stir 150811054731 lva1 app6891 2

Fat suppressi on imaging By Roshan Shah B.Sc MIT 3rd year 3rd batch Introductio n  Fat saturation is an MRI technique used to suppress the signal from normal adipose tissue  To suppress the fat signal for a given MR sequence a fat suppression module is typically inserted at the beginning of an otherwise normal MRI sequence Indicatio n  It is used in MRI for mainly two purpose  To suppress the signal from normal adipose tissue to reduce chemical shift artifact or improve visualization of uptake of contrast material  Tissue characterization, particularly in adrenal gland tumors, bone marrow infiltration, fatty tumors, etc Physics for fat suppression  (FAT SAT) A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequence, so that they produce a negligible signal  This technique requires a homogeneous magnetic field and homogeneous volume of tissue  To prepare this type of sequence, the following properties should be used  Fat and water have different resonant frequencies  They have different Larmor precession frequencies  They have different T1 relaxation times Advantages  This method is reliable for contrast material enhanced T1 weighted imaging  It is useful in tissue characterization particularly in area with a large amount of fat  It also useful for avoiding chemical shift misregistration artifact  Allows good visualization of small anatomical details Disadvantage Inhomogeneities of the static magnetic field Inhomogeneities In the radiofrequency field Inhomogeneities in volume of tissue Methods  Fat suppression can be achieved in a number of different ways  Short tau inversion recovery (STIR)  Chemical shift Selective (CHESS) FatSat  Spectral Pre-saturation with Inversion Recovery (SPIR)  Spectral Attenuated Inversion Recovery (SPAIR)  Water excitation  DIXON-based STI R  It is an inversion recovery pulse sequence with specific timing so as to suppress the signal from fat Cont.… Advantages: 1.It suppresses whole of the adipose tissue including water fraction 2.This is only method which can be used even in magnetic field inhomogeneities 3.It can be used with low magnetic field strength Disadvantages: 4.Beginning at TI null most of the proton have not completely relaxation, and are therefore still partially saturated, in this situation will overall produce signal loss and SNR ratio will decrease It cannot be used post gadolinium to demonstrate contrast enhancement Long acquisition time MRI image appearance  Fluids normally appear bright and fat appear very dark in a STIR images  Pathological processes normally increase the water content in tissues Due to the added water component this results in a signal increase on STIR images Consequently pathological processes are usually bright on STIR images MRI image appearance SPI R  It is the combination of the spectral saturation and STIR routines MRI image appearance SPAI R  It is a hybrid technique combining features of both CHESS and STIR  SPAIR uses an adiabatic pulses Cont.…  Advantages  SPAIR provide better and more homogeneous fat suppression than SPIR  Tissue contrast is not affected  Disadvantages  The inversion time is longer  Reduce number of slice for a given TR MRI image appearance Water excitation This technique is based on the chemical shift Instead of suppressing fat, these techniques use a short series of RF pulses(binomial pulse) to selectively excite water protons No spoilers are needed Cont.…  Advantages  No additional preparation pulse is necessary  Reduced sensitivity to B1 Inhomogeneities  Disadvantages  Increased total measurement time  Reduced maximum number of slices Application  used primarily in the musculoskeletal system, especially for evaluation of cartilage Some 3D applications in the breast and liver have also been reported DIXON-based This technique is based on the chemical shift It uses ‘In phase’ and ‘Out phase’ cycling of fat and water Acquiring 2, or more echoes at different TE's, "water only" and "fat only" images can be extracted In-phase water Out-phase fat Cont.…  Advantages  Insensitive to B0 and B1 Inhomogeneities  contrasts delivered in one measurement  Disadvantages  Increases minimal TR because inand opposed phase data must be acquired Application The Dixon technique is widely used in abdominal imaging, imaging of the extremities, and the spine The opposed phased imaging is useful for detection of small amounts of fat Exadrenal gland tumors or steatosis ... appear very dark in a STIR images  Pathological processes normally increase the water content in tissues Due to the added water component this results in a signal increase on STIR images Consequently... signal increase on STIR images Consequently pathological processes are usually bright on STIR images STIR sagittal sequence used in C spine imaging and knee imaging CHES S  Based on the chemical... combination of the spectral saturation and STIR routines MRI image appearance SPAI R  It is a hybrid technique combining features of both CHESS and STIR  SPAIR uses an adiabatic pulses Cont.…