TRUS applications in anal and rectal diseases

Abstract

Transrectal ultrasound (TRUS), sometimes named endoanal ultrasound, is performed with dedicated endoprobes and in the hands of an experienced user, can represent an indispensable tool in evaluation of anal fistulae and abscesses in the perianal and – rectal region plus staging of anal and rectal cancer. The spatial resolution of TRUS is unsurpassed, however, MRI or CT should be used to provide a complementary overview of the entire region. The anal canal constitutes the distal approximately 4 cm of the bowel system and represents the transition between mucosal lining in the rectum and squamous cell lining of the skin. The ultrasound appearance is dominated by the change in anatomy of the muscular layers of the wall which in this small segment is made up solely of the circular, concentric muscles representing the internal and external anal sphincter. The anal canal is divided into 3 levels relating to the sphincter muscles. In rectum 5 distinctive layers corresponding to the underlying anatomy can be identified with TRUS and correlated to disease. TRUS guided intervention by means of biopsy or drainage can be performed with a rigid US probe as well as with flexible echoendoscopes. The rigid US probe with an end-fire design is optimal for endo-rectal interventions and in most cases can visualize the entire rectum including perirectal structures. With use of a dedicated needle guide mounted on the probe this technique enables precise puncture of most rectal or perirectal target seen on TRUS. TRUS guided biopsy of rectal masses is an important supplement to the conventional endoscopic biopsy if the latter technique does not provide the final diagnosis. Indications are staging of rectal cancer or suspicions of local recurrence. In these cases, a TRUS guided biopsy of the rectal masse with a 1.2 mm needle (18 gauge automatic or semi-automatic biopsy systems) can provide a deep tissue sample that often results in a final histologic diagnosis. In addition, TRUS guided biopsy is an obvious technique to provide histologic or cytologic information about enlarged lymph nodes or other suspicious perirectal masses. TRUS guided drainage of perianal and deep pelvis abscesses or other fluid collections is a safe approach and effective treatment in cases where the transabdominal percutaneous access cannot be utilized because of overlying structures. This technique provides an alternative to other access routes such as transvaginal, transgluteal or transperineal technique Smaller abscesses can often be treated with needle aspiration followed by saline irrigation, whereas larger abscesses may require continuous drainage through a catheter, which can be inserted with Seldinger technique. A preprocedure CT examination of the lower abdomen is recommended to visualize the exact location and size of the abscess in order to optimize drainage strategy.