From the Guidelines
T1, T2, and DWI sequences differ in their ability to visualize tissue characteristics, with T1-weighted images showing anatomical detail, T2-weighted images detecting pathology and edema, and DWI measuring water molecule diffusion to identify areas of restricted diffusion, such as in acute stroke. The differences between these sequences are based on their physical principles, with T1 measuring longitudinal relaxation time, T2 measuring transverse relaxation time, and DWI measuring water molecule diffusion 1. Key characteristics of each sequence include:
- T1-weighted images: fat appears bright (hyperintense) and water appears dark (hypointense), making them excellent for visualizing brain structure and fat-containing tissues.
- T2-weighted images: water appears bright and fat appears as an intermediate signal, making them ideal for detecting pathology, edema, and inflammation.
- DWI: measures the random movement of water molecules in tissue, appearing bright in areas where diffusion is restricted, such as in acute stroke or cellular-dense tumors. According to the guidelines for the early management of patients with ischemic stroke, DWI allows visualization of ischemic regions within minutes of onset of symptoms, providing early identification of lesion size, site, and age, with a high sensitivity and specificity for detecting acute ischemia 1. In clinical practice, these sequences are complementary and are typically used together in imaging protocols to provide comprehensive diagnostic information, with the choice of sequence depending on the specific clinical question and the tissue characteristics being evaluated.
From the Research
Overview of MRI Sequences
- T1, T2, and Diffusion-Weighted Imaging (DWI) are different MRI sequences used to evaluate various aspects of tissue structure and function 2, 3, 4.
- T1 and T2 sequences provide information on tissue morphology, while DWI measures the diffusion of water molecules in tissues, which can indicate pathological changes 2.
Diffusion-Weighted Imaging (DWI)
- DWI is based on the echo planar imaging (EPI) technique and uses strong magnetic field gradient pulses to sensitize the MR signal to microscopic motion of water molecules 2.
- DWI can measure diffusion along any direction, but in clinical routine, only directionally averaged DWI images (trace maps) are used 2.
- DWI is useful in detecting and characterizing lesions in the brain and whole body, particularly in stroke diagnostics 2, 4.
Comparison of T1, T2, and DWI Sequences
- T1 and T2 sequences provide different image contrasts, with T1-weighted images showing bright fat and dark water, and T2-weighted images showing bright water and dark fat 3.
- DWI provides a unique image contrast that is sensitive to the diffusion of water molecules, which can indicate tissue pathology 2.
- Studies have shown that DWI can detect lesions not visible on T1 or T2 sequences, particularly in cases of acute ischemic stroke and closed head injury 4, 5.
Clinical Applications of DWI
- DWI has been used to evaluate acute ischemic stroke, detect and characterize lesions, and predict cerebral edema after stroke 3, 4, 6.
- DWI has also been used to evaluate diffuse axonal injury in closed head injury, and has been shown to be valuable in detecting shearing injuries not visible on conventional MRI sequences 5.