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Neuroimaging 1 > MR Physics 3: Parameters for Image Contrast > Flashcards

MR Physics 3: Parameters for Image Contrast Flashcards

1
Q

Why are hydrogen nuclei important in MRI?

A

Used for MRI because of their magnetic susceptibility and their vast amount in the human body.

An intrinsic property of the hydrogen nuclei is their rotation (spin) which makes them magnetic along the rotational axis.

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2
Q

What is proton density?

A

Proton density (PD) =most basic MRI measure, representing the concentration of water protons (mobile hydrogen atoms) in each voxel.

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3
Q

How is MRI contrast developed?

A

Developed due to differences in:
T1, T2, T2* and proton density between different tissue types

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4
Q

What does the recovery time of T1 and T2 depend on?

A

Molecular motion of each spin near the Larmor frequency causes local magnetic field fluctuations with both T1 and T2 relaxation

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5
Q

What is relaxation?

A

The process in which spins release the energy received from a radiofrequency pulse

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6
Q

What are the two types of relaxation?

A

T1 - spin-lattice or longitudinal relaxation
T2 - spin-spin or transverse relaxation

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7
Q

How can MRI contrast be changed?

A

By changing TR and TE

-By altering the time at which the signal is measured after excitation
-By altering the time allowed to recover between pulses

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8
Q

What is TR?

A

Time to repetition

i.e. how quickly the entire pulse signal is repeated

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9
Q

What is TE?

A

Time to echo

i.e. the time between signal excitation and maximum signal

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10
Q

Can TR and TE be manipulated by the researcher?

A

Yes, they are both under operator control

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11
Q

How does proton density affect T1 contrast?

A

Controls the image appearance by altering the TR

Differences in PD and T1 between tissues in each image voxel provide image contrast

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12
Q

What does a long TR produce?

A

Proton density weighted image

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13
Q

What does a short TR produce?

A

T1 weighted image

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14
Q

What type of image does a spin echo sequence generate?

A

T2 weighted

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15
Q

What type of image does a gradient echo sequence generate?

A

T2* weighted

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16
Q

What is a spin echo sequence?

A

The simplest form of the spin-echo (SE) pulse sequence consists of 90°-pulse, a 180°-pulse, and then an echo.

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17
Q

How is a gradient echo sequence different to spin echo sequence?

A

Gradient echo uses magnetic gradients to generate a signal, instead of using 180 degrees radiofrequency pulse like spin echo = faster image acquisition time

The gradient echo formation results from applying a dephasing gradient before the frequency-encoding or readout gradient.

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18
Q

What causes the difference between T2 and T2*?

A

The type of echo sequence they use

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19
Q

What does increasing echo time do?

A

Increases T2 weighting

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20
Q

What does does decreasing echo time do?

A

Increases PD weighting

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21
Q

Summarise TR and TEs effect on images.

A

A long TR and short TE sequence is usually called Proton density-weighted

A short TR and short TE sequence is usually called T1-weighted

A long TR and long TE sequence is usually called T2-weighted

A short TR and long TE is TOO NOISY - dark image not much signal

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22
Q

What is signal to noise ratio?

A

The Signal to Noise Ratio (SNR) is a measure of the image signal in an area of tissue with respect to the background tissue.

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23
Q

What is contrast to noise ratio?

A

The Contrast to Noise Ratio (CNR) in an MRI image is the contrast between the average image values in a tissue of interest relative to the background (i.e. the surrounding tissue).

24
Q

What do we do with SNR and CNR?

A

We have to compromise between SNR and CNR to capture the optimal signal

25
What is isointensity?
When the amount of decay of SNR and amount of decay od CNR hit the same point together = creates an isointense point which will have the same brightness even if the T2 values are different
26
What has a long T1 and long T2?
Water/CSF
27
What has a long T2 and short T1?
Extracellular methemoglobin
28
What has an intermediate T2 and intermediate T1?
Muscle Grey matter White matter
29
What has a short T2 and a long T1?
Air Cortical bone Deoxy Haemoglobin Tendons Fibrosis
30
What has a short T2 and short T1?
Fat Paramagnetic agents Proteinaceous solution
31
What 3 pulse sequences change image contrasts?
Spin echo sequence Gradient echo sequence Susceptibility weighted imaging
32
What are three contrast agents?
Gandolinium Chemical Enhanced Saturation Transfer (CEST) Hyperpolarisation
33
What is a pulse sequence?
A series of events we use to control the magnetisation it contains RF pulses and magnetic gradients How magnetisation (signal) is prepared changes the contrast
34
What are the pros of gradient echo imaging?
Faster imaging Shorter TE
35
What are the cons of gradient echo imaging?
Poor magnetic field (e.g. air pockets)- homogeneity distorts the image
36
What are the pros of spin echo imaging?
Less susceptible to magnetic field homogeneity
37
What are the cons of spin echo imaging?
Ususally longer TE Slower imaging that gradient echo imaging
38
What is magnetic susceptibility?
How the magnetic is induced in a substance when exposed external magnetic field Substance can be diamagnetic, paramagnetic or ferromagnetic
39
What are diamagnetic substances?
Diamagnetic materials are the most unique as they repel both poles of magnets Not attracted to external magnetic fields
40
What are paramagnetic substances?
Paramagnetic materials are weakly attracted to a single pole Attracted to external magnetic fields
41
What are ferromagnetic substances?
Ferromagnetic materials are strongly attracted to both poles of magnets Highly attracted to external magnetic fields
42
What are gradient echo T2* sequences sensitive to?
Tissue susceptibility
43
What can susceptibility weighted imaging (T2*) do?
Enhance detection of calcifications and haemorrhage/blood.
44
What are 3D FatSat FLASH images?
Images that exploit a chemical shift difference between water and fat for image editing A chemical shift selective pulse before the acquisition saturates the fat signal Very thin slice acquisitions (<= 1mm) allow cartilage to be highlighted
45
How does the 3D Fat/Water separation work?
Chemical shift difference between water and fat can be used to produce in-phase and out-phase images For a particualr TE the fat magnetisation will be 180 degrees out of phase Aquire in-phase (water + fat) and out-phase (water- fat) images
46
How is the Dixon technique linked to 3D FatSat FLASH images?
It consists of the acquisition of in-phase and out-of-phase images, from which water-only (WO) and fat-only (FO) images are reconstructed This produces four sets of images (or more) per acquisition
47
How do we use contrast agents?
Inject these substances that change the T1/T2 of tissue
48
What is gandolinium-chelate?
Paramagnetic substance (weakly magnetic) It shortens T1 Increases signal on T1 weighted image Produces dynamic contrast enhanced MRI
49
What is gandolinium-chelate useful for detecting?
Lesions and tumours
50
What is CEST contrast?
CEST measures the transfer of magnetization from molecular protons to the solvent water protons, an effect that becomes apparent as an MRI signal loss (“saturation”) This allows molecular information to be accessed with the enhanced sensitivity of MRI
51
How does CEST work?
Irradating tissue with long RF pulse saturates the signal macromolecule proteins Protein protons exchange with mobile protons, reducing the water signal CHanges the image contrast dependent on protein concentration
52
How does hyperpolarisation contrast work?
Creates substances with dramatically increased nuclear spin polarization (and thus, greatly enhanced MR signal intensity) to enhance the information and contrast available on an MR image.
53
What does EPI stand for?
Echo-planar imaging
54
What is EPI?
Very fast gradient echo imaging sequence <<1s Whole of k-space is acquired in a single TR period But its susceptible to image distortion
55
How does EPI work?
EPI = fast-imaging technique whereby a 2-D image can be obtained in less than a second, by quickly switching the magnetic gradients back and forth after a single excitation pulse

Neuroimaging 1 (18 decks)

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