Imaging-Based Approach to Extradural Infections of the Spine

A Comparative Factor Analysis and New Magnetic Resonance Imaging Scoring System for Differentiating Pyogenic Versus Tuberculous Spondylodiscitis

OBJECTIVE

This study aimed to compare and analyze differences in clinical and magnetic resonance imaging (MRI) findings between tuberculous spondylodiscitis (TbS) and pyogenic spondylodiscitis (PyS), and to develop and validate a simplified multiparameter MRIbased scoring system for differentiating TbS from PyS.

METHODS

We compared predisposing factors in 190 patients: 123 with TbS and 67 with PyS, confirmed by laboratory tests, culture, or pathology. Data encompassing patient demographics, clinical characteristics, laboratory results, and MRI findings were collected between 2015 and 2020. Data were analyzed using logistic regression methods, and selected coefficients were transformed into an MRI-based scoring system. Internal validation was performed using bootstrapping method.

RESULTS

Univariate analysis revealed that the significant risk factors associated with TbS included thoracic lesions, vertebral destruction > 50%, intraosseous abscess, thin-walled abscess, well-defined paravertebral abscess, subligamentous spreading, and epidural abscess. Multivariate analysis revealed that only thoracic lesions, absence of epidural phlegmon, subligamentous spreading, intraosseous abscesses, well-defined paravertebral abscesses, epidural abscesses, and absence of facet joint arthritis were independent predictive factors for TbS (all p < 0.05). These potential predictors were used to derive an MRI scoring system. Total scores ≥ 14/29 points significantly predicted the probability of TbS, with a sensitivity of 97.58%, specificity of 92.54%, and an area under the curve of 0.96 (95% confidence interval, 125.40-3,257.95).

CONCLUSION

This simplified MRI-based scoring system for differentiating TbS from PyS helps guide appropriate treatment when the causative organism is not identified.

Bioorthogonal Radiolabeling of Azide-Modified Bacteria Using [18F]FB-sulfo-DBCO

Purpose: This study was motivated by the need for better positron emission tomography (PET)-compatible tools to image bacterial infection. Our previous efforts have targeted bacteria-specific metabolism via assimilation of carbon-11 labeled d-amino acids into the bacterial cell wall. Since the chemical determinants of this incorporation are not fully understood, we sought a high-throughput method to label d-amino acid derived structures with fluorine-18. Our strategy employed a chemical biology approach, whereby an azide (-N3) bearing d-amino acid is incorporated into peptidoglycan muropeptides, with subsequent "click" cycloaddition with an 18F-labeled strained cyclooctyne partner. Procedures: A water-soluble, 18F-labeled and dibenzocyclooctyne (DBCO)-derived radiotracer ([18F]FB-sulfo-DBCO) was synthesized. This tracer was incubated with pathogenic bacteria treated with azide-bearing d-amino acids, and incorporated 18F was determined via gamma counting. In vitro uptake in bacteria previously treated with azide-modified d-amino acids was compared to that in cultures treated with amino acid controls. The biodistribution of [18F]FB-sulfo-DBCO was studied in a cohort of healthy mice with implications for future in vivo imaging. Results: The new strain-promoted azide-alkyne cycloaddition (SPAAC) radiotracer [18F]FB-sulfo-DBCO was synthesized with high radiochemical yield and purity via N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB). Accumulation of [18F]FB-sulfo-DBCO was significantly higher in several bacteria treated with azide-modified d-amino acids than in controls; for example, we observed 7 times greater [18F]FB-sulfo-DBCO ligation in Staphylococcus aureus cultures incubated with 3-azido-d-alanine versus those incubated with d-alanine. Conclusions: The SPAAC radiotracer [18F]FB-sulfo-DBCO was validated in vitro via metabolic labeling of azide-bearing peptidoglycan muropeptides. d-Amino acid-derived PET radiotracers may be more efficiently screened via [18F]FB-sulfo-DBCO modification.

Imaging in spinal infections: Current status and future directions

Imaging plays an important role in the diagnosis of spinal infections. Early diagnosis is paramount in the treatment of spinal infections and leads to improved outcomes. This article reviews the imaging and relevant clinical details of infections of the spine: pyogenic spondylodiscitis, tuberculous spondylodiscitis, septic facet arthritis, epidural abscess, and subdural abscess. Though radiographs can reveal subtle changes with infections, advanced imaging modalities have increased sensitivity to aid in early diagnosis. Magnetic resonance imaging (MRI) is emphasized given it is generally the most sensitive and specific advanced imaging modality. However, nuclear medicine imaging and computer tomography (CT) play a role diagnosis in cases where MRI is not available or contra-indicated. Additionally, CT is also important for image-guided biopsy to guide antimicrobial treatment.

Evaluating the Performance of Pathogen-Targeted Positron Emission Tomography Radiotracers in a Rat Model of Vertebral Discitis-Osteomyelitis

BACKGROUND

Vertebral discitis-osteomyelitis (VDO) is a devastating infection of the spine that is challenging to distinguish from noninfectious mimics using computed tomography and magnetic resonance imaging. We and others have developed novel metabolism-targeted positron emission tomography (PET) radiotracers for detecting living Staphylococcus aureus and other bacteria in vivo, but their head-to-head performance in a well-validated VDO animal model has not been reported.

METHODS

We compared the performance of several PET radiotracers in a rat model of VDO. [11C]PABA and [18F]FDS were assessed for their ability to distinguish S aureus, the most common non-tuberculous pathogen VDO, from Escherichia coli.

RESULTS

In the rat S aureus VDO model, [11C]PABA could detect as few as 103 bacteria and exhibited the highest signal-to-background ratio, with a 20-fold increased signal in VDO compared to uninfected tissues. In a proof-of-concept experiment, detection of bacterial infection and discrimination between S aureus and E coli was possible using a combination of [11C]PABA and [18F]FDS.

CONCLUSIONS

Our work reveals that several bacteria-targeted PET radiotracers had sufficient signal to background in a rat model of S aureus VDO to be potentially clinically useful. [11C]PABA was the most promising tracer investigated and warrants further investigation in human VDO.

Understanding a mass in the paraspinal region: an anatomical approach

The paraspinal region encompasses all tissues around the spine. The regional anatomy is complex and includes the paraspinal muscles, spinal nerves, sympathetic chains, Batson's venous plexus and a rich arterial network. A wide variety of pathologies can occur in the paraspinal region, originating either from paraspinal soft tissues or the vertebral column. The most common paraspinal benign neoplasms include lipomas, fibroblastic tumours and benign peripheral nerve sheath tumours. Tumour-like masses such as haematomas, extramedullary haematopoiesis or abscesses should be considered in patients with suggestive medical histories. Malignant neoplasms are less frequent than benign processes and include liposarcomas and undifferentiated sarcomas. Secondary and primary spinal tumours may present as midline expansile soft tissue masses invading the adjacent paraspinal region. Knowledge of the anatomy of the paraspinal region is of major importance since it allows understanding of the complex locoregional tumour spread that can occur via many adipose corridors, haematogenous pathways and direct contact. Paraspinal tumours can extend into other anatomical regions, such as the retroperitoneum, pleura, posterior mediastinum, intercostal space or extradural neural axis compartment. Imaging plays a crucial role in formulating a hypothesis regarding the aetiology of the mass and tumour staging, which informs preoperative planning. Understanding the complex relationship between the different elements and the imaging features of common paraspinal masses is fundamental to achieving a correct diagnosis and adequate patient management. This review gives an overview of the anatomy of the paraspinal region and describes imaging features of the main tumours and tumour-like lesions that occur in the region.

Cervical spine spondylodiscitis: Review of literature on current treatment strategies

Infections of the spine are an ever-increasing health concern requiring an often complex and prolonged treatment that can lead to significant morbidity. Of particular interest is the cervical spine where there is an increase rate of post-infectious deformity, secondary neurological deficits and substantially higher rates of associated morbidity and mortality than the thoracic or lumbar spine. In this review, we explore the diagnosis and treatment of spondylodiscitis with particular focus on the cervical spine.

Magnetic Resonance Imaging for Spine Emergencies

This article is devoted to the MR imaging evaluation of spine emergencies, defined as spinal pathologic conditions that pose an immediate risk of significant morbidity or mortality to the patient if not diagnosed and treated in a timely manner. MR imaging plays a central role in the timely diagnosis of spine emergencies. A summary of MR imaging indications and MR imaging protocols tailored for a variety of spinal emergencies will be presented followed by a review of key imaging findings for the most-encountered emergent spine pathologic conditions. Pathologic conditions will be broadly grouped into traumatic and atraumatic pathologic conditions. For traumatic injuries, a practical and algorithmic diagnostic approach based on the AO Spine injury classification system will be presented focused on subaxial spine trauma. Atraumatic spinal emergencies will be dichotomized into compressive and noncompressive subtypes. The location of external compressive disease with respect to the thecal sac is fundamental to establishing a differential diagnosis for compressive emergencies, whereas specific patterns of spinal cord involvement on MR imaging will guide the discussion of inflammatory and noninflammatory causes of noncompressive myelopathy.

Imaging of Spine Infections

The incidence of spondylodiskitis has increased over the last 20 years worldwide, especially in the immunodepressed population, and it remains a complex pathology, both in terms of diagnosis and treatment. Because clinical symptoms are often nonspecific and blood culture negative, imaging plays an essential role in the diagnostic process. Magnetic resonance imaging, in particular, is the gold standard technique because it can show essential findings such as vertebral bone marrow, disk signal alteration, a paravertebral or epidural abscess, and, in the advanced stage of disease, fusion or collapse of the vertebral elements. However, many noninfectious spine diseases can simulate spinal infection. In this article, we present imaging features of specific infectious spine diseases that help radiologists make the distinction between infectious and noninfectious processes.

A Clinical Prediction Tool for MRI in Emergency Department Patients with Spinal Infection

Introduction

Patients with pyogenic spinal Infection (PSI) are often not diagnosed at their initial presentation, and diagnostic delay is associated with increased morbidity and medical-legal risk. We derived a decision tool to estimate the risk of spinal infection and inform magnetic resonance imaging (MRI) decisions.

Methods

We conducted a two-part prospective observational cohort study that collected variables from spine pain patients over a six-year derivation phase. We fit a multivariable regression model with logistic coefficients rounded to the nearest integer and used them for variable weighting in the final risk score. This score, SIRCH (spine infection risk calculation heuristic), uses four clinical variables to predict PSI. We calculated the statistical performance, MRI utilization, and model fit in the derivation phase. In the second phase we used the same protocol but enrolled only confirmed cases of spinal infection to assess the sensitivity of our prediction tool.

Results

In the derivation phase, we evaluated 134 non-PSI and 40 PSI patients; median age in years was 55.5 (interquartile range [IQR] 38–70 and 51.5 (42–59), respectively. We identified four predictors for our risk score: historical risk factors; fever; progressive neurological deficit; and C-reactive protein (CRP) ≥ 50 milligrams per liter (mg/L). At a threshold SIRCH score of ≥ 3, the predictive model’s sensitivity, specificity, and positive predictive value were, respectively, as follows: 100% (95% confidence interval [CI], 100–100%); 56% (95% CI, 48–64%), and 40% (95% CI, 36–46%). The area under the receiver operator curve was 0.877 (95% CI, 0.829–0.925). The SIRCH score at a threshold of ≥ 3 would prompt significantly fewer MRIs compared to using an elevated CRP (only 99/174 MRIs compared to 144/174 MRIs, P <0.001). In the second phase (49 patient disease-only cohort), the sensitivities of the SIRCH score and CRP use (laboratory standard cut-off 3.5 mg/L) were 92% (95% CI, 84–98%), and 98% (95% CI, 94–100%), respectively.

Conclusion

The SIRCH score provides a sensitive estimate of spinal infection risk and prompts fewer MRIs than elevated CRP (cut-off 3.5 mg/L) or clinician suspicion.

The Diagnosis and Management of Pediatric Spine Infections

The management of pediatric spine infections requires a multidisciplinary approach that includes orthopedic surgeons, infectious disease specialists, interventional radiologists, and others. The prevalence of the disease has increased in frequency, virulence, and degree of soft tissue involvement over the past several years; there has also been a resurgence of some types of infections, such as tuberculosis, fungal, and viral pathogens. The diagnosis can often be reached with a detailed history, physical examination, laboratory tests, and imaging studies. Pathologies mimicking infection require a more invasive approach for diagnosis, including core or open biopsy. The treatment of discitis, spondylodiscitis, vertebral osteomyelitis, spinal epidural, and intramedullary abscesses in children is at times complex, and although many infections can be treated non-surgically with antibiotic therapy, some more extensive infections require surgical management. A timely diagnosis is important as it allows the initiation of the appropriate antimicrobial therapy and would decrease the complexity of the subsequent surgical intervention.

Small Molecule Sensors Targeting the Bacterial Cell Wall

This review highlights recent efforts to detect bacteria using engineered small molecules that are processed and incorporated similarly to their natural counterparts. There are both scientific and clinical justifications for these endeavors. The use of detectable, cell-wall targeted chemical probes has elucidated microbial behavior, with several fluorescent labeling methods in widespread laboratory use. Furthermore, many existing efforts including ours, focus on developing new imaging tools to study infection in clinical practice. The bacterial cell wall, a remarkably rich and complex structure, is an outstanding target for bacteria-specific detection. Several cell wall components are found in bacteria but not mammals, especially peptidoglycan, lipopolysaccharide, and teichoic acids. As this review highlights, the development of laboratory tools for fluorescence microscopy has vastly outstripped related positron emission tomography (PET) or single photon emission computed tomography (SPECT) radiotracer development. However, there is great synergy between these chemical strategies, which both employ mimicry of endogenous substrates to incorporate detectable structures. As the field of bacteria-specific imaging grows, it will be important to understand the mechanisms involved in microbial incorporation of radionuclides. Additionally, we will highlight the clinical challenges motivating this imaging effort.

Sensing Living Bacteria in Vivo Using d-Alanine-Derived 11C Radiotracers

Incorporation of d-amino acids into peptidoglycan is a unique metabolic feature of bacteria. Since d-amino acids are not metabolic substrates in most mammalian tissues, this difference can be exploited to detect living bacteria in vivo. Given the prevalence of d-alanine in peptidoglycan muropeptides, as well as its role in several antibiotic mechanisms, we targeted this amino acid for positron emission tomography (PET) radiotracer development. d-[3-11C]Alanine and the dipeptide d-[3-11C]alanyl-d-alanine were synthesized via asymmetric alkylation of glycine-derived Schiff-base precursors with [11C]methyl iodide in the presence of a cinchonidinium phase-transfer catalyst. In cell experiments, both tracers showed accumulation by a wide variety of both Gram-positive and Gram-negative pathogens including Staphylococcus aureus and Pseudomonas aeruginosa. In a mouse model of acute bacterial myositis, d-[3-11C]alanine was accumulated by living microorganisms but was not taken up in areas of sterile inflammation. When compared to existing clinical nuclear imaging tools, specifically 2-deoxy-2-[18F]fluoro-d-glucose and a gallium citrate radiotracer, d-alanine showed more bacteria-specific uptake. Decreased d-[3-11C]alanine uptake was also observed in antibiotic-sensitive microbes after antimicrobial therapy, when compared to that in resistant organisms. Finally, prominent uptake of d-[3-11C]alanine uptake was seen in rodent models of discitis-osteomyelitis and P. aeruginosa pneumonia. These data provide strong justification for clinical translation of d-[3-11C]alanine to address a number of important human infections.

Diagnostic performance of 18F-FDG PET/CT in patients with spinal infection: a systematic review and a bivariate meta-analysis

PURPOSE

Diagnosis of spinal infection (SI) is challenging and usually requires multiple tests. We aimed to perform a systematic review and a bivariate meta-analysis on the diagnostic role of ¹⁸F-FDG PET/CT in patients with SI.

METHODS

A comprehensive literature search of studies published through February 2019 in PubMed/MEDLINE and Cochrane library databases was carried out. Studies investigating the diagnostic performance of ¹⁸F-FDG PET/CT in patients with SI were eligible for inclusion in the qualitative analysis. For the quantitative analysis, pooled sensitivity, specificity, positive and negative likelihood ratio (LR+ and LR-) and diagnostic odds ratio (DOR) of ¹⁸F-FDG PET/CT in patients with suspected SI were calculated on a per examination-based analysis. Pooled data were presented with 95% confidence intervals (95% CI).

RESULTS

Twenty-six articles (833 patients) using ¹⁸F-FDG PET/CT were eligible for the qualitative analysis. Twelve studies (396 patients) were selected for the meta-analysis. Overall, ¹⁸F-FDG PET/CT demonstrated a very good diagnostic performance in patients with SI and several studies underlined the value of ¹⁸F-FDG PET/CT in assessing the response to treatment. The bivariate meta-analysis on ¹⁸F-FDG PET/CT in patients with suspected SI provided the following results: sensitivity 94.8% (95% CI 88.9-97.6%) and specificity 91.4% (95% CI 78.2-96.9%). The pooled LR+, LR- and DOR were 4.7 (95% CI 2.9-7.7), 0.11 (95% CI 0.07-0.16) and 63.4 (95% CI 28.9-139), respectively. No significant heterogeneity or publication bias was found.

CONCLUSION

¹⁸F-FDG PET/CT demonstrated a very good diagnostic performance in patients with SI and can be used in patients in which MRI cannot be performed or is non-diagnostic or inconclusive. Several studies underlined the value of ¹⁸F-FDG PET/CT in assessing the response to treatment in patients with SI. Overall, larger multicentre and prospective studies and cost-effectiveness analyses are warranted.