Relaxation

LEVEL 3

T1 recovery, T2 decay, and how they tell tissues apart.

Need-to-know for clinical practice

Tissue Contrast

Adjust TR/TE to create contrast

T1 Recovery (Longitudinal)

T2 Decay (Transverse)

Fat

Myocardium

CSF/Water

Tissues

TR (Repetition Time)500ms

TE (Echo Time)14ms

Image Weighting

T1-Weighted

Simulated Image

Fat72%

Myocardium24%

CSF/Water12%

The Two Clocks: T1 & T2

When the RF pulse stops, the signal doesn't just cut out. It fades. This fading is governed by two independent mechanical processes.

The T1 Clock

Longitudinal Relaxation: How fast protons align back with B₀.

Analogy: A compass in syrup. Tighter syrup (fat) helps the needle snap back faster than loose water.

The T2 Clock

Transverse Relaxation: How fast protons lose their shared rhythm.

Analogy: Dancers losing step. In pure water, they stay in sync for a long time. In solid muscle, they bump into each other and lose rhythm quickly.

Relaxation in the Clinic

In cardiovascular imaging, we don't just "look" at the image. We measure the physics using Parametric Mapping.

The T1 Mapping Lab

Fit the recovery curve to find the T1 time

Guess T1 Value (ms)1000 ms

Fit Accuracy:FITTING...

Clinical Note: Healthy myocardium T1 is usually ~1000ms at 1.5T. High T1 values (e.g. 1200ms) suggest fibrosis or edema.

T1 Mapping

The Fibrosis Hunter: We measure the exact T1 Recovery time for every pixel in the heart muscle. By fitting the recovery curve, we get an absolute number in milliseconds (ms). High values suggest diffuse disease like fibrosis or amyloidosis.

T2* (T2-Star) & Iron

T2* is ultra-fast T2 decay sensitive to the B₀ field. Iron deposits in the heart (common in Thalassemia) disturb the B₀ field, causing the signal to die almost instantly. We use this to prevent heart failure in these patients.

Knowledge Check1 / 2

To produce a T1-weighted image (where fat is bright and fluid is dark), what settings do you need?

Loading Clinical Data...