EOS Imaging of Scoliosis, Leg Length Discrepancy and Alignment
Introduction
The EOS machine (EOS Imaging, Paris, France) is a biplanar slot scanning system used to obtain standing radiographs of the spine and lower extremities, and is used primarily in the evaluation of scoliosis, leg length discrepancy, and malalignment of the lower extremities, often in the pediatric population. The simultaneous biplanar imaging allows the creation of 3D surface reconstructions of the spine and lower extremities, and the highly efficient detector system permits imaging at relatively low radiation dose. These features of the EOS offer advantages over computed radiography (CR), digital radiography (DR), and computed tomography (CT). In this article, we will review the unique EOS imaging technology, the imaging goals in the evaluation of scoliosis and lower extremity deformity in pediatric patients, as well as the benefits and challenges of EOS compared to other imaging options.
Section snippets
EOS
EOS imaging system first became commercially available in Europe in May 2007, with the first unit installed in the United States a year later.1 In addition to being an upright scanner, EOS has technological advances not seen in other radiography systems: (1) A slot scanner and multiwire proportional chamber (MWPC), which combined produce high resolution, low dose images. (2) Biplanar imaging (Fig 1).
Biplanar Imaging
The EOS scanner is not the first to use the concept of the MWPC to produce low dose images. Kalifa et al reported its use in an upright slot scanning x-ray beam system in 1998.5 However, unlike earlier slot scanning systems, the EOS scanner has two perpendicularly oriented source-collimator-detector units, allowing 2 orthogonal (frontal and lateral) radiographs to be obtained at the same time. In addition to decreasing the imaging time for the examination, these concurrent orthogonal views
Applications of EOS imaging
The most common applications of EOS scanner are imaging of scoliosis and assessing alignment abnormalities of the lower extremities.
How to perform EOS imaging
For entire spine imaging, EOS studies are typically performed with patients standing in the scanner, with neutral spine alignment. Ideally, the patient's isocenter is positioned in the center of the machine, with the aid of red laser light guides (Fig 6). In the transverse plane, isocenter is defined as the mid spine, and in the sagittal plane, by convention, it is a line from the external auditory canal to the lateral malleolus. In patients with severe curvature, lordosis or kyphosis, these
EOS Versus Alternative Imaging Methods
EOS is the only currently available imaging option that offers simultaneous acquisition of frontal and lateral views which allows for 3D post processing. In most imaging departments, entire spine and full-length lower extremity exams are performed with either CR or DR equipment. For CR imaging, a long cassette with 3 embedded phosphor plates to cover the desired longitudinal field of view is positioned anteriorly or posteriorly and a single x-ray image is obtained. The 3 images are stitched
Scoliosis
Frontal and lateral radiographs can give important information about lateral spine curvature and sagittal balance. However, two dimensional radiographs are of limited value in demonstrating the axial rotation of vertebral bodies, which has been linked to the primary deformity in idiopathic scoliosis.4,21 Also, complex segmentation abnormalities can be difficult to appreciate on radiographs. The three-dimensional deformity of a scoliotic curvature has to be accurately demonstrated to allow for
Radiation Dose and Dose Reduction Techniques
Scoliosis is the most common skeletal disorder in the pediatric population that requires serial imaging, with 1 in 300 adolescents treated for scoliosis.1 Depending on the severity of the scoliosis and the treatment plan, patients may have radiographic assessment of their spine every 3-12 months.35, 36, 37 Not surprisingly, patients with more severe, complex scoliosis tend to have higher imaging radiation exposure. Patients with surgically treated adolescent idiopathic scoliosis (AIS) have a
Motion artifact
Artifacts have long been a problem in the imaging of scoliosis. Most commonly, this is stitch artifact in digital imaging systems incapable of imaging the entire spine in one field of view. When separate images of the thoracic and lumbar spine or obtained and then spliced together there is potential for malalignment of the images, particularly if the patient moves between the two exposures. 16% of scoliosis images obtained with this technique had stitch error that could be interpreted as an
Edge Enhancement
Edge enhancement artifacts are not unique to EOS imaging; it is inherent in digital radiographs. Edge enhancement is seen as radiolucency at the margins of a high contrast object, such as a spinal hardware, where it can simulate hardware loosening, or the left heart border, where it can simulate pneumomediastinum (Fig 14).
Cost
In addition to efficacy, it is important to consider cost effectiveness of a technology.
The current cost to install a new EOS unit is approximately $950,000. A large, dedicated space is required for the unit which has a foot print of 6.5 × 6.5 feet and is 8.9 feet high. In contrast, the single-exposure long DR panel is approximately $125,000 added to an existing DR unit. The least expensive option is the multi-exposure DR detector wall-mount which can be added to most current DR units for less
Conclusion
EOS offers high quality, low radiation dose imaging of the spine and lower extremities that requires substantial monetary and facilities expenditure. The advantages associated with extremely low radiation dose in patients with significant potential cumulative exposure as well as the potential benefits of 3D rendering make it an important capital consideration for imaging departments with high pediatric orthopedic volume. Many of the diagnostic advantages of the EOS scanner are dependent on the
Acknowledgement
We would like to thank our model, Erica Oliveri, RT.
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