The value of the flow phase in a three phase bone scintigraphy performed with Technetium-99m methyl diphosphonate (99mTc-MDP) in a patient being worked up for infection involving the musculoskeletal system

Abstract

Bone scintigraphy is one of most frequently performed investigation in many Nuclear Medicine departments. A significant portion of these are performed for the diagnosis of infection involving the musculoskeletal system. Most departments use preset protocols for planar bone scintigraphy, which involves injection of a diphosphonate labelled with Technetium-99m (99mTc-MDP). This is followed by imaging over three phases; viz. the initial flow phase, followed immediately by the blood pool phase, and the eventual delayed bone phase performed 2-4 hours after injection. The sensitivity is high if there is increased uptake in all 3 phases. However, it has been noticed that –especially in the paediatric patient –all 3 phases are not possible due to lack of co-operation; thus, the flow phase is often omitted. Furthermore, in our department it is not always possible to obtain good quality images of the flow phase due to technical reasons, and thus the flow phase is sometimes ignored whilst reporting. Finally, we noticed that if there is increased flow to a specific region during the initial flow phase, there is almost always increased uptake during the blood pool phase as well. This is in relation to its mechanism of uptake being hyperaemia, altered capillary permeability, increased vascularity etc. Thus, we attempted to determine if the flow phase offered any real advantage over and above the blood pool and delayed phases in this group of patients. To the best of our knowledge, no study has been performed previously in the literature looking at the role of the flow phase as compared to the blood pool and delayed phases, and comparing these to determine congruency. There has also been no study performed previously to assess if the flow phase offered any real added advantage in patients being worked up for infection involving the musculoskeletal system. Objective: The aim of this study was to determine the value of the flow phase in three-phase bone scintigraphy performed with technetium-99m methyl diphosphonate (99mTc-MDP) in a patient being worked up for infection involving the musculoskeletal system. Methods: This was a case-controlled study comparing retrospectively flow versus blood pool phases, to determine the value of the flow phase in the routine bone scan for patients being worked up for infection involving the musculoskeletal system at two University Hospitals over a 36-month period. Results: The sample population comprised of 120 patients, ranging from a mere 5-month-old baby to elderly patients in their seventies, with the median age being 56 years. On analysis of the three-phase bone scintigraphy, we discovered that the findings on the flow phase were congruent with findings on the blood pool phase in 94/120 patients (78%). In all these 94 patients the flow phase could have been easily omitted as it provided no REAL ADDED advantage over the blood pool phase. The balance of the 26/120 patients (22%) demonstrated INCONGRUENT findings between the flow and blood pool phases. However, even in these cases, the reporting and management decisions were based more on the findings of the blood pool phase. Part of the reason may be the better delineation that is possible on the blood pool phase, which allows the reporting nuclear physician to provide a better description of uptake with a greater deal of confidence. Again, in this group of patients, the flow phase could have been easily omitted as it provided no REAL ADDED advantage over the blood pool phase. Conclusion: On analysis of the congruent and incongruent scans combined, it can be concluded that not a single patient of the 120 patients analysed benefitted additionally from the flow phase. Thus, our recommendation is that the flow phase can be safely omitted in patients being worked up for infection involving the musculoskeletal system without fear of losing crucial information. This may be beneficial in patients who find it difficult to lie still for the entire duration of the scan. Due to the time constraints applicable to the flow phase, it would thus be more prudent to pay more attention in obtaining good quality blood pool images and omit the flow phase. This would reduce the overall time required for a patient to be lying motionless on the camera, which is especially difficult in the paediatric population. Ultimately, this would also make management of the camera time more efficient in a busy department with limited capacity. A further prospective study with a larger patient base may yield more conclusive results. Such a study should consider analysing three-phase bone scintigraphy performed for other indications as well. It would also be prudent to include scans with negative findings for a more thorough analysis. If such a study were to concur with our findings, this may change the practice of medicine in many centres in future, specifically in patients being worked up for infection involving the musculoskeletal system