Gradient echo (GE) in spinal MR imaging does not use a 180° pulse to achieve the echo. This gradient-driven echo allows for rapid imaging with very short repetition time (TR). Intrinsic to good image quality in GE imaging is the choice of flip angle, which has optimal values for specific TRs and tissue types, the Ernst angle (the longer the T1 of the tissue, the smaller the best flip angle). There are two types of GE imaging; spoiled and steady-state. Spoiled sequences (fast low-angle shot [FLASH] and spoiled gradient-recalled acquisition in steady state [GRASS]) destroy the residual transverse magnetization after each alpha pulse. In steady-state sequences (fast-imaging steady precession [FISP], steady-state free precession [SSFP], and GRASS), this transverse magnetization is maintained and stabilizes after a few pulses. For tissue with a short T2 (e.g., fat, muscle), or sequences requiring long TR, the spoiled and steady-state sequences look the same. If the T2 of interest is long (e.g., CSF), then the steady-state sequence will give the familiar CSF myelogram effect. Flip angle is a powerful modifier of GE contrast. Spoiled GE sequences will be more T1-weighted with higher flip angles approaching 90°. For steady-state sequences where the TR is shorter than the T2, tissues with long T1 and T2 will show preferentially increased signal with increasing flip angle. Spin-density images can be obtained with a GE technique with short TR if a small flip angle is used. T2-like contrast (T2*) can be obtained with increasing echo time (TE), as with conventional SE imaging.
-->
Subscribe to:
Post Comments (Atom)
Popular Posts
-
Like any technique, fMRI has advantages and disadvantages, and in order to be useful, the experiments that employ it must be carefully desi...
-
--> Routine shoulder protocols are the most variable, but most include SHOULDER MRI SEQUENCES ...
-
MRI Prostrate Spectroscopy SINGLE VOXEL Planning --> MRI Prostrate Spectroscopy MULTI VOXEL Planning
-
MRI OF THE RIGHT ANKLE JOINT. Clinical details: Fall and twisting injury to the Right Ankle 8 months ago. Now pain around the Ank...
-
MRI of the Right Lower Leg Clinical details: Technique: Multiplanar multisequence study including T1,T2, STIR and GRE weigh...
-
Normal MRI Anatomy The pectoralis tendon is best seen on axial T1 and T2-weighted images as a curvilinear low-signal band inserting o...
-
Axial Plane: Prescribe plane perpendicular to midshaft of the proximal phalanx of the thumb. Scan from 1st carp-metacarpal joint th...
-
T2-weighted images are used for both detection and characterization of focal liver lesions. A routine liver imaging protocol includes a...
No comments:
Post a Comment