<P> This table does not include uncommon and experimental sequences . </P> <Table> <Tr> <Th> Group </Th> <Th> Sequence </Th> <Th> Abbr . </Th> <Th> Physics </Th> <Th> Main clinical distinctions </Th> <Th> Example </Th> </Tr> <Tr> <Td> Spin echo </Td> <Td> T1 weighted </Td> <Td> T1 </Td> <Td> Measuring spin--lattice relaxation by using a short repetition time (TR) and echo time (TE) </Td> <Td> <Ul> <Li> Lower signal for more water content, as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage </Li> <Li> High signal for fat </Li> <Li> High signal for paramagnetic substances, such as MRI contrast agents </Li> </Ul> <P> Standard foundation and comparison for other sequences . </P> </Td> <Td> </Td> </Tr> <Tr> <Td> T2 weighted </Td> <Td> T2 </Td> <Td> Measuring spin--spin relaxation by using long TR and TE times . </Td> <Td> <Ul> <Li> Higher signal for more water content . </Li> <Li> Low signal for fat . </Li> <Li> Low signal for paramagnetic substances . </Li> </Ul> <P> Standard foundation and comparison for other sequences . </P> </Td> <Td> </Td> </Tr> <Tr> <Td> Proton density weighted </Td> <Td> PD </Td> <Td> Long TR (to reduce T1) and short TE (to minimize T2) </Td> <Td> Joint disease and injury . <Ul> <Li> High signal from meniscus tears (pictured) </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Td> Gradient echo </Td> <Td> Steady - state free precession </Td> <Td> SSFP </Td> <Td> Maintenance of a steady, residual transverse magnetisation over successive cycles . </Td> <Td> Creation of cardiac MRI videos (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Inversion recovery </Td> <Td> Short tau inversion recovery </Td> <Td> STIR </Td> <Td> Fat suppression by setting an inversion time where the signal of fat is zero . </Td> <Td> High signal in edema, such as in more severe stress fracture . Shin splints pictured: </Td> <Td> </Td> </Tr> <Tr> <Td> Fluid attenuated inversion recovery </Td> <Td> FLAIR </Td> <Td> Fluid suppression by setting an inversion time that nulls fluids . </Td> <Td> High signal in lacunar infarction, multiple sclerosis (MS) plaques, subarachnoid haemorrhage and meningitis (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Double inversion recovery </Td> <Td> DIR </Td> <Td> Simultaneous suppression of cerebrospinal fluid and white matter by two inversion times . </Td> <Td> High signal of multiple sclerosis plaques (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Diffusion weighted (DWI) </Td> <Td> Conventional </Td> <Td> DWI </Td> <Td> Measure of Brownian motion of water molecules . </Td> <Td> High signal within minutes of cerebral infarction (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Apparent diffusion coefficient </Td> <Td> ADC </Td> <Td> Reduced T2 weighting by taking multiple conventional DWI images with different DWI weighting, and the change corresponds to diffusion . </Td> <Td> Low signal minutes after cerebral infarction (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Diffusion tensor </Td> <Td> DTI </Td> <Td> Mainly tractography (pictured) by an overall greater Brownian motion of water molecules in the directions of nerve fibers . </Td> <Td> <Ul> <Li> Evaluating white matter deformation by tumors </Li> <Li> Reduced fractional anisotropy may indicate dementia </Li> </Ul> </Td> <Td> </Td> </Tr> <Tr> <Td> Perfusion weighted (PWI) </Td> <Td> Dynamic susceptibility contrast </Td> <Td> DSC </Td> <Td> Gadolinium contrast is injected, and rapid repeated imaging (generally gradient - echo echo - planar T2 weighted) quantifies susceptibility - induced signal loss . </Td> <Td> In cerebral infarction, the infarcted core and the penumbra have decreased perfusion (pictured). </Td> <Td> </Td> </Tr> <Tr> <Td> Dynamic contrast enhanced </Td> <Td> DCE </Td> <Td> Measuring shortening of the spin--lattice relaxation (T1) induced by a gadolinium contrast bolus . </Td> </Tr> <Tr> <Td> Arterial spin labelling </Td> <Td> ASL </Td> <Td> Magnetic labeling of arterial blood below the imaging slab, which subsequently enters the region of interest . It does not need gadolinium contrast . </Td> </Tr> <Tr> <Td> Functional MRI (fMRI) </Td> <Td> Blood - oxygen - level dependent imaging </Td> <Td> BOLD </Td> <Td> Changes in oxygen saturation - dependent magnetism of hemoglobin reflects tissue activity . </Td> <Td> Localizing highly active brain areas before surgery . </Td> <Td> </Td> </Tr> <Tr> <Td> Magnetic resonance angiography (MRA) and venography </Td> <Td> Time - of - flight </Td> <Td> TOF </Td> <Td> Blood entering the imaged area is not yet magnetically saturated, giving it a much higher signal when using short echo time and flow compensation . </Td> <Td> Detection of aneurysm, stenosis or dissection . </Td> <Td> </Td> </Tr> <Tr> <Td> Phase - contrast MRA </Td> <Td> PC - MRA </Td> <Td> Two gradients with equal magnitude but opposite direction are used to encode a phase shift, which is proportional to the velocity of spins . </Td> <Td> Detection of aneurysm, stenosis or dissection (pictured). </Td> <Td> (VIPR) </Td> </Tr> <Tr> <Td_colspan="2"> Susceptibility weighted </Td> <Td> SWI </Td> <Td> Sensitive for blood and calcium, by a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to exploit magnetic susceptibility differences between tissues . </Td> <Td> Detecting small amounts of hemorrhage (diffuse axonal injury pictured) or calcium . </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Group </Th> <Th> Sequence </Th> <Th> Abbr . </Th> <Th> Physics </Th> <Th> Main clinical distinctions </Th> <Th> Example </Th> </Tr> <Tr> <Td> Spin echo </Td> <Td> T1 weighted </Td> <Td> T1 </Td> <Td> Measuring spin--lattice relaxation by using a short repetition time (TR) and echo time (TE) </Td> <Td> <Ul> <Li> Lower signal for more water content, as in edema, tumor, infarction, inflammation, infection, hyperacute or chronic hemorrhage </Li> <Li> High signal for fat </Li> <Li> High signal for paramagnetic substances, such as MRI contrast agents </Li> </Ul> <P> Standard foundation and comparison for other sequences . </P> </Td> <Td> </Td> </Tr>

How many set of paired gradient are present in clinical mr scanner