Modified contrast-enhanced ultrasonography with the new high-resolution examination technique of high frame rate contrast-enhanced ultrasound (HiFR-CEUS) for characterization of liver lesions: First results

1Introduction

Contrast-enhanced sonography (CEUS) of the liver has a high diagnostic value in the hands of experienced examiners [1–7]. Already in the DEGUM multicenter study, under appropriate examination conditions, CEUS was able to achieve a diagnostic reliability comparable to contrast-enhanced computed tomography (ceCT) with regard to the detection and characterization of solid liver lesions [8–13]. Compared with contrast-enhanced magnetic resonance imaging (ceMRI), there may be diagnostic advantages for MRI over CEUS with respect to the detection and characterization of solid liver lesions due to the use of liver-specific MRI contrast agents.

Ultrasound contrast agents of the 2nd generation are based on the principle of echo signal amplification by oscillation of microbubbles using Contrast Harmonic Imaging (CHI) with a low mechanical index (MI) < 0.2 [5]. The EFSUMB Guidelines describe a large number of applications relating to liver diagnostics and non-liver indications [1]. Regarding liver tumor diagnostics, there is a high classification of diagnostic relevance also by the FDA with recommendations and own experience also for applications in pediatric questions [1, 17].

The possible side effects of CEUS are considered to be low with increasingly worldwide clinical use. There is no restriction of renal perfusion [14, 15] or renal function and no influence of ultrasound contrast agents on thyroid metabolism. This still allows CEUS diagnosis of the liver even when impaired renal function makes this no longer feasible for the use of CT or MRI contrast agents.

The most common ultrasound contrast agent currently used in Europe is based on the Sulphur hexafluoride microbubbles contrast agent SonoVue™ (BRACCO, Italy), which are usually administered intravenously as a bolus of 1 to 2.4 ml followed by 5 to 10 ml of NaCL. Liver-specific ultrasound contrast agents are being researched but are currently not approved for routine use for liver ultrasound diagnosis in Germany and many European countries [1].

In order to optimize CEUS in the application of Sulphur-hexafluoride microbubbles contrast agent with respect to image quality, a large number of technical developments have been carried out recently. Increasingly, high-resolution multi-frequency transducers as sector or linear transducers with significantly increased crystal number, modified programs of the CHI with improved amplitude modulation or pulse inversion techniques (PIHI) and improved digital image spreading especially of CINE sequences are used [5]. Furthermore, efforts are being made to increase the frame rate to achieve even greater detail. A possible novel contrast medium ultrasound technique, which tries to take advantage of all the latest technical developments, is the new technique of high frame rate contrast-enhanced ultrasound (HiFR).

Whether there may be diagnostic advantages for CEUS with the novel mode of investigation such as HiFR must be based on prior studies regarding diagnostic confidence. The extent to which the detection and characterization of solid liver tumors is possible must be clarified, complicated cystic lesions can be differentiated, and the dynamics of lesions unclear on CT or MRI can be dynamically recorded and digitally stored from the early arterial phase after 5–15 s to a late phase of 5 to 6 min for independent assessment.

The following clinical investigation examined the extent to which the new technique of HiFR-CEUS detects and characterizes complicate cystic, solid, and microvascular liver lesions?

2Material and methods

Written informed consent was obtained from patients for all examinations using contrast-enhanced sonography (CEUS). Evaluation was performed independently using the image data stored in PACS by experienced readers in consensus. For the scientific evaluation of the data of contrast medium sonography for the diagnosis of liver lesions, an ethics vote of the ethics committee of the University Hospital Regensburg is available (approval number: 21-2761-104). The procedure for CEUS liver tumor diagnostics complies with the guidelines of the DEGUM studies [1].

N = 100 Patients with unclear liver lesions were subsequently included in the study. The examinations were performed on referral of the liver consultations for the clarification of unclear liver lesions on the basis of previous findings of B-mode sonography, computed tomography (CT) or magnetic resonance imaging (MRI). The aim of CEUS was to allow a final diagnosis, to describe the type, location and number of possible liver lesions or perfusion changes and, if necessary, to determine the further procedure. In the case of clearly benign findings, a follow-up after 3–6 months was performed as a reference, and in the case of suspected lesions corresponding to hepatocellular carcinoma (HCC) a grading according to LI-RADS IV-V was performed according to the probability of malignancy, and reference imaging with contrast-enhanced multislice CT and/or MRI with liver-specific contrast agent (Primovist™, Bayer, Germany) was performed promptly. If metastases were indicated, a primary tumor search was performed with CT and, if necessary, positron emission tomography (PET-CT). If necessary, a biopsy was performed after tumor board decision. If these were available, comparison to the surgical result of a liver tumor resection was performed.

All ultrasound examinations were performed by an experienced examiner (> 3000 examinations/year > 20 years) with a multifrequency convex probe (C1-6 MHz) on a high-end ultrasound machine (Resona 7, Mindray, China). The detectable lesions were digitally documented in B-mode in two planes under optimization of the examinations depth-dependent over the frequency, complementary with Tissue Harmonic Imaging (THI), ultrasound CT technique and averting Photopic technique (B-Colour). Assessment of macrovascularization of the liver was performed with color-coded duplex sonography (CCDS) via optimizations with the HR flow technique and glazing flow [34] for the hepatic veins (LV), hepatic artery (HA), portal vein (PV), and, if possible, tumor lesions with parameters mostly adapted to low flow (low scale/pulse repetition frequency (PRF), highest possible color gain, and low wall filter). Diagnostic indications were a wheel spoke pattern in focal nodular hyperplasia (FNH), marginal vascularization in adenomas (hepatic adenoma) or irregular central vascularization in HCC. It was also necessary to detect vascular tumor infiltration, especially in relation to the hepatic veins and portal vein intrahepatic. In the case of vascular changes, it was necessary to detect shunt with CCDS or a vascular breakdown in emboli and infarcts.

3Results

N = 100 patients (42 women, 58 men, from 18 years to 90 years, mean 63±13 years) with various entities of focal and diffuse liver parenchymal changes were studied, all of which could be detected with high image quality with HiFR-CEUS. There were n = 19 cases with proportionally septated cysts, in n = 5 cases scars after hemihepatectomy with local reduced fat, in n = 19 cases scars after microwave ablation, in n = 9 cases hemangiomas, focal nodular hyperplasia (FNH) in n = 8 cases, colorectal metastases in n = 15 cases, hepatocellular carcinoma (HCC) in n = 11 cases n = 8 cases, Osler disease in n = 8 cases. Table 1 shows the classification by the diagnostic procedures used in the 32 patients with malignant tumors of the 100 patients with suspected liver tumor.

The size of the lesions ranged from 5 mm to 200 mm, with a mean of 33.1±27.8 mm. The tumor sizes are classified in Table 2 for the three imaging methods.

The tumor sizes correlated quite well: CEUS –CT: 0.743 and CEUS –MRI: 0.745.

Final diagnoses could be made by the experienced examiner in 97/100 cases with CEUS, confirmed by reference imaging, in parts by histopathology or follow-up. Image quality for HiFR CEUS was rated 3 to 5, with an average of good 4 in a total of 62 cases, very good in 27 cases, and still satisfactory in 11 cases despite severe sonic conditions.

Fig. 1

Case of a 76 years old patient with colorectal cancer. Detection of a small liver metastasis of the right liver lobe, less than 1 cm in diameter using the HiFR CEUS technology after bolus injection of 1.5 ml ultrasound contrast agent. Wash out of the non-cystic lesion, image after 3 minutes in the late venous phase (left side, arrow). The lesion is not visible on fundamental B-mode (right side).

In all cases, contrast-enhanced sonography (CEUS) was technically feasible with good image quality in 62% (scored 4), very good image quality in 27% (scored 5). Due to difficult sonographic conditions, such as meteorism, colon interposition, inhomogeneous clear steatosis hepatis in obesity per magna and due to motion unrest, the digital cine sequences were limited in image quality but still sufficient image quality in 11 % (evaluation with 3 points). In some cases with extremely difficult sonic conditions for segment VIII lesions, image quality was significantly limited by distances to lesions > 20 cm (lesions were very difficult to detect). In MRI, limited image quality occurred in 20 % due to respiratory artifacts especially in the contrast sequences with a rating < 3 points. In CT, inhomogeneous contrast with limited assessment due to mixed contrast of the liver occurred in 15% even when using fast multislice technique.

High frame rate contrast-enhanced ultrasonography (HiFR-CEUS) was able to detect the typical contrast uptake from the rim to the center, nodular in hemangiomas, and from the center to the rim in the sense of a wheel spoke pattern in FNH in all cases of benign liver lesions, such as hemangiomas and focal nodular hyperplasia (FHN), leading to diagnosis. Contrast uptake increased toward the late phase to 5 min and was complete in typical hemangiomas, whereas a central recess remained in partially thrombosed atypical hemangiomas. In typical cases of FNH, a central scar was comparably found.

Typical for scarring changes after micro wave ablation or postoperatively in CEUS was the lack of contrast enhancement from the arterial to the late phase. In marginal recurrences, irregular nodular contrast enhancement arterially in the marginal area and a wash out of these nodular tumor parts beginning in the portal-venous phase would typically occur. With HiFR-CEUS, the defects were correctly assessed in all cases. This was also true for postoperative defects with local reduced fat. In these cases, the echo-deficient areas on the B-scan appeared garland-shaped with wavy margins. MRI examinations with dynamic contrast sequences (Vibe-3D) and diffusion sequences at 1.5 or 3 Tesla with specific contrast agent (Primovist™) or a 2-phase multislice CT with arterial and later portal venous phase served as reference.

Typical for malignant lesions was an arterial irregular contrast enhancement in HiFR-CEUS and a wash out starting in the portal venous phase and increasing to the late phase. Thereby, in all cases of colorectal metastases the more reliable detection was achieved in correlation to CT or MRI. Evidence of irregular marginal vascularization was detectable for all lesions. With high image quality, metastases as small as 5 mm in diameter could be delineated based on the wash out in the portal venous phase after 60 s up to the late phase of 5 to 6 min. The image quality was in some cases superior to CT and reached that of MRI with liver-specific contrast agent in almost all cases. In cases of small septated cysts up to 10 mm in diameter, HiFR-CEUS proved superior to CT in differentiation from metastases.

Fig. 2

Case of a 34 years old female patient with focal nodular hyperplasia (FNH). Detection in the early arterial phase using the HiFR CEUS technology after bolus injection of 1.0 ml ultrasound contrast agent hyperenhancement from the center to the margin (arrow). Excellent visualization by HiFR CEUS. The lesion is difficult to detect on fundamental B-mode (right side).

In the detection of hepatocellular carcinoma (HCC) HiFR-CEUS was able to detect irregular, almost chaotic early hypervascularization in the tumor foci and wash out from the portal-venous phase, in HCC often clearly only in the late phase from 4 min, when the tumor foci were > 15 mm in diameter with good or very good image quality. In the case of multiple suspicious foci, the detection of even small tumor foci > 10 mm in the late phase is crucial for the detection of all malignant lesions of a CCC or HCC in HiFR-CEUS. This was possible with HiFR-CEUS in almost all cases with good image quality, in some cases even very good. However, the image quality was often significantly limited by the echo inhomogeneous cirrhosis.

Fig. 3

A 73 years old patient with a small lesion of a hepatocellular carcinoma. Detection of a small tumor lesion of the right liver lobe, at maximum 1 cm in diameter using the HiFR CEUS technology after bolus injection of 1.5 ml ultrasound contrast agent (left side, arrow). Clear detectable by HiFR CEUS with irregular arterial hypervascularization (a) and wash out after 4 minutes (b) and by MRI using liver specific contras agent (c). The lesion is not visible on fundamental B-mode (right side).

More difficult was the evaluation of tumor foci with a maximum diameter of only 10 mm. Under difficult sonic conditions, it may be difficult to assess both hypervascularization and partial washout as reliably malignant at depths < 15 cm. Thus, one case was evaluated as regenerative dysplastic with HiFR-CEUS. In this case, the evaluation as HCC lesion was finally performed by MRI with liver-specific contrast agent. The diagnostic test performance of the methods used is shown in Table 3.

Fig. 4

A 61 years old patient with post ablation defect after MWA. Detection of an avascular defect on the left liver lobe, at maximum 4 cm in diameter using the HiFR CEUS technology after bolus injection of 1.5 ml ultrasound contrast agent (left side, arrow). Clear detectable MWA defect by HiFR CEUS with central devascularization during early arterial phase (a), arterial phase (b), portal venous phase (c) and up to late phase after 3 minutes (d) in comparison to contrast enhanced CT scans (e) during arterial phase (left) and portal venous phase (right) as a vascular defect on the left liver lobe (arrows). The lesion is not clear visible on fundamental B-mode.

4Discussion

Numerous publications exist on the diagnosis of focal liver lesions (FLL) using contrast-enhanced ultrasound (CEUS) in comparison to CT, MRI and histopathology. A critical factor for the characterization of FLL is the assessment of the enhancement pattern of the wash-in and wash-out of the contrast agent. The novelty of the provided research is the use of a new ultrasound technique based on the ‘Zone technology’, which uses broader zones insonation instead of line-by-line insonation. This allows significantly higher frame rates (giving a better representation of the contrast inflow dynamics) and more homogeneous acoustic power distribution across the image field (reducing microbubble destruction). Such new technical developments have made contrast-enhanced sonography (CEUS) an alternative with high diagnostic accuracy and fewer risks in liver diagnostics compared with contrast-enhanced computed tomography (ceCT) or magnetic resonance imaging (ceMRI) [18–22]. However, the success of CEUS always depends on the examination conditions, patient compliance, organ structure, depth localization, and ultimately the technical capabilities in the hands of experienced investigators [23]. Under favorable conditions, CEUS can achieve diagnostic confidence significantly > 90% in the localization and characterization of focal liver lesions, comparable to reference imaging.

Fig. 5

Case of a 54 years old female patient with Osler’s disease. CT scan of the lung shows irregular arterial hypervascularization of the central liver (a), after bolus injection of 1.5 ml ultrasound contrast agent HiFR-CEUS shows during early arterial phase (b), arterial phase (c), late arterial phase irregular dilatation of the hepatica artery (arrow) and hyperenhancement of hemangiomas (d). Late phase of HiFR-CEUS after 1 min shows homogenous contrast of the liver parenchyma without wash out (e).

Using second generation ultrasound contrast agents SonoVue™ as a bolus injection intravenously of only 1 to 2.4 ml, continuous dynamic acquisition of microvascularization from the early arterial phase after 5 to 15 s to the late phase of 5 to 6 min can be imaged with CEUS and stored in DICOM cine sequences. The acquisition of smaller tumor foci is directed by the use of optimized multifrequency probes, by high frame rate and optimization of the harmonic imaging of Contrast Harmonic Imaging (CHI) with pulse inversion techniques and amplitude modulation. One approach to this is offered by the new technique of HiFR.

CEUS is dynamic microvascular imaging down to the level of tumor capillaries. Detection of tumor nodular foci, complicated cysts with septal enhancement and evaluation into benign or malignant was based on wash in kinetics and regular or irregular arterial vascular patterns and wash out kinetics to late phase [1–9, 10–22, 30, 31]. Contrast CT is vascular and parenchymal and shows different dynamics. For MRI, liver-specific contrast agents are available with vascular, parenchymal, and RES-specific contrast, which may be beneficial in detecting smaller HCC foci. CEUS, however, does not affect renal function, which may be a critical advantage when creatinine clearance is altered [1, 14, 15].

Fig. 6

Case of a 45 years old female patient with a liver lesion on the right liver lobe, segment V, 5 cm in diameter. Correlation of the tumor lesion by MRI T2, T1, arterial phase and late contrast phase (a, b, c, d) using liver specific contrast agent as a benign lesion. Using HiFR-CEUS with bolus injection of 1.5 ml ultrasound contrast agent shows typical signs of a focal nodular hyperplasia (FNH) with contrast enhancement from the center to the margin during early arterial phase up to arterial phase (e, f, g) and no wash out after 3 min in the late phase with a small central scar (h) (arrow).

Characterization of malignant and benign foci of the liver is based on detailed assessment of arterial vascularization with CEUS as regular in benign lesions or irregular in malignant lesions. Regular patterns include nodular enhancement from the margin in hemangiomas, wheel spoke pattern from center to margin in an FNH, or from margin to center in adenomas. In complicated cysts that are reactively altered, narrow septal contrast enhancement is found in septa < 2 mm. These contrast agent patterns can sometimes be detected much better with HIFR-CEUS than with ceCT or ceMRI. Thus, CEUS is also important here as a reference imaging modality when the findings on CT are inconclusive and MRI with liver-specific contrast agent cannot be performed [24–28]. CEUS should always be performed before a possible biopsy, since the size, localization, and number of possible lesions, including benign ones, can be detected much better than with B-mode. In addition, CEUS enables also interventional procedures [29].

In malignant lesions, irregular hypervascularization is found in the arterial phase, such as in hepatocellular carcinoma (HCC), or irregular marginal vascularization, such as in metastases, most commonly in colorectal metastases. To detect this irregular microvascularization, the use of HiFR-CEUS may also be beneficial. This is also true for the characterization of tumor cysts with irregular septa > 3 mm or tumor nodules. Especially in smaller lesions of 10 mm or less in diameter, the detection of irregular microvascularization can be crucial in the differentiation of regenerated nodules, dysplastic foci, or HCC findings according to LI-RADS classification [5]. Thus, a high-resolution contrast mode in CEUS such as HiFR-CEUS is co-decisive in determining tumor entity.

To characterize a malignant or benign lesion, it is crucial to determine the dynamic washout kinetics. In metastases, this often begins in the portal venous phase (after 60 to 90 s), in CCC in the late portal venous phase, and in HCC not infrequently after 3 min, but may also only be detectable in the late phase after 5 min [1, 5, 27]. To detect this late washout behavior, particularly high-resolution CEUS techniques are required. HIFR-CEUS can also be helpful for this purpose. This also applies to the detection of small malignant tumor foci of less than 10 mm in diameter. Under favorable acoustic conditions, CEUS can achieve a sensitivity higher than CT, even comparable to ceMRI.

In the evaluation of vascular changes with CEUS, it is crucial to detect reduced perfusion, such as in infarcts or behind tumors, increased perfusion, such as in hyperemia, or micro- and macro-shunts or tumor thrombosis with transfer of microbubbles into the thrombus. In Osler's disease, signs of hepatic changes include elongation and dilatation of the hepatic artery (A hepatica) with early contrast often < 10 s, rapid contrast of the portal vein (V portae through shunts) < 40s, and hepatic veins < 50 s. Infarcts and scars appear as avascular wedge-shaped defects [32, 33]. Comparably, postoperative or post-ablative defects may impose after radiofrequency ablation (RFA), microwave ablation (MWA), or electroporation (IRE). In tumor recurrences, irregular nodular marginal changes arterially hyper-vascularized with wash out in the late phase are then not uncommon [24, 26]. HiFR-CEUS can be helpful to visualize micro-shunts, smaller tumor focus findings and to visualize the penetration of microbubbles into possible tumor thromboses in cases of aggressive malignant tumors.

Atypical partially thrombosed hemangiomas may be difficult to classify species-diagnostically, because the central thrombus may suggest a partial wash out. This is also true of adenomas with central fat or necrosis. The central scar of FNH cannot always be imaged. After chemotherapy, partial necrosis may occur in metastases, with an avascular appearance. Similarly, partial avascular necrosis occurs after chemoembolization (TACE) for HCC [1, 24, 26]. Under favorable acoustic conditions, HiFR-CEUS can image these partial necroses with high detail.

In cases where a defect was detected after ablation, dynamic CEUS was able to assess the dynamic microvascularization of only one defect continuously from the early arterial phase to the portal venous phase over one minute. Late washout is typical for malignant tumors. In small HCC lesions, only irregular arterial enhancement could be detected by CEUS or CT with contrast. In these cases, MRI with liver-specific contrast agents might be superior, or biopsies after 3 to 6 months. New liver-specific ultrasound contrast agents are prepared for in vivo or clinical evaluation but could not be used in clinical practice in Europe so far.

Limitations of CEUS examinations even with HiFR are the clear dependence on the experience of the examiner [23]. HIFR-CEUS was interpreted in independent blinded fashion by the MRI, CT or biopsy. The depth localization of possible foci at a depth of more than 15 cm, liver parenchymal changes due to fatty liver tissue (steatosis hepatis) or liver fibrosis up to cirrhosis, especially in ascites, worsen the acoustic conditions. In some cases, HiFR can still help to map the relevant findings. Unfavorable ultrasound conditions due to air superposition, meteorism, adiposity per magna and poor compliance cannot be changed.

5Conclusion

Based on the results the study indicates that the new HiFR-CEUS technology (high frame rate contrast-enhanced ultrasound) might further improve the diagnostic accuracy of CEUS leading to comparable diagnostic accuracy as with other imaging techniques.