(T) Times between successive RF pulses used in pulse sequences. Particularly important are the inversion time (TI) in inversion recovery, and the time between 90° pulse and the subsequent 180° pulse to produce a spin echo, which will be approximately one half the spin echo time(TE). The time between repetitions of pulse sequences is the repetition time(TR).

Knee MRI, with its high soft tissue contrast is one of the main imaging tools to depict knee joint pathology. MRI allows accurate imaging of intra-articular structures such as ligaments, cartilage, menisci, bone marrow, synovium, and adjacent soft tissue.
Knee exams require a dedicated extremity coil, providing a homogenous imaging volume and high SNR to ensure best signal coverage. A complete knee MR examination includes for example sagittal and coronal T1 weighted, and proton density weighted pulse sequences +/- fat saturation, or STIR sequences. For high spatial resolution, maximal 4 mm thick slices with at least an in plane resolution of 0.75 mm and small gap are recommended. To depict the anterior cruciate ligament clearly, the sagittal plane has to be rotated 10 - 20° externally (parallel to the medial border of the femoral condyle). Retropatellar cartilage can bee seen for example in axial T2 weighted gradient echo sequences with Fatsat. However, the choice of the pulse sequences is depended of the diagnostic question, the used scanner, and preference of the operator.
Diagnostic quality in knee imaging is possible with field strengths ranging from 0.2 to 3T. With low field strengths more signal averages must be measured, resulting in increased scan times to provide equivalent quality as high field strengths.
More diagnostic information of meniscal tears and chondral defects can be obtained by direct magnetic resonance arthrography, which is done by introducing a dilute solution of gadolinium in saline (1:1000) into the joint capsule. The knee is then scanned in all three planes using T1W sequences with fat suppression. For indirect arthrography, the contrast is given i.v. and similar scans are started 20 min. after injection and exercise of the knee.
Frequent indications of MRI scans in musculoskeletal knee diseases are:
e.g., meniscal degeneration and tears, ligament injuries, osteochondral fractures, osteochondritis dissecans, avascular bone necrosis and rheumatoid arthritis.

See also Imaging of the Extremities and STIR.

From Siemens Medical Systems;
the 3 T MAGNETOM Allegra is a dedicated MR headscanner, perfect as a research system in cognitive and neuroscience with MRS and fMRI. MAGNETOM Allegra is a full member of the MAGNETOM product family. It uses many common components, i.e. electronics, computer system, software and pulse sequence concepts.

The mapping of the magnetic field by measuring or imaging the spatial distribution of magnetic field strength, can be performed by scanning with a probe and handles a large range of field strengths, but is slow and tedious. Accurate field maps can be made by measuring the Larmor frequency as a function of position.
The field must be homogeneous enough to allow MR imaging to be performed, than the magnetic field can be mapped by different methods.
1. The adaptation of chemical shift imaging.
2. The faster one measures the change in signal phase in an image obtained with a gradient echo pulse sequence resulting from a change in echo time TE, which is proportional to the local field strength.
Also useful is a spin echo pulse sequence with data collection from two time locations of the readout gradient and the data acquisition interval, where each having a known shift of the acquisition center away from the spin echo.

MR myelography is studying the spinal canal and subarachnoid space by high-resolution MRI with a technique in which a sequence with strong T2 weighting is used to provide high contrast between the "dark" spinal cord and its nerves and the surrounding "bright" cerebrospinal fluid. MR myelography as part of an entire MR examination has virtually replaced X-ray myelography. Used sequences are T2 weighted fast spin echo pulse sequences or a refocused gradient echo pulse sequence with strong T2 weighting.

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'