Select QGraphicsItem - qt

Question

I have problems selecting my QGraphicsItem.
I have multiple Items (Line and Ellipse) and Itemgroups (containing lines).
In some cases they are overlaying each other but are still visible because of transparency.
For example: I first create a small circle and later i create a bigger circle around the small one.
How can I select the small circle?
I only want an item beeing selected if i click on a visible nontransparent part of it (maybe with some tolerance for lines of 1px width). How can I do that?
A small example how i create a circle:
circleToDraw = new QGraphicsEllipseItem;
this->addItem(circleToDraw);
circleToDraw->setPen(QPen(Qt::black, 1, Qt::SolidLine));
circleToDraw->setPos(origPoint);
circleToDraw->setRect(origPoint.y() - event->scenePos().y(), origPoint.y() - event->scenePos().y(), (event->scenePos().y() - origPoint.y())*2, (event->scenePos().y() - origPoint.y())*2);
Later i will also implement some QImages with transparent parts. Here the problem will be again, that I want to select Items under the transparent part.
Thank you!
Edit
My code for now:
QList<QGraphicsItem *> items = this->items(event->scenePos());
for each(QGraphicsItem* item in items)
{
if(item->opacity() == 1){
//opacity() does not work as expected
//circle will still be deleted when I click the (transparent) inside of it
//item->setSelected(true); //selects all items while pressing mouse, but only top item after releasing mouse. why?
this->removeItem(item);
delete(item);
}
}