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Evaluation of a dedicated MDCT protocol using iterative image reconstruction after cervical spine trauma
Clinical Radiology 68 (2013) e391ee396
Contents lists available at SciVerse ScienceDirect
Clinical Radiology journal homepage: www.clinicalradiologyonline.net
Evaluation of a dedicated MDCT protocol using iterative image reconstruction after cervical spine trauma € rner, R. Hempel, Z. Deak, F.G. Mueck, U. Linsenmaier, M.F. Reiser, L.L. Geyer*, M. Ko S. Wirth Department for Clinical Radiology, University Hospital LMU Munich, Munich, Germany
art icl e i nformat ion Article history: Received 20 June 2012 Accepted 9 November 2012
AIM: To evaluate radiation exposure for 64-row computed tomography (CT) of the cervical spine comparing two optimized protocols using filtered back projection (FBP) and adaptive statistical iterative reconstruction (ASIR), respectively. MATERIALS AND METHODS: Sixty-seven studies using FBP (scanner 1) were retrospectively compared with 80 studies using ASIR (scanner 2). The key scanning parameters were identical (120 kV dose modulation, 64 0.625 mm collimation, pitch 0.531:1). In protocol 2, the noise index (NI) was increased from 5 to 25, and ASIR and the high-definition (HD) mode were used. The scan length, CT dose index (CTDI), and doseelength product (DLP) were recorded. The image quality was analysed subjectively by using a three-point scale (0; 1; 2), and objectively by using a region of interest (ROI) analysis. ManneWhitney U and Wilcoxon’s test were used. RESULTS: In the FBP group, the mean CTDI was 21.43 mGy, mean scan length 186.3 mm, and mean DLP 441.15 mGy cm. In the ASIR group, the mean CTDI was 9.57 mGy, mean scan length 195.21 mm, and mean DLP 204.23 mGy cm. The differences were significant for CTDI and DLP (p < 0.001) and scan length (p ¼ 0.01). There was no significant difference in the subjective image quality (p > 0.05). The estimated mean effective dose decreased from 2.38 mSv (FBP) to 1.10 mSv (ASIR). CONCLUSION: The radiation dose of 64-row MDCT can be reduced to a level comparable to plain radiography without loss of subjective image quality by implementation of ASIR in a dedicated cervical spine trauma protocol. These results might contribute to an improved relative risk-to-benefit ratio and support the justification of CT as a first-line imaging tool to evaluate cervical spine trauma. Ó 2013 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction About 2e6% of all admitted blunt trauma patients suffer from suspected neck or spinal injuries.1 In the US more than 1 million patients with suspected spinal injury are treated in the emergency department annually.2 Missed
* Guarantor and correspondent: L.L. Geyer, Department for Clinical Radiology, University Hospital LMU Munich, Nussbaumstrasse 20, 80336 Munich, Germany. Tel.: þ49 89 5160 9210; fax: þ49 89 5160 9212. E-mail address: [email protected] (L.L. Geyer).
spinal injuries can result in damage to the cord and neurological disability, and can have severe consequences for the patient.3 Imaging of the spine is often requested by surgeons and emergency physicians as a precaution, to avoid missed injuries to the cervical spine (c-spine), and this results in 98% of the radiographs requested showing no abnormality.4 The National Emergency X-Radiography Utilization Study (NEXUS) and the Canadian C-Spine Rules (CCR) established a decision instrument based on clinical criteria to help physicians to identify patients who need imaging.5,6 Studies have shown that using CCR and NEXUS to aid
0009-9260/$ e see front matter Ó 2013 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.crad.2012.11.025
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clinical decision-making helped to reduce the number of images of the c-spine.7,8 For the radiographic evaluation of trauma to the c-spine, a plain film radiograph in two or three planes, computed tomography (CT), or magnetic resonance imaging (MRI) are available. For the evaluation of ligamentous injuries, MRI and dynamic flexioneextension radiographs (DS) are standard.9 In 2007, the American College of Radiology (ACR) Appropriateness Criteria stated that if imaging was indicated by clinical criteria such as NEXUS or CCR, CT of the cspine should be the first-line imaging test because of its high sensitivity in the detection of c-spine injuries.10 In 2002, the guidelines still indicated plain films at this point because of the higher radiation dose and limited availability of CT.9,11 Although the availability of CT is no longer an issue, the greater exposure to radiation is still a problem. Filtered back projection (FBP) reconstruction of an image has been used as the standard method so far to compute images for viewing from the acquired CT raw data. Meanwhile, iterative reconstruction algorithms are available from different vendors; the so-called Adaptive Statistical Iterative Reconstruction (ASIR) was evaluated in this study. ASIR better compensates for image noise and can be used to reduce the patient’s exposure to radiation.12e14 The purpose of this retrospective study was to evaluate exposure to radiation for 64-row CT examinations of the cspine comparing two optimized protocols using FBP and ASIR, respectively.
that dose reduction of up to 25% is possible in abdominal CT examinations.13 Previous publications provide a detailed description of the ASIR algorithm.12,14
Patients A waiver of consent was granted by the local Institutional Review Board because of the retrospective design of the study. During periods of 3 months patients with clinically suspected osseous injury to the c-spine, who subsequently underwent CT of the area, were included in the study. Patients in both groups were included if the examination was performed using a specific protocol for imaging trauma to the bony c-spine. Patients with dedicated CT examinations to rule out disc herniation were excluded.
Imaging protocol (Table 1) The applied imaging protocols were optimized for each CT system. The main variables were kept constant. According to the vendor’s recommendation, the noise index (NI) was increased from 5 to 25 with an ASIR level of 30% at the Discovery CT 750HD system and the HD mode was used. To standardize the scan length, the maximum range was defined from the orbital roof to the second thoracic vertebral body. Patients with extended scan fields were excluded. The patient’s head was placed in a dedicated head mount with arms close to the body, and the patient was instructed to keep the shoulders down.
Materials and methods
Image quality
CT systems
All CT examinations were reviewed on a picture archiving and communication system (PACS) workstation (Syngo, Siemens Medical Solutions, Erlangen, Germany) equipped with 3k liquid crystal displays (LCDs). For objective evaluation of image noise, a region of interest (ROI) of 1 cm2 was defined in the musculature of the neck at the level of cervical vertebra 3 (CV3) and a ROI of 1 cm2 was defined in the sternocleidomastoid muscle at the level of cervical vertebra 7 (CV7) on each patient for each group. The standard deviation of the actual mean CT density
Two clinical standard protocols for dedicated CT examinations of the c-spine using different 64-row multidetector (MDCT) systems were compared: the LightSpeed VCT XT and the Discovery CT 750HD (both GE Healthcare, Waukesha, WI, USA), which replaced the first system at our institution. The first system used a standard FBP algorithm for reconstruction of raw data images, and the second was capable of ASIR, which allowed compensation of image noise more effectively.12,14 This algorithm is a vendorspecific, but not scanner-specific reconstruction method, so that even already-installed CT systems of the same manufacturer can be upgraded. Moreover, the Discovery CT 750HD was capable of the so-called high-definition (HD) mode, which allowed for higher spatial resolution of the image by increasing the number of single views per rotation from 984 to 2460. ASIR generates an estimated “forward projection” first and then compares these data to the actual measurements of density. The aim is to keep the differences between forward projection data and measured data as small as possible. Additionally, values of adjacent voxels are taken into account and adjusted if necessary. The result is a more homogeneous image with less noise or a preserved image quality with less radiation dose as there is a trade-off between noise and dose. Recent studies using ASIR showed
Table 1 Imaging protocols. LightSpeed VCT Collimation Rotation time (s) Pitch Table feed (mm/rotation) Tube voltage (kV) Tube current (modulated, mA) Noise index Image reconstruction Kernel Section thickness (mm) Reformations
Discovery HD 750
64 0.625 mm 64 0.625 mm 0.5 0.5 0.531:1 0.531:1 10.62 10.62 120 120 max. 300 max. 300 5 25 FBP ASIR VS 30% Bone Plus HD Bone Plus 3 Axial, coronal, sagittal
FBP, filtered back-projection; ASIR, Adaptive Statistical Iterative Reconstruction; VS, volume mode.
L.L. Geyer et al. / Clinical Radiology 68 (2013) e391ee396
value was recorded and taken as a measurement of image noise. All examinations were reviewed subjectively for quality by two radiologists in consensus with 8 and 10 years of professional experience in reading c-spine CT examinations. The readers were blinded to the image reconstruction and CT device used. The images were rated with a three-scale score at levels CV3 and CV7 regarding the overall image quality, visibility, contour, and texture of bony structures (2 ¼ diagnostic without limitations; 1 ¼ diagnostic with slight limitations; 0 ¼ not diagnostic). The score was given after images had been viewed in all three planes (transverse, coronal, and sagittal). As the protocol used with both CT devices was a dedicated bone trauma scan, the readers should only evaluate the quality of trabecular and cortical bone. Soft-tissue structures, in particular intervertebral discs, were not included in the rating process. Artefacts were included in the overall score and did not have to be rated separately.
Estimation of patients’ exposure to radiation To estimate the radiation dose of both patient groups, dose reports that were generated automatically after each examination were retrieved from the PACS. The volume CT dose index (CTDIvol), doseelength product (DLP), and scan length of all studies were recorded. For estimation of the effective radiation exposure (mSv), the DLP was multiplied by a conversion factor of 0.0054 mSv/mGy cm.15 As the dose modulation is dependent on the patient’s constitution, the coronal and sagittal diameters were measured in the scout views and compared between both groups.
Statistical analysis All statistical tests were performed with a dedicated software package (SPSS v18.0, SPSS, Chicago, IL, USA). Significant differences in age distribution, patients’ diameters, dose parameters, and mean effective dose were tested by Student’s t-test. The gender distribution was analysed by the Chi-square test. The significance of mean values of image noise was compared for both patient groups using the ManneWhitney U-test, and for subjective evaluation with Wilcoxon’s test. The significance level was p < 0.05.
Radiation exposure (Table 2) Mean CTDI and DLP were significantly lower in the ASIR group (p < 0.001) with a reduction of 55.3% for CTDI and 53.7% for DLP. The scan length was significantly increased by 4.6% in the ASIR group (p ¼ 0.01).
Noise measurements (Table 3) The image noise in the soft-tissue adjacent to the c-spine significantly increased by 52.6% at the level of CV3 (p < 0.001) and by 21.2% at CV7 (p ¼ 0.002) in the ASIR group.
Subjective image quality (Table 3) The subjective rating of the image quality did not differ significantly between the two CT systems (CV3 p ¼ 1, CV7 p ¼ 0.231). None of the examinations produced any images that were not diagnostic. In the ASIR group, there were slightly more examinations with minor limitations (five compared with two).
Estimation of effective dose Estimation of mean effective dose resulted in 2.38 mSv in the FBP group and 1.10 mSv in the ASIR group. Consequently, the effective dose was reduced by 53.8% in the ASIR group. CTDI as an indicator of radiation exposure, which is independent of the scan length, was reduced by 55.3%.
Discussion The reduction in exposure and the resulting image quality were compared between two 64-row CT systems, one using FBP and the other using ASIR for image computing of the raw data. The use of the low-dose protocol with ASIR resulted in a considerable reduction of the estimated effective dose from 2.4 to 1.1 mSv, whereas subjective image quality remained comparable (Fig 1) and scan length was even larger in the ASIR patient cohort. Table 2 Results from the analysis of the dose reports.
Results Patients Sixty-seven patients (37 women and 30 men) were included in the control group (LightSpeed VCT XT, FBP, standard exposure); their mean (SD) age was 54 (20) years. Eighty patients (31 women and 49 men) were included in the study group (Discovery CT 750HD, ASIR, HD mode, reduced exposure); their mean (SD) age was 51 (23) years. There were no significant differences between patients’ age (p ¼ 0.352), gender distribution (p ¼ 0.05), sagittal (13.1 2.0 cm versus 13.0 2.0 cm; p ¼ 0.729) and coronal diameter (12.8 1.4 cm versus 13.0 1.5 cm; p ¼ 0.427).
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Patients CTDIvol (mGy) Mean (SD) Range Scan length (mm) Mean (SD) Range DLP (mGy cm) Mean (SD) Range Estimated mean effective dose (mSv)
LightSpeed VCT (FBP)
Discovery HD 750 (ASIR)
67
80
21.4 (1.4) 14.6e21.9
9.6 (3.4)a 5.0e20.9
186.3 (23.6) 142.0e261.8
195.2 (26.8)b 118.8e261.8
441.2 (51.7) 286.5e591.3 2.38
204.2 (68.1)a 86.7e418.4 1.10
CTDIvol, volume CT dose index; DLP, doseelength product. a p < 0.001. b p ¼ 0.013.
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Table 3 Noise measurements and subjective image quality. LightSpeed VCT (FBP) Noise measurements (HU) CV3 Mean (SD) 24.5 (18.9) Range 12.7e114.0 CV7 Mean (SD) 28.8 (9.3) Range 11.2e49.0 Subjective evaluation CV 3 Mean (SD) 2.00 (0) CV 7 Mean (SD) 1.97 (0.17)
Discovery HD 750 (ASIR)
37.4 (6.2)a 26.3e50.7 34.9 (6.7)b 23.8e55.0
2.00 (0)c 1.93 (0.27)c
CV3, cervical vertebra 3; CV7, cervical vertebra 7. a p < 0.001. b p ¼ 0.002. c p > 0.05.
The objective measurements showed a significant increase of the noise in the soft tissue in the ASIR group. However, as the study focused on bony structures, the greater image noise in the soft tissue seemed to have no relevant influence on the evaluation of these structures. In this context, the use of the HD mode has to be discussed. On the one hand, the improved spatial resolution allowed for preserving the image quality of the bony structures. On the other hand, it can be hypothesized, that the HD mode might contribute to the increased image noise in the soft tissue, based on initial study results and the finding that highresolution kernels usually result in higher noise levels.16 Nonetheless, further studies are under investigation to clarify this topic. Decision criteria such as NEXUS or CCR help physicians to clear c-spine injuries clinically. For imaging, the recommendations of the Appropriateness Criteria of the ACR have been established. However, there is still discussion about whether a plain radiograph or CT is the preferable first-line
imaging test if there is a suspicion of trauma to the cspine.17,18 Plain radiography has the advantage of lower exposure of the patient and better availability, but it is often difficult to get adequate and quick radiography of the cspine in three planes. 98% of the requested radiographs show no fractures.4 CT has better sensitivity for the detection of fractures,17 and studies have a reported sensitivity between 36% and 94% for radiographs and between 98% and 100% for CT for the detection of fractures.17,18 CT is also faster and more cost-effective.3 In high-risk patients such as those over 50years-old or those with high-velocity trauma, CT is more accurate and effective because it helps to reduce the number of cases of paralysis, the time spent in hospital, and cervical immobilization.3 During the past few years, CT has gained more and more clinical importance. In Germany, it makes up 7% of all radiological examinations, but accounts for 56% of the overall radiation dose because it usually delivers considerably more radiation to the patient than a plain radiograph.19 With alternating values between 2.9 and 26 mSv for a standard CT examination of the c-spine, the radiation dose is reported to be up to 10-times higher than that of a conventional radiograph in two planes.17,20 The mean effective dose for two-plane radiography of the c-spine is reported to be between 0.9 and 4.0 mSv.17,21 The results of the present study indicate that with the use of a dedicated CT protocol, the exposure is comparable with a series of conventional radiographs to rule out bony injuries to the c-spine. Given that CT is more accurate, the recommendations of the ACR are strongly enforced. The ACR guidelines for imaging of c-spine trauma show that the relative radiation level for radiographic examinations of the c-spine is low (0.1e1 mSv), whereas that of CT of the c-spine is medium (1e10 mSv).10 In addition, Theocharopoulos et al.22 evaluated the relative risk-to-benefit ratio of c-spine CT compared with plain radiography. As the higher diagnostic accuracy of c-spine CT outbalanced
Figure 1 Comparison of images with FBP (left) and ASIR (right). Although the subjective image quality shows no significant difference, there is an obvious difference in the parameters of radiation dose: FBP: CTDIvol ¼ 21.93 mGy, DLP ¼ 420.25 mGy cm versus ASIR: CTDIvol ¼ 6.54 mGy, DLP ¼ 156.23 mGy cm.
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the increased radiation dose, the authors emphasized the use of CT to assess the c-spine regardless of the patient age, sex, and fracture risk. The present results indicate that CT of the c-spine no longer gives a higher radiation dose than radiography in two planes. The radiation exposure is comparable to a standard radiography examination and strongly supports the ACR criteria to recommend CT as the first-line imaging test for suspected c-spine trauma. Plain radiography still has a higher recommendation level in the ACR Appropriateness Criteria for follow-up imaging on a patient with no initially unstable injury, but who is kept in a collar for neck pain and returns for evaluation.10 This concept must also be discussed in light of the present results. The retrospective study design, which did not allow for randomization of the groups, has to be discussed in its context. Because the CT device capable of ASIR replaced the other 64-row CT at our hospital, retrospective analysis was undertaken. The study groups were not balanced for the sexes. The higher, but not significant male percentage of the study group is probably the reason why there was a significant difference in the scan length. Although the sagittal and coronal diameters were not different, the CTDI was significantly reduced. As the CTDI is an indicator of radiation exposure, which is independent of scan length, the results are also valid if the scan length does not differ. Although image quality was assessed both objectively and subjectively, most of the images showed no lesions, e.g., fractures. Consequently, the sensitivity and specificity for detection of fractures for the two groups cannot be discussed. In addition, the image quality of the soft tissue was not further evaluated. Nevertheless, the study methods were justified based on the following observations: the key parameter of CT in the diagnostic work-up of patients with c-spine trauma is its high sensitivity for the detection of osseous injuries.23 In contrast, studies have shown that softtissue injuries (such as ligamentous or spinal cord injuries) might be missed by CT.24 Unfortunately, there are no exact data about the incidence of this. Conversely, MRI is the currently most sensitive imaging technique of choice to rule out soft-tissue injuries of the c-spine25; however, the value of additional MRI might be incremental. Hogan et al.26 have shown that CT allows for a reliable identification of patients with unstable c-spine injury. This is also supported by the very low estimated incidence of 0.04e0.6% of unstable injuries without osseous defects.1,27 Following practical guidelines, MRI might be indicated in patients with a high clinical suspicion of soft-tissue injury or with neurological symptoms.28 However, as the field of iterative reconstruction algorithms is rapidly evolving, future studies have to evaluate the performance of even more sophisticated reconstruction algorithms with respect to soft-tissue injuries. Based on the results of recent studies,12e14 it can be assumed that ASIR is an important parameter of the dosereduced CT protocol in the present study group and allows the subjective image quality to be preserved. As the focus of this study was to present a CT protocol that was applicable
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in daily routine, the detailed analysis of each scan parameter, such as ASIR or the HD mode, was not included in the study. In addition, the second CT system (ASIR group) was equipped with more recent hardware components (e.g., different detector), which are considered to provide improved image characteristics, such as a higher signalto-noise ratio, and might also contribute to potential dose savings.16 Therefore, further studies are required that analyse their individual contributions independently. Moreover, it would have been possible to reduce the dose of the first CT device (FBP group) to some extent. However, with a mean effective dose of 2.38 mSv, the dose was already in the lower third of the range compared to other studies that have investigated the diagnosis of c-spine injuries with effective dose values for CT between 1.08 and 26 mSv.17,20 In conclusion, the results of the present study indicate that CT of the c-spine using a dedicated ASIR protocol can help to reduce the exposure to a comparable or even lower level than plain radiography. With this in mind, and considering the higher diagnostic accuracy provided by CT compared with plain radiography, the present results may contribute to justify the use of CT as the first-line imaging method in suspected injuries to the c-spine. Even if these results are promising, larger studies are needed to validate the accuracy, safety, and reproducibility of this recommendation.
Acknowledgements L.G. has received payment for lectures, including for services on speakers bureaus, from GE Healthcare. M.K. has received a research grant from GE Healthcare.
References 1. Saltzherr TP, Fung Kon Jin PH, Beenen LF, et al. Diagnostic imaging of cervical spine injuries following blunt trauma: a review of the literature and practical guideline. Injury 2009;40:795e800. 2. McCaig LF, Nawar EW. National hospital ambulatory medical care survey: 2004 emergency department summary. Adv Data 2006:1e29. 3. Blackmore CC, Ramsey SD, Mann FA, et al. Cervical spine screening with CT in trauma patients: a cost-effectiveness analysis. Radiology 1999;212:117e25. 4. Stiell IG, Wells GA, Vandemheen K, et al. Variation in emergency department use of cervical spine radiography for alert, stable trauma patients. CMAJ 1997;156:1537e44. 5. Canadian CT, Head and C-Spine (CCC) Study Group. Canadian C-Spine Rule study for alert and stable trauma patients: II. Study objectives and methodology. CJEM 2002;4:185e93. 6. Hoffman JR, Mower WR, Wolfson AB, et al. Validity of a set of clinical criteria to rule out injury to the cervical spine in patients with blunt trauma. National Emergency X-Radiography Utilization Study Group. N Engl J Med 2000;343:94e9. 7. Mower WR, Hoffman J. Comparison of the Canadian C-Spine rule and NEXUS decision instrument in evaluating blunt trauma patients for cervical spine injury. Ann Emerg Med 2004;43:515e7. 8. Stiell IG, Clement CM, McKnight RD, et al. The Canadian C-spine rule versus the NEXUS low-risk criteria in patients with trauma. N Engl J Med 2003;349:2510e8. 9. Sciubba DM, Petteys RJ. Evaluation of blunt cervical spine injury. South Med J 2009;102:823e8. 10. American College of Radiology ACR Appropriateness CriteriaÒ: Suspected Spine Trauma. (1999; updated 2009). Available at: http://www. acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria/pdf/
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ExpertPanelonMusculoskeletalImaging/SuspectedCervicalSpineTrauma Doc22.aspx [accessed 29.07.10]. Daffner RH, Hackney DB. ACR appropriateness criteria on suspected spine trauma. J Am Coll Radiol 2007;4:762e75. Hara AK, Paden RG, Silva AC, et al. Iterative reconstruction technique for reducing body radiation dose at CT: feasibility study. AJR Am J Roentgenol 2009;193:764e71. Prakash P, Kalra MK, Kambadakone AK, et al. Reducing abdominal CT radiation dose with adaptive statistical iterative reconstruction technique. Invest Radiol 2010;45:202e10. Silva AC, Lawder HJ, Hara A, et al. Innovations in CT dose reduction strategy: application of the adaptive statistical iterative reconstruction algorithm. AJR Am J Roentgenol 2010;194:191e9. Huda W, Ogden KM, Khorasani MR. Converting doseelength product to effective dose at CT. Radiology 2008;248:995e1003. Yanagawa M, Tomiyama N, Honda O, et al. Multidetector CT of the lung: image quality with garnet-based detectors. Radiology 2010;255:944e54. Antevil JL, Sise MJ, Sack DI, et al. Spiral computed tomography for the initial evaluation of spine trauma: a new standard of care? J Trauma 2006;61:382e7. Bailitz J, Starr F, Beecroft M, et al. CT should replace three-view radiographs as the initial screening test in patients at high, moderate, and low risk for blunt cervical spine injury: a prospective comparison. J Trauma 2009;66:1605e9. € r Strahlenschutz BfS B IV Radiation Exposures From Bundesamt fu Medical Applications (2009). Available at: http://www.bfs.de/de/bfs/ druck/uus/JB08TeilB_IV.pdf [accessed 19.12.10].
20. ICRP. Managing patient dose in multi-detector computed tomography (MDCT). ICRP Publication 102. Ann ICRP 2007;37:1e79. 21. Regulla DF, Eder H. Patient exposure in medical X-ray imaging in Europe. Radiat Prot Dosimetry 2005;114:11e25. 22. Theocharopoulos N, Chatzakis G, Damilakis J. Is radiography justified for the evaluation of patients presenting with cervical spine trauma? Med Phys. 2009;36:4461e70. 23. Holmes JF, Akkinepalli R. Computed tomography versus plain radiography to screen for cervical spine injury: a meta-analysis. J Trauma 2005;58:902e5. 24. Holmes JF, Mirvis SE, Panacek EA, et al. Variability in computed tomography and magnetic resonance imaging in patients with cervical spine injuries. J Trauma 2002;53:524e9. discussion 530. 25. Muchow RD, Resnick DK, Abdel MP, et al. Magnetic resonance imaging (MRI) in the clearance of the cervical spine in blunt trauma: a metaanalysis. J Trauma 2008;64:179e89. 26. Hogan GJ, Mirvis SE, Shanmuganathan K, et al. Exclusion of unstable cervical spine injury in obtunded patients with blunt trauma: is MR imaging needed when multi-detector row CT findings are normal? Radiology 2005;237:106e13. 27. Panczykowski DM, Tomycz ND, Okonkwo DO. Comparative effectiveness of using computed tomography alone to exclude cervical spine injuries in obtunded or intubated patients: meta-analysis of 14,327 patients with blunt trauma. J Neurosurg 2011;115:541e9. 28. Como JJ, Diaz JJ, Dunham CM, et al. Practice management guidelines for identification of cervical spine injuries following trauma: update from the eastern association for the surgery of trauma practice management guidelines committee. J Trauma 2009;67:651e9.
Contents lists available at SciVerse ScienceDirect
Clinical Radiology journal homepage: www.clinicalradiologyonline.net
Evaluation of a dedicated MDCT protocol using iterative image reconstruction after cervical spine trauma € rner, R. Hempel, Z. Deak, F.G. Mueck, U. Linsenmaier, M.F. Reiser, L.L. Geyer*, M. Ko S. Wirth Department for Clinical Radiology, University Hospital LMU Munich, Munich, Germany
art icl e i nformat ion Article history: Received 20 June 2012 Accepted 9 November 2012
AIM: To evaluate radiation exposure for 64-row computed tomography (CT) of the cervical spine comparing two optimized protocols using filtered back projection (FBP) and adaptive statistical iterative reconstruction (ASIR), respectively. MATERIALS AND METHODS: Sixty-seven studies using FBP (scanner 1) were retrospectively compared with 80 studies using ASIR (scanner 2). The key scanning parameters were identical (120 kV dose modulation, 64 0.625 mm collimation, pitch 0.531:1). In protocol 2, the noise index (NI) was increased from 5 to 25, and ASIR and the high-definition (HD) mode were used. The scan length, CT dose index (CTDI), and doseelength product (DLP) were recorded. The image quality was analysed subjectively by using a three-point scale (0; 1; 2), and objectively by using a region of interest (ROI) analysis. ManneWhitney U and Wilcoxon’s test were used. RESULTS: In the FBP group, the mean CTDI was 21.43 mGy, mean scan length 186.3 mm, and mean DLP 441.15 mGy cm. In the ASIR group, the mean CTDI was 9.57 mGy, mean scan length 195.21 mm, and mean DLP 204.23 mGy cm. The differences were significant for CTDI and DLP (p < 0.001) and scan length (p ¼ 0.01). There was no significant difference in the subjective image quality (p > 0.05). The estimated mean effective dose decreased from 2.38 mSv (FBP) to 1.10 mSv (ASIR). CONCLUSION: The radiation dose of 64-row MDCT can be reduced to a level comparable to plain radiography without loss of subjective image quality by implementation of ASIR in a dedicated cervical spine trauma protocol. These results might contribute to an improved relative risk-to-benefit ratio and support the justification of CT as a first-line imaging tool to evaluate cervical spine trauma. Ó 2013 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction About 2e6% of all admitted blunt trauma patients suffer from suspected neck or spinal injuries.1 In the US more than 1 million patients with suspected spinal injury are treated in the emergency department annually.2 Missed
* Guarantor and correspondent: L.L. Geyer, Department for Clinical Radiology, University Hospital LMU Munich, Nussbaumstrasse 20, 80336 Munich, Germany. Tel.: þ49 89 5160 9210; fax: þ49 89 5160 9212. E-mail address: [email protected] (L.L. Geyer).
spinal injuries can result in damage to the cord and neurological disability, and can have severe consequences for the patient.3 Imaging of the spine is often requested by surgeons and emergency physicians as a precaution, to avoid missed injuries to the cervical spine (c-spine), and this results in 98% of the radiographs requested showing no abnormality.4 The National Emergency X-Radiography Utilization Study (NEXUS) and the Canadian C-Spine Rules (CCR) established a decision instrument based on clinical criteria to help physicians to identify patients who need imaging.5,6 Studies have shown that using CCR and NEXUS to aid
0009-9260/$ e see front matter Ó 2013 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.crad.2012.11.025
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L.L. Geyer et al. / Clinical Radiology 68 (2013) e391ee396
clinical decision-making helped to reduce the number of images of the c-spine.7,8 For the radiographic evaluation of trauma to the c-spine, a plain film radiograph in two or three planes, computed tomography (CT), or magnetic resonance imaging (MRI) are available. For the evaluation of ligamentous injuries, MRI and dynamic flexioneextension radiographs (DS) are standard.9 In 2007, the American College of Radiology (ACR) Appropriateness Criteria stated that if imaging was indicated by clinical criteria such as NEXUS or CCR, CT of the cspine should be the first-line imaging test because of its high sensitivity in the detection of c-spine injuries.10 In 2002, the guidelines still indicated plain films at this point because of the higher radiation dose and limited availability of CT.9,11 Although the availability of CT is no longer an issue, the greater exposure to radiation is still a problem. Filtered back projection (FBP) reconstruction of an image has been used as the standard method so far to compute images for viewing from the acquired CT raw data. Meanwhile, iterative reconstruction algorithms are available from different vendors; the so-called Adaptive Statistical Iterative Reconstruction (ASIR) was evaluated in this study. ASIR better compensates for image noise and can be used to reduce the patient’s exposure to radiation.12e14 The purpose of this retrospective study was to evaluate exposure to radiation for 64-row CT examinations of the cspine comparing two optimized protocols using FBP and ASIR, respectively.
that dose reduction of up to 25% is possible in abdominal CT examinations.13 Previous publications provide a detailed description of the ASIR algorithm.12,14
Patients A waiver of consent was granted by the local Institutional Review Board because of the retrospective design of the study. During periods of 3 months patients with clinically suspected osseous injury to the c-spine, who subsequently underwent CT of the area, were included in the study. Patients in both groups were included if the examination was performed using a specific protocol for imaging trauma to the bony c-spine. Patients with dedicated CT examinations to rule out disc herniation were excluded.
Imaging protocol (Table 1) The applied imaging protocols were optimized for each CT system. The main variables were kept constant. According to the vendor’s recommendation, the noise index (NI) was increased from 5 to 25 with an ASIR level of 30% at the Discovery CT 750HD system and the HD mode was used. To standardize the scan length, the maximum range was defined from the orbital roof to the second thoracic vertebral body. Patients with extended scan fields were excluded. The patient’s head was placed in a dedicated head mount with arms close to the body, and the patient was instructed to keep the shoulders down.
Materials and methods
Image quality
CT systems
All CT examinations were reviewed on a picture archiving and communication system (PACS) workstation (Syngo, Siemens Medical Solutions, Erlangen, Germany) equipped with 3k liquid crystal displays (LCDs). For objective evaluation of image noise, a region of interest (ROI) of 1 cm2 was defined in the musculature of the neck at the level of cervical vertebra 3 (CV3) and a ROI of 1 cm2 was defined in the sternocleidomastoid muscle at the level of cervical vertebra 7 (CV7) on each patient for each group. The standard deviation of the actual mean CT density
Two clinical standard protocols for dedicated CT examinations of the c-spine using different 64-row multidetector (MDCT) systems were compared: the LightSpeed VCT XT and the Discovery CT 750HD (both GE Healthcare, Waukesha, WI, USA), which replaced the first system at our institution. The first system used a standard FBP algorithm for reconstruction of raw data images, and the second was capable of ASIR, which allowed compensation of image noise more effectively.12,14 This algorithm is a vendorspecific, but not scanner-specific reconstruction method, so that even already-installed CT systems of the same manufacturer can be upgraded. Moreover, the Discovery CT 750HD was capable of the so-called high-definition (HD) mode, which allowed for higher spatial resolution of the image by increasing the number of single views per rotation from 984 to 2460. ASIR generates an estimated “forward projection” first and then compares these data to the actual measurements of density. The aim is to keep the differences between forward projection data and measured data as small as possible. Additionally, values of adjacent voxels are taken into account and adjusted if necessary. The result is a more homogeneous image with less noise or a preserved image quality with less radiation dose as there is a trade-off between noise and dose. Recent studies using ASIR showed
Table 1 Imaging protocols. LightSpeed VCT Collimation Rotation time (s) Pitch Table feed (mm/rotation) Tube voltage (kV) Tube current (modulated, mA) Noise index Image reconstruction Kernel Section thickness (mm) Reformations
Discovery HD 750
64 0.625 mm 64 0.625 mm 0.5 0.5 0.531:1 0.531:1 10.62 10.62 120 120 max. 300 max. 300 5 25 FBP ASIR VS 30% Bone Plus HD Bone Plus 3 Axial, coronal, sagittal
FBP, filtered back-projection; ASIR, Adaptive Statistical Iterative Reconstruction; VS, volume mode.
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value was recorded and taken as a measurement of image noise. All examinations were reviewed subjectively for quality by two radiologists in consensus with 8 and 10 years of professional experience in reading c-spine CT examinations. The readers were blinded to the image reconstruction and CT device used. The images were rated with a three-scale score at levels CV3 and CV7 regarding the overall image quality, visibility, contour, and texture of bony structures (2 ¼ diagnostic without limitations; 1 ¼ diagnostic with slight limitations; 0 ¼ not diagnostic). The score was given after images had been viewed in all three planes (transverse, coronal, and sagittal). As the protocol used with both CT devices was a dedicated bone trauma scan, the readers should only evaluate the quality of trabecular and cortical bone. Soft-tissue structures, in particular intervertebral discs, were not included in the rating process. Artefacts were included in the overall score and did not have to be rated separately.
Estimation of patients’ exposure to radiation To estimate the radiation dose of both patient groups, dose reports that were generated automatically after each examination were retrieved from the PACS. The volume CT dose index (CTDIvol), doseelength product (DLP), and scan length of all studies were recorded. For estimation of the effective radiation exposure (mSv), the DLP was multiplied by a conversion factor of 0.0054 mSv/mGy cm.15 As the dose modulation is dependent on the patient’s constitution, the coronal and sagittal diameters were measured in the scout views and compared between both groups.
Statistical analysis All statistical tests were performed with a dedicated software package (SPSS v18.0, SPSS, Chicago, IL, USA). Significant differences in age distribution, patients’ diameters, dose parameters, and mean effective dose were tested by Student’s t-test. The gender distribution was analysed by the Chi-square test. The significance of mean values of image noise was compared for both patient groups using the ManneWhitney U-test, and for subjective evaluation with Wilcoxon’s test. The significance level was p < 0.05.
Radiation exposure (Table 2) Mean CTDI and DLP were significantly lower in the ASIR group (p < 0.001) with a reduction of 55.3% for CTDI and 53.7% for DLP. The scan length was significantly increased by 4.6% in the ASIR group (p ¼ 0.01).
Noise measurements (Table 3) The image noise in the soft-tissue adjacent to the c-spine significantly increased by 52.6% at the level of CV3 (p < 0.001) and by 21.2% at CV7 (p ¼ 0.002) in the ASIR group.
Subjective image quality (Table 3) The subjective rating of the image quality did not differ significantly between the two CT systems (CV3 p ¼ 1, CV7 p ¼ 0.231). None of the examinations produced any images that were not diagnostic. In the ASIR group, there were slightly more examinations with minor limitations (five compared with two).
Estimation of effective dose Estimation of mean effective dose resulted in 2.38 mSv in the FBP group and 1.10 mSv in the ASIR group. Consequently, the effective dose was reduced by 53.8% in the ASIR group. CTDI as an indicator of radiation exposure, which is independent of the scan length, was reduced by 55.3%.
Discussion The reduction in exposure and the resulting image quality were compared between two 64-row CT systems, one using FBP and the other using ASIR for image computing of the raw data. The use of the low-dose protocol with ASIR resulted in a considerable reduction of the estimated effective dose from 2.4 to 1.1 mSv, whereas subjective image quality remained comparable (Fig 1) and scan length was even larger in the ASIR patient cohort. Table 2 Results from the analysis of the dose reports.
Results Patients Sixty-seven patients (37 women and 30 men) were included in the control group (LightSpeed VCT XT, FBP, standard exposure); their mean (SD) age was 54 (20) years. Eighty patients (31 women and 49 men) were included in the study group (Discovery CT 750HD, ASIR, HD mode, reduced exposure); their mean (SD) age was 51 (23) years. There were no significant differences between patients’ age (p ¼ 0.352), gender distribution (p ¼ 0.05), sagittal (13.1 2.0 cm versus 13.0 2.0 cm; p ¼ 0.729) and coronal diameter (12.8 1.4 cm versus 13.0 1.5 cm; p ¼ 0.427).
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Patients CTDIvol (mGy) Mean (SD) Range Scan length (mm) Mean (SD) Range DLP (mGy cm) Mean (SD) Range Estimated mean effective dose (mSv)
LightSpeed VCT (FBP)
Discovery HD 750 (ASIR)
67
80
21.4 (1.4) 14.6e21.9
9.6 (3.4)a 5.0e20.9
186.3 (23.6) 142.0e261.8
195.2 (26.8)b 118.8e261.8
441.2 (51.7) 286.5e591.3 2.38
204.2 (68.1)a 86.7e418.4 1.10
CTDIvol, volume CT dose index; DLP, doseelength product. a p < 0.001. b p ¼ 0.013.
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Table 3 Noise measurements and subjective image quality. LightSpeed VCT (FBP) Noise measurements (HU) CV3 Mean (SD) 24.5 (18.9) Range 12.7e114.0 CV7 Mean (SD) 28.8 (9.3) Range 11.2e49.0 Subjective evaluation CV 3 Mean (SD) 2.00 (0) CV 7 Mean (SD) 1.97 (0.17)
Discovery HD 750 (ASIR)
37.4 (6.2)a 26.3e50.7 34.9 (6.7)b 23.8e55.0
2.00 (0)c 1.93 (0.27)c
CV3, cervical vertebra 3; CV7, cervical vertebra 7. a p < 0.001. b p ¼ 0.002. c p > 0.05.
The objective measurements showed a significant increase of the noise in the soft tissue in the ASIR group. However, as the study focused on bony structures, the greater image noise in the soft tissue seemed to have no relevant influence on the evaluation of these structures. In this context, the use of the HD mode has to be discussed. On the one hand, the improved spatial resolution allowed for preserving the image quality of the bony structures. On the other hand, it can be hypothesized, that the HD mode might contribute to the increased image noise in the soft tissue, based on initial study results and the finding that highresolution kernels usually result in higher noise levels.16 Nonetheless, further studies are under investigation to clarify this topic. Decision criteria such as NEXUS or CCR help physicians to clear c-spine injuries clinically. For imaging, the recommendations of the Appropriateness Criteria of the ACR have been established. However, there is still discussion about whether a plain radiograph or CT is the preferable first-line
imaging test if there is a suspicion of trauma to the cspine.17,18 Plain radiography has the advantage of lower exposure of the patient and better availability, but it is often difficult to get adequate and quick radiography of the cspine in three planes. 98% of the requested radiographs show no fractures.4 CT has better sensitivity for the detection of fractures,17 and studies have a reported sensitivity between 36% and 94% for radiographs and between 98% and 100% for CT for the detection of fractures.17,18 CT is also faster and more cost-effective.3 In high-risk patients such as those over 50years-old or those with high-velocity trauma, CT is more accurate and effective because it helps to reduce the number of cases of paralysis, the time spent in hospital, and cervical immobilization.3 During the past few years, CT has gained more and more clinical importance. In Germany, it makes up 7% of all radiological examinations, but accounts for 56% of the overall radiation dose because it usually delivers considerably more radiation to the patient than a plain radiograph.19 With alternating values between 2.9 and 26 mSv for a standard CT examination of the c-spine, the radiation dose is reported to be up to 10-times higher than that of a conventional radiograph in two planes.17,20 The mean effective dose for two-plane radiography of the c-spine is reported to be between 0.9 and 4.0 mSv.17,21 The results of the present study indicate that with the use of a dedicated CT protocol, the exposure is comparable with a series of conventional radiographs to rule out bony injuries to the c-spine. Given that CT is more accurate, the recommendations of the ACR are strongly enforced. The ACR guidelines for imaging of c-spine trauma show that the relative radiation level for radiographic examinations of the c-spine is low (0.1e1 mSv), whereas that of CT of the c-spine is medium (1e10 mSv).10 In addition, Theocharopoulos et al.22 evaluated the relative risk-to-benefit ratio of c-spine CT compared with plain radiography. As the higher diagnostic accuracy of c-spine CT outbalanced
Figure 1 Comparison of images with FBP (left) and ASIR (right). Although the subjective image quality shows no significant difference, there is an obvious difference in the parameters of radiation dose: FBP: CTDIvol ¼ 21.93 mGy, DLP ¼ 420.25 mGy cm versus ASIR: CTDIvol ¼ 6.54 mGy, DLP ¼ 156.23 mGy cm.
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the increased radiation dose, the authors emphasized the use of CT to assess the c-spine regardless of the patient age, sex, and fracture risk. The present results indicate that CT of the c-spine no longer gives a higher radiation dose than radiography in two planes. The radiation exposure is comparable to a standard radiography examination and strongly supports the ACR criteria to recommend CT as the first-line imaging test for suspected c-spine trauma. Plain radiography still has a higher recommendation level in the ACR Appropriateness Criteria for follow-up imaging on a patient with no initially unstable injury, but who is kept in a collar for neck pain and returns for evaluation.10 This concept must also be discussed in light of the present results. The retrospective study design, which did not allow for randomization of the groups, has to be discussed in its context. Because the CT device capable of ASIR replaced the other 64-row CT at our hospital, retrospective analysis was undertaken. The study groups were not balanced for the sexes. The higher, but not significant male percentage of the study group is probably the reason why there was a significant difference in the scan length. Although the sagittal and coronal diameters were not different, the CTDI was significantly reduced. As the CTDI is an indicator of radiation exposure, which is independent of scan length, the results are also valid if the scan length does not differ. Although image quality was assessed both objectively and subjectively, most of the images showed no lesions, e.g., fractures. Consequently, the sensitivity and specificity for detection of fractures for the two groups cannot be discussed. In addition, the image quality of the soft tissue was not further evaluated. Nevertheless, the study methods were justified based on the following observations: the key parameter of CT in the diagnostic work-up of patients with c-spine trauma is its high sensitivity for the detection of osseous injuries.23 In contrast, studies have shown that softtissue injuries (such as ligamentous or spinal cord injuries) might be missed by CT.24 Unfortunately, there are no exact data about the incidence of this. Conversely, MRI is the currently most sensitive imaging technique of choice to rule out soft-tissue injuries of the c-spine25; however, the value of additional MRI might be incremental. Hogan et al.26 have shown that CT allows for a reliable identification of patients with unstable c-spine injury. This is also supported by the very low estimated incidence of 0.04e0.6% of unstable injuries without osseous defects.1,27 Following practical guidelines, MRI might be indicated in patients with a high clinical suspicion of soft-tissue injury or with neurological symptoms.28 However, as the field of iterative reconstruction algorithms is rapidly evolving, future studies have to evaluate the performance of even more sophisticated reconstruction algorithms with respect to soft-tissue injuries. Based on the results of recent studies,12e14 it can be assumed that ASIR is an important parameter of the dosereduced CT protocol in the present study group and allows the subjective image quality to be preserved. As the focus of this study was to present a CT protocol that was applicable
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in daily routine, the detailed analysis of each scan parameter, such as ASIR or the HD mode, was not included in the study. In addition, the second CT system (ASIR group) was equipped with more recent hardware components (e.g., different detector), which are considered to provide improved image characteristics, such as a higher signalto-noise ratio, and might also contribute to potential dose savings.16 Therefore, further studies are required that analyse their individual contributions independently. Moreover, it would have been possible to reduce the dose of the first CT device (FBP group) to some extent. However, with a mean effective dose of 2.38 mSv, the dose was already in the lower third of the range compared to other studies that have investigated the diagnosis of c-spine injuries with effective dose values for CT between 1.08 and 26 mSv.17,20 In conclusion, the results of the present study indicate that CT of the c-spine using a dedicated ASIR protocol can help to reduce the exposure to a comparable or even lower level than plain radiography. With this in mind, and considering the higher diagnostic accuracy provided by CT compared with plain radiography, the present results may contribute to justify the use of CT as the first-line imaging method in suspected injuries to the c-spine. Even if these results are promising, larger studies are needed to validate the accuracy, safety, and reproducibility of this recommendation.
Acknowledgements L.G. has received payment for lectures, including for services on speakers bureaus, from GE Healthcare. M.K. has received a research grant from GE Healthcare.
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