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The role of ultrasound in the detection of non-radiopaque foreign bodies
clinical Radiology (1990) 41, 109-112
The Role of Ultrasound in the Detection of Non-Radiopaque Foreign Bodies F. J. GILBERT, R. S. D. C A M P B E L L and A. P. BAYLISS
Radiology Department, Aberdeen Royal Infirmary Fifty consecutive ultrasound examinations performed for suspected non-radiopaque foreign bodies in extremities were reviewed to establish the value of this procedure. Ultrasound detected 21 of 22 foreign bodies found at operation. There were three false positive examinations. The sensitivity was 95.4%, the specificity 89.2%, the positive predictive value 87.5% and the negative predictive value was 96.2%. This study demonstrates that ultrasound is a reliable method for detecting-~non-radiopaque foreign bodies and makes a significant contribution to patient management.
The detection and localisation of non-radiopaque foreign bodies in soft tissues is a recurring problem in the Accident and Emergency Department, frequently resulting in lengthy and often unfruitful exploratory procedures. While ultrasound is accepted in the evaluation of muscles, tendons and soft tissues, there are only a few reports of the detection of foreign bodies in soft tissues (Fornage et al., 1985; Fornage and Schernberg, 1986; Fornage and Schernberg, 1987; Gooding et al., 1987) although its application is well recognised in ocular trauma (Gordon, 1985). While this is a potentially valuable application of ultrasound, the reliability of the examination is not known. A retrospective analysis of 50 consecutive referrals for investigation of patients suspected of harbouring a non-radiopaque foreign body was made to establish the sensitivity, specificity and predictive value of ultrasound scanning.
METHOD Fifty patients, aged 16-78 years, were referred from the Accident and Emergency Department with the clinical suspicion of a non-radiopaque foreign body, between May 1987 and August 1988. All patients had soft tissue radiographs of the appropriate areas, and seven had negative surgical exploration prior to referral. Ultrasound was performed on all patients by one of three radiologists, using a real time, high resolution (10 MHz) sector scanner (Diasonics D R F 406) with an inbuilt oil 'stand off'. Lateral resolution was 0.6 mm and axial resolution 0.15 mm (company's literature). The radiologists did not see the plain radiographs prior to the ultrasound examination but had a verbal report from the senior casualty officer. The clinical information and outcome was obtained from the records held by the Accident and Emergency Department. The soft-tissue radiographs were also reviewed to ascertain whether all the foreign bodies had in fact been non-radiopaque. Correspondence to: Dr Fiona Gilbert, Radiology Department, AberdeenRoyal Infirmary, Foresterhill,Aberdeen AB9 2ZB.
RESULTS Forty-eight patients had no evidence and two doubtful evidence of a foreign body on soft tissue radiographs. On review of these radiographs one showed a linear soft tissue opacity representing the wooden splinter, the remaining radiographs were all normal. The site of the suspected foreign body varied: the finger (25), web space of fingers (4), volar aspect of hand (10), wrist (2), leg (3) and foot (6). The foreign body was thought to be wood (25), thorn (5), glass (8), fishbone (3), metal (4), cement (1) and unknown (4). Ultrasound (US) detected a foreign body in 24 patients. All underwent operation and the foreign body was found and removed in 21 cases. The sizes varied from 1 mm to 2.5 cm, the smallest being a rose thorn and the largest a wooden splinter. All were located at less than 2 cm depth from the skin surface. No foreign body could be found at surgery in three patients, who were all thought to have a foreign body of vegetable origin; their symptoms settled and did not recur over a 6 month follow up period. Thus there were 21 true positive and three false positive ultrasound examinations. N o foreign body was found by US in 26 patients, of whom 11 underwent an exploratory operation: a foreign body was found in one patient, across the web space between the fingers where it had proved difficult to scan. The remaining 15 patients were discharged without exploration. All were advised to return if their symptoms recurred, but there were no representations over a 6 month period. Thus there were 25 true negative and one false negative ultrasound examinations. The sensitivity of the test was 95.4%, the specificity 89.2%, the positive predictive value 87.5% and the negative predictive value 96.2%. The average length of time for the examination was 10 minutes, but some examinations took up to half an hour when a small foreign body was suspected or if it was situated in a difficult location.
Ultrasound findings In all cases the foreign body was demonstrated as a bright hyperechoic focus (Fig. la, b) and was best imaged with the scan plane parallel to the long axis of the foreign body, or at right angles to its long axis. An acoustic shadow was seen in 52% of all cases, and the shadow was best demonstrated when the object was directly within the scan plane and lying in the focal zone of the transducer. Wood produced an acoustic shadow in 11 out of 17 cases (64%). No acoustic shadow was seen with rose thorns or glass fragments. No foreign body produced acoustic enhancement. A hypoechoic area was found surrounding six of 17 wooden foreign bodies and in both the thorn foreign bodies, i.e. 65% of vegetable foreign bodies. The hypoechoic area averaged 0.5-1 cm in size and was not related
110
CLINICAL RADIOLOGY
(a) Fig. 3 - 2.0 mm bright echogenic focus (arrow) in the sole of the foot with reverberation artefact due to a (3.0 ram) fragment of glass. Table 1 - Analysis o f positive ultrasound findings confirmed at operation
(b) Fig. 1 - (a) 1.9 cm linear bright echogenicarea with acoustic shadow. (b) 1.9 cm wooden splinter found at exploration.
Fig. 2- 0.8 cm linear bright echogenic area with surrounding hypoechoic area representing inflammatory tissue. No acoustic shadow. to the length of time the foreign body had been present in the tissue (24 h - 6 months) (Fig. 2). A hyperechoic comet tail artefact, was found with both the glass foreign bodies (Fig. 3). No hypoechoic area or acoustic shadow was found with the small glass fragments. The nature of the foreign bodies and their US appearances are summarised in Table 1.
DISCUSSION This study has shown that high-resolution ultrasound is a
Nature o f FB
Number o f cases
Acoustic shadow
Hypoechoic area
Reverberation artefact
Wood Thorn Glass
17 2 2
11 0 0
6 2 0
0 0 2
reliable method for detecting and localising foreign bodies in the superficial tissues of the extremities. However as with all US examinations the figures are operator dependent and considerable time and patience is sometimes required. As described in the results a foreign body is best imaged with the scan plane parallel to the long axis of the foreign body. The transducer head used in our study measured 6 x 3 cm, making it difficult to access web spaces, especially when the foreign body lay at right angles to the phalanges. This accounted for the one false negative examination. The use of a smaller transducer head would certainly reduce the risk of missing a foreign body, and some manufacturers do produce 7.5 M H z transducers which can match the practical performance of the Diasonics 10 M H z unit. All foreign bodies were visualised as bright hyperechoic foci, concurring with previous reports (Fornage and Schernberg, 1986; Gooding e t al.,1987). The acoustic shadow has also been reported in experimental models (Gooding et al., 1987). In our study the quality of the acoustic shadow varied, being most obvious when the long axis of the foreign body is parallel to, or at right angles to the scan plane and lies within the focal zone of the transducer. The hypoechoic area surrounding the foreign body has been documented (Fornage and Schernberg, 1987) and probably represents inflammatory tissue or pus. Foreign bodies producing this appearance were all vegetable in origin and had been present for a varying length of time (24 h-6 months). The presence of a hypoechoic area should ensure a diligent search for a foreign body. N o r m a l tendons are echogenic when the scan plane is perpendicular to the axis of the tendon, but the tendon m a y appear falsely hypoechoic when it lies obliquely to the ultrasound beam (Fornage and Rifkin, 1988). This artefact should not be mistaken for an area of inflammation.
111
US FOR NON-RADIOPAQUE FOREIGN BODIES
Fig. 4 - Hyperechoic artefact presumably due to fresh bleeding or the introduction of air following recent exploration.
The hyperechoic comet tail artefact seen in the patients with glass foreign bodies, is thought to be due to reverberations inside the dense echogenic material, and has been reported both with metal objects (Fornage et al., 1986) and with glass objects (Gooding et al., 1987). The three false positive cases were reviewed to try to find the reasons for misdiagnosis. Case l: A patient with a collection of pus, had incision and drainage prior to US, which showed a 1.5 cm diffuse hyperechoic area (Fig. 4); this may have been due to fresh bleeding or gas introduced during the drainage procedure. No foreign body was found at a second surgical exploration. Case 2: A patient with a suspected splinter in the thenar eminence had a negative surgical exploration two months before referral for US, which showed a 1.8 cm bright echogenic focus seen in two planes (Fig. 5 a,b). A second exploration still revealed no foreign body. The echogenic focus may have represented scar tissue from the first exploration, although the appearance was identical to that seen with other foreign bodies. Case 3: The suspected foreign body was thought to lie in close relation to the third MCP joint of the hand. The object imaged, with no acoustic shadow or hypoechoic area, may well have been the head of the metacarpal. In two cases, prior surgical exploration lead to difficulty in interpreting the US appearances arrd we would recommend that ultrasound be performed prior to operative procedures. Alternatively rescanning after 48 h, would allow reabsorption of air introduced at the time of operation. In a review of 200 patients with retained foreign bodies in the hand, wood was the commonest material found accounting for 36% of cases (Anderson et al., 1982). Wood is normally only radiopaque in 15 % of cases and in our study only one long wooden splinter was visible on the soft tissue radiograph. While the use of xeroradiography (which is not available in many centres) increases the chance of detection of wooden splinters, small objects such as thorns may be missed (3Voesner and Saunders, 1972). The importance of ultrasound as a diagnostic tool is highlighted by the fact that patients were referred only When there was a strong clinical suspicion of a nonradiopaque foreign body, the majority of which were of
(a)
(b) Fig. 5 - (a), (b) False positive examination, Case 2. Linear echogenic area mimicking a foreign body, but possibly due to scar tissue.
vegetable origin. Since they frequently cause severe inflammatory reactions, their recognition and removal is important. Metal and glass are radiopaque (Tandberg, 1982; de Lacey et al., 1985) and the majority of metal and glass foreign bodies can be detected by adequate soft tissue radiography. However, ultrasound still has a useful role as it can be used to describe the exact location of the foreign body. This reduces operative time and postoperative morbidity (Fornage and Schernberg, 1987). In conclusion, high resolution ultrasound is a valuable aid in the detection and localisation of non-radiopaque foreign bodies with a high sensitivity and specificity. It should be performed prior to surgical exploration. The typical sonographic appearance of a wooden foreign body is a bright hyperechoic focus often with an acoustic shadow. A hypoechoic area surrounding the foreign body suggests the presence of inflammatory tissue.
Acknowledgements.We wish to thank Mr A. B. Matheson and Mr G. Page, Consultants in the Accident and Emergency Department, and Mr R. Crawford, Senior Registrar in Accident and Emergency, Aberdeen Royal Infirmary for their encouragement and allowing access to their patients' records. We also thank Mrs Joanne Seawright and Miss Audrey Walker and the Department of Medical Illustration for their help in the preparation of the manuscript.
112
CLINICAL RADIOLOGY
REFERENCES
Anderson, MA, Newmeyer III, WL & Kilgore, ES (1982). Diagnosis and treatment of retained foreign bodies in the hand. American Journal of Surgery, 144, 63-67. De Lacey, G, Evans, R & Sandin, B (1985). Penetrating injuries: how easy is it to see glass (and plastic) on radiographs?British Journal of Radiology, 58, 27-30. Fornage, BD & Rifkin, MD (1988). Ultrasound examination of tendons. Radiologic Clinics of North America, 26:1, 87-107. Fornage, BD & Schernberg, FL (1986). Sonographic diagnosis of foreign bodies of the distal extremities. American Journal of Roentgenology, 147, 567-569. Fornage, BD & Schernberg, FL (1987). Sonographic pre-operative
localisation of a foreign body in the hand. Journal of Ultrasound in Medicine, 6, 217-219. Fornage, BD, Sehernberg, FL & Rifkin, MD (1985) Ultrasound examination of the hand. Radiology 155, 785-788. Gooding, GAW, Hardiman, T, Sumers, M, Stress, R, Graf, P & Grunfield, C (1987). Sonography of the hand and foot in foreign body detection. Journal of Ultrasound in Medicine, 6, 441-447. Gordon, D (1985). Letter-non-metallic foreign bodies. BritishJournalof Radiology, 58, 574. Tandberg, D (1982). Glass inthe hand and foot. Will an X-ray film show it? Journal of American Medical Association, 248, 1872-1874. Woesner, ME & Sanders, I (1972). Xeroradiography: A significant modality in the detection of non-metallic foreign bodies in soft tissues. American Journal of Roentgenology, 115, 636-640.
The Role of Ultrasound in the Detection of Non-Radiopaque Foreign Bodies F. J. GILBERT, R. S. D. C A M P B E L L and A. P. BAYLISS
Radiology Department, Aberdeen Royal Infirmary Fifty consecutive ultrasound examinations performed for suspected non-radiopaque foreign bodies in extremities were reviewed to establish the value of this procedure. Ultrasound detected 21 of 22 foreign bodies found at operation. There were three false positive examinations. The sensitivity was 95.4%, the specificity 89.2%, the positive predictive value 87.5% and the negative predictive value was 96.2%. This study demonstrates that ultrasound is a reliable method for detecting-~non-radiopaque foreign bodies and makes a significant contribution to patient management.
The detection and localisation of non-radiopaque foreign bodies in soft tissues is a recurring problem in the Accident and Emergency Department, frequently resulting in lengthy and often unfruitful exploratory procedures. While ultrasound is accepted in the evaluation of muscles, tendons and soft tissues, there are only a few reports of the detection of foreign bodies in soft tissues (Fornage et al., 1985; Fornage and Schernberg, 1986; Fornage and Schernberg, 1987; Gooding et al., 1987) although its application is well recognised in ocular trauma (Gordon, 1985). While this is a potentially valuable application of ultrasound, the reliability of the examination is not known. A retrospective analysis of 50 consecutive referrals for investigation of patients suspected of harbouring a non-radiopaque foreign body was made to establish the sensitivity, specificity and predictive value of ultrasound scanning.
METHOD Fifty patients, aged 16-78 years, were referred from the Accident and Emergency Department with the clinical suspicion of a non-radiopaque foreign body, between May 1987 and August 1988. All patients had soft tissue radiographs of the appropriate areas, and seven had negative surgical exploration prior to referral. Ultrasound was performed on all patients by one of three radiologists, using a real time, high resolution (10 MHz) sector scanner (Diasonics D R F 406) with an inbuilt oil 'stand off'. Lateral resolution was 0.6 mm and axial resolution 0.15 mm (company's literature). The radiologists did not see the plain radiographs prior to the ultrasound examination but had a verbal report from the senior casualty officer. The clinical information and outcome was obtained from the records held by the Accident and Emergency Department. The soft-tissue radiographs were also reviewed to ascertain whether all the foreign bodies had in fact been non-radiopaque. Correspondence to: Dr Fiona Gilbert, Radiology Department, AberdeenRoyal Infirmary, Foresterhill,Aberdeen AB9 2ZB.
RESULTS Forty-eight patients had no evidence and two doubtful evidence of a foreign body on soft tissue radiographs. On review of these radiographs one showed a linear soft tissue opacity representing the wooden splinter, the remaining radiographs were all normal. The site of the suspected foreign body varied: the finger (25), web space of fingers (4), volar aspect of hand (10), wrist (2), leg (3) and foot (6). The foreign body was thought to be wood (25), thorn (5), glass (8), fishbone (3), metal (4), cement (1) and unknown (4). Ultrasound (US) detected a foreign body in 24 patients. All underwent operation and the foreign body was found and removed in 21 cases. The sizes varied from 1 mm to 2.5 cm, the smallest being a rose thorn and the largest a wooden splinter. All were located at less than 2 cm depth from the skin surface. No foreign body could be found at surgery in three patients, who were all thought to have a foreign body of vegetable origin; their symptoms settled and did not recur over a 6 month follow up period. Thus there were 21 true positive and three false positive ultrasound examinations. N o foreign body was found by US in 26 patients, of whom 11 underwent an exploratory operation: a foreign body was found in one patient, across the web space between the fingers where it had proved difficult to scan. The remaining 15 patients were discharged without exploration. All were advised to return if their symptoms recurred, but there were no representations over a 6 month period. Thus there were 25 true negative and one false negative ultrasound examinations. The sensitivity of the test was 95.4%, the specificity 89.2%, the positive predictive value 87.5% and the negative predictive value 96.2%. The average length of time for the examination was 10 minutes, but some examinations took up to half an hour when a small foreign body was suspected or if it was situated in a difficult location.
Ultrasound findings In all cases the foreign body was demonstrated as a bright hyperechoic focus (Fig. la, b) and was best imaged with the scan plane parallel to the long axis of the foreign body, or at right angles to its long axis. An acoustic shadow was seen in 52% of all cases, and the shadow was best demonstrated when the object was directly within the scan plane and lying in the focal zone of the transducer. Wood produced an acoustic shadow in 11 out of 17 cases (64%). No acoustic shadow was seen with rose thorns or glass fragments. No foreign body produced acoustic enhancement. A hypoechoic area was found surrounding six of 17 wooden foreign bodies and in both the thorn foreign bodies, i.e. 65% of vegetable foreign bodies. The hypoechoic area averaged 0.5-1 cm in size and was not related
110
CLINICAL RADIOLOGY
(a) Fig. 3 - 2.0 mm bright echogenic focus (arrow) in the sole of the foot with reverberation artefact due to a (3.0 ram) fragment of glass. Table 1 - Analysis o f positive ultrasound findings confirmed at operation
(b) Fig. 1 - (a) 1.9 cm linear bright echogenicarea with acoustic shadow. (b) 1.9 cm wooden splinter found at exploration.
Fig. 2- 0.8 cm linear bright echogenic area with surrounding hypoechoic area representing inflammatory tissue. No acoustic shadow. to the length of time the foreign body had been present in the tissue (24 h - 6 months) (Fig. 2). A hyperechoic comet tail artefact, was found with both the glass foreign bodies (Fig. 3). No hypoechoic area or acoustic shadow was found with the small glass fragments. The nature of the foreign bodies and their US appearances are summarised in Table 1.
DISCUSSION This study has shown that high-resolution ultrasound is a
Nature o f FB
Number o f cases
Acoustic shadow
Hypoechoic area
Reverberation artefact
Wood Thorn Glass
17 2 2
11 0 0
6 2 0
0 0 2
reliable method for detecting and localising foreign bodies in the superficial tissues of the extremities. However as with all US examinations the figures are operator dependent and considerable time and patience is sometimes required. As described in the results a foreign body is best imaged with the scan plane parallel to the long axis of the foreign body. The transducer head used in our study measured 6 x 3 cm, making it difficult to access web spaces, especially when the foreign body lay at right angles to the phalanges. This accounted for the one false negative examination. The use of a smaller transducer head would certainly reduce the risk of missing a foreign body, and some manufacturers do produce 7.5 M H z transducers which can match the practical performance of the Diasonics 10 M H z unit. All foreign bodies were visualised as bright hyperechoic foci, concurring with previous reports (Fornage and Schernberg, 1986; Gooding e t al.,1987). The acoustic shadow has also been reported in experimental models (Gooding et al., 1987). In our study the quality of the acoustic shadow varied, being most obvious when the long axis of the foreign body is parallel to, or at right angles to the scan plane and lies within the focal zone of the transducer. The hypoechoic area surrounding the foreign body has been documented (Fornage and Schernberg, 1987) and probably represents inflammatory tissue or pus. Foreign bodies producing this appearance were all vegetable in origin and had been present for a varying length of time (24 h-6 months). The presence of a hypoechoic area should ensure a diligent search for a foreign body. N o r m a l tendons are echogenic when the scan plane is perpendicular to the axis of the tendon, but the tendon m a y appear falsely hypoechoic when it lies obliquely to the ultrasound beam (Fornage and Rifkin, 1988). This artefact should not be mistaken for an area of inflammation.
111
US FOR NON-RADIOPAQUE FOREIGN BODIES
Fig. 4 - Hyperechoic artefact presumably due to fresh bleeding or the introduction of air following recent exploration.
The hyperechoic comet tail artefact seen in the patients with glass foreign bodies, is thought to be due to reverberations inside the dense echogenic material, and has been reported both with metal objects (Fornage et al., 1986) and with glass objects (Gooding et al., 1987). The three false positive cases were reviewed to try to find the reasons for misdiagnosis. Case l: A patient with a collection of pus, had incision and drainage prior to US, which showed a 1.5 cm diffuse hyperechoic area (Fig. 4); this may have been due to fresh bleeding or gas introduced during the drainage procedure. No foreign body was found at a second surgical exploration. Case 2: A patient with a suspected splinter in the thenar eminence had a negative surgical exploration two months before referral for US, which showed a 1.8 cm bright echogenic focus seen in two planes (Fig. 5 a,b). A second exploration still revealed no foreign body. The echogenic focus may have represented scar tissue from the first exploration, although the appearance was identical to that seen with other foreign bodies. Case 3: The suspected foreign body was thought to lie in close relation to the third MCP joint of the hand. The object imaged, with no acoustic shadow or hypoechoic area, may well have been the head of the metacarpal. In two cases, prior surgical exploration lead to difficulty in interpreting the US appearances arrd we would recommend that ultrasound be performed prior to operative procedures. Alternatively rescanning after 48 h, would allow reabsorption of air introduced at the time of operation. In a review of 200 patients with retained foreign bodies in the hand, wood was the commonest material found accounting for 36% of cases (Anderson et al., 1982). Wood is normally only radiopaque in 15 % of cases and in our study only one long wooden splinter was visible on the soft tissue radiograph. While the use of xeroradiography (which is not available in many centres) increases the chance of detection of wooden splinters, small objects such as thorns may be missed (3Voesner and Saunders, 1972). The importance of ultrasound as a diagnostic tool is highlighted by the fact that patients were referred only When there was a strong clinical suspicion of a nonradiopaque foreign body, the majority of which were of
(a)
(b) Fig. 5 - (a), (b) False positive examination, Case 2. Linear echogenic area mimicking a foreign body, but possibly due to scar tissue.
vegetable origin. Since they frequently cause severe inflammatory reactions, their recognition and removal is important. Metal and glass are radiopaque (Tandberg, 1982; de Lacey et al., 1985) and the majority of metal and glass foreign bodies can be detected by adequate soft tissue radiography. However, ultrasound still has a useful role as it can be used to describe the exact location of the foreign body. This reduces operative time and postoperative morbidity (Fornage and Schernberg, 1987). In conclusion, high resolution ultrasound is a valuable aid in the detection and localisation of non-radiopaque foreign bodies with a high sensitivity and specificity. It should be performed prior to surgical exploration. The typical sonographic appearance of a wooden foreign body is a bright hyperechoic focus often with an acoustic shadow. A hypoechoic area surrounding the foreign body suggests the presence of inflammatory tissue.
Acknowledgements.We wish to thank Mr A. B. Matheson and Mr G. Page, Consultants in the Accident and Emergency Department, and Mr R. Crawford, Senior Registrar in Accident and Emergency, Aberdeen Royal Infirmary for their encouragement and allowing access to their patients' records. We also thank Mrs Joanne Seawright and Miss Audrey Walker and the Department of Medical Illustration for their help in the preparation of the manuscript.
112
CLINICAL RADIOLOGY
REFERENCES
Anderson, MA, Newmeyer III, WL & Kilgore, ES (1982). Diagnosis and treatment of retained foreign bodies in the hand. American Journal of Surgery, 144, 63-67. De Lacey, G, Evans, R & Sandin, B (1985). Penetrating injuries: how easy is it to see glass (and plastic) on radiographs?British Journal of Radiology, 58, 27-30. Fornage, BD & Rifkin, MD (1988). Ultrasound examination of tendons. Radiologic Clinics of North America, 26:1, 87-107. Fornage, BD & Schernberg, FL (1986). Sonographic diagnosis of foreign bodies of the distal extremities. American Journal of Roentgenology, 147, 567-569. Fornage, BD & Schernberg, FL (1987). Sonographic pre-operative
localisation of a foreign body in the hand. Journal of Ultrasound in Medicine, 6, 217-219. Fornage, BD, Sehernberg, FL & Rifkin, MD (1985) Ultrasound examination of the hand. Radiology 155, 785-788. Gooding, GAW, Hardiman, T, Sumers, M, Stress, R, Graf, P & Grunfield, C (1987). Sonography of the hand and foot in foreign body detection. Journal of Ultrasound in Medicine, 6, 441-447. Gordon, D (1985). Letter-non-metallic foreign bodies. BritishJournalof Radiology, 58, 574. Tandberg, D (1982). Glass inthe hand and foot. Will an X-ray film show it? Journal of American Medical Association, 248, 1872-1874. Woesner, ME & Sanders, I (1972). Xeroradiography: A significant modality in the detection of non-metallic foreign bodies in soft tissues. American Journal of Roentgenology, 115, 636-640.