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90 M.G. Mutch, E.H. Birnbaum, and C.O. Menias
FIGURE 6-34. CT scan of SBO with evidence of ischemia.
information that can indicate concern for vascular compromise. Thickened, congested bowel with increased attenuation
at the site of obstruction associated with engorgement of the
mesenteric vasculature is concerning for strangulation (Figure
6-34). The mesentery may become hazy or the vasculature
may be obliterated as the inflammation progresses and it
becomes filled with fluid or even blood. Other findings of
ischemia include lack of enhancement after IV contrast
administration or the presence of ascites. The presence of
pneumatosis and portal venous gas are the more ominous
signs of intestinal ischemia. Finally, a spiral pattern of
engorged mesenteric blood vessels may indicate an internal
hernia or rotation of small intestine around fixed adhesions.
Postoperative Evaluation
CT has greatly impacted the postoperative evaluation of the
surgical patient. It is typically used to evaluate a patient with
abdominal pain, fevers, leukocytosis, or persistent ileus in the
postoperative period. The yield of a CT scan is greatest when
it is obtained 5 days or more after surgery. Before postoperative day five, it is difficult to differentiate normal postoperative intraperitoneal free air or fluid from air or fluid that
represents a leak from a hollow viscus or infected fluid. It
usually takes more than 5 days for an abscess to organize into
a walled-off, contained collection. Once again, the findings of
the CT scan must be interpreted in the context of the clinical
condition of the patient. Therefore, the yield will be greatest
when the scan can address a specific question.
Findings highly suggestive of an anastomotic leak
include an inappropriate volume of free air or fluid in the
abdomen. The presence of extraluminal oral contrast
FIGURE 6-35. CT scan of colorectal anastomotic leak.
confirms a perforation of a hollow viscus. The presence of
localized fluid and air around an anastomosis are concerning
for a leak but must be taken in context to the postoperative
period and the condition of the patient. As mentioned above,
water-soluble enemas are more sensitive than a CT with rectal contrast at detecting a colorectal anastomotic leak.
However, a CT is often more easily and readily obtained. An
abscess is defined as an organized fluid collection with or
without air that has an enhancing rim (Figure 6-35). As
mentioned above, CT is very good at distinguishing between
an ileus and a mechanical bowel obstruction, which is an
important distinction in the perioperative period.
Other Colitides
There are a handful of inflammatory processes that affect the
colon that have not been addressed. The CT findings are very
similar for all inflammatory processes of the colon. However,
their clinical presentations are different, so combining the
presenting signs and symptoms with the distribution of CT
findings will usually lead to the correct diagnosis. This section will briefly address some these remaining processes.
Neutropenic enterocolitis or typhlitis typically occurs in
patients who are neutropenic either from cytotoxic chemotherapy or severe immunosuppression. The terminal ileum,
cecum, and right colon are most frequently affected. CT is the
study of choice for the diagnosis. Circumferential thickening
of the terminal ileum, cecum, and variably the right colon
are the common CT findings consistent with typhlitis (Figure
6-36). The bowel wall may become so thickened because of
edema that a hypodense ring develops between the mucosa
and musculature. Complications such as pneumatosis or perforation can also be detected.
6. Diagnostic Evaluations—Radiology, Nuclear Scans, PET, CT Colography 91
FIGURE 6-36. CT scan of neutropenic enterocolitis.
Ischemic colitis is the most common vascular abnormality
of the colon. Presenting symptoms include abdominal pain
associated with bloody diarrhea. The age of the patient and
onset of symptoms will help to differentiate between IBD,
infectious colitis, and ischemic colitis. Endoscopy is the gold
standard for diagnosing ischemic colitis. CT is much more
readily available so it is often the first test ordered. The colitis may be segmental or diffuse, typically occurring in the
watershed areas of the right colon, splenic flexure, and rectosigmoid. CT findings consist of thickened, edematous colon
in these areas (Figure 6-37). The typical “thumbprinting” in
the colonic mucosa can be seen on CT scan as well as
plain films. There may be a halo sign of either low attenuation
caused by edema or high attenuation caused by hemorrhage
within the bowel wall. A pericolonic inflammatory response
is often present as well. Thrombus within the colonic
FIGURE 6-37. CT scan of ischemic colitis.
FIGURE 6-38. CT scan of pseudomembranous colitis.
mesenteric vessels may also be seen. Finally, pneumatosis or
portal venous gas may be present indicating bowel infarction.
Pseudomembranous colitis resulting from the toxins produced by Clostridium difficile can cause profound inflamma-
tion of the colon. Computed tomographic findings include
nonspecific thickening and edema of the colon and pericolonic inflammation. Generally, the edema and thickening of
the colon is greater than that seen with infectious colitis or
other inflammatory processes. The presence of pancolitis also
tends to suggest pseudomembranous colitis versus other colitides (Figure 6-38). Once again, the CT results must be interpreted in the clinical context of the patient.
Radionuclide Imaging
Radionuclide imaging studies base their imaging on physiology rather than anatomy, and have a wide spectrum of use in
clinical medicine. Radiopharmaceuticals and gamma cameras
are the mainstay of radionuclide imaging. The specific
radionuclides are chosen based on either the biologic properties of the element (i.e., iodine has an affinity for thyroid tissue) or the physical and chemical properties that allow
linkage to appropriate compounds. These radiolabeled compounds are given to a patient to localize within a specific
organ system (such as the thyroid) or identify the sight of an
ongoing physiologic process (as in GI bleeding). The quality
of a scan depends on how well the agent targets the organ or
the physiologic process. A gamma camera is used to acquire
images once the agent is given to the patient. Gamma and
X-ray photons are absorbed and converted into flashes of
38
light.
The location and intensity of these scintillation events
are determined and recorded. Spot images are generally taken
in 10-minute intervals and the completed image reflects the
92 M.G. Mutch, E.H. Birnbaum, and C.O. Menias
distribution of scintillation events (counts) detected during the
acquisition phase. Image quality improves as the number of
counts increases.
Radionuclide imaging studies are widely used in the diagnosis of lower GI bleeding. The principle is that the intravascular tracer will be extravasated into the bowel lumen
during active bleeding. Concentration of the tracer on the
acquired images allows identification of the bleeding sight.
Technetium
99m
Tc is the radionuclide used in bleeding scans.
This radiopharmaceutical can label colloid or red blood cells
for scanning purposes. Radiolabeled colloid is readily available, but is metabolized rapidly. Red blood cells take longer
to label but clearance of the tagged cells is prolonged and the
tracer can remain active up to 24 hours after injection. Most
centers prefer to use tagged red blood cells because lower GI
bleeds are characteristically intermittent and the opportunity
to identify the active bleeding sight lasts only a few minutes
with labeled colloid. The preparation for a tagged red blood
cell scan requires an aliquot of the patient’s blood to be
labeled with
99m
Tc. Once labeled,
99m
Tc red blood cells are
injected back into the patient and the patient is imaged with
10-minute acquisition intervals for approximately 60–120
minutes. Focal areas of increased activity identified within the
lumen of the bowel indicate that active bleeding occurred during this acquisition period (Figure 6-39). A positive scan may
localize the region of bowel that contains the bleeding site,
but may not accurately localize the specific site, if the bleeding is slow or intermittent. The labeled red blood cells remain
in circulation as long as the cells are viable and the limiting
factor to imaging is the half-life of the
99m
Tc. If no bleeding
occurs during the initial acquisition phase, then delayed views
can be obtained up to 24 hours later to determine whether
active bleeding has occurred. Bleeding scans have greater
prognostic value than diagnostic value when the tracer is only
seen on delayed images.
39,40
The location of activity seen on
delayed images does not reflect the exact bleeding sight but
does indicate that active bleeding occurred during this observation period. The accuracy for localizing the actual bleeding
site increases if the extravasation of tracer is identified within
the first 15–30 minutes.
36
The longer it takes the tracer to
accumulate, the less likely the bleeding site will be accurately
identified by angiography. Backwash and washout of blood
caused by peristalsis account for some of this inaccuracy.
Bleeding scans are more sensitive than angiograms in the
detection of lower GI bleeding. The required rates of bleeding
for detection are lower for bleeding scans (0.1–0.2 mL per
37,41
minute) than for angiograms (0.5 mL per minute).
The
early (within 3 minutes) detection of intraluminal tracer indicates a high likelihood of successful arteriographic localization of the bleeding site. For this reason, some interventional
radiologists require a positive scan before performing angiography. The addition of early colonoscopy to the diagnostic
algorithm requires a bowel preparation. Cathartics will
remove any intraluminal tracer making delayed images
worthless. If the patient is hemodynamically unstable and rapidly bleeding, some centers may prefer to go directly to arteriograms because the time involved in pretest preparation for
tagged red blood scans may be too lengthy.
A Meckel’s scan, although not used as often as the tagged
red blood scan, can be useful in the evaluation of patients with
occult bleeding with no identifiable colonic source. These
scans are generally limited to the evaluation of children and
young adults who have complaints of abdominal pain and
intestinal bleeding. The abnormal bleeding from a Meckel’s
diverticulum is caused by the aberrant gastric mucosa that
lines the diverticulum.
by mucous secreting cells in gastric mucosa.
scans are performed with
99
Tc pertechnetate is actively extracted
99m
Tc per pertechnetate as a radio-
42,43
Meckel’s
label for the detection of ectopic gastric mucosa. Imaging is
usually done for approximately 30–60 minutes after injection
of the tracer. All views should be obtained early because the
tracer is extracted into the stomach and then into lumen of the
GI tract. Rapid transit of the tracer through the GI tract will
obscure extravasation on later images.
A focus of increased radioactivity outside of the stomach
indicates ectopic gastric mucosa (Figure 6-40). Typically, the
collection of activity is identified in the right lower quadrant
within 10–20 minutes. The sensitivity of the Meckel’s scan is
85% and the specificity is 95%.
43
Arteriography
FIGURE 6-39.
pool activity within the ascending colon in this patient with bleeding
after a recent polypectomy.
99m
Tc-tagged red blood cell study shows early blood
Arteriography is an invasive procedure performed by specialty trained physicians and is used in the diagnosis and
treatment of a variety of colorectal diseases. The arteriogram
is performed through a percutaneous approach under sterile
conditions. The femoral artery is a preferred puncture sight
although axillary and brachial arteries may be used. A
guidewire is introduced through the needle and a catheter is
introduced over the guidewire. Various catheters and guide
wires allow the interventional radiologist to access the vessels
in question.
Arteriography is an invasive procedure with an overall mortality of one in 40,000.
44
Complications from the performance
of the procedure and manipulation of the wires and catheters
6. Diagnostic Evaluations—Radiology, Nuclear Scans, PET, CT Colography 93
FIGURE 6-40.
focus of increased uptake in the right lower quadrant, with approximately the same intensity as the stomach indicating gastric mucosa
is present within this Meckel’s diverticulum.
99m
Tc-pertechnetate scan (Meckel’s) shows a discrete
are more common than reactions to the contrast itself.45The
most common complications are related to hematomas or
pseudoaneurysms at the puncture sight, dissection or
embolization secondary to catheter manipulation. Contrast
reactions and contrast toxicity (renal failure) occur in <1% of
studies done. Experience and technique can minimize many of
the complications. Hydration and IV mannitol can reduce the
nephrotoxicity. If the patient has allergies to iodine or has had
a prior contrast reaction, premedication with methyl prednisolone is done 12 and 2 hours before arteriography.
The arteriogram is a useful diagnostic and therapeutic
modality in the treatment of active lower GI bleeding. If a
radionuclide scan is performed and localizes the site of bleeding, a selective angiogram can then be performed. For bleeding
localized to the left colon on tagged RBC study, the inferior
mesenteric artery is selected first. The superior mesenteric
artery is selected first for those bleeds that occur in the right
colon. If the bleeding site is not identified after injection of
both the superior and inferior mesenteric arteries, a celiac run
is performed looking for an upper intestinal bleeding source.
Active bleeding can be diagnosed by the accumulation of contrast in the arterial phase that persists through the venous phase
(Figure 6-41). Bleeding needs to occur at a higher rate for a
positive angiogram (0.5 mL per minute) than for nuclear imaging (0.1–0.2 mL per minute). Because lower GI bleeding can
be intermittent, the bleeding site is sometimes not identified at
the time of the angiogram.
Diverticulosis and vascular ectasias are presumed to be
the leading cause of lower GI bleeding in most patients.
FIGURE 6-41. Mesenteric angiogram shows pooling of contrast in the
sigmoid colon in this patient with surgically proven diverticular
bleeding.
Diverticular bleeds appear as a blush of contrast contained
within a diverticulum. Vascular ectasias often occur in the
right colon and appear as small vascular clusters, a blush in
the wall of the colon and early opacification of a draining
46
vein.
Arteriovenous malformations are developmental in
origin and are often seen in the small bowel. They appear as
tortuous, dilated arteries and early prominent veins. Capillary
telangiectasias (common in Osler Weber Rendu syndrome)
appear as multiple, tiny areas of blush and no arteriovenous
shunting. Postpolypectomy bleeding has been diagnosed and
treated with angiography. A rapid blush of dye occurs at the
site of bleeding and often stops with direct infusion of vasopressin or embolization (Figure 6-42).
Acute mesenteric ischemia is one of the most common intestinal disease processes for which arteriography is used for diagnosis and treatment. Acute mesenteric ischemia can be either
nonocclusive or occlusive. Nonocclusive mesenteric ischemia
arises from a “low flow” state typically secondary to reduction
in mesenteric blood flow from cardiac failure or hypotensive
shock. This diagnosis can frequently be made with clinical
symptoms and computer tomography images. The typical early
angiographic images show diffuse vasoconstriction of mesenteric arterial branches and decreased parenchymal vascularity
(Figure 6-43). In the late stage there is increased accumulation
of contrast in the bowel wall. Treatment includes volume resuscitation and cardiac support. The diagnostic percutaneous
catheter can be used to treat the mesenteric phase of constriction
with IV glucagon or intraarterial infusion of the papaverine in an
intensive care unit setting.
Occlusive acute mesenteric ischemia is a medical emergency, thus early diagnosis and treatment may prevent bowel
94 M.G. Mutch, E.H. Birnbaum, and C.O. Menias
FIGURE 6-43. Mesenteric angiogram shows vasoconstriction and
pruning of the superior mesenteric artery and its branches in this
patient who presented with mesenteric ischemia secondary to severe
hypotension.
FIGURE 6-42. Mesenteric angiogram shows extravasation of contrast
A indicating an acute bleed that was successfully treated after infusion of pitressin B.
necrosis and perforation. These patients typically have severe
abdominal pain with nonspecific physical findings.
47
An
arteriogram is the most useful diagnostic examination for
patients in whom one has a high clinical suspicion of acute
occlusive mesenteric ischemia.
48
A catheter is inserted into
the aorta and an aortogram is obtained. The celiac and superior mesenteric arteries are catheterized and injected with
contrast in order to identify the level of occlusion and document collateral circulation. A superior mesenteric artery
embolus typically lodges just proximal or distal to the take
off of the middle colic artery and is seen as a meniscus at the
site of occlusion and blockage of contrast (Figure 6-44).
Atherosclerotic occlusion will often involve the origin of the
superior mesenteric artery seen as stenosis or plaque with a
trickle of glow beyond (Figure 6-45). Collaterals will
develop from the inferior mesenteric artery through the marginal artery. If the inferior mesenteric artery is occluded or
absent, the collaterals will develop from the middle or inferior hemorrhoidal arterial branches of the internal iliac
48,49
artery.
CT Colonography
CT colonography is rapidly developing as a noninvasive total
colonic examination for the detection of colon polyps and cancers. This technique uses volumetric data acquired by helical
CT scanners and workstations which use two- and threedimensional images to evaluate data. Since 1994, there have
been technical improvements in the CT hardware and software
allowing better visualization and discrimination of the reconstituted images. The three-dimensional endoluminal imaging
is better at evaluation of surface morphology and discriminating between polyps and haustral folds (Figure 6-46).
50
The
6. Diagnostic Evaluations—Radiology, Nuclear Scans, PET, CT Colography 95
FIGURE 6-46. Three-dimensional image confirms the presence of the
polyp in Figure 6-45.
FIGURE 6-44. Mesenteric angiogram shows a large filling defect
within the proximal superior mesenteric artery consistent with an
embolism in this patient with ischemic bowel.
FIGURE 6-45. Axial two-dimensional image from a CT colon study
shows a well-defined 6-mm polyp in the sigmoid colon.
two-dimensional images help in the correlation of images
seen on the three-dimensional fly-through.
Image processing and interpretation has improved with
newer software and an experienced radiologist can generally
complete the examination in <15 minutes.
Although the colon is evaluated in a noninvasive way,
a bowel preparation is still required the day before the examination to eliminate formed fecal matter. Air insufflation is
done via a small tube placed within the rectum to distend the
bowel and to minimize folds within the colonic wall.
Insufflation with a handheld bulb, or with a CO
insufflator is
2
performed. A CT tomogram is obtained to confirm adequate
insufflation in both the prone and supine positions and further
air is insufflated as needed. Unlike colonoscopy, IV sedation
is not required. Patient satisfaction after colonoscopy and CT
colonography are similar because bowel preparation is
needed for both tests.
51
Rapid scanning can be done in a single breath hold.
Volumetric data are acquired twice, once with the patient
prone and once with the patient supine. The change in position
allows any fluid within the bowel lumen to shift, revealing
abnormalities within the contralateral wall. Most tests can be
completed in <15 minutes. Once the images are acquired, a
trained radiologist reviews them at the workstation and the
images are recreated in such a way as to give an endoluminal
view of the colon similar to that seen on colonoscopy. Several
studies have compared CT colonography to colonoscopy for
high- and low-risk patients. The sensitivity of this technique
per individual patient ranges from 75% to 100% and the specificity ranges from 72% to 100%.
50–54
Both the sensitivity and
specificity are dependent on the polyp size. In randomized,
96 M.G. Mutch, E.H. Birnbaum, and C.O. Menias
55
of flat lesions.
Acquiring images after a change from the
prone to the supine position can frequently unmask hidden
polyps and tagging residual stool with subsequent digital subtraction is being evaluated.
58
An added benefit of the technique is the potential for the discovery of incidental
extracolonic findings. The dose of radiation used for CT
colonography is less than for conventional CT with the result
that the scanned images are not the same. Nonetheless, the
incidence of clinically important extracolonic findings is
approximately 11%.
59
Whether CT colonography can be used for mass screening
of average-risk patients has yet to be determined. Currently,
most centers are using CT colonography for those patients
who have had incomplete colonoscopies or who cannot
undergo colonoscopy for medical reasons. Clinical trials are
ongoing as educational efforts and technical improvements
are made in an attempt to improve this potential screening
technique.
Positron Emission Tomography
Whole body positron emission tomography (PET) was originally developed as a research technique in the 1970s. The
clinical use of this technique has evolved over the past several
FIGURE 6-47. Axial attenuation corrected PET image A and fusion
PET CT image B show an area of intense FDG uptake in the right
hepatic lobe consistent with hepatic metastatic disease in this patient
with cecal adenocarcinoma.
controlled trials, the specificity is greater for polyps larger than
1 cm than for polyps larger than 5 mm.
53
The detection of
small (< 5 mm) polyps is poor in most studies with sensitivities as low as 11.5%.
54,55
The importance of these small lesions
continues to be debated by the medical community. A recent
nonrandomized multicenter blinded study comparing CT
colonography with colonoscopy found that the sensitivity for
CT colonography detecting polyps ≥6 mm was 39% and those
≥10 mm was 55%.
56
This study acknowledged that the accuracy of CT colonography varied between centers and with the
experience of the radiologists. Interobserver variability can be
significant and is evidence of the steep learning curve.
57
The main limitation of CT colonography has been distin-
guishing polypoid tissue from fecal matter and the detection
decades. This technique uses [18F] 2-fluoro-2-deoxyglucose (FDG) which is a radiopharmaceutical glucose analog to measure increased glucose uptake and metabolism in
rapidly dividing cells. Malignant and other rapidly dividing
cells that have a high metabolic rate will take up FDG for use
as a glucose substrate. The first metabolite of FDG is FDG-6phosphate which is not a substrate for glucosephosphate
isomerase because of the configuration of FDG. FDG-6phosphate has a low membrane permeability and thus the
labeled substrate accumulates intracellularly.
60
The imaging
technique of PET utilizes differences in uptake of FDG in
malignant versus benign cells. The intracellular accumulation
concentrates the radiopharmaceutical analog, which appears
“bright” upon imaging (Figure 6-47).
The performance of a PET scan requires that the patient
fast for 4–6 hours before the injection of FDG. A urinary
catheter is placed to minimize the effect of the accumulation
of tracer in the bladder. Emission scans are then performed
with the patient motionless shifting the table between scans to
alter the field of view. Older techniques required that a patient
remain motionless for approximately 1–2 hours. Newer techniques have reduced the scan per bed position to approximately 2 minutes. Scans are then enhanced through a
segmentation calibration and the scattered events outside the
body are removed. Attenuation in each area is altered depending on tissue within the region.
61
FDG PET has been used to evaluate metastatic disease and
to improve staging accuracy (Figure 6-48). This technique
images the whole body and is more sensitive than CT for
the detection of hepatic and extrahepatic colorectal cancer
D-
6. Diagnostic Evaluations—Radiology, Nuclear Scans, PET, CT Colography 97
current CT and MRI scans. FDG PET is extremely useful in
this arena because a positive FDG PET scan in the setting of no
inflammation would indicate a local recurrence of a rectal cancer. Furthermore, detection of small extrapelvic metastases is
more accurate using FDG PET than CT or MRI. Thus, the use
of FDG PET for staging or recurrent cancers may help plan or
avoid expensive and possibly more morbid surgical procedures.
False-positive and false-negative tests have been reported to
occur in several distinct situations. FDG is not a tumor-specific
substance and increased FDG activity is seen in the normal
urinary and GI tracts. The cellular glucose metabolism is also
increased in inflammation as the increased uptake of FDG can
be seen in leukocytes and macrophages. Inflammatory
processes such as diverticulitis and pneumonia can lead to
false-positive readings thus making it imperative to correlate
positive PET findings with the clinical picture and conventional radiologic evaluation.
67,68
Detection of metastatic disease is dependent on the size and
degree of metabolic activity. Limited spatial resolution may
lead to false-negative readings for small, <1-cm lesions.
Adenocarcinomas with a high mucinous content may result in
false-negative readings because of the low cellularity of these
cancers. Sensitivity can be as low as 59% for mucinous carci-
70
nomas.
The combination of CT and FDG PET imaging has
reduced some of the inaccuracies and makes the study more
readily correlated anatomically.
The routine use of FDG PET for primary cancers is more
problematic. Although the risks and radiation doses are low
and the technique is noninvasive, the cost per scan is very
high. Thus, for primary cancers in which the information
would not alter the planned surgical procedure, it is probably
not indicated. However, in patients that are poor surgical risk,
the findings in FDG PET may help avoid or alter the surgical
procedure and might change the goal from a curative to palliative intent. Current CMS (HCFA) recommendations for
reimbursed PET imaging in colorectal cancer include
1) evaluation of patients with a question of recurrent disease
as indicated by rising CEA, 2) evaluation of resectability, and
3) evaluation of patients with locally advanced disease to
determine unresectability on the basis of metastasis when the
operation is a large otherwise debilitating procedure.
MRI is a continually evolving field of radiology. The tech-
FIGURE 6-48. CT scan in a patient who had prior rectal resection for
carcinoma shows soft tissue mass in the surgical bed of the perirectal fat A. Follow-up PET examination B shows intense FDG uptake
within this soft tissue mass consistent with recurrence.
metastases and the detection of local recurrence. The reported
sensitivities for PET detection of liver metastases range from
89% to 95% and for extrahepatic metastases 87% to 92%.
The use of FDG PET versus CT or MRI is based on the premise that functional differences in tumor appear before size
changes. Postoperative changes, particularly for rectal cancers,
are difficult to evaluate using standard modalities. The distinction between tumor and scar is not easily defined with the
nique was developed in the early 1980s and currently there is
a wide range of MRI systems in use. This technique relies on
the difference in tissue contrast or signal intensity. High signal intensity appears white on the image whereas low signal
intensity is dark. T
1
and T2refer to specific tissue properties
that describe the way protons behave after being excited by a
radiofrequency pulse in a strong magnetic field.
62–66
cific parameters chosen to acquire an image on an MR magnetic system determine whether an image is T
weighted. T1refers to the longitudinal relaxation rate and T
refers to the transverse relaxation rate. Structures containing
water appear black on T
1
-rated images and structures con-
taining fluid (cysts or gallbladder) are white on T
71,72
The spe-
2
-weighted
1
or T
69
2
2
98 M.G. Mutch, E.H. Birnbaum, and C.O. Menias
images. Unlike CT scans, iodinated contrast is not used for
the performance of these scans. The contrast agents that have
been developed can be used in patients with renal insufficiencies, and those with iodinated contrast allergic reaction. The
risk from MRI is attributed to the interaction between the
strong magnetic field and certain implantable devices such as
cardiac pacemakers, cerebral aneurysms clips, and cochlear
implants.
The use of MR for intraabdominal bowel anatomy is limited because of the peristaltic action of the bowel wall and
motion of the abdominal cavity caused by respirations. MRI
has evolved to be better than CT for tissue characterization
and evaluation of tissues planes within the pelvis. The layers
of the bowel wall can be visualized easily for evaluation of
rectal cancers. The muscularis propria is low signal intensity
and the submucosa has higher signal intensity.
73,74
The accuracy of MRI for preoperative staging for rectal carcinoma
continues to be evaluated. Contrast enhancement improves
the correlation with histologic stage.
75
Endorectal MRI is sim-
ilar to endoluminal ultrasound for determination of tumor
FIGURE 6-50. Endoanal MRI image demonstrates a nondisrupted
(normal) signal of the internal and external sphincter.
depth and nodal staging although some studies have shown
ultrasound to be more accurate in determining local inva-
76,77
sion.
able preoperative staging difficult.
Overstaging and interobserver variation make reli-
75,78
MRI more accurately
predicts the circumferential resection margin (Figure 6-49).
Several parameters help distinguish pathologic tissues. After
pelvic irradiation, the radiation edema or fibrosis can be differentiated from tumor on T
appears low signal in the T
2
-weighted images. The fibrosis
2
images and enhances slowly.
Recurrent tumors have a higher signal and enhance quickly
during dynamic gadolinium-enhanced scanning.
80,81
Changes
in postradiation normal tissue result in slow tissue enhancement with gadolinium.
80
The anal sphincter and pelvic anatomy have been imaged
with MR using an internal coil (Figure 6-50). The internal
sphincter has higher signal intensity than the external sphinc-
79
ter. Pelvic muscle morphology, sphincter injuries, and
abscesses can be identified. Clinical experience with MR has
not been as extensive as with endoluminal ultrasound but
comparative studies have been favorable. MRI of the puborectalis is better than endorectal ultrasound and capable of
showing atrophy.
82
It remains to be seen what the role of MR
will be in the evaluation of fecal incontinence because of variability of scanner capabilities among institutions and limited
access to high-performance scanners.
FIGURE 6-49. Endorectal MRI examination shows an ulcerated
plaque-like cancer arising from the mucosa and extending to the first
muscular layer of the muscularis propria.
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