To answer this question, we first need to discuss briefly how X-rays work.
When X-rays travel through the body, they are absorbed in different amounts by different tissues, depending on the properties of these tissues, in particular, how radiologically dense these tissues are.
Structures such as bone contain calcium which helps them absorb X-rays more readily. This, in turn, produces high contrast on the X-ray detector causing bony structures to appear whiter than other tissues on an X-ray image.
Conversely, X-rays travel more easily through less radiologically dense tissues such as fat and muscle, as well as structures that are air-filled, like our lungs. These structures are displayed in shades of grey on a radiograph.
Phleboliths and calcification have differing radiologic densities and, depending on how white they appear on a film, radiologists are usually able to guess at their likely consistency and, hence, what they are likely to be when a white image appears on a bladder X-ray.
Unfortunately, bacterial biofilms are too small and aren't radiologically dense. Hence, they would not appear as an image on an X-ray.
Identifying phleboliths/calcifications is important for UTIs because they are a nidus for infection to grow (which can result in recurrent infections). Calcifications in the wrong places (like the ureter) can also result in discomfort, obstruction and infection and, depending on size and location, may even require surgical removal.
