T1, T2 and Proton Density Weighting | MRI Weighting and Contrast | MRI Physics Course #6

T1 and T2 Relaxation Rates

This section introduces the concepts of T1 and T2 relaxation rates in different tissues and how they contribute to contrast differences in an image.

Manipulating Pulse Sequence for Weighting Images

• The pulse sequence can be manipulated to highlight either the T1 or T2 differences in tissues, creating a weighted image.

• A T1-weighted image emphasizes contrast due to T1 relaxation differences between tissues.

• A T2-weighted image emphasizes contrast due to T2 relaxation rates that differ within tissues.

Transverse Decay (T2 Relaxation)

This section explains the process of transverse decay or T2 relaxation, where protons lose net transverse magnetization at different rates in different tissues.

• Transverse decay occurs when protons deface, resulting in the loss of net transverse magnetization vector.

• Different tissues have varying rates of loss of transverse magnetization, leading to differences in T2 contrast within an image.

• The rate of loss is represented by the T2 constant, which is unique for each tissue.

Longitudinal Recovery (T1 Relaxation)

This section discusses longitudinal recovery or T1 relaxation, where there is a recovery of the longitudinal magnetization vector at different rates depending on the tissue.

• Longitudinal recovery involves the recovery of the longitudinal magnetization vector after it has been flipped 90 degrees into the transverse plane.

• Different tissues have varying rates of recovery, leading to differences in T1 contrast within an image.

• The rate of recovery is represented by the T1 time constant, which is unique for each tissue.

Pulse Sequence Parameters: Time of Echo and Time of Repetition

This section explores the two parameters that can be changed in a pulse sequence: time of echo (TE) and time of repetition (TR).

• The time of echo is the duration between the RF pulse and sampling the transverse magnetization signal.

• Changing the TE highlights T2 differences within tissues.

• The time of repetition is the duration between consecutive RF pulses.

• Changing the TR determines the amount of T1 contrast contributing to the images.

Contrast in Images with Different TE

This section explains how changing the TE affects image contrast by highlighting T2 differences in tissues.

• A short TE results in high signal but little contrast, as transverse magnetization vectors have not undergone significant T2 relaxation.

• Increasing TE allows for better visualization of T2 differences, as shown by contrasting signals from different tissues.

Considerations for Image Contrast

This section discusses factors that affect image contrast, such as proton availability and longitudinal magnetization vector magnitude.

• Proton availability varies between tissues, affecting longitudinal magnetization vector magnitude and subsequently transverse magnetization vector magnitude after a 90-degree RF pulse.

• In reality, CSF and fat have more free hydrogen protons available compared to muscle, resulting in different initial signal levels.

• Selecting a short TE would result in high signal but low contrast due to limited T2 relaxation. Increasing TE enhances contrast by highlighting T2 differences.

Timestamps are approximate and may vary slightly.

T1 and T2 Relaxation in MRI

This section discusses the concepts of T1 and T2 relaxation in MRI and how they affect the image contrast.

Short Time of Repetition (TR)

• When using a short time of repetition (TR), tissues have not fully regained their longitudinal magnetization after the 90-degree RF pulse.

• The differences in tissue contrast at this point are predominantly due to T1 differences.

• Fat regains longitudinal magnetization faster than water or CSF.

Long Time of Repetition (TR)

• With a longer TR, tissues regain more longitudinal magnetization before the next 90-degree RF pulse.

• If sampled immediately after the 90-degree RF pulse with a short echo time (TE), there is no contrast but high signal between tissues.

• Waiting for a longer TR allows for full recovery of longitudinal magnetization, resulting in loss of T1 contrast.

Creating a T1 Weighted Image

• To create a T1 weighted image with contrast predominantly from T1 relaxation differences, keep TE short to avoid allowing transverse relaxation (T2) differences to occur.

• Reduce TR time to highlight differences in T1 between tissues.

• Lower TR values result in higher contrast based on differential rates of longitudinal recovery.

Creating a T2 Weighted Image

• To create a T2 weighted image and highlight T2 relaxation differences, use long TR values.

• Increase TE time to further emphasize differences in transverse decay within different tissues.

• Water has slower T2 relaxation compared to fat and muscle.

Contrast Differences

• Contrast in images can be attributed to both T1 and T2 relaxation properties of tissues.

• A shorter TE emphasizes T1 differences, while a longer TE highlights T2 differences.

• CSF appears bright in the T2 weighted image due to its slower rate of longitudinal magnetization recovery compared to fat.

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New Section

This section discusses the differences in contrast between T1-weighted and T2-weighted images based on the number of protons available for nuclear magnetic resonance.

Contrast Differences in T1-Weighted and T2-Weighted Images

• In a T2-weighted image, CSF and fat appear brighter compared to a T1-weighted image due to different TE and TR times in the pulse sequence.

• Contrast differences in tissues are purely based on the number of protons available for nuclear magnetic resonance when using a long TR time and short TE time.

• Differences in transverse magnetization vectors are purely due to variations in the number of protons available for nuclear magnetic resonance within a specific voxel.

• Proton density weighted images create contrast solely based on proton density within the image, resulting in bright fat and fluid signals.

• Muscle signal appears intermediate because its longitudinal magnetization vector is smaller compared to muscle and fat, resulting in lower transverse magnetization after flipping 90 degrees with an RF pulse.

New Section

This section highlights the excellent contrast achieved by differentiating bright fluid from surrounding low signal structures. It also mentions how advanced pulse sequences can help identify tears or abnormalities more easily.

Achieving Contrast with Bright Fluid

• The image shows clear differences between bright fluid (such as synovial fluid or water signal) and darker structures like menisci, ligaments, and subchondral bone plates.

• Advanced pulse sequences can negate signal from fatty tissue, allowing for better identification of tears or abnormalities by focusing on water signal contrast.

New Section

This section emphasizes the importance of TR and TE times in creating different image weightings. It provides ballpark figures for TR and TE values and explains how to identify the type of weighting based on these values.

Understanding Image Weighting Based on TR and TE Times

• Changing the TR time highlights T1 differences in tissues, while changing the TE time highlights T2 differences.

• A long TR time and short TE time result in a proton density weighted image.

• Ballpark figures for TR times are in the late thousands to 2000 range, while short TE times are in the tens range (10 to 30).

• Longer TE times are closer to the hundreds range (80 to 160).

• Identifying image weighting can be done by considering whether TR or TE values fall within certain ranges.

New Section

This section concludes the discussion on T1 and T2 differences in tissue contrast and introduces the localization of signal within an MRI image.

Signal Localization Within an Image

• The focus shifts from T1 and T2 differences to localizing where exactly signal is coming from within an MRI image.

• Further topics related to selecting specific slices in an MRI will be covered.