MAP.11 Pulse Sequences and Natural Tissue Contrast

This set of 50 flashcards covers MRI physics, including T1 and T2 relaxation times for various biological tissues, pulse sequence parameters (TE and TR), and the characteristics of T1-weighted, T2-weighted, and Proton Density weighted images.

What is the definition of Time to Echo (TE)?

The time between the $90^\circ$ pulse and receipt of the echo signal from the sample.

What is the definition of Time to Repeat (TR)?

The time between successive $90^\circ$ radio frequency (RF) pulses.

At a magnetic field of $1\,Tesla$, what is the T1 relaxation time for fat?

$$250\,ms$$

At a magnetic field of $1\,Tesla$, what is the T2 relaxation time for fat?

$$80\,ms$$

What is the T1 relaxation time for liver tissue at $1\,Tesla$?

$$450\,ms$$

What is the T2 relaxation time for liver tissue at $1\,Tesla$?

$$40\,ms$$

At a magnetic field of $1\,Tesla$, what is the T1 relaxation time for kidney?

$$550\,ms$$

At a magnetic field of $1\,Tesla$, what is the T2 relaxation time for kidney?

$$60\,ms$$

What are the T1 and T2 relaxation times for muscle at $1\,Tesla$?

$T1 = 800\,ms$ and $T2 = 45\,ms$

What is the T1 relaxation time for white matter in the brain?

$$650\,ms$$

What is the T2 relaxation time for white matter in the brain?

$$90\,ms$$

What is the T1 relaxation time for grey matter in the brain?

$$800\,ms$$

What is the T2 relaxation time for grey matter in the brain?

$$100\,ms$$

What are the characteristic relaxation times for cerebrospinal fluid (CSF)?

$T1 = 2000\,ms$ and $T2 = 150\,ms$

What are the T1 and T2 relaxation times for water?

Both are $3000\,ms$ ($T1 = T2$).

What are the relaxation characteristics for bone and teeth?

T1 is very long and T2 is very short.

What is the general relationship between T1 and T2 in most tissues?

$T1 > T2$, except for water where they are equal.

How much more water does grey matter contain relative to white matter?

$10\text{-}15\%$ more water.

What two effects contribute to the decay of transverse magnetization ($B_T$)?

Intrinsic and extrinsic effects.

How can the extrinsic effect be suppressed to recover the true amplitude of the decaying $B_T$ signal?

By using a spin-echo sequence to force dephasing proton spins to precess in phase again.

What specific RF pulse is applied to create an 'echo' signal after a $90^\circ$ pulse?

A $180^\circ$ RF pulse.

How is the timing of the $180^\circ$ RF pulse related to the Time to Echo (TE)?

It is applied at half the time to echo ($\frac{1}{2}\text{TE}$).

How is T2 measured in NMR instrumentation?

By measuring the decay of the induced radio signals (echoes) detected in the radio frequency coil or antennae.

Why can T1 not be measured directly through the transverse magnetization signal?

T1 requires a different RF pulse sequence such as varying the Time to Repeat (TR).

What happens to the magnitude of $B_T$ if the Time to Repeat (TR) is shorter than the T1 of the tissue?

The magnitude of $B_T$ is inhibited (reduced) after the subsequent $90^\circ$ pulse.

In pulse timing parameters, what is TE used to probe?

Transverse magnetization ($B_T$) decay.

In pulse timing parameters, what is TR used to probe?

Longitudinal magnetization ($B_L$) recovery.

In T1-weighted images (T1WI), how does a short T1 affect signal and brightness?

Short T1 results in a larger signal and appears bright in the image.

In T1-weighted images (T1WI), how does a long T1 affect signal and brightness?

Long T1 results in less signal and appears dark in the image.

Why does a short T1 result in a larger signal in T1WI?

It allows for a larger longitudinal magnetization ($B_L$) to be recovered before the next pulse.

Which TR setting is used to maximize contrast between T1 properties of different tissues?

A mid TR.

What TE setting is used to minimize the contribution of T2 contrast in T1-weighted images?

A short TE.

In T2-weighted images (T2WI), how does a short T2 affect signal and brightness?

Short T2 results in less signal and appears dark in the image.

In T2-weighted images (T2WI), how does a long T2 affect signal and brightness?

Long T2 results in a stronger signal and appears bright in the image.

Which TE setting is used to maximize T2 contrast in MRI?

A mid TE.

What TR setting is used to minimize the contribution of T1 contrast in T2-weighted images?

A long TR.

What is the primary measurement focus of Proton Density (PD) weighted images?

The number of H-1 protons in an area.

How is T1 contrast minimized for Proton Density weighted imaging?

By using a long TR.

How is T2 contrast minimized for Proton Density weighted imaging?

By using a short TE.

What are the specific pulse timing settings for a T1-weighted image?

Short TE and mid TR.

What are the specific pulse timing settings for a T2-weighted image?

Mid TE and long TR.

What are the specific pulse timing settings for a Proton Density image?

Short TE and long TR.

How does grey matter appear compared to white matter on a T1-weighted MRI?

Grey matter appears dark gray while white matter appears lighter gray.

Describe the appearance of CSF on a T1-weighted MRI.

It is void of signal and appears black.

Which substance appears bright on a T2-weighted MRI?

CSF (cerebrospinal fluid).

On a T2-weighted MRI, which tissue appears as 'darker gray'?

White matter.

On a PD-weighted MRI, which tissue appears bright?

Grey matter.

How much contrast is there between brain and CSF in a PD-weighted image?

There is little contrast between brain and CSF.

In MRI diagnostics, what can structural brain alterations highlight in an older individual?

A mild form of white matter disease.