Intro to Neuroradiology Jenna Ford, MD PGY 3 Neurology

Learning Objectives Learn the fundamentals of common neuroimaging modalities Review the pros and cons of different imaging modalities Learn to identify common neurologic disease based on imaging characteristics Learn the basic method of CT scan windowing

CT Scan A series of XR images obtained at various angles and reformatted using software Radiodensity measured in Hounsfield Units (HU) related to how much the XR beam is weakened/attenuated by the material it is passing through All relative to water which is 0 HU CT Terminology (typically relative to brain) White/Bright = Hyperdense Black/Dark = Hypodense

CT Scan Indications Pros: Cons: Unstable patients, emergency room Acute stroke Looking for pathology that requires immediate intervention Pros: Fast!! Usually the first step Cheap Helpful for excluding large pathology (i.e. bleed, large ischemic stroke, mass, abscess, hydrocephalus) Cons: Less detail/information Radiation (not ideal in young and pregnancy) Low sensitivity in posterior fossa

CT Windowing Air -1000 Black Water Gray Cortical Bone +1000 White Gray Cortical Bone +1000 White Substance HU CSF +15 White Matter +20-30 Grey Matter +35-45 Acute Blood +45-65 Fat –100-(–15) Bone +700-3000

CT Windowing W = Window C = Center; L = Level (depending on the software) C Air -1000 Black Water Cortical Bone +1000 White W

Window Wide = >400 Lets you compare areas with different HU’s (bone) +300 +700 Water Black White

Window Narrow = 50-350 Lets you compare areas with similar HU’s (brain vs blood) 55 Water Black White 40

Center/Level This is what you set to be the middle of your window (i.e. gray) Dark Gray Light Gray 50 L 45 55 Black White Gray

CT Scan: First Signs of Stroke Hypodensity (edema) Vasogenic – fluid leaking out of damaged blood vessels, integrity of BBB has been compromised. Restricted to the white matter or highly vascular areas Cytotoxic – induced by cytokines and inflammatory cells that are present when actual cell death/damage occurs. Extends to the cortex, does not preferentially affect highly vascularized tissue (white and gray matter affected)

CT Scan: First Signs of Stroke Sulcal Effacement Loss of gray-white differentiation

CT Scan: First Signs of Stroke Hyperdense MCA sign Can be a soft call so be careful If present will be the first thing to appear on imaging, if you get imaging early enough this may be the only thing you see

CT Scan: ASPECT Score Alberta Stroke Program Early CT Score A 10-point pretreatment score that divides the MCA territory into 10 regions and identifies patients with stroke who are unlikely to have good outcome from thrombolysis The ASPECTS value is calculated from two standard axial CT cuts, one at the level of the thalamus and basal ganglia, and one just rostral to the ganglionic structures. 10 = normal CT scan 0 = diffuse ischemic involvement throughout the complete MCA territory A score of 7 or less predicts worse outcome at 3 months and symptomatic hemorrhage

CT Scan: ASPECTS Score

Case #1 ASPECTS 5

Case #2 ASPECTS 8

Case #3

Old Stroke vs New Stroke Encephalomalacia Abnormal tissue looks shriveled rather than plump No mass effect, can see ex-vacuo hydrocephalus Hypodense relative to new strokes; CSF is less dense than edema

CT Perfusion Useful in acute stroke to determine how much salvageable tissue is left; i.e. the penumbra CBV – Cerebral blood volume; amount of blood in that region CBF – Cerebral blood flow; amount of blood passing through that region in a given period of time TTP – Time to peak; MTT – Mean transit time (CBV/CBF); the amount of time that the blood spends in a give volume of circulation Infarct – no blood flow, dead Since blood is not really getting to this area the transit time will be very very long and MTT/TTP will be markedly increased Ischemic penumbra – flow is less but the body is actively compensating to continue perfusion Cerebral blood flow is only mildly diminished because cerebral blood volume is elevated to compensate for delayed collateral perfusion manifest as increased mean transit times. This pattern suggests robust collateral circulation at early stages of acute ischemic stroke.

CT Perfusion

CT Perfusion

MRI Patient lies in a large magnet which aligns all the protons (i.e. Hydrogen or water molecules) in the body. Radiowaves are then shot through the patient and returning signals are converted into an image MRI Terminology: White/Bright = hyperintense Black/Dark = Hypointense Enhancing refers to contrast only! Many different sequences and protocols depending on what you want to look at

MRI Indications Pros Cons: Acute stroke Further evaluation of CT findings Chronic/subacute symptoms Tumors/masses Infection (abscess, meningitis, encephalitis) Aging of blood Pros More detailed Safe in Pregnancy Can obtain vessel imaging without contrast Cons: Slower More contraindications (i.e. metal/implantable device, PPM, stents, AICD, bullet fragment) Expensive Patient must hold still, difficult if claustrophobic

T1 CSF – dark (hypointense) White Matter – bright (hyperintense) Good image for anatomy

T2 FLAIR – Fluid Attenuated Inversion Recovery CSF – bright (hyperintense) White Matter – dark (hypointense) T2 FLAIR – Fluid Attenuated Inversion Recovery Removes CSF! Great for looking at edema since that has a different protein make up than CSF and will remain bright Also will show other fluids with increased protein content such as certain cysts and abscesses

DWI – Diffusion Weighted Image Shows restricted diffusion of water molecules…i.e. ischemia! Dying neurons will swell as their Na-K pumps stop working and water accumulates within the cell…diffusion of water is restricted intracellularly and this lights up on DWI Bright spots on DWI are either: Ischemia “T2 shine-through” Must compare to ADC and FLAIR

ADC – Apparent Diffusion Coefficient Dark areas that are bright on DWI confirm ischemia Very susceptible to artifact so this is never the first scan you look at when looking for ischemia Areas bright on DWI that are not dark on ADC are most likely “T2 shine-through”, i.e. chronic Look at T2 FLAIR sequence to confirm

SWI – Susceptibility Weighted Image Shows hemosiderin deposits (old blood) and calcifications Useful when looking for old hemorrhages that don’t show up on CT Stroke with hemorrhagic conversion Old hemorrhage of brain tumors Cerebral amyloid angiopathy Different institutions have different versions/names for this type of sequence GRE (gradient echo), STIR, SPIN, SWAN

Post Contrast (Typically T1) Contrast Study vs Angiogram…Not the same thing! Contrast is to evaluate breakdown in the blood brain barrier and identify abnormal brain parenchyma or tissue Angiograms are to evaluate the blood vessels T1+C vs T1+C Fat sat Fat sat just makes all of the fat tissue have decreased signal so that it doesn’t obscure any contrast enhancing lesions

CT w/wo contrast

CT Angiogram

MRI w/wo contrast

MRI w/wo contrast