1.4 Introduction to Magnetic Resonance Imaging Flashcards Preview

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Flashcards in 1.4 Introduction to Magnetic Resonance Imaging Deck (11)
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1
Q

Describe the mechanism of acquisition for an MRI image.

A

MRI machine has a superconducting magnet which creates a permanent magnetic field of 1-2 T (which is greater than earth’s natural magnetic field of 50 uT).

All hydrogen atoms in the body align either parallel or anti-parallel to this field.

Coils in the magnet emit radiofrequency pulses. These cause the protons to change their orientation.

The coils emitting the RF pulses turn off, and as they do so, the protons realign to the magnetic field, releasing signals in the form of RF pulses.

These signals are read by receiver coils, which then turn the pulses released by the tissue into an image.

2
Q

Explain the terms T1 and T2 with regards to proton realignment time.

A
3
Q

Explain how T1 and T2 weighted images are created.

A
4
Q

Define the terms ‘hyperintense’ and ‘hypointense’ for an MRI.

A
5
Q

What appears bright on T1 image vs. on T2 image?

A

‘Proteinaceous fluid’ - e.g. blood in haemorrhagic cyst.

6
Q

Note the differences in imaging on the brain and abdomen for T1 vs. T2 image.

A

T1 Image

  • CSF and kidney cystic fluid are both dark
  • Subcutaneous fat is bright
  • Liver is rather bright

T2 Image

  • CSF and kidney cystic fluid are both bright
7
Q

Explain what ‘suppression’ in MRI imaging.

A

Suppression of signals from a certain normally bright tissue can be used in MRI, e.g. fat suppression or fluid suppression.

This is to allow better characterisation of other hyperintense tissues.

Fat suppression is used in contrast-enhanced images to better see the bright contrast.

The image shows a T1 image with and without fat suppression - you can see it helps us to determine that the scrotal mass of the patient contains mainly fat.

8
Q

What is ‘FLAIR’ imaging?

A

FLAIR imaging is where fluid is signal-suppressed in MRI.

Stands for ‘fluid attenuation inversion recovery’.

T2 weighted MRI sequence in which the CSF fluid is suppressed so other T2 hyperintensities are easier to see.

9
Q

What is the contrast fluid of choice in MRI? How does it work?

A

Gadolinium

  • Heavy metal chelated with other compounds
  • Injected IV but can also be injected intra-articularly
  • Excreted renally
  • Shortens T1 relaxation which results in structures appearing brighter on postcontrast
    T1 weighted images than on pre-contrast T1 weighted images

Especially useful in picking up structures of increased blood flow or inflammation, e.g. tumours. (https://www.drugwatch.com/gadolinium/)

10
Q

What is a major contraindication for gadolinium use in patients?

A

eGFR <30, as it can cause nephrogenic systemic fibrosis.

11
Q

Name a major contraindication for MRI.

A

Patients with metal implants or devices, e.g. cochlear implants, pacemakers, pumps.

OR

Metal shrapnel and objects in sensitive locations, e.g. eyes, where movement can cause serious damage.

The MRI magnet is always ON and can either cause these objects to overheat, or to become airborne, or move, causing damage.

Because of this, many non-ferromagnetic devices, joint replacements, aneurysm clips and more are being developed, with some already in circulation.