basic modalities for CNS imaging
-CT with our without contrast
-Nuclear Imaging (PET or SPECT)
what to look for in each slide?
-brain symmetry (shift/compression)
-brain tissue-focal changes?
-skull and soft tissues
-secondary windows (look for fractures or blood)
-size, shape etc
describe computerized tomography
-distinguishes among tissues based on x-ray absorption
-Hounsfield units determine level of contrast (air=-1000, water-0, bone = 1000)
makes bone white
highlights blood-makes it white
makes brain tissue lighter so you can see structures better
-distinguishes among tissue characteristics at the molecular level
-signal intensity determines pixel value
what does signal intensity depend on?
-weighting of the image and composition of the tissue
describe T1 tissue contrast
-fat appears bright (short T1)
-skin and bone appear gray (medium T1)
-gray matter and fluids appear dark (long T1)
-contrast appears bright
based on relaxation time
describe T2 contrast:
-fat appears dark (short T2)
-bone appears dark
-gray matter and fluids (long T2) are bright
-blood vessels are dark
based on energy decay time
common CNS injuries?
degenerative disorders (of brain and spine)
general findings with most brain/spine disorders?
what does edema in the brain look like on CT?
-areas of hypodensity
-greatest in white matter
-mass effect-->flattening of gyri, displacement/deformation of ventricles/midline shift
-note that the brain can return to normal from nonhemmorrhagic edea or brain atrophy from shearing
edema on MRI?
-decreased signal on T1, increased on T2
-enhancement with contrast
what does brain atrophy look like?
-ipsilateral dilation of basal cisterns and ventricles
-thinning of gyri
-ex vacuo (?) dilation of ventricles
man hits head on curb with no helmet. what are the potential injuries and what is the time frame in which they occur?
1) closed head trauma/diffuse axonal injury-->immediate to delayed
2) Coup/Contra Coup-->immediate with late findings
4) epidural hematoma-->minutes to hours
5) subdural hematoma-->minutes to weeks
6) secondary edema--> 24 hours to 1 week
7) secondary infection from open skull fx-->24 hours to 1 week
best imaging modality to determine foreign objects in brain?
-if it came from the outside in-->best is the CT scan, because it brings a little bit of air with it, creating a halo around the object
guidelines for when to obtain imaging
-feel depressed skull fracture
-observation in the ER is in 15 minute increments (billed by 15 minutes)
-thinking of this, the CT scan is the cheaper way to go
patient is declining after CT scan-now what? pt does not respond to sternal rub
-looking for edema now so you want to get an MRI for the second image
common areas for trauma?
-frontal + temporal lobe
what to look for on CT for cortical contusions:
-look for scalp swelling to focus on where the coup may be
what do focal lesions look like?
-multiple poorly defined areas of low attenuation with irregular contour (edema), intermixed with irregular areas of increased density (petechial hemorrhage)
-diffuse cerebral swelling without hemorrhage in immediate post-trauma period bc of hyperemia and ischemic edema
-some amount of contrast enhancement from leaking caps
-isodense hemorrhage after 2-3 weeks
what is the best modality for initial detection of contusional edema:
-MRI gives accurate portrayal of extent of the lesion
describe hemorrhagic lesions on MRI
-initially decreased signal density, surrounded by hyperintense edema on T2
-hyperintense on T1 and T2 in subacute phase due to methemoglobin
-chronic phase: hyperintense gliosis and hypointense hemosiderin on T2
nonhemorrhagic lesions on MRI?
-hypointense on T1 and hyperintense on T2
what does diffuse axonal injury look like on ct vs mri
ct=foci of decreased density
mri= multiple small oval.round foci of decreased intensity on T1 and increased on T2
CT of epidural hematoma
-look for fracture line
--lens shaped (lenticular, elliptical, biconves)
-does not cross suture lines
-stretching of vessels
mri of epidural hematoma
-not as good early on as CT
-hyperintense stripes between blot and dura on T1 and T2
-->T1= isointense to minimally hypointense
-->T2= markedly hypointense
-->hyperintense on T1 (deoxyhemoglobin converted to methemoglobin)
acute subdural hematoma
-cresent shaped fluid collection between skull and cerebral hemisphere
-concave inner margin
-convex outer margin
CT of acute subdural hematoma
less than 1 week= hyperdense
1-2 weeks = isodense
3-4 weeks= hypodense
subacute subdural hematoma
-mass effect! (squish things)
-can become lens shaped later
-displacenement of sulci, ventricles and parenchyma, midline shift etc.
-if bilateral, no midline shift
just flip card over
-most likely cause= hemorrhagic stroke (angry, stressed, high BP)
-best imaging modality=CT scan is best for hemorrhagic stroke
-choice of imaging: ischemic=TPA
-need to demonstrate blood which is a contraindication for TPA (would not give TPA for hemorrhagic stroke)
-80-85% of strokes
-associated with intermittant claudication, angina, carotoid bruits or peripheral vascular disease (start with MRI, then get CT)
-intracerebral or subarachnoid
-15% of strokes
-associated with HTN
-CT first then MRI later
CT findings in brain infarct
-early=obscured gray/white matter
-follows watershed distribution
-may see calcification
when is mass effect maximal for brain infarct?
24 hours (sometimes starts 2 hours after onset)
when does mass effect disappear for a brain infarct?
after 1 month
ct hemorrhagic stroke
hematoma=solid, homogenous, in one place
how does extracelular met-Hb appear on mri?
white in both T1 and T2
how does oxy-hb appear?
gray in T1 and T2
how does haemosiderin apear?
black in T1 and T2
-most frequently result of a ruptured aneurysm
-best seen on CT scans
-blood diffuses into CSF and thins out and sinks to the lowest point
info on imaging seizures:
-exam findings: out of the blue seizure=meningitis-->stiff neck (brudzinski sign leg comes up when you move the neck)
-seizures do not show up on neuroimaging. they show up on EEG
-guidelines for ordering imaging: CT...
black in T2, white in T1
gray in T1
black in T2
gray in both
CT brain abscess
-look for abscess with infection or possible meningitis
describe CNS neoplasms
-1/3 are metastatic lesions
-1/3 are gliomas
-1/3 is non-glial origin
what is a glioma?
indicates that tumor originates from glial cells
most common glioma
1) low grade pilocytic type
2) intermediate anaplastic type
3) high grade malignant glioblastoma multiforme (most common type of astrocytomas at 50%)
most common non-glial cell tumor?
what does extra axial mean?
outside of the brain-enhance these with contrast
-do not enchance intra-axial tumors
meningioma of the brain
-buckling of underlying brain
-calcifications circular or radial
-hyperostosis of adj. bone
this is for CT
mri brain meningioma
-arcuate bowing of white matter
case 4 info-dude is irritable and confused-gotten worse over 6 years
DDX=alzheimers, dementia, alcoholism, delirium, falling
-CT/MRI will not show dementia-->only can see it late in the disease when atrophy sets in.
-no need for PET
general brain atrophy findings
-ipsilateral dilation of basal cisterns and ventricles
-ex vacuo dilation of ventricles
-thinning of gyri
-non-specific imaging usuall
-cracked walnut appearance-enlarged sulci
what image to do for degenerative disc disease
degenerative disc disease
-not always apparent
-dessication (drying out) of nucleus pulposus
-fissuring and cracking of disc end plates
-loss of disc height
common signs of desiccating disc on CT
-sclerosis of facet joints
-neural foramina narrowing
-cyst formation on the disc
-liberation of nitrogen from surrounding tissues into areas with abnormal nucleus pulposis to annulus fibrosis attachment
- can see in x rays
-see on up to 50% of pts over the age of 40 on CT
stages of disc herniation:
stage 1) disc degeneration: disc herniates from aging
stage 2) prolapse: impingement into the spinal canal-basically more pronounced bulge
stage 3) extrusion: nucleus pulposus breaks through the wall, but remains within the disc
stage 4) sequestration: pulposus breaks through and lies outside the disc in the spinal canal