You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

MRI morphometric characterisation of the paediatric cervical spine and spinal cord in children with MPS IVA (Morquio-Brailsford syndrome). Nearly all children with MPS IVA develop skeletal deformities affecting the spine. At the atlanto-axial spine, odontoid hypoplasia occurs. GAG deposition around the dens, leads to peri-odontoid infiltration. Transverse/alar ligament incompetence causes instability. Atlanto-axial instability is associated with cord compression and myelopathy, leading to major morbidity and mortality. Intervention is often required. Does the presence of widened bullet shaped vertebra in platyspondily encroach on the spinal canal and cause spinal stenosis in MPS IVA? So far, there have been no standardised morphometric measurements of the paediatric MPS IVA cervical spine to evaluate whether there is pre-existing spinal stenosis predisposing to compressive myelopathy or whether this is purely an acquired process secondary to instability and compression. This study provides the first radiological quantitative analysis of the cervical spine and spinal cord in a series of affected children. MRI morphometry indicates that the MPS IVA spine is narrower at C1-2 level giving an inverted funnel shape. There is no evidence of a reduction in the Torg ratio (canal-body ratio) in the cervical spine. The spinal canal does not exceed 11 mm at any level, significantly smaller than normal historical cohorts (14 mm). The sagittal diameter and axial surface area of both spinal canal and cord are reduced. C1-2 level cord compression was evident in the canal-cord ratio but the Torg ratio was not predictive of cord compression. In MPS IVA the reduction in the space available for the cord (SAC) is multifactorial rather than due to congenital spinal stenosis.
{"Chemical": []}