Clinical Applications

4.3.1. MR-Imaging Parameters for SPIO-MRI

Enhancement with SPIO depends on susceptibility effects, which vary with different pulse sequences, so pulse sequence choice is critically important in determining the diagnostic effectiveness of SPIO, and optimized parameters are essential to maximize lesion detection. Pulse sequence optimization for SPIO-enhanced MR imaging has recently been discussed (Ward et al., 2000; Kim et al., 2002a,b). Basically, pre- and post-contrast MR images consist of three imaging parameters: T2-weighted (fast) spin echo (SE), T1 -weighted gradient echo (GRE), and T2-weighted GRE. We compared signal-to-noise ratio (SNR) and lesion detectability among T2-weighted fast spin echo (FSE), echo planar imaging (EPI), short TE fast spoiled GRASS (SPGR), and long TE fast SPGR. Long TE fast SPGR exhibited the best SNR and detectability. This sequence provides little lesion-liver contrast before contrast enhancement, but a large decrease in signal occurs after SPIO administration (Figure 4.1). Therefore, lesion-liver contrast after SPIO is independent of inherent T1 and/or T2 difference between lesions and liver tissue, but highly dependent on Kupffer cell distribution and phagocytic function. The flip angle on long TE fast SPGR was set at 45-60 degrees, such that solid masses retain higher signal intensity than the surrounding liver and are distinguishable from liver cysts, which become almost as low in intensity as the surrounding liver tissue. SPIO-enhanced MR imaging is basically a modality for delineating phagocytic activity and cannot be used to assess the lesion vascularity and/or viability. This weakness could be circumvented with the use of perfusion MR imaging.

Perfusion study using echo planar imaging (EPI) yields negative enhancement of hypervascular tumors (Ichikawa et al., 1999), and one-stop shop diagnosis (involving both dynamic and RES-targeted MR imaging) for hypervascular hepatocellular carcinoma (HCC) may be feasible in the future. Parallel imaging will improve the image quality of single-shot EPI by correcting magnetic field inhomogeneity. On dynamic MR imaging using T1-weighted GRE, enhancement was positive in the liver for at least 30 seconds after bolus injection of SPIO (Reimer, 1998). However, positive enhancement of hypervascular HCC in early phase of T1W-GRE is too weak to assess the tumor viability. Recent advances in parallel imaging techniques have allowed for further improvements in image quality by reducing blurring and minimizing susceptibility-induced artifacts. One trial study of whole-body cancer screening using positron emission tomography (PET)-like projection images based on diffusion-weighted imaging (DWI) data has been reported (Takahara et al., 2004). Recently, combined use of diffusion-weighted imaging and SPIO was attempted for a small number of patients, and post-SPIO DWI exhibited improved contrast-to-noise ratio between malignant lesions and liver (Naganawa et al., 2005). Further study is needed to determine whether the combination of DWI and SPIO can improve the detection of malignant liver lesions.

Figure 4.1. Lesion-liver contrast on FSPGR with long echo time (TE). On T2-weighted FSE, a hyperintense mass is noted before SPIO (a, arrow). The lesion is more clearly visualized after SPIO (b, arrow). However, T2-weighted FSE images mask the effect of SPIO because of its inherent T2 contrast. On FSPGR with long TE, liver SNR is excellent but lesion-liver contrast is poor before SPIO (c). After SPIO administration, liver signal is markedly decreased and HCC (d, arrow) with satellite nodules (d, arrowheads) is clearly demonstrated. Because the inherent lesion-liver contrast is weak, the contrast after SPIO is due to Kupffer cell function alone.

Figure 4.1. Lesion-liver contrast on FSPGR with long echo time (TE). On T2-weighted FSE, a hyperintense mass is noted before SPIO (a, arrow). The lesion is more clearly visualized after SPIO (b, arrow). However, T2-weighted FSE images mask the effect of SPIO because of its inherent T2 contrast. On FSPGR with long TE, liver SNR is excellent but lesion-liver contrast is poor before SPIO (c). After SPIO administration, liver signal is markedly decreased and HCC (d, arrow) with satellite nodules (d, arrowheads) is clearly demonstrated. Because the inherent lesion-liver contrast is weak, the contrast after SPIO is due to Kupffer cell function alone.

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