MRI - MIC Flashcards Module 1

Which dephasing mechanism is NOT rephased by a 180-Degree RF pulse?

Spin-spin interactions (T2 relaxation)

What determines the precessional frequency of protons’ net magnetization?

The magnetic field strength (Via the Larmor equation)

Why are subsequent echoes smaller than the first after a 90-degree RF pulse?

Due to spin-spin interactions that cause signal loss (T2 relaxation)

What happens to proton magnetization when RF energy is applied at their precessional frequency?

The net magnetization tilts into the transverse plane, producing a measurable signal.

As protons move closer to the MR magnet, what happens to their precessional frequency?

it increases (Due to higher magnetic field strength)

What are the three requirements for MRI?

Strong magnetic field, external RF energy source, and hydrogen protons

What is T1 relaxation?

Recovery of longitudinal magnetization (Spin-lattice relaxation)

What happens when TR is decreased in a spin-echo sequence?

T1 contrast increases

What RF frequency is needed for protons in a 4T field?

170.4 MHz

What RF flip angle maximizes transverse magnetization?

90-degrees

How many echoes are formed by 6 successive 180-degree RF pulses after a 90-degree pulse?

6 echoes

How do T1-weighted images appear for brain tissues?

White matter = brightest

Gray matter = Intermediate / Medium

CSF = Darkest

What is the time from the 180-degree RF pulse to the echo called?

Echo Time (TE)

How does I.V. contrast affect vascular lesion appearance on T1-weighted images?

Lesion appears brighter due to shortened T1

What gives protons their magnetic properties

Their charge and Spin

Which mechanisms affect spin echo size?

Spin-spin interactions, magnetic field inhomogeneities, chemical shift, susceptibility

If the magnetic field is along the Z-axis, what is the transverse plane?

XY plane

If the magnetic field is along the Y-axis, what is the transverse plane?

XZ plane

How do hydrogen protons behave in an MR magnet?

They align parallel or antiparallel to the field, creating net magnetization.

What is the time from RF excitation to echo called?

Echo Time (TE)

How does edema appear on T2-weighted images?

Bright

How is the MR echo detected?

Induced voltage in an RF coil from precessing transverse magnetization

If no spin-spin interactions exist, how do the first and second echoes compare?

They would be equal

How is energy released from tissues after RF pulse?

Through T1 and T2 relaxation

Why is CSF bright on T2-weighted images?

It has a long T2 relaxation time

What is another name for T1 relaxation?

Spin-lattice relaxation

What parameter controls contrast from T2 relaxation?

TE (Echo Time)

What happens if a single 45-degree RF pulse is applied to resting protons?

Partial net magnetization enters transverse plane; smaller signal is generated

What determines the precessional frequency of protons?

Magnetic field strength and gyromagnetic ratio (Larmor equation)

How do T1 and T2 times differ?

T1 = regrowth of longitudinal magnetization
T2 = decay of transverse magnetization

How do T1 times affect image brightness?

Shorter T1 = brighter signal on T1-weighted images

How does a short T1 time affect tissue brightness on T1-weighted images?

The tissue appears bright

What is the time from one RF excitation to the next called?

Repetition time (TR)

What is energy transfer between protons called?

Spin-spin interaction

What is T2 relaxation time?

Time for transverse magnetization to decay to 37% due to spin-spin interactions

What happens to precessional frequency if magnetic field increases?

It increases

In a 6-echo spin echo sequence, which echo has the most signal?

The first echo

Which parameter controls T2 contrast in an image?

TE (Echo Time)

What flip angle maximizes transverse magnetization at spin echo time?

90-degrees

What happens if TE is increased from 80ms to 120ms?

T2 contrast increases