IVD3 - MRI Sequences Flashcards by Katharine Russell

How should voxel dimension be aligned with an object of interest in order to maximize the resolution of that object?

Align the long axis of the voxel to the log axis of the object

Example: the long axis of the voxel should be in the sagittal plane when imaging the spine, so the transverse image has good resolution (this is largely the same for all neuro images - transverse is typically the most important plane)

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How is signal to noise ratio (SNR) related to voxel size?

Signal to noise ratio is proportional to voxel volume. The larger the voxel, the more signal that voxel receives meaning it has a higher signal to noise ratio (since noise stays constant) but this comes at the cost or poorer resolution

A smaller voxel will give you better resolution but you have less signal per voxel therefore your SNR will be lower (poorer) and your image will have more of the “static” overlying it

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What does noise look like on an MRI with a poor signal to noise ratio (SNR)?

Like TV static overlying the image
This is separate from the resolution of that image

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How does echo time (TE) affect the signal to noise ratio (SNR)?

A long TE results in more noise, because you have given the surrounding environment more time to have an impact on the imaging (e.g. stray radiofrquency pulses)

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How does a gradient echo sequence differ from a spin echo sequence in how the signal is generated?

The 180 degree refocusing pulse that is used during a spin echo sequence is swapped out and replaced by a “rephasing gradient” which has a similar effect on the hydrogen ions

The frequency encoding gradient gets applied twice during this sequence, once in the flipped direction as the ‘rephasing gradient’ and again during readout of the signal (like spin echo)

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Based on the below TR and TE, what image weighting would the MRI sequence have?

TR = 1500 - 3500
TE = 60 - 150

T2 weighting

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Based on the below TR and TE, what image weighting would the MRI sequence have?

TR = 400 - 600
TE = 5 - 30

T1 wieghting

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Based on the below TR and TE, what image weighting would the MRI sequence have?

TR = 1500 - 3000
TE = 5 - 30

Proton density

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What are the two parameters that make up imaging weighting on gradient echo sequences?

Flip angle
Echo time (TE)

Flip angle essentially takes the place of TR but has the inerve relationship with weighting (a low flip angle is used in T2 weighting whereas a high flip angle is used in T1 weighting)

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Based on the below Flip angle and TE, what image weighting would the MRI sequence have?

Flip angle = <40 degrees
TE = > 30

T2 GRE

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Based on the below TR and TE, what image weighting would the MRI sequence have?

Flip angle = > 50
TE = < 15

T1 GRE

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Based on the below TR and TE, what image weighting would the MRI sequence have?

Flip angle = < 40
TE = < 15

proton density GRE

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What are the pros and cons of gradient echo sequences as they relates to time and image quality?

GRE are fast sequences but have a high sensitivity to certain artefact

Example: susceptibility artefacts

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How are inversion recovery MRI sequences achieved (as in how do they differ from spin echo)?

Inversion recovery is created by sending out a 180 degree “inversion RF pulse” prior to the initial 90 degree excitation pulse

The time between the inversion pulse and the excitation pulse in the inversion time (TI) which can be altered to achieve the different types of inversion sequences (FLAIR vs STIR)

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How does inversion time differ between FLAIR and STIR sequences?

STIR (short tau inversion recovery) uses a short inversion time (TI)
FLAIR uses a long inversion time (TI)

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What are the benefits of chemical fat saturation (FS) techniques on MRI and what are the downsides?

What are the disadvantages of fat sat techniques?

It can be applied to nearly any sequence type including those collected post contrast (this differs from STIR which would be impacted by contrast)

It increases the amount of noise (salt and pepper) in the image

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How does contrast work on a molecular/ physics level for MRI?

Contrast results in faster proton relaxation, meaning the affected protons will have a short T1 relaxation time and therefore be brighter on T1 imaging

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What sequences or techniques may improve the sensitivity of post-contrast effects on an MRI image?

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Fat saturation
Inversion recovery
Subtraction images

What is brownian motion?

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The random movement of small molecules due to thermal collisions

How is an apparent diffusion coefficient (ADC) map created?

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Created by running two gradient echo sequences back to back (DWI) and mapping the changes between those images

If there is no movement of water (the images are identical) then there will be no signal (black) = restricted diffusion

If there is lots of movement of water then there will be lots of signal (bright) = normal or increased diffusion

What substances would be bright on DWI and dark on ADC map based on the basics of MRI physics?

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This pattern is consistent with restricted diffusion (lack of water flow) which is most common associated with cytotoxic edema (frequently associated with infarcts)

What are the advantages and disadvantages of T1 weighting on MRI?

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Advantages:
* Fast scan which means less noise and better anatomic detail
* Can be utilized with contrast to improve lesion conspiquity

Disadvantages:
* Poor tissue contrast (generally appears more greyed out)

What is the expected appearance of bone on T1 weighted sequences?

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Black

What is the expected appearance of air on T1 weighted sequences?

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Black

What is the expected appearance of fat on T1 weighted sequences?

White

What is the expected appearance of CSF on T1 weighted sequences?

black

What is the expected appearance of white matter on T1 weighted sequences?

Grey (lighter)

What is the expected appearance of grey matter on T1 weighted sequences?

Grey (darker)

Name 6 substances that are hyperintense on T1 weighted images?

1) Fat 2) Gadolinium 3) Methemoglobin (both intra- and extracellular) 4) Proteinacious substances (vasopressin, cysts) 5) Melanin 6) Manganese (other minerals)

What are the advantages and disadvantages of T1 weighting on MRI?

Advantages: * Good image contrast * Most lesions are high in water content and are highlighted well on T2W images Disadvantages: * Takes a longer time than other sequences to acquire * Anatomic detail is not as good as other sequences (higher TE = higher noise)

What is the expected appearance of manganese on T1 weighted sequences?

Bright

What is the expected appearance of manganese on T2 weighted sequences?

Neutral

What is the expected appearance of melanin on T2 weighted sequences?

Dark

What is the expected appearance of CSF on T2 weighted sequences?

Bright

What is the expected appearance of fat on T2 weighted sequences?

Bright

What is the expected appearance of gadolunium on T2 weighted sequences?

Neutral or dark (dark at higher concentrations)

What is the expected appearance of minerals on T2 weighted sequences?

Dark

What is the expected appearance of air on T2 weighted sequences?

Dark

What are the advantages and disadvantages of using STIR MRI sequences?

Advantages: * Can improve conspicuity of lesions located adjacent to or within fat Disadvantages: * Cannot be used after administering contrast media due to the effects of contrast change tissue properties to act more like fat - therefore contrast enhancing objects may be suppressed

For what reason should STIR not be run after administration of contrast agent?

Contrast changes tissue properties to act more like fat - therefore contrast enhancing lesions may be suppressed on STIR sequences similar to fat

What are the advantages and disadvantages of using FLAIR MRI sequences?

Advantages: * Water rich lesions remain bright while pure fluids are suppressed which allows for better highlighting of lesions in or near CSF Disadvantages: * Few disadvantages

What substances will typically void on gradient echo sequences?

Hemorrhage Occult mineralization Gas-fluid interfaces

Why are T2 GRE sequences preferred over T1 or other types of GRE sequences?

Susceptibility artefact is more pronounced on T2 GRE sequences making it better than other at picking up hemorrhage

What other types of lesions (aside from infacts) may show up bright on DWI and dark on ADC (restricted diffusion)?

Very dense/cellular tumors (may correlate with a higher grade) Intracranial abscesses

IVD3 - MRI Sequences Flashcards

(44 cards)