Factors affecting images

What is signal to noise ratio?

Ratio between signal detected and amplitude of noise

What are five factors afffecting the SNR?

Magnetic field strength

Coil types and position

TR, TE and flip angle

Number of signal averages

Voxel column and slice thickness

How does increasing slice thickness affect the signal?

Increases number of hydrogen atoms that can be used to produce a signal, producing more information/signal

What are the disadvantages of increasing Slice thickness?

Loss of spatial resolution

What is the matrix size?

Number of pixels in the image

How does increasing the matrix size affect the signal?

Decreases the signal to noise ratio

What are the disadvantages of increasing matrix size?

Increase the time

What is the average number of excitations?

Number of times the data is acquired repeatedly to form the same image

What is the disadvantage of increasing the number of excitations?

Increases scan time

How does average number of excitations increase snr?

Superimposes multiple pictures and removes moving artefacts

What are the four types of artefacts in MRI?

Motion

Radiofrequency

Image processing

Magnetic susceptibility

What is phase mismapping?

Replication of anatomy in the phase encoding direction caused by motion

What are four ways to compensate for motion artefacts?

Phase swap

Pre-saturation bands

Respiratory navigator

Patient comfort

What is phase swap?

Swapping phase encoding direction to avoid creating artefact in area of interest

What are the disadvantages of phase swap?

May cause wrap around artefact

What is wrap around artefact?

Signal from anatomy outside of field of view appears flipped on the opposite side of the image

What are presaturation bands?

Null the signal from the area the band is placed on

What is a respiratory navigator?

Monitors signal when anatomy is motionless and data acquired outside these boundaries

What are the disadvantages of respiratory navigator?

Can be inconsistent if patients respiratory rate is inconsistent

What is a zipper artefact?

Interference of external radiofrequency waves disrupting the magnetic field

What is the cause of wrap around artefacts?

Anatomy close to the receiver coil produces a signal

What are the three remedies for wrap around artefacts?

Increase field of view

Saturation bands

Change phase encoding direction

What are the two values determining field of view?

Field of view

Phase field of view

What is field of view?

Total area being imaged

What is the phase field of view?

Portion of field of view used in phase encoding direction

How does changing phase encoding direction reduce wrap artefact?

Avoids anatomy involved in the artefact

How do magnetic artefacts affect images?

Produce signal void

How does magnetic interference work?

Metal objects become magnetised and causes inhomogenities

Dephase hydrogen in surrounding tissues, causing signal loss

What are the two remedies to reduce magnetic susceptibility?

Spin echo sequences

Metal Artefact Reduction Sequences

What are the benefits of paramagnetic properties of deoxygenated blood?

Identify haemorrhages in gradient echo pulses