When a study is digital stored, it is sub-categorized in the following hierarchy:
Patient [Patient level]
Exam [Study instance level]
Series [Series instance level]
Image [SOP instance level]
For example:
John Doe
CT of the Head
Axial
Image
Patient Level
In the example above, in the real world the patient is John Doe. In PACS, we have archived John Doe, organized by his name.
Study Level or Exam Level
John Doe had a CT of the Head performed in the real world. In PACS, we have obtained the study, CT of the Head.
Series Level
When John underwent the CT, multiple series of images were reconstructed in the real world. A scout series, coronal, sagittal and axial views. In PACS, when the CT of the Head study is opened, the user can see the 4 distinct series.
Image Level or SOP Instance Level
In the real world, each series contained a multitude of images. In PACS, the user can scroll through each image of a series.
In the hierarchy, a patient may have several exams. The exam may have several series. Each series may have several images. The image level is referred to as the SOP instance.