Gadolinium-based hepatocyte-specific contrast agents have been widely used in liver imaging to detect liver tumors in clinics. However, tumor-to-liver contrasts are low in the case of chronic liver disease because contrast agents accumulate less to hepatocytes [1]. In this study, we applied 3D phase-sensitive inversion recovery (PSIR) sequence to liver imaging because this sequence is expected to improve tumor-to-liver contrasts. Moreover, this sequence can simultaneously provide a T1 map of the liver without any additional scan, which has been recently reported to be useful for liver-function assessments [2]. The purpose of this study was to determine whether the PSIR sequence can improve contrast enhancements of Gd-based agents and liver contrast while simultaneously providing an accurate T1 map of liver parenchyma.

Phantom study

The segmented 3D PSIR sequence was performed with a 3.0T clinical MRI system using an IR TFE readout and centric k-space reordering [3]. Thirteen phantom tubes containing a mixture of 1.0% agarose solution and different concentrations of Gd contrast agents were used. To determine the optimal TI for PSIR, the phantom tubes were imaged by PSIR with four different TIs (300, 400, 500, and 600 ms), and then the contrast of each phantom was calculated in the comparison to those of a control phantom with a T1 value of 764 ms, which is the same as in a liver tumor. These phantoms were also imaged by conventional TFE with FA = 15° and 30° for comparison with PSIR. For simultaneous T1 calculation by PSIR, we used two time-point images acquired from IR prepared image data and reference data with FA = 10°. From the relationships between two IR magnetization evolutions, the fully relaxed magnetization and T1 value were estimated using the iterative process reported by Warntjes MJB et al. [4]. Finally, these T1 values determined by PSIR were validated with two conventional T1 calculation methods: the standard IR-SE (10 points) and look-locker methods (L-L).

In vivo study

Seven healthy volunteers (mean age: 29.6 years) participated in this study. T1-weighted images were obtained using PSIR with optimal TI and conventional TFE (FA = 15° and 30°), and regions of interest were drawn over the liver, spleen, and muscle in each image. Liver-to-spleen and liver-to-muscle contrast were calculated and compared with each sequence. For validating the simultaneous T1 calculation by PSIR, T1 values of liver parenchyma, spleen, and muscle were compared with those obtained with the look-locker method.

PSIR showed a higher contrast of Gd-containing phantoms than those of conventional TFE (Fig. 1). The estimated T1 values measured by PSIR in each TI setting were linearly correlated with those measured by IR-SE. However, shorter TI values (300 and 400 ms) showed higher gradients in the range of longer T1 values (Fig. 2a); thus, the optimal TI for PSIR was determined as 500 ms. The estimated R1 (= 1/T1) of the phantom measured by PSIR were also well correlated with those measured by IR-SE and L-L (Fig. 2b). In the in vivo study, PSIR demonstrated significantly higher liver-to-spleen and liver-to-muscle contrast than those of TFE with FA = 15° and 30° (PSIR: 1.16 ± 0.14, TEF with FA = 15: 0.30 ± 0.12, TFE with FA = 30: 0.44 ± 0.23 for liver-to-spleen contrast, and PSIR: 1.30 ± 0.26, TEF with FA = 15: 0.22 ± 0.03, TFE with FA = 30: 0.22 ± 0.05 for liver-to-muscle contrast) (Fig. 3). The representative images of each sequence are shown in Figure 4. The estimated T1 values of liver parenchyma, spleen, and muscle in each subject measured by PSIR were clearly correlated with those measured by L-L (Fig. 5).The PSIR sequence improved contrast enhancements of Gd-based agents and liver-to-spleen or muscle contrasts while simultaneously providing an accurate T1 map of liver parenchyma. Although a clinical study is required to evaluate its clinical utility, this sequence may have potential to improve liver MR imaging.