Neuroimaging Lab (Week 3--Hathout)

Question 1

2 advantages of CT over plain film x-rays

Answer 1

1) CT can image in slices
2) CT avoids problems of superposition of structures

Question 2

Is spatial resolution better in CT or plain film X-ray?

Answer 2

Spatial resolution is actually better in plain film X-ray

However, CT has extremely good low contrast resolution, enabling detection of very small changes in tissue type (CT can distinguish soft tissue, fluid, blood, fat, calcification)

Question 3

What does CT measure?

Answer 3

CT measures linear X-ray attenuation of each pixel, which depends mostly on electron density in that pixel

Higher electron density = higher X-ray attenuation = brighter on CT

Question 4

Hounesfield units

Answer 4

X-ray attenuation coefficients

There is an x-ray attenuation coefficient assigned to each pixel

Higher number = brighter on CT!

Water = 0

CSF = 4

White matter = 28

Gray matter = 34

Air = 1000

Bone = ~1000

(Fat = -87??)

Question 5

CT with contrast

Answer 5

Can inject iodine-based contrast to detect leaky BBB

Iodine has high atomic number and is bright on CT

Normal brain does not enhance but vessels, tumors, infections do

Question 6

Average linear attenuation coefficient for CT

Answer 6

The average linear attenuation coefficient between tube and detectors, “mu”

Reflects the degree to which x-ray intensity is reduced by a material

Each pixel is given a shade of gray corresponding to its “mu”

Generate Hounesfield numbers from “mu”

Question 7

We get more info from CT compared to plain film x-ray, but it comes at a cost–what is that?

Answer 7

Much bigger doses of radiation with CT

Question 8

What is the leading cause of atraumatic subarachnoid bleed?

Answer 8

Ruptured aneurysm causes subarachnoid hemorrhage (SAH)

Can see basal cisterns in front of brainstem have hyperdense (white) material

Question 9

Baby with ventricles that are too big

Answer 9

Hydrocephalus because of congenital aqueductal stenosis

Large 3rd ventricle but normal 4th ventricle

Males have this more often

Question 10

Ring enhancing lesions

Answer 10

Bacterial abcess: bright on DWI (ring enhancing lesion on MRI)

Tumor: dark on DWI (ring enhancing lesion on MRI)

Question 11

Three easy steps of MRI

Answer 11

1) Bring hydrogen protons into equilibrium with big magnetic field
2) Disturb equilibrium (with radiofrequency pulses; create horizontal magnetization that wasn’t there before)
3) Allow system to “relax” back to equilibrium

Question 12

What does MRI depend on?

Answer 12

Hydrogen protons and their magnetic properties

Speed of relaxation varies for hydrogen protons based on their chemical environment (hydrogen in water vs. fat vs. protein)

Relaxation is governed by exponential time constants (T1 or T2)

Question 13

At equilibrium, what are protons doing?

Answer 13

Protons spinning “out of phase” and produce net vertical magnetization but no horizontal magnetization

Question 14

T1 vs. T2

Answer 14

T1: exponential time constant for spin-lattice relaxation; how quickly vertical magnetization recovers in that tissue; shorter T1 means brighter (fat bright and water dark)

T2: exponential time constant for spin-spin relaxation; how quickly horizontal magnetization disappears in that tissue; longer T2 means brighter (fat dark and water bright)

Question 15

3 things that MR signal and appearance on MRI images depend on

Answer 15

1) Proton density per voxel (how many hydrogen protons are there to generate a signal)
2) T1 time of protons
3) T2 time of protons

Question 16

What has greater tissue contrast, MRI or CT?

Answer 16

MRI has better tissue contrast!

Question 17

Why is MRI gold standard imaging technique?

Answer 17

Even though it has lower spatial resolution (only 1mm), much higher contrast resolution

Question 18

3 main types of MR images

Answer 18

1) Proton density (PD) weighted
2) T1 weighted (reflect differences in PD and T1 times)
2) T2 weighted (reflect differences in PD and T2 times)

Question 19

How does MRI contrast (gadolinium) work?

Answer 19

Gadolinium shortens the T1 time of adjacent hydrogen protons, making them brighter

Note: MRI cannot “see” gadolinium because it can only see hydrogen protons, BUT gadolinium changes the environment of the hydrogen protons around it to create bright contrast in the area that it’s in!

Question 20

When is CT preferred over MRI, and why?

Answer 20

CT is fast and less motion sensitive so is used for fast screening

CT is better at detecting Ca2+ and acute blood so used for trauma

Used to rule out SAH, subdural hematomas, hydrocephalus

Question 21

Positron emission tomography (PET)

Answer 21

Use radioactive tracer that emits protons, which annihilate electrons to create 2 photons given off at 180 degree angle and are detected to create an image

Measures tissue metabolism using F18 attached to glucose molecule (18-FDG)

More metabolically active group of cells will be “hot” on PET

Used to study brain activity, oncologic imaging, dementias, seizures, neurodegenerative disease, tumors

Lower spatial resolution (6mm)

Red = hypermetabolic; blue (and yellow) = hypometabolic

Question 22

CT vs. PET

Answer 22

On CT, nodule looks nonspecific but on PET can see hypermetabolism which tells you that it’s a tumor

(If no metabolic activity it might just be a benign nodule)