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Background
Osteogenesis imperfecta (OI) is a heterogeneous group of rare genetic skeletal disorders, characterized by bone fragility. OI has both skeletal and extraskeletal manifestations and variable severity, from mild to lethal. Although the diagnosis remains clinical, imaging plays a key role in the diagnostic pathway of OI, as well as in monitoring the disease over time and assessing response to therapy. In addition to traditional imaging modalities like conventional radiography, CT, MRI, ultrasound (US), and dual-energy x-ray absorptiometry (DXA), innovative techniques with the potential to improve the characterization of bone fragility in OI are emerging, including high-resolution peripheral quantitative CT (HR-pQCT) and quantitative ultrasound (QUS).

Educational Goals / Teaching Points
The goals of this pictorial essay are to: 1) review the main imaging techniques used for diagnosis and monitoring of OI with their associated findings; and 2) define the most appropriate applications of each technique in current practice.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
For prenatal diagnosis of severe OI, US is the first-line examination. Suggestive features include bone fractures or deformities (bowing/angulation) and decreased skeletal echogenicity due to insufficient mineralization. In difficult cases, as second-line techniques, when a greater certainty is needed, low-dose CT with 3D reconstructions or (rarely) MRI can help confirm diagnosis. On the other hand, in the postnatal setting, plain radiographs are the preferred initial imaging modality. The main radiographic features of OI are generalized osteopenia/osteoporosis, bone fractures, and bone deformities (including bowing of long bones). Zebra lines are also observed following cyclical treatment with bisphosphonates in children with OI. Hyperplastic callus formation and calcification of interosseous membranes is typical of OI type V. CT and MRI are appropriate as problem-solving techniques or for extraskeletal manifestations. DXA is commonly used to identify and monitor low bone mineral density, but its accuracy is limited by previous fractures, bone deformities, metallic hardware, and short stature. Innovative imaging modalities may improve the characterization of bone fragility in OI, such as HR-pQCT, which is a 3D technique capable of quantifying bone microstructure and geometry, or QUS, which is a portable, inexpensive, and radiation-free modality to gain information on bone quality in terms of mineralization and microstructure.

Conclusion
To conclude, after highlighting the specificities and peculiarities of all these different techniques and their possible use in the diagnosis and monitoring of OI, it is important to underline the emerging need for continuous active research in the field as well as for the definition of international and standardized guidelines able to improve patient care by enhancing the use of these imaging modalities strategically and synergically.