Imaging assessment of penetrating craniocerebral and spinal trauma

Imaging assessment of penetrating craniocerebral and spinal trauma

Clinical Radiology (2009) 64, 1146e1157 REVIEW Imaging assessment of penetrating craniocerebral and spinal trauma C. Offiah*, S. Twigg Department of...

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Clinical Radiology (2009) 64, 1146e1157

REVIEW

Imaging assessment of penetrating craniocerebral and spinal trauma C. Offiah*, S. Twigg Department of Neuroradiology, The Royal London Hospital, Barts and The London NHS Trust, Whitechapel, London, UK Received 10 April 2009; received in revised form 16 June 2009; accepted 26 June 2009

Craniocerebral and spinal penetrating trauma, which may be either missile (most typically gun-related) or non-missile (most typically knife-related), is becoming an increasingly common presentation to the urban general and specialized radiology service in the UK. These injuries carry significant morbidity and mortality with a number of criteria for prognosis identifiable on cross-sectional imaging. Potential complications can also be pre-empted by awareness of certain neuroradiological features. Not all of these injuries are criminal in origin, however, a significant proportion will be, requiring, on occasion, provision of both ante-mortem and post-mortem radiological opinion to the criminal investigative procedure. This review aims to highlight certain imaging features of penetrating craniocerebral and spinal trauma including important prognostic, therapeutic, and forensic considerations. ª 2009 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Introduction Much of the literature on penetrating injury of the neural axis, particularly relating to radiological evaluation, has stemmed largely from military experience. In the post-computed tomography (CT) era, this includes the Gulf war and, more recently, the Kosovo and Iraqi conflicts. Nevertheless, it remains difficult to apply the protocol-driven, shrapnel-predominating battlefield conditions of the military setting where gunshot wounds are characteristically high-velocity, to the civilian setting where the vast majority of gunshot penetrating injuries of the neural axis are low-velocity. However, such civilian trauma, low-velocity or otherwise, is becoming an increasing occurrence in daily radiological practice in the UK. The experience in the

* Guarantor and correspondent: C. Offiah, Department of Neuroradiology, The Royal London Hospital, Barts and The London NHS Trust, Whitechapel, London E1 1BB, UK. Tel.: þ44 207 377 7000x14 2456; fax: þ44 207 377 7165. E-mail address: [email protected] (C. Offiah).

diagnostic evaluation and management of such injuries in the civilian sector is considerable in the US, particularly given the prevalence and availability of firearms, but recent increased media coverage of such crimes in the UK has drawn much attention to the rising tide of violent gun and knife-related penetrating injuries. Recent Home Office statistics for crime in England and Wales recorded 49 firearm-related fatalities in the year ending December 2007 compared with 56 in the year ending December 2006, a decrease of 13%.1,2 However, total injuries (categorized as fatal, serious, or slight), rose by 2% to a total of 3048. In the year ending December 1999, this total was a relatively low 864. Home Office figures also indicate an average of 60 knife offences per day in England and Wales for 2007/2008 with the highest number of offences recorded in London at 7409.1 Additionally, both accidental and selfinflicted penetrating injuries of the neural axis, remain challenging, although infrequent, imaging requests for the radiologists. The present review aims to highlight some of the salient imaging appearances of penetrating craniocerebral and spinal trauma, important

0009-9260/$ - see front matter ª 2009 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2009.06.004

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prognostic, therapeutic, and forensic considerations, and potential secondary complications of such trauma.

philosophy is that the soldier wounded, but not killed by a bullet on the frontline is a far greater drain on the economical and manpower resources of the enemy than is a fatality.6,8,9 Jacketed bullets are more likely to ricochet than unjacketed ones and tissue damage (and, therefore, lethality) is a function of bullet fragmentation and deformation. Although the majority of military ammunition does not exhibit these features in accordance with the Hague Convention ruling, there are a few rare types of military bullets that have been specifically designed at great cost to fall within the accords of the Convention yet still display some of these features.9 The jacket carries a ‘‘finger-print’’ of the firearm it has been discharged from and, therefore, carries the greatest ballistic information, which can be invaluable to the firearms forensic expert and criminal investigation team. Penetrators may also fragment and have a characteristic appearance at radiography and when recovered at post-mortem. In general, bullet wounds (and, in fact, any projectile wound) are more severe when the missile yaws through tissue, fragments, deforms (into a mushroom shape for example), is large, or is travelling at high velocity.8 One of the ‘‘benefits’’ of the hollow-point and soft-point bullet (the former are used by specialist fire-arms officers in the UK) is that it is less likely to exit the target, which poses smaller risk to by-standers; it exhibits significant deformation as a result of its construct transferring most or all of its kinetic energy to the tissue, usually stopping within the body target. Therefore, the lethality of the hollow- and softpoint bullet is very high, particularly when deployed in a ‘‘critical’’ shot. It is worth briefly commenting on the manufacture of shotgun ammunition: spherical shots are held within the shell by a ‘‘wadding,’’ which is a plastic (radioloucent) seal containing the propellant gases. The wadding is expelled with the shot and so becomes a short-range projectile. Presence within the victim of the wadding is suggestive of shooting at short range in which case the probability of survival is low10 and casualties demonstrating such usually present to the pathologist as opposed to the clinician. Not all shotgun ammunition utilizes shot pellets: some consists of slug ammunition made up of a solid core of lead. Explosive bullets (for example, the devastator round) are available, which are designed to explode on impact. However, it has been noted that these bullets frequently fail to explode on impact, but may do so during surgical debridement or

Craniocerebral gunshot injury Bullets and wound (terminal) ballistics Bullets usually consist of a lead core (with tin or antimony added for hardness3), which may be partly or completely covered (‘‘jacketed’’) with another metal (frequently a copper alloy, but steel or aluminium is sometimes used; Fig. 1).4 Penetrators are small conical pieces of steel that may be located in the tip of a bullet designed to stiffen the bullet to augment penetration of the target.5e7 Open-tip bullets may have a soft point or a hollow point; the soft point is a rounded lead tip and the hollow point has a small conical hole in the lead core (Fig. 1).6 Civilian bullets are more damaging to tissue than military bullets, the reason being that, unlike military bullets (which are under strict specifications of the Hague Convention 1889), civilian bullets are not required to have (and frequently do not have) a full metal jacket, which is the metal jacket or case that completely covers the bullet tip resisting deformation and fragmentation. As a result civilian bullets are much more likely to fragment or deform in tissue particularly on striking bone. True full-metal-jacketed military projectiles are, under the Hague Convention specifications, in fact designed to wound not kill. The harsh reality of warfare

Figure 1 Internal and terminal ballistic appearances. Hollow-point bullet: (from left to right) unfired; in cross-section to demonstrate hollowed construct; and deformation following soft tissue simulant impact. Note the grooves created on the jacket of the discharged bullet rendered by the gun of source. Acknowledgements to Dr Derek Allosop (Cranfield Forensic Institute).

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during retrieval at post-mortem thereby posing a threat to surgeon or pathologist respectively. Unfortunately, they cannot be distinguished radiographically from hollow-point bullets. Rubber and plastic bullets used by some riot control police units are usually non-penetrating and are not radio-opaque. Fragments from explosive munitions, such as grenades and bombs (for example crudely constructed domestic nail bombs), as well as public building explosions due to gas leaks, for example, can produce devastating effects, although for pieces of shrapnel to cause death the casualty must be at close range to the explosive device. This is because fragments are usually irregularly shaped and have non-aerodynamic orientation and, therefore, tend to lose velocity and kinetic energy quickly in flight.6

Mechanisms of injury There are two major mechanisms of tissue damage recognized: tissue crushing (or permanent cavitation) and temporary cavitation (or tissue stretching). The sonic pressure wave leading the projectile missile has been proven to play no part in tissue injury mechanisms.6,8,11 In tissue crushing, the bullet crushes the tissue it strikes creating a permanent channel or cavity.

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The pressure at the tip of an advancing bullet is very high and can reach thousands of atmospheres.6,12 This juxta-missile pressure creates a permanent cavity and it is this that fills with blood, dead tissue, debris (such as skin, dirt, and pieces of clothing) as well fragments of the metal jacket and bone. The volume of tissue crushed and damaged is increased if the bullet yaws along its trajectory, fragments (if it strikes bone for example), or deforms (hunting bullets by law are manufactured to deform (‘‘mushroom’’) in order to increase the extent of tissue damage and the likelihood of a quick death, thereby averting prolonged suffering of the animal).6,12 Comminuted bone fragments created by the bullet, themselves become secondary missiles, increasing the extent of crushed tissue (Fig. 2).8 In temporary cavitation, the more a bullet yaws the greater the surface area that impacts with tissue. The force moves tissue radially creating a temporary cavity, the size of which will be related to the surface area exposed to the yawing missile. The temporary cavity collapses and reopens in a pulsatile manner for a few cycles. The tissue damage and temporary cavity are caused by the outward movement of the tissue, which stretches and tears parenchyma and produces localized blunt trauma and shear injury.6 Because forces follow paths of least resistance, brain

Figure 2 Unenhanced CT images of the brain of a 17-year-old female shot at close range with a handgun. (a) There is amelioration of some of the streak artefact when images are viewed on bone windows. Fragmentation of the bullet associated with in-driven bone fragments are present in the right parietal lobe and assist in determining the trajectory of the penetrating bullet. The entry site can be identified by the pattern of beveling of the outer and inner table of the right parietal calvarium. A large fragment of the bullet has remained superficial in the extracranial soft tissues. (b) The bullet was ‘‘jacketed’’ and this has separated and lodged intracranially in the midline superiorly close to the lambda. This should not be confused with a second bullet.

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parenchyma is one of the most sensitive tissues to the effects of temporary cavitation even with small handgun bullets, which produce small cavities. The degree of tissue damage produced by weapons causing large temporary cavitation depends on the tissue characteristics: high-density tissue, such as bone, and less elastic, less dense tissue, such as brain, incur more damage.8 The temporary conical cavity created pulsates several times before collapsing creating negative pressure that may also suck debris including clothing material, hair, and bone fragments into the wound. There are four basic classifications of penetrating cranial gunshot injury described, each of which relates to the degree of penetration of the cranial vault. (1) Superficial injuries: these relate to bullets trapped within the scalp or skull at the time of injury (delayed migration of bullets has been described3,4,13) without intracranial penetration. (2) Tangential injuries: these relate to bullets that graze the head without penetration of the calvarium and without lodging in the extracalvarial soft tissues. (3) Penetrating injuries: these result from bullets that enter and lodge within the cranium. In penetrating and superficial injuries, therefore, the ‘‘spent’’ bullet comes to lie within the victim. (4) Perforating injuries: these relate to through-and-through injuries in which the bullet has entered and exited the skull vault. Therefore, the cranial vault demonstrates both entry and exit wounds. More than 80% of gunshot wounds to the head are penetrating or perforating.7,13,14 The distinction between the extent of velocityrelated damage associated with penetrating injury as opposed to perforating injury is not clear cut. An early paper in the pre-CT era reported 100% perforation of brain parenchyma once penetration of the cranial vault had occurred suggesting that bullets lodged within the brain had in fact ricocheted off the opposing skull vault inner table before coming to rest.13,15 It is worth noting that bullets are not sterilized by the heat of firing and can carry bacteria from the body surface or body organ deep into the wound.

parenchymal herniation (‘‘fungus cerebri’’) and associated mass effect.

Imaging methods These are directed at assessing the neuroimaging findings with potential management and prognostic implications in penetrating craniocerebral trauma. Such findings include: exit and entry sites; intracranial fragments; missile track and its relationship to both vessels and air-containing skullbase structures; intracranial air; transventricular injury; missile track crossing the midline; multilobar injury; basal cistern effacement; brain

Plain films These can be useful in providing an overview of fractures as well as missile trajectory and location. However, the ante-mortem role is limited particularly when the scout views performed as part of the CT examination can be equally valuable. These scout views should always be assessed in addition to the formal CT images. If plain films are deemed necessary, they should only be obtained if delays to CT examination and further management will not be incurred. Post-mortem radiography is routine and invaluable, particularly where death has occurred prior to the instigation of any emergent medical management and imaging. It has proved invaluable in forensic investigation of gunshot wounds used to locate the bullet, identify the type of ammunition, document the path of the bullet, and assist in retrieval. In such cases this will be the only form of imaging available in the criminal investigative and judicial process. Non-projectile penetrating injury due to embedded implements, such as knives, scissors, or screwdrivers, for example, is also well assessed on the scout views of CT of the brain. CT CT of battle casualties was first used by Israeli medical teams in the Lebanon conflict in 1982.16 Portable CT machines were used by the US military on the ground and on hospital ships in the Gulf war.16 CT is now the primary technique used in the radiographic evaluation of civilian penetrating craniocerebral injury. All patients with craniocerbral gunshot injury should be imaged emergently by unenhanced CT whether or not there is evidence of penetration on clinical examination. Volume acquisition, as opposed to contiguous sequential sections, is the protocol of choice on current multi-detector helical systems as scanning time is rapid and the volume dataset obtained allows retro-formatting of images to variable section thickness (for skull-base assessment for example) as well as three-dimensional surface-rendered fracture depiction. The latter can be extremely illustrative for criminal investigation teams, legal professionals, and jury personnel where crosssectional imaging details may prove more difficult to convey. However, CT is not without its problems in the scenario of penetrating missile injury. Probably the most significant of these is metallic streak artefact. The high atomic weight metals, such as lead and copper, which are the typical component parts

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of bullets contrast greatly with the relative radiodensity of brain parenchyma. Repeat imaging with the gantry angled may ameliorate this effect. Post-mortem CT (and magnetic resonance) imaging is an emerging area of forensic investigation with many advantages as an adjunct to formal autopsy where gross post-mortem access may inevitably destroy tissue planes.5,17,18 Magnetic resonance imaging (MRI) The majority of military and paramilitary ammunition contains ferromagnetic materials, usually in the jacket covering the lead or antimony core. Conversely, commercial sporting bullets are usually non-ferromagnetic as are the vast majority of those used in civilian and police shootings. Theoretically, therefore, this latter group are safe for imaging with MRI and would not be subject to rotation and displacement in the strong magnetic fields of the MRI machine. In addition, non-ferromagnetic bullets do not give rise to as much susceptibility artefact.13 It has been suggested that bullets showing less deformation on CT or plain film imaging (some sporting bullets for hunting) more likely have a hard steel ferromagnetic component compared with the more easily deforming non-ferromagnetic bullets.19 Nevertheless, owing to the uncertainty of bullet construct in the vast majority of civilian shootings, the use of MRI would seem imprudent, and CT should continue to be the primary imaging mode. Angiography Catheter angiography has a very selective role to play in the imaging assessment of penetrating projectile missile injuries of the cranium. The frequency of requirement and use of the technique has been reduced somewhat by multisection CT technology and the inherent capabilities of CT angiography (CTA). Nevertheless, CTA remains susceptible to streak artefact. Angiography (either CT or catheter) may be required in those patients where there is increased risk of vascular injury: this would include those cases where the wound trajectory is through or near the Sylvian fissure and, therefore, M1 and M2 segments of the middle cerebral artery, the supraclinoid carotid artery, the vertebrobasilar vessels, the cavernous sinus region or the major dural venous sinuses.20 Peripheral branches of the middle cerebral artery followed by the anterior cerebral artery are more vulnerable in craniocerebral penetrating injury than the internal carotid artery.21 Angiography has a significant role to play in delayed vascular complications occurring following

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craniocerebral penetrating injury, most notably traumatic aneurysm formation, which can go on to rupture. Between 0.4 and 0.7% of intracranial aneurysms are caused by trauma. The exact incidence of traumatic intracranial aneurysms occurring as a result of high-velocity missile penetrating brain injury remains unclear.19,21,22 Patients with faciocranial, orbitocranial, and pterional entry wounds with concomitant haematoma or a wound profile in which fragments crossed dural compartments have been reported to have four to 10 times the risk of developing an aneurysm.20,22

Imaging findings Scalp Even in superficial and tangential injuries, all five layers of the scalp are lacerated. The three potential spaces bound by scalp layers may accumulate haemorrhage. Which potential space is most affected is somewhat age-dependent: the subcutaneous (caput succedaneum) and subperiosteal haematomata are more common in the paediatric population usually in relation to birth trauma (e.g., assisted deliveries); subgaleal haematomata are more common in adults in all types of trauma. Unlike subperiosteal haematomata, subgaleal haematomata are not confined by sutures. Subgaleal hygroma may also be evident on imaging if there has been fracture of the calvarium (or craniotomy) and dural breach. Evaluating entrance and exit wounds in perforating injuries is valuable particularly in imaging of post-mortem cases d entry wounds are usually smaller than exit wounds.14,23,24 Calvarium The point of impact and entry can be distinguished from an exit site by fracture pattern and bone bevelling. The entry site is identified by irregular stellate fractures radiating from the point of impact (cantilever fracture pattern). In penetrating, high-velocity injury, the projectile creates a rapidly advancing space-occupying volume in what is essentially a water-filled incompressible container generating extensive skull fracturing at a distance from the impact site. The fractures propagate across the skull vault faster than the bullet travels through the brain parenchyma; Puppe’s rule states that later formed fractures cannot cross, but must stop at previously formed ones. As a result, where there is an exit wound, there should be shorter fracture lines that stop at the previously formed entrance wound fractures.5,7,8,13,14 In this way, the chronological order of fracture formation (entrance versus exit) can

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frequently be derived. The inner table is usually more comminuted than the outer table at the site of impact and conversely, the outer table may be more comminuted than the inner table at the exit site. Bevelling of bone tends to occur in the direction of travel: bevelled edges are directed inwards at the site of entry (Fig. 2a) and directed outwards at the exit site.5,7,8,13,14,23,25,26 Because of the complex forces that can be generated in penetrating cranial injury, particularly of the missile type, other types of fracture pattern may be identified: isolated fracturing of the inner (or occasionally just the outer) table is referred to as spallation. Here the forces are insufficient to cause complete fracture (Fig. 3); depressed skull fractures, which are usually comminuted, may occur with missile and non-missile (e.g., knife or screwdriver) penetrating injury in which depressed fractures are associated with a higher incidence of complications, such as haemorrhage, infection, cerebrospinal fluid (CSF) leak and epilepsy; in particular, an attempt should be made to identify in-driven fragments. A type of perforating projectile calvarial injury called keyhole fracture in which the entry and exit sites lie very close together demonstrates rather dramatic focal comminution.7,14,26 Such keyhole fractures can be useful in identifying the line of fire in decomposed or fire-damaged post-mortem studies

including those performed with cross-sectional imaging. Intracranial ricochet of bullets is a particularly common phenomenon with resultant fracture of the inner table at the point of rebound: a bullet lodged within brain parenchyma with evidence of inner table spallation at some site, the author usually assumes to have ricocheted within the cranium. Superficial and tangential injuries deserve a specific mention: extensive fracturing can occur remote from the impact site, depressed and comminuted fractures are not uncommon, spallation of the inner table may be seen and a fracture type specific to tangential injury called guttering (a groove created by the bullet as it course along the outer portion of the calvarium) may be demonstrated. Skull-base fractures associated with penetrating trauma carry high early and late morbidity and mortality. CT assessment in the acute setting is clearly mandatory. There is a high incidence of CSF rhinorrhoea and otorrhoea, infection (both extraaxial and intra-axial), cranial nerve injury (in particular optic and facial nerve injury), and vascular injury to the internal carotid arteries and the cavernous sinuses.

Figure 3 Unenhanced CT images of the brain of a patient who sustained a superficial gunshot injury to the head involving the left temporo-occipital region. The bullet is lodged predominantly in the extracranial soft tissues but there is spallation of the inner table of the skull vault evident at the site of bullet impaction.

Intracranial The injuries incurred in the acute setting can to some degree be anticipated radiologically. These are: pneumocephalus, extra-axial haemorrhage (which may be extradural, subdural, subarachnoid, and intraventricular), parenchymal laceration, intraaxial parenchymal haemorrhage, haemorrhagic and non-haemorrhagic contusion, dffuse axonal or shear injury, infarction, oedema, and herniation (Fig. 4). Parenchymal air remote from the missile track (permanent cavity) carries a poor prognosis.27 Extradural haematomas are somewhat underreported in the setting of penetrating injury be it missile or non-missile. This may, in part, be due to misidentification as a subdural collection as the typical lenticular configuration may be absent. Delayed subdural haemorrhage or subarachnoid haemorrhage (SAH) suggests the presence of delayed or late complications, such as traumatic aneurysm formation and rupture or consumptive coagulopathy.13,28,29 Parenchymal laceration is generally conical in shape with the base situated at the entrance site and the laceration tapering inwards along the missile track. Not all bullet tracks are haemorrhagic so the presence of hyperdensity cannot be relied upon to identify the entire course of the bullet intracranially, particularly where

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Figure 4 Close range shotgun injury to the face. The billiard-ball effect of the pellets within a single shotgun shot causes dramatic devastating injury as a result of pellet spread despite close-range of the shot. The billiard-ball effect also means that the range of the shot cannot be estimated from the radiographic pattern of pellet spread. Unenhanced CT images of the brain: (a) soft-tissue windows illustrate the marked obscuration of detail created by streak artefact from the innumerable pellets. However, right hemispheric subdural haematoma, intraventricular blood, and subfalcine herniation to the left is evident. (b) Bone windows clarify the location of intracranial and extracranial pellets. Marked orbito-cranial injury was evident (images not shown). (c) Imaging performed 9 days after injury. An extensive right craniectomy has been performed to accommodate cerebral swelling and alleviate elevated intracranial pressure. Mild ‘‘fungus cerebri’’ is evident as brain parenchyma ‘‘mushrooms’’ through the craniectomy site as a result of persisting brain swelling.

ricocheting has occurred within the cranial cavity. The length of bullet track, the location, and the number of ricochets have a direct correlation with prognosis. Multilobar involvement carries a poor prognosis.30,31 Parenchymal haematomata and contusions may be seen at a distance from the missile track.

Intraventricular haemorrhage confers a poor prognosis30,31; its presence in any trauma scenario signifies major brain injury.13,32 Carey12 reported respiratory effects in animal models even with low energy projectile injury and even with bullets that were more than 2 cm from the brainstem. It would appear that

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respiratory arrest and haemodynamic lability may still occur as a result of brainstem dysfunction, even if a missile enters the brain without causing significant disruption presumably through transfer of cavitating (temporary) energy. Secondary features of such insults include varying degrees of hypoxic ischaemic encephalopathy in those casualties that survive. The most important and most frequently encountered delayed complications that can occur in penetrating craniocerebral injury patients are infection, CSF fistula, hydrocephalus, and delayed haemorrhage. Any form of intracranial and extracranial infection may occur: scalp wound infection, osteomyelitis, epidural and subdural empyema, meningitis, ventriculitis, cerebritis, and frank cerebral abscess. Despite the antibiotics era, the incidence of complicating infection is between 5e7% with 90% of cases developing within 3e5 weeks, although occurrence may be delayed for years if the foreign body is retained.16,36e38 Significant risk factors include external wound infection, CSF fistula, paranasal sinus injury, and retained organic material, such as hair or clothing. Post-traumatic epilepsy following penetrating cranial injury warrants exclusion radiologically of evolving intracranial infection. In this setting (i.e., non-acute), MRI may be the most appropriate imaging method and is not necessarily contraindicated at this stage. CSF fistula is common following bony breach and fractures of the anterior skull base with involvement of the paranasal sinuses or of the lateral skull base with involvement of the middle ear cavity and mastoid. It is most frequently encountered in orbitocranial penetrating gunshot injury.39 CT cisternography remains the mainstay of radiological assessment. Other reported complications include bullet migration and embolization and lead toxicity. Bullet migration usually indicates development of an intracranial (usually intraparenchymal) abscess, but they may also move in evolving haematoma and if intraventricular in location. Such migration may lead to secondary delayed complications, such as obstructive hydrocephalus.13,19,33e35 Embolization of bullets, and in particular, because of their small size, shot gun pellets, may occur when the projectile enters the arterial or venous circulation even in solely peripheral non-intracranial gunshot injury. Arterial embolization into the intracranial circulation with subsequent infarction has been recorded.4,19 The majority of lead fragments in soft tissue become walled-off by chronic inflammatory and fibrous tissue and do not cause

problems. However, lead toxicity, including lead encephalopathy, from retained bullets has been reported.19 In addition, copper-jacketed lead bullets surgically implanted into cats have resulted in copper resorption and neurological deterioration.19

Non-missile craniocerebral penetrating injury Stab wounds of the brain are relatively uncommon, particularly because the osseous calvarium usually provides an effective protective barrier. However, there are areas of relatively thin bone where nonmissile penetrating implements, such as knives, screwdrivers, scissors, pencils, and even stiletto heels may penetrate. Particular areas include the orbita and the temporal regions. Nevertheless, if a sharp implement is thrust forcefully enough it will penetrate any part of the skull vault. Because brain injury is usually restricted to the wound track, casualties usually make it to hospital as opposed to succumbing at the scene, and many survive, but with varying morbidity. The main focus in the immediate assessment clinically and radiologically is the location of the entire intracranial component of the penetrating implement, most importantly in relation to vital areas, particularly the brainstem, and the relationship to intracranial (and where affected, extracranial) vessels. Assessment of cerebral oedema, particularly where penetrating injury involves the posterior fossa with the well-recognized potential for rapid deterioration due to secondary complications, such as obstructive hydrocephalus and coning, must also be assessed. Despite potential problems associated with streak artefact from metallic penetrating weapons, unenhanced CT of the brain with attention to the planning scout images remains the first-line imaging investigation (Fig. 5).40,41 This should be supplemented by CT angiography of the head to attempt to identify vascular conflict even despite the potential for artefact, and particularly if the course of the penetrating implement traverses major vessel territory or if there is significant haemorrhage. If indicated, formal catheter angiography of the intracranial vessels can be performed on discussion with neurosurgical and critical care clinicians. Immediate imaging should also address the potential for early or delayed complications, including progressive haemorrhage, arterial dissection injury, and secondary ischaemic complications, brainstem related apnoea and associated hypoxic ischaemic encephalopathy (Fig. 6), infection, traumatic aneurysm formation, and rupture and CSF fistula.42,43

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Figure 5 A 16-year-old male who sustained a non-missile craniocerebral penetrating injury. Unenhanced CT images of the brain: (a) the implement is clearly identifiable on the scout image. (b) Bone window settings enable identification of a tiny intraparenchymal in-driven bone fragment adjacent to the blade anteriorly. These can act as foci of inflammation and infection, as well as epileptogenic source postoperatively, if not removed during debridement. The treating neurosurgeon should be alerted to their presence.

Penetrating spinal injury The incidence of penetrating gunshot injuries to the spine and spinal cord has increased in urban areas.44 Gunshot wounds to the spine account for 13e17% of all spinal cord injuries each year.45 The commonest site of involvement is the thoracic region but clearly most devastating is the cervical spine. The two major primary mechanisms of injury are identical to craniocerebral projectile penetrating injury, namely, tissue crushing or permanent cavity formation and temporary cavity formation.46 Approximately one-third of spinal cord injuries occurring secondary to penetrating missile trauma will involve retention of bullet fragments within the spinal canal. Lead poisoning has been reported with retained bullets, particularly where lodged in synovial joints,19 such as facet joints, but also documented with bullets lodged in the intervertebral discs.45,47 Spinal cord necrosis around copper fragments implanted within the dura of rabbit spinal cords has been demonstrated in in-vivo experiments (with minimal changes with aluminium fragments and extradural fragments), which has implications for retained jacketed bullets. As with craniocerebral gunshot penetrating injury, MRI remains controversial in the evaluation of these injuries. The main concern remains migration of potentially ferromagnetic components of the bullet. Another concern is heating around the bullet. Consequently, plain films and CT remain the imaging methods of choice for localization and

Figure 6 A 16-year-old male who presented with a penetrating knife wound to the left occiput (imaging not shown). Unenhanced CT brain image of the examination performed less than 12 h after the knife was surgically removed. The intraparenchymal haemorrhagic knife track, as well as intraventricular and extra-axial blood, is evident, but the generalized bland greyewhite matter appearance demonstrated is a poor prognostic sign indicative of severe hypoxic ischaemic decompensation. The patient died a few hours after this examination.

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assessment of associated bone injury and in-driven bone fragments. As an adjunct to unenhanced CT assessment, CT myelography can also be performed particularly to identify complicating intraspinal extramedullary haematomata as well as cord swelling or disruption (Fig. 7). Complications of gunshot injury and surgical decompression include CSF leaks, infection, such as osteomyelitis or meningitis, and arachnoiditis.48 Post-traumatic syringomyelia may occur earlier than in other types of blunt spinal cord injury.49 Spinal arachnoid cysts, cord atrophy, and tethering may also occur as delayed features.49 Other delayed complications include disc herniation where the posterior longitudinal ligament has been disrupted, pain (particularly with conus and cauda equina level injuries), which appears unaffected by bullet removal, neuropathic (Charcot) arthropathy, and finally bullet migration.50 Non-missile penetrating injury of the spine may be accidental impalement injury or criminal inflicted injury. Stab wounds remain relatively infrequent: in one series by Peacock et al.,52 they accounted for 25% of spinal cord injuries (after motor vehicle accidents and gunshot injuries) with

the thoracic region most commonly affected. However, the lead author has encountered more cervical spine stab injuries compared to thoracic (Fig. 8). Both stab wounds and gunshot wounds may result in neurological deficit from spinal cord compression by extramedullary haematoma, herniated disc fragments and displaced bone fragments, as well as direct cord parenchymal damage.46 Recovery rates are higher in stab wound injuries to the spine compared with gunshot injuries to the spine as tissue injury and oedema are less marked.51 Even though a BrowneSequard type of neurological deficit can occur due to hemitransection of the cord ipsilateral to the wound,53 transient neurological deficit can occur on the side contralateral to the stab wound due to contrecoup parenchymal contusion as the cord is forced into the contralateral side of the spinal canal.46 MRI is invaluable in assessing parenchymal damage if the ferromagnetic penetrating implement has not been retained. Contusion, haematoma, or a linear knife track may be identified. Haematoma may be intramedullary, subarachnoid, subdural, epidural, and extraspinal. STIR (short tau inversion recovery) or T2-weighted, fat-saturated sequences are particularly useful for identifying spinal ligamentous disruption involving the anterior and posterior

Figure 7 A 28-year-old male shot with a handgun. Axial image through the T12 vertebra level of a CT myelogram performed 1 day after injury as an alternative to MRI because of a retained bullet. The bullet (not shown) had traversed the left hemithorax and upper abdomen, fractured the left posterior elements of L1 vertebra, and lodged within the spinal canal. As well as subdural and subarachnoid haematoma most notable on the left, note the associated in-driven bone fragments. There is an epidural collection of contrast medium on the right as a result of CSF leak secondary to dural tear.

Figure 8 Stab injury sustained to the back of the neck with a knife. The patient presented with BrowneSequard syndrome. Sagittal, T2-weighted, MRI image demonstrates surgical emphysema in the paraspinal soft tissues and a linear transverse fracture of the posterior cortex and body of C4 with marrow oedema caused by penetration of the blade of the weapon. A high signal track consistent with the hemi-transection of the cervical spinal cord, as well as surrounding parenchymal oedema is present.

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longitudinal ligaments, ligamentum flavum, interspinous ligaments, and ligamentum nuchae. This aids in localizing the course of the penetrating track in stab injuries.54,55 The majority of penetrating spinal injuries (missile and non-missile) are stable. CSF leaks occur in up to 4% of stab wounds to the spine, but the majority stop without surgical intervention. Post-traumatic meningocoeles may also be demonstrated. MRI may identify small foci of susceptibility artefact caused by metallic fragments from the offending weapon, which are not revealed on plain film assessment or CT. These areas of susceptibility can be a help, not a hindrance, by determining the depth of penetration of the weapon. However, such susceptibility artefact should be distinguished from bone fragments and air bubbles associated with traumatic pneumomyelogram.51

Conclusion Penetrating trauma to the neural axis is a challenging case load that presents not infrequently to the practising general and specialising radiologist. The incidence, particularly in relation to criminal penetrating injuries, remains significant. Given the fact that the majority of these patients will require cross-sectional imaging as part of acute management, as well as for the management of potential medium to long-term complications, it is imperative that the radiologist is aware of pertinent imaging features and protocols relevant to both medical management and jurisprudence.

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