Cesium Iodide is the preferred material for the scintillation layer because it can be formed into:

Cesium iodide is favored for the scintillation layer in digital radiography primarily because it can be formed into columns that effectively focus light photons. This columnar structure allows for improved light collection efficiency and directional light emission, which enhances the performance of the imaging system significantly. When x-rays interact with the cesium iodide, the resulting light photons are emitted preferentially along the direction of the columns, allowing for more precise and concentrated detection by the underlying imaging receptor. This leads to better image quality with higher resolution and improved contrast.

In contrast, other structural forms such as granules or thin sheets do not offer the same level of light collection efficiency and focusing capability, which can lead to scattering and loss of image definition. Dense layers, while important for other aspects of imaging, do not specifically address the need for optimal light channeling that the columnar form provides. This unique property of cesium iodide makes it the optimal choice for scintillation layers in digital radiography systems.