Week 9 - Magnetic Resolution Imaging

What are three components that an MRI uses?

• Magnetic field

• Radio-waves

• Computer

How are patients kept still during MRI?

Sedation or GA

Why is GA or sedation needed for MRIs?

If a patient moves it messes with the image and generally have to restart

What is the Gauss line?

Perimeter of magnetic field

What must stay out of the gauss line?

Ferrous metal

What protons are used and where are they found in the body?

Hydrogen protons found in the tissues

What are 3 precautions to take with MRIs despite no ionising radiation?

• Patients with metal implants

• Operators with pace makers, implants or pregnant

• Ear protection for high field unit as very noisy

How much water is the body made of?

70%

What is the magnetic property of hydrogen?

Protons

How is an image produced? - 7 steps

1. Magnetic field is applied to the structure

2. Spinning protons rotate to align with the magnetic

3. A series of radio frequency waves are applied

4. This forces the proton away from the magnet to 90-180 degrees

5. Transmission of RFW ceased, the protons return to their original position with the magnet

6. This movement and energy this causes is detected via sensors (RFC) within the system

7. These signals are then converted from analogue to digital images

What does this picture show?

T1 relaxation

What does this picture show?

T2 relaxation

What collects the data in an MRI machine

Radiofrequency coil

What is the purpose of a radiofrequency coil?

Sends and receives radio waves

What is the job of the radiofrequency waves?

• Sends the RF pulse that tips the protons

• Listens for the MRI signal coming back

What is the purpose of a gradient coil?

Creates magnetic field variations

What is the job of the gradient coil?

• Changes the magnetic field slightly in different directions

• Allows the MRI to know where the signal is coming from

What is produced from an MRI?

Cross sectional images in three planes

What are the clinical application of MRI?

• Predominately soft tissue

• Inflammatory process in bones

• Sensitive to bone density changes and remodelling

• Pre-fractural plains

• Single lobe damage to tendon and ligament tissue

What are do fields refer to?

The strength of the magnetic field?

3 main points about low field?

• 0.27 tesla

• Temperature controlled by the environment

• Less sensitive than high field but tolerates more movement

1 pro and 1 con of low field?

• Cheap to install and maintain

• Slight reduction to image quality BUT may tolerate more movement better

2 main points of high field

• > 1 tesla

• Uses liquid helium to enable superconducting

1 pro and 3 cons of high field

1. High image quality

2. Very sensitive - even to breathing

3. More expensive than low field

4. Artefacts increased around metal objects

What should the building/housing be cased in?

Copper lining

How does hygiene correlate with MRIs?

• Keeps patients safe

• Protects equipment

• Improves image quality

• Supports infection control