First-generation contrast media such as Levovist have been used in the diagnosis of hepatic focal lesions [12
]; however, this agent has some limitations [19
]. Levovist must be insonated with high acoustic power, which destroys the microbubbles. The wide-band frequency signals from the destroyed microbubbles can then be imaged, resulting in intermittent images that have a marked mixture of tissue and microbubble harmonics. The advent of second-generation contrast media such as SonoVue (Bracco, Milan, Italy) has enabled real-time imaging using low-MI mode. Many studies report that CEUS with SonoVue has high performance in detecting the vascularity of HCC, even for small nodules <2 cm in diameter [20
Sonazoid is a second-generation contrast agent composed of perfluorobutane microbubbles with a shell of phosphatidyl serine natrium; the microbubbles are 3 μm in mean diameter, similar to the size of SonoVue (2.5 μm in mean diameter) [13
]. The optimal method for evaluating the vascular phase utilises low-MI imaging that minimises bubble destruction. As well as SonoVue, Sonazoid can provide real-time imaging at the vascular phase using low-MI modes. The enhancement pattern of HCC by CEUS with SonoVue is hyperenhancement at the early vascular phase (arterial dominant) and hypo-enhancement at the portal or parenchymal phase. Hypo-enhancement at the portal or parenchymal phase is caused by washout of contrast agent from the nodule, owing to perfusion of the surrounding liver parenchyma.
Gaiani et al [21
] report that 91% of HCC nodules that are hypervascular on dynamic CT demonstrate hyperenhancement at the early vascular phase with SonoVue and that 75% demonstrate slight hypo-enhancement during the portal or parenchymal phase. Xu et al [24
] reported that 87% of small HCC (≤2 cm) demonstrated hyperenhancement at the early vascular phase with SonoVue and that 54% demonstrated hypo-enhancement in the portal phase, with 46% demonstrating iso-enhancement. In the present study, 88% of HCC that were hypervascular on dynamic CT, regardless of tumour size, showed hyperenhancement at the early vascular phase of CEUS with Sonazoid. Therefore, the detection rate of hypervascular HCC by Sonazoid is equal to that by SonoVue.
Recent studies have shown that the detectability of tumour vascularity by CEUS with SonoVue is comparable to that by dynamic CT [25
]. The present study demonstrated that Sonazoid has the same performance as dynamic CT, both for newly developed HCC and local recurrence, and that the detection rate of tumour vascularity by Sonazoid in hypervascular nodules was not affected by lesion size. We previously demonstrated that the detection rate of vascularity with Levovist is not affected by tumour size [12
]. In addition, Xu et al [24
] reported that SonoVue retains high detectability for small HCC nodules (≤2 cm). On the basis of these results, the detectability of vascularity by CEUS does not depend on tumour size, regardless of the contrast media used. Tumour location, however, is an important factor in tumour detection: the detection rate of vascularity using Sonazoid was less for nodules located deeper than 9 cm from the abdominal wall than for nodules located within 9 cm, as we previously demonstrated using Levovist [12
Sonazoid can also provide Kupffer imaging at the post-vascular phase; this phase occurs more than 10 min after administration of the contrast agent, because most of the Sonazoid microbubbles (99%) are phagocytosed by Kupffer cells. By contrast, SonoVue is barely phagocytosed and cannot create a Kupffer image [15
]. As only half of the Levovist microbubbles are phagocytosed by Kupffer cells, Levovist provides only intermittent images on high-MI, whereas Sonazoid can provide real-time continuous images on low-MI. Therefore, Kupffer imaging at the post-vascular phase using Sonazoid is more effective than that using Levovist and is easier to scan repeatedly. The number of Kupffer cells in cancerous tissues is markedly lower than that in non-cancerous liver tissues and numbers decrease with decreasing histological grade [26
]. Thus, quantitative evaluation of Kupffer cells can be useful in the differential diagnosis of focal liver lesions and histological grade of HCC. Imai et al [7
] studied SPIO-enhanced MRI in patients with HCC and showed that enhancement reflected Kupffer cell numbers. Although we did not evaluate the correlation between enhancement in the post-vascular phase of CEUS with Sonazoid and histological grade of HCC, Inoue et al [27
] and Korenaga et al [28
] reported that the post-vascular phase of CEUS with Sonazoid correlates closely with SPIO-enhanced MRI. In the present study, the detection rate of hypo-enhancing nodules at the post-vascular phase by Sonazoid was 83%, which is as high as that demonstrated by the previous study [27
]. By contrast, the detection of hypo-enhancement at the portal or parenchymal phase by SonoVue is 54–75% [21
]. These findings suggest that Sonazoid might be superior to SonoVue in tumour detection at the post-vascular and parenchymal phases.
The present study demonstrated that hypo-enhancement at the post-vascular phase was less frequent in small nodules (≤2 cm) than in large nodules (>2 cm) and less frequent in isovascular nodules than in hypervascular nodules. Liu et al [26
] showed that the number of Kupffer cells in HCC decreases with increasing tumour size. Moreover, these authors reported that the number of Kupffer cells is reduced in moderately differentiated HCCs (which are hypervascular), as compared with dysplastic nodules or well-differentiated HCCs (which are iso- or hypovascular). Our results are in good agreement with these previous findings. Because well-differentiated HCCs have as many Kupffer cells as non-cancerous liver tissues, they have an isoechoic pattern at the post-vascular phase by Sonazoid [28
CEUS with Sonazoid is useful not only for the assessment of HCC, but also for the differential diagnosis of benign hepatic nodules. Some benign hepatic focal nodules, mainly focal nodular hyperplasia (FNH) and haemangioma, have been shown to demonstrate hypervascularity [29
]. FNHs show marked homogenous enhancement and a “central stellate” or “spoke-wheel” appearance at the early vascular phase; at the post-vascular phase these nodules show iso-enhancement. The sensitivity and specificity of the FNH pattern are both 100% [30
]. By contrast, haemangiomas show a subtle non-specific enhancement at the early vascular phase and peripheral globular enhancement at the late vascular phase, together with a progressive centripetal fill-in pattern [31
]. The sensitivity and specificity of the haemangioma pattern are reported to be 90% and 99.6%, respectively [30
]. Sonazoid-enhanced ultrasound is therefore useful to differentiate HCCs from these benign nodules. Recent studies have reported that HCC nodules not detected by baseline B-mode ultrasound could be detected at the vascular phase and/or at the post-vascular phase of CEUS with Sonazoid [32
]. Furthermore, Sonazoid-enhanced ultrasound has been shown to be a sensitive and accurate modality for evaluating the response of HCCs following arterial embolisation [33
]. In the clinical setting, we routinely use CEUS with Sonazoid from diagnosis to therapy evaluation.