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Introduction to Neuroimaging

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Introduction to Neuroimaging Introduction to Neuroimaging Aaron S. Field, MD, PhD Assistant Professor of Radiology Neuroradiology Section University of Wisconsin–Madison Updated 7/17/07 Neuroimaging Modalities Radiography (X-Ray) Magnetic Resonance (MR) Fluoroscopy (guided procedures) • MR Angiography/Venography (MRA/MRV) • Angiography • Diffusion and Diffusion Tensor • Diagnostic MR • Interventional • Perfusion MR • Myelography • MR Spectroscopy (MRS) Ultrasound (US) • Functional MR (fMRI) • Gray-Scale Nuclear Medicine ―Duplex‖ • Color Doppler • Single Photon Emission Computed Tomography (SPECT) Computed Tomography (CT) • Positron Emission Tomography • CT Angiography (CTA) (PET) • Perfusion CT • CT Myelography 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 Digital Subtraction Angiography Indications: • Aneurysms, vascular malformations and fistulae • Vessel stenosis, thrombosis, dissection, pseudoaneurysm • Stenting, embolization, thrombolysis (mechanical and pharmacologic) Advantages: • Ability to intervene • Time-resolved blood flow dynamics (arterial, capillary, venous phases) • High spatial and temporal resolution Disadvantages: • Invasive, risk of vascular injury and stroke • Iodinated contrast and ionizing radiation 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 Indications: • Spinal stenosis, nerve root compression • CSF leak • MRI inadequate or contraindicated Advantages: • Defines extent of subarachnoid space, identifies spinal block Disadvantages: • Invasive, complications (CSF leak, headache, contrast reaction, etc.) • Ionizing radiation and iodinated contrast • Limited coverage Ultrasound US transducer carotid Ultrasound Indications: • Carotid stenosis • Vasospasm - Transcranial Doppler (TCD) • Infant brain imaging (open fontanelle = acoustic window) Advantages: • Noninvasive, well-tolerated, readily available, low cost • Quantitates blood velocity • Reveals morphology (stability) of atheromatous plaques Disadvantages: • Severe stenosis may appear occluded • Limited coverage, difficult through air/bone • Operator dependent Ultrasound – Gray Scale Gray-scale image of carotid artery Ultrasound – Gray Scale Plaque in ICA Gray-scale image of carotid artery 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 Brain Gray matter 30–40 Blood clot 55–75 Calcification >150 Bone 1000 Metallic foreign body >1000 Computed Tomography Attenuation: High or Low? High: Low: 1. Blood, calcium 1. Fat, air 2. Less fluid / more tissue 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-slice ―2D Recons‖ in any plane 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 CT Angiography 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 - Head CT Angiography - Head Circle of Willis Vascular Malformations Aneurysms CT Angiography - Neck Carotid Vertebral bifurcations arteries Aortic arch CT Angiography 3D Volume Rendering CT Angiography - Indications • Atherosclerosis • Thromboembolism • Vascular dissection • Aneurysms • Vascular malformations • Penetrating trauma CT Perfusion CBV CBF MTT Hemodynamic Parameters Derived From Concentration-Time Curves Bolus Vein arrival Artery Hemodynamic Parameter Maps Transit Time Blood Flow Blood Volume (sec) (mL/min/g) (mL/g) 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) CT Myelography CT Myelography Magnetic Resonance (MR) Hydrogen proton MRI in water or fat Magnetic Resonance Imaging Magnetic Resonance Imaging Transmitter Receiver RF RF = Radio Frequency energy Received signal magnetic COMPUTER field MRI Safety: The Magnet is Always On! 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*: Typically unsafe*: • Orthopedic hardware • Cardiac pacemakers (and other electrical devices) • Surgical clips, staples, sutures (older devices must be checked!) • Some older aneurysm clips • Intravascular stents/filters • 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! Magnetic Resonance Excited protons relax back to equilibrium T2 T1 Relaxation rates depend on local molecular environment Magnetic Resonance “T1-weighted” “T2-weighted” w/ fat suppression Magnetic Resonance T1 T2 Arachnoid Cyst 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 Accentuating blood/calcium “blooming” T2 T2* Diffusion MR Imaging NORMAL CYTOTOXIC EDEMA (Acute Ischemia) Diffusion MR Signal 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 Magnetic Resonance Angiography Axial ―source‖ images… …reformatted to ―maximum intensity projections‖ (MIP) Multiple projections allow No need for IV contrast! 3D-like display Time-Resolved MRA (TRICKS) IV contrast bolus reveals temporal dynamics Magnetic Resonance Angiography with Perfusion MR MRA 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) IV Contrast in Neuroimaging 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 Enhancement: 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 Enhancement T1 T1+C Hemorrhagic melanoma metastasis IV Contrast: Is it Indicated? Typically not Typically yes • Trauma • Neoplasm • R/O hemorrhage • Infection • Hydrocephalus • Vascular disease • Dementia • Inflammatory disease • Epilepsy Always best to provide detailed indication! Radiologist will protocol exam accordingly MR vs. CT CT MR Advantages: Advantages: • Simpler, cheaper, more accessible • Much broader palette of tissue contrasts (including functional and molecular) yields • Tolerated by claustrophobics greater anatomic detail and more • No absolute contraindications comprehensive analysis of pathology • Fewer pitfalls in interpretation • No ionizing radiation • Better than MR for bone detail • Direct multi-planar imaging Disadvantages: • IV contrast better tolerated (in most pts.) • Ionizing radiation Disadvantages: • IV contrast complications • Higher cost, limited access • Need recons for multi-planar • Difficult for unstable patients • Limited range of tissue contrasts • 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.
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