The appearance of which structure indicates the beginning of the venous phase in a cerebral blood flow study?

The appearance of the superior sagittal sinus indicates the beginning of the venous phase in a cerebral blood flow study. During a cerebral perfusion study, there are distinct phases of blood flow that can be visualized: the arterial phase and the venous phase. The superior sagittal sinus is a major venous structure in the brain that drains blood away from the cerebral hemisphere. When this sinus becomes visible on imaging, it signifies that the arterial supply is transitioning to the venous drainage phase of blood flow.

In the arterial phase, structures like the middle cerebral artery or basilar artery would be more prominent as they are involved in the arterial supply to the brain. However, these do not reflect the onset of the venous phase. The inferior vena cava, while a major vessel for venous return in the body, is not related to the cerebral venous drainage and thus would not be part of cerebral blood flow studies. Therefore, the observed presence of the superior sagittal sinus serves as a clear marker that the study has entered the venous phase.