British Journal of Radiology (2005) 78, 791-795
© 2005 British Institute of Radiology
doi: 10.1259/bjr/17137072
16 detector multislice CT versus skeletal scintigraphy in the diagnosis of wrist fractures: value of quantification of 99Tcm-MDP uptake
A M Groves, FRCR1,
H K Cheow, FRCR2,
K K Balan, FRCPI2,
P W P Bearcroft, FRCR1 and
A K Dixon, FRCR1
Departments of 1 Radiology and 2 Nuclear Medicine, Addenbrooke's Hospital NHS Trust and the University of Cambridge, Hills Road, Cambridge CB2 2QQ, UK
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Abstract
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To compare the measured uptake of 99Tcm-methylene diphosphonate (99Tcm-MDP) in those scaphoid fractures seen on both 16 detector multislice CT and scintigraphy, with those seen only on scintigraphy. Over a 12 month period a total of 51 patients with suspected fracture underwent both conventional 99Tcm-MDP scintigraphy and 16 detector multislice CT on the same day. The 99Tcm-MDP uptake was then quantified in patients with identified fracture. This was measured by placing a region of interest (ROI) over the fracture site and the mean and maximum number of counts were compared with those in a similar size ROI placed over background bone activity. A total of 23 fractures were identified on scintigraphy of which 16 were also detected on CT (concordant). In seven cases the fracture was not seen on CT, even in retrospect (discordant). In the discordant cases, follow-up radiographs and MRI (where available) also failed to demonstrate a fracture. The mean fracture count to background bone activity ratio averaged 7.7 (range 3.2–18.5) for concordant fractures and 3.8 (range 1.7–5.3) for discordant fractures (t-test p=0.04). The maximum fracture count to background bone activity ratio averaged 12.7 (range 4.3–27.7) for concordant fractures and 6.3 (range 2.6–9.5) for discordant fractures (t-test p=0.03). It is speculated whether these discordant fractures with less 99Tcm-MDP uptake may represent a less severe injury such as bone bruise.
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Introduction
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Scintigraphy has been used to diagnose fractures since the 1960s [1]. It has become increasingly recognised that conventional 99Tcm-methylene diphosphonate (99Tcm-MDP) bone scintigraphy is more sensitive than plain radiographs in fracture detection and hence it is used as a routine investigation when there is a clinical suspicion of a wrist fracture, but the radiograph is equivocal or normal [2, 3]. With the development of newer imaging techniques, ultrasound [4], CT [5] and MRI [6] have all been employed to aid fracture identification. Of the newer techniques MRI has received the most attention and there are reports of scintigraphic false negative MRI positive fractures [7].
The use of MRI in the investigation of traumatic bone injuries has led to the development of new terminologies. Accordingly, in trauma patients undergoing MRI examination the term fracture is generally used when there is increased T2 weighted and decreased T1 weighted marrow signal changes in the presence of a cortical breach or fracture line. If there is marrow signal change in the absence of cortical disruption or fracture line detection the phrase "bone bruise" has been used [8]. The description of bone bruise has been used since the 1980s [9] and in the wrist from the early 1990s [10]. However, the term is yet to be validated by other imaging techniques or histology and, with one exception [11], there has been little scintigraphic support of the concept. Indeed, in the context of trauma, it is suggested that all focal "hot spots" are likely to represent fracture [12]. However, it is also recognized that a proportion of "hot spots" are not identifiable through other techniques such as CT [13].
In order to investigate the spectrum of bone trauma in the wrist further, we identified fractures that were diagnosed on scintigraphy and performed quantification of 99Tcm-MDP in these lesions. We then compared that uptake in those patients with CT identifiable fractures (concordant group) with those that had no CT evidence of fracture (discordant group) to see if there was a quantifiable difference between the groups.
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Material and methods
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Patients
Over a 12 month period 51 patients (17 male and 34 female) with suspected acute fracture were prospectively recruited for CT and skeletal scintigraphy performed on the same day. The average age of the patients was 40.2 (range 17–81) years. All patients gave their informed consent to participate and the local ethics committee approved the study.
Skeletal scintigraphic technique
Patients presenting to the nuclear medicine department were injected with 400 MBq of 99Tcm-MDP. 3 h post injection local view images (800 000 counts per image) of the hands/wrists were obtained on a single headed gamma camera with a high resolution collimator. The scintigram images were displayed on a 128 by 128 pixel matrix. The study was reported routinely by the duty nuclear medicine physician with input from a senior nuclear medicine trainee, without knowledge of the CT result.
CT technique
CT images of the suspected fracture site were obtained using a 16 detector multislice unit. The patient was positioned prone with the hand held above the head and the wrist placed flat on the CT table. Acquisition occurred using the 0.75 mm detectors and images were reconstructed in 0.5 mm slice widths. The images underwent multiplanar reconstruction, displayed using a bony algorithm and viewed in interactive cine mode. CT images were reported by a musculoskeletal radiologist and a senior CT trainee in consensus and without knowledge of the nuclear medicine images.
Concordant versus non concordant fracture determination
After initial blind reporting, any discordant cases were retrospectively reviewed by a senior nuclear medicine physician and a senior CT radiologist. Both these reporters were made aware of the original interpretations and any follow up of clinical or imaging data. The latter consisted of 6 week post trauma radiographs in 5 patients and MRI in one further patient. At the end of this process, if a fracture could still not be identified, the fracture was termed discordant (Figures 1 and 2
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Figure 1. A concordant fracture. (a) The local view bone scintigram planar image of the wrist shows focal increased MDP uptake in the region of the right trapezium. (b) On 16 detector CT the fracture was best identified (arrow) in the coronal plane.
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Figure 2. An example of a discordant fracture. (a) The local view bone scintigram planar image of the wrist shows focal increased MDP uptake in the region of the left trapezium. No fracture was identified on the 16 detected CT examined fully in all planes. (b) The trapezium is shown (arrow) on the coronal reconstructed CT image.
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Quantification of 99Tcm-MDP update
Quantification was performed on the routine planar local view images (Figure 3). The site of scintigraphic fracture was identified and a region of interest was carefully hand-drawn around the area. A second region of interest was then created in order to estimate normal bone 99Tcm-MDP uptake. The region was of exactly the same size and area as that created for the fracture. This was placed on the opposite limb in a mirror image position over the bone. Commercially available software was then used to quantify the mean and maximum of counts within the fracture region and the mean counts over normal bone. By dividing the mean and maximum counts in the fracture region by the mean counts of normal bone, the fracture count to normal bone and the maximum fracture count to normal bone ratio was calculated.
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Figure 3. Local view bone scintigram planar image of the wrist illustrating placement of regions of interest used to calculate MDP uptake.
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Statistical analysis
The count rates between discordant and not discordant fracture groups were compared using Student's unpaired t-test.
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Results
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After final review there were 23 fractures in total. These consisted of 16 (70%) concordant fractures and 7 (30%) discordant fractures. All the discordant cases were scintigraphic positive CT negative. The sites of fracture are summarized in Table 1.
The mean fracture count to background bone activity ratio averaged 7.7 (range 3.2–18.5) for concordant fractures and 3.8 (range 1.7–5.3) for discordant fractures (t-test p=0.04). The maximum fracture count to background bone activity ratio averaged 12.7 (range 4.3–27.7) for concordant fractures and 6.3 (range 2.6–9.5) for discordant fractures (t-test p=0.03).
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Discussion
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Approximately one third of the patients in our study group had discordant scintigraphic positive CT negative fractures. This would suggest a reasonably high prevalence of such fractures. One interpretation of our findings might be that scintigraphy is more sensitive at detecting occult wrist fractures than CT. Indeed, the suggestion that scintigraphy has a higher sensitivity than CT in detecting these fractures has been forwarded by some for many years [12]. Such a finding has now also been reported using modern multidetector CT machines [14]. However, it is difficult to comment on sensitivity when there is lack of histiological proof or a recognized gold standard. In order to investigate other explanations for these discordances, we quantified the uptake of 99Tcm-MDP. We have convincingly demonstrated that these patients with discordant findings have significantly less 99Tcm-MDP uptake at their trauma site compared with those with concordant lesions.
In order to explain the difference in 99Tcm-MDP uptake between the concordant and discordant fracture groups, it may be helpful to try and examine the significance at a metabolic level. It is recognized that 99Tcm-MDP uptake is proportional to osteoblastic metabolism [15]. However, it is also recognized (as is the case in most scintigraphic techniques) that although this increased uptake is sensitive to metabolic change, it has a relatively poor specificity regarding the underlying cause [16]. One documented cause of increased 99Tcm-MDP is bony trauma [17]. It may follow that the degree of metabolic activity reflects the severity of bony trauma (i.e. a graded response to injury). In such a way, the argument may be extended and the so-called discordant fractures in our study may represent lesser forms of bony injury than those with CT concordant lesions. Given the ability of CT to image the bones and that the axial resolution of 16 detector multislice CT is close to 0.75 mm [18] then alteration in bony trabecula detail might be expected in patients with fracture. The fact that no such CT changes were seen amongst the discordant fracture group may add to the argument that these patients might be suffering from a less severe injury. Whether patients with these less severe injuries might need less clinical intervention is open to speculation. This would be of clinical interest, but would require a study with a detailed clinical follow-up. Detailed clinical follow up was not part of our study design and all the patients were treated using scintigraphy as a gold standard (as is routine practice in our institution), so patients with positive scintigrams were clinically treated as fractures.
As previously mentioned, in the MRI bone trauma literature there is some recognition of bone injury gradation, reflected by terminology such as "bone bruise". The changes in signal intensity produced by the latter have been associated with histological evidence of trabecular microfracture [19, 20]. Consequently, the terms are sometimes used interchangeably: the presence of "bone bruise" MRI signal change (bone marrow oedema) is often reported as a microfracture. However, this may not be the complete picture. The MRI signal characteristics of bone bruise are not specific for microfracture and similar changes have been histologically shown to occur with other lesions such as marrow necrosis or fat cell lysis [19, 21, 22]. Whilst, some investigators have attempted to differentiate microfracture from other abnormalities by the pattern and localization of the bone marrow oedema [23], there still remains a lack of evidence confirming that these marrow signal changes are specific to microfracture. Scintigraphy is not usually associated with the term bone bruise. However, the term has been quoted during the 1980s in association with focal tracer accumulation in the presence of trauma and it was speculated that this entity may well be part of a spectrum of bone injury [11]. The scintigraphic concept of a spectrum of bone injury has generally not been pursued, with perhaps the exception of stress fractures [24]. An alternative explanation for the reduced uptake of tracer in the discordant patient group is the possibility that the uptake is due to overlying soft tissue injury rather than bone trauma. However, in these circumstances the tracer uptake might be expected to be more diffuse. In this setting, early blood pool imaging might have been useful, but this is not routinely used for traumatic lesions in our institution. Another possibility is that inflammation at an adjacent joint may mimic bone trauma on scintigraphy. One strategy that might help differentiate soft tissue or joint lesions from bony ones is the use of coregistration techniques. Indeed, we did employ this technique in a few selected cases. Although the technique was sometimes helpful, it is recognized that the relative fine anatomical detail of the wrist is challenging since small movements can result in clinically significant misregistration [25].
In conclusion, an interesting cohort of patients has been investigated with scintigraphic positive but CT negative fractures. Patients within the discordant group have significantly lower focal 99Tcm-MDP uptake than those individuals with concordant lesions. It would be intuitive that the discordant lesions represent a less severe form of injury such as a bone bruise, which is a common MRI diagnosis. As such, this group of patients warrants further investigation and work is continuing at our institution to explore these ideas further.
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Acknowledgments
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The authors are indebted to the kind efforts of the technical/radiographic and clerical staff of both the nuclear medicine and radiology departments. The authors would like to thank Nick Bird, Nuclear Medicine Physicist, for his helpful suggestions and Siemens Plc for ongoing scientific support.
Received for publication September 10, 2004.
Revision received January 12, 2005.
Accepted for publication April 14, 2005.
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Abstract
Introduction
