Neuroradiology Part I Flashcards
What are window level and window width akin to in CT imaging?
Window level - Brightness - think level of brightness
Window width - contrast - think with of particles which can be differentiated / contrasted
What are the units which determine brightness on CT and what is the baseline?
Degree with which a substance attenuates the X-ray beam, expressed in Hounsfield units
-1000 (pure air) to +1000 (pure metal), with pure water set at 0
How does fat appear on CT? Blood? Brain tissue?
Fat - hypodense to water, appears dark
Blood - contains iron -> bright
Brain tissue - heavier than water, bright, but not as bright as blood
What determines signal strength in MRI?
Generally, the amount of water in a tissue, because energy is dissipated as protons realign with the magnetic field is what determines the signal
What appears bright and dark in a T1 weighted sequence in the brain and ventricles?
T1 = true to advertising
White matter appears bright
Gray matter appears dark
Water appears dark (ventricles dark)
Fat appears bright
What appears bright and dark in a T2 weighted sequence?
Opposite of T1, which is true to advertising
Gray matter appears light, white matter appears dark
How can you tell if something is a T2 flair rather than T1?
Just look at the gray / white matter
Any T2 sequence will have gray matter appearing more bright than white matter (central cortex)
What are some pros and cons of CT?
Pros:
Quick and easy, less patient compatibility issues, good sensitivity
Cons: Ionizing radiation
What are some pros and cons of MRI?
Pros: Much better resolution, sensitive for early dz detection and characterization
Cons: Long exams which can cause motion and image degradation
Patient clearance issues (metal pacemakers and implants)
How can the central sulcus be found on transverse MRI? What lobes does it separate?
Inverted omega sign
- Separates frontal and parietal lobes
How does edema appear on CT, T1 MRI, and T2 MRI?
CT - hypodense (dark)
T1 MRI - hypointestine (dark) (water)
T2 MRI - hyperintesnse (bright)
How does vasogenic vs cytotoxic edema appear differently via imaging?
Vasogenic - extracellular edema due to BBB breakdown, follows white matter tracts in finger-like projections
Cytotoxic - intracellular edema from disruption of Na/K pump - involves both gray and white matter (seen in acute stroke from ischemia) - no finger-like projections, involves large areas
What contrast type is used for CT and for MRI? What are the risks for each?
CT - iodine-based -> may cause nephropathy
MRI - gadolinium-based -> may cause nephrogenic systemic fibrosis, a scleroderma-like syndrome. Happens less now that we do MRIs less.
How does contrast appear on CT / MRI?
Contrast appears bright on CT and T1 MRI -> we want to keep water dark so that we can tell when contrast is seeping thru BBB
What would be some examples of extrinsic / extra-axial causes of brain herniation?
Epidural / subdural hematoma
Masses - i.e. meningioma
What often happens to the contralateral ventricle in a midline shift and why?
Also known as a subfalcine herniation
- > cingulate gyrus crosses midline and compresses the opposite foramen of Monro
- > contralateral ventricle is compressed
Note, ACA can also be compressed in this lesion
What are the complications of descending transtentorial hernation?
Aka bilateral “uncal”
Displacement and compression of brainstem, CN3
Displacement and compression of posterior cerebral artery as it courses through the tentorial notch adjacent to CN3
Also obviously compression of contralateral cerebral peduncle by herniated tissue causing a false localization of the lesion
What can cause an upward transtentorial hernation?
Masses, hemorrhage, or edema in the cerebellum leads to upward displacement of cerebellar vermis thru tentorial notch into the supratentorial area
Is CT or MRI better for diagnosing acute hemorrhage and why?
CT -> blood will always appear hyperdense due to iron
MRI will depend on the degradation state of the hemoglobin and is much more variable
What is a myelography?
CT with subarachnoid contrast
What are normal X-rays / radiographs still important for in spine evaluation? Where have they been supplanted?
Important in LIGAMENTOUS instability with flexion / extension films
-> very cheap and easy to use. Can see bony fractures, deformities, and osteophytes
Also okay for evaluation of radiculopathy (spinal nerve root impingement)
Have been supplanted in evaluation of trauma -> CT shows improved fracture detection.
What is a common site for instability in flexion and extension of the neck?
predental space - between C1 anterior arch and the dens of C2
What are the anterior and posterior vertebral lines, and spinal laminar line?
Checking for alignment in radiographs of the neck
Anterior - alignment of anterior segment of vertebral bodies
Posterior - alignment of posterior surface of vertebral bodies
Spinal laminar line - alignment of spinal processes
What is the only thing which CT scan is better than MRI for in evaluation of spinal cord?
Detection of fractures
Everything else, including seeing osteophytes off of joints in degenerative disc disease, is better done by MRI
