TOO Short a TE

Coronal 3D GRE is an excellent sequence to evaluate the intrinsic ligaments of the wrist, as well as the triangular fibrocartilage. However, a TE of 2.8 msec. was chosen as the min. TE defaulted by the magnet according to the strength of the magnet and the gradients.

The resultant image displays normal ligamentous structures as high in signal intensity: the low signal expected from normal ligaments is a result of the highly organized collagen matrix; however, ligaments and tendons are intrinsically rich in hydrogen.


Coronal GRE with a TE of 17 msec. allows visualization of an intact luno-triquetral ligament as a dark triangular structure as opposed to the high signal intensity fluid-filled gap in the torn scapo-lunate ligament.	On a different cut, the centrally torn triangular fibrocartilage is clearly distinguishable from the peripherally intact portion.



Coronal T1 with a TE of 15 msec. renders the rotator cuff tendons isointense with the adjacent muscle. This effect is in part related to the magic angle effect of the curving fibers of the rotator cuff as they are about to attach to the greater tuberosity. This effect is TE dependent: the shorter the TE, the more prominent the magic angle effect is.	On the same patient, the T2 weighted sagittal sequence, clearly proves that the rotator cuff is intact, despite the spurious high signal on T1.



On a different example, merely bringing up the TE to 20 msec. reduces significantly the magic angle effect within the normal supraspinatus tendon.	On T2 the supraspinatus tendon shows consistent low signal, aside from a tiny focus of high signal anteriorly, which needs to be evaluated on consecutive slices not shown.

Comments

Magnets tend to default to the minimum TE whenever a T1 weighted sequence is selected: the shorter the TE, the more the contrast between structures depends on their T1 values. Also, bringing down the TE values is an indirect measure of the strength of the gradient platform.

However, with MSK imaging, a pure T1 image is not only not diagnostically necessary, but it can also be counterproductive: short TE's tend to show the signal that the highly organized spins within normal tendinous and ligamentous collagen matrices emit, allowing not enough time for them to dephase. Further, magic angle effects tend to be more pronounced with shorter TE's.

Therefore, it is good practice to strike a balance between a TE that is short enough to provide T1 contrast, but long enough to allow normal tendons and ligaments to be displayed as dark structures when they are normal. Of course, too long a TE may limit the ability to detect abnormal signal in truly pathologic structures; as always, there is a compromise between sensitivity and specificity.

In addition, when increasing the TE values, the TR will have to be lengthened, resulting in a time penalty.