MRI

What property of atomic nuclei makes MRI possible?

Nuclear spin

Why is hydrogen (¹H) the most important nucleus in MRI?

It is abundant in the body and has a strong magnetic moment

What causes net magnetization in a tissue placed in a magnetic field?

A slight excess of spins in the low-energy state

Why does increasing the main magnetic field strength increase MRI signal?

It increases spin polarization and net magnetization

What is nuclear spin polarization?

The population difference between spin energy states

What is Larmor frequency?

The precession frequency of nuclear spins in a magnetic field

How does Larmor frequency depend on magnetic field strength?

It is proportional to field strength

What is resonance in MRI?

Energy exchange between spins and RF field at the Larmor frequency

What happens to net magnetization during a 90° RF pulse?

It is tipped from the z-axis into the transverse (xy) plane

Why is transverse magnetization necessary for signal detection?

Only changing transverse magnetization induces a signal in coils

What is relaxation in MRI?

Return of magnetization to equilibrium after excitation

What is T1 relaxation?

Recovery of magnetization along B0

What is T2 relaxation?

Loss of phase coherence in the transverse plane

What physical process causes T1 relaxation?

Energy exchange between spins and their surroundings

What causes T2 relaxation?

Random magnetic field fluctuations between nearby spins

What is the basic structure of a spin-echo sequence?

→ 90° RF pulse → 180° RF pulse → echo.

What is the role of 180 RF pulse in spin-echo imaging?

Refocuses dephasing due to field inhomogeneities

How is an echo formed in a gradient-echo sequence?

By reversing the polarity of a gradient

What advantages do gradient-echo sequences have?

Faster imaging and shorter TR/TE

Which operator-controlled parameters influence image contrast?

TR, TE, TI and flip angle

T1 weighted image?

Short TR and short TE, fat is bright and CSF is dark

T2 weighted image?

Long TR and long TE, bright CSF

PD weighted image?

Long TR and short TE

What is inversion recovery and its purpose?

A sequence starting with a 180 inversion pulse, to null signal from specific tissues

Which signal does FLAIR suppress and how?

CSF signal and by using TI = CSF null point

Which signal does STIE suppress and how?

Fat, exites tissue when fat magnetization is 0

What is the basic idea behind the Dixon method?

Separating fat and water using phase differences

Why are gadolinium-based contrast agents effective in MRI?

They are strongly paramagnetic

What are magnetic field gradients used for in MRI?

Spatial localization of signal

What is slice selection and how is it achieved?

Exciting a specific slice using RF + gradient

What is phase encoding?

Encoding spatial position using phase shifts

What is frequency encoding?

Encoding position using frequency differences (applied during signal acquisition)

What is k-space?

Raw spatial frequency data matrix

Why is FFT essential for MRI image reconstruction?

It converts frequency dat into spatial data

Why are MRI magnets superconducting?

To generate strong stable field with low power loss

What are the three types of magnetic fields used in MRI?

Static, gradient and RF fields

What is Fast spin echo?

Multiple echoes collected per TR

What is Echo planar imaging (EPI)?

Very fast imaging acquiring whole k-space per excitation

What is the basic idea behind SENSE or GRAPPA?

Using coil sensitivity profiles to reconstruct missing data