Bone Scintigraphy and Computed Tomography: Advanced Diagnostic Imaging Techniques in Endangered Sea Turtles
Advanced diagnostic imaging techniques have been underutilized in reptile medicine,5 particularly scintigraphy and computed tomography (CT). Based on the lack of published data regarding the clinical applications of these techniques in reptiles, our laboratory decided to investigate the value of bone scintigraphy and CT for the characterization of bone lesions in endangered sea turtles. Specifically, this study focused on bone lesions in three Kemp’s Ridley sea turtles (Lepidochelys kempi), and compared the impact of plain radiographs, scintigraphic images, and CT scans on the determination of diagnosis and treatment plan for each animal.
Bone scintigraphy is commonly used in equine medicine to detect functional bone abnormalities.6 Scintigraphy can reveal the activity and extent of a bone lesion, which enables the clinician to make a more accurate diagnosis and formulate a specialized treatment plan.7 This imaging technique begins with an intravenous injection of a gamma ray emitting isotope, which can be visualized with a gamma camera as it is metabolically distributed throughout the body.2 In this study, we injected a diphosphonate compound linked to radioactive 99MTechnetium (Tc-99M HDP; Mallinckrodt Corp., Hazelwood, MO, USA) into the left dorsal cervical sinus of each animal. Once administered, the Tc-99M HDP incorporated into bone at a rate proportional to that of bone turnover and regional blood flow.1,2 To optimize the detection of the radiopharmaceutical uptake, each animal was scanned approximately 2 hours post-injection with a gamma camera (Pho-Gamma, 75PMT; Siemens, New York, NY, USA). Areas of excessive bone turnover appeared as regions of increased radiopharmaceutical uptake. Increased uptake could indicate either a fracture, infection, periosteal reaction, arthritic joint, or growth plate. Areas of deficient bone turnover appeared as regions of decreased radiopharmaceutical uptake. Decreased uptake could demonstrate either a bone cyst, sequestrum, or inactive bone in a non-union fracture.
In both human and veterinary medicine, axial and spiral CT scans provide superior anatomic detail when compared to plain radiographs, particularly when analyzing bone. This is partially due to the fact that sectional images of the subject are taken, which eliminate the superimposition of structures.3 In addition, spiral CT scans allow for rapid scan times, which decrease motion artifact and provide higher resolution of images. For this study, two of the animals were scanned using either an axial CT scanner or a spiral CT scanner to delineate bone lesions. The axial CT scan was performed on a Shimadzu 3000-TEE scanner (Shimadzu, Braintree, MA, USA) and lasted approximately 12 minutes. Sectional images were 2 mm thick, and the scan was acquired using settings of 120 kvp, 250 mA, and 2.5 ms. The spiral CT scan was performed on a Picker Venue PQ 5000 scanner (Marconi Corp., Cleveland, OH, USA) and lasted approximately 2 minutes. Depending on the resolution desired, sections were either 2, 3, or 5 mm thick, and scans were acquired with settings of 120 kvp and 250–275 mA. When appropriate, three-dimensional reconstruction was performed on the areas of concern to better visualize the bone abnormalities.
Case One: MH98-771Lk
A juvenile Kemp’s Ridley sea turtle (3.6 kg, straight carapace length of 28.9 cm) washed up in Eastham, MA, USA on 7 November 1998. The animal was collected by the Massachusetts Audubon Society and transported to the New England Aquarium for assessment. Physical examination revealed moderate hypothermia and dehydration, and radiographs demonstrated pneumonia. These conditions resolved with appropriate treatment, which included parental fluids, antibiotics, and antifungal therapy. On 14 December 1998, the animal developed external lesions on the right fore flipper, concentrated primarily over the carpal joint. On 5 January 1999, the primary lesion had developed into an abscess, which was lanced and debrided under local anesthesia. On 10 February 1999, the animal became reluctant to use the right flipper, and radiographs revealed geographic osteolysis of the right distal ulna. The abscess was surgically debrided for a second time and cultures were taken of the distal ulna, which grew Enterococcus fecalis and a coagulase positive Staphylococcus. Appropriate antibiotic therapy was administered to treat the osteomyelitis, which consisted of ampicillin/sulbactam at 10 mg/kg, IV, SID (Unasyn®; Pfizer, Parsippany, NJ, USA) and amikacin at 2.5 mg/kg, IV, ETD (Amiglyde-V®; Fort Dodge, IA, USA). In addition, a physical therapy regime was implemented to prevent ankylosis of the joint. By 6 March 1999, the animal was showing clinical signs of improvement, but radiographs on 18 March 1999 did not demonstrate significant improvement of the ulnar lesion.
Axial CT scanning and bone scintigraphy were performed on 1 April 1999 to determine the status of the right distal ulnar lesion. The animal was anesthetized for the CT scan with medetomidine at 15 µg/kg, IV (Dormitor®; Pfizer, Exton, PA, USA) and ketamine at 2.5 mg/kg, IV (Ketaset®; Fort Dodge, IA, USA). This level of sedation produced apnea, but was not ideal for intubation, so higher doses of medetomidine (30 µg/kg, IV) and ketamine (3 mg/kg, IV) were used in later studies. The CT findings were consistent with the previous radiographic findings, but they did not offer any additional structural information. The bone scintigraphy, however, revealed a slight increase in bone turnover at the distal end of the ulna that was indicative of active bone healing. The decision was made to discontinue antibiotics, monitor the animal closely, and perform follow-up scintigraphy on 19 May 1999. The follow-up scintigraphy study, which was conducted without anesthesia, exhibited no increase in bone turnover in the right ulna when compared to the left. Therefore, the bone lesion was considered to be inactive and, therefore, noninfected. Since the animal was deemed clinically healthy, it was transported to Florida on 5 June 1999 to be released back into the wild.
Case Two: MH99-842Lk
A juvenile Kemp’s Ridley sea turtle (2.1 kg, straight carapace length of 24.9 cm) was found cold-stunned in Brewster, MA, USA on 18 November 1999. The animal presented severely hypothermic and dehydrated, with a penetrating wound on the right fore flipper over the humeroradial joint. Radiographs were taken on 21 November 1999, which revealed osteolysis at the distal end of the right humerus, as well as pneumonia. Broad-spectrum antibiotic and antifungal therapy was started, and the animal was placed on ceftazidime at 22 mg/kg, IM, ETD (Fortaz®; Glaxo Wellcome, Research Triangle Park, NC, USA), clindamycin at 5 mg/kg, IM, SID (Cleocin®; Upjohn, Kalamazoo, MI, USA), and fluconazole at 0.75 mg/kg, SC, EOD (Diflucan®; Pfizer, New York, NY, USA). By 26 December 1999, the penetrating skin wound was epithelializing and granulation tissue had formed, but the humeroradial joint remained swollen. On 9 January 2000, plain radiographs were taken, which demonstrated evidence of bone remodeling at the distal end of the right humerus.
To determine the activity of the humeral lesion, scintigraphy was scheduled for 1 February 2000. The animal was anesthetized for the study with medetomidine at 30 µg/kg, IV and ketamine at 3 mg/kg, IV, and then intubated and ventilated with room air for the short procedure. The scintigraphic image showed a mildly intense radiopharmaceutical uptake associated with the distal end of the right humerus, but the intensity was similar to the distal end of the left humerus. The humeral lesion was considered to be a slow healing process, and there was no evidence of an active bone or joint infection. Based on both the radiographic findings and scintigraphy results, the decision was made to discontinue antibiotic therapy and monitor the animal. Prior to release, a follow-up scintigraphy study will be performed to ensure that the lesion has remained inactive.
Case Three: MH99-831Lk
A juvenile Kemp’s Ridley sea turtle (2.9 kg, straight carapace length of 27.8 cm) washed up in Barnstable, MA, USA on 18 November 1999. The animal was severely hypothermic, dehydrated, and extremely dull. The animal had been struck by a boat propeller on both the head and carapace. The head wound was just dorsal to the right orbit, and the carapace wound was on the right craniolateral aspect of the shell and communicated with the coelomic cavity. The wounds were extensively debrided, and fragments of bone were removed from both sites. Broad-spectrum antibiotic and antifungal therapy was started, and the animal was placed on ceftazidime at 22 mg/kg, IM, ETD, clindamycin at 5 mg/kg, IM, SID, and fluconazole at 0.75 mg/kg, SC, EOD. The wounds were cleaned, flushed, and packed daily with triple antibiotic ointment, and then covered with Ilex® protective paste (Medcon, Grafton, MA, USA). Radiographs were taken on 21 November 1999, which showed simple skull and carapace fractures, as well as pneumonia. In addition, there were osteolytic lesions on both the left and right fore flipper tips, with the right flipper having more numerous and severe digital lesions than the left. On 6 December 1999, the wounds were surgically debrided and bacterial cultures were taken of all areas, which were negative. On 16 February 2000, radiographs were repeated, and although the head wound was healing superficially, there was no evidence of new bone formation on the radiographs. In addition, the flipper tip lesions had worsened, particularly the right fore flipper, fourth digit, second and third phalanges. Contrast media (2.0 ml of Hypaque®; Nycomed Inc., Princeton, NJ, USA) was injected into the carapace wound, which confirmed that the wound still communicated with the coelomic cavity. Follow-up radiographs on 2 March 2000 and 17 March 2000 were taken, and showed no improvement of the flipper tip lesions and no new bone formation in the propeller wounds.
Spiral CT scanning and bone scintigraphy were performed on 21 March 2000 to determine the severity and activity of the lesions. Due to the short amount of time required for the spiral CT scan (approximately 2 minutes), the animal was not anesthetized for the procedure. The CT delineated a bone fragment in association with the right orbital skull fracture. This fragment was not seen on any of the previous plain radiographs. The scintigraphy results were also significant, and illustrated that there was a mild increase in bone turnover associated with the orbital fragment, indicating that the bone was viable. In addition, the CT scan revealed a second bone fragment within the carapace wound that was also not evident on plain radiographs. The scintigraphic images established that this second bone fragment had a decreased amount of radiopharmaceutical uptake, so the fragment was most likely a sequestrum. In addition, the scintigraphy also showed an increase in uptake at the right fore flipper, fourth digit, that was consistent with the most severe flipper tip lesion. This finding was suggestive of osteomyelitis, but the radiopharmaceutical uptake was not intense enough to indicate a severe condition. After receiving the results of both the CT scan and bone scintigraphy, a surgical plan was made to remove the sequestrum and to continue the animal on antibiotic and antifungal therapy. A follow-up CT scan and scintigraphy study have been scheduled in order to monitor the progress of these lesions.
These three cases have demonstrated that bone scintigraphy was invaluable in the clinical evaluation of osteopathies, particularly osteolysis, in sea turtles. Historically, lytic bone lesions in reptiles have been difficult to interpret on plain radiographs, and these lesions can be present following the resolution of osteomyelitis.5 Scintigraphy offered functional versus structural information, therefore, it was critical for determining if a lesion was active or inactive, and infected or noninfected. This imaging technique greatly impacted the diagnosis and treatment plan for each animal, therefore, we recommend bone scintigraphy for evaluation of osteopathies in sea turtles.
CT was also a useful diagnostic tool, particularly in defining fractures and identifying obscure bone fragments. Both axial and spiral CT scans provided exceptional bone resolution, and the three-dimensional reconstruction aided in the identification of bone fragments that could not be elucidated on plain radiographs. In addition, the spiral CT scanner was able to perform faster scans than the axial CT scanner, resulting in reduced motion artifact, enhanced image quality, and improved lesion conspicuity. This study demonstrated that CT scans offered more meaningful data than plain radiographs when evaluating fractures. Conversely, CT scanning did not provide supplementary information for the clinical assessment of osteolytic changes.
In 1994, Robert Twardock reviewed the use of nuclear medicine in equine practice and stated that scintigraphic images of bone demonstrated dynamic pathophysiologic processes but provided poor anatomic detail. Radiography exhibited excellent anatomic detail, but usually did not offer insight into the dynamic state of osteopathic processes. Therefore, he concluded that the two imaging techniques were extremely complementary and should be used together to efficiently achieve a diagnosis.6 The cases presented in this paper have demonstrated that this same principle applies to sea turtle medicine. In addition, we have added insight into the usefulness of CT scanning as a diagnostic tool in sea turtle osteopathies. We conclude that both scintigraphy studies and CT scans are extremely valuable for fracture management, but only bone scintigraphy is recommended for evaluation of osteolytic lesions in sea turtles.
We would like to thank Dr. Andrew Stamper, Connie Merigo, Belinda Rubinstein, Jim Rice, Kristen Patchett, Melissa Hodge, Deana Edmunds, Casey Sugarman, Robert Cooper, Susan Goodridge, Katarina Peterson, Dr. Sonia Mumford, John Dayton, and the volunteers of the New England Aquarium for their support of this project. We would also like to thank the Tufts University School of Veterinary Medicine’s Nuclear Medicine Department for their generous donation of the scintigraphy and computed tomography services.
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