Gas is a normal and common sign in imaging in different body tissues and internal cavities. Usually, it can be a benign incidental find, but in other cases may disclose life-threatening pathology. Abnormal gas can be found from trauma, iatrogenic injuries, interventional procedures or infection to inflammation.
1. HOW IS GAS SHOWN IN EACH IMAGING TECHNIQUE?
The air is one of the five basic radiographic densities: (air, fat, water, bone, and metal) and constitutes a physiological and pathological marker. It is its location that indicates whether it is normal or not.
Radiologically, the air manifests itself differently according to the imaging technique we use.
On X-Ray, gas is the most radiolucent material visible on a film.
On ultrasound, gas appears as a bright reflective surface with shadowing that obscures the underlying anatomy, with either long path reverberation artifacts (for large gas collections) or short path ‘ringdown’ artifacts (for small gas collections). We should remember that, due to it's low density, gas will generally move in an anti‐gravitational direction to the most elevated areas, so its location changes depending on the position of the patient.
Fig. 1: Reverberation artifact due to intestinal normal gas on abdominal ultrasound.
On CT air is assigned a value of −1000 Hounsfield units, this low density allows us to recognize gas bubbles without the administration of any exogenous contrast agents. Air shows as “black” in all the windows, but moreover we can optimize the visualization of small volumes of gas using the lung window.
Fig. 2: Normal luminogram in a contrast enhanced abdominal CT scan.
This low density also proportionally reduces the available signal intensity of the gas in MRI, appearing as an absence of signal in all sequences.
Fig. 3: Normal air in the respiratory tract in MR image.
2. WHERE CAN WE FIND GAS UNDER NORMAL CONDITIONS?
Physiologically, gas can be found in the paranasal sinuses, mastoid cells, respiratory tract (pharynx, larynx, trachea, bronchi, lungs) and the gastrointestinal tract (esophagus, stomach, small bowel and colon).