Differences

This shows you the differences between two versions of the page.

--- library:time_to_echo [2025/05/01 01:32] – scott
+++ library:time_to_echo [2025/05/01 01:57] (current) – scott
@@ Line 19: / Line 19: @@
 ==TE for T2* weighting==
-Without the strong refocusing pulses of a fast spin echo, transverse magnetization will decay rapidly due to the compounding effect of both T2 and T2* processes. This much shorter decay time means that TE's need to be much shorter, generally less than 30ms, or all the signal will have been lost.
+Without the strong refocusing pulses of a fast spin echo, transverse magnetization will decay rapidly due to the compounding effect of both T2 and T2* processes. This much shorter decay time means that TE's need to be much shorter, generally less than 30ms, or all the signal will have been lost. The most common method of achieving T2* weighting is to use a 2D gradient echo with a TE of ~20ms. It may be necessary to reduce the receiver bandwidth to recover SNR as well as allow for later TE selections. There are gradient echo sequences with special modifications that allow for enhanced T2* imaging such as Multi-echo GRE and SWAN. Note the T2* weighted image below demonstrating hemorrhage, TE ~18ms.
+{{:library:t2star_weighting.png?600|}}
+==In and Out of Phase TE==
+When utilizing fast 3D gradient echo sequences, a TE time may no longer be selectable, and instead the options are limited to an 'in-phase' or 'out-of-phase' option. In-phase and out-of-phase describe the relationship between fat and water vectors as they rapidly precess; at certain time points, fat and water vectors may be pointing in the same direction (in-phase) or opposite directions (out-of-phase) and the resulting images will demonstrate characteristic differences. In out-of-phase images, voxels containing both fat and water will will have signals that cancel out, resulting in Chemical Shift artifact. The specific times where fat and water are in and out of phase depend on the field strength, but at 1.5T are approximately 2.2ms, 4.4ms, 6.6ms etc. with the out of phase TE occurring earliest. Unless imaging the abdomen, typically select in-phase whenever possible.
+{{:library:ax_op_fspgr.gif|}}  {{:library:ax_ip_fspgr_2_.gif|}}