Clarification: T1 and T2 Weighting on MRI, Not CT
T1 and T2 weighting are MRI (Magnetic Resonance Imaging) concepts, not CT concepts—CT does not use T1 or T2 weighting. 1 CT relies on x-ray attenuation through tissues of variable density to produce images, while MRI uses radiofrequency pulses and magnetic fields to manipulate protons in tissue, generating images based on different relaxation properties. 1
Understanding T1 and T2 Relaxation Times
T1 (longitudinal relaxation time) represents the time required for tissue to become magnetized after being placed in a magnetic field, or alternatively, the time to regain longitudinal magnetization following a radiofrequency pulse. 1 Tissues with short T1 times (like fat) appear bright on T1-weighted images, while tissues with long T1 times (like water/CSF) appear dark. 1
T2 (transverse relaxation time) measures how long transverse magnetization persists in a uniform external magnetic field, or how long resonating protons remain coherent ("in phase") following a 90° radiofrequency pulse. 1 Tissues with long T2 times (like water/CSF) appear bright on T2-weighted images, while tissues with short T2 times appear dark. 1
Key Differences Between T1-Weighted and T2-Weighted MRI
Signal Characteristics
T1-weighted images provide superior anatomic detail and soft tissue contrast, particularly after gadolinium contrast administration, which shortens T1 relaxation time and causes enhancing lesions to appear bright. 1, 2
T2-weighted images are more sensitive for detecting pathology, as most disease processes (edema, inflammation, tumors) contain increased water content and appear hyperintense (bright). 1
The MR signal is greatest when T1 is short and T2/proton density are high; signal decreases when T1 is long and T2/proton density are low. 1
Acquisition Techniques
T1-weighted sequences are typically acquired using gradient recalled echo (GRE) sequences, which offer shorter acquisition times (5-10 minutes) and the ability to acquire 3D image volumes with isotropic resolution ≤1.5mm. 1
T2-weighted sequences commonly use fast spin-echo (FSE) or turbo spin-echo (TSE) techniques, with acquisition times varying based on 2D versus 3D protocols. 1
T1-weighted GRE sequences can utilize the Dixon technique to produce multiple image contrasts (in-phase, opposed-phase, fat-only, water-only) within a single acquisition, enabling fat-fraction mapping for lesion characterization. 1
Clinical Applications
For contrast-enhanced imaging:
Contrast-enhanced T1-weighted sequences are the gold standard for detecting and delineating enhancing parenchymal lesions, measuring tumor burden, and assessing treatment response, with sensitivity of 90% and specificity of 80%. 1, 2
Gadolinium-enhanced T1-weighted images should be acquired 5-8 minutes after contrast injection for optimal lesion visualization and maximum contrast uptake. 1, 2
Contrast-enhanced T1-weighted images demonstrate significantly higher contrast-to-noise ratios for tumor detection compared to T2-weighted images. 3, 4, 5
For non-contrast imaging:
T2-weighted sequences (particularly with fat suppression like FLAIR or STIR) are superior for detecting edema, inflammation, and lesions near CSF spaces (juxtacortical and periventricular regions). 1
T2-weighted FLAIR combines T1 and T2 weighting with CSF suppression, improving lesion conspicuity near cortex and ventricles, with sensitivity of 95% for leptomeningeal disease. 1, 2
Specific Tissue Appearances
Fat appears bright on T1-weighted images and intermediate on T2-weighted images. 1
Water/CSF appears dark on T1-weighted images and bright on T2-weighted images. 1
Acute hemorrhage (hemosiderin deposits) appears as signal loss on T2-weighted or T2 gradient echo sequences due to magnetic susceptibility effects, but may not be as conspicuous on T1-weighted images.* 1
Practical Protocol Recommendations
For brain tumor imaging, acquire both sequences:
Precontrast 3D isotropic T1-weighted GRE (1mm isotropic resolution) followed by postcontrast T1-weighted GRE with identical parameters for accurate comparison. 1, 2
Axial 2D T2-weighted FLAIR (3-4mm slice thickness, no gap) acquired between contrast injection and postcontrast T1-weighted imaging to utilize the 5-minute delay period efficiently. 1, 2
For multiple sclerosis imaging:
Acquire T2-weighted sequences in at least two planes (axial T2/proton-density plus sagittal T2-FLAIR) to compare different T2 sequences and improve lesion detection. 1, 6
Gadolinium-enhanced T1-weighted sequences are mandatory when T2 lesions are present to distinguish acute from chronic lesions and demonstrate dissemination in time. 1, 6