Neuroradiology

Computerized Tomography

·      detects tissue densities

·      X-ray passes through the patient and is partially absorbed by tissues

·      Amount of energy absorbed depends on the density of the tissues traversed

·      Signal Densities

·      Hyperdense structures = white (i.e. bone)

·      Isodense = gray

·      Hypodense structures = black (i.e. air)

·      White matter – High myelin (fat) content  à hypodense

·      Gray matter – brighter than white matter

CT-Myelography

·      Iodinated contrast dye is introduced into the CSF via lumbar puncture

·      Allows better visualization of nerve roots and spinal cord impingement

Magnetic Resonance Image

·      detects tissue intensities

T1 Weighted Images

·      Good for structural anatomy

·      CSF - dark

·      Tissues

o   Gray matter - dark gray

o   White matter – light gray

·      Air - dark (no hydrogen atoms)

T2 Weighted Images

·      Good for edema and pathologies

·      T2 = think water image; water is bright; look for dark disease

·      CSF - bright

·      Tissues – looks like a negative image

o   gray matter – light gray

o   white matter – dark gray

·      Air - dark (no hydrogen atoms)

FLAIR

·      obtains T2-weighted contrast while keeping the CSF dark

·      effective to identify periventricular ischemic foci and multiple sclerosis plaques and to differentiate perivascular spaces (dark) from ischemic zones (bright)

Diffusion-Weighted Imaging (DWI)

·      assesses the ease of water diffusion through cells

·      Ischemic Stroke – Immediately bright on DWI and dark on ADC

o   cytotoxic edema following stroke restricts movement of water

o   area of restricted diffusibility corresponds to high signal intensity on DWI images and a reduction in the apparent diffusion coefficient (ADC)

T2 shine-through

·      refers to high signal on DWI images that is not due to restricted diffusion, but rather to high T2 signal which 'shines through' to the DWI image

·      bright areas on dwi can be from stroke or t2 shinethrough

·      stroke areas will be dark on adc while t2 shinethrough will continue to be bright

Understanding MRI

·      MRI has a superconducting coil that produces a magnetic field

·      Magnetic field causes protons (Hydrogen atoms) in the body to align toward or away from it

o   Those facing opposite direction will cancel out

o   Small excess will align with the magnetic field, creating the MR signal

o   The higher the magnetic field strength (higher Tesla scanners), the more net protons that will align with the field contributing to a higher signal-to-noise ratio

·      Radio frequency (RF) pulse is applied and the energy is absorbed by the proton pushing it to a higher energy state

·      When the RF pulse is turned off, the proton slowly realigns back with magnetic field

·      As is realigns it gives off electromagnetic radiation in the form of RF waves which are picked up by the MRI receiving coil

·      The gradual return to thermal equilibrium is called the T1 Relaxation

·      T2 Relaxation is the random interaction between two spinning protons that causes the loss of signal

Hemorrhage

CT

·      Hyperdense immmediately from coagulation of blood

·      Isodense after 1 week  once clot breaks down

·      Hypodense after 2-3 weeks

·      suspected intracranial hemorrhage should receive non-contrast CT scan

o   small hemorrhages appear as whitish areas at the base of the brain

o   hemorrhages can be masked by the normally hyperdense contrast material in blood vessels and meninges at the brain base

MRI

·      T1: George Washington Bridge (Gray/White/Black)

·      T2: Oreo cookie (Black/White/Black).

Ischemic Stroke

CT

·      Cannot immediately be seen

·      Cell death and edema must occur to be detected

·      Persistent areas of hypodensity may remain with eventual gliosis and necrosis

MRI

·      Cells increase in water content (edema) within a few hours after stroke and are immediately visible

Neoplasms

CT

·      Hypodense, hyperdense or isodense depending on type and stage

·      May contain areas of calcification, hemorrhage or fluid-filled cysts

·      May produce surrounding edema that is hypodense

·      IV contrast dye is often helpful in visualization

Intravenous contrast

·      Denser than brain, hyperdense (white) on CT

·      Appears in areas of increased vascularity or break-down of the BBB

·      gadolinium used in MRI is less nephrotoxic and less prone to allergic reactions than iodinated contrast agents used in CT

CT versus MRI

·      dense bones at the base of the skull obscure adjacent areas with "shadowing" artifact on CT

·      MRI provides clear images of basilar structures such as brain-stem, cerebellum and pituitary fossa

·      spinal cord is also more clearly visible on MRI for similar reasons

·      CT depends on overall tissue density

·      MRI depends on proton density

·      Two cases when CT scan is better than MRI: head trauma, hemorrhagic stroke

Imaging planes

·      Angle of the axial CT slice is sometimes adjusted by a few degrees off the true plane

·      enables the whole brain to be covered using fewer slices

·      decreases radiation exposure to the eyes

·      Scout image – included on all CT and MRI films so that the exact angle of scanning can be documented