Postprocessing Imaging Techniques of the Computed Tomography Angiography in Trauma Patients for Preprocedural Planning

Abstract

Computed tomography provides a wealth of diagnostic information in the trauma patient including the presence of organ, bone, and vasculature injuries for the rapid triage of trauma patients. In the context of interventional radiology, appropriately protocoled studies can be reviewed for vascular injury and help focus the angiographic assessment of bleeding patients to ideally achieve earlier hemostasis. This article outlines various image-processing techniques such as multiplanar reformats, curved planar reformats, maximum intensity projections, and volume rendering to identify and more thoroughly characterize vascular injuries as a preprocedural planning tool to expedite endovascular hemostasis in a case-based format.

Blunt and penetrating trauma may lead to vascular injury with associated hemorrhage and hemodynamic instability. Uncontrolled hemorrhage is life-threatening and accounts for nearly 40% of trauma deaths. ^{1 2} Hemorrhage is the most common cause of death within the first hour of trauma care center and accounts for nearly 50% of deaths in the first 24 hours. ² One of the principles of damage-control resuscitation is to “avoid delays in surgical, endoscopic, or angiographic hemostasis.” ³ While delays in hemostasis may result in frank exsanguination, the sequelae of hemorrhage may also result in coagulopathy and higher rate of mortality. ^{3 4} Temporizing measures to reduce the rate of hemorrhage include tourniquets, pelvic binders, and the resuscitative endovascular balloon occlusion of the aorta (REBOA) catheter which can be applied in the interval between presentation and definitive management. ⁵

In the context of trauma, definitive management may be achieved through either operative or endovascular means. The critical importance of early hemostasis achieved through endovascular treatment is recognized by the American College of Surgeons Committee on Trauma in the criteria for trauma center verification which states: “In Level I and II trauma centers qualified radiologists must be available within 30 minutes to perform complex imaging studies, or interventional procedures.” ⁶ Significant efforts to this end have been made to improve communication and workflows to reduce barriers that impede time to angiography. ^{7 8}

Computed tomography (CT) has transformed the trauma workup of the patient beyond the primary survey and initial chest and pelvic radiographs, allowing, if desired, for the detection of traumatic injuries from head to toe. ⁹ CT angiography with the rapid acquisition of high-resolution images following the intravenous administration of iodinated contrast material further allows for the survey of the vasculature. The presence of active hemorrhage on CT is recognized as a risk factor for increased morbidity and mortality.

Vascular injury on CT is most ominously characterized by contrast extravasation which indicates active hemorrhage. Additional CT findings, however, also suggest vascular injury and may be clinically significant. These findings include direct signs of injury such as pseudoaneurysm, occlusion, arteriovenous fistula, thrombus formation, intimal dissection, spasm, and external compression. Indirect signs of injury may include perivascular hematoma, projectile near a neurovascular bundle, or shrapnel less than 5 mm from a vessel.

This review will focus on CT imaging in the context of trauma and suspected vascular injury with discussion on postprocessing imaging techniques to quickly identify the presence and site of vascular injury. In theory, these techniques may focus on angiographic procedural efforts for more rapid embolization and reduce time to hemostasis.

Computed Tomography and Trauma

CT studies performed in the context of trauma must be optimized to evaluate for bony injury, injury to the visceral organs, as well as vascular injury. The extent of vascular injury may be more easily characterized with a multiphasic scan as active hemorrhage would be expected to accumulate over time on delayed images relative to a single phase of arterial phase images. An example of a trauma protocol CT would be noncontrast images acquired in the axial plane followed by the acquisition of images after the administration of iodinated contrast material in the arterial and delayed phase. Alternatively, to achieve the arterial phase and venous phase in the same set of images, a split bolus of contrast may also be performed as is commonly done at our institution. First, a 70-mL of contrast bolus is administered at 4 mL/second. Thirty seconds later, a second bolus of 60 mL contrast is injected at 4 mL/second. Images are obtained 70 seconds after the initial contrast bolus. This provides images with both arterial and venous opacification in a single acquisition. Delayed phase images can be obtained on an as needed basis. Details of the trauma protocol can be found in Table 1 .

Table 1. Trauma body computed tomographic protocol.

Parameter	Specifications
Contrast injection protocol	Split bolus. 70 cc bolus at 4 cc/sec followed by 60 cc bolus at 4 cc/sec with a 30-second pause in between. 30 cc saline bolus chase
Scan trigger	70 seconds after the start of first contrast bolus
Coverage	Thoracic inlet to pubis symphysis
Imaging mode	Helical
Tube rotation time	0.5 second
Pitch	0.984
kVp	120
mA	Auto; noise index 16
Reformats	Axial, sagittal, and coronal. 3D reconstructions as required
Slice thickness	One axial reconstruction at 1-mm slice thickness. Other axial, sagittal, and coronal reformats at 2.5 mm

Abbreviations: kVp, kilovoltage peak; mA, milliampere; 3D, three-dimensional.

Vascular Injury

CT angiography has a high sensitivity for the detection of active hemorrhage with an estimated threshold of 0.35 mL/min, which is more sensitive than the estimated threshold in digitally subtracted angiography of 0.5 mL/min. ¹⁰ Thus, CT with the rapid acquisition times and universal availability in trauma centers is routinely performed in patients deemed stable enough to tolerate the examination.

Multiplanar Reformats

Multiplanar reformats (MPRs) refer to the presentation of three-dimensional (3D) data acquired in one plane reconstructed in an alternate plane. MPRs in the coronal and sagittal planes are routinely included in the standard stacks of images for interpretation of CT studies acquired in the axial plane. The MPRs in orthogonal planes provide an efficient means to demonstrate complex anatomic relationships. Several picture archiving and communication systems (PACS) now support the dynamic reconstruction of MPR images and these can be performed in any desired obliquity to optimize anatomic relationships for the detection of vascular irregularities.

Curved Planar Reformats

As arteries rarely remain within the confines of a single plane, they can be difficult to fully evaluate with MPR. Reconstruction of curved planar reformats (CPRs) can be performed along the course of a selected artery using contiguous axial images, MPR images, or volume rendering as the reference image to define the course of the CPR. ^{11 12}

Maximal Intensity Projection

Maximal intensity projection (MIP) is a method to depict 3D data in a two-dimensional (2D) image. With this technique, parallel mathematical rays are cast through a 3D structure and projected onto a 2D surface in the desired viewing direction. The density in the voxel depicted in the 2D image represents the maximum voxel intensity (density in the case of CT) encountered by the ray as it traverses the 3D data. ¹³ The 3D data may represent the entire volume of the structure or may be a specified slab thickness. This technique is particularly well suited to the CT angiography as the contrast-enhanced arteries are high density. MIP images may be especially useful in preprocedural planning as the MIP images may demonstrate subtle evidence of hemorrhage or vessel injury that is not readily apparent on source images.

Volume Rendering

Volume rendering is a data visualization technique to represent a 3D volume in a 2D image. Relative to the MIP technique, volume rendering has the advantage that it can also represent soft tissues in addition to bones and arteries. The volume rendering technique maintains the 3D relationships that may be dropped with MIP technique. ¹⁴

Cinematic Rendering

Cinematic rendering is a technique inspired by the computer animation and entertainment industries that generates photorealistic images of the human body from CT or MR source images. The technique allows for the integration of light scattered from all possible directions along a ray; thus, the technique more faithfully reproduces the path of light and scatter of light between adjacent anatomic structures leading to photorealistic images.

Case Presentations

Case 1

A 23-year-old man involved in a motorcycle crash presents with pelvic fracture and transient hypotension and is brought for emergent pelvic angiography. Immediately prior to pelvic angiography, MIP images reconstructed from images obtained in the portovenous phase in the contralateral oblique plane demonstrated abrupt cutoff of a branch of the anterior division of the left internal iliac artery. Pelvic angiography confirmed the finding and the artery was subsequently coil embolized. No further arterial injuries were noted angiographically ( Fig. 1 ).

Fig. 1.

A 23-year-old man with traumatic injury following motorcycle accident. ( a ) Oblique maximal intensity projection (MIP) images of the pelvis demonstrating abrupt cutoff of a branch of the anterior division of the left internal iliac artery (white arrowhead). ( b ) Pelvic angiography confirms abrupt cutoff of the branch noted on MIP images (white arrowhead). ( c ) Postembolization angiographic images demonstrating coil embolization of the arterial stump (arrow—embolization coil). Spasm (*) noted in the distal branch on postembolization images.

Case 2

A helmeted 60-year-old motorcyclist involved in a motorcycle versus car collision. He was noted to have multiple pelvic fractures but remained hemodynamically stable and was initially managed nonoperatively in the intensive care unit. A CT angiography was performed at the time of admission without evidence of contrast extravasation. The following day, he became hemodynamically unstable with dropping hemoglobin and need for transfusion. An emergent pelvic angiography was performed. Review of the axial images from the CT scan demonstrated possible injury of the right inferior gluteal artery. Vessel injury is more easily recognized on images that provide a longitudinal view of the vessel as with oblique MPR, MIP, and CPR images. Pelvic angiography confirmed the presence of an inferior right gluteal artery injury and the injured vessel was treated with coil embolization ( Figs. 2 and 3 ).

Fig. 2.

A 60-year-old man involved in motorcycle versus car collision with right inferior gluteal artery injury. ( a ) Two images of the inferior gluteal artery. The top image demonstrates a central filling defect consistent with thrombus and an image 5 mm distal demonstrating more extensive intraluminal thrombus (arrowheads). ( b ) An oblique multiplanar reformat image demonstrates abrupt cutoff of the inferior gluteal artery (arrow) and ( c ) MIP images (8 mm slab) more clearly demonstrate length of occluded inferior gluteal artery (arrow). ( d ) Two curved planar reformat images in orthogonal planes demonstrating the occluded right inferior gluteal artery (arrows). Angiographic images with gluteal artery injury pre- ( e ) (arrow—injured artery) and post- ( f ) coil embolization (asterisk).

Fig. 3.

Volume rendering of the pelvis and right inferior gluteal artery injury. ( a ) Standard volume rendering and ( b ) volume-rendered image inspired by cinematic rendering. Arrow—level of arterial injury.

Volume rendering of the bony pelvis and arterial vasculature similarly demonstrates the abrupt cutoff of the likely dissected right inferior gluteal artery. The volume renderings may be susceptible to artifact from suboptimal contrast opacification that may mimic vascular injury and thus findings should be confirmed with source images. The volume rendered images more clearly demonstrate the 3D relationships between bone and artery which may be obscured with MIP images.

Case 3

A 55-year-old restrained driver involved in a motor vehicle collision presented with right chest pain. CT of the chest demonstrated a large right breast hematoma. The right breast hematoma continued to enlarge and the patient was brought for angiography. CT images were reviewed and MIP images were constructed to more easily identify the injured vessel arising from a perforator branch off of the right internal mammary artery. Selective catheterization and angiography of the right internal mammary artery of same configuration are seen on MIP images. The perforating vessel was embolized with glue ( Fig. 4 ).

Fig. 4.

A 55-year-old woman, restrained driver, in a motor vehicle accident with increasing right breast hematoma. ( a ) MIP images demonstrate contrast pooling in right breast hematoma (open arrow). Solid small white arrow marks the origin of the supplying perforator vessel arising from the right internal mammary artery. ( b ) Focused MIP at the site of extravasation and ( c ) angiographic image following selective catheterization of the right internal mammary artery demonstrating the same configuration as MIP images (arrow—origin of feeding vessel).

Case 4

A 54-year-old woman with history of atrial fibrillation on chronic anticoagulation presented to the hospital following a fall. CT demonstrated a large left retroperitoneal hematoma with active contrast extravasation. Axial MIP images more clearly demonstrate contrast extravasation arising from the iliolumbar artery. Other sites of extravasation were noted on axial images (not shown). Full-thickness MIP and volume rendering further demonstrated multifocal extravasation in the distribution of the left L4 lumbar and left iliolumbar arteries, which was confirmed by selective angiograms. Both bleeding arteries were treated with gelatin slurry embolization followed by coil embolization. Also evident on coronal MIP images, volume rendering and aortogram is residual dissection in patient with history of Stanford type A dissection with proximal surgical repair ( Fig. 5 ).

Fig. 5.

A 54-year-old woman on chronic anticoagulation for atrial fibrillation with history of repaired type A dissection with residual abdominal component status post fall. ( a ) Axial CT image demonstrates retroperitoneal hematoma with focus of contrast extravasation (white arrowhead). ( b ) Axial MIP image (8-mm slice) which better demonstrates contrast extravasation with better demonstration of feeding artery. ( c ) Full-thickness MIP with bone subtraction and ( d ) volume-rendered image with multifocal extravasation (arrowheads). Feeding arteries appear to be left iliolumbar (small white arrow) and left lumbar artery (large white arrow). ( e ) Aortogram with multifocal extravasation (arrowheads) likely arising from the left iliolumbar artery (small arrow) and L4 lumbar artery (large arrow). Note the close correlation of vascular anatomy between the CTA ( c and d ) and catheter angiography ( e ). ( f ) Selective angiogram of the iliolumbar artery demonstrating active extravasation (arrowheads).

Case 5

A patient presents following blunt abdominal trauma. A CT was performed in the hemodynamically stable patient. Both MIP and volume rendered (VR) images reveal a right hepatic artery pseudoaneurysm with excellent correlation with angiographic images ( Fig. 6 ).

Fig. 6.

A patient presents with blunt abdominal trauma. ( a ) MIP and ( b ) Volume-rendered images from a contrast-enhanced CT show a contrast-filled outpouching adjacent to the right hepatic artery (arrows). ( c ) Digitally-subtracted angiogram (DSA) image demonstrates a pseudoaneurysm arising from the right hepatic artery (arrows) with excellent correlation of the findings on both modalities.

Case 6

A 66-year-old man motorcyclist involved in a motorcycle versus car collision was brought by the emergency medical services to the emergency department and found to be hypotensive. An MPR in the candy cane oblique provides excellent views of the aorta and with easy characterization of the position of the traumatic aortic transection. Mediastinal hematoma and flap-like projections into the aortic lumen are well visualized on axial images. Angiographic images correlated well with findings seen on CT exam ( Fig. 7 ).

Fig. 7.

A 66-year-old male motorcycle driver who was hit by a car and presented with hypotension. ( a ) Oblique (candy cane oblique) reformatted image of chest CT demonstrates discontinuity in the aortic wall with a circumferential contour bulge, suggesting aortic transection, at isthmus (arrow). The ( b ) axial image demonstrates flap-like projections in the aortic lumen (arrow) and surrounding mediastinal hematoma. The digitally-subtracted angiogram (DSA) images ( c, d ) show corresponding contrast-filled outpouching at the inferior aspect of the aorta (arrows). An aortic stent graft was placed.

Case 7

A 79-year-old woman with left blunt abdominal trauma secondary to fall with her left side striking the bathtub. She suffered a grade 5 injury to splenic laceration characterized by CT. Coronal MPR images accurately characterize and predict subsequent findings on angiography ( Fig. 8 ).

Fig. 8.

A 79-year-old female, who fell while getting up from the toilet and hit her left side on her bathtub, presented with hypotension. Complex splenic injury with active extravasation (arrows; a, b ) and hemoperitoneum consistent with an AAST grade 5 injury. ( c ) Digitally-subtracted angiogram (DSA) image confirms the active contrast leakage at corresponding location (arrow).

Case 8

A 53-year-old man pedestrian struck by a motor vehicle with polytrauma including bilateral lower extremity fractures of the bilateral tibia and fibula. Lower extremity runoff CT angiography showed vascular injury involving the right popliteal artery and proximal three-vessel runoff characterized by absence of contrast opacification. MIP images accurately depict the popliteal artery cutoff and reconstitution of the three-vessel runoff observed on the subsequent angiography. The popliteal artery was repaired with a self-expanding stent with restoration of three-vessel runoff following peroneal artery angioplasty ( Fig. 9 ).

Fig. 9.

A 53-year-old male pedestrian struck by vehicle with polytrauma including fractures of bilateral tibia and fibula presented with a cold right lower extremity. ( a and b ) Sequential axial CTA images show absence of contrast in the popliteal artery and proximal run-off vessels in the right lower extremity (arrows). ( c, d ) Bilateral tibia and fibular fractures are seen on sequential coronal CT images (arrows). ( e ) MIP and ( f ) curved planar reformat images demonstrate occlusion/dissection of the distal right popliteal artery with poor distal run-off (arrows). The digitally-subtracted angiogram (DSA) images ( g, h ) demonstrate the segmental occlusion with reformation at tibioperoneal trunk (arrows). The occlusion was crossed by a guidewire (arrow; I ) and a balloon angioplasty with stenting was performed (arrow; J ).

Case 9

A 19-year-old man with penetrating trauma to the bilateral lower extremities secondary to three gunshots. A CT of the lower extremity runoff study was performed to characterize suspected vascular injury. Axial images and CPR of the popliteal artery and tibioperoneal trunk show pseudoaneurysm at the tibioperoneal trunk and no flow of contrast distally. Surgical exploration found injury to 4-cm segment of artery which required repair with reversed saphenous vein graft. Completion angiogram, after surgical repair, shows restoration of flow in the repaired segment with three-vessel runoff is shown in Fig. 10 .

Fig. 10.

A 19-year-old male, who was brought to hospital with three gunshot wounds to bilateral lower extremities. ( a ) Axial CTA image demonstrates a fracture in the fibula with a contrast-filled lobular structure at the expected location of tibioperoneal trunk (arrow). A pseudoaneurysm at the tibioperoneal trunk is seen on b—arrow . Curved planar reformat image with absence of contrast flow distally. A 4-cm segment of tibioperoneal trunk was repaired with a reversed saphenous venous graft. ( c ) A completion angiography was done via the great saphenous vein (GSV) conduit (arrow; 4-Fr micropuncture catheter) which revealed a widely patent proximal and distal anastomosis with intact three-vessel tibial runoff.

Conclusion

Uncontrolled active hemorrhage may be life-threatening depending on the rate and location of the hemorrhage. CT angiography has a high sensitivity for the detection of vascular injury and active bleeding with a more sensitive threshold of detection than first-order digitally subtracted angiography. Diagnostic radiologists routinely employ imaging processing techniques such as MIP images and volume rendering to characterize suspected sites of vascular injury. These advanced image-processing techniques are also powerful tools in the hands of the interventional radiologist, as the emergent nature of these studies may dictate that the patient is transported emergently for angiography prior to the complete evaluation by the diagnostic radiologist. Moreover, these tools provide the potential to focus the angiography to the precise site of hemorrhage and reduce the time to embolization and hemostasis yielding the highest probability of survival.