by Tim B. Hunter, MD and Mihra S. Taljanovic, MD, PhD
Blast injuries encountered in terrorist attacks using improvised explosive devices (IEDs) are fortunately rare even in this age of global terrorism. Such injuries are most commonly encountered on the battlefield but may be seen in an urban terrorist attack, such as the Boston Marathon bombing (Kellerman, 2013; Singh, 2014; Singh, 2016). In principle, they are no different than more routine injuries. The forces involved in blast injuries, however, are usually of much greater magnitude than more common injuries from automobile accidents, gunshot wounds, and other daily trauma.
Blast forces may produce extensive soft tissue and bony injury with frequent foreign bodies composed of metallic shrapnel as well as non-metallic debris (paper, cardboard, wood fragments, dirt, rocks, plastic) from the bomb itself or from nearby objects caught up in the blast. Blast injuries are described as a spectrum of primary, secondary, tertiary, and quaternary blast patterns (Horrocks, 2001; Garner, 2007; Champion, 2009; Proud, 2013; Singh, 2016). These include the barotrauma of the explosive detonation (primary blast injury), debris in the blast wind (secondary blast injury), physical displacement of the victim with blunt or penetrating trauma (tertiary blast injury), and burns, crush, and inhalation injuries (quaternary blast injury).
A more extensive discussion of blast injuries is beyond the scope of this essay. It is safe to say blast injuries whether occurring on the battlefield, in a terrorist attack, or in an industrial accident are likely to produce severe soft tissue and bony abnormalities with a great potential for foreign bodies, both metallic and non-metallic. In nearly all cases, standard radiography and cross-sectional imaging, especially CT, play an important role in the evaluation of foreign bodies and skeletal trauma (Singh, 2016).
Foreign Bodies and MR Imaging
All patients should be properly screened before undergoing an MR imaging study. Patients should be asked about prior surgery, foreign body injuries, the presence of a medical device or implant, and the possibility of a pregnancy (Shellock, 2001; Hunter, 1996; Eshed, 2010; Shellock, 2013). Metallic foreign bodies such as bullets or shrapnel may present a hazard to the patient, because they can be moved or twisted by the strong magnetic ﬁelds encountered in MR imaging studies. The seriousness of the risk depends on the ferromagnetic characteristics of the foreign body, its location, and the strength of the MR magnetic ﬁelds (Dula, 2014). See the following sites for more specific and timely information: MRI Safety Information; MRIMaster.com; RadiologyInfo.org.
Small intraocular ferromagnetic fragments are a contraindication to MR imaging. They have a signiﬁcant risk of causing vitreous hemorrhage and possible blindness. However, if a patient with a possible foreign body in the eye, such as a metal worker constantly exposed to tiny metallic slivers, has no symptoms and radiographs of the orbits show no recognizable foreign bodies, then MR imaging is considered safe (Shellock, 2001). Some eyeliners applied with a tattooing process and some eye makeup may contain enough ferrous pigment to produce MR imaging artifacts. They may also interact with the magnetic ﬁelds enough to cause skin irritation and swelling.
In general, tattoos in other parts of the body cause no problems. Microscopic pieces of metal may be deposited into subcutaneous and muscular tissues after orthopedic surgery. These fragments are often invisible on radiographs, but they will produce visible MR imaging artifacts that are usually minor, although they can impair the diagnostic utility of a study (figure: MR artifacts from tiny metallic fragments).
Radiographs should be obtained to determine the location of any bullet, bullet fragment, shrapnel, acupuncture needle, or other possible retained ferromagnetic material from a past injury or therapy. It is necessary to determine if the foreign body is near a vital structure. Even if a piece of metal is located in a safe subcutaneous site, it may cause painful symptoms during an MR imaging study.
Most jewelry is nonferromagnetic, usually being composed of gold and silver. However, some alloys used in jewelry may be ferromagnetic and cause discomfort from being heated or torqued during an MR imaging examination (Hunter, 1996). Whenever possible, patients should be asked to remove all jewelry, including eye rings, nipple rings, tongue rings, labial rings, necklaces, and bracelets before the study.
Postmortem Imaging in Trauma Patients
Radiologic imaging, particularly CT, has been advocated as a possible alternative to traditional autopsy, though it is probably better thought of as complementary to autopsy (Roberts, 2012; Scholing, 2009). It has been especially advocated for trauma patients and for patients who die unexpectedly from an unknown cause. Radiologic imaging may help determine the cause of death, and it can also be of important educational value for radiologists and trauma teams providing feedback regarding the location of support lines. Improperly placed lines and tubes may be noted leading to improved training and better skills of the medical staff (Lotan, 2015) (figure: postmortem abdominal radiograph).