Clinical Impact of SPECT/CT With In-111 Biotin on the Management of Patients With Suspected Spine Infection

Exploration of Coincidence Detection of Cascade Photons to Enhance Preclinical Multi-Radionuclide SPECT Imaging

We proposed a technique of coincidence detection of cascade photons (CDCP) to enhance preclinical SPECT imaging of therapeutic radionuclides emitting cascade photons, such as Lu-177, Ac-225, Ra-223, and In-111. We have carried out experimental studies to evaluate the proposed CDCP-SPECT imaging of low-activity radionuclides using a prototype coincidence detection system constructed with large-volume cadmium zinc telluride (CZT) imaging spectrometers and a pinhole collimator. With In-111 in experimental studies, the CDCP technique allows us to improve the signal-to-contamination in the projection (Projection-SCR) by ~53 times and reduce ~98% of the normalized contamination. Compared to traditional scatter correction, which achieves a Projection-SCR of 1.00, our CDCP method boosts it to 15.91, showing enhanced efficacy in reducing down-scattered contamination, especially at lower activities. The reconstructed images of a line source demonstrated the dramatic enhancement of the image quality with CDCP-SPECT compared to conventional and triple-energy-window-corrected SPECT data acquisition. We also introduced artificial energy blurring and Monte Carlo simulation to quantify the impact of detector performance, especially its energy resolution and timing resolution, on the enhancement through the CDCP technique. We have further demonstrated the benefits of the CDCP technique with simulation studies, which shows the potential of improving the signal-to-contamination ratio by 300 times with Ac-225, which emits cascade photons with a decay constant of ~0.1 ns. These results have demonstrated the potential of CDCP-enhanced SPECT for imaging a super-low level of therapeutic radionuclides in small animals.

The use of MRI, PET/CT, and nuclear scintigraphy in the imaging of pyogenic native vertebral osteomyelitis: a systematic review and meta-analysis

BACKGROUND CONTEXT

Native vertebral osteomyelitis (NVO) is a severe infection with an increasing incidence globally. Although there is no widely agreed upon reference standard for diagnosis of the disease, imaging plays a crucial role. Magnetic resonance imaging (MRI) is currently the imaging modality of choice. In recent years, advances in imaging have allowed for a larger role for alternative imaging techniques in the setting of NVO.

PURPOSE

Our aim was to evaluate the diagnostic accuracy of MRI, PET/CT, and nuclear imaging, namely 67Gallium and 99mTechnetium scintigraphy, in the diagnosis of pyogenic NVO.

STUDY DESIGN/SETTING

We conducted a systematic review of five medical databases and included all studies from 1970 to September 2021 that compared imaging techniques and provided sufficient data for diagnostic test accuracy meta-analysis.

METHODS

Abstract screening, full text review, and data extraction were done by a pair of independent reviewers. Nonnative and nonpyogenic patients were excluded. A bivariate random effect model was used for meta-analysis.

RESULTS

Twenty studies were included in the meta-analysis, encompassing a total of 1,123 imaging studies. The meta-analysis sensitivity and specificity of MRI were 90% and 72% respectively; those of PET/CT were 93% and 80%; those of 67Ga were 95% and 88%; those of 99mTc were 86% and 39%; and the sensitivity and specificity of combined Ga and Tc were 91% and 92% respectively in the setting of suspected NVO.

CONCLUSIONS

67Ga has the highest sensitivity for NVO, and its specificity is augmented when combined with 99mTc. MRI and PET/CT are both highly sensitive modalities, although the specificity of PET/CT is slightly better. MRI remains an appropriate initial test depending on the availability of other modalities.

Imaging of Spondylodiscitis: An Update

Spondylodiscitis is an infection of the vertebral body and/or intervertebral disc, which can also involve the epidural space, posterior elements, and paraspinal soft tissues. Due to high morbidity and mortality, prompt diagnosis and treatment of spondylodiscitis is critical. However, diagnosis can be challenging due to nonspecific signs and symptoms. Magnetic resonance imaging with and without contrast is the imaging modality of choice due to high sensitivity and specificity. Intravenous administration of gadolinium contrast can better demonstrate the extent of soft tissue and bone abscesses. However, magnetic resonance imaging is less useful in the evaluation of treatment response. When magnetic resonance imaging cannot be performed or is not diagnostic, radionuclide imaging is a useful alternative. Although bone scintigraphy frequently is used as a screening test, false negative results can occur early in the course of the infection and in the elderly. This test is not useful for detecting the soft tissue infections that often accompany or mimic spondylodiscitis. Gallium-67 citrate improves the specificity of the bone scan, can detect infection earlier than the bone scan, may be more sensitive, especially in elderly patients, and identifies accompanying soft tissue infection. Performing SPECT and SPECT/CT improve accuracy. The 2-3 day delay between radiopharmaceutical administration, poor image quality, and relatively high patient radiation dose are significant disadvantages of gallium-67. Furthermore gallium-67 is no longer as readily available as it once was. ¹⁸F-FDG imaging is the radionuclide test of choice for spondylodiscitis. The procedure, completed in one day, is sensitive, has a high negative predictive value, and reliably differentiates degenerative from infectious vertebral body end-plate abnormalities. ¹⁸F-FDG has outperformed bone and gallium-67 imaging in comparative studies. ¹⁸F-FDG may be able to provide an objective means for monitoring response to treatment. The potential of other agents for diagnosing spondylodiscitis has been studied. Although indium-111biotin accurately diagnoses spondylodiscitis, this radiopharmaceutical has never been commercially available. Gallium-68 citrate and ^99mTc-radiolabeled antimicrobial peptides have been investigated, but their role in the diagnosis of spondylodiscitis has not been established. Labeled leukocyte scintigraphy has no role in the diagnosis of spondylodiscitis.

Recent Progress in the Molecular Imaging of Tumor-Treating Bacteria

Bacterial cancer therapy (BCT) approaches have been extensively investigated because bacteria can show unique features of strong tropism for cancer, proliferation inside tumors, and antitumor immunity, while bacteria are also possible agents for drug delivery. Despite the rapidly increasing number of preclinical studies using BCT to overcome the limitations of conventional cancer treatments, very few BCT studies have advanced to clinical trials. In patients undergoing BCT, the precise localization and quantification of bacterial density in different body locations is important; however, most clinical trials have used subjective clinical signs and invasive sampling to confirm bacterial colonization. There is therefore a need to improve the visualization of bacterial densities using noninvasive and repetitive in vivo imaging techniques that can facilitate the clinical translation of BCT. In vivo optical imaging techniques using bioluminescence and fluorescence, which are extensively employed to image the therapeutic process of BCT in small animal research, are hard to apply to the human body because of their low penetrative power. Thus, new imaging techniques need to be developed for clinical trials. In this review, we provide an overview of the various in vivo bacteria-specific imaging techniques available for visualizing tumor-treating bacteria in BCT studies.

Joint EANM/ESNR and ESCMID-endorsed consensus document for the diagnosis of spine infection (spondylodiscitis) in adults

PURPOSE

Diagnosis of spondylodiscitis (SD) may be challenging due to the nonspecific clinical and laboratory findings and the need to perform various diagnostic tests including serologic, imaging, and microbiological examinations. Homogeneous management of SD diagnosis through international, multidisciplinary guidance would improve the sensitivity of diagnosis and lead to better patient outcome.

METHODS

An expert specialist team, comprising nuclear medicine physicians appointed by the European Association of Nuclear Medicine (EANM), neuroradiologists appointed by the European Society of Neuroradiology (ESNR), and infectious diseases specialists appointed by the European Society of Clinical Microbiology and Infectious Diseases (ESCMID), reviewed the literature from January 2006 to December 2015 and proposed 20 consensus statements in answer to clinical questions regarding SD diagnosis. The statements were graded by level of evidence level according to the 2011 Oxford Centre for Evidence-based Medicine criteria and included in this consensus document for the diagnosis of SD in adults. The consensus statements are the result of literature review according to PICO (P:population/patients, I:intervention/indicator, C:comparator/control, O:outcome) criteria. Evidence-based recommendations on the management of adult patients with SD, with particular attention to radiologic and nuclear medicine diagnosis, were proposed after a systematic review of the literature in the areas of nuclear medicine, radiology, infectious diseases, and microbiology.

RESULTS

A diagnostic flow chart was developed based on the 20 consensus statements, scored by level of evidence according to the Oxford Centre for Evidence-based Medicine criteria.

CONCLUSIONS

This consensus document was developed with a final diagnostic flow chart for SD diagnosis as an aid for professionals in many fields, especially nuclear medicine physicians, radiologists, and orthopaedic and infectious diseases specialists.

Bone Scintigraphy: A Review of Technical Aspects and Applications in Orthopedic Surgery

Due to its high sensitivity, low cost, accessibility, and ease of use, bone scintigraphy is used in orthopedic surgery for the diagnosis and management of varied pathology. It is commonly used for insufficiency fractures, metastatic neoplasia, staging and surveillance of sarcoma, and nonaccidental trauma. It augments diagnoses, including stress or occult fractures, musculoskeletal neoplasia or infection, and chronic regional pain syndrome, in patients presenting with normal results on radiographs. Bone scan images are resistant to metal-based implant artifact, allowing effective evaluation of failed total joint prostheses. Bone scintigraphy remains an underused tool in the evaluation and management of orthopedic patients. [Orthopedics. 2019; 42(1):e14-e24.].

Two decades of SPECT/CT - the coming of age of a technology: An updated review of literature evidence

PURPOSE

Single-photon emission computed tomography (SPECT) combined with computed tomography (CT) was introduced as a hybrid SPECT/CT imaging modality two decades ago. The main advantage of SPECT/CT is the increased specificity achieved through a more precise localization and characterization of functional findings. The improved diagnostic accuracy is also associated with greater diagnostic confidence and better inter-specialty communication.

METHODS

This review presents a critical assessment of the relevant literature published so far on the role of SPECT/CT in a variety of clinical conditions. It also includes an update on the established evidence demonstrating both the advantages and limitations of this modality.

CONCLUSIONS

For the majority of applications, SPECT/CT should be a routine imaging technique, fully integrated into the clinical decision-making process, including oncology, endocrinology, orthopaedics, paediatrics, and cardiology. Large-scale prospective studies are lacking, however, on the use of SPECT/CT in certain clinical domains such as neurology and lung disorders. The review also presents data on the complementary role of SPECT/CT with other imaging modalities and a comparative analysis, where available.

Currently Available Radiopharmaceuticals for Imaging Infection and the Holy Grail

Infection is ubiquitous. However, its management is challenging for both the patients and the health-care providers. Scintigraphic imaging of infection dates back nearly half a century. The advances in our understanding of the pathophysiology of disease at cellular and molecular levels have paved the way to the development of a large number of radiopharmaceuticals for scintigraphic imaging of infection. These include radiolabeling of blood elements such as serum proteins, white blood cells (WBCs), and cytokines, to name a few. Infectious foci have also been imaged using a radiolabeled sugar molecule by taking advantage of increased metabolic activity in the infectious lesions. Literature over the years has well documented that none of the radiopharmaceuticals and associated procedures that facilitate imaging infection are flawless and acceptable without a compromise. As a result, only a few compounds such as 99mTc-hexamethylpropyleneamineoxime, 18F-FDG, the oldest but still considered as a gold standard 111In-oxine, and, yes, even 67Ga-citrate in some countries, have remained in routine clinical practice. Nonetheless, the interest of scientists and physicians to improve the approaches to imaging and to the management of infection is noteworthy. These approaches have paved the way for the development of numerous, innovative radiopharmaceuticals to label autologous WBCs ex vivo or even those that could be injected directly to image infection or inflammation without direct involvement of WBCs. In this review, we briefly describe these agents with their pros and cons and place them together for future reference.

Pathogen-Specific Bacterial Imaging in Nuclear Medicine

When serious infections are suspected, patients are often treated empirically with broad-spectrum antibiotics while awaiting results that provide information on the bacterial class and species causing the infection, as well as drug susceptibilities. For deep-seated infections, these traditional diagnostic techniques often rely on tissue biopsies to obtain clinical samples which can be expensive, dangerous, and has the potential of sampling bias. Moreover, these procedures and results can take several days and may not always provide reliable information. This combination of time and effort required for proper antibiotic selection has become a barrier leading to indiscriminate broad-spectrum antibiotic use. Exposure to nosocomial infections and indiscriminate use of broad-spectrum antibiotics are responsible for promoting bacterial drug-resistance leading to substantial morbidity and mortality, especially in hospitalized and immunosuppressed patients. Therefore, early diagnosis of infection and targeted antibiotic treatments are urgently needed to reduce morbidity and mortality caused by bacterial infections worldwide. Reliable pathogen-specific bacterial imaging techniques have the potential to provide early diagnosis and guide antibiotic treatments.

Imaging of Spondylodiscitis

Spondylodiscitis is an infection of the vertebral body or disc and may also involve the epidural space, posterior elements, and paraspinal soft tissues. It is a cause of morbidity and mortality, and warrants early diagnosis and prompt treatment. Diagnosis can be difficult because of nonspecific signs and symptoms. Magnetic resonance imaging is sensitive and specific and is the imaging modality of choice for spondylodiscitis. Gadolinium contrast can show the extent of soft tissue and bone phlegmon and abscess. The test is less useful for evaluating treatment response. When magnetic resonance imaging cannot be performed or is not diagnostic, radionuclide imaging is a useful alternative. Although bone scintigraphy frequently is used as a screening test, false-negative results can occur, especially in the elderly. This test is not useful for detecting soft tissue infections that accompany or mimic spondylodiscitis. Gallium-67 citrate improves the specificity of the bone scan, can detect infection earlier than the bone scan, may be more sensitive, especially in elderly patients, and identifies accompanying soft tissue infection. Performing SPECT and SPECT/CT improves accuracy. The 2- to 3-day delay between radiopharmaceutical administration and the relatively poor image quality are significant disadvantages of gallium-67. Indium-111 biotin, alone or in combination with streptavidin, accurately diagnoses spondylodiscitis; unfortunately, this agent is not widely available. Currently, ¹⁸F-FDG imaging is the radionuclide test of choice for spondylodiscitis. The procedure, which is completed in a single session, is sensitive, has a high negative predictive value, and reliably differentiates degenerative from infectious vertebral body end plate abnormalities. In comparative investigations, ¹⁸F-FDG has outperformed bone and gallium-67 imaging. Preliminary data suggest that ¹⁸F-FDG may be able to provide an objective means to measure response to treatment. Gallium-68 citrate and 99mTc-radiolabeled antimicrobial peptides have been investigated, but their role in spondylodiscitis has yet to be established.

Evaluation of a Novel Tc-99m Labelled Vitamin B12 Derivative for Targeting Escherichia coli and Staphylococcus aureus In Vitro and in an Experimental Foreign-Body Infection Model

Purpose

Vitamin B₁₂ (cyanocobalamin, Cbl) is accumulated by rapidly replicating prokaryotic and eukaryotic cells. We investigated the potential of a Tc-99m labelled Cbl derivative ([^99mTc]PAMA(4)-Cbl) for targeting infections caused by Escherichia coli and Staphylococcus aureus. In vitro binding assays were followed by biodistribution studies in a mouse model of foreign body infection.

Procedures

E. coli (ATCC 25922) and S. aureus (ATCC 43335) were used as test strains. [⁵⁷Co]Cbl, [⁶⁷Ga]citrate and [^99mTc]DTPA served as reference compounds. The in vitro competitive binding of [⁵⁷Co]Cbl or [^99mTc]PAMA(4)-Cbl, and unlabeled Cbl, to viable or killed bacteria, was evaluated at 37 and 4 °C. A cage mouse model of infection was used for biodistribution of intravenous [⁵⁷Co]Cbl and [^99mTc]PAMA(4)-Cbl in cage and dissected tissues of infected and non-infected mice.

Results

Maximum binding (mean ± SD) of [⁵⁷Co]Cbl to viable E. coli was 81.7 ± 2.6 % and to S. aureus 34.0 ± 6.7 %, at 37 °C; no binding occurred to heat-killed bacteria. Binding to both test strains was displaced by 100- to 1000-fold excess of unlabeled Cbl. The in vitro binding of [^99mTc]PAMA(4)-Cbl was 100-fold and 3-fold lower than the one of [⁵⁷Co]Cbl for E. coli and S. aureus, respectively. In vivo, [^99mTc]PAMA(4)-Cbl showed peak percentage of injected dose (% ID) values between 1.33 and 2.3, at 30 min post-injection (p.i.). Significantly higher retention occurred in cage fluids infected with S. aureus at 4 h and with E. coli at 8 h p.i. than in non-infected animals. Accumulation into infected cages was also higher than the one of [^99mTc]DTPA, which showed similar biodistribution in infected and sterile mice. [⁵⁷Co]Cbl gradually accumulated in cages with peaks % ID between 3.58 and 4.83 % achieved from 24 to 48 h. Discrimination for infection occurred only in E. coli-infected mice, at 72 h p.i. [⁶⁷Ga]citrate, which showed a gradual accumulation into cage fluids during 12 h, was discriminative for infection from 48 to 72 h p.i. (P < 0.05).

Conclusion

Cbl displayed rapid and specific in vitro binding to test strains. [^99mTc]PAMA(4)-Cbl was rapidly cleared from most tissues and discriminated between sterile and infected cages, being a promising candidate for imaging infections in humans.

Radionuclide Imaging of Musculoskeletal Infection: A Review

There are numerous imaging tests for diagnosing musculoskeletal infection. Radiographs are routinely performed, because even when not diagnostic, they provide an anatomic overview of the region of interest that could influence subsequent procedure selection and interpretation. MRI is sensitive and provides superb anatomic detail. Bone scintigraphy accurately diagnoses osteomyelitis in bones not affected by underlying conditions. (67)Ga is used primarily for spondylodiskitis. Although in vitro labeled leukocyte imaging is the radionuclide test of choice for complicating osteomyelitis such as diabetic pedal osteomyelitis and prosthetic joint infection, it is not useful for spondylodiskitis. Antigranulocyte antibodies and antibody fragments have limitations and are not widely available. (111)In-biotin is useful for spondylodiskitis. Radiolabeled synthetic fragments of the antimicrobial peptide ubiquicidin are promising infection-specific agents. (18)F-FDG is the radiopharmaceutical of choice for spondylodiskitis. Its role in diabetic pedal osteomyelitis and prosthetic joint infection is not established. Preliminary data suggest (68)Ga may be useful in musculoskeletal infection. (124)I-fialuridine initially showed promise as an infection-specific radiopharmaceutical, but subsequent investigations were disappointing. The development of PET/CT and SPECT/CT imaging systems, which combine anatomic and functional imaging, has revolutionized diagnostic imaging. These hybrid systems are redefining the diagnostic workup of patients with suspected or known infection and inflammation by improving diagnostic accuracy and influencing patient management.

Vertebral osteomyelitis in adults: an update

INTRODUCTION

The incidence of vertebral osteomyelitis is increasing, attributed to an ageing population with inherent co-morbidities and improved case ascertainment.

SOURCES OF DATA

References were retrieved from the PubMed database using the terms 'vertebral osteomyelitis' and 'spondylodiscitis' between January 1, 2009 and April 30, 2014 published in English as checked in May 2014 (>1000 abstracts checked).

AREAS OF AGREEMENT

Blood cultures and whole spine imaging with magnetic resonance imaging are essential investigations. Thorough debridement is the mainstay of surgical management, although placing metalwork in active infection is becoming increasingly common.

AREAS OF CONTROVERSY

The extent of pursuing spinal biopsies to determine aetiology, antimicrobial choices and duration, monitoring the response to treatment, and surgical techniques and timing all vary widely in clinical practice with heterogeneous studies limiting comparisons. Surgery, rather than conservative approaches, is being proposed as the default management choice, because it can, in carefully selected patients, offer faster reduction in pain scores and improved quality of life.

AREAS TIMELY FOR DEVELOPING RESEARCH

Further studies are needed to define the most effective technique for spinal biopsies to maximize determining aetiology. High-quality trials are required to provide an evidence base for both the medical and surgical management of vertebral osteomyelitis, including challenging medical management as the default option.

Nuclear medicine imaging of bone infections

Osteomyelitis is a broad group of infectious diseases that involve the bone and/or bone marrow. It can arise haematogenously, via extension from a contiguous infection, or by direct inoculation during surgery or trauma. The diagnosis is not always obvious and imaging tests are frequently performed as part of the diagnostic work-up. Commonly performed radionuclide tests include technetium-99m ((99m)Tc)-diphosphonate bone scintigraphy (bone), and gallium-67 ((67)Ga) and in vitro labelled leukocyte (white blood cell; WBC) imaging. Although they are useful, each of these tests has limitations. Bone scintigraphy is sensitive but not specific, especially when underlying osseous abnormalities are present. (67)Ga accumulates in tumour, trauma, and in aseptic inflammation; furthermore, there is typically an interval of 1-3 days between radiopharmaceutical injection of and imaging. Currently, this agent is used primarily for spinal infections. Except for the spine, WBC imaging is the nuclear medicine test of choice for diagnosing complicating osteomyelitis. The in vitro leukocyte labelling process requires skilled personnel, is laborious, and is not always available. Complementary marrow imaging is usually required to maximise accuracy. Not surprisingly, alternative radiopharmaceuticals are continuously being investigated. Radiolabelled anti-granulocyte antibodies and antibody fragments, investigated as in vivo leukocyte labelling agents, have their own limitations and are not widely available. (111)In-biotin is useful for diagnosing spinal infections. Radiolabelled synthetic fragments of ubiquicidin, a naturally occurring human antimicrobial peptide that targets bacteria, have shown promise as infection specific radiopharmaceuticals. 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG) positron-emission tomography (PET) with or without computed tomography (CT) is very useful in musculoskeletal infection. Sensitivities of more than 95% and specificities ranging from 75-99% have been reported in acute and subacute bone and soft-tissue infection. FDG is the radionuclide test of choice for spinal infection. It is sensitive, has a high negative predictive value, and can differentiate degenerative from infectious vertebral body end-plate abnormalities. Data on the accuracy of FDG for diagnosing diabetic pedal osteomyelitis and prosthetic joint infection are inconclusive and its role for these indications remains to be determined. Other PET radiopharmaceuticals that are under investigation as infection imaging agents include gallium-68 citrate ((68)Ga) and iodine-124 fialuridine ((124)I -FIAU).

Radionuclide imaging of osteomyelitis

Radionuclide procedures frequently are performed as part of the diagnostic workup of osteomyelitis. Bone scintigraphy accurately diagnoses osteomyelitis in bones not affected by underlying conditions. Degenerative joint disease, fracture, and orthopedic hardware decrease the specificity of the bone scan, making it less useful in these situations. Gallium-67 scintigraphy was often used as an adjunct to bone scintigraphy for diagnosing osteomyelitis. However, now it is used primarily for spinal infections when (18)F-FDG imaging cannot be performed. Except for the spine, in vitro-labeled leukocyte imaging is the nuclear medicine test of choice for diagnosing complicating osteomyelitis. Leukocytes accumulate in bone marrow as well as in infection. Performing complementary bone marrow imaging with (99m)Tc-sulfur colloid facilitates the differentiation between osteomyelitis and normal marrow and improves test overall accuracy. Antigranulocyte antibodies and antibody fragments, such as (99m)Tc-besilesomab and (99m)Tc-sulesomab, were developed to eliminate the disadvantages associated with in vitro-labeled leukocytes. These agents, however, have their own shortcomings and are not widely available. As biotin is used as a growth factor by certain bacteria, (111)In-biotin is useful to diagnose spinal infections. Radiolabeled synthetic fragments of ubiquicidin, a naturally occurring human antimicrobial peptide that targets bacteria, can differentiate infection from sterile inflammation and may be useful to monitor response to treatment. (18)F-FDG is extremely useful in the diagnostic workup of osteomyelitis. Sensitivity in excess of 95% and specificity ranging from 75%-99% have been reported. (18)F-FDG is the radionuclide test of choice for spinal infection. The test is sensitive, with a high negative predictive value, and reliably differentiates degenerative from infectious vertebral body end-plate abnormalities. Data on the accuracy of (18)F-FDG for diagnosing diabetic pedal osteomyelitis are contradictory, and its role for this indication remains to be determined. Initial investigations suggested that (18)F-FDG accurately diagnoses prosthetic joint infection; more recent data indicate that it cannot differentiate infection from other causes of prosthetic failure. Preliminary data on the PET agents gallium-68 and iodine-124 fialuridine indicate that these agents may have a role in diagnosing osteomyelitis.

Multiagent imaging of inflammation and infection with radionuclides

Molecular imaging with single photon- and positron-emitting tracers plays an important role in the evaluation of inflammation and infection. Although supplanted by labeled leukocyte imaging for most indications, gallium-67 remains useful for opportunistic infections, pulmonary inflammation and interstitial nephritis and, when [¹⁸F]FDG is not available, spinal infection and fever of unknown origin. In vitro labeled leukocyte imaging is the radionuclide procedure of choice for most infections in immunocompetent patients. When performed for musculoskeletal infection, complementary bone marrow imaging usually is necessary. Recent data suggest that dual time point imaging might be an alternative to marrow imaging. Several methods of labeling leukocytes in vivo, with agents including antigranulocyte antibodies and antibody fragments, peptides and cytokines, have been investigated, with variable results. These agents are not widely available and none of them are available in the USA. Radiolabeled antibiotics have been investigated as “infection-specific” tracers, but the results to date have been disappointing. Conversely, radiolabeled antimicrobial peptides do hold promise as infection-specific tracers. The use of positron-emitting tracers for diagnosing inflammation and infection has generated considerable interest. [¹⁸F]FDG is useful in fever of unknown origin, spinal osteomyelitis, vasculitis and sarcoidosis. Other positron-emitting tracers that have been investigated include [¹⁸F]FDG-labeled leukocytes, copper-64-labeled leukocytes, gallium-68 citrate and iodine-124 FIAU. Although radiolabeled tracers are used primarily for diagnosis, they also offer objective biomarkers for assessing response to therapeutic interventions in inflammatory diseases. They could also potentially be used to target cells and molecules with specific receptor expression for histological characterization, select patients for receptor-targeted therapy and predict response to treatment.

Radiosynthesis of 68Ga-labelled DOTA-biocytin (68Ga-r-BHD) and assessment of its pharmaceutical quality for clinical use

OBJECTIVES

Biocytin analogues labelled with indium-111, yttrium-90 and lutetium-177 have shown their effectiveness in the imaging of infections/inflammation in patients with osteomyelitis and function as efficient tools in pretargeted antibody-guided radioimmunotherapy. In this study, the labelling of a biocytin analogue coupled with DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), namely, r-BHD, with gallium-68 (68Ga) was optimized, and the quality and stability of the preparations were assessed for clinical use.

MATERIALS AND METHODS

Synthesis of 68Ga-r-BHD was carried out by heating a fraction of the 68Ge/68Ga eluate in a reactor containing the biocytin analogue with the appropriate buffer. The influence of the precursor amount (from 2.5 to 140 nmol), the pH of the reaction (from 2 to 5.5) and the buffer species (1.5 mol/l sodium acetate, 1.5 mol/l sodium formate, 4.5 mol/l HEPES) on radiochemical yield and radiochemical purity was assessed. Studies on stability and binding to avidin (Av) were also conducted in different media.

RESULTS

Under the best labelling condition (56 nmol of precursor, 3.8 pH, sodium formate buffer) synthesis of 68Ga-r-BHD resulted in a yield of 64 ± 3% (not decay corrected). Radiochemical purity was around 95% because a 68Ga-coordinated sulfoxide form of the ligand was detected as a by-product of the reaction (68Ga-r-SBHD). The by-product was identified and characterized by liquid chromatography-electrospray ionization tandem mass spectrometry. At the natural 1 : 4 Av/68Ga-r-BHD molar ratio, affinity results were 62 ± 2 and 80 ± 2% in saline and human serum, respectively. Stability of 68Ga-r-BHD and of the radiotracer/Av complex remains almost constant over 180 min. 68Ga-r-BHD appears to be a good candidate for clinical applications.

Prosthetic joint infections: radionuclide state-of-the-art imaging

Prosthetic joint replacement surgery is performed with increasing frequency. Overall the incidence of prosthetic joint infection (PJI) and subsequently prosthesis revision failure is estimated to be between 1 and 3%. Differentiating infection from aseptic mechanical loosening, which is the most common cause of prosthetic failure, is especially important because of different types of therapeutic management. Despite a thorough patient history, physical examination, multiple diagnostic tests and complex algorithms, differentiating PJI from aseptic loosening remains challenging. Among imaging modalities, radiographs are neither sensitive nor specific and cross-sectional imaging techniques, such as computed tomography and magnetic resonance imaging, are limited by hardware-induced artefacts. Radionuclide imaging reflects functional rather than anatomical changes and is not hampered by the presence of a metallic joint prosthesis. As a result scintigraphy is currently the modality of choice in the investigation of suspected PJI. Unfortunately, there is no true consensus about the gold standard technique since there are several drawbacks and limitations inherent to each modality. Bone scintigraphy (BS) is sensitive for identifying the failed joint replacement, but cannot differentiate between infection and aseptic loosening. Combined bone/gallium scintigraphy (BS/GS) offers modest improvement over BS alone for diagnosing PJI. However, due to a number of drawbacks, BS/GS has generally been superseded by other techniques but it still may have a role in neutropenic patients. Radiolabelled leucocyte scintigraphy remains the gold standard technique for diagnosing neutrophil-mediated processes. It seems to be that combined in vitro labelled leucocyte/bone marrow scintigraphy (LS/BMS), with an accuracy of about 90%, is currently the imaging modality of choice for diagnosing PJI. There are, however, significant limitations using in vitro labelled leucocytes and considerable effort has been devoted to developing alternative radiotracers, such as radiolabelled HIGs, liposomes, antigranulocyte antibodies and fragments, as well as more investigational tracers such as radiolabelled antibiotics, antimicrobial peptides, bacteriophages and thymidine kinase. On the other hand, positron emission tomography (PET) is still growing in the field of PJI imaging with radiotracers such as (18)F-fluorodeoxyglucose (FDG), (18)F-FDG white blood cells and (18)F-fluoride. But unfortunately this superb tomographic technique will only receive full acceptance when specific PET uptake patterns can be successfully developed. The emergence of hybrid modality imaging using integrated single photon emission computed tomography (SPECT) and PET with computed tomography (SPECT/CT and PET/CT) may also have a contributing role for more accurate assessment of joint replacement complications, especially combined with new radiotracers such as (68)Ga and (64)Cu. Finally, in searching for infection-specific tracers, currently there is no such diagnostic agent available.

111In-DTPA-Biotin uptake by Staphylococcus aureus

The potential of indium-111 labelled diethylenetriaminepentaacetic acid α,ω-bis(biocytinamide) (In-DTPA-Biotin) as a specific tracer in nuclear medicine imaging of vertebral osteomyelitis has been shown in a large series of consecutive patients. Biocytin is known to serve as a biotin source for a number of different microorganisms and quantitative studies on staphylococci indicated that on a molar basis biocytin seemed to have an activity equal to that of biotin. In this study, we evaluated the possibility of an illicit transport of In-DTPA-Biotin in cultures of Staphylococcus aureus on continued incubation for 24 h. Radiolabelled biocytin was prepared as described earlier and the stability and radiochemical purity was assessed in vitro for 24 h after labelling. Our data seem to demonstrate a passive transport of In-DTPA-Biotin into the cells of the microorganisms.