1. Introduction
In 2016 nearly 146 million patients sought treatment in emergency departments (ED’s) in the United States. More than 10.5 million (7.2%) of these patients presented with injuries to the extremities, and 4.6 million (3.2%) of these had open wounds of the extremities [
1]. Broken glass-induced injuries account for 13% of patients with traumatic wounds [
2]. While no published articles explicitly enumerated the total number of broken glass-induced injuries in the US, the authors estimated the prevalence of these injuries by applying the reported rates (4.6 × 10
6 × 13% ≈ 600,000 injuries per year). Of course, this represents an approximation because the authors employed rates derived from different sources. While this number only represents a small percentage of the total number of annual ED visits, it still is a significant number of cases fraught with potential liability and the possibility of attracting malpractice suits. Kaiser et al. [
3] reviewed closed malpractice claims in Massachusetts from 1988 to 1994 and reported that of all cases involving retained FBs, 53% were glass fragments. Only 31% of patients underwent X-ray imaging as part of their wound evaluation: for those who were imaged, 5 of 6 glass FBs (83%) remained undiscovered and undiagnosed. Among physicians who were sued, 60% of those who failed to take initial radiographs lost their cases, either in court or by settlement; furthermore, indemnity payouts were significantly increased for cases in which the physician failed to image the patient [
3]. When treating a patient with a wound that may contain FBs, such as broken glass, standard emergency medicine practice requires that the practitioner evaluate the patient by taking a detailed history, performing a physical examination, and obtaining imaging. Most practitioners and facilities will get conventional radiographs to screen for glass FBs. Radiopaque objects, such as metal pieces, are easier to detect than more radiolucent objects, such as glass, which can often be missed [
4,
5,
6]. The accuracy of detection of glass FBs by standard X-ray is generally not dependent on the type or chemical composition of the glass. In a study of 430 patients by Montano et al. [
7], window and bottle glass account for 30% and 25% of all cases, respectively. Although these glass types are relatively low density (in comparison to lead-oxide glasses), these glass FBs can be detected by conventional radiographs.
The probability of detection is more dependent on the size of the FBs. The likelihood of detecting glass FBs with a more than 2 mm diameter approaches 99% using conventional X-rays. For smaller glass FBs that are >1 mm and >0.5 mm, the likelihood of detection decreases to 84% and 61%, respectively [
8]. Unfortunately, many glass FBs in wounds are smaller, resulting in a ‘missed foreign body’ rate of up to 50% after a physical examination and radiographs increasing the probability of wound infection, complications, poor clinical outcomes, and legal liability [
3].
Most wounds treated in the ED that contain glass FBs are hand injuries, where infection, complication, and poor clinical outcomes have more significant consequences for the patient whose hands are vital to their occupational and personal functioning [
9]. The complex bone structure of the hand further complicates accurate X-ray imaging of these very small pieces of glass.
Point of Care testing (POCT) is increasingly becoming desirable and trenchant in multiple clinical settings, including EDs, Urgent Care Centers, and various austere medical settings. POCT increases the efficiency of patient care in these settings, decreasing patient waiting times and increasing patient satisfaction while improving clinicians’ ability to make diagnoses and provide treatment without requiring the input of off-site specialists. The quality of standard ultrasound machines used in most emergency departments (non-hand-held devices) is generally superior to available hand-held devices. However, the study focused on resource-limited areas and austere settings where portability is a significant factor. For example, hand-held u/s devices today are used in rural Africa, up on Mt. Everest, and this summer, launching into space with NASA; these places would not be as readily feasible with a traditional bulkier and more costly machine. Thus, the study looked at two portable imaging modalities. An MXFI device is small, light, and can easily be taken to the patient’s bedside for POCT. Portable ultrasound equipment like the Butterfly iQ used for this research is much less expensive than standard X-ray devices. It allows the clinician to examine the patient at the bedside instead of requiring that the sick or injured patient be transported to the radiology suite.
The cost for the latest model of the Butterfly iQ+ is less than $3000 plus the cost of a smartphone or tablet. In contrast, the cost for non-hand-held ultrasound machines is between $40,000 and $100,000. The cost for hand-held X-ray imagers is similar.
POCT can involve blood and urine testing as well as imaging. As more Emergency Medicine clinicians gain skills in using POCUS, and as ultrasound devices become smaller, more portable, and less expensive, POCUS is increasingly perceived as a powerful and clinically useful modality for diagnosing and initiating patient treatment at the bedside. For instance, the value of POCUS has been highlighted during the current SARS-CoV-2 pandemic; POCUS has been shown to be able to identify patients with significant and concerning pulmonary involvement at the first point of clinical evaluation, thus permitting rapid triage and initiation of appropriate treatment.
In some clinical circumstances, POCUS is now considered the Standard of Care. One of these is in the identification of FBs in wounds [
6,
10]. POCUS has demonstrated significant efficacy and accuracy in detecting radioopaque and radiolucent FBs in wounds. Several studies have reported that radiolucent objects are poorly visualized by X-ray with sensitivities of less than 10%; some glass and almost all wooden FBs are radiolucent [
11,
12]. POCUS, employing high-resolution devices, successfully detected wood and thorns with great accuracy. In an analysis of 120 patients with wood and thorn FBs in their wounds, trained practitioners using high-resolution POCUS devices identified wood FBs and thorns with a 94.5% sensitivity and a 53.8% specificity [
13]. However, size remained an issue, as larger wooden objects were more easily detected with POCUS; objects smaller than 2.5 mm were often missed [
14].
Furthermore, several ultrasound-guided soft tissue foreign body removal techniques have been developed [
15,
16,
17,
18].
POCUS continues to evolve with lighter probes offering increased resolution at decreasing prices, thus bringing radiological imaging to remote and extreme environments, such as rural Africa, on top of Mount Everest, Antarctica, and this summer in space with NASA [
19,
20,
21]. In 2020, the WHO published several guidelines and handbooks on using portable medical imaging in the context of the COVID-19 pandemic, TB, and pneumonia [
22].
Traditional mainstream portable X-ray devices are cumbersome, weighing hundreds to thousands of pounds. In the past few years, the market has seen new innovative, lightweight X-ray devices, including Micro C (Oxos; Atlanta, GA, USA), Maven Handheld (Maven Imaging; Aliso Viejo, CA, USA), and Smart-C (Turner Imaging Systems; Orem, UT, USA). To date, there has not been published peer-reviewed data on these systems. New studies have looked at improving the resolution of traditional X-rays by using dark-field radiography, increasing the detection of FB by a factor of 6 [
23]. MFXI has a higher resolution (about 60 μm) than newer lightweight devices (100–150 μm).
The investigators employed a home-built micro-focus X-ray imaging device with a 10 μm focus size with an up to 100-fold increased resolution compared to conventional X-ray systems. This MFXI has the potential to be incorporated into an imaging device that could easily be employed at the patient’s bedside in the ED, Urgent Care setting, and austere medical settings. The MXFI device is small, light, and can easily be taken to the patient’s bedside for POCT. As a first step in investigating the clinical potential of the MFXI for POCT in the emergency and urgent care setting, the investigators created a glass foreign body/chicken wing model to compare the relative accuracy of MFXI to POCUS in detecting small glass FBs in wounds. The hypothesis was that MFXI images allow non-radiologist, emergency medicine clinicians to more accurately identify small and very small FBs than POCUS utilized by these same Emergency Medicine clinicians. The clinicians regularly read plain X-rays as part of their clinical practice but did not have specific radiology training. They had been trained and certified as POCUS operators and had significant Emergency Medicine experience using POCUS in their practice.
MFXI provides sufficient resolution for the detection of small glass fragments. It is particularly advantageous in comparison to ultrasound imaging. This is illustrated in
Figure 1. These images were taken during the early phase of our study. Examples of glass fragments are shown. The MFXI image clearly shows seven glass fragments. The ultrasound image of the same sample only shows one fragment.
2. Materials and Methods
2.1. Study Design and Setting
Small glass fragments ranging from 1 mm to 5 mm were embedded in 58 chicken wings and thighs. The technicians inserting the glass pieces carefully mapped the insertion sites on diagrams with location and count. Each sample contained between 0 and 7 FBs. On average, 5.3 glass bodies were implanted in each positive sample (58 samples). Two control samples were prepared with no FBs. The total number of samples was 60. Each sample was placed in a closed polystyrene container with 1 mm thick walls. All X-ray images were taken through the closed boxes. Formalin solution was added to each container to preserve the samples for the duration of the study. A trained laboratory technician acquired the X-ray images. Next, the samples were transferred to five emergency medicine physicians with POCUS training and certification and up to three years of POCUS hands-on clinical practice. These clinicians regularly read X-ray images as part of their clinical practice but had no specific training as radiologists. The clinicians performed POCUS on the chicken wings and thighs and recorded the number of glass FBs detected. The clinicians were then given the X-ray images of the wings and thighs randomly with no correlation to the chicken pieces that they had previously ultra-sounded. The clinicians then recorded the number of FBs found in each image. These clinicians did not participate in preparing the chicken legs or thighs and were blinded to the number and location of FBs in each sample.
Although the operators could have moved the objects during ultrasound scanning, they did not touch the chicken wings/thighs. Formalin solution was added to each container to preserve the samples, thus unintentionally deterring manipulation. Minimal rotation of the polystyrene container with the sample contained did occur.
In real patients, the clinician could and would manipulate the body part in question to obtain the best views for both u/s and X-rays. For example, hands and feet are often placed in a water bath for u/s. Areas of interest are moved and scanned on multiple planes. Additionally, traditional X-rays involve multiple views and manipulation to achieve those views. Only one view was obtained with MFXI.
2.2. US Imaging
US-imaging of the samples was done using a Butterfly iQ (Butterfly; Guilford, CT, USA) connected to an iPad. The chicken pieces with glass FBs were scanned using a water bath technique; the polystyrene container with the samples was placed in another water-filled basin,
Figure 2. The transducer was held approximately 0.5–1 cm from the skin surface; thus, the scans were performed without any probe–skin contact to avoid potential bias from contact between the probe and subcutaneous glass pieces (tactile clues). Scans were performed under “soft tissue” presets. The center frequencies were between 1 and 10 MHz. Scanners could change depth, gain, and other presets deemed necessary. The clinicians counted and recorded the number of FBs visualized in each chicken wing and thigh.
2.3. X-ray Imaging
A trained laboratory technician used a micro-focus X-ray tube to acquire images of the chicken wings and thighs into which small glass FBs had been embedded. The samples were placed 1.2 m below a 12-bit remote RadEye 200 CMOS detector in a vertical imaging arrangement with a 1.6 m source-to-detector distance. The X-ray source was a True Focus X-ray tube, model TFX-3110EW with a Tungsten anode and a 10 μm focus size. The tube operated at 80 kV and 0.2 mA. For image processing, three different image types must be acquired the background (BG), the flatfield (FF), and the sample (SA) images. The background is taken by reading out the detector without X-ray exposure. The flatfield is generated by exposing the detector to X-rays without any sample in the viewing field. These two images were taken once and used for the workup of all sample images. The clinicians were given the X-ray images and asked to record the number of glass FBs seen in each image.
2.4. Analysis
X-ray image analysis: all measured sample images were corrected for the detector and electronic noise and any inhomogeneity of the X-ray imaging system resulting in a corrected image (CI) using the formula CI = (SA – BG)/(FF – BG). All image processing was done in ImageJ ver.1.53 [
24]. The corrected images were read by the physicians who counted the number of FBs. The raters only analyzed one X-ray image per sample.
2.5. Statistical Analysis
The statistical analysis was carried out separately on the level of the FBs and the level of the sample. For instance, since each sample had multiple glass bodies, the number of true positive glass bodies (P = 307) was well-known, but the number of true negatives was ill-defined. In contrast, the number of positive samples (P = 58) and the number of true negative samples (N = 2) was well known at the sample level. Consequently, metrics such as accuracy were calculated on the sample but not on the foreign-body level.
2.6. Statistical Analysis, Detailed Foreign Body Ratings
Any distribution that is symmetrical around a true value will have an average close to the true value because false positives (FPs) will cancel out false negatives (FNs). A more detailed statistical analysis was carried out to better characterize both imaging modalities. The cancellation of FPs and FNs was largely avoided by calculating the numbers of found minus real FBs in each sample and for each reviewer. The results were categorized as FN for negative numbers, FP for positive numbers, and true positive (TP) for zero deviation from the real numbers. The total number in each category was summed and normalized, yielding the corresponding rates as follows: true positive rate (TPR, sensitivity) = TP/P, and the positive predictive value (PPV, precision) = TP/(TP + FP).
2.7. Statistical Analysis, Detailed Sample Ratings
Many patients present in the emergency departments with multiple suspected FBs in open wounds. For this reason, not just the probability of finding the FBs was analyzed but also how accurately an entire sample with multiple FBs was evaluated. The statistical procedures are largely identical to that for the foreign body, but now the total real number of objects (sample) to be evaluated was P + N = 60; 58 (=P) had on average 5.3 FBs implanted and 2 (=N) samples had no FBs implanted. The total number in each category was summed and normalized, yielding the corresponding rates as follows: true positive rate (TPR, sensitivity) = TP/P, and true negative rate (TNR, specificity) = TN/N. Additionally, we calculated the positive predictive value (PPV, precision) = TP/(TP + FP), the negative predictive value (NPV) = TN/(TN + FN), and the accuracy = (TP + TN)/(P + N).
4. Discussion
4.1. Clinical Significance
EM physicians trained in the use of POCUS, who regularly use this modality in their clinical practice, and who had not received any additional radiology training, were able to identify accurately a significantly greater number of glass FBs in chicken pieces using MFXI than using POCUS and with much greater inter-rater reliability. An MFXI device is lightweight, portable, and can be brought to the patient’s bedside. There is virtually no radiation scatter, thus no need for lead screening of patient or clinician. We did not measure the X-ray dose absorbed in the samples. However, the absorbed dose for hand imaging is typically less than 10 µGy. This dose corresponds to 3 h of equivalent dose by natural background radiation in the USA for hand imaging. Images were read accurately by clinicians not trained as radiologists. The images may be read accurately by artificial intelligence software. Thus, MFXI has the potential to serve as a Point of Care diagnostic modality in a variety of hospital and outpatient clinical settings as well as a more austere milieu including but not limited to emergency departments, urgent care clinics, so-called “minute clinics,” rural health centers, disaster intervention sites, and military casualty clearing sites.
4.2. Weakness of the Design
Ultrasound has several limitations, including detection depth. This study uses chicken wings and legs. These models are significantly smaller than their human counterparts, thus improving the resolution. Using a water bath adds some simulated distance between the ultrasound probe and the sample. However, water is an excellent acoustic medium for ultrasound transmission. In the clinical setting, a water bath may not be feasible, depending on injury and location.
FBs were inserted with a scalpel, potentially creating a very small tract. Although the chicken wings and legs were imaged in a water bath, it is possible that tiny air bubbles had been placed inadvertently along the track, and the sonographer was seeing the track leading up to the foreign body and not the actual foreign body. Although this concern is theoretical, with FBs < 1 mm, it may be difficult to distinguish air bubbles from actual FBs.
For MFXI, only one projection was used. Thus, the results are likely to show lower accuracy than in a clinical setting where multiple projections are routinely done, which helps to eliminate false readings. Thus, MFXI in future clinical practice may be even more accurate than demonstrated here.
4.3. Limitations
Only five clinicians were involved in the study. Although they had identical training and similar clinical experience in using POCUS, there was variation in their ability to identify glass FBs using POCUS. A larger number of clinician operators/readers with a more varied experience level may be useful in future research. Furthermore, the investigators must include more true negative samples in subsequent investigations to calculate a more reliable and robust sensitivity and specificity for MFXI.
4.4. Future Directions
The investigators will repeat this research with more operators/readers representing a broader emergency medicine clinical experience sample. The investigators will also correlate the experience and background of each operator/reader with their performance and define overall group performance as a function of clinical experience. The investigators will likely include several clinicians trained in radiology as well. The investigators will also likely expand their investigation to include small fractures in the bones of the chicken wings and thighs, as POCUS is also used to define bony fractures. The investigators are currently working with AI developers to incorporate this modality in the reading of the images created by the MFXI. They will compare AI readings of the MFXI images with those provided by clinicians.