High Spatial Resolution Chemical Imaging of Implant-Associated Infections with X-ray Excited Luminescence Chemical Imaging Through Tissue

In the United States, about 2 million fracture fixation devices are inserted annually, and 5%-10% of them lead to implant-associated infections¹. These infections are harder to treat with antibiotics at later stages due to the heterogeneity and antibiotic-resistant nature of the biofilms^2,3. If they are diagnosed early, infections can be treated with antibiotics and surgical debridement to prevent extra medical costs for a second surgery to replace hardware at the treated fracture site. Plain radiography and other advanced radiographic techniques are applied in the diagnosis of orthopedic implant-associated infections, non-unions, and related complications. Although these techniques are used frequently to acquire structural information of the surrounding bone and tissue at the orthopedic implant, they are unable to provide biochemical information in the specific environment. Thus, we developed a novel X-ray excited luminescence chemical imaging (XELCI) technique for high-resolution imaging of biochemical information noninvasively at the implant site. Diagnosis of orthopedic implant-associated infections is commonly carried out by one or a combination of different means. Clinical observations (pain, swelling, redness, wound discharge, etc.) suggest the first signs of infection. Later, radiological and laboratory experiments are carried out to confirm the failure of bone healing progression and identify the pathogenic organism^4,5. Nuclear medicinal techniques such as computerized tomography (CT), magnetic resonance imaging (MRI), and radionucleotide methods such as Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) are in use for better visualization of the infected implant and the associated infection^6,7. CT and MRI are advantageous in determining bone necrosis and soft tissue abnormalities, respectively but cause interferences at a close distance to the metal implants⁸. Different X-ray methodologies such as SPECT and PET in combination with radioisotope-labeled analytes as in vivo imaging contrast agents are widely utilized to diagnose implant-associated osteomyelitis². Current applications combine both data from CT scanning and labeling data from either SPECT or PET to generate anatomical information⁹. Although one or more of these imaging modalities are used to aid infection diagnosis, they cannot detect the pH variations associated with infection early to initiate the treatments with antibiotics to avoid extra medical and surgical expenses.

The main advantage of utilizing the imaging system used in this study for monitoring implant-associated infections is its ability to reveal biochemical information about the biofilm microenvironment with a spectral reference. Although the main focus is on imaging and mapping pH at the infected site, this method can be altered to monitor other biomarkers specific to implant-associated infections. Thus, XELCI allows understanding the pathophysiology of the infection. The high spatial resolution imaging allows mapping heterogeneity as the infection grows. pH at the surface where the biofilm formation occurs is very important to understanding biochemical changes. Also, other microenvironment changes can occur due to antibiotic-related stress responses by bacteria^10,11. Due to surface-specific and high spatial resolution imaging, the antibiotic effect on the biofilm microenvironment can be monitored. The technique can also be used to study the biofilm environment for targeted drug delivery experiments. We can study targeted low pH drug release or raising pH to make them more susceptible to work at higher pH.

Three specific characteristics of this imaging technique are X-ray resolution, Implant surface specificity, and chemical sensitivity (Figure 1A). These characteristics can be compared with the currently available imaging techniques for imaging orthopedic implant-related infections (Figure 1B). Once irradiated with X-rays, phosphor particles coated on the implant surface generate red and near-IR (NIR) light that can penetrate through a few centimeters of tissue (albeit with some attenuation)^12,13. Table 1 shows some of the features of the developed imaging system compared to other ways that have been used to measure pH in biofilms or through tissue.

XELCI is a novel imaging technique to acquire high spatial resolution chemical information optically near implanted medical devices in combination with X-ray excitation, as shown in Figure 2. Here the selective excitation and optical detection of X-ray excitable phosphor particles is utilized. The implant is coated with two layers, a pH-sensitive dye incorporated polymer layer over a layer of scintillator particles. Once a sequence of focused X-ray beams irradiates the implant, the scintillator layer generates visible light (620 nm and 700 nm). This produced light passes through the pH-sensitive layer modulating the luminescence spectrum depending on the pH of the surrounding environment. Low pH is generally associated with infection and biofilm formation; as the infection progresses, the pH changes from physiological pH (pH 7.2) to acidic (less than pH 7), and the pH dye in the sensor changes color and thus absorbance. The variation of the luminescence spectrum is shown in Figure 2E for Bromocresol green pH dye at pH 7 and pH 4. The transmitted light through tissue and bone is collected and the spectral ratio determines pH. To generate a pH image, the focused X-ray beam irradiates a point at a time in the scintillator film and scans the beam point-by-point across the sample. Previously, this technique was applied to image pH variation on the surface of the orthopedic implants^14,15 and have tested it to monitor pH variations in the intramedullary canal through bone and tissue.

Figure 3 below shows a schematic of the imaging system. Basic components of the imaging system are the X-ray excitation source with poly capillary optics, a one-piece acrylic light guide connecting to two photomultiplier tubes, the x, y, and z motorized stage (30 cm x 15 cm x 6 cm travel) and the computer connected for data acquisition. The X-ray source, x,y,z stage, and collection optics (elbow, light guide, photomultiplier tubes (PMTs)) are in the X-ray proof enclosure, while the X-ray controller, power source for PMTs, function generator connected to the data acquisition (DAQ) board and computer are kept outside. A push-button, normally open switch, placed between the enclosure and the front of the door serves as an interlock. If the door is not fully closed (the interlock switch is open), the X-ray source will not turn on, and it will automatically turn off the X-ray source if it is opened during operation. The motors can execute a continuous scan as well as they can be moved to any discrete location. The scan speed for y-axis is usually 1-5 mm/s, while the step size on the x-axis can be chosen typically from 150-2000 µm. The parameters can be chosen depending on the required spatial resolution. Even exposure times are confirmed by consistent speed throughout a continuous scan.

Once the focused X-ray beam is irradiated on the X-ray luminescence particles, the generated light will pass through the pH-sensitive film by modulating the light depending on the surrounding pH. The transmitted light will interact (scatter and absorb partially) with a tissue, while the light attenuation by scattering and absorption will increase as the tissue thickness increases. The collection optics includes a one-piece bifurcated acrylic light guide fitted with a reflective aluminum elbow (with a 90° bend and polished reflective interior surface) at the beginning. This is to ensure the light is collimated as soon as light reaches the light guide. These additions significantly improved the light collection efficiency. For further details, Figure 4 shows the machine drawings of the elbow and light guide. The 90° elbow was machined out of aluminum with the internal surface polished to a mirror finish and the light guide was machined with Acrylic. We have also attached a broad range long-pass blue light filter (blocking 350-450 nm light) at the beginning of the elbow to ensure that only red light will pass through. The end of the one-piece acrylic light guide bifurcates into two streams leading to two different PMTs. The PMTs are enclosed in a small light-tight metal box that is in contact with a thermoelectric cooler to cool down the PMTs to ~5 °C. At the beginning of one of the PMTs, a narrow range long-pass filter (blocking 570-640 nm light and passing 640-740 nm light) is attached to measure only the 700 nm light. Therefore, the 620 nm and 700 nm light can be calculated separately. The PMTs are set up in photon counting mode, and they generate transistor-transistor logic (TTL) pulses for each photon detected. A DAQ system counts the pulses (saturation point 20 million pulses per second) using USB communication. Two separate intensity maps are generated after processing the data, and a final image is created by considering the ratio of the signal wavelength intensity (620 nm) to the reference wavelength intensity (700 nm). This ratio accounts for differences in total light collection efficiency, which depend strongly on the position of collection optics, X-ray irradiation intensity, and tissue thickness. In addition, a spatially separated reference region without any pH indicator dye accounts for spectral distortion from wavelength-dependent tissue penetration. A graphics-based programming language is used for controlling the imaging system, and a basic flow chart of the operation is shown below. The imaging setup, except for the computer, X-ray controller, and DAQ unit, is enclosed in a safe X-ray enclosure to minimize radiation exposure.