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42 Moshe Schein · Sai Sajja · Hans Ulrich Elben
improves your diagnostic yield. One exception to this is when ureteric calculi are
at the top of the differential diagnosis list and a noncontrast study gives almost all
the information required.
Contraindications to Intravenous Contrast Medium
Impaired renal function
History of prior allergic reaction to iodinated contrast medium
Severe asthma or congestive heart failure
Diabetic patient on metformin (if renal function is normal, you can use intra-
venous contrast, but metformin should be stopped for 2 days thereafter)
Multiple myeloma or sickle-cell anemia
Reviewing the Abdominal CT
It is important to note the distance between two CT “slices.” Usually, the technologists use 5-mm intervals between the slices, but it is sometimes helpful to
request 3-mm cuts of the appendiceal area in a clinically challenging case. Also, it
is essential to ensure that you have all the images by looking at the image numbers.
Many hospitals have done away with hard copies and introduced instead picture
archiving and communication systems (PACSs), which make access to images
easier. In this last case, scrolling through the scan gives information that is much
easier to interpret than if individual films are examined.
With individual films, we always begin with a good look at the scout film;
it provides similar information to a flat plate of the abdomen and provides a
“global view.” The visualized portions of the lower lung fields should also be
looked at in both mediastinal and lung windows. Pulmonary infiltrates and pleu-
ral effusions can be easily identified and at times are a reflection of an acute
subdiaphragmatic process. An unsuspected pneumothorax in a trauma patient
will also be obvious in the lung windows.
Although it is easier to concentrate on the area of interest (e.g., the right
lower quadrant in a patient with suspected appendicitis) and look for findings to
support or exclude the diagnosis, it is essential to look at the rest of the abdomen.
One needs to look specifically for the presence of free gas and free fluid and to see
all the solid organs (liver, spleen, kidneys), stomach, small and large bowel, the
pancreas, and blood vessels. One key point is to follow the structure in question
in serial images—stacking—to obtain as much information as possible.
Viewing the images with the PACS you can calculate the Hounsfield units
(HU) for the various structures you see. To remind you:
5 Abdominal Imaging 43
Structure HU
Bone 1,000
Liver 40–60
Blood
a
40
Muscle 10– 40
Kidney 30
Wat er 0
Fat −50 to −100
Air −1,000
a
A fresh clot could measure over 70 HU. Fresh blood about 40 HU, but if you come back the
next day or two, it is as little as 20 HU
Pneumoperitoneum
While an erect chest film can identify a straightforward case of pneumoperitoneum, CT scan is the most sensitive means available for its detection. On a CT scan, gas
collects beneath the two rectus muscles around the falciform ligament (> Fi g. 5.10). It
also collects between the liver and anterior abdominal wall and within the “leaves” of the
mesentery (> Fi g. 5.11). The findings are at times subtle, and only a few bubbles of ex-
traluminal gas are all that is required to make the diagnosis of pneumoperitoneum. The
key to the identification of extraluminal gas is inspection of all the scans of the abdomen
in lung windows. It is easier with PACS as we can manipulate the window settings. Even
if your hospital does not have PACS, the CT scan station will have the ability to do that.
Fig. 5.10. CT: two pockets of extraluminal gas in the epigastric region (arrows)
44 Moshe Schein · Sai Sajja · Hans Ulrich Elben
Fi g. 5 .11. CT in a patient with perforated duodenal ulcer: free gas between the liver and
anterior abdominal wall (arrow). Gas is also seen around the gallbladder (GB) and leakage of
orally administered contrast is seen around the liver
Free Fluid
Free fluid from any source tends to accumulate in the most dependent parts
of the peritoneal cavity, Morrison’s hepatorenal pouch and the pelvis. When there
is a large amount of fluid, the bowel loops float to the midline. In addition to identifying the presence of fluid, measurement of the fluid density offers some clues
regarding its nature: less than 15 HU for transudative ascites and more than 30 HU
for exudative ascites or blood.
Solid Organs
While solid organ pathology is a rare cause of nontraumatic acute abdominal
conditions, CT is the modality of choice in the investigation of the hemodynamically
stable victim of blunt abdominal trauma. Lacerations of the solid organs appear
as linear or branching low-attenuation areas. Subcap sular hematomas appear as
crescentic low-attenuation areas at the periphery. Intra parenchymal hematomas
appear as round or oval collections of blood within the parenchyma.
Hollow Organs
The entire gastrointestinal tract from the stomach to rectum can be traced
in serial sections, and abnormalities should be sought. In case of small bowel obstruction, the cause (e.g., tumor or inflammatory mass) and the site of obstruction
5 Abdominal Imaging 45
(the transition point) can be identified (> F ig. 5.12). The presence of pneumatosis
can be identified more readily with CT than a plain film and, if present, suggests
intestinal ischemia. CT is also sensitive for identifying inflammation, which is suggested by the appearance of tissue infiltration or stranding (> Figs. 5.13 and 5.14).
If intravenous contrast has been administered, then reduced enhancement of
loops of bowel may signal ischemia. Similarly, the origins of the mesenteric vessels
may be inspected to get some idea about patency.
Fig. 5.12. CT in a patient with small bowel obstruction showing the transition
point between the distended proximal and collapsed distal bowel (arrow)
Fig. 5.13. CT scan through the upper abdomen shows a distended thick-walled GB
with marked pericholecystic stranding (arrow) suggestive of acute cholecystitis
46 Moshe Schein · Sai Sajja · Hans Ulrich Elben
Fig. 5.14. CT scan through the right lower quadrant showing thickened appendix
(thin arrow) with periappendiceal fat infiltration (thick arrow), confirming the diagnosis
of acute appendicitis
The various CT scan findings that are associated with acute appendicitis
are as follows:
Appendiceal signs
Appendix >6 mm in diameter
Failure of the appendix to fill with oral contrast or gas to its tip
Enhancement of the appendix with intravenous contrast
Appendicolith
Periappendiceal signs
Increased fat attenuation (stranding) in the right lower quadrant
Cecal wall thickening
Phelgmon in the right lower quadrant
Abscess or extraluminal gas
Fluid in the right lower quadrant or pelvis
Similarly, stranding in the left lower quadrant or thickening of the sigmoid
>
colon suggests d ive rticu litis (
Fig. 5.15). Diffuse thickening of the colon suggests
an inflammatory process like colitis, whether infective or ischemic (> Fig. 5.16).
The retroperitoneum, including the pancreas, should then be looked at; the
presence of stranding and fluid collections around the pancreas suggests pancreatitis.
Retroperitoneal hematoma next to an abdominal aortic aneurysm suggests a leak.
5 Abdominal Imaging 47
It is also important to look at the pelvic organs in female patients. Particular
attention should be paid to any large cystic masses in the adnexa, which may
suggest a complicated cyst, ovarian torsion, or a tubo-ovarian abscess.
Your patient does not require a CT ticket to enter the OR (> Fig . 5.17), but occasionally CT will change your operative plans or even cancel the need for the operation.
Fig. 5.15. Contrast-enhanced CT scan of the lower abdomen showing thickening of
the sigmoid colon with diverticula and surrounding inflammation (acute diverticulitis)
Fig. 5.16. Contrast-enhanced CT scan showing thickening of the hepatic flexure of
the transverse colon (arrow) suggestive of colitis
48 Moshe Schein · Sai Sajja · Hans Ulrich Elben
Fig. 5.17. “Where is the CT?”
Invited Commentary by a Radiologist: How to Read and Interpret
the Abdominal CT for an Acute Abdomen
Hans Ulrich Elben
How to Order a CT Examination
Contrary to what you may think, “some radiologists” understand something
about medicine and surgery. And, a few of us know something about CT scans. We
therefore respectfully request that you please provide us with an accurate clinical
picture and your tentative diagnosis when requesting a scan. You should tell us
also about any relevant previous operations or injuries (like cholecystectomy,
appendectomy, hysterectomy).
Technically State-of-the-Art CT Examination
A good CT examination is performed with a spiral CT after intravenous
administration of a contrast medium. If possible, we also like to use an oral
diluted Gastrografin medium. The latter can also be given rectally, especially
when suspecting acute diverticulitis, an obstructing colonic lesion, or colonic
trauma. In women with suspected gynecological pathology, you should mark
the position of the vagina with a normal vaginal tampon. An important exception: in case of suspected ureteric colic, the use of oral contrast is not
necessary.
5 Abdominal Imaging 49
Interpretation
Start with a scout view, similar to a plain AXR in a supine patient. Look at the
distribution of gas in the stomach and the small and large intestine. Are there signs
of free gas outside the intestinal lumen? It is absolutely necessary to look at the CT
images in a special window for chest examination (center −700 HU, window width
2,000 HU) as well as in a normal window (center 40 HU, window width 400 HU).
Thus, you will recognize free gas outside the intestinal lumen much better.
Step-by-Step Interpretation of Images by Organs
Try to examine every organ from cranial to caudal direction completely.
Especially note the limits and the structures of the tissues.
Liver
Look at edges of the liver, homogeneous enhancement, and luminal contrast
within the portal vein and its branches. Important diagnoses are blunt trauma
with rupture of the liver, abscesses, portal vein thrombosis (
>
Fig. 5.18).
Gallbladder and Bile Ducts
The intrahepatic bile ducts accompany the branches of the portal vein.
Normally, they are hardly recognized unless dilated. If there is cholangiectasis,
Fig. 5.18. Abdominal CT: intrahepatic abscess. Note the enhancement of the wall
50 Moshe Schein · Sai Sajja · Hans Ulrich Elben
Fig. 5.19. Abdominal CT: acute cholecystitis
follow the common bile duct down to the duodenal papilla. Do you see any signs
of tumor-associated obstruction or choledocholithiasis?
Normally, the wall of the GB is thin (about 2–3 mm). A distended GB, thickened wall, a pericholecystic layer of fluid, a “halo” sign, and intramural air are
strong indications of cholecystitis (> Fig. 5.19).
Spleen
Notice the size and form of the spleen. Is there homogeneous enhancement?
Important diagnoses include traumatic or spontaneous rupture with lack of contrast and fluid around the spleen and infarct of the spleen with a hypoperfused
wedgelike area.
Pancreas
The position of the pancreas is from the hilum of the spleen (cauda pancreatici), in front of the contrast-enhanced splenic artery and vein and superior
mesenteric artery and vein to the duodenal loop (caput pancreatici). Normally, the
pancreas shows uniform homogeneous enhancement. In pancreatitis, the organ is
enlarged diffusely. In pancreatic necrosis, parts of the gland do not light up with
5 Abdominal Imaging 51
contrast. The surrounding fatty tissue is not dark and inconspicuous by comparison
but shows bright streaks. Fluid around the pancreas signifies inflammatory exudate.
Kidneys, Ureters, Urinary Bladder, and Urethra
Kidney stones you will see best in a native (i.e., not contrasted) scan within
the renal pelvis or one of the ureters. The ureters have to be examined along their
entire course from the renal pelvis to the bladder. Is there any dilatation? Any tissue reaction surrounding calcification (rim sign)? Irregular spotty contrast of the
renal tissue refers to nephritis, and wedge-shaped absence of contrast implies a
renal infarct. In renal vein thrombosis, the renal vein does not enhance with contrast. Streaky changes in the perirenal fatty tissue suggest inflammation.
Organs of the Pelvis
For women: examine the uterus and the adnexa positioned laterally to it. Do
you see cystic structures (ovarian cysts)? Do you recognize inflammatory signs in
the surrounding fatty tissue, or is there fluid concentration with enhancement of
its wall (tubo-ovarian abscess)? Are there signs of bleeding?
For men: identify bladder, prostate gland, and seminal vesicles.
Stomach, Gut, and Peritoneal Cavity
Examine the whole intestinal tract starting with the stomach and following
the small bowel from duodenum to jejunum, ileum down to the ileocecal valve, the
cecum and the ascending, transverse, descending, and pelvic colon to the rectum.
CT features of obstruction and inflammation and other specific conditions are
discussed elsewhere in this book. An inflamed Meckel’s diverticulum can be identified by a diverticulation of the intestinal lumen with streaky reactions of the
surrounding tissue (> Fig . 5.20). In the right lower quadrant, look for the cecum
and the vermiform appendix; signs of acute appendicitis are well described in the
previous section. In active Crohn’s disease, you will often recognize a considerably
thickened wall of the terminal ileum.
In the descending and pelvic colon, you should look for diverticula and
signs of inflammation: thickened wall and streaky thickened structures in the
pericolic fat. Complicated diverticulitis is suggested by extraluminal gas, leakage
of contrast, and an abscess (> Fig. 5.21). Appendagatis is an inflammation of the
appendix epiploica and needs no operation (> Fig. 5.22). Colonic diverticula tend
to perforate in the high-pressure zone above an obstructing carcinoma. CT is not a
good tool for distinguishing a colonic inflammatory mass from a malignant one.
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