by Tim B. Hunter, MD and Mihra S. Taljanovic, MD, PhD
Blast Injuries
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).
The American College of Radiology (ACR) has designated four safety zones within MRI facilities (Mitchell, 2022):
MRI Zone 1. Area that is freely accessible to the general public without supervision. The magnetic fringe fields in these regions are considered very low to negligible.
MRI Zone 2. A public area where MRI patient screening and preparation takes place.
MRI Zone 3. This is a restricted area with access only for screened patients and health care personnel. This is near the magnet room and may present a hazard to those not properly screened.
MRI Zone 4. The Magnet Room. This area houses the MRI machine and has a very high strength magnetic field. It is a region for greatest risk for hazards, such as projectiles or injuries to patients with implanted devices or hazardous foreign bodies.
MRI in the presence of a cardiac implantable electronic device (CIED) raises significant concerns for patient safety. CIEDs include permanent pacemakers and implantable cardioverter defibrillators. A primary concern is for disruption of the normal electronic function of the device with erroneous signals sent to the heart resulting in a life-threatening arrythmia, cardiac asystole, or an inappropriate defibrillating shock. In addition to these risks to the patient, there is concern for other patient injuries such as burns and severe pain from torquing of the device during the MRI exam. A cardiac device may also cause considerable artifact greatly hindering the diagnostic useful of an MRI study. Close examination of clinical data in this regard, however, shows MRI can be safely performed in the presence of at least some CIEDs when properly monitored (Markman, 2018; Muthalaly, 2018). None-the-less, it is incumbent on the radiologist and the patient's physician to be fully aware of the risks and safety measures needed to prevent harm.
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 (Callaghan, 2019). 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).
Blankets containing metallic threads or traces of metal may be hazardous in the magnetic resonance imaging (MRI) environment (Bertrand, 2018). Clothing with invisible metallic components can also present a hazard with the possibility of significant cutaneous burns (Pietryga, 2013). Textiles that contain metal, metallic threads, or traces of metal include radiofrequency (RF) shielding blankets (Krainak, 2018). They may cause fires or thermal injuries to patients. It is best if patients change out of street cloathes for their MR exam, and any cloathing or other materials furnished to them during the exam must be guaranteed to be free of metallic wiring or traces of metal.
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.
One interesting potential source of MRI artifacts is magnetic eyelashes. False eyelashes are common and are usually glued to the native eyelash. Reusable magnetic eyelashes attach via a pair of thin strips of magnets which adhere to each other and are attached to the native upper eyelash. A phantom study showed these eyelashes create significant MRI artifacts and may put a patient at risk during an MRI exam by detaching (Slonimsky, 2019). Patients should be questioned about the use of magnetic eyelashes, and such eyelashes should be removed prior to a patient undergoing an MRI study. In fact, all persons with access to an MRI scanner room, patients, technologists, nurses, physicians, and other personnel should avoid the use of magnetic eyelashes (Slonimsky, 2019).
Implant based breast reconstruction is often used for patient postmastectomy rehabilitation. Breast tissue expanders are often incorporated as part of the first stage of the mastectomy rehabilitation. These contain an injection port which has a magnet that allows the surgeon to identify the site for injection of normal saline to achieve expansion. Tissue expanders with magnetic ports are classified as MRI unsafe and are contraindicated for MRI (Bayasgalan, 2020). Breast tissue expanders with an injection port incorporating passive radiofrequency indentification (RFID) technology allows the surgeon to detect the filling port's position for tissue expansion. Such implants have metallic content and must undergo appropriate testing to ensure patient safety and also to ascertain the reliabilitiy of the RFID device after undergoing the electromagnetic fields associated with MRI. Initial results with an early RFID tissue expander port suggests it is acceptable or "MR Condtional" using an MRI examination at 1.5 T or 3 T (Bayasgalan, 2020).
On October 12, 2017 the United States Food and Drug Administration provided 510(k) premarket clearance for clinical use of 7T MRI systems. These systems are have considerably stronger electromagnetic fields than the 1.5T and 3T systems currently in widespread use. There is greater force on metallic devices, increased potential for functional deficiency of active implants, and unpredictable radiofrequency heating (Hoff, 2019). Also more pronounced at 7T are patient effects like vertigo, dizziness, false feelings of motion, nause, nystagmus, and other untoward effects compared to lower field strength systems. These 7T systems more than likely will magnify the MRI effects described above for foreign bodies. Safety guidelines need to be established specific to this field strength and possible strengths higher than 7T. These questions are reviewed in some detail by Hoff, 2019.
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).
