Imaging description

In and out of phase T1-weighted gradient-echo MRI of the liver is mainly used to evaluate diffuse fatty infiltration. Normal liver has the same signal intensity on in and out of phase images (Figure 21.1). Fatty liver shows a loss of signal on out of phase versus in phase images (Figure 21.2), due to chemical-shift-related signal cancellation between fat and water protons. Occasionally, the liver is brighter on out of phase versus in phase images (Figure 21.3). This is known as paradoxical signal gain and is due to hepatic iron overload causing T2* related signal loss on in phase imaging [1], since in phase images are typically acquired with a longer echo time (in a 1.5T scanner, out of phase images are usually acquired at a TE of 2.1 milliseconds with in phase images acquired at a TE of 4.2 milliseconds). With a longer echo time, iron-induced signal loss becomes more pronounced (such T2* effects are negligible in livers unaffected by iron overload).

Importance

Paradoxical signal gain on out of phase MRI can be seen with liver iron concentrations of 80 μ iron per gram and above [2]. This threshold is about twice the upper limit of normal liver iron concentration (36 μmol per gram) and is considered clinically significant [3], so that the detection of this phenomenon merits recognition and reporting. Liver iron overload also confounds the detection of diffuse fatty infiltration on out of phase MRI [4], because iron and fat have the opposite effect on signal dropout.