MRI image formation

What is repetition time (TR) determine?

How much longitudinal magnetisation occurs before the next radiofrequency pulse

What does the echo time (TE) determine?

How much transverse magnetisation occurs before the next radiofrequency pulse

What are the four parts of spin echo sequence?

Excitation pulse

Dephasing

Signal loss

Signal regeneration

What occurs when the 90 degree excitation pulse is applied?

90 degree flip angle into the transverse plane induces a voltage in the receiver coil

What occurs when the excitation pulse is removed?

Free induction decay

What is dephasing?

Hydrogen nuclei stop spinning in phase

What is free induction decay?

Loss of signal intensity when vectors of magnetic spins dephase

Why does dephasing occur?

Due to inhomogeneities within the magnetic field

What are inhomogeneities in the magnetic field?

Areas that vary slightly from the magnetic field strength

What is the relationship between precessional frequency and magnetic field strength?

Precessional frequency increases as magenetic field strength increases

What occurs when an 180 degree rephasing signal is applied?

Faster spins catch up to slower spins so that they come to the same point in the precessional path

Why is signal regeneration necessary for image acquisition?

Maximum signal is induced in receiver coil by reducing T2 dephasing allowing for longer relaxation time

What type of radiofrequency pulse do gradient echo sequences use?

Variable excitation pulses so not all longitudinal magnetisation is converted to transverse plane

What occurs when the radiofrequency pulse is withdrawn in gradient echo sequences?

Free induction decay signal is produced

T2 star dephasing occurs

What happens to magnetic moments in gradient echo sequences when they dephasing?

Released by magnetic gradients

What is t2 relaxation?

Time it takes for the transverse magnetisation to decay 37% of its original value due to interactions between nearby spins

What is the cause of t2 relaxation?

Microscopic magnetic field variations from neighbouring nuclei

What is T2 star relaxation?

Observed decay of transverse magnetisation including T2 decay and magnetic field inhomogeneities

What is the cause of t2 star relaxation?

Intrinsic spin spin interactions and external magnetic field variations

What is the function of gradient coils?

Generate linearly varying magnetic fields along three axes

Are superimposed on the main magnetic field

How do gradient fields affect resonance frequency of hydrogen?

Varies with position across the gradient field causing them to emit different frequencies

What is slice selection?

Rf pulse applied at a specific frequency to only excite spins in a thin slice along the z axis

What is frequency encoding?

Gradient applied so spins in different positions process at different frequencies along the x axis

What is phase encoding?

Gradient applied before signal causing spins in different positions to accumulate different phase shifts along the y axis

What are the three gradient coils?

Z plane coil

Y plane coil

X plane coil

How are gradient echo sequences induced?

Low flip angle radiofrequency pulse applied

Spins begin to dephase

Reversal of frequency encoding gradient to rephrase spins

Signal detection

What is spatial encoding?

Scanner determines where signal is coming from by turning gradient coils on and off

What is the k space?

Data space where raw MRI signals are stored before image reconstruction

What does each point in the k space represent?

Specific spatial frequency and phase of the mri signal

How are frequencies transferred into images?

Using Fourier Transform

What is stored in the centre of the k space?

Low spatial frequencies which create contrast

What is stored at the edges of the k space?

High spatial frequencies which create fine details

What is an inherent property of k space and how can it be utilised?

Mathematical symmetry can be used to reconstruct a full image with half of a k space

What are the five steps of image formation?

Excitation

Spatial encoding

Signal acquisition

Fourier transform

Image reconstruction