Gradients in MRI

What is the purpose of gradients?

Gradients are used for spatial encoding to localize signal in the body by generating spatial variations in B0 or the main magnetic field

What are the three components of Gradients?

Gradient Coils, amplifiers and a cooling system

Z gradient

alters the long axis

Y gradient

alters the vertical axis

X gradient

alters the horizontal axis

x

sagittal

y

coronal

z

axial

what is the function of gradient amplifiers?

Gradient amplifiers supply the necessary current to the gradient in the coils during data acquisition

older gradient amplifiers used what?

Linear Analog Amplifiers (LAA) - prone to power loss and heating

Newer Models of gradient amplifiers use?

Pulse Width Modulated (PWM) design - improve efficiency by applying voltage in bursts (pulses), reducing heating and allowing the coil to interpret the supply as smooth and continuous if pulses are closely spaced

Amplitude - in other words means

strength

What does amplitude mean of gradients?

defines the power of the gradient and how steep or strong the gradient slope can be

What does amplitude determine?

spatial resolution

The units for amplitude are

mT/m or G/cm

The values of amplitude typically are

< 40 mT/m

rise time is another name for

speed

rise times is defined as

the time it takes a gradient to reach maximum amplitude

rise time is typically in units of

ms (milliseconds)

The typical values for rise time is

< 1000 ms

A diffusion is typically what value?

130ms

low amplitude gradients typically have ___________ times so its not a sole indicator of performance

shorter rise

Slew rate is

the speed(rise time) and strength(amplitude) of the gradient

slew rate is calculated as

amplitude/rise time

the units for slew rate are

mT/m/s

What does slew rate describe?

it describes how quickly the gradient magnetic field can be applied at a given amplitude over a given distance

What does duty cycle mean?

defines the percentage of time the gradient is permitted to work or spends at maximum amplitude during a TR period

High duty cycle =

heating

What helps reduce high duty cycle?

parallel imaging helps reduce RF exposure and scan time

What is a balanced gradient system in MRI, and what does it mean when "net gradient dephasing over the TR interval is zero?

A balanced gradient system is one where all gradient-induced phase shifts are fully reversed within each TR cycle.
"Net gradient dephasing over TR is zero" means that any dephasing caused by gradients is canceled out by the end of the TR, allowing spins to return to their original phase.
This results in a strong, steady signal and is used in sequences like bSSFP (e.g., TrueFISP, FIESTA) for high-contrast imaging of fluids and soft tissues.

Gradients alternately perform three main tasks for spatial localization:

slice thickness, slice selection, and frequency + phase encoding

Slice thickness is determined by what?

by the slope of the slice select gradient and the transmit bandwidth of RF

A steeper slope =

thinner slice

A shallower slope =

thicker slice

Why is the slice select gradient applied during both the 90° and 180° RF pulses in a spin echo sequence?

To ensure that both the initial excitation and the refocusing occur in the same imaging slice. The gradient selects the correct spatial location (slice) by making only the protons in that region resonate with the RF pulse.

During a spin echo sequence

slice select is on during the 90 degree and 180 degree pulses

dephasing and rephasing do what?

gradients cause magnetic moments to dephase (precessional frequencies slow down or speed up depending on location along the gradient) and then can be used to rephase them to generate an echo

What are eddy currents?

induced by rapidly switching gradients on and off in nearby conducting components (coils, main magnet, cryoshields)

eddy currents can cause an artifact called

Nyquist N/2 ghost

Eddy current can effect certain sequences:

DWI, MR Angio, Gradient echo, and sequences w/ very short TE.

Faradays law of induction

a changing magnetic field through a circuit induces an EMF (voltage)

Lenz law states

the induced current will create a magnetic field that opposes the change in the original magnetic flux

lorentz force

describes the force exerted on a charged particle moving in electric and magnetic fields