3:2 and 4:3 equivalent how?

Ok inscribed sounds to me how sensors are ie the sensor is in the image circle. Circumscribed has the entire image circle on the sensor?

Wouldn't the corners of the senor be outside the image circle and thus the image would kind of suck without cropping if that was the case? Do both these methods actually exist or are you describing a theoretical camera? Now that I think about it I suppose a circular fish eye would fit this description.