What is the difference between T1 and T2 weighted images on MRI?
T1- or T2-weighting typically refers to the spin echo MR sequence. T1-weighted images are short TR (300–1000 ms) and short TE (10–30 ms) and provide excellent anatomic detail. In contrast, T2-weighted images are long TR (1800–2500 ms) and long TE (40–90 ms), sensitive for detecting fluid and edema. An intermediate-weighted sequence or proton density sequence combines T1 and T2 weighting by having a long TR (>T1) and short TE (<T2). This technique has some advantages of both T1 and T2-weighted sequences and is commonly used in musculoskeletal imaging.
Two other sequences commonly used are gradient echo and short tau (T1) inversion recovery (STIR). Gradient echo sequences can be either T1- or T2-weighted and permit very rapid acquisition with thin-section, high-resolution images. STIR is effectively a fat-suppression technique, very sensitive in detection of fluid or edema. These images greatly aid in the detection of subtle marrow and soft tissue disease, such as muscle tears, but have relatively poor spatial resolution.
Gadolinium is a paramagnetic element used as a contrast agent in MRI. It adds to the cost of MRI but depicts areas of increased blood flow on T1-weighted images, which is helpful in the detection of early erosions and in distinguishing between effusion and synovial inflammation. Gadolinium should be generally be avoided in patients with significant renal insufficiency (creatinine clearance < 30 cc/minute) due to the risk of nephrogenic systemic fibrosis.