Carotid body tumors (CBTs) are non-chromaffin and slowly growing paragangliomas, located at the carotid artery bifurcation area. CBTs represent more than 50% of head and neck paragangliomas (1, 2), and 4%–6% of these neoplasms have a malignant tendency to develop distant metastasis (3). Surgical resection is the most common treatment for CBTs (4), and surgical treatment as early as possible is advisable.

Imaging modalities are essential for the diagnosis of CBTs. Currently, color Doppler ultrasound (CDU) is widely used in the diagnostic workup for CBTs due to the absence of radiation and its convenience. Identifying CBTs relies primarily on their specific location at carotid bifurcation, which leads to the splaying of carotid vessels, as well as the hyervascularity nature of CBTs (4). However, CDU was reported to have lower sensitivity in detecting CBTs (5), as well as poorer accuracy in characterizing the size of the lesion areas and Shamblin type of CBTs (6, 7) than computed tomography angiography (CTA). CTA, however, although widely accepted as the initial diagnostic tool of CBTs (4, 8), poses the risk of adverse effects, including exposure to ionizing radiation and nephrotoxicity from iodinated contrast material, and is unsuitable for vulnerable populations such as pediatric patients, pregnant patients, or those with chronic renal disease. Magnetic resonance angiography cannot be performed when a patient has a pacemaker or stainless stell prosthesis. Therefore, the utilization of new imaging modalities, such as contrast-enhanced ultrasonography (CEUS), has been considered as potentially beneficial to characterizing CBTs.

Studies have shown that CEUS using second-generation microbubble agents substantially improves the ability of CDU for detecting and characterizing lesion areas in several organs (e.g., liver, breast) (9) but not yet extensively in detecting CBTs. Sasan et al. demonstrated that CEUS could better identify liver lesions that were not visualized by CDU and promoted the ultrasound-guided liver biopsy (10). Moreover, time–intensity curve (TIC) analysis could quantitatively obtain the tumor perfusion feature and vascularity, which had been used in differential diagnosis (11). Other studies reported that CEUS was superior to CDU and equal to contrast-enhanced CT in assessing tumor vessels (12), which has great clinical significance to the preoperative evaluation of CBTs. Therefore, CEUS might be able to assess the morphology and vascularity of CBTs more accurately and provide a better preoperative assessment than CDU. However, the utilization and performance of CEUS on the evaluation of CBTs remain to be investigated.

This study aimed to prospectively explore the characteristics and enhanced patterns of CBTs at CDU and CEUS qualitatively and quantitatively and compare CDU and CEUS for their morphology and vascularity signature.

With the introduction of ultrasonography contrast agents, CEUS has overcome the limitations of CDU by allowing a dynamic enhancement assessment of lesions similar to CTA and also providing the benefits of a paucity of contrast material side effects (16). The contrast agents of CEUS consist of gas molecules stabilized by lipid or albumin; they do not contain iodine and are extracted from the body through the pulmonary system, thus avoiding exposure to ionizing radiation and iodine contrast-related nephrotoxicity (17). In general, the contrast is very safe to patients, with a low rate of anaphylactoid reactions compared to the other types of contrast agents (18). Although some mild adverse events have been reported with the use of a CEUS contrast agent, such as injection site pain or rash, light headache, chest discomfort, nausea, and hyperventilation (19), the ultrasound contrast is considered the safest agent among other types of contrast media, with a very low frequency of severe adverse events (0.007%–0.0086%) (18, 20), which is lower than that of iodinated media and lower than or similar to that of gadolinium agents (21, 22). The value of CEUS in the evaluation of several tumors and vascular disease has been proven by other studies (10, 23, 24), while its utilization in CBT assessment remains to be investigated.

Our study firstly used CEUS to investigate the contrast enhancement feature of CBTs. We found that CBTs showed a fast wash-in, slow wash-out pattern with high AUC and a peak intensity enhancement signature, with a mean WIT and WOT of 3.00 ± 1.10 s and 58.79 ± 24.21 s (mean ± SD), respectively. Among all TIC parameters, WIT had the most consistent result, which was most likely due to their hypervascularity feature and close connection to carotid arteries. We also compared CEUS to conventional CDU for the evaluation of the lesion morphology and Shamblin type. We found that, first, CEUS was superior to gray-scale ultrasound in evaluating the morphology of lesion areas. It better enhanced and thus visualized the margins of the CBTs, given that the adventitia of carotid vessels in gray-scale ultrasound sometimes interferes with the discrimination of those margins. Besides, CEUS was also found to be more accurate in describing the Shamblin type of CBT lesions, which might contribute to the preoperative preparation and surgical planning of CBTs because a higher Shamblin type has been shown to be correlated with a higher risk of vascular injury and surgical bleeding during a surgical excision of the lesions (25).

In addition, we found that CEUS could better evaluate the vasculature abundance of CBTs in comparison to CDU in that CDU showed no significant difference in the AUC between the different vascularity groups it obtained. The discrepancy between the vascularity grades and enhancement intensity might have resulted from the lower sensitivity to small feeding vessels of CDU. CEUS, in contrast, uses microbubbles that are small enough (∼3 μm) to remain intact when flowing through arterioles and big enough to stay within the vascular compartment without passing through capillary fenestrations (26). Therefore, CEUS is a great tool for the visualization of tumor microvessels using a very small amount of contrast agents (27, 28). The vascularity classified by CEUS was found to correlate more with the AUC values, and a significant difference was also observed between different groups. These findings indicate that CEUS could provide a better evaluation of the vascularity than CDU both qualitatively and quantitatively.

Contrastly, CDU showed a higher sensitivity in the present study as compared to CEUS, in detecting the feeding vessels of CBTs (100% vs. 88%), while other studies had reported the opposite findings (29). It was difficult for both CEUS and CDU to identify the origin of the microvessels when the lesion contained abundant blood vessels. When the feeding vessel was detectable, however, CEUS was superior to CDU in determining the origin of the vessel since it could visualize the dynamic contrast enhancement within the micro-vessels, as revealed by our study (χ² = 9.162, p=0.010). Although superior, CEUS image acquisition relied on two-dimensional (2D) long and transverse section imaging and displayed the blood feeding vessels in a single plane. Our study concluded that 2D-CEUS could not fully capture the vascular spatial heterogeneity if the lesion was detected in a suboptimal position, a conclusion that is in accordance with previous studies (30). Neverthless, since preoperative embolization or early ligation of the feeding vessels prior to the resection of the tumor could benefit from early detection of the origins of feeding vessels, CEUS could potentially contribute to a bloodless operative field and therefore reduce the operative morbidity, due to its superiority in identifying the origin of feeding vessels (31, 32).

Our study had several limitations. First, there was no reference tissue for CBT lesions owing to their particular location, and the lack of the reference limited the characteristic description of the contrast enhancement pattern as in other solid organs. Second, 2D-CEUS could not fully capture the morphology and vascularity spatial heterogeneity when the lesion was detected in a suboptimal position and the largest dimension could not be acquired within the plane with the highest Shamblin grade sometimes. Neverthless, this is, to the best of our knowledge, the first prospective comparative study that investigated the performance of CEUS on the evaluation of CBTs, and our findings might give new perspectives on preoperative evaluation of the lesions when computed tomography or magnetic resonance imaging modalities are unsuitable or not available. Based on our findings, CEUS is superior to CDU in evaluating the morphology, Shamblin typing, vascularity, and origins of the feeding vessels of CBT lesions. More importantly, the advantages of CEUS are especially in monitoring the evolution of the CBTs including the location, size, growth rate, vascular encasement, and dynamic evaluation of the vasculature with a low cost, practicality, and lack of ionizing radiation. Beyond that, CEUS might show advantages in the differential diagnosis of CBTs. For example, our preliminary studies showed a distinct enhancement pattern of schwannomas, another neoplasm located at carotid bifurcation, as compared to CBTs, which exhibited a ring-like enhancement pattern and revealed a slower wash-in, faster wash-out pattern with lower peak intensity in TIC analysis ( Supplementary Figures 1 , 2 ). Future studies might be conducted to complement the current findings by introducing the 3D analysis of CEUS (30) and including adjacent tissues such as carotid arteries as reference. Besides, future comparative studies with larger sample size will help to validate the advantage of CEUS in the differential diagnosis between CBTs and other neoplasms at carotid bifurcation including schwannomas.

TIC analysis showed that CBT displayed consistently a fast wash in, slow wash-out pattern with high AUC. Both CDU and CEUS are appropriate to use in evaluating the morphology and vascularity of CBTs. In comparison with CDU, CEUS can better assess the Shamblin type of the lesion areas, especially for Shamblin I and III types. CEUS can also define the vascularity grades that are correlated better with TIC values. CEUS is superior to CDU in visualizing the origin of feeding vessels due to its excellent temporal resolution, but it has lower sensitivity than CDU.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the institutional review board of Peking Union Medical College Hospital. The patients/participants provided their written informed consent to participate in this study.

GG, XZ, and JS: conceptualization, methodology, formal analysis, investigation, data curation, manuscript drafting. SG, QG, JS, and BL: investigation, data curation and manuscript editing. BZ: conceptualization, methodology, supervision, formal analysis, manuscript review and editing. YZ: conceptualization, methodology, manuscript review and editing, supervision and funding acquisition. All authors contributed to the article and approved the submitted version.

This work was supported by grant from the Major Research Program of Natural Science Foundation of China (51890892), The Natural Science Foundation of China (82070492 & 81770481), The Non-profit Central Research Institute Fund of Chinese Academy of Medical Sciences (2021-JKCS-027), The Chinese Academy of Medical Sciences Innovation Fund for Medical Sciences (Grant No.CIFMS2021-I2M-1-016) and The CAMS Innovation fund for Medical Science (CIFMS, Grant No.2021-I2M-C&T-A-006).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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