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Scientific Foundations > Neuroradiology > Flashcards

Neuroradiology Flashcards

1
Q

What does CT measure?

A

Tissue density

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2
Q

How is tissue density measured?

A

Hounsfield units (HU)

Each pixel (voxel) has a number that is a ratio compared to water. Each pixel is then coloured a shade of grey to represent the number.

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3
Q

How many Hounsfield units is water?

A

0

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4
Q

How many Hounsfield units is air?

A

-1000

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5
Q

How many Hounsfield units is bone?

A

1000+

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6
Q

How many Hounsfield units is grey matter?

A

40

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7
Q

How many Hounsfield units is white mater?

A

35

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8
Q

How many Hounsfield units is fresh blood?

A

80

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9
Q

What is a CT window and why is it necessary?

A

A set range of Hounsfield units that the image contains, adjusted depending on the specific tissues of interest.

This helps you pick up more subtle tissue density differences as the human eye can only reliably distinguish between 25 shades of grey

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10
Q

What is the window level?

A

The midpoint of the range of CT numbers displayed

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11
Q

What is the standard brain WW/WL?

A

80/35

(range = - 5 to 75)

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12
Q

How does fresh blood typically appear on a CT scan?

A

Bright (HU 80)

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13
Q

How does bone and calcification show on a CT scan?

A

Very bright because they are very dense

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14
Q

How do the ventricles typically appear on a CT scan?

A

Clear, dark areas (Fluid; HU = 0)

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15
Q

How does oedema appear on a CT scan?

A

Dark - low density fluid

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16
Q

Increased __________ increases density.

A

Cellularity

Useful for spotting tumours - appear brighter

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17
Q

What is spiral/helical scanning?

A

Multiple detectors rotate around the patient as they are moved through, allowing multiple slices to be acquired simultaneously which can reconstructed by a computer to great a 3D image

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18
Q

How many slices are taken in a typical CT?

A

64-128

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19
Q

How does a CT work?

A

▪️Heated cathode releases high-energy electrons
▪️Energy is released as X-ray radiation
▪️Radiation passes through tissue to the detectors
▪️More dense tissue = more X-ray absorbed = less reaches detector

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20
Q

What is contrast enhancement?

A

Intravenous injection of a contrast medium which distributes in the extracellular space.

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21
Q

What contrast medium is usually used in CT and how does it show up?

A

Iodinated contrast medium

Shows up bright

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22
Q

What is contrast enhancement a good marker of in the CNS?

A

▪️BBB breakdown (e.g. neoplasia, infection, inflammation, necrosis)
▪️Vascularity

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23
Q

What contrast is most commonly used for MR scanning and how does it show up?

A

Gadolinium chelate

Metal-based so shows up bright on T1W images

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24
Q

What are the main risks associated with the use of both IV contrast?

A

▪️Anaplylaxis (immediate reaction) (more common with iodinated)
▪️Allergy (delayed reaction)
▪️Post contrast acute kidney injury

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25
Q

What other side effects are associated with the use of gadolinium chelate?

A

▪️Neohrogenic systemic fibrosis in those with renal failure
▪️Gadolinium accumulation in the brain

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26
Q

What is perfusion?

A

Blood flow through the brain

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27
Q

How can you measure perfusion with CT?

A

With a tissue time/density curve

Cerebral blood flow = cerebral blood volume (space under curve) ÷ mean transit time

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28
Q

What are parameter maps?

A

Visual representations of cerebral blood flow, cerebral blood volume, and mean transit time

(Red = higher CBF, blue =slower etc)

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29
Q

How does blood flow differ between white and grey matter?

A

Grey matter needs higher blood flow

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30
Q

When are perfusion parameter maps useful?

A

▪️Detect stroke - where is perfusion reduced?
▪️Detect malignancy of a brain tumour (glioblastomas have high CBV as they create new vessels)

31
Q

What are the smallest structures we can see on a scan?

A

Dependent on:

▪️Pixel/voxel size (typically 0.5mm, 512 matrix)
▪️Signal to noise ratio
▪️Contrast resolution

32
Q

What is the signal to noise ratio for CT?

A

CT noise inversely rated to radiation exposure

(More radiation = better image but riskier)

33
Q

What is the ALARA principle?

A

Radiation exposure should be “as low as reasonably achievable”

(IRMER, 2000)

34
Q

What is radiation dose measured in?

A

Millisievert (mSv)

35
Q

What is the UK annual background radiation?

36
Q

How long do patients typically stay in the scanner before movement begins to affect the images?

A

30-40 minutes

37
Q

What is signal to noise ratio for MRI?

A

MRI noise inversely related to scan time and field strength

(Longer and stronger scan = better but more prone to movement artifacts)

38
Q

What are the five stages of CT?

A
  1. Density/voxel
  2. Exposure (kV mAs)
  3. Physical processes (linear attenuation coefficient μ)
  4. Tissue processes (cellularity, fluid, haemorrhage, myelination, etc)
  5. Diagnosis/pathology
39
Q

How does MRI work?

A

▪️Generates very strong magnetic field
▪️Pulses radiofrequency energy
▪️Nuclei (hydrogen) in tissues and fluid align in magnetic field with a resonant frequency proportional to the strength of the field
▪️Aerial records nuclei remitting radiofrequency
▪️Signal produced by subsequent loss of alignment with time

40
Q

What are the main safety considerations for MRI?

A

▪️Metallic foreign bodies (e.g. from eye trauma)
▪️Implants (e.g. pacemakers, DBS, aneurysm clips, joint replacements)
▪️Pregnancy
▪️Claustrophobia
▪️Acoustic noise
▪️Peripheral nerve stimulation?

41
Q

Adjusting sequence parameters alters ____________

42
Q

What are the three main types of basic MRI pulse sequences?

A

▪️Spin echo
▪️Inversion recovery
▪️Gradient echo

43
Q

What type of pulse sequence is contrast enchanted MRI?

A

Angiographic

44
Q

How are MRIs most commonly used clinically?

A

With contrast

45
Q

What is a T1 weightef MRI sequence?

A

A basic pulse sequence that enhances the signal of fatty tissue and suppresses water

Water/CSF = dark
Fat = white
Grey matter is darker than white matter

46
Q

What is T1 weighted MRI best used for?

A

Anatomy and soft tissue structures

47
Q

What is a T2 weighted MRI sequence?

A

A basic pulse sequence that enhances the signal of fatty tissue AND water

Water/CSF = bright
Grey matter is lighter than white matter

48
Q

What is diffusion weighted imaging?

A

Measure diffusion restriction

Areas of greater restriction = dark

49
Q

What is T2 weighted MRI good for?

A

Pathology such as inflammation or oedema

50
Q

What is diffusion weighted imaging useful for?

A

▪️Ischaemoc stroke
▪️Abscesses
▪️Tumours

51
Q

What is FLAIR?

A

Fluid Attenuated Inversion Recovery

Like T2 but water appears dark
Grey matter = lighter than white

52
Q

What is FLAIR imaging useful for?

A

MS and periventricular lesions

53
Q

What is MRA and what is it used for?

A

Magnetic resonance angiography

Useful for arteriovenous malformations and aneurysms as vessels show up bright

54
Q

What is the main process of MRI?

A
  1. Signal intensity (voxel)
  2. Sequence (e.g. gradient echo)
  3. Physical processes (e.g. T1, T2, SWI)
  4. Tissue processes
  5. Diagnosis/pathology
55
Q

What is SWI and what is it useful for?

A

Susceptibility weighted imaging

Highlights paramagnetic and/or diamagnetic substances so particularly sensitive to venous blood (shows dark)

56
Q

What MRI sequence is best for imaging for the days/years after an initial haemorrhage and why?

A

T2 - haemosiderin is left behind and shows up as very dark patches

57
Q

How does extracellular methaemoglobin show up on MRI?

A

Very bright patches

58
Q

How does intracellular methaemoglobin show on MRI?

A

Very bright on T1 and very dark on T2

59
Q

What is methaemoglobin?

A

A product released a few days after initial haemorrhage as red blood cells burst

60
Q

What type of MRI should be used immediately after initial haemorrhage and what does it show?

A

Either T1 or T2

Oxyhaemglobin shows up as dark patches

61
Q

What type of MRI should be used in the hours after initial haemorrhage and how does it show on the scan?

A

Either T1 or T2

Deoxyhaemoglobin shows as a dark patch, possibly clearer on T1

62
Q

What do bright patches on T1 weighted MRI suggest about a haemorrhage?

A

The patches are likely methaemoglobin (intracellular or extracellular), indicating it’s been days/months since the initial haemorrhage

63
Q

What do bright patches on T2 weighted MRI suggest about a haemorrhage?

A

Likely extracellular methaemoglobin, suggesting the haemorrhage is in its late stage, after days/months

64
Q

What do very dark patches on T2 weighted MRI suggest about a haemorrhage?

A

Likely intracellular methaemoglobin or haemosiderin suggesting the haemorrhage occurred either a few days ago or is in its final stage after days-years

65
Q

If a patch shows up as bright on T1 but dark on T2, what does that suggest about the haemorrhage?

A

It’s around 3 days old. The patch is intracellular methaemoglobin

66
Q

If a patch shows up as bright on T1 and on T2, what does that suggest about the haemorrhage?

A

It is days/months old. The patches are extracellular methaemoglobin

67
Q

How do the blood byproducts from a haemorrhage show up on gradient echo SWI?

A

Dark patches

Heamosiderin is the darkest

68
Q

What pathological processes show up as bright on T2 MRI and dark on T1?

A

▪️Oedema
▪️Gliosis
▪️Necrosis

69
Q

What pathological processes show up bright on T2 imaging?

A

▪️Demyelination
▪️Extracellular methaemoglobin
▪️Oedema
▪️Gliosis
▪️Necrosis

70
Q

What pathological processes show up bright on T1 imaging?

A

▪️Fatty change
▪️Methaemoglobin
▪️Contrast
▪️Oxygen
▪️Macromolecules

71
Q

What pathological processes show up dark on T2 imaging?

A

▪️Calcium/bone/iron
▪️High cellularity (tumour)
▪️Myelination
▪️Flow

72
Q

What pathological processes show up dark on T1 imaging?

A

▪️Oedema
▪️Gliosis
▪️Necrosis
▪️Flow

73
Q

What might necrosis indicate and what would it look like on MRI?

A

Stoke/infarction

Dark on T1, bright on T2

Scientific Foundations (27 decks)

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