Avascular necrosis of the knee: 99mTc-HDP bone SPECT/CT and MRI

Siemens Symbia Intevo, Injected dose: 680 MBq
SPECT: 360°, 32 frames, 25 s/frame, 3D iterative SPECT
CT: 130 kVp, 10 eff mAs, 2mm slice thickness, DLP 280 mGy*cm, multiplanar reconstructions in high resolution kernel and bone window

During the first two phases of 3-phase bone scan no relevant perfusion changes were noticed. In the 3rd phase, however, markedly increased regional bone metabolism was seen, which corresponded to the areas of the typical curvilinear signs on MRI. In CT slight subchondral sclerosis were correlated to the area of increased regional bone metabolism and geographic defect, but no collapse, osteolytic or cystic changes could be detected.Concluding from all imaging procedures, the changes in MRI, 99mTc-HDP 3-phase bone scan, SPECT and CT indicated an early stage and reversible stage of the disease and allowed an option for nonsurgical treatment.

Of note, MRI is the first method of choice for the diagnosis of avascular necrosis. In selected cases 3-phase 99mTc-HDP imaging with SPECT/CT may help to tailor the treatment and avoid unnecessary surgery by showing the active, viable metabolism of the so called geographic defects in MRI.Bone scanning with SPECT/CT may assist to further differentiate the stages of avascular necrosis by metabolic information: the geographic defects in MRI may show increased, normal or decreased HDP uptake in SPECT/CT.