introduction to neuroimaging · • cross-sectional view of spinal canal along with spinal cord and...
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Introduction to Neuroimaging
Aaron S. Field, MD, PhD
Assistant Professor of Radiology
Neuroradiology Section
University of Wisconsin–Madison
Updated 7/17/07
Neuroimaging ModalitiesRadiography (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
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)
―Duplex‖
Radiography (X-Ray)
Radiography (X-Ray)
Primarily used for spine:
• Trauma
• Degenerative Dz
• Post-op
Fluoroscopy (Real-Time X-Ray)
Fluoro-guided procedures:
• Angiography
• Myelography
Fluoroscopy (Real-Time X-Ray)
Fluoroscopy (Real-Time X-Ray)
Digital Subtraction Angiography
Fluoroscopy (Real-Time X-Ray)
Digital Subtraction Angiography
• 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
Indications:
Advantages:
Disadvantages:
Digital Subtraction Angiography
Fluoroscopy (Real-Time X-Ray)
Myelography
Lumbar or cervical puncture
Inject contrast intrathecally
with fluoroscopic guidance
Follow-up with post-myelo CT
(CT myelogram)
Myelography
• 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
Indications:
Advantages:
Disadvantages:
Ultrasound
carotid
US transducer
Ultrasound
• 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
Indications:
Advantages:
Disadvantages:
Ultrasound – Gray Scale
Gray-scale image of carotid artery
Ultrasound – Gray Scale
Gray-scale image of carotid artery
Plaque in ICA
Ultrasound - Color Doppler
Peak Systolic Velocity (cm/sec) ICA Stenosis (% diameter)
125 – 225 50 – 70
225 – 350 70 – 90
>350 >90
Computed Tomography (CT)
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‖
Computed Tomography
Typical HU Values:
Air –1000
Fat –100 to –40
Water 0
Other fluids (e.g. CSF) 0–20
White matter 20–35
Gray matter 30–40
Blood clot 55–75
Calcification >150
Bone 1000
Metallic foreign body >1000
Brain
Computed Tomography
Attenuation: High or Low?
High:
1. Blood, calcium
2. Less fluid / more tissue
Low:
1. Fat, air
2. More fluid / less tissue
Air –1000
Fat –100 to –40
Water 0
Other fluids 0–20
White matter 20–35
Gray matter 30–40
Blood clot 55–75
Calcification >150
Bone 1000
Metallic foreign body >1000
Computed Tomography
“Soft Tissue Window” “Bone Window”
Computed Tomography
Computed Tomography
Scan axially…
…stack and re-slicein any plane―2D Recons‖
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)
Skull and skull base, vertebrae
Fractures
Skull and skull base, vertebrae
Multiple Myeloma Osteoma
Ventricles
Hydrocephalus
Intracranial masses, mass effects
Solid mass Cystic mass
Intracranial masses, mass effects
L hemisphere swelling Generalized swelling
Acute Hemorrhage
Intraparenchymal Subarachnoid Subdural Epidural
Acute Ischemia
Loss of gray-white distinction and swelling in known arterial territory
Calcification
Hyperparathyroidism
1. Rapid IV contrast bolus
2. Dynamic scanning during arterial phase
3. Advanced 2D and 3D Reconstructions:
2D multi-planar (sagittal, coronal)
Volume–rendered 3D recons
CT Angiography
CT Angiography - Head
CT Angiography - HeadCircle of Willis
Aneurysms
Vascular Malformations
CT Angiography - Neck
Carotid
bifurcations
Vertebral arteries
Aortic arch
CT Angiography
3D Volume Rendering
• Atherosclerosis
• Thromboembolism
• Vascular dissection
• Aneurysms
• Vascular malformations
• Penetrating trauma
CT Angiography - Indications
CT Perfusion CBV
CBF
MTT
Hemodynamic Parameters Derived From Concentration-Time Curves
Artery
VeinBolus
arrival
Hemodynamic Parameter Maps
Transit Time (sec)
Blood Flow (mL/min/g)
Blood Volume (mL/g)
• 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)
CT Myelography
CT Myelography
CT Myelography
Magnetic Resonance (MR)
Hydrogen proton in water or fat
MRI
Magnetic Resonance Imaging
COMPUTER
Magnetic Resonance Imaging
magnetic
field
RF
Transmitter Receiver
RF = Radio Frequency energy
Received signal
MRI Safety: The Magnet is Always On!
Magnetic Resonance Safety
Typically safe*:
• Orthopedic hardware
• Surgical clips, staples, sutures (older devices must be checked!)
• Intravascular stents/filters
* This is an incomplete list and there are many exceptions to every ―rule‖When in doubt, check it out!
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.
MRI Safety Test:
Will it: Move? Torque? Get hot? Pass a current? Malfunction? Become a projectile? Get stuck in scanner?
Magnetic ResonanceExcited protons relax back to equilibrium
Relaxation rates depend onlocal molecular environment
T1
T2
“T1-weighted” “T2-weighted” w/ fat suppression
Magnetic Resonance
Magnetic Resonance
Arachnoid Cyst
T2T1
Magnetic Resonance
T2 T2 w/ fat suppression
Magnetic Resonance
T2 T2 w/ fat suppression
Magnetic Resonance
T2 T2 w/ water suppression
(T2-FLAIR)
Magnetic Resonance
T2 T2*
Accentuating blood/calcium
“blooming”
NORMAL CYTOTOXIC EDEMA (Acute Ischemia)
Diffusion
MR Signal
Diffusion MR Imaging
Magnetic Resonance
Imaging Diffusion
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
DWI
Magnetic Resonance Angiography
Axial ―source‖ images… …reformatted to ―maximum intensity projections‖ (MIP)
Multiple projections allow 3D-like displayNo need for IV contrast!
Time-Resolved MRA (TRICKS)
IV contrast bolus reveals temporal dynamics
MRA Perfusion MR
Magnetic Resonance Angiography with Perfusion MR
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)
1. 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)
2. Tissue that gets brighter with IV contrast is said to “enhance”
(Brightness, in and of itself, is not enhancement!)
3. 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
IV Contrast in Neuroimaging
1. Vessels
2. Meninges
pachy = dura
lepto = pia-arachnoid
3. Circumventricular organs(structures outside BBB)
Pineal gland
Pituitary gland
Choroid plexus
4. Absent/leaky BBB
Some tumors
Inflammation
Infarction
IV Contrast in Neuroimaging
Enhancement:
EnhancementT1 T1+C
Hemorrhagic melanoma metastasis
IV Contrast: Is it Indicated?
• Trauma
• R/O hemorrhage
• Hydrocephalus
• Dementia
• Epilepsy
• Neoplasm
• Infection
• Vascular disease
• Inflammatory disease
Typically not Typically yes
Always best to provide detailed indication!
Radiologist will protocol exam accordingly
MR vs. CT
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
CT MRAdvantages:
• 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
Introduction to Neuroimaging
Aaron S. Field, MD, PhD
Assistant Professor of Radiology
Neuroradiology Section
University of Wisconsin–Madison