*2.1.1 Spondylodiscitis*

The role of 18F-FDG in the diagnosis of spondylodiscitis has been extensively studied. The pooled sensitivity and specificity of 18F-FDG PET/PET-CT were 97% and 88% in one meta-analysis [8]. In another meta-analysis, the pooled sensitivity was 94.8% and the pooled specificity was 91.4% (**Figure 3**) [9] . In intraindividual comparisons, 18F-FDG has outperformed bone and gallium-67 scintigraphy both alone and in combination [10, 11].

Postoperative spondylodiscitis often has an indolent, nonspecific presentation. Prompt diagnosis is imperative because a delay may lead to involvement of the bone, epidural space, and paravertebral soft tissues, and may necessitate hardware removal, which can lead to instability and pseudoarthrosis [12]. In a meta-analysis of 18F-FDG for diagnosing postoperative spondylodiscitis, the summary AUC for spondylodiscitis was 0.92 in patients with versus 0.98 in patients without spinal hardware. Falsepositive results were more common in patients with than in patients without hardware (12.8% vs. 7%), presumably due to hardware-induced aseptic inflammation. Performing PET/CT rather than PET alone reduces hardware-associated false-positive results [8]. Analyzing uptake patterns may facilitate the differentiation between aseptic inflammation and infection. Confluent increased 18F-FDG uptake in soft tissue and bone immediately adjacent to the hardware at multiple contiguous levels

#### **Figure 2.**

*Sacral osteomyelitis. There is 18F-FDG uptake in a sacral decubitus ulcer extending into the distal sacrum (arrow).*

#### **Figure 3.**

*Spondylodiscitis. There is abnormal 18F-FDG activity in the T12-L1 vertebrae corresponding to erosive changes on the CT component, with extension into the prevertebral space (arrow).*

#### **Figure 4.**

*Spondylodiscitis thoracic spine. On the pretreatment 18F-FDG PET/CT (left) there is intense uptake in the T2-T3 vertebrae (arrow). On the posttreatment study, performed about 3 months later, the abnormal uptake had resolved. Persistent esophageal activity (arrowhead) was thought to be secondary to a foreign body reaction or metastatic disease in this patient with esophageal carcinoma (reproduced with permission from Seminars in Nuclear Medicine: Raghavan M, Palestro CJ: Imaging spondylodiscitis: an update. 53:152-166. DOI: 10.1053/j.semnuclmed.2022.11.005).*

is suggestive of infection, while focal uptake adjacent to one or two hooks, screws, or anchors, usually at the upper or lower aspects of the spinal hardware is more suggestive of noninfectious complications [13].

18F-FDG may be useful for monitoring treatment response in spondylodiscitis (**Figure 4**). Some investigators have reported that changes in standardized uptake value (SUV) reliably differentiate responders from nonresponders, while other investigators have observed that changes in uptake patterns are useful for monitoring treatment response [14–19].
