In-out-of-phase
An in/out-of-phase is a gradient sequence used as a tool to detect microscopic fat in a lesion/organ. It is used in particular to evaluate adrenal masses (fat-containing adenoma vs adrenal carcinoma) and fatty infiltration of the liver.
Background: the protons in fat and water have a slight difference in Larmor frequency, which may cause a so-called chemical shift artifact (details on chemical shift artifact will not be discussed in this module). The series has two components: the in-phase sequence and the out-of-phase sequence. When reading the signal on the in-phase sequence, the protons of the fat and water are in exactly the same phase (despite the slight difference in Larmor frequency). The protons are in-phase and therefore give off signal. The signal in the out-of-phase sequence is read at a specific different moment, the moment that the protons of the fat and water are not exactly in the same phase. This eventually leads to signal loss (Note: the protons are out-of-phase).
Example: a fat-containing adrenal lesion has high signal intensity on the in-phase sequence and low signal intensity on out-of-phase (fig. 25).
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Figure 25. T2 weighted image & in/out-of-phase of the abdomen in transversal direction. A spherical mass originating in the right adrenal can be seen on the T2 weighted image. As compared to the in-phase, signal loss of the lesion occurs on the out-of-phase series, a sign of microscopic fat. Diagnosis: high-fat adrenal adenoma.
Coincidental finding: note also the buildup of fats in the liver (fatty liver).
The in/out-of-phase sequences are therefore useful to detect microscopic fat in a lesion. Information on whether a tumor does or does not contain fat may help in the differential diagnosis.
