History: 25 year old asymptomatic male.
There are many pitfalls to the portable chest radiograph. The difference is in where the x-ray beam is being shot from and where the film is placed. In a normal PA radiograph, the x-ray beam is coming from behind the patient and the heart is closest to the film. In this manner, there is minimal magnification of the heart. However, during a portable chest radiograph, the beam is shot from the front of the patient, and the heart is relatively further from the film, with greater magnification. Therefore, on a AP (portable) chest radiograph, the heart can falsely appear to be enlarged. A good resource for this phenomenon is found here.