but it's not
If your sphere is a class deriving from graphics item (and it should be), and it lives in a scene (from what you said, so I assume it's QGraphicsScene) then in the constructor of the sphere set setAcceptHoverEvents( true ), and reimplement hoverEnterEvent and hoverLeaveEvent.
i draw my spheres in the window of qt but i using ogre
and i store my spheres in vector to draw it i use
ent_sphere = ogreSceneManager->createEntity( sphere[i].name,Ogre::SceneManager:: PT_SPHERE);
sphereNode= ogreSceneManager->getRootSceneNode()->createChildSceneNode(sphere[i].name, Ogre::Vector3(kk, kkk, kkkk));
sphereNode->setScale(0.1f, 0.1f, 0.1f);
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