Vascular pattern and diagnostic accuracy of contrast-enhanced ultrasound (CEUS) in spleen alterations

2.1 Study cohort

This retrospective study was approved by the local institutional ethical committee (EA4/148/18). Oral and written informed consent of all patients was obtained before examination. All study data were collected in compliance with the principles expressed in the Declaration of Helsinki 2002.

A total of 56 patients between 2012 and 2018 received an ultrasound examination of the spleen using B-mode, CCDS and CEUS. While only patients with complete follow up data were included in the final retrospective analysis, six patients were excluded due to loss of follow up. An active malignancy was defined as active cancer or anti-tumor therapy within the last 6 months or metastatic state at time of examination.

2.2 CEUS examination protocol

For gray-scale B-mode, the spleen was examined with a 1–6 MHz convex array transducer looking for detection, lesion size, echogenicity and homogeneity of the lesions using high-end ultrasound systems (Aplio500/Aplio i900; Canon Medical Systems Corporation, Tochigi, Japan). Colour-coded and Power Doppler ultrasound were employed to determine vascularization of the masses. Spleen was examined in modified longitudinal and transversal planes, if necessary, also by inter- and trans-costal scanning in deep inspiration and with optimized scanning positions.

CEUS examinations were performed during clinical routine using high-end ultrasound systems with up-to-date CEUS specific protocols available at the time of the examination. The examinations were performed at 1–6 MHz using i8X convex array transducer (Canon, Otawara, Japan). Ultrasound B-mode images were optimized using spatial compounding, frequency-based compounding, ApliPure™ level 5, differential Tissue Harmonic Imaging (dTHI©), and Precision Imaging^© with level 4 Speckle Reduction (SR). CEUS images were performed at 1 to 5 MHz, 10fps. Each ultrasound system was configured with a very low MI (0.06 to 0.07) to avoid early microbubble-destruction. A bolus of 1.2–2.4 ml of ultrasound contrast agent (SonoVue^®, Bracco Imaging, Milan, Italy) was repeatedly injected up to three times until satisfying cine loops were acquired. Baseline B-mode US, CCDS and CEUS (i.e. qualitative assessment of contrast enhancement pattern) were performed by a single high-experienced radiologist with more than ten years’ experience in CEUS (EFSUMB level 3). The loops and images were read by two experienced radiologists in consensus.

2.3 Image interpretation

All B-mode US images, CCDS and CEUS loops were retrospectively evaluated in consensus by two experienced radiologists (M.H.L., T.F.), one of them EFSUMB level 3 examiner. B-mode US features were homogenicity, echogenicity, lesion shape, maximal lesion size and number of lesions. In case of more than one lesion, the reference lesion in CEUS examination was used for evaluation of lesion size and vascular pattern. CCDS included central, peripheral or no hypervascularization. CEUS features included early arterial contrast enhancement, contrast enhancement compared to the surrounding splenic tissue (hyper- or hypoenhancement), wash-out during parenchyma phase and contrast enhancement pattern (peripheral-nodular, centrifugal, centripetal, general hypoenhancement compared to regular surrounding tissue).

2.4 Reference standard

All pathologies underwent imaging follow-up (35/50 ceCT, 12/50 ceMRI, 3/50 CEUS) and clinical surveillance including paraclinical parameters (laboratory tests, tumor markers etc.) served as reference standard. Five out of these 50 patients underwent both additional imaging and surgical exploration/biopsy with the respective histopathologic results serving as reference standard. A minimum follow-up was performed up to one year. Negative tumor markers with stable or diminishing lesion size after one year were considered to indicate a benign nature of the lesion. All radiological reports were approved by board-certified radiologists.

2.5 Multisclice CT protocol

Overall, 35/50 patients underwent multiphase CT examinations (arterial, and/or portal-venous and venous/delayed phase) using a standard protocol with a 64–128 detector CT scanner (Siemens Medical Systems, Forchheim, Germany), a minimum slice thickness of 0.625 mm and reconstruction interval 2 mm. The contrast agent was injected into an antecubital vein as a bolus, using a dual head power injector with a flow rate of 4,5–5 ml/s. 100–120 ml (Ultravist®, Bayer Healthcare, Berlin, Germany) with an iodine concentration of 370 mg/ml was administered, followed by 50 ml saline. The acquisition direction was cranio-caudal.

2.6 MRI protocol

MRI was performed with 1.5 T and 3.0 T systems using phased-array body coils in 12/50 patients. The examination protocols comprised T2-weighted (w) standard 2D sequences with and without fat saturation (FS) and T1-w unenhanced 2D sequences with and without FS (including in-/opposed phase technique). T1-w 3D sequences with FS were acquired in breath hold technique before and during arterial, portal venous, and venous phase, respectively after intravenous application of Gadubutrol acid (Gadovist, Bayer Pharma, Berlin Germany; body weight adapted; manual or automatic injection at approximately 1–2 mL/s flow rate followed by 40 mL saline flush).

2.7 Statistical analysis

Continuous variables were tested for normal distribution using the Kolmogorov-Smirnov-test; variables not following a normal distribution are reported as median and interquartile range (IQR) and were compared using the Mann-Whitney U-Test. Categorical variables were compared using student’s t-test or chi² test, as appropriate. The obtained CEUS results and matched histopathological reports from each patient were analyzed retrospectively for diagnostic accuracy by testing sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV).

A two-sided significance level of α 0.05 was defined appropriate to indicate statistical significance. All statistical analyses were performed using the SPSS software (IBM Corp. Released 2016. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.).

4.1 Traumatic pathologies

B-mode US and CCDS can demonstrate morphologic and hemodynamic changes in splenic lacerations. Additionally, CEUS is a well-established criterion for the diagnosis of traumatic splenic lesions [18]. Therefore real time contrast enhancement pattern on CEUS may provide further diagnostic information if B-Mode ultrasound appearances are uncertain [19, 20]. In our study, CEUS ruled out splenic laceration in all patients and may serve as a feasible tool especially for young patients with isolated splenic trauma. During follow-up CEUS can depict a change in size or shape of infarction [21]. Changes in lesion features during follow-up confirm the differential diagnosis from other splenic lesions and allow conservative management in these patients.

4.2 Evaluation of unclear splenic lesions

CEUS has shown advantages over contrast-enhanced MRI including unmatched temporal resolution due to continuous real-time imaging in detecting unclear renal lesions [22]. Higher sensitivity has been shown in combining CEUS with MRI in the characterization of focal liver lesions [23] while CEUS and MRI alone have shown equal results in differentiating liver lesions [9]. Compared to CEUS, MRI is more time consuming especially in the diagnostic workup of acute alterations and may not be widespread available in smaller non-university hospitals. Our study showed excellent results by CEUS in combination with prior ceCT or ceMRI and knowledge of clinical information, indicating that incidental findings on B-mode US may get a fast and easy additional imaging evaluation using CEUS as secondary imaging modality. Furthermore, incidental findings on single-phase ceCT could benefit from additional fast and cost-effective CEUS imaging for evaluation of lesion perfusion.

Data of the differentiation of focal splenic lesions using CEUS is rare. CEUS allows visualization of splenic microvascularization and may thus aid in the preoperative assessment of splenic lesions with hypovascularization in the parenchymal phase as an important feature in the diagnosis of malignancy [24]. Lesion washout in parenchyma phase was present in all malignant alterations (n = 5) and also in benign lesions such as two hemangiomas, one hamartoma and a rare case of sarcoidosis, all confirmed by cross sectional imaging and histopathology. Similar to Bachmann et al., CEUS was superior to CCDS in the visualization of vascularization especially in benign splenic lesions (43.3% hyperenhancement by CEUS vs. 31% hypervascularization by CCDS in benign lesions) [25].

Caremani et al. demonstrated that hypoenhancement of a focal splenic lesion had a positive predictive value of 87% for a neoplasia. Although late phase washout is a typical sign of malignant lesions, less pronounced washout could also be visualized in many benign lesions [1]. Therefore, clear differentiation between benign and malignant lesions due to contrast enhancement in CEUS is currently challenging [1, 13]. CEUS may be useful in the assessment of splenic lesions where CCDS comes to its limits because splenic tumours may appear avascular.

Benign lesions appear as anechoic masses or echogenic lesions due to gas or calcification [26]. Vascular tumors (hemangioma and hamartoma) are the most common benign lesions while secondary tumors (lymphoma and metastases) are the most common malignant lesions [27, 28]. While only one case of Hodgkin lymphoma presented with early contrast enhancement in arterial phase and progressive early washout during parenchyma phase, early enhancement was more frequently seen in benign FSL (n = 12; six hemangiomas, three splenomas, two hamartomas and one case of sarcoidosis). Although progressive centripetal filling is unusual in splenic hemangiomas [1], 11/35 FSL showed centripetal contrast filling (all benign lesions) whereof four hemangiomas demonstrated with centrifugal contrast filling.

While especially small situated hypoechoic tumors appear with short filling time or circumferential vessel with a rapid centripetal filling on CEUS, the visualization of specific contrast enhancement pattern in small lesion is limited [23, 26]. Lymphoma of the spleen could show a variety of morphologic findings and benign alterations like septic embolism could also mimic a malignant FSL. CEUS and evaluation of perfusion kinetics therefore allows a better detection especially of lymphoid lesions within surrounded splenic tissue [29, 30]. While two patients had inconclusive findings (focal lesion) on ceCT, CEUS revealed one incidental finding of a diffuse B-cell lymphoma in a patients after bike accident two weeks before examination with persistent left upper abdominal pain (presented with enhanced solid tissue compared to splenic tissue and washout during parenchyma phase, Fig. 2). Incidentally, lymphoid lesions may show a small irregular rim of enhancement during the early arterial phase [31, 32]. Furthermore, intralesional vessels and large areas of necrosis due to irregular neovascularization favor a diagnosis of malignancy, which may explain the different perfusion kinetics on time intensity curves [1].

While malignant lesions represented greater median tumor size (p = 0.342) and more often complete hypoenhancement (p = 0.421) in this study without reaching statistical significance (due to a small number of rare malignant lesions), washout on CEUS was seen in all malignant lesions (p = 0.001). Thus, differentiation of benign and malignant lesion may base on a combination of echogenicity, heterogenicity and contrast enhancement. The use of multiparametric US including shear wave elastography, microflow imaging and CEUS may be a promising tool in the diagnosis of splenic alterations in the future [33, 34]. CEUS could potentially avoid unnecessary biopsy of the spleen, which are associated with higher risk for bleeding due to high blood flow compared to liver biopsy. Furthermore, it represents a preferred modality for young patients with a low rate of allergic reaction and without use of radiation exposure.