The inability to straight leg raise should raise clinical suspicion of an injury to the extensor mechanism.  Ultrasound can be a useful first investigation, and further care must be taken when scanning those who can still straight leg raise, as partial or near-complete full-thickness tears as shown can present as so. 

Large knee joint effusions can also cause an inability to straight leg raise, and large effusions or hemarthrosis in younger patients should warrant MRI to assess for internal knee injuries.  If a lipohemarthrosis is identified, both radiographs and CT should be considered.