Image Weighting in MRI

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Questions and Answers

What is T1 recovery primarily caused by?

Spin lattice energy transfer (correct)

Dephasing of nuclei

Spin–spin energy transfer

External magnetic field inhomogeneity

Which statement about T2 decay is correct?

T2 decay is faster than T1 recovery in all tissues.

T2 decay involves the loss of phase coherence. (correct)

T2 decay is caused by interactions with surrounding tissue Gs.

The T2 time is invariant across different types of tissues.

How does T1 recovery differ between fat and water?

Both have the same T1 recovery time.

Water absorbs energy rapidly compared to fat.

Fat recovers energy more quickly than water, resulting in a shorter T1 time. (correct)

Fat has a longer T1 time than water.

What happens to the NMV during saturation?

It is pushed beyond the transverse plane. Signup and view all the answers

Which factor determines how much T1 recovery occurs in a particular tissue?

Repetition Time (TR) Signup and view all the answers

Which sequence describes the relationship between T2* and T2?

T2* happens before T2 and is affected by external field inhomogeneities. Signup and view all the answers

What is the time taken for 63% of the longitudinal magnetization to recover known as?

T1 recovery time Signup and view all the answers

Which process primarily contributes to T2 decay?

Spin–spin energy transfer Signup and view all the answers

What predominantly influences the contrast in T1 weighted images?

Differences in T1 recovery times of tissues Signup and view all the answers

What is the typical TR range for T1 weighted imaging?

300–600 ms Signup and view all the answers

How do short TEs affect the appearance of fat and water in T2 weighted imaging?

They prevent full dephasing, resulting in similar transverse components. Signup and view all the answers

In T2 weighted images, which tissue exhibits a high signal due to its long T2 decay time?

Water Signup and view all the answers

What are the typical TE parameters for T2 weighted imaging?

70 ms + Signup and view all the answers

What does proton density weighting emphasize in MR imaging?

The number of hydrogen protons in tissues Signup and view all the answers

Which tissue type is always dark on MR images because of low proton density?

Cortical bone Signup and view all the answers

What is a key characteristic of T2 weighted images regarding tissue with short T2 decay times?

They appear dark and contribute to low signals. Signup and view all the answers

What is the main component that determines the contrast in MR images?

Transverse component of magnetization Signup and view all the answers

Which intrinsic contrast mechanism is characterized by the return of longitudinal magnetization?

T1 recovery Signup and view all the answers

Which parameter is NOT an extrinsic contrast parameter in MR imaging?

T2 decay Signup and view all the answers

How do fat and water differ in terms of molecular motion and energy absorption?

Fat has a slow rate of molecular motion and low inherent energy. Signup and view all the answers

What effect does faster molecular motion have on energy release?

It complicates energy release to surroundings. Signup and view all the answers

What does the term 'Free Induction Decay (FID)' refer to in MR imaging?

The decay of NMV in the transverse plane. Signup and view all the answers

Which component is most affected by relaxation processes in MR imaging?

Transverse component signals Signup and view all the answers

How do intrinsic contrast mechanisms influence MR imaging?

They account for the relaxation rates of different tissues. Signup and view all the answers

Study Notes

Image Contrast Mechanisms

High signal areas (large transverse magnetization) appear white in images.
Low signal areas (small transverse magnetization) appear dark in images.
Intermediate signal areas are displayed as shades of gray.

Contrast in MR vs. CT

MR contrast is influenced by two types of parameters: extrinsic (operator-controlled) and intrinsic (not operator-controlled).

Extrinsic Contrast Parameters

Repetition Time (TR): Time interval between RF pulse applications.
Echo Time (TE): Duration from RF pulse to signal collection.
Flip Angle: Angle of the NMV movement due to an RF pulse.
Turbo-Factor (ETL/TF): Influences echo train length.
Time from Inversion (TI): Time elapsed following an inversion pulse.

Intrinsic Contrast Mechanisms

T1 Recovery: Time for longitudinal magnetization recovery.
T2 Decay: Time for loss of transverse magnetization.
Proton Density: Variation based on number of hydrogen protons.
Flow: Movement contributing to signal changes.
Apparent Diffusion Coefficient (ADC): Reflects tissue diffusion properties.

Molecular Motion

Molecules exhibit rotational and translational motion.
Faster motion complicates energy release to surroundings.

Composition of Fat and Water

Fat:
- Large molecules, slow molecular motion, low energy absorption efficiency.
Water:
- Small molecules, high molecular motion, high energy absorption inefficiency.

Relaxation Processes

RF pulse induces resonance, followed by signal decay upon pulse removal (free induction decay, FID).
Differences in relaxation rates among tissues create contrast during imaging.

T1 Recovery

T1 recovery involves energy transfer (spin-lattice) as NMV returns to alignment with B0.
Recovery is exponential and varies by tissue type.
Short TRs result in incomplete recovery for both fat and water.

T2 Decay

T2 decay represents the time for 63% of transverse magnetization loss due to dephasing.
Caused by spin-spin energy transfer and results in signal decay.
T2 time varies among different tissues.

Imaging Techniques

T1 Weighted Images:
- Short TR, short TE; highlights anatomical detail.
- Fat appears bright; water appears dark (high contrast).
T2 Weighted Images:
- Long TR, long TE; emphasizes pathology.
- Fat appears dark; water appears bright.
Proton Density (PD) Weighted Images:
- Long TR, short TE; highlights differences in hydrogen proton density.
- Low PD areas are dark; high PD areas are bright.

Contrast Examples in T1 and T2 Weighting

T1: Bright high signals from fat compared to water.
T2: Dark appearance of fat against bright water background, emphasizing pathology.

Key Parameters for Imaging

T1 Weighted: TR 300-600 ms, TE 10-30 ms.
T2 Weighted: TR 2000 ms+, TE 70 ms+.
PD Weighted: Long TR and short TE to reduce T1 and T2 effects.

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Description

This quiz explores the concepts of image contrast in Magnetic Resonance Imaging (MRI), including mechanisms such as T1 recovery, T2 decay, and various weighting techniques. Understand the differences between high, low, and intermediate signal areas on MR images compared to CT images. Test your knowledge on how these elements contribute to effective imaging.