Bruhl et al found an unexpectedly poor correlation between MRI and clinical findings in achondroplastic children with regard to indications for decompressive surgery at the craniocervical junction (CCJ) and suggested the role of MRI is limited to demonstrating spinal cord compression in individual cases. ^[9]

In a study of atlantoaxial subluxation using MRI, Yamashita et al demonstrated that the severity of myelopathy had a poor correlation with the atlantodental interval on conventional radiography; however, there was a high correlation between magnetic resonance grading and the degree of myelopathy. ^[10]

Kao et al suggested that MRI is useful in delineating the many abnormalities of the cranial, cerebral, and cervicomedullary junction present in children with achondroplasia. ^[11] All 10 children in their study had narrowing of the subarachnoid space at the level of the foramen magnum, and 5 had compressive deformities of the cervicomedullary junction. In addition, apparent upward displacement of the brainstem and a relatively vertical course of the optic nerve were seen in all patients, dilated lateral and third ventricles were seen in 5 patients, bifrontal widening of the subarachnoid space was evident in 4 children, skull asymmetry was seen in 2 patients, and an empty sella (confirmed on metrizamide cisternography) was present in 1 child. In 1 patient, foci of abnormal signal intensity were seen in the cervicomedullary region. ^[11]

The amount of blood flow in the superior sagittal sinus is correlated with brain maturation. Hydrocephalus associated with achondroplasia is closely related to reduction in blood flow in the superior sagittal sinus — a finding that supports the hypothesis that in patients with achondroplasia, hydrocephalus results from a restriction of venous outflow.

Cine phase-contrast MRI is a convenient and effective method of measuring volumetric flow rates in vivo. In healthy children, the flow velocity is 92-196 mm/s (mean, 136 mm/s), and the flow rate is 189-688 mL/min (mean, 484 mL/min). The flow rates change in a manner that is statistically related to age: flow rate rapidly increases during the first 2 years and reaches a peak by 6-8 years of age. The flow velocity shows a similar pattern but without significant statistical correlation. In all cases of achondroplasia with hydrocephalus, both flow values are lower than the reference values by 1 standard deviation. In cases of achondroplasia without hydrocephalus, as well as in obstructive hydrocephalus, the values are not reduced.^{[12, 13, 14]}

High-signal intramedullary cord lesions in achondroplasia on T2-weighted MRI just below the craniocervical junction are a common finding. These lesions are usually asymptomatic and unaccompanied by cord compression signs. The lesions occur in a third of achondroplasia population. Their etiology is unclear and seems to be unrelated to mechanical causes. ^[15]