue,
  but
• The blood-brain barrier keeps IV contrast out of
  the brain!
• Enhancement implies BBB is absent or
  dysfunctional
• Remember Some brain anatomy lives outside the
  BBB

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IV Contrast in Neuroimaging
Enhancement

• Vessels
• Meninges
• pachy dura
• lepto pia-arachnoid
• Circumventricular organs (structures outside BBB)
• Pineal gland
• Pituitary gland
• Choroid plexus
• Absent/leaky BBB
• Some tumors
• Inflammation
• Infarction

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Enhancement
T1
T1C
Hemorrhagic melanoma metastasis
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IV Contrast Is it Indicated?
Typically not
Typically yes

• Trauma
• R/O hemorrhage
• Hydrocephalus
• Dementia
• Epilepsy

• Neoplasm
• Infection
• Vascular disease
• Inflammatory disease

Always best to provide detailed
indication! Radiologist will protocol exam
accordingly
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MR vs. CT
CT
MR

• Advantages
• Simpler, cheaper, more accessible
• Tolerated by claustrophobics
• No absolute contraindications
• Fewer pitfalls in interpretation
• Better than MR for bone detail
• Disadvantages
• Ionizing radiation
• IV contrast complications
• Need recons for multi-planar
• Limited range of tissue contrasts

• Advantages
• Much broader palette of tissue contrasts
  (including functional and molecular) yields
  greater anatomic detail and more comprehensive
  analysis of pathology
• No ionizing radiation
• Direct multi-planar imaging
• IV contrast better tolerated (in most pts.)
• Disadvantages
• Higher cost, limited access
• Difficult for unstable patients
• Several absolute contraindications (cardiac
  pacer, some aneurysm clips, etc.)
• Claustrophobics may need sedation
• Image interpretation more challenging
• Lacks bone detail

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Introduction to Neuroimaging
Aaron S. Field, MD, PhD Assistant Professor of
Radiology Neuroradiology Section University of
WisconsinMadison