When a patient is placed in the magnet the hydrogen atoms in the water of their body tissues line up along the magnetic field. Radiofrequency pulses are sent in, causing the atoms to ‘flip’ into another plane and then ‘relax’ back when the pulse is turned off. This recovery process is known as relaxation. This relaxation time varies from one type of tissue to another. This difference in relaxation times is used in MRI to distinguish normal and pathologic tissues. Each tissue is characterized by two relaxations times: T1 (longitudinal relaxation time) and T2 (transverse relaxation time). Most of the images are created by one of these two characteristics being the predominant source of contrast. This means when an image is described as a T1-weighted image T1 is the main source of contrast.
T1 image characteristics
When an MRI sequence is set to produce a T1-weighted image, it is the tissues with the short T1 values that produce the highest magnetization and which appear brightest in the image. A T1-weighted sequence produces T1 contrast mainly by de-emphasizing the T2 contributions. This is normally achieved by using short repetition times TR (300-600ms) to maximize the difference in longitudinal relaxation during the return to equilibrium, and a short echo time TE (10-15ms) to minimize T2 dependency during signal acquisition.
Click here to read detailed physics of T1, T2 and PD
MRI image appearance
The easiest way to identify T1 weighted images is to look for fluid filled spaces in the body (e.g. Cerebrospinal fluid in the brain ventricles and spinal canal, free fluid in the abdomen, fluid in the gall bladder and common bile duct, synovial fluid in joints, fluid in the urinary tract and urinary bladder, oedema or any other pathological fluid collection in the body). Fluids normally appear dark in a T1 weighted image.
Tissues and their T1 appearance
Bone marrow : - equal to or higher than that of muscle (fatty marrow is usually bright)
Moving blood : - dark
White matter : - whiter
Gray matter : - gray
Fluids : - dark
Bone : - dark
Fat : - bright
Air : - dark
Pathological processes normally increase the water content in tissues. Due to the added water component this results in a signal loss on T1 weighted images and signal increase on T2 weighted images. Consequently pathological processes are usually bright on T2 weighted images and dark on T1 weighted images.
Useful for pelvic imaging (only used for pelvic infections pre contrast pelvis T1 imaging)
Useful for abdominal imaging (T1 tse respiratory gated scans)
Useful for chest imaging (T1 tse respiratory gated scans)
Very useful for brachial and lumbar plexus imaging
Very useful for anterior neck orbits and face imaging
Very useful for any musculoskeletal imaging
Very useful for extremity imaging
Very useful for brain imaging
Very useful for spine imaging