Assignment 4 (Ch2) review

Flashcards for review of lecture notes in preparation for quiz 2.

What role does TR play in determining MRI contrast?

TR controls the amount of T1 recovery that takes place by controlling the time from one excitation pulse to the next.

How does TE influence T2 weighted imaging?

It controls how much dephasing or decay occurs in the transverse plane prior to echo collection.

Explain the difference between intrinsic and extrinsic contrast parameters.

Intrinsic parameters are inherent to the body tissues and cannot be directly controlled, while extrinsic parameters can be directly controlled at the tech console.

Describe the physiological process of T1 recovery and its significance in image formation.

Nuclei give off energy to the surrounding lattice, causing them to lose energy needed to withstand the pull of B0 and start going back up.

What happens to nuclei during T2 decay, and why is it called spin-spin relaxation?

Nuclei interact with each other, causing them to lose their phase. This interaction is one of the two processes of relaxation.

How do differences in molecular tumbling rates contribute to MRI contrast?

The closer the molecular tumbling rate is to the precessional frequencies of the nuclei, the more efficient the relaxation process will be.

Why does fat appear bright and fluid appear dark in T1-weighted images?

Fat has a faster T1 recovery rate compared to fluids, so it absorbs and gives off more energy over repetitive excitation pulses.

How do you optimize pulse sequence parameters to produce a T2 weighted image?

A long TR allows full recovery of all tissues, and you use a long TE to allow fat to fully decay, so only fluids are in phase.

What makes proton density (PD) contrast different from T1 and T2 contrast?

Proton density contrast is based on the amount of protons physically present in the tissue and is not based on recovery or decay rates.

How do B0 inhomogeneities contribute to T2 star decay?

B0 inhomogeneities enhance or quicken the decay process.

Why is a short TE used to minimize T2 contrast in PD weighted imaging?

A short TE keeps everything in phase in the transverse plane at the time of echo collection, maximizing signal.

What is meant by saturation in MRI, and how can it be both useful and harmful?

Saturation is giving protons so much energy that they stop responding and don't give off a signal because they are now antiparallel.

What are the essential components of a pulse sequence in MRI?

RF pulses, slice select gradient, phase encoding gradient, frequency encoding gradient, and signal collection.

How does using a long TR reduce T1 contrast in an MRI image?

A long TR allows all tissues to fully recover and get excited, so there is no difference in T1 recovery.

Why is it important for all nuclei to be in phase in the transverse plane before echo collection?

If they are in phase, we will get a strong signal because they're pointing at the coil.

Explain how signal intensity affects the appearance of tissues on an MRI image.

The stronger the signal from a tissue, the brighter it appears (hyperintense); the weaker the signal, the more hypointense.

In terms of contrast, how do fat and water differ in their T1 and T2 relaxation properties?

Fat has a short T1 recovery rate and a short T2 decay rate, whereas water has a long T1 recovery rate and a long T2 decay rate.

How is MRI able to differentiate between various soft tissues with such high detail?

Different tissues have different T1, T2, and PD contrast due to their inherent contrast parameters and our ability to manipulate these parameters.

What role does a flip angle play in a pulse sequence?

A flip angle controls the degree to which we tilt the net magnetization vector (NMV).

Describe how different pulse sequences can be used to image specific pathologies or anatomy.

Different contrast mechanisms allow us to increase the signal from fat versus fluid, which allows us to visualize anatomy or pathology.