Lecture 10 - Brain Imaging Flashcards Preview

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Flashcards in Lecture 10 - Brain Imaging Deck (93):
1

CT is essentially a

Rotating x-ray beam where the brain is imaged from several directions

2

For CT what happens to the beam as it passes through brain tissue?

It attenuates (ie weakens)

3

In CT, what is the significance of beam attenuation as it passes through tissue?

Detectors on opposite side of source of beam pick up rate of attenuation (ie., rate at which x-ray beam weakens as it passes through tissue)

4

Rate of attenuation in CT

varies by tissue composition

5

Radiodensity information in CT

(ie., attenuation rate) is detected

6

How might CT allow for reconstruction of a 3D image?

If patient is moved through the scanner slowly, allows us to acquire information at multiple levels of the brain, and therefore reconstruct 3-D image

7

CT enhances visualization of what?

1) bony anatomy 2) acute hemorrhagte or stroke 3) elements with high atomic numbers show up better (calcium, iron, iodine, barium, lead)

8

For CT what are the advantages?

It is faster and less expensive than MRI, can be used as an initial screening and assessment tool (bone fragments, etc)

9

What are CT disadvantages compared to MRI?

1) Use of X-ray; 2) less contrast differences between soft tissues 3) lower spatial resolution, several millimeters in CT scan VS one millimeterin MRI scan

10

hypodense (dark) structures in CT can indicate

edema and infarction

11

hyperdense (bright) structures in CT

indicate calcifications and hemorrhage and bone trauma can be seen as disjunction in bone windows.

12

CT can detect tumors by

Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema

13

Infarct in CT VS MRI

1) CT: loss of gray / white differentiation in CT scan 2) Infarct in MRI: greater resolution to see the difference

14

Anatomic MRI is based on

principles of nuclear magnetic resonance

15

Anatomic MRI produces what?

High resolution images of the brain and spine

16

Advantage of MRI

no radiation; radio frequency waves instead of x-rays

17

Clinical applications of MRI

1) high resolution and detailed visualization of soft tissue 2) visualizes anatomy (gray and white matter; CSF) 3) identifies a wide range of pathological processes

18

What the body is comprised of that the MRI takes advantage of?

63% hydrogen atoms

19

What property of H atoms in the body does the MRI use?

protons in H atoms have a spin, like a top that produces a small magnetic field which aligns with or against the large externally applied magnetic field resulting in a precess or wobble at a frequency propotional to the magnetic field

20

What is the effect of H atoms lining up with the external magnetic field?

Net magnetization of the tissue

21

In MRI, how do you detect the net magnetization of the tissue?

By applying a radiofrequency pulse which tips the protons away from the direction of the magnetization

22

In MRI, what happens after you turn the RF pulse off?

The protons realign with the external magnetic field resulting in the decay of the energy that each spinning proton absorbed from the RF pulse

23

In MRI, what happens as the energy decays?

RF signal is emitted which is picked up with an antennae and decoded into images with Fourier transform algorithms

24

In MRI, explain the digital image construction

1) spin of proton decays 2) emits RF signals at different rates 3) Pixel = RF signal strength on a grey scale

25

What does the rate of RF emission depend on in MRI?

The composition of the tissue in which they are located

26

In MRI, what is the significance of pulse sequences?

1) vary timing of the RF pulse which attenuates the tissue of interest

27

Give an example of pulse sequences in MRI.

Rapid repetitions of the RF pulse enhances grey – white contrast

28

What is the best image strategy for the MRI of lesions?

Infrequent repetitions of RF pulse enhances signal from water which is usually increased in pathologic conditions

29

What are the diagnostic applications of anatomic MRI?

1) inflammatory disease (multiple sclerosis) 2) neoplastic disease (tumors) 3) epilepsy 4) cerebrovascular disease (stroke)

30

What is MRS?

1) Permits us to study chemical structure of the brain 2) separates out chemical mixtures in the brain 3) can derive concentrations and ratios of chemicals (metabolites)

31

How does MRS work?

1) an RF pulse is applied, after which each chemical component (metabolite) emits a specific frequency

32

What happens to the signals of MRS?

They are analyzed with Fourier transforms to generate NMR spectra which consists of multiple peaks

33

For MRS, what does the size of the peak correspond to?

The concentration of each chemical component

34

What are the MRS diagnostic markers?

1) NAA 2) Choline 3) Creatine 4) Lactate

35

MRS diagnostic marker NAA

located in cell bodies and dendrites; considered a neuronal marker

36

MRS diagnostic marker choline

cell membrane synthesis and degradation: marker for demyelination

37

MRS diagnostic marker creatine

glial marker

38

MRS diagnostic marker lactate

found following ischemic events

39

MRS shows what for MS?

1) acute stage shows presence of Lactate, slight decrease in N-acetylaspartate and an increase in choline 2) longitudinal study = disappearance of lactate at 3 months, persistent low levels of NAA, progressive choline increase during the first weeks follo

40

Diffusion weighted imaging and diffusion tensor imaging allows for what?

Allows us to visualize and measure the integrity of white matter tracts in the brain

41

In diffusion weighted images, what is a voxel?

The intensity of each image element

42

In diffusion weighted images, what does a voxel reflect?

the best estimate of the rate of water diffusion at that location

43

DWI

3 gradient gradient-directions are applied, sufficient to estimate the trace of the diffusion tensor or 'average diffusivity', a putative measure of edema

44

Anisotropy

measurement of water diffusion along different orientations within axons

45

DTI measures

water diffusion along different orientations within axons.

46

What is the assumption for DTI in an axon?

it is that features of the axon, such as cell membranes and myelin, restrict the flow of water through the axon, so that the water flows in a relatively organized manner along the longitudinal and perpendicular orientations of the axon

47

In DTI, if water is not diffusing along these axes, but instead seems to be diffusing in many directions, what is it thought to represent?

Pathology

48

Measurement is of diffusion along length of axis: the larger the coefficient

the greater the diffusion of water along the longitudinal axis

49

Measurement is of diffusion along length of axis: the smaller the coefficient

the less diffusion along longitudinal axis, and presumably the more the water is diffusing in many different directions

50

Greater diffusion along the longitudinal axis is thought to represent what?

intactness of axonal development

51

In DTI, if the axon is long what do you expect?

If relatively long, represented as an ellipsoid and indicates that water is diffusing along long axis and is thought to represent the integrity of myelin

52

In DTI, Radial diffusivity represents what?

diffusion along perpendicular axes of voxel, in both directions. It is thought to represent integrity of myelin; myelin would restrict the flow of water in the perpendicular direction

53

Fractional anisotriopy, or FA, is calculated based on

both axial and radial diffusivity and represents the overall direction of water diffusion, and is thought to reflect the overall organization and integrity of the axon

54

High degree of anisotropy indicates that

more water is diffusing along the longitudinal axis of the voxel relative to the perpendicular axis, and would be represented by an ellipsoid

55

low degree of anisotropy indicates

that water is diffusing equally in all directions, and would be represented as a sphere

56

In DTI, red is

lateral to medial

57

In DTI, green is

anterior to posterior

58

In DTI, blue is

superior to inferior

59

fMRI

allows us to acquire images of the brain while patients are performing cognitive tasks in the MRI scanner

60

what are the advantages of fMRI?

1) allows evaluation of brain functioning in vivo 2) Understand the neural systems that make performance of cognitive tasks possible 3) Understand changes in brain function associated with disorders and with aging 4) Understand sites of neural reorganizat

61

What is the basis for BOLD in an fMRI?

Oxyhemoglobin is paramagnetic and can be measured by small changes in intensity values on the MR images

62

How does fMRI work?

1) Measure blood-oxygen-level dependant signal with MRI 2) compare baseline and experimental conditions

63

In the fMRI, why do we measure the changes in intensity values?

to determine how much a task is stimulating neural activity and in what regions of the brain neural activity is stimulated

64

What are the limitations of fMRI?

1) limitations of temporal resolution 2) limitations of spatial resolution 3) relation b/w neuronal activity, blood flow and fMRI signals has not been definitively established

65

What is PET?

1) Use of tracers that are incorporated into a biologically active molecule 2) scanner images the positron-emitting tracer upon its decay

66

What are the applications of PET?

1) blood flow and perfusion 2) Metabolism (resting and task dependant)

67

PET 18-FDG

traces glucose uptake, thought to reflect synaptic activity

68

PET 15-O

traces oxygen uptake

69

Ligands and neuroreceptor imaging?

1) radiotracers bind to pre and post synaptic neuroreceptors 2) EG dopamine synthesis and reuptake 3) advanced pharmacological treatments

70

What are the limitations for PET?

1) need cyclotron 2) injection of radioactive tracer 3) poor spatial resolution

71

How does PET apply to tumor management?

Although [18F]-FDG–PET seems to be useful in grading brain tumors and determining their prognosis, PET also has another advantage over anatomic imaging. Unlike CT or MR imaging, PET can distinguish radiation necrosis from tumor recurrence

72

How can PET distinguish radiation necrosis from tumor recurrence?

areas of radiation necrosis are hypometabolic, whereas tumor recurrence appears hypermetabolic on FDG-PET

73

radiation necrosis was associated with what?

hypometabolism in the white matter only

74

necrosis caused by chemotherapy was associated with what?

gray matter changes in addition to white matter abnormalities

75

Can you you distinguish an area of tumor recurrence among necrotic changes?

Yes with PET

76

fMRI showed activation of what area in mapping emotion?

Amygdala activation during presentation of all emotions

77

What does fMRI mapping of declarative memory show?

Indicates that prefrontal cortex regions are important for memory formation and they have prolonged maturational trajectory

78

PET and Alzheimers.

It has been used in the management of AD, since hypometabolism in various brain regions have been associated with severity of clinical symptoms. Has the ability to aid in the diagnosis and the determination of the course and severity of the disease.

79

fMRI and Alzheimers

1) APOE-4 allele carriers show increased brain activation during memory tasks 2) After 2 years degree of baseline brain activity correlated with memory decline 3) shows clinical predictive significance

80

Intraoperative MRI

awake real time neurosurgery, don’t forget to ask the patient to count in their native tongue

81

Measurements provided for CT

brain structure

82

Measurements provided for MRI

1) brain structure 2) vasculature 3) chemical structure 4) fiber tracts

83

Measurements provided for PET

1) perfusion 2) metabolism 3) neurotransmitter integrity

84

What are the disorders for CT?

1) hemorrhages 2) trauma 3) generalized atrophy

85

What are the disorders for MRI?

1) neoplasm (tumor) 2) demyelination (MR spectroscopy) 3) degenerative disorders (cortical atrophy)

86

What are the disorders for PET?

1) psychiatric 2) addictive 3) degenerative disorders 4) epilepsy

87

What are the advantages of CT?

1) bone imaging 2) hemorrhage detection 3) patient flexibility

88

What are the advantages of MRI?

1) high spatial resolution 2) no radiation 3) excellent contrast 4) functional / chemical imaging

89

What are the advantages of PET?

Functional imaging 2) physiological variables

90

What are the disadvantages for CT?

1) ionizing radiation 2) not as good for soft tissue

91

What are the disadvantages for MRI?

1) long study duration 2) no ferromagnetic or electronic devices 3) claustrophobic

92

What are the disadvantages for PET?

1) ionizing radiation 2) costly, limited access 3) tracer production

93

Neuroimaging and Knowledge

1) increased understanding of neural mechanisms in learning, aging and disease 2) development of effective, well-targeted pharmacological agents 3) identification of functionally important areas prior to neurosurgery 4) understanding of neural reorganizat