introduction to neuroimaging · • cross-sectional view of spinal canal along with spinal cord and...

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