Magnetic Resonance Imaging (MRI)
MRI is seldom used to study the mesentery
since currently, its spatial resolution is inferior to that of CT or US. In
addition, MRI is still strongly hampered by respiratory artefacts. On the other
hand, MRA is an attractive technique of investigation for vessels because it
avoids direct injection of a iodized contrast media. So, in most of "diagnosis"
indications, MRA progressively replaces the traditional angiography.
The setting of the sequence's parameters is a compromise meanwhile
of acquisition and resolution. One obtains as the case may be a "T1",
"T2" or "gradient echo" sequence.
Each sequence requires 4 to 8 minutes of acquisition depending on the
number of slices and the resolution.
The contrast media is Gadolinium, a rare earth with exceptional side
Fat suppression techniques (Fat-Sat, STIR, SPIR,...) improve the contrast
after injection of Gadolinium (figure 32b).
Angiographic sequences by MR (MRA) are realized with (better resolution)
or without injection of Gadolinium, and are based on fast 2D or 3D "gradient
echo" acquisition protocols (in one breathhold).
The semiology is variable depending on the T1 , T2 or EG weight of
each sequence; each structure having its own curves of signal decreasing
after a RF excitement :
- The fat appears in iso or hyper signal on T1 or T2 sequences.
- Vessels with slow stream are in T1 hypersignal.
- Vessels with fast stream are in T1 hyposignal.
- After injection of gadolinium, the arterial or venous vessels are
in hypersignal on T1 weighted sequences.
- Lymph nodes appear in T1 hyposignal
Figure 32: Abdominal IRM. Axial native cups(cuttings). A) images
balanced in T1. The fat appears in hypersignal; the structures vascular
and ganglionic in hyposignal ( no injection) b) images balanced in
T1 after Gadolinium's injection ( vascular raising) and abolition(deletion)
of the signal of the fat (the fat appears in hyposignal, what improves
the vascular contrast)
Figure 32 : Abdominal MRI. Native
a) T1. Fat is hypersignal ; vessels and
lymph nodes in hyposignal.(no injection)
b) Gadolinium enhanced T1 sequence with
fat suppression (fat is hyposignal improving the vascular contrast)