A 45 yo Female presents to the ED with sudden onset of chest pain, described as worse when taking a deep breath.  She is significantly short of breath and appears distressed.  She recently underwent a total right knee arthroplasty and reports having been bedridden secondary to pain.  Physical exam is remarkable for a right lower extremity with surgical incisions that clean, dry, intact; however, her left lower extremity is swollen with significant tenderness along the popliteal fossa and calf.  

Vitals: Temp 99.2HR 120RR: 28 BP:  130/80 SpO2 90% on RA.

A CTPA is ordered...