Introduction to Neuroimaging (Basak)

Question 1

Describe basic physics of CT (DENSITY).

Answer 1

x-ray beams rotate around patient and pass through patient to be collected by detectors on opposite side

more x-ray absorbed in bone vs. water

data translated into density values

Question 2

Understand the concept of Hounsfield Units (HU).

Answer 2

how DENSITY is expressed for CT

Question 3

Identify basic CT densities including that of fluid, brain, calcification, and fat.

Answer 3

Air -> -1000 (black)
Fat -> -100
Water -> 0
Brain -> +30-40
Blood -> +50-100
Bone -> +1000 (BRIGHT)

CSF is HYPODENSE to bone.

Question 4

Understand the usefulness of and indications for basic CT examinations.

Why do you not give contrast for hemorrhage?

Answer 4

x-ray for brain (first line imaging)

• Intracranial hemorrhage
• Acute trauma (head, neck, body, spine)
• Stroke imaging (not so much to find stroke itself but to find other reasons for stroke like acute hemorrhage)
• Fractures
• Sinusitis
• Bone lesions
• Dental imaging (odontogenic tumors, planning for dental implants)
• Myelography (to look for problems in spinal cord)
• CT angiography
• CT perfusion
• Temporal bone

Don’t give contrast when intracranial hemorrhage is suspected because blood is bright and contrast is bright - won’t be able to tell the difference

Question 5

Describe the basic physics of MRI (INTENSITY).

Answer 5

MRI uses magnets and radio frequency waves - NOT XRAY

all MRI techniques are based on receiving and processing signals from protons

When proton moves, it spins around an axis which induces a magnetic field.

Proton also wobbles and the extent to which it does is dependent on the strength of the magnetic field.

Radio pulse disturbs equilibrium of these protons -> causes protons to flip into a higher E state

When radio pulse stops, protons spin back into alignment parallel with main magnetic field

Give off E in form of radio frequency magnetic field -> this E can be detected by receiver coil and measured

Question 6

Identify T1 and T2 weighted images (MRI).

Answer 6

Intensity determined by WATER and FAT

T1 and T2 weighting dependent on characteristics and strength of E released by H ions

T1 = anatomy
FAT = BRIGHT (hyper intense)
WATER (CSF) and EDEMA = DARK (hypo intense)

T2 = pathology
FAT / white matter = DARK (hypo intense)
WATER (CSF) and EDEMA = BRIGHT (hyper intense)

Question 7

What is Diffusion Weighted Image (DWI) in MRI?

Answer 7

Used to detect acute or subacute infarcts (to up 14 days old)

If abnormal, will see a bright spot

Question 8

Most effective uses of MRI

Answer 8

• Intracranial tumor, TIA/stroke, infection, demyelination
• Epilepsy
• Dementia
• Hearing loss
• Pituitary gland (CT is worthless)
• Cranial nerve abnormalities (1-2 mm thick)
• Pediatrics: metabolic disease, development delay, congenital anomalies
• Fetal brain and spine
• Trauma of brain or spine
• Spine tumors
• Spine degenerative change
• Evaluation fo arteries or veins inc rain or spine
• Head and neck tumor or infection

Question 9

What is FLAIR?

Answer 9

Fluid Attenuation Inversion Recovery -> pathology

Similar to T2W images but CSF hypo intense

Only CSF is suppressed so that you can look for plaques in the surrounding ventricles (MULTIPLE SCLEROSIS)

Question 10

fMRI?

Answer 10

Functional MRI

used to assess brain metabolic activity

uses BOLD (blood oxygen level dependent) imaging