5.1. MRI

What is the Net Magnetic Vector (NMV)?

The net magnetic moment of hydrogen nuclei that produces a significant magnetic vector useful in clinical MRI

What happens to the NMV at equilibrium?

It aligns with the external magnetic field (B0)

What is precession in MRI?

The additional spin or wobble of magnetic moments of hydrogen produced by the influence of an external magnetic field (B0)

What is the Larmor (precessional) frequency?

The rate of rotation of magnetic moments around the axis under the influence of B0

What is the formula for Larmor frequency?

w0 = γ × B0 (precessional frequency = gyromagnetic ratio × magnetic field strength)

What is the gyromagnetic ratio of hydrogen?

42.57 MHz/T

What is the Larmor frequency of hydrogen at 1.5T?

63.86 MHz

What is resonance in MRI?

An exchange of energy between two systems at a specific frequency

What are the effects of resonance on the NMV?

The NMV gains energy and flips at a flip angle — typically 90 or 180 degrees

What determines the magnitude of the flip angle?

The amplitude and duration of the RF pulse

What two components are produced after the RF pulse?

Longitudinal magnetisation (Mz) and transverse magnetisation (Mxy)

What happens to longitudinal magnetisation (Mz) when the RF pulse is turned off?

It increases and recovers — this is T1

What happens to transverse magnetisation (Mxy) when the RF pulse is turned off?

It decreases and decays — this is T2* and T2

What are T1 and T2?

Two independent processes happening simultaneously that produce different signal characteristics on MRI images

What is Faraday's law of induction in the context of MRI?

If a receiver coil is placed in the area of a moving magnetic field, a voltage is induced — this becomes the MRI signal

What are the advantages of MRI?

Non-invasive, no ionising radiation, multi-planar 3D imaging, excellent soft tissue contrast, non-contrast angiographic exams available (TOF), contrast agents well tolerated

What are the disadvantages of MRI?

Expensive, time-consuming, claustrophobia, not suitable for everyone, noisy, risk of heating the patient, body habitus limits

What is an RF burn in MRI?

A burn caused by small currents passing through areas of skin contact, dissipated as heat — the most reported side effect of MRI

How do radiographers prevent RF burns?

By ensuring minimal skin contact and using low absorption rates on the scanner

What are the MRI safety implications/contraindications?

Metal in eyes or shrapnel, cardiac pacemakers, insulin pumps, ferromagnetic aneurysm clips, temporary pacing leads, medicine patches, contrast-associated risks

How are MRI safety incidents prevented?

The patient completes a safety form prior to the scan, which is checked by a radiographer to determine contraindications

What are examples of metal artefacts in MRI?

Fixed dental braces, hair extensions, airgun pellets, knee replacements

What are examples of motion artefacts in MRI?

Breathing motion, head movement, blinking

What are the clinical applications of MRI?

Tumours, bones and joints, brain, spine, vascular imaging, treatment planning

Why is MRI usually used as part of a diagnostic pathway rather than first-line?

It is not as accessible as other modalities and is used as a problem-solving tool to complement other imaging

What is gadolinium used for in MRI?

As a contrast agent to demonstrate vascularity, delineate surrounding tissues, and show infection and inflammation

How does gadolinium contrast work?

It changes the signal of tissues with high vascularity, making them appear differently on T1 post-contrast sequences

What are typical MRI scanner field strengths used in healthcare?

1.5 Tesla or 3 Tesla

What are typical MRI bore sizes?

55-70 cm

What types of MRI scanners exist?

Static scanners and mobile vans

What are the magnet wires in an MRI scanner made of?

Niobium-titanium wires encased in copper

What principle does MRI work on?

The interactions between a magnetic field and water (hydrogen atoms)

Why is hydrogen used in MRI?

It is the simplest atom (1 proton, 0 neutrons, 1 electron), is unstable and therefore MRI active, and is present in large amounts in the body in water and fat

What are the three types of atomic motion relevant to MRI?

Electrons spinning on their own axis, electrons orbiting the nucleus, and the nucleus spinning about its own axis

What happens to spinning nuclei according to electromagnetic induction?

As the nucleus spins it induces a magnetic field, producing angular momentum (net spin)

What is the difference between stable and unstable atoms in MRI?

Stable atoms have equal spin-up and spin-down nucleons that cancel out; unstable atoms have a net spin or angular momentum

What is B0?

The external magnetic field in MRI, equal to 1 Tesla or 10,000 Gauss

How are magnetic moments aligned outside B0?

They are randomly aligned

How are magnetic moments aligned inside B0?

They align parallel or anti-parallel to the external magnetic field

What is multiplanar imaging in MRI?

The ability to image in axial, sagittal, and coronal planes

What is MRI not beneficial at looking at?

Cortical bone

The net energy produced by hydrogen protons aligning within an external magnetic field is called:

NMV (Net Magnetisation Vector)

What is Susceptibility Weighted Imaging (SWI) sensitive to?

The presence of metal-like components

What is the intra-venous contrast used in MRI called?

Gadolinium