What is the recommended source-to-image distance (SID) for a transthoracic lateral projection, Lawrence method of the humerus?

In radiography, particularly for a transthoracic lateral projection using the Lawrence method of the humerus, the recommended source-to-image distance (SID) is typically 40 inches (102 cm). This specific distance is chosen because it optimizes the balance between image resolution and radiation exposure to the patient, ensuring that the image captures the anatomy clearly while minimizing scatter and maintaining appropriate exposure levels.

Utilizing a SID of 40 inches allows for adequate magnification of the humerus while still providing a clear and diagnostic image. This distance aids in reducing geometric distortion, particularly important when imaging areas that can be overlapped by other anatomical structures, as is common with the humerus.

Setting the SID at distances shorter or longer than 40 inches could potentially result in poorer image quality due to increased magnification or distortion, respectively. Therefore, the commonly accepted SID for this particular projection is 40 inches, making it the correct answer.