Penetration and Exposure in Chest Xray

 Penetration is assessed by visualising  the lower thoracic vertebral bodies, whose outline should just be visible through the heart  on a PA projection

If  the outline of  spine cannot be visualised  through the heart, the film is underpenetrated

In underpenetrated film,pulmonary vessels and interstitial markings appear more prominent, loss of detail at the lung bases and vertebrae, results in increased density.