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Fundamentals of P > MRI & MRS > Flashcards

MRI & MRS Flashcards

1
Q

CT contrast is due to what

A

CT contrast is due to tissue density dependent

attenuation of x-rays

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2
Q

MR image contrast - define

A

MR image contrast – i.e. the relative signal intensities between different tissue types and pathologies – depends on physical properties of the tissue such as water and fat content, cellular structure, cell density

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3
Q

How is a magnetic moment produced

A

The positive charge of a spinning proton produces a magnetic moment μ

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4
Q

Describe precess of magnetic moment

A

In a magnetic field Bo the magnetic moment of a proton precesses at the Larmor frequency νL

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5
Q

Describe formation of MR imaging

A

MR Imaging is formed using a radiofrequency pulse to generate an MR signal from a slice of tissue

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6
Q

Describe use of magnetic field gradients

A

Magnetic field gradients are used to encode the signal in space so that the computer can generate an image

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7
Q

MRI safety issues

A

No ferromagnetic objects in the exam room

Scissors, stethoscopes, wheel chairs, gas cylinders
Hearing aids, watches, spectacles, (dentures – image quality)

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8
Q

MRI contraindications

A

Pacemakers
Infusion pumps
1st trimester pregnancy
Aneurysm clips (refer to manufacturers specifications
Metallic foreign bodies (orbit x-ray, shrapnel)

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9
Q

Describe the magnetisation in MRI - source, use

A

The strong magnetic field creates magnetisation in all the tissue

This magnetisation is from the protons in water and fat in the tissue

The magnetisation can be manipulated by radiofrequency pulses to produce an MRI signal to create an image

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10
Q

MRI image - intensity dependent on what

A

The intensity in the image depends on water content, tissue structure, blood flow, perfusion, diffusion, paramagnetics etc

The image signal intensity depends on T1 and T2 and provides contrast between tissue in an MR image.

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11
Q

T2 signal decay description

A

Mxy decays according to T2 which affects how long the MR signal lasts

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12
Q

T1 signal decay description

A

Mz recovers according to T1 which affects how much M there is available to be excited to give the next signal

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13
Q

T1/T2 relaxation times difference due to what

A

T1 and T2 relaxation times vary between different tissues and pathology

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14
Q

MR image built up from what

A

An MR image is built up from a series of signal acquisitions

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15
Q

T2 of tissue determines what

A

The T2 of tissue determines how quickly the MRI signal decays away after the radiofrequency pulse

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16
Q

T2 dependent on what

A

T2 is very dependent on how mobile the water is in the tissue and increases with
Oedema, an increase in water content
Demyelination, a loss of brain tissue structure

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17
Q

T2 reduced by what

A

T2 is reduced by the presence of paramagnetic ions
Fe from blood breakdown products
Gd from contrast agents

18
Q

Describe signal acquisitions in MRI

A

Radiofequency pulse -
MRI signal from tissue
Signals from tissue and csf are reduced depending on T1
Mz recovery with time after the RF pulse depends on the T1 relaxation time

19
Q

CSF vs Mz T1

A

Tissue has a shorter T1 than csf so its Mz recovers more quickly

CSF has a very long T1

20
Q

Formation of saturated MRI signal

A

When the repetition time (TR) between pulses is much shorter than T1 the magnetisation that can produce the MRI signal is reduced (“saturated”)

The MR signal is then T1-weighted.

21
Q

Tissue sizes - effect on T1

A

Tissue with long T1 produces a smaller signal than tissue with short T1.

22
Q

Define relaxation time

A

The time constant which determines the rate at which excited protons return to equilibrium

23
Q

How is T1 effected in grey/white matter

A

T1 is lower in white matter than grey matter because of myelinated neurones

24
Q

How is T1 effected in water

A

T1 is also dependent on how mobile the water is in the tissue and T1 increases slightly with oedema

25
Q

T1 dependent on what

A

T1 is very dependent on the presence of paramagnetic ions which reduce T1

Fe from blood breakdown products
Gd from contrast agents

26
Q

Contrast agent purpose

A

Used to increase the contrast of structures or fluids within the body in medical imaging. absorb or alter external electromagnetism or ultrasound.

Water in the vicinity of the contrast agent experiences strong fluctuating magnetic fields hence T1 and T2 are reduced.

27
Q

Contrast agent definition

A

Paramagnetic (unpaired electrons) or superparamagnetic (ferrites)

Chelated to reduce toxicity

28
Q

Define chelate

A

A compound containing a ligand (typically organic) bonded to a central metal atom at two or more points

29
Q

Effect of shielded nucleus

A

A stronger shielded nucleus has a lower resonant frequency

30
Q

Define MRS and it’s purpose

A

magnetic resonance spectroscopy = non-invasive, ionizing-radiation-free analytical technique to study metabolic changes in brain tumors, strokes, seizure disorders, Alzheimer’s disease, depression, and other diseases affecting the brain.

31
Q

MRS process

A

Acquire signal from hydrogen protons (other endogenous nuclei also used),

MRS very frequently only acquires signal from a single localized region, referred to as a “voxel”.

32
Q

What does MRS determine

A

MRS can be used to determine the relative concentrations and physical properties of a variety of biochemicals frequently referred to as “metabolites” due to their role in metabolism

33
Q

Describe where NAA is found by MRS

A

(N-acetyl aspartate) found predominantly in neurons, marker for viable neurons, reduced in pathology

34
Q

Describe where tCr is found by MRS

A

(Cr and PCr)

cell energy metabolism, marker of viable cell density in some tissue

35
Q

Describe where tCho is found by MRS

A

(Cho, GPC, PC)

cell membrane metabolism (growth and degradation), elevated in tumors and gliosis

36
Q

Describe where Glx is found by MRS

A

(glutamate & glutamine) amino acids

37
Q

Describe where ml is found by MRS

A

(myo-Inositol) osmolyte found in glial cells, marker for gliosis inflammation, elevated cell membrane synthesis

38
Q

Describe where Lac is found by MRS

A

(lactate) (not visible in normal brain spectra)

the end product of anaerobic metabolism, high in tumors & stroke etc.

39
Q

Describe where Ala is found by MRS

A

Ala (alanine) (not visible in normal brain spectra) amino acid, marker for meningiomas

40
Q

Describe where lipids are found by MRS

A

Lipids (not visible in normal brain spectra) membrane breakdown products, macrophages

Fundamentals of P (62 decks)

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