A primer to common major gastrointestinal post-surgical anatomy on CT—a pictorial review

The post-operative abdomen can be challenging and knowledge of normal post-operative anatomy is important for diagnosing complications. The aim of this pictorial essay is to describe a few selected common, major gastrointestinal surgeries, their clinical indications and depict their normal post-operative computed tomography (CT) appearance. This essay provides some clues to identify the surgeries, which can be helpful especially when surgical history is lacking: recognition of the organ(s) involved, determination of what was resected and familiarity with the type of anastomoses used.


Introduction
The post-operative abdomen can be challenging. Familiarity with normal post-operative anatomy can be helpful in diagnosing complications. The introduction of mechanical stapling devices using radio-opaque metallic sutures has greatly facilitated the identification of surgical anastomoses. However, occasionally some or part of anastomoses can be hand-sewn and if no radio-opaque markers are left in place, this is occult on computed tomography (CT), rendering the radiologist's task difficult in identifying an anastomosis. There are three main types of anastomoses [1]. These include: (1) end to end-used when there is adequate luminal diameter on both sides; (2) end to side-used when there is a size discrepancy between the parts to be joined; (3) side to side- Fig. 1 Anastomotic configurations depicted as enteric hand-sewn anastomoses. a End-to-end anastomosis between two segments of small bowel. b End-to-side anastomosis between two segments of small bowel. c Side-to-side anastomosis between small and large bowel used in creation of a large anastomosis in the setting of a narrow lumen (Fig. 1).
A typical staple line has either two or three staggered rows of staples which cannot be individually resolved by CT. In addition there are three basic types of staplers: (1) circular staplers that produce a continuous ring of staples, often for end-to-end or end-to-side anastomoses; (2) linear staplers that produce one staple line, often used to transect a lumen, resulting in a stapled blind end and an open end which is resected; (3) linear staplers that produce a double staple line and include a knife to cut between the lines, used for side-to-side anastomoses, and to transect a lumen but sealing both sides simultaneously. This explains the finding on CT of two long linear staple lines at the site of a gastro-jejunostomy.
The standard practice at our institution is to image patients in the early postoperative period only when there is clinical suspicion of postoperative complications. Follow-up imaging is generally performed at 3-to 6month intervals. At our institution, oral contrast is used on a per patient basis, according to suspicion of anastomotic leak by the clinician. When needed, we administer 750 ml of water-soluble oral contrast agent beginning approximately 1 h prior to imaging, with an additional 250 ml just prior to the start of CT. Additionally, 120 ml of a non-ionic contrast agent is administered intravenously at a rate of 3 ml/s, unless    Gastrojejunal anastomosis (dashed black arrow) between gastric pouch (GP) and the Roux limb (R). In this case, the Roux limb is antegastric. S remnant stomach, BPL bilio-pancreatic limb, black arrow side-to-side jejuno-jejunal anastomosis, CC common channel distal to jejuno-jejunal anastomosis Depending on the extent of disease, the Whipple procedure can be modified. a When feasible, a pylorus-preserving procedure is performed, where the duodenum is not resected, creating a duodenojejunostomy rather than the classic gastrojejunostomy (as illustrated in b). b Classic Whipple anatomy with gastrojejunostomy (dashed black arrow), choledochojejunostomy (black arrow), and an end-to-side pancreaticojejunostomy (white arrow). S stomach, J jejunum   partial gastrectomy (antrectomy) with enteric continuity restored by an end-to-end anastomosis between the remnant stomach and duodenum-a gastroduodenostomy [2]. The entire incised end of the stomach may be used for anastomosis (Polya procedure) or only a part of the lumen may be used and the remainder closed (Hofmeister modification) (Figs. 2 and 3). In Billroth 2 surgery, the anastomosis is made between the remnant stomach and the jejunum-a gastrojejunostomy-with the duodenal stump closed [2]. Variations may include the type of anastomosis (Polya or Hofmeister modification), the type of gastrojejunostomy (Roux-en-Y or Loop-type) and the placement of anastomotic loop (ante-or retro-colic), depending on the clinical situation and surgeon's preference. A retrocolic gastrojejunostomy is the preferred method, as it creates a shorter afferent loop, resulting in less postoperative retrostomal herniation and better nutritional status compared with the antecolic gastrojejunostomy (Figs. 4 and 5). A Roux-en-Y anastomosis typically implies a proximal segment of small bowel and a distal segment of small bowel (which is often a blind end) anastomosed together, representing the two upper limbs of the "Y" and the distal small bowel (beyond the anastomosis) representing the lower part of the "Y".

Bariatric surgery
Bariatric surgeries are a vast subject and may be classified as restrictive, malabsorptive or mixed. Restrictive procedures involve the creation of a small gastric pouch which induces weight loss through satiety. Examples include adjustable gastric banding and vertical-banded gastroplasty. Malabsorptive procedures bypass portions of the small bowel, limiting the digestive processes and absorption to a short segment of ileum. Examples of malabsorptive surgeries include the jejuno-ileal bypass, abandoned because of the severe malnutrition it induces, and biliopancreatic diversion with or without a duodenal switch, which has been slow to gain popularity. Mixed procedures are a combination of both [3].
The Roux-en-Y gastric bypass (RYGBP) is a common procedure that combines restrictive (small gastric pouch) and malabsorptive properties (Roux limb) [4,5]. The standard set-up involves four steps: (1) the stomach is divided, thus creating a small gastric pouch and an excluded stomach; (2) the jejunum is divided distal to the ligament of Treitz by creating a bilio-pancreatic limb and a Roux limb; (3) the Roux limb is joined to the bilio-pancreatic limb via a side-to-side jejuno-jejunal anastomosis. Small bowel distal to this anastomosis is

Puestow
This procedure is indicated in patients with intractable chronic pancreatitis associated with pancreatic duct dilation beyond 6 mm [9]. The pancreas is filleted, incised longitudinally, exposing the main pancreatic duct from neck to tail with extraction of ductal calculi. A Roux-en-Y loop of jejunum is brought up to create a lateral side-to-side pancreaticojejunostomy with direct drainage of the pancreatic duct into the jejunum (Figs. 10 and 11).

Small bowel
Ileal pouch-anal anastomosis The ileal pouch-anal anastomosis (IPAA) preserves fecal continence in patients who require a proctocolectomy for chronic ulcerative colitis and familial adenomatous polyposis [10,11]. Multiple configurations exist for the ileal reservoir, with the J pouch being the most common. The surgical steps include: (1) performing a colectomy and proctectomy; (2) stapling the distal end of ileum to form a blind end "stump"; (3) folding the distal ileum on itself and creating an apical enterotomy; (4) creating a side-to-side anastomosis by cutting the ileal walls centrally and connecting the two ileal loops to form a large pouch with two parallel staple lines; (5) creating an anastomosis between the anus and enterotomy with a circular stapling device passed trans-anally; (6) performing a temporary loop ileostomy which is usually reversed 6-12 weeks later. Normal CT findings include: a staple line at the blind end of the distal ileum, parallel staple lines along the ileal pouch and a circular staple line at the ileal pouch-anal anastomosis within the sling of the levator ani muscles (Figs. 12 and 13).  Fig. 17 Lower anterior resection. a Axial contrast-enhanced CT images of a 59-year-old man with lower anterior resection for rectal cancer 13 cm from the anal border shows the low colorectal anastomosis (white arrow). There is preservation of the distal rectum (with rectal tube) in the pre-coccygeal area (b). The anus is also spared (not shown) Fig. 19a, b Abdomino-perineal resection. Axial contrast-enhanced CT images of a 78-year-old woman with abdomino-perineal resection for cancer of the lower rectum located 2.5 cm from the anal border.
Note the absence of rectum in the pre-coccygeal space in (a). The anus, anal sphincters and levator ani muscles were all resected and patient had a permanent end colostomy in left iliac fossa (white arrow)

Hartmann procedure
The Hartmann procedure is indicated in patients with complicated diverticulitis, obstructive or perforated sigmoid cancer or penetrating trauma operated on an emergent basis [12,13]. It is performed instead of a primary re-anastomosis because of the risk of anastomotic rupture secondary to inadequate bowel preparation and/or complicating peritonitis. After a partial colectomy or sigmoidectomy, it involves the creation of a temporary diverting colostomy and a blindending rectal or colonic stump closed by suture (Hartmann's pouch). The colostomy can then be taken down in the second stage of the procedure, 4-6 months later, after the peritoneal inflammation subsides (Figs. 14 and 15).

Lower anterior resection
Lower anterior resection is an anal-sparing procedure indicated for lesions of the recto-sigmoid junction and proximal rectum that are over 5 cm from the anal border [13]. It involves resection of the distal left colon, sigmoid and proximal rectum with a low colo-rectal anastomosis or resection including the entire rectum with a colo-anal anastomosis (Figs. 16 and 17).

Abdomino-perineal resection
This resection is indicated for lesions of the distal rectum, less than 5 cm from the anal border, and involves resection of the distal portion of the descending colon, sigmoid, rectum and entire anus (anus, sphincters, levator ani muscles) with creation of a permanent end colostomy (Figs. 18 and 19) [13].

Conclusion
The post-operative abdomen can be complex and challenging. This pictorial essay describes the appearance of a few selected common gastrointestinal surgeries. Familiarity with normal post-operative anatomy can aid in the detection of complications.