T1 recovery, T2 decay, and how they tell tissues apart.
Need-to-know for clinical practice
Learning Objectives
Distinguish between T1 recovery and T2 decay as independent processes
Predict tissue brightness for given TR/TE combinations
Classify images as T1-weighted, T2-weighted, or PD-weighted
Explain the clinical role of T1 mapping in detecting diffuse fibrosis
Describe how T2* reflects iron deposition in the myocardium
Tissue Contrast
Adjust TR/TE to create contrast
T1 Recovery (Longitudinal)
T2 Decay (Transverse)
Fat
Myocardium
CSF/Water
Field Strength
T1 values increase at higher field strength
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.
Clinical Cutoffs:T2* < 20ms = cardiac iron loading. T2* < 10ms = severe cardiac risk requiring urgent chelation therapy.
Late Gadolinium Enhancement (LGE)
LGE is the gold standard for detecting myocardial scar. It relies on Inversion Recovery (IR) physics to null healthy myocardium and make scar tissue bright.
The TI Selection Rule
Board Fact: The Inversion Time (TI) must be set to the exact moment when healthy myocardium crosses the zero-point of its T1 recovery curve. At this TI, healthy tissue has zero signal (appears black) while scar tissue (which has shorter T1 due to gadolinium accumulation) has already recovered and appears bright.
Finding the Right TI
TI Scout: Run a series of images at different TI values. Choose the TI where normal myocardium is darkest.
Typical Range: 250-350ms at 10 min post-contrast (varies with dose, time, and clearance).
PSIR (Phase-Sensitive IR): Removes the need for perfect TI selection by preserving the sign of magnetization. Scar is always bright regardless of TI.
Board Pitfall: TI must be increased over time as gadolinium washes out and myocardial T1 lengthens.
Clinical Correlation
Scar Pattern Recognition
The pattern of LGE tells you the etiology: Subendocardial = ischemic (follows coronary territory). Mid-wall = non-ischemic (myocarditis, DCM). Epicardial = myocarditis or sarcoid. Diffuse subendocardial = amyloid.
Fat Suppression Techniques
Fat can obscure pathology and mimic disease. Multiple techniques exist to suppress or separate fat signal, each with distinct physics and clinical indications.
STIR (Short-TI IR)
Uses an inversion pulse with TI ~150ms (1.5T) to null fat. Advantages: Works even with B₀ inhomogeneity. Clinical use: Gold standard for myocardial edema detection (T2-STIR). Limitation: Also suppresses short-T1 tissues like gadolinium-enhanced tissue.
Spectral Fat Sat (CHESS)
Applies a narrow-bandwidth RF pulse at the fat frequency (3.5 ppm offset) followed by a spoiler gradient. Advantage: Selective — only suppresses fat. Limitation: Fails at field boundaries and with shimming issues.
Dixon (Water-Fat Separation)
Acquires images at in-phase and opposed-phase echo times. Mathematically separates water and fat into individual images. Board Advantage: Provides both water-only AND fat-only images simultaneously. Modern multi-point Dixon is robust to B₀ inhomogeneity.
Knowledge Check1 / 2
To produce a T1-weighted image (where fat is bright and fluid is dark), what settings do you need?
