Neuroradiology Part I

Question 1

What are window level and window width akin to in CT imaging?

Answer 1

Window level - Brightness - think level of brightness

Window width - contrast - think with of particles which can be differentiated / contrasted

Question 2

What are the units which determine brightness on CT and what is the baseline?

Answer 2

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

Question 3

How does fat appear on CT? Blood? Brain tissue?

Answer 3

Fat - hypodense to water, appears dark
Blood - contains iron -> bright
Brain tissue - heavier than water, bright, but not as bright as blood

Question 4

What determines signal strength in MRI?

Answer 4

Generally, the amount of water in a tissue, because energy is dissipated as protons realign with the magnetic field is what determines the signal

Question 5

What appears bright and dark in a T1 weighted sequence in the brain and ventricles?

Answer 5

T1 = true to advertising

White matter appears bright
Gray matter appears dark

Water appears dark (ventricles dark)
Fat appears bright

Question 6

What appears bright and dark in a T2 weighted sequence?

Answer 6

Opposite of T1, which is true to advertising

Gray matter appears light, white matter appears dark

Question 7

How can you tell if something is a T2 flair rather than T1?

Answer 7

Just look at the gray / white matter

Any T2 sequence will have gray matter appearing more bright than white matter (central cortex)

Question 8

What are some pros and cons of CT?

Answer 8

Pros:
Quick and easy, less patient compatibility issues, good sensitivity

Cons: Ionizing radiation

Question 9

What are some pros and cons of MRI?

Answer 9

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)

Question 10

How can the central sulcus be found on transverse MRI? What lobes does it separate?

Answer 10

Inverted omega sign

- Separates frontal and parietal lobes

Question 11

How does edema appear on CT, T1 MRI, and T2 MRI?

Answer 11

CT - hypodense (dark)
T1 MRI - hypointestine (dark) (water)
T2 MRI - hyperintesnse (bright)

Question 12

How does vasogenic vs cytotoxic edema appear differently via imaging?

Answer 12

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

Question 13

What contrast type is used for CT and for MRI? What are the risks for each?

Answer 13

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.

Question 14

How does contrast appear on CT / MRI?

Answer 14

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

Question 15

What would be some examples of extrinsic / extra-axial causes of brain herniation?

Answer 15

Epidural / subdural hematoma

Masses - i.e. meningioma

Question 16

What often happens to the contralateral ventricle in a midline shift and why?

Answer 16

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

Question 17

What are the complications of descending transtentorial hernation?

Answer 17

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

Question 18

What can cause an upward transtentorial hernation?

Answer 18

Masses, hemorrhage, or edema in the cerebellum leads to upward displacement of cerebellar vermis thru tentorial notch into the supratentorial area

Question 19

Is CT or MRI better for diagnosing acute hemorrhage and why?

Answer 19

CT -> blood will always appear hyperdense due to iron

MRI will depend on the degradation state of the hemoglobin and is much more variable

Question 20

What is a myelography?

Answer 20

CT with subarachnoid contrast

Question 21

What are normal X-rays / radiographs still important for in spine evaluation? Where have they been supplanted?

Answer 21

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.

Question 22

What is a common site for instability in flexion and extension of the neck?

Answer 22

predental space - between C1 anterior arch and the dens of C2

Question 23

What are the anterior and posterior vertebral lines, and spinal laminar line?

Answer 23

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

Question 24

What is the only thing which CT scan is better than MRI for in evaluation of spinal cord?

Answer 24

Detection of fractures

Everything else, including seeing osteophytes off of joints in degenerative disc disease, is better done by MRI

Question 25

What is MRI best for with spinal cord?

Answer 25

Evaluation of soft tissues, seeing nerve root exits between bones, discs, ligaments, thecal sac, and spinal cord abnormalities (i.e. tumor or demyelination as in MS)

Question 26

What is the only thing which CT has no efficacy in at all and cannot be done for?

Answer 26

Cord signal abnormalities -> cannot detect demyelinating diseases of the cord via CT

Question 27

What is the best way to workup arthritis / degenerative disk disease in imaging?

Answer 27

Best: MRI if patient able

Best alternative: CT / CT myelography if patient is not MRI compatible