B. CT protocols for the spine Poster No.: A-003 Congress: ECR 2010 Type: Invited Speaker Topic: Neuro Authors: B. Tins; Oswestry/UK Keywords: CT, spine, diagnostic imaging protocol DOI: 10.1594/ecr2010/A-003 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 7
Learning objectives 1. To describe state-of-the art CT protocols for the spine 2. To become familiar with technical factors (acquisition, reconstruction) and parameters for CT examination of the spine 3. To recognise artifacts and errors in analysing CT of the spine Main 1. Introduction CT is the examination of choice for the assessment of the bone structure of the spine. It is fast and tolerated by almost every patient. The choice of imaging parameters and also the reconstruction algorithm and reformatting and the mode of display can make a significant difference to the perceived image quality and diagnostic accuracy. While generally a robust imaging method, artefacts can occur in CT imaging. In particular movement artefact and streak artefact due to very high attenuation areas can cause a problem for image interpretation. Ways to minimize or limit these artefacts will be discussed. Generally improvement in image quality results in an increased radiation dose and possibly longer imaging times. Depending on the clinical question the imaging parameters should be varied accordingly. Usually a common sense approach is all that needed to make CT imaging of the spine a relatively simple and highly reliable examination. 2. Positioning and scan parameters The patient position of choice for CT imaging of the spine is supine, ie the patient is lying on its back. This ensures minimal respiratory movement of the spine and usually good patient comfort resulting in less patient movement. If other positions are required care should be taken to stabilize and secure the patient to prevent movement on, or even a Page 2 of 7
fall from the CT table. Pressure points must be avoided especially during interventional procedures by using padding. Respiratory compromise must be avoided. Ideally the spine of the patient is aligned along the z axis of the scanner, the head/neck should be in neutral position. External high attenuation material (especially metal) should be removed if possible; otherwise it can result in artefact. The choice of imaging parameters determines the image quality. Generally examinations with high kv and mas settings, thin collimation and low pitch result in the best image quality. The downside of this is relatively high radiation exposure of the patient and increased examination time and tube loading. The latter two issues, time and tube loading, are no longer relevant for the newer multidetector row CTs (MD-CT). For older generation scanners the technical ability of the scanner will determine the achievable practical collimation, pitch and exposure factors. For newer generation MD-CT the main challenge is to achieve images allowing accurate and confident diagnosis while avoiding excessive radiation exposure. 3. Image postprocessing For orthopaedic imaging scanning with thin slice collimation is preferable, ideally 1mm or less. The pitch has to be less than 2, otherwise not the whole volume gets imaged. Lower pitch generally means better image quality. On modern scanners with adaptive tube current modulation the patient dose is largely determined by the selection of the effective mas. The tube current is then being modulated to achieve the preset effective mas which in essence defines the signal to noise ratio in the data. Altering the pitch will result in tube current adaption and usually will not lead to significant changes in the patient dose. After acquisition of the scanogram modern scanners will calculate the expected patient dose according to the scanning parameters. On older scanners with fixed tube currents the dose is strongly dependent on the pitch. CT imaging of the spine with a kv of 120-140, mas setting of 350-450, collimation or 1mm or less and a pitch between 1 and 1.5 should result in a high quality raw data set. For viewing the raw image data has to be reconstructed. Typically the data is reconstructed as 1mm axial slices using a soft tissue and a bone kernel. When reconstructing the first set of images from the raw data care should be taken to include an overlap to reduce the chance of missing a z-plane fracture. Sometimes only a soft tissue kernel is used and the bone is viewed using bone window settings but this practice results in suboptimal bone imaging. Page 3 of 7
Multiplanar reconstructions can performed from the raw data or the reformats from the reconstructed image data. Using the raw data results in prolonged processing times and offers no significant qualitative advantage over reformating from the axial reconstructions. The coronal or sagittal reformats are often done in 2-3mm slice thickness and this is usually sufficient for accurate image interpretation. Volume shaded reconstruction area routine in a number of centres but are not universally acquired. They are often requested by non imaging doctors as they find these easier to understand. There is some controversy regarding the necessary slice thickness required to confidently identify bony injuries. It has been proven that a slice thickness of 3mm misses a significant number of bony injuries when compared with 1mm slice thickness. Furthermore fractures in the xy plane, ie a base of dens axis fracture, can be overlooked with thick slices and lack of overlap in the reconstruction algorithm (Link, Meier et al. 1996; Obenauer, Alamo et al. 2002; Hauser, Visvikis et al. 2003; Wintermark, Mouhsine et al. 2003; Roos, Hilfiker et al. 2004; Mulkens, Marchal et al. 2007; Phal, Riccelli et al. 2008). Generally it is preferable to review the imaging on a 3d workstation rather than a viewing station because on a workstation the imaging planes and parameters are easily adjusted for each specific case. When reviewing CTs of the spine bone and soft tissue windows should be used routinely. For assessing neural compromise specialised window settings with narrow width and relatively low centering points can be useful. For patients with metalwork implants the routine CT protocols for the spine are usually sufficient to obtain good quality images. To image metal implants high kv, high mas, thin collimation and low pitch are advised, this is usually already the case for CT protocols for the spine. For the evaluation of neural structures contrast enhanced techniques such as CT myelography are useful. Reversed gantry CT is no longer necessary when using modern MD-CT, it is preferable to acquired axial images and review appropriate reformats. 4. Clinical issues Imaging of the whole spine is not commonly done to primarily image the spine, MRI is generally preferred if possible. If the whole spine is imaged it is more commonly part of a trauma protocol. In tumour imaging large parts of the spine are frequently imaged, typically the thoracic, lumbar and sacral spine as part of a CT chest/abdomen/pelvis. In PET-CT imaging the whole body is frequently imaged, including the whole spine. Page 4 of 7
CT of the spine is not sensitive for neural abnormalities or tumour infiltration. CT can show lytic or sclerotic bone lesions but marrow infiltration without bony response is difficult to image. The strong point of CT is bone imaging, therefore it is excellent for trauma imaging, it can also assess bone change in reaction to tumour infiltration. CT of the spine does demonstrate bony abnormalities in developmental anomalies such as scoliosis or dysraphism well, but imaging of the whole spine is results in a high patient dose. The estimates about the received effective dose when performing CT of the whole spine vary. A recent publication put it as high as 41.5 msv (though the exact imaging protocol was not provided) (Biswas, Bible et al. 2009). The cervical spine was found to be exposed to an effective dose of about 4.4 msv, the thoracic spine 18.0 msv and the lumbosacral spine to 19.2 msv. The estimated effective doses in this study were higher than in most other publications. Most studies use the calculated exposure factors as obtained from the scan parameters for dose assessment; in the study of Biswas et al the dose was actually measured. The reason for the discrepancy is not clear and unfortunately a comparison with the dose as calculated by the imaging parameters was not done. It has been suggested to use low dose CT protocols for trauma imaging of the cervical spine to reduce the patient dose (Mulkens, Marchal et al. 2007). The authors have assessed the impact of dose reduction by the reduction of the kv and mas and found low dose protocols still to be satisfactory despite reduced image quality. Most radiologists will prefer good imaged quality resulting in high diagnostic confidence. Also the impact of low dose protocols on soft tissue depiction was not assessed. In the interest of radiation protection judicious use of spine CT particularly in younger patients is necessary. Generally for serial exams and exams covering large parts of the spine MR imaging is preferable. 5. Conclusion In conclusion MD-CT imaging of the spine allows for excellent bone imaging. The patient dose depends on the imaging parameters, in particular the soft tissue contrast resolution suffers when lower dose protocols are used. The highest image quality is achieved with thin collimation, high mas and low pitch. For many applications this might not be necessary and it is part of the radiologist's remit to choose an appropriate imaging protocol. References Page 5 of 7
Biswas, D., J. E. Bible, et al. (2009). "Radiation exposure from musculoskeletal computerized tomographic scans." J Bone Joint Surg Am 91(8): 1882-9. Hauser, C. J., G. Visvikis, et al. (2003). "Prospective validation of computed tomographic screening of the thoracolumbar spine in trauma." J Trauma 55(2): 228-34; discussion 234-5. Link, T. M., N. Meier, et al. (1996). "Artificial spine fractures: detection with helical and conventional CT." Radiology 198(2): 515-9. Mulkens, T. H., P. Marchal, et al. (2007). "Comparison of low-dose with standard-dose multidetector CT in cervical spine trauma." AJNR Am J Neuroradiol 28(8): 1444-50. Obenauer, S., L. Alamo, et al. (2002). "Imaging skeletal anatomy of injured cervical spine specimens: comparison of single-slice vs multi-slice helical CT." Eur Radiol 12(8): 2107-11. Phal, P. M., L. P. Riccelli, et al. (2008). "Fracture detection in the cervical spine with multidetector CT: 1-mm versus 3-mm axial images." AJNR Am J Neuroradiol 29(8): 1446-9. Roos, J. E., P. Hilfiker, et al. (2004). "MDCT in emergency radiology: is a standardized chest or abdominal protocol sufficient for evaluation of thoracic and lumbar spine trauma?" AJR Am J Roentgenol 183(4): 959-68. Wintermark, M., E. Mouhsine, et al. (2003). "Thoracolumbar spine fractures in patients who have sustained severe trauma: depiction with multi-detector row CT." Radiology 227(3): 681-9. Personal Information Dr. Bernhard Tins Robert Jones and Agnes Hunt Orthopaedic and District Hospital Oswestry Shropshire SY10 7AG UK Page 6 of 7
Bernhard.Tins@rjah.nhs.uk Page 7 of 7