Ultrasonography is an essential imaging modality in diagnosing patients with gallbladder disease. If an ultrasound examination reveals a thickened gallbladder wall, malignant and benign gallbladder lesions, such as cholecystitis, benign cholesterolosis, and adenomyomatosis, among others, should be distinguished [1,2]. It is important to distinguish between benign and malignant gallbladder lesions because the treatment methods and prognoses are different for each.

Imaging features including invasion of the surrounding tissue, growth of large-sized polypoid lesions, and restriction on diffusion-weighted magnetic resonance imaging (MRI) are well-known features of malignant gallbladder lesions [3-5]. Many attempts have been made to distinguish between benign and malignant gallbladder lesions using various imaging modalities, such as contrast-enhanced computed tomography and MRI [6-10]. However, gallbladder cancer exhibits a variety of imaging features [3]. Occasionally, it is difficult to distinguish between benign and malignant gallbladder lesions in cases that do not exhibit specific malignant features, such as minimal wall thickening and comparable enhancement to that of the adjacent normal wall of the gallbladder.

The comet tail artifact is a well-known useful reverberation artifact on sonograms [11]. It is often accompanied by a thickened gallbladder wall lesion, such as gallbladder adenomyomatosis. The presence or absence of the comet tail artifact depends on the configuration of the gallbladder lesions. However, no research has investigated whether the comet tail artifact appears in malignant gallbladder lesions. The aim of this study was to evaluate whether the comet tail artifact on ultrasonography can be used as a reliable finding in the diagnosis of benign gallbladder diseases.

The Institutional Review Board approved this retrospective study and waived the requirement for patients to provide informed consent.

Ultrasonography images are created by a strong reflected beam that is first reflected from the surfaces of reflectors. However, if the ultrasound beam is reflected repeatedly between the front and back surfaces of the reflector, multiple reflected beams appear in the image, as if multiple reflectors are present in the posterior row. When the process is repeated, a smaller amplitude and the appearance of an inverted triangular shape are attained; this pattern is known as the comet tail or V-shaped artifact [11]. Comet tail artifacts are known to occur at interfaces where areas of different acoustic impedance are found; for example, at the valve prosthesis on echocardiography, or in areas of abdominal extraluminal free gas on abdominal ultrasonography [12,13].

In addition, the artifact is often present in adenomyomatosis of the gallbladder. Comet tail artifacts are the result of cholesterol crystals deposited in Rokitansky-Aschoff sinuses (RASs), which is a consequence of increased endoluminal pressure [14,15]. However, RASs are also present in cases of chronic cholecystitis [16,17]. Xanthogranulomatous cholecystitis is caused by extravasation of bile into the gallbladder wall through ruptured RASs; hence, this condition is considered to be rich in RASs [16-18]. Most cases with chronic cholecystitis, and approximately 80% of cases with xanthogranulomatous cholecystitis, show accompanying cholelithiasis [18,19]. Despite a lack of studies regarding the component ratio of cholesterol and pigmented stone in patients with gallbladder disease, one study reported that approximately 90% of cases of chronic cholecystitis and approximately 97% of cases of xanthogranulomatous cholecystitis had accompanying cholesterol or mixed stones [20]. Cholesterol crystals are a precursor of cholesterol stones, created and grown through cholesterol supersaturation and nucleation in the gallbladder [21,22]. Thus, patients with cholesterol stones show large amounts of cholesterol crystals. In summary, comet tail artifacts are caused by an accumulation of cholesterol crystals in RASs and can be seen in cases of both chronic cholecystitis and xanthogranulomatous cholecystitis, as well as in cases of adenomyomatosis.

In the present study, gallbladder lesions showing comet tail artifacts were diagnosed as chronic cholecystitis, adenomyomatosis, cholesterolosis, and xanthogranulomatous cholecystitis, and there was no difference in the proportion of each diagnosis, even when groups were divided according to the extent of the involved lesion. Thus, except for cholesterolosis, gallbladder diseases with comet tail artifacts are consistent with the diseases in which RASs exist. Cholesterolosis or cholesterol polyp is caused by the accumulation of cholesterol-rich macrophages in the lamina propria of the gallbladder. No studies have confirmed a relationship between cholesterolosis and RAS presence [23]. However, results from our study indicated a significantly lower rate of gallstone involvement in cases of cholesterolosis (33.3%) than in cases of chronic cholecystitis (89.2%). Although not statistically significant, the rate of accompanying gallstones in cholesterolosis was lower than in adenomyomatosis (71.8%) or xanthogranulomatous cholecystitis (100%). Notably, there was a small number of patients with cholesterolosis and xanthogranulomatous cholecystitis, which limited the statistical robustness of the results; the association between cholesterolosis and gallstones remains controversial in the literature [23]. This study could not find any association between cholesterolosis and RASs, or between cholesterolosis and gallstones. Moreover, the study could not identify other significant causes of comet tail artifacts in cholesterolosis. Future studies regarding the relationship between cholesterolosis and the comet tail artifact or RASs are required.

The ultrasonographic diagnoses of gallbladder disease were acute cholecystitis, adenomyomatosis, chronic cholecystitis, cholesterolosis, benign polyp, and gallbladder cancer. Of these diagnoses, adenomyomatosis, cholesterolosis, and benign polyp are not usually included in the indications for cholecystectomy. However, these patients were found to have acute cholecystitis or gallstone on ultrasonography, or biliary colic symptoms, which led to surgery. For this reason, the list of preoperative ultrasonographic diagnoses and the list of clinical indications of cholecystectomy were slightly different. Meanwhile, on preoperative ultrasonographic examination, gallbladder cancer was suspected in five patients. Although the thickness of the lesions was less than 10 mm, they were relatively thick and showed loss of the wall layer. Three of these lesions showed an irregular margin. The final diagnoses of these lesions were adenomyomatosis, chronic cholecystitis, and cholesterolosis. Thus, even if several ultrasonographic findings, including irregular margins and loss of the wall layer, suggest that a lesion is malignant, benignity should be considered in lesions with a comet tail artifact.

In the present study, there were two cases of coexistent chronic cholecystitis and low-grade dysplasia in a thickened gallbladder lesion. The time interval between the gallbladder ultrasonography and cholecystectomy was 14 and 7 days, respectively, which was within the approximately 2-week interval in which no significant changes in the lesion are expected. Nevertheless, because low-grade dysplasia is indicative of a pre-malignant lesion, the possibility that gallbladder lesions would be diagnosed as gallbladder cancer if the time interval is larger cannot be ruled out [24]. In this study, 204 days was the maximum time interval between ultrasonography and cholecystectomy, which may be too short for the development of gallbladder malignancies. Further research investigating longer time intervals in this regard is warranted.

In this study, there were no statistically significant differences in clinical and imaging findings, with the exception of gallstones, among the four diagnoses. The proportion of gallstones in cases of chronic cholecystitis was significantly higher than the corresponding proportion in cases of adenomyomatosis. However, because of the high rates of accompanying gallstones in cases of both chronic cholecystitis (89.2%) and adenomyomatosis (71.8%), the use of gallstones to distinguish between these diagnoses is limited. Although cholesterolosis showed a low percentage (33.3%) of accompanying gallstones and a statistically significant difference compared with chronic cholecystitis, the number of patients diagnosed with cholesterolosis in this study was too small to generalize the result. Thus, gallstones in chronic cholecystitis may be an important associated finding that is often seen; however, it is unreasonable to argue that gallstones may help differentiate other gallbladder diseases. To investigate the proportion of malignant lesions and the ratios among the other benign diseases according to the extent of involvement, this study divided the extent of gallbladder lesions into two categories. However, there were no malignant lesions in either category, and there was no statistically significant difference in the proportion of final pathologic diagnoses between the categories. As a result, it is unlikely that lesion extent can help to differentiate gallbladder lesions with a comet tail artifact. This study compared the length, thickness, and number of comet tail artifacts among the four final pathologic diagnoses, and found no statistically significant differences. Therefore, these clinical and imaging findings, including the shape and number of comet tail artifacts, may not be helpful in differentiating among these four benign diseases.

Meanwhile, there were actually two or more pathologic diagnoses in many patients. However, for convenience of analysis, the final diagnosis was simplified to a single pathologic diagnosis. The pathologic reports did not mention how, or to what extent, the pathologic findings were mixed. However, the reports did contain an indication of which pathologic findings seemed to be more widespread and prominent. The largest or most prominent pathological findings were regarded as the primary pathologic findings, and only the corresponding disease was chosen as the final diagnosis. Thus, simplifying the diagnosis may have led to a limitation in the comparison of the clinical or imaging findings among the 4 diagnoses.

In previous studies, comet tail artifacts were produced by cholesterol crystals and observed in adenomyomatosis of the gallbladder [14,25-27]. However, no study has investigated the relationship between the pathologic diagnosis of gallbladder lesions and the presence of a comet tail artifact. This study is meaningful in that it investigated the pathologic diagnosis of gallbladder lesions with comet tail artifacts on ultrasonographic imaging in a relatively large number of patients, and all cases were benign gallbladder lesions.

This study has a few limitations. First, it was a retrospective study. The pathologic diagnoses were obtained from the whole gallbladder specimens, rather than from the exact lesions corresponding to the comet tail artifacts. As a result, there is a limited one-to-one correspondence between the pathologic diagnosis and imaging findings in the exact area with the comet tail artifact. In addition, whether the portion of the specimen with dysplasia corresponded to the exact area accompanying the comet tail artifact remains unknown. Second, this study simplified the pathologic diagnoses, of which two or more could co-exist in each patient, to a single final diagnosis. Third, the study used a single-center design. There were too few cases of cholesterolosis (n=3) and xanthogranulomatous cholecystitis (n=2), which limited the statistical power of the results. Selection bias was also a limitation of this study. This study was conducted only among patients with comet tail artifacts. In future studies, it is necessary to collect consecutive patients regardless of the presence of a comet tail artifact, and to make a direct comparison between the group with the artifacts and the other group without the artifacts. A prospective study with a multicenter design and large cohort is needed in the future.

In conclusion, in this study, all gallbladder lesions with a comet tail artifact on ultrasonographic imaging, regardless of lesion extent and other suspicious ultrasonographic findings such as irregular margin and loss of the wall layer, were benign, including causes of chronic cholecystitis, adenomyomatosis, xanthogranulomatous cholecystitis, and cholesterolosis. Therefore, the comet tail artifact in gallbladder lesions may potentially be a reliable sign suggesting the presence of a benign lesion. Our results may be applicable to the differential diagnosis of gallbladder diseases through gallbladder ultrasonography.