Distention.
Adequate colonic distension is necessary for virtual colonoscopy.
This is obvious as in a partially or non-distended colonic segment tumoral lesions can be overlooked.
Optimal colonic distention is achieved with:
insufflation of the colon with CO2,
scanning the patient in supine and prone position (dual positioning).
Most observers agree however that the use of hyoscin buthylbromide (Buscopan®) improves colonic distension. Furthermore it also improves patient comfort during VC by reducing abdominal cramps. It is usual to administer 20 mg of hyoscin buthylbromide intravenously immediately before starting colonic inflation.
Colonic inflation can be obtained by manual inflation of room air. However, before the use of CO2 to inflate the colon patients frequently suffered from post-procedural discomfort s. a. abdominal fullness, bloating, cramps,… due to the insufflation of the colon with air. In fact the air had to be expelled after the exam. The use of hioscin buthylbromide made this a difficult task.
The introduction of CO2 resolved these post-procedural inconveniences. In fact CO2 is reabsorbed 35 times faster than room air. So even before the patients leave the CT suite the insufflated CO2 is completely reabsorbed.
Furthermore an automated CO2-injector can be used making colonic inflation rapid and more uniform with improved colonic distension . The use of CO2 with an injector is now considered as the standard method to insufflate the colon.
Finally to further improve colonic distention the patient is scanned in both supine and prone position. This technique of dual positioning allows to distend parts of the colon who were collapsed because of a dependent position in the supine which becomes non-dependent in prone and vice versa.
All experts agree that the patient is to be scanned in supine and prone position. Alternatively, patients can be scanned in supine and left decubitus position, which is especially helpful in the frail and elderly .
The act of scanning twice in a different position is called dual positioning. In fact the lower or dependent parts of the colon tend to collapse: the recto sigmoid colon in the supine and the transverse colon in the prone position. By turning the patients in the other position these segments become non-dependent and show good expansion because of the CO2 moving to the higher parts of the colon.
Furthermore polyp conspicuity improves because lesions tend to prolapse more or less into the colonic lumen in one of the two positions.
