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Ann Thorac Surg 2006;81:1339-1346
© 2006 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Critical Evaluation of Chest Computed Tomography Scans for Blunt Descending Thoracic Aortic Injury

^a Michael E. DeBakey Department of Surgery, Baylor College of Medicine, Houston, Texas
^b Department of Radiology, Baylor College of Medicine, Houston, Texas

Accepted for publication November 3, 2005.

^_* Address correspondence to Dr Wall, Michael E. DeBakey Department of Surgery, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030 (Email: mjwall{at}houston.rr.com).

This article has been selected for the open discussion forum on the CTSNet Web Site: http://www.ctsnet.org/sections/newsandviews/discussions/index.html

 

	Abstract

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

 
BACKGROUND: Although aortography has been the long-held "gold standard" for diagnosis of traumatic blunt aortic injury, advances in imaging technology offer less-invasive, more-rapid, and potentially more cost-effective evaluation. The purpose of this study was to review this hospital's experience with the screening and diagnosis of blunt aortic injury with emphasis on the critical evaluation of computed tomography (CT) scans for defining descending thoracic aortic injury.

METHODS: A retrospective single-center analysis of all patients undergoing aortography to evaluate for blunt aortic injury between January 1, 1997, and August 31, 2004, was performed. A policy of relying on CT scans to definitively diagnose blunt aortic injury was not in force, and all patients with positive, equivocal, and negative screening CT scans with significant injury mechanism underwent subsequent aortography; this contributed to an unbiased analysis. A subgroup of patients imaged with the latest generation multislice CT scanners (July 1, 2003, to August 31, 2004) was separately analyzed with rapid three-dimensional reconstruction.

RESULTS: Of 856 aortograms, 206 (24.1%) were preceded by chest CT scan. Of 31 patients with confirmed aortic injury, 20 had undergone CT scan with 16 positive for definite injury, 3 positive for possible injury, and 1 false-negative study. Of the 206 patients scanned, 114 (55.3%) showed possible injury, 76 (36.9%) were negative, and 16 (7.8%) were positive. Only 3 of the 114 with possible injury (2.6%) were true positives whereas 1 of the 76 negative scans (1.3%) was a false negative and all 16 positive scans were true positives. These data for CT scan imaging result in a sensitivity of 95%, a specificity of 40%, a positive predictive value of 15%, and a negative predictive value of 99%.

CONCLUSIONS: Chest CT is an acceptable screening tool based on prerequisite high sensitivity and ease of performance in the trauma patient suspected of having a descending thoracic aortic injury. Although the excellent negative predictive value resulted in an algorithm change at this institution, there were a significant number of equivocal scans that required subsequent aortography. Three-dimensional software reconstruction of the aorta can aid in diagnosing blunt aortic injury when findings are equivocal, but there will continue to be artifacts and limitations that require aortography for clarification.

Blunt aortic injury (BAI) is second only to head injury as the most common cause of death in blunt trauma [1]. Mortality remains high, with most deaths occurring at the scene of the accident (85%). Those who do survive until presentation at a trauma center often have other associated injuries that can distract from this life-threatening but treatable entity [2]. With the advent of rapid, high-resolution multislice computed tomographic (CT) scanners, a stable trauma patient can undergo a head, cervical spine, abdomen, and pelvis scan in less than 10 minutes. Because very little time is required to obtain additional slices, patients with significant mechanism of injury or questionable mediastinum by chest radiograph can now undergo screening chest CT scanning in addition to more traditional CT scan imaging. Although several studies have demonstrated that CT scan of the chest is an excellent screening tool for BAI, tremendous controversy surrounds its use as a sole diagnostic modality [3–11].

Computed tomography of the aorta is not without disadvantages and can be affected by technique of study, cardiac and lung movement, volume averaging, inconsistent vascular opacification, and congenital anomalies [7, 12]. Difficulty is encountered when relying on CT scanning for standalone diagnostic imaging, particularly because imaging artifacts or overinterpretation of findings may lead to a high number of false-positive studies and a low positive predictive value. Secondary to this bias, longstanding protocol at the Ben Taub General Hospital calls for aortography to confirm aortic injury after a positive or equivocal screening chest CT.

The purpose of this study is to review this institution's experience with the screening and diagnosis of BAI, with emphasis on the critical evaluation of the true predictive value of positive or equivocal chest CT scans. The latest experience with three-dimensional aortic reconstructions is reported, including their limitations. Finally, a practical imaging algorithm is presented that includes CT scanning with three-dimensional reconstruction to aid in the suspected diagnosis of descending aortic injury and relies on aortography for confirmation of injury.

	Material and Methods

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

 
All patients who underwent aortogram as a screening modality for BAI from January 1, 1997, to August 31, 2004, were included in the analysis. This study was not submitted for institutional review board approval because individual patient consent was waived and this was a quality review study with no patient identifiers used. During the study period any patient who had a widened mediastinum by chest radiograph or significant mechanism with suspicion of injury was studied either directly by aortogram alone or in combination with screening CT scan. Because of a new technology evaluation and a policy of not relying on CT scans to diagnose BAI, all patients with direct signs of aortic injury by CT (positive study), equivocal CT studies (including mediastinal hematoma), and negative CT studies with significant injury mechanism underwent subsequent aortography. This presented a unique opportunity to evaluate for both false-positive and false-negative scan results and to obtain meaningful data using aortogram as a gold standard. Aortogram was not used selectively, based on the likelihood of injury as read by CT scan.

Demographic, imaging, and outcome data such as age, sex, mechanism of injury, chest radiograph findings, CT scan findings, management of aortic injury, operative findings, and outcomes were also collected and stored in database format. Two types of CT scanners were used during the study period; a spiral scanner (January 1, 1997, to September 1, 2003) and a newer generation, multislice helical scanner (July 1, 2003, to August 31, 2004). With aortography being the gold standard for defining injury, the results of the chest radiographs, CT scans, aortograms, and operative findings were compared to determine their relative utility.

Technique of Aortography
In all cases, the common femoral artery was cannulated using modified Seldinger technique, and a 5F pigtail catheter was advanced to the proximal thoracic ascending aorta. The examination consisted of three digital subtraction angiography runs, in the anteroposterior, left anterior oblique, and steep right anterior oblique–right lateral projections. Each run was typically performed with an injection of 20 to 30 mL/s for a total 40 to 50 mL of nonionic contrast, for an average of 120 to 150 mL of contrast used, with images captured at 3 to 6 frames/s. When available, findings of the screening studies (chest radiograph or CT scan) were used by the radiologist in helping to make a final interpretation of the aortogram.

Technique of Chest Computed Tomography
From January 1, 1997, until June 1, 2003, a General Electric single-slice spiral CT scanner was used in the emergency department at Ben Taub General Hospital. Starting in July of 2003 a high-speed multislice scanner (Somatom Sensation 16 slice, Siemens Medical Solutions, Malvern, PA) was used for all trauma imaging. The protocol for chest CT included a bolus of 100 mL of contrast administered intravenously at a rate of 3 to 4 mL/s. The single slice CT images were acquired with 3.0 to 5.0 mm slice thickness. With the new generation scanners, the images were acquired with 0.75 to 1.5 mm slice collimation, reconstructed as 1.5 to 2.0 mm slice thickness at 1.0 to 1.5 mm intervals for advanced reconstruction and as 3.0 to 5.0 mm slices for axial interpretation. Any abnormalities (ie, mediastinal hematoma, aortic contour irregularities, hemothorax) noted on the axial images prompted immediate two-dimensional and three-dimensional reconstruction using Leonardo workstations running Inspace software and a Syngo environment (Siemens Medical Solutions). Both multiplanar maximal intensity projection and volume rendering technique reconstructed images were reviewed by the most senior radiologist and surgeon. Successive images of a patient with aortic transection are shown in Figure 1 using software reconstruction and eventual subtraction of surrounding anatomic structures to better define the injury. For the purposes of our clinical program and for this study, direct signs of aortic injury include vessel lumen-filling defects, contour abnormality, false aneurysm, or extravasation of contrast. Because the majority of aortic injuries occur in the proximal descending aspect, injuries to the branch vessels, ascending aorta, aortic arch, distal descending aorta, and abdominal aorta were not analyzed and represent a limitation of the current study.

View larger version (77K): [in this window] [in a new window]   Fig 1. Successive steps in software three-dimensional reconstruction of an aortic injury. A three-dimensional reconstruction of the patient's thorax is recreated on a computer workstation. Using sophisticated software image subtraction techniques, the surrounding anatomic detail is cleared from the image to reveal the aortic anatomy, and a transection in the descending aorta is seen.

	Results

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

Utility of Chest Radiograph for Blunt Aortic Injury
Of the 539 radiographs read as abnormal or indicating a widened mediastinum, 28 had aortic injury (28 of 539, 5%) and 511 did not (511 of 539, 95%). Of the 317 radiographs that were initially read as normal, 3 of these patients (3 of 317, 1%) had BAI and 314 did not (314 of 317, 99%). Of the 31 patients with confirmed BAI, 28 (28 of 31, 90%) had a widened mediastinum by chest radiograph and 3 (3 of 31, 10%) were interpreted as negative for mediastinal hematoma. These data result in a sensitivity of 90%, a specificity of 38% for screening value, a positive predictive value of only 5%, and a negative predictive value of 99% (Table 1). The raw calculations for these values are presented in Appendix 1.

View larger version (16K): [in this window] [in a new window]   Fig 2. Total number of aortograms performed for blunt chest trauma per year at the Ben Taub General Hospital from years 1997 through 2004.

View larger version (17K): [in this window] [in a new window]   Fig 3. Total number of aortograms performed for blunt chest trauma per year with percentage undergoing screening computed tomography scan before aortogram (gray bars) from 1997 through 2004.

Using the latest generation CT scanner and three-dimensional reconstruction software on the 11 most recent patients with aortic injury, 10 (91%) unequivocally demonstrated descending thoracic aortic injuries, and we believe that these images provide enough anatomic information for operative planning. Additionally, the average time from scan to fully reconstructed images took less than 15 minutes when performed by one of two radiologists who had extensive experience with the imaging software. The number of radiologists with expertise in using the workstation software for image reconstruction was a limitation of the study. At the time of this study, more radiologists, including senior residents, were training to use the imaging software so that someone would be available at all times. Figure 4 demonstrates some examples of this technology. Patient A underwent screening CT scan with coronal, sagittal, and three-dimensional reconstructions after a high-speed motor vehicle accident. The reconstructed images clearly show a pseudoaneurysm at the aortic isthmus with the aortogram images shown for comparison. Patient B was also involved in high-speed motor vehicle accident, and the reconstructed aortic images demonstrate a long pseudoaneurysm of the proximal descending aorta with the aortogram images shown for comparison.

View larger version (113K): [in this window] [in a new window]   Fig 4. (A) Patient A was involved in high-speed motor vehicle accident. The three-dimensional software reconstructions are compared with the aortograms, demonstrating an identical aortic pseudoaneurysm and anatomy. (B) Patient B was also involved in a high-speed motor vehicle accident. Three-dimensional reconstruction images demonstrate a long pseudoaneurysm involving the proximal descending aorta, identical to that seen on the aortogram (middle image).

	Comment

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

This study confirms data of numerous other investigators that CT scanning of the chest for BAI is an acceptable screening tool. Only 1 patient with aortic injury had a normal CT scan such that a negative predictive value of 99% and a sensitivity of 95% were obtained, which is ideal for a screening test. The false-negative CT scan was performed in 1997 on the older generation scanner, and the aortogram confirmed a subtle aortic injury that was successfully managed nonoperatively. In the stable patient undergoing transport to the CT scanner for blunt trauma evaluation, it requires very little additional time to obtain a chest scan in addition to the abdominal, head, and other indicated scans with the latest scanner technology, which is also prerequisite for a screening test. Other information, including lung and bony thorax injuries, can be obtained from chest CT.

Literature consensus is that if axial images are unremarkable with regard to aortic anatomy and the presence of hematoma, then further workup is not indicated. However, operator experience, institutional experience, and unusual anatomic locations of injury are important limitations of this developing technology. Controversy remains, however, over the role of CT scanning as a definitive study for aortic injury. There are several recent reports that support the role of chest CT scan as a definitive study (thus basing the decision on whether to operating entirely on the scan results) provided that the scan clearly shows injury and is not equivocal, which would probably require an aortogram depending on the practice guidelines at that institution [5, 9, 11]. The specificity of CT scanning for BAI has improved with advances in reconstruction software that can rapidly create coronal, sagittal, oblique planar, and three-dimensional images. This was observed when comparing the older and newer generation scanners, and although the sample size is currently small with the new scanners, this trend is expected to continue as the radiologists gain more experience with the imaging software.

Patients with positive CT scans can be stratified into two groups: scans that clearly show direct signs of injury (luminal defect, pseudoaneurysm, contrast extravasation) and those that have indirect signs or equivocal findings (mediastinal hematoma, movement artifacts, anatomic anomalies). Stratification into these two groups leads to a significant improvement in the specificity of the positive study regarding scans that show direct signs of injury. Studies that are equivocal are frustrating to the surgeon, and one particularly common finding is mediastinal hematoma without direct signs of aortic injury. Of 74 patients in the current series with mediastinal hematoma as the only significant finding on CT, 3 had a confirmed aortic injury by aortography, a 4% missed injury rate. An earlier study revealed a missed injury rate of 20% in patients with only indirect signs who had aortic injury [3]. Aortography is strongly advocated for patients with any suspicious CT findings, even in the absence of direct signs of injury.

Pulsation artifacts from the heart and lungs and suboptimal contrast bolus are two major obstacles for screening chest CT in BAI. Until the image acquiring times become substantially faster or unless the heart is transiently stopped, there will always be some degree of motion artifact (Fig 5A). Because of their smaller size and surrounding structures, branch vessel imaging is also difficult, and clear, unobstructed images are hard to create on the graphics workstation even with extensive software subtraction techniques (Fig 5B). Caution must exist as major limitations of even the current multislice CT scanners are their ability to detect injuries to the ascending aorta, aortic root, or branch vessels. This is a significant problem in evaluating the patient with significant blunt chest trauma, and missed injuries to these structures may occur with overreliance on CT scans. Consider that up to 15% of patients with descending aortic injuries also have injuries to the ascending aorta or branch vessels as determined at autopsy series [13].

View larger version (124K): [in this window] [in a new window]   Fig 5. Problem areas for computed tomographic scanning of blunt aortic injury. (A) Three examples of motion artifact seen in three-dimensional reconstruction of ascending aorta. (B) Two examples of the branch vessels with surrounding structures and artifacts that make identification of injury difficult.

For patients without definite signs of aortic injury on plain radiograph, CT scan of the chest is an acceptable screening tool but cannot be used solely as a definitive study. Although the number of aortograms performed has dramatically decreased across the country and at Ben Taub General Hospital, they are still required for equivocal studies or injuries may be missed. Aortography has several pitfalls including risk of femoral arterial injury and time required to perform study, which can delay treatment. The average wait time for completing an aortogram was 3 hours, which included mobilizing technicians and faculty, and procedure and equipment setup. Therefore, the new imaging technology is welcomed, but one should be cautious about the information obtained and making operative decisions based on these studies. However, using the new imaging technology, direct signs of injury with software reconstruction will probably be the only study needed in the future, with aortograms reserved for equivocal studies.

A practical algorithm for imaging patients with suspected blunt aortic injury is proposed (Fig 6). After the initial radiograph, stable patients should have screening chest CT if indicated according to mechanism or radiograph findings, and some can proceed directly to aortogram if overt radiographic signs of aortic injury are present. For those patients undergoing screening chest CT scan, if the axial images of the chest are negative, no further workup is necessary for descending thoracic aortic injury, and software image reconstruction is not performed. If the images show any abnormality, software reconstruction should be undertaken by a radiologist experienced with this technology. Clear, unequivocal descending aortic injury, at this time, should still be confirmed by aortogram to precisely define the surrounding anatomy and evaluate for other associated injuries including the ascending aorta and the branch vessels. Computed tomographic scan findings that remain equivocal after reconstruction also require an aortogram to definitively exclude aortic injury. As imaging technology continues to progress in quality and speed, the need for aortogram will continue to decrease but will continue to remain the true, definitive test for years to come.

View larger version (27K): [in this window] [in a new window]   Fig 6. A practical algorithm for suspected blunt aortic injury. *Obvious chest x-ray signs of aortic injury do not need CT chest at this time (if the surgeon is comfortable). Following other CT scans, the stable patient should proceed to aortogram. **At this time, aortogram should be obtained to confirm injury and delineate the operative anatomy. In the future, as technology improves, the surgeon will probably be able to proceed to operation without aortogram. (Abd = abdominal; CT = computed tomography; 3-D = three-dimensional.)

	Appendix 1

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

	Appendix 2

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

	References

Top Abstract Introduction Material and Methods Results Comment Appendix 1 Appendix 2 References

 

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This Article Abstract Full Text (PDF) Online Discussion Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Brian A. Bruckner Daniel J. DiBardino Shanda H. Blackmon Richard G. Fisher Kenneth L. Mattox Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Bruckner, B. A. Articles by Wall, M. J. Search for Related Content PubMed PubMed Citation Articles by Bruckner, B. A. Articles by Wall, M. J. Related Collections Great vessels