MRI Physics Week 2 Chapter 2

Intrinsic contrast parameters are factors that can be changed at the MRI console.

True/False

False

T1 recovery, T2 decay, and proton density are intrinsic parameters.

True/False

True

TR and TE are extrinsic contrast parameters.

True/False

True

Relaxation refers to the process where spins gain energy.

True/False

False

Relaxation results in recovery of longitudinal magnetization and decay of transverse magnetization.

True/False

True

T1 recovery is caused by spin-spin interactions.

True/False

False

T2 decay is caused by spin-spin relaxation.

True/False

True

T1 relaxation time is the time it takes to regain 63% of longitudinal magnetization.

True/False

True

T2 relaxation time is the time it takes to lose 63% of transverse magnetization.

True/False

True

Proton density refers to the number of mobile hydrogen protons per unit volume.

True/False

True

Tissues with higher proton density give a higher signal.

True/False

True

High proton density

Hyperintense signal

Low proton density

Hypointense signal

Fat

Has short T1 and short T2 relaxation times

Water

Has long T1 and long T2 relaxation times

Short T1

Characteristic of fat

Long T1

Characteristic of water

Short T2

Tissue where transverse magnetization fades rapidly (signal lost quickly)

Long T2

Tissue where transverse magnetization persists longer (signal lasts longer)

Fat appears bright on T1

High signal intensity on T1-weighted images

Water appears dark on T1

Low signal intensity on T1-weighted images

Water appears bright on T2

High signal intensity on T2-weighted images

Fat appears dark on T2

Low signal intensity on T2-weighted images

Fat appears bright on T1-weighted images because its longitudinal magnetization recovers quickly.

True/False

True

Water has a long T1 and appears bright on T1-weighted images.

True/False

False

On T2-weighted images, water appears bright because it maintains signal longer.

True/False

True

Tissues with low proton density have low signal and appear hypointense.

True/False

True

T1-weighted

Short TR & Short TE

T2-weighted

Long TR & Long TE

Proton Density (PD)

Long TR & Short TE

Diffusion

Movement of molecules due to thermal motion

ADC (apparent diffusion coefficient)

Measures net displacement of molecules

DWI (diffusion-weighted images)

Produces images based on ADC contrast

BOLD (blood oxygenation level dependent)

Uses oxy- vs. deoxyhemoglobin to map brain activity

Functional MRI (fMRI)

Rapid imaging to assess function based on blood oxygenation

Magnetization Transfer contrast (MTC)

Uses energy exchange between bound/free nuclei

Susceptibility Weighting imaging (SWI)

Involves differences in magnetic susceptibility between tissues

Contrast Agents

Substances that indirectly affect T1 or T2 times

In radiography, intrinsic contrast parameters include the density of structures through which the X-ray beam passes and is attenuated by, while_____contrast parameters include the exposure factors.

extrinsic

If there is no contrast, it is impossible to identify _____ or detect abnormalities within the body.

anatomy 

If a hydrogen nucleus lies in an area of ________with lower field strength, the precessional frequency of its magnetic moment decreases, i.e. it slows down.

inhomogeneity 

The term _____refers to the loss of coherent transverse _______,and T2 relates to the fact that it is the secondary______process.

decay, magnetization, relaxation 

According to classical theory, the ______gradually realigns itself in the longitudinal plane as the proportion of spin-up and spin-down hydrogen nuclei changes.

NMV

Is this the correct definition of Echo time?

time taken for 63% of the longitudinal magnetization to recover.

no

Is this the correct definition of T1 recovery time?

time taken for 63% of the longitudinal magnetization to recover.

yes

The precessional frequency of magnetic moments of the hydrogen nuclei within a tissue varies slightly, but efficient energy exchange, due to ______motion, is optimal at the Larmor frequency.

molecular 

A tissue has a high signal if it has a large transverse component of_____magnetization at time TE.

coherent 

T2 decay in water is less efficient than in fat, as the molecules are spaced apart, and spin–spin ______are less likely to occur.

interactions 

In water, molecular mobility is high, resulting in less efficient T1 ______because the molecular ________rate does not match the Larmor frequency and does not allow efficient energy exchange from hydrogen nuclei to the surrounding molecular ________.

recovery, tumbling, lattice 

Image contrast derived from the differences in the T1 recovery times of the tissues rather than any other mechanism.

T1 contrast 

If the TR is longer than the ________times of the tissues, full recovery occurs in all tissues, and, therefore, it is not possible to produce an image that demonstrates contrast based on the differences in their T1 recovery times.

relaxation 

If the TE is short, then little _______occurs, and therefore it is not possible to produce images that demonstrate differences in T2 decay times of the tissues.

dephasing 

The magnitude of the transverse component of magnetization for fat and water depends only on their individual proton densities rather than the rate of_________of their longitudinal components.

recovery 

If the TR is too long, both the _______in fat and water return to B0 and fully recover their longitudinal magnetization.

vectors 

Image that demonstrates the differences in the T2 decay times of tissues.

T2-weighted image 

PD-weighted image is one where differences in the number of mobile hydrogen nuclei per unit volume of tissue are the main ________factor in forming image contrast Table (2.10).

determining 

Fat is an unreliable marker, as it is bright in many types of _______depending on the pulse sequence.

weighting 

A short TE does not give the _______in fat or water time to dephase and so diminishes T2 contrast (Equation (2.7)).

vectors

The area labeled A, which has a high signal, is not _______but slow-flowing blood in the superior sagittal sinus.

fat

In diffusion imaging, normal tissue that exhibits a high ADC has a lower signal _______than abnormal tissue that has a low ADC, as the molecules within it are free to move.

intensity

BOLD exploits differences in the magnetic ________of oxyhemoglobin and deoxyhemoglobin because of increased cerebral blood flow and little or no increase in local _______consumption that occurs during ____.

susceptibility, oxygen, stimulation 

Diffusion is a term used to describe the movement of _______in the extracellular space due to random thermal motion.

molecules 

T1 agent

agents that shorten T1 relaxation in tissues that take up the agent

How is longitudinal recover referred to?

T1 relaxation 

Decay of coherent transverse magnetization is:

T2 weighting

What is the effect of a substance on the relaxation rate called?

Relaxivity

What is the term that is used to describe the movement of molecules in the extracellular space due to random thermal motion? 

Diffusion 

The voltage that is induced in the receiver coil is the:

Signal

The first few radio frequency (RF) excitation pulses are:

Preparatory/Dummy

BOLD is the image contrast that depends on a _____ process?

Physiological

Saturation occurs when the NMV is flipped to ____ degrees.

180

Which of the following is an intrinsic contrast parameter in MRI?

T1 recovery time

What defines intrinsic contrast parameters in MRI?

Tissue-dependent properties that cannot be changed

Which of the following is an extrinsic contrast parameter in MRI?

TE

What defines extrinsic contrast parameters in MRI?

Settings that can be adjusted at the console

What does the term "relaxation" refer to in MRI physics?

The process by which spins lose energy

What causes T1 recovery in fat tissue to be fast?

Rapid NMV realignment with B₀

What percentage of longitudinal magnetization must be regained to reach T1 relaxation time?

63%

In which plane does T1 recovery occur?

Longitudinal

What causes T2 decay in MRI?

Spin–spin interactions

What percentage of transverse magnetization must be lost to reach T2 relaxation time?

63%

In which plane does T2 decay occur?

Transverse

What does proton density measure in MRI?

The number of protons per unit volume of tissue

How does proton density affect MRI signal?

Higher proton density yields more signal

What does T1 and T2 relaxation depend on in MRI?

Tissue characteristics

Why is the T1 recovery time of fat short?

NMV of fat realigns rapidly with B₀

Why is the T1 recovery time of water long?

Water has high inherent energy and resists lattice absorption

Why is the T2 decay time of fat short?

Closely packed molecules cause rapid dephasing

Why is the T2 decay time of water long?

Molecules are spaced apart, causing slow dephasing

Which tissue appears bright on T1-weighted images?

Fat

Which tissue appears dark on T1-weighted images?

Water

Why does fat appear bright on T1-weighted images?

More longitudinal and transverse magnetization

Why does water appear dark on T1-weighted images?

Less longitudinal and transverse magnetization

Which tissue appears bright on T2-weighted images?

Water

Which tissue appears dark on T2-weighted images?

Fat

Why does water appear bright on T2-weighted images?

Large magnitude of transverse magnetization

Why does fat appear dark on T2-weighted images?

Transverse magnetization decays quickly due to short T2

How does tissue with high proton density appear on PD-weighted images?

Bright

How does tissue with low proton density appear on PD-weighted images?

Dark

What TR and TE combination produces a T1-weighted image?

Short TR (300–700 ms), Short TE (10–25 ms)