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1 All authors: Department of Radiology, Mayo Clinic, 4500 San Pablo Rd., Jacksonville, FL 32224-3899.
Received May 21, 2001;
accepted after revision September 17, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Seattle, April—May 2001.
Abstract
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MATERIALS AND METHODS. The imaging studies of 12 patients with surgically confirmed wooden foreign bodies were retrospectively reviewed. The study group consisted of seven females and five males, 10-65 years old (mean age, 36 years). All patients underwent radiography. Nine patients were evaluated with sonography, eight with MR imaging, three with CT, and one with CT arthrography. Gadolinium-enhanced MR imaging was performed in six patients. Three patients presented with a draining sinus and nine with painful swelling. Only three patients presented with a history of penetrating injury.
RESULTS. Lesions were located in the foot (n = 4), hand (n = 3), thigh (n = 2), calf (n = 2), and elbow (n = 1). Radiographs failed to reveal the retained foreign bodies in all patients. With MR imaging, wooden foreign bodies displayed a variable signal intensity that was equal to or less than that of skeletal muscle on both T1- and T2-weighted images. MR imaging showed the surrounding inflammatory response in all patients. CT showed the retained wood as linear cylindric foci of increased attenuation. Wood was highly echogenic and revealed pronounced acoustic shadowing on sonography. Arthrography in one patient showed an associated reactive synovitis.
CONCLUSION. The imaging appearance of wooden foreign bodies is variable; however, imaging can be quite specific, and when taken in the appropriate clinical setting, the imaging should reliably suggest the diagnosis. Sonography is frequently underused but proved most useful for the evaluation of retained wooden foreign bodies.
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Patient presentation was variable. The most common presenting symptoms were pain and swelling, which were observed in 10 patients. Three patients presented with a draining sinus. Only three patients presented prospectively with a history of penetrating trauma. The foot was involved in four patients, the hand in three, the thigh in two, the calf in two, and the elbow in one.
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MR Imaging
MR imaging showed the retained foreign bodies to be hypointense to skeletal
muscle on both T1- and T2-weighted sequences. Two patients revealed the
retained fragment as a signal void (Fig.
2A,2B).
The surrounding inflammatory response was seen in all patients. The response
was hypointense on T1-weighted images and isointense to hyperintense on
T2-weighted images in relation to surrounding skeletal muscle and fat,
respectively (Figs.
2A,2B,3A,3B,3C,4A,4B,4C,4D).
Gadolinium-enhanced imaging in six patients showed enhancement of the
surrounding inflammatory response.
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CT
CT depicted the retained wooden foreign body as a linear or cylindric area
of high attenuation compared with the surrounding skeletal muscle and fat
(Fig.
5A,5B).
All foreign bodies were best visualized on images viewed at a wide (bone)
window. The surrounding inflammatory response seen on MR imaging was difficult
to differentiate from surrounding skeletal muscle, although effacement of
surrounding fat planes, indicative of an inflammatory response, was seen in
two patients.
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Sonography
Sonography showed the retained wooden foreign bodies as linear echogenic
structures with pronounced acoustic shadowing (Figs.
4A,4B,4C,4D
and
5A,5B).
In two patients with larger pieces of wood, only the hyperechoic,
crescent-shaped leading edge of the wood was seen with acoustic shadowing
obscuring the trailing edge.
CT Arthrography
In one case in which the wooden foreign body was suspected to have
traversed the elbow joint, athrography revealed an irregular margin of the
joint with a thickened, corrugated synovium (Fig.
1A,1B,1C).
Contrast material was seen about the retained foreign body.
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Xeroradiography has been reported as slightly more sensitive than conventional radiography for the detection of retained wood; however, xeroradiographs show negative results in 80% of patients and are not available in most radiology departments [2].
CT has been shown to be useful in the evaluation for suspected wooden matter. In our series, retained wooden foreign bodies were more subtle when using the standard window and level setting. When the settings were altered by increasing the window width, the foreign bodies were more easily identified. Previous authors have suggested using wide window widths up to 1,000 H with a level of -500 H for optimization of detection [5]. The attenuation of a retained wooden foreign body varies in relation to the content of air and fluid in the interstices of the wood. When dry wood enters the body, it is predominantly filled with air. Within approximately 1 week, the wood absorbs surrounding blood products and exudate and increases its attenuation [6]. In addition, different types of wood have been shown to have variable attenuations, and surface coating such as paint or sealant will affect the degree and timing of this increase in attenuation [7]. Dry wood, with a high air content, has been reported to mimic a gas collection [5]. The attenuation values for smaller objects may vary relating to partial volume averaging. When compared with MR imaging, CT has the advantage of being less expensive, more readily available, and faster to perform [7].
The identification of wooden foreign bodies may be exceedingly difficult on MR imaging, especially when foreign bodies are small and there is no associated abscess or fluid collection. In such cases, the foreign body may appear as a signal void with surrounding nonspecific granulation tissue. In this series, all foreign bodies appeared hypointense on all sequences. Two cases showed the lesions to be signal voids. Retained wood, in contrast to metal, does not reveal susceptibility artifact, and linear signal voids may be mistaken for tendons or dense collagenous structures (Fig. 2A,2B). It has been reported with MR imaging, just as with CT, that wood in soft tissue may absorb the surrounding hematoma and exudate, prolonging T1 and T2 relaxation times [1].
Some degree of surrounding inflammatory tissue is usually associated with a foreign body. In the acute setting, surrounding hemorrhage and hematoma may be seen, being replaced in time with granulomatous tissue. In this study, the inflammatory reaction associated with retained wood showed prolonged T1 and T2 relaxation times and prominent contrast enhancement. Identification of the inflammatory response can assist the viewer in identifying the retained foreign body because the actual splinter may be difficult to visualize. The surrounding foreign body reaction may be mistaken for a soft-tissue mass or a tumor if the central foreign body is not identified [8].
Sonography has been well studied in the evaluation of retained foreign bodies and has proved both sensitive and specific [9, 10]. Given the markedly different acoustic impedance of wood and soft tissues, retained wooden foreign bodies are easily identified, with the leading edge of the echogenic wood resulting in marked acoustic shadowing [11] (Figs. 4A,4B,4C,4D and 5A,5B). Sonography proved to be the best modality in the detection of retained wooden foreign bodies. However, the evaluation was often performed to confirm findings first seen on other modalities. In our experience, only 25% of patients presented with a history of penetrating injury. In patients presenting with nonspecific pain and swelling, MR imaging or CT is often performed first to evaluate for the presence of an underlying mass or inflammatory process. At our institution, if there is any reason to suspect a retained foreign body or if an equivocal case imaged with another modality presents, sonography is routinely performed. When compared with MR imaging or CT, sonography is less expensive, more readily available, and superior in the detection of small wooden foreign bodies. Sonography is the modality of choice in patients who present with a history of antecedent skin puncture or when a penetrating injury is suspected.
Arthrography may be helpful in cases in which the wooden foreign body has penetrated a joint cavity. In one patient in our series, the retained wooden foreign body penetrated the elbow joint. Distention of the elbow joint with contrast material showed marked irregularity of the synovium consistent with reactive synovitis (Fig. 1A,1B,1C), which was also seen on contrast-enhanced MR imaging. If intraarticular, the foreign body may be identified on arthrography as a filling defect in the contrast-filled joint.
When a soft-tissue mass is seen and there is a possibility of a retained foreign body, the shape of the lesion can be helpful. On cross-sectional imaging, retained wooden foreign bodies tend to be cylindric with a long length and minimal width. Imaging perpendicular to the foreign body often reveals a target appearance with the central foreign body appearing as a signal void or hypointense area in contrast to the surrounding hyperintense inflammatory tissue [3] (Figs. 2A,2B and 4A,4B,4C,4D). If a wooden splinter is large and the slice thickness is thin, the lesion can be seen in a profile that allows a confident diagnosis (Fig. 3A,3B,3C). Unfortunately, imaging parallel to a thin foreign body with thick slices or a large gap may miss the wood completely and render the foreign body inconspicuous.
In summary, the detection of retained wooden foreign bodies can be exceedingly difficult because patients typically present with nonspecific symptoms without a reported history of penetrating injury. Radiographs are frequently the first study obtained, but they are usually unremarkable. MR imaging and CT are often performed to evaluate nonspecific symptoms; however, accurate identification of retained wood with these modalities can be difficult. Wood usually shows a linear hypointense signal on MR imaging with an associated inflammatory mass. CT typically shows the retained wood as a linear area of increased attenuation, which is best seen on wide window settings. Sonography has proved the most useful modality, easily identifying the retained wood as a linear echogenic focus with marked acoustic shadowing. Unfortunately, sonography is often underused if the appropriate history is not presented. At our institution, if there is any reason to suspect a retained wooden foreign body, sonography is routinely performed. Radiologists need to be aware of the various imaging appearances of wooden foreign bodies and should be able to successfully localize the retained wood if the appropriate history is obtained.
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