Just to add to my response, while the transmission using the above mechanism will happen using SOAP over HTTP, the response from your target system will be ignored by the Idoc in SAP. The IDoc will go into successful status if the response contains a HTTP 200 in the HTTP Header.
So technically, you can make a synchronous call by using a SOAP adapter to send the IDoc from SAP but in reality this continues to be a asynchronous as the response cannot be processed.
Also, if you decide to use this approach, your Sender Interface can be any Interface - as this is treated as a SOAP Interface in PI.
Regards
Bhavesh
PS: I haven't tried this personally and this is my theoretical understanding, will also give a shot over the next day or so and revert.