72 Part I Chapter 3 SPECT/CT in post-operative knee pain Bone SPECT/CT acquisition protocol The European Association of Nuclear Medicine (EANM) practice guidelines for bone scintigraphy canbe summarized as followswith respect to the post-operative knee [40]: - Blood pool images in suspected inflammation or infection, disturbed blood supply and all patients with prosthesis (UKA and TKA): anterior and posterior images, 256 × 256 matrix, LEHR collimator, 2–5 minutes per view. - Static delayed images: anterior, posterior and lateral images, 256 × 256 matrix, LEHR collimator, 5 minutes or 500 kcounts per view. - SPECT/CT images if planar is non-diagnostic, particularly useful in knee prosthesis. 128 x 128 matrix, LEHR collimator, 20 seconds / angle, step mode, non-circular rotation. For attenuation correction/localization CT: 2.5 to 40 mA; 80 – 130 keV; 1-5mm slice thickness. For diagnostic CT: 40 to 335 mA; 80 – 130 keV; 0.33-2.0mm slice thickness. Pain in the post-operative knee may also be the result of extra-articular causes, of which nerve entrapment in the spine and hip OA or femoral head avascular necrosis are most frequently identified as a cause for knee pain [29]. This emphasizes the continued need for whole-body planar imaging as part of a SPECT/CT study of the knee in order to identify relevant hip and spine pathology and guide subsequent cross-sectional imaging (additional bone-SPECT/CT bed position, an MDCT or MRI). Still, a large portion of the extra-articular causes for persistent post-operative knee pain may remain elusive even after thorough evaluation of the hip, spine, or vascular problems [29]. In periprosthetic fractures, first line imaging consists of X-rays. Additionally, boneSPECT/CT may be warranted in order to assess the fracture healing process and to detect non-union, which is especially important in case of comminuted fractures to diagnose or exclude necrotic bone fragments [41]. Increased BTO is usually seen in all 3 phases early after onset and after 6 months or more, and is most pronounced on the late phase images. Bone SPECT/CT may detect necrotic bone fragments by decreased BTO, which can cause pain and may need removal. Optimal CT quality improves the accuracy of the SPECT/CT study, especially enhancing specificity. In patients with a painful post-operative knee, metal artefact suppression techniques are often beneficial to ensure adequate CT quality. Various algorithms have been published with promising results in correcting metal artefacts, such as linear interpolation, multi-dimensional adaptive filtering in the sinogram, or modified