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Original Report |
1 Department of Radiology, Indiana University School of Medicine, 550 N
University Blvd., Indianapolis, IN 46202.
2 Department of Radiology, University Hospitals of Leicester NHS Trust,
Leicester General Hospital, Leicester, United Kingdom LE5 4PW.
Received January 20, 2004;
accepted after revision May 17, 2004.
Address correspondence to K. Sandrasegaran
(ksandras{at}iupui.edu).
Abstract
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CONCLUSION. Gelatin absorbable sponge may mimic a postoperative abscess on CT. Findings that may help differentiate the hemostatic agent from abscess include linear arrangement of tightly packed gas bubbles, fixed position of gas bubbles on subsequent examinations, shape, lack of air–fluid level, and lack of enhancing wall.
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CT studies were performed using a 4-MDCT scanner (Mx8000, Philips Imaging), a 16-MDCT scanner (IDT, Philips Imaging), or a 2-slice scanner (CT Twin, Philips [formerly Elscint]). Contrast medium was given orally on the initial CT examination in 17 patients (500–750 mL of 2% diatrizoate meglumine, Gastrografin, Bracco). Contrast medium was given IV to all patients (150 mL of iopamidol, Isovue-300, Bracco). The effective slice width was 6.5 mm for the 2-slice scanner and 5 mm for the MDCT scanners with longitudinal reconstructions of 3 and 2.5 mm, respectively.
The CT images were reviewed by two abdominal radiologists to confirm the
presence of low-density material at the operative bed (density, 
–100
H). The size, shape, Hounsfield density, homogeneity, presence of
air–fluid levels, and appearance of surrounding fat were recorded by the
radiologists blinded to the postoperative course of the patients. Soft-copy
images were viewed on an MxView Station (Philips Imaging). The clinical
findings in all patients were reviewed from the hospital information system.
Patient demographics, type of surgery, date of operation, and postsurgical
progress were recorded.
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On the initial CT examinations, the size of the gelatin sponge ranged from 1.5 to 7.4 cm (mean, 3.2 cm). The density of the sponges measured from –104 to –458 H, with a mean of –245 H. The overall density of gelatin sponges was between that of gas and fluid. The shape of the sponges was variable. In eight patients, the sponge appeared predominantly polygonal, principally as rectangular strips (Fig. 1A, 1B, 1C). In 10 patients, the sponge was predominantly rounded (Fig. 2A, 2B). In all patients, the gas bubbles within the sponge were tightly packed and were not dispersed randomly (Fig. 3). No intervening fluid or soft-tissue density was present between gas pockets. The gas pockets within the gelatin sponge were uniform in size without discrete dominant bubbles.
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Five patients in the series had clinical and CT evidence of infection at the operative site. In four of these five patients, a drainage catheter was placed and purulent material was aspirated. These patients showed the presence of a well-defined low-density region, the hemostatic sponge, surrounded by fluid (Fig. 3). One of these five patients had a low-grade fever (up to 39.5°C), and hemorrhagic fluid was detected at aspiration. Results for the fluid and blood cultures were negative in this patient.
A gelatin sponge was visible on all follow-up CT scans obtained less than 38 days after surgery. The sponge was not seen on scans obtained 56, 74, and 156 days after surgery. In all patients with serial CT examinations, the gelatin sponge was smaller on subsequent scans. This change is best illustrated in a patient who had three CT examinations at postoperative day 6, 15, and 22 (Fig. 4A, 4B, 4C). The gas pockets in the gelatin sponge maintained their close spatial arrangement without dispersion on subsequent scans (Fig. 5A, 5B). This appearance was disparate from that of abscesses in which discrete gas bubbles were seen in different and random positions relative to each other on follow-up examinations.
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Gelatin bioabsorbable sponge is a purified porcine skin product that is water-insoluble. It aids in rapid hemostasis by forming an artificial clot and by its physical property of providing a mechanical matrix for platelet aggregation. Gelatin absorbable sponge contains numerous gas pockets that are not displaced immediately by interstitial body fluids. The presence of a gas-containing structure on early postoperative scans raises the possibility of abscess. It is interesting to note that researchers of original CT studies mentioned the presence of a gas-containing structure consistent with an abscess in 27 of the 34 scans. In 14 cases, the possibility of hemostatic agent was raised; such cases were the more recent studies when our radiologists became more aware of the CT appearances of gelatin sponge.
Our results show that it is possible to differentiate between these entities using CT. Gas pockets in gelatin sponge are packed tightly and are not discrete. They often are lined up in a linear fashion. Air–fluid levels, which typify abscess, usually are not seen in gelatin sponge without coexisting infection. Rim enhancement of the gelatin sponge generally is not seen in the absence of coexisting abscess, although rare exceptions occur. In one of 14 patients who did not clinically exhibit any signs of infection, the site of gelatin bioabsorbable sponge packing showed rim enhancement. The CT examination was performed 12 days after surgery. It is not clear if the enhancement was due to a foreign-body reaction or if there was subclinical infection.
Gelatin sponge usually is cut in geometric shapes to be placed against the bleeding surface during surgery; its shape may help differentiate it from an abscess. Follow-up CT examinations can reveal useful information. The gas bubbles in gelatin bioabsorbable sponge maintain their spatial position, which would be unusual in an abscess. CT performed for up to 38 days after surgery shows the sponge as a predominantly gas-containing structure. Over time, blood and interstitial fluid penetrate the sponge and a more homogeneous appearance of soft-tissue rather than gas-containing structure is seen.
The presence of gelatin sponge does not preclude an abscess, and the two can coexist. As seen in some of our cases, a well-defined geometric-shaped gas-containing structure within a fluid collection may be seen. Other CT findings of abscess such as scattered discrete large gas bubbles, rim enhancement, and air–fluid levels may be seen in this collection.
Surgical sponges are made of materials such as cellulose that do not become oxidized. They are nonabsorbable and must be removed from the surgical field [2]. In contrast, gelatin sponges are absorbed eventually and are left intentionally in the surgical field after surgery to continue to function as a hemostatic agent. The mechanism of absorption is not clear. Macrophages have been implicated in the absorption of other bioabsorbable hemostatic agents [3]. Of the eight patients for whom we had serial CT examinations, gelatin bioabsorbable sponge was seen as an identifiable structure on CT for up to 38 days after surgery. Our review shows that eventually gelatin sponges become resorbed at a reasonably rapid pace after the first week of surgery.
The use of hemostatic gelatin sponge is not without complications. The incidence of infection, which may be due to presence of a foreign body or superinfection of hemorrhage, is increased. In our series, 22.2% of the patients had superadded infection. Foreign-body granulomas can occur with excess use of gelatin sponges, particularly in the cranial cavity [4–6]. Neurologic dysfunction including quadriparesis may be caused by the use of these agents in the spinal canal [7]. Toxic shock syndrome has been reported after nasal surgery in which bioabsorbable gelatin sponges were used [8, 9].
A literature review reveals sparse information about the CT appearances of these hemostatic agents. Two groups have reviewed the CT examinations of a total of six patients [10, 11] who had hemostasis achieved with the use of oxidized cellulose (Surgicel) during neurologic and abdominal surgery. Like our findings, their findings suggest that linear arrangement of gas bubbles that maintain their position on subsequent scans should raise the possibility of hemostatic agents in contradistinction from abscess. It is probable that the CT findings seen with gelatin absorbable sponges are common to other hemostatic bioabsorbable agents. In a case report, researchers suggested the use of indium-labeled WBC scanning to differentiate a sterile mediastinal air collection associated with the use of a gelatin bioabsorbable sponge from abscess in a patient after aortic surgery [12]. However, careful evaluation of the CT findings can help differentiate gelatin sponge from abscess in most cases. We believe that such an analysis and a review of operative notes or consultation with the referring surgical team could prevent unnecessary antibiotic therapy or drainage catheter placement.
We are aware of the limitations of our study. The study was retrospective and depended on postoperative CT examinations reporting a foreign body containing gas. Many patients with the appropriate CT examinations probably were not included in this series. It is our experience that the gelatin sponge frequently is mistaken for abscess. Indeed, in many patients, the CT images from a second or third examination raised the possibility of gelatin sponge when the finding had been overlooked or misinterpreted on the initial CT examination. Consequently, the number of patients in this series was modest. Only a proportion of patients in our series had serial CT examinations to assess the natural history of gelatin sponge; it would not have been ethical to perform additional examinations without clinical need. Nevertheless, we were able to describe the CT features of gelatin absorbable sponge and determine criteria that may help in differentiation from postoperative abscess.
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