A total of 43 patients who presented to the emergency department (ED) and underwent radiologic investigation within 7 hours of the time of the initial explosion on April 15, 2013, were included in this study. These 43 patients evaluated were from Massachusetts General Hospital, Brigham and Women's Hospital, and Brigham and Women's Faulkner Hospital. Institutional review board approval was obtained with waiver of informed consent for the retrospective medical record review. Several critically injured patients who were transported directly to the operating theater from the ED resuscitation bay without undergoing initial imaging evaluation within 7 hours of the blast were excluded from the study. Patients who did not undergo any radiologic investigation were also not included in the study.

The radiographic and CT features of these patients were evaluated for imaging findings of primary, secondary, tertiary, and quaternary blast injury. The primary blast injuries included ear, lung, and bowel injuries caused by high-pressure blast wave. The presence of shrapnel was considered diagnostic of secondary blast wave. The presence of fractures without any evidence of soft-tissue breech and shrapnel were considered features of tertiary blast wave. Burn injuries and toxic gas inhalation were considered features of quaternary blast wave. The radiographic and CT images were evaluated for the following: presence of shrapnel; number of shrapnel; penetration of pleural or peritoneal surface; presence of soft-tissue or osseous injury; type of shrapnel (ball bearing, nail, screw, metal fragment from casing, etc.); and fractures.

The age of the patients ranged from 19 to 65 years (mean, 37.1 years) in the 43 patients included in this study. There were 21 men and 22 women in the study group. Seven patients presented within 60 minutes of the blast for the radiologic assessment, and 22 patients presented between 60 and 120 minutes. Nine patients presented between 2 and 3 hours, and another five presented 3–7 hours after the blast. The first patient imaged in radiology was 28 minutes from the time of the blast.

Abdomen and pelvis CT studies were performed in 13 patients. Chest CT studies were performed in two patients. Cross-sectional imaging of the head was performed in 10 patients, most often with CT (n = 9). Four patients underwent a cervical spine CT study. All 43 patients underwent radiographic evaluation.

Blast injuries have the potential to cause soft tissue as well as osseous injuries from the blast wave, shrapnel fragments, and blast wind [4–8]. Primary blast injury is caused by a blast wave produced by the explosion and has the potential to cause injuries to the air-containing cavities, such as ear, lungs, and abdominal hollow viscera. The tissue damage in the lungs is caused by the pressure difference at the air-tissue interface, leading to pulmonary contusion, hemothorax, pneumothorax, traumatic emphysema, bronchopleural fistula, air embolism, and arteriovenous fistulas. The positive-pressure wave, which lasts 5–10 ms, can damage the tympanic membrane at relatively lower pressures of 5–15 pounds per square inch. Damage to the lungs and bowel loops requires relatively higher pressures of 40–75 pounds per square inch [6]. Because the colon contains the most air in the abdomen, colonic perforations are the most common site of bowel perforation. In our study, we did not encounter any pulmonary or bowel injury from the primary blast wave. The likely reason for the lack of pulmonary and bowel injury is believed to be the open space where the explosion occurred and the relatively smaller intensity of the blast as compared with high-explosive ordnance seen with improvised explosive devices in other parts of the world. There were 11 tympanic membrane perforations, 10 of which were believed to be related to the primary blast wave and one related to shrapnel. The tympanic membrane perforation rate of 32.5% (14/43) is comparable to 30.7% (1223/3981) prevalence rate for blast-related ear injuries among U.S. military personnel during Operation Iraqi Freedom [7]. The tympanic membrane perforation rates have been variously reported to range from 9% to 47% in different studies [7–10].

Secondary blast injury is caused by shrapnel fragments traveling at a high speed, secondary to the explosion. These penetrating injuries are the most common cause of explosion-related injury [11]. The radiologic evaluation of secondary blast injury involves identification of the shrapnel fragments and the associated soft-tissue injury caused by the penetrating force. Tertiary blast injuries are caused when victims are thrown by the force of the blast wind. These injuries are typically blunt traumatic injuries and often manifested as fractures. Quaternary blast injuries include burns, crush injuries, toxic fume inhalation, and posttraumatic stress disorder.

Because the explosive devices were left on the ground, lower extremity injuries were the most common, with five patients in our study group requiring lower limb amputations as a result of their injuries. Shrapnel fragments in the leg and thigh were identified in 36.5% and 31.2% of the patients, respectively. Shrapnel in the pelvis, thigh, leg, and foot accounted for approximately 86.2% of the total shrapnel count. This injury distribution is different from the retrospective cohort study of explosion injuries documented in the Israeli National Trauma Registry, in which the majority of patients had head and brain injuries [12]. In the same study, the most common site of internal injury was chest, followed by abdomen. However, the study of blast injury survivors in Pakistan, Lebanon, and Afghanistan showed a predilection for extremity injuries [13–15]. The location of the injuries after bomb blast is likely dependent on the height at which the bomb is placed, the incendiary nature of the accelerant, and the matériel packaged inside the improvised explosive device.

Because of the lower intensity of the blast injury at the Boston Marathon, the shrapnel fragments did not penetrate the pleural or peritoneal space. The lack of bowel or solid visceral injuries in the study group is different from the study by Bala et al. [16] from Israel, where bowel injury was seen in 85.7% of the terror-related blast victims.

The majority of the injuries in the study cohort were secondary blast injuries, caused by shrapnel, which most commonly were ball bearings (66.1%) and metal fragments (23.3%). These metal fragments are believed to be primarily from the pressure-cooker casing of the bomb. This is consistent with what is reported in the literature [7].

A potential bias in this study was the absence of more critically injured patients from the study group, who were taken to the operating room without initial radiologic evaluation. It is likely that a disproportionate number of these patients had secondary blast injuries leading to limb amputations, which were clinically obvious and therefore did not require initial imaging. We also did not include any patients who had minor injuries and did not require any imaging. It is very likely that those patients who presented with limbs missing would have had additional fragments in their injured limbs. It is doubtful that the limb loss seen at the Boston Marathon bombing was secondary to a primary blast injury but more likely was because of fragmentation.

The extensive use of radiologic investigations in the patients from the Boston Marathon bombing reaffirms its role in the evaluation of foreign bodies and skeletal trauma. Although the primary blast injuries were confined to the ear, the most common radiographically apparent injury was from shrapnel (most often ball bearings) lodged in the lower half of the body. The radiologic evaluation with radiography and contrast-enhanced CT played an integral part in the diagnostic assessment of the blast injury and played a critical role in achieving the most optimal management of the stable patients.