In the era of COVID-19, large numbers of patients are tested with nasopharyngeal swabs at drive-through locations remote to the hospital setting. Swab fracture with retained foreign body is unusual but has been reported in recent months as a possible complication [4, 5]. The types of nasopharyngeal swabs used to collect the specimens for COVID-19 testing may vary; however, FDA-authorized instructions for several of the widely used tests recommend specimen collection using swabs with aluminum or plastic shafts . In this case, the shaft of the swab was made of aluminum.
While the nasopharyngeal swab went undetected on initial radiographic evaluation, review of the lateral radiographs that covered the skull base and neck soft tissues in retrospect did reveal a minimally radiopaque thin line at the inferior aspect of the nasal cavity representing the aluminum shaft of the swab (Fig. 3). For this radiograph, exposure was set at 65.9 kVp and 28 mAs, and the field of view extended from the skull base to the upper chest without collimation. Notably, the patient was wearing a surgical-type face mask during the examination, and the bridge piece and staples retaining the bands of the mask were easily visualized as dense radiopaque objects.
The visibility of aluminum foreign bodies on radiographs has long been an area of interest to radiologists. The adoption of aluminum can pull tabs by soft drink manufacturers in the late twentieth century introduced the issue of identifying accidentally swallowed can tabs on radiographs, which often went undetected as has been reported several times in children [7, 8] and in adults [9, 10]. In one such case, the can tab was not visible on radiographs but was later identified on computed tomography (CT) . Additional cases have been reported of aluminum-based foreign bodies that are not readily detected radiographically, including aluminum-jacketed ammunition and Japanese 1-yen coins [11, 12]. Interestingly, a 1973 proposal by the US Mint to replace the copper penny with a cheaper aluminum alloy alternative was eventually denied due to concerns raised by physicians that ingested coins, a commonly encountered issue in children, may also not be detectable with X-ray [13, 14]. In a study by Ellis in 1993 where aluminum pieces of different sizes were implanted in chicken wings to determine if they are radiopaque, the study author concluded that the pieces were radiographically visible in soft tissue but warned that superimposition of bony structures may make detection more difficult . Similarly, in a 2005 study by Valente et al., aluminum can tabs were placed in the posterior pharynx and upper esophagus of cadavers, and radiographs were obtained to be assessed by two radiologists blinded to the knowledge of which cadavers contained the tabs and the number of tabs present. The study authors concluded that while the aluminum foreign bodies were often detected, the sensitivity of detection was not adequate to rule out their presence if not seen . Although not recommended as a primary modality for detection of foreign bodies, CT has been demonstrated superior to radiographs at detection of inhaled or ingested radiolucent foreign bodies and has been recommended in situations when a suspected aluminum foreign body is not detected radiographically [9, 17].
The difficulty with radiographic imaging of aluminum stems from its physical properties whereby its ability to attenuate diagnostic X-rays through photoelectric absorption is intermediate compared with soft tissue and bone. This is likely further exacerbated by the use of aluminum filtration in some modern X-ray machines to improve image quality and minimize radiation dose, which effectively eliminates the photons just above the k-edge of aluminum that would give the best contrast between aluminum and soft tissue from the X-ray beam .
Recognizing this, we can tailor the radiograph technique in our advantage to give the best chance for detection of a suspected aluminum foreign body such as a nasopharyngeal swab. We therefore performed an experiment to identify the best imaging technique to visualize an aluminum shaft nasopharyngeal swab used for COVID-19 drive-through testing. First, we image a standard BBL ™ rayon-tipped aluminum shaft CultureSwab TM at 66 kVp and 1 mAs (Fig. 4). Then, using a phantom model, we taped the swab onto the side of a skull phantom at the level of the nasal cavity (note that the phantom has no nasal cavity to place the swab into) and obtained radiographs covering the entire skull without collimation with exposure of 66 kVp and 5 mAs (Fig. 5a), 121 kVp and 1 mAs (Fig. 5b), and 56 kVp and 10 mAs (Fig. 5c). We then collimated the field of view to the nasal bones and obtained additional radiographs at 121 kVp and 3 mAs (Fig. 6a) and at 56 kVp and 77 mAs (Fig. 6b). Those parameters were chosen to illustrate the effect of high versus low tube voltage (kVp) and collimation on swab conspicuity. The parameters matched the tube voltage used for the clinical radiograph that was obtained in the emergency department (66 kVp) and the tube voltage routinely used for chest radiographs (121 kVp) and for extremity radiographs (56 kVp) at our institution. The mAs was adjusted to provide similar exposure and noise in the radiographs. After review, it was clear that the collimated images obtained at lower tube voltages produced the best visualization of the aluminum shaft of the nasopharyngeal swab. Collimation reduces scatter, which in turn improves image contrast and shaft conspicuity. Low tube voltage results in lower mean energy of the X-ray beam, which increases the probability of photoelectric interaction with the swab shaft and thus also improves contrast and swab conspicuity.
Recognition that aluminum foreign bodies may be difficult to visualize radiographically and optimization of the radiograph acquisition technique could play a key role in management of unusual cases of nasopharyngeal swab fracture with suspected ingested, aspirated, or retained nasopharyngeal foreign body, which may be asymptomatic. In the setting of negative radiographs, further evaluation should be considered with CT imaging or endoscopy, as was critical in this case to prevent complications.