This “target” appearance can be seen in a variety of disease processes but is a common finding in metastatic colon cancer and hepatocellular carcinoma.. limitations of colorectal imagi
Trang 1called to do so The US beam will be completely reflected by
bone and sufficiently scattered by air to thwart imaging distal to
these substances When the transmitted sound wave reflects off
a moving target, the returning echo will have a slightly different
frequency (the Doppler Effect) Doppler US capitalizes on this
principle and allows thedetermination of direction and
veloc-ity of a mobile target.(250) The most frequent application for
Doppler US is the detection and quantification of blood flow
Specifically, Doppler US is extremely helpful in evaluating the
upper and lower extremities for deep venous thrombosis
US has many advantages It is an inexpensive, widely available
modality that provides real time, multiplanar images with no
radiation exposure to the patient The US equipment is mobile,
allowing critically ill patients to be imaged within the ICU The
structures that can be studied by US include arteries, veins, liver,
spleen, gallbladder, bile ducts, pancreas, kidneys, bladder, uterus,
and ovaries Transabdominal US is typically limited in its
evalu-ation of the gastrointestinal tract Intraluminal bowel gas will
obscure the surrounding anatomy Therefore, patients should be
NPO for 4 to 8 hours before being imaged to reduce the volume
of intraluminal gas.(251) Nonetheless, US can detect abnormal loops of bowel Wall thickening, hyperemia, fecoliths, bowel dis-tention, wall edema, and noncompressibility all can be detected by ultrasound and suggest intestinal pathology US can be helpful in diagnosing a wide variety of disease processes including appendi-citis (Figure 11.49), intussusception, inflammatory bowel disease, colitis (from numerous causes), and neoplasm (Figure 11.50) Due to the superior sensitivity and specificity of other imaging modalities, US evaluation of the bowel is typically reserved for situations where limitation of radiation exposure is desired (i.e., pediatric and pregnant patients)
Figure 11.50 Colon Cancer Liver Metastasis US of the liver demonstrates an
isoechoic mass with a hypoechoic peripheral halo This “target” appearance can
be seen in a variety of disease processes but is a common finding in metastatic
colon cancer and hepatocellular carcinoma.
Figure 11.51 Normal Layers of Colon on Intrarectal ultrasound (Graphic
representation of 5 layers).
Figure 11.52 Normal Endoluminal Ultrasound.
Figure 11.53 Ultrasound of uT3 rectal mass
Trang 2limitations of colorectal imaging studies
Intraoperative ultrasound (IUS) can provide important
infor-mation to the surgeon and is commonly used to evaluate the liver
for metastatic disease and guide the subsequent metastasectomy
IUS is particularly useful in delineating the relationship between
hepatic tumors and adjacent vasculature.(252) Studies have
shown that IUS provides vital information to the surgeon during
the procedure that will affect surgical decision making in up to
38% hepatic metastasectomy.(251)
Endoluminal Ultrasound
Endoluminal ultrasound’s (EUS) impact on the workup for
col-orectal cancer continues to expand Transrectal US appears to be
the most accurate imaging modality in determining the extent
of local invasion of rectal cancer.(253) EUS can delineate the
components of the intestinal wall Images typically consist of five
rings of different echogenicity (3 hyperechoic and 2 hypoechoic)
that allow the localization of the mucosa, muscularis mucosa,
submusoca, muscularis propria, and serosa (254)) (Figure 11.51)
Colorectal tumors will appear as a hypoechoic mass that distorts
the normal bowel architecture (Figure 11.52 and 11.53) EUS can
accurately identify the specific layers of the bowel wall invasion,
thereby elucidating the tumor stage.(255) Recent studies have
shown that transrectal US has difficulty differentiating between
tumor and peritumoral inflammation, thereby producing a
ten-dency to over stage a recently diagnosed cancer EUS is often used
in conjunction with traditional endoscopy to allow direct
visual-ization of the mucosa, assess the depth of wall involvement,
facili-tate biopsy, and evaluate for pericolonic lymphadenopathy While
EUS has the ability to detect local lymph node involvement, cross
sectional imaging (CT, MRI, or PET) is still needed to evaluate for
regional and distant metastatic disease.(255)
Nononcologic applications of EUS include the evaluation of
the colon, rectum, and anus for strictures, fistulas, and abscesses
Transanal US is often used in the evaluation of incontinence as it
can detect defects within the internal anal sphincter, external anal
sphincter, puborectalis sling, and pelvic musculature.(254)
magnetic resonance imaging (mri)
In magnetic resonance imaging, strong magnetic fields and
tar-geted radiofrequency pulses are harnessed to map the location
of protons within the body Depending on the specific imaging
parameters utilized, protons within fat (T1 MRI sequences), or
water (T2 MRI sequences) can be selectively displayed Ionizing
radiation and iodinated contrast agents are not used MRI images
are degraded by motion and the combination of bowel peristalsis
and diaphragmatic movement has traditionally limited the
appli-cation of MRI in the evaluation of gastrointestinal pathology
(254) Ferromagnetic metals cannot be taken into the magnetic
field and therefore most surgical implanted devices have been
transitioned to MRI compatible materials Care must still be
taken with certain implanted devices as the strong magnetic field
may cause malfunction Confirmation of MRI compatibility with
the manufacturer is required for implanted devices such as
car-diac pacemakers, cochlear implants, spinal cord stimulators, and
basal ganglion stimulation devices Nephrogenic systemic fibrosis
(NSF) is a disorder seen exclusively in patients with chronic renal
insufficiency that presents with diffuse systemic sclerosis with
particularly severe cutaneous fibrosis In 1997, NSF was linked
to gadolinium exposure in patients with renal insufficiency The FDA has recently placed a black box warning on gadolinium containing MRI contrast agents.(256)
Technological advancement with quicker image acquisition has reduced motion blurring and has allowed the diagnostic assessment
of the sigmoid colon and rectum (anatomically fixed structures) (257) While MRI can be useful in the diagnosis of inflammation of the GI tract (for example, appendicitis, Crohn’s disease, and ulcer-ative colitis) (Figure 11.54), the largest advances have been made
in evaluation of colorectal cancer.(254) The effectiveness of MRI
is similar to CT for the initial staging of colorectal tumors.(258) MRI is very accurate evaluating the pelvis for local rectal tumor
Figure 11.54 A and 11.54B Terminal Ileitis in Crohn’s Disease Axial (A) and
coronal (B) T1-fat saturated MRI images demonstrate mucosal enhancement within the terminal ileum (arrow) with no enhancement in the adjacent normal ileum (arrow head) The mucosal enhancement indicates active terminal ileitis.
(a)
(b)
Trang 3extension (Figure 11.55), and has an advantage over CT in the
evaluation of tumoral invasion of the levator ani, mesorectal fascia,
internal and external sphincter muscles.(258–259) Endorectal MRI
is a promising new technique that can help evaluate the depth of
local tumor invasion Endorectal ultrasound has been shown to be
equally sensitive and specific as our currently available endorectal
MRI and can be performed in a fraction of the time.(258, 260)
MRI is also a valuable tool in detecting distant metastatic
disease Metastatic foci within the brain, skeleton, and liver are
readily detected with MRI Local tumor recurrence can be
dif-ferentiated from mature fibrosis if the surgical resection was at
least 1 year prior Unfortunately, immature fibrosis (<1 year old)
cannot be successfully distinguished from recurrent tumor with
MRI.(258, 259)
nucLear medicine imaging
Positron Emission Tomography
Positron Emission Tomography (PET) has been approved by
Medicare for the diagnosis, staging, and restaging of colorectal
cancer since 2001.(261) Unlike other imaging modalities that
rely on architectural distortion, PET scans detect neoplasm
based on physiologic differences between normal tissue and
cancer cells Malignant cells have a higher baseline metabolic
state, increased mitotic activity, and consume more glucose
PET scans utilize the glucose analog F-18 fluorodeoxyglucose
(F-18 FDG) F-18 FDG is transported into the cell through
transmembrane glucose transporters but, unlike glucose, it
does not undergo further metabolism.(261, 262) This causes an
accumulation of F-18 FDG within the tumor cell Fluorine-18
emits positrons that subsequently undergo annihilation when
contacted by electrons This annihilation produces gamma
photons that are summated by specialized detectors and allow
image generation
The photon count and inferred amount of glucose uptake is
reported in standard uptake values (SUVs) The SUV takes into
consideration the dose of F-18 FDG injected and body surface
Figure 11.55 Perirectal Mass Fluid sensitive (STIR) T2 MRI of the pelvis shows
a hyperintense mass adjacent to the rectum, worrisome for rectal carcinoma
However, after resection, this mass was found to be a high grade liposarcoma.
(a)
(b)
Figure 11.56 A–11.56C Comparison between CT and PET Figure 11.56A
demonstrates multiple discrete areas of hypermetabolism within the liver on PET scan, representing metastatic colon adenocarcinoma Figure 11.56B shows a noncontrast CT scan of the same patient The multiple metastatic foci are nearly impossible to detect without contrast Figure 11.56C Iodinated contrast helps to delineate between normal hepatic tissue and hypodense metastatic disease.
(c)
Trang 4limitations of colorectal imaging studies
area.(261) In general, a SUV value above 2.5 is suspicious for
malignancy but may also be secondary to an inflammatory or
infectious process.(262) Care must be taken when relying on
SUVs as they are only semi-quantitative and many variables affect
the reported numeric value One particularly strong variable is
the serum glucose A high serum glucose level will reduce tumor
uptake of F-18 FDG and lower SUV values Patients typically fast
overnight and avoid carbohydrates before the procedure.(262)
Blood glucose levels are checked before the examination with a
level below 200 mg/dl desired
PET imaging of the colon is very sensitive (>90%) but lacks
specificity (40–60%) due to physiologic bowel glucose uptake and
hypermetabolic benign lesions, including colitis and benign
pol-yps.(262) The main advantage of PET is its superiority over CT
in the detection of metastatic colorectal cancer PET will detect
increased glucose metabolism in regional lymph nodes or
dis-tant metastatic sites (Figure 11.56) that do demonstrate enough
architectural distortion to be detected as abnormal by CT
exami-nation PET has also been shown to be superior to CT in the
eval-uation of colorectal cancer recurrence (Figure 11.57) (263) PET
can help monitor response to chemotherapy and radiation
treat-ment but does not have the ability to detect microscopic residual
disease (262)
One of the main limitations of PET is low spatial resolution
This problem has largely been overcome by a new technique that
allows the concurrent acquisition of PET and CT images
dur-ing a sdur-ingle examination PET/CT augments the localization
of malignancy in contiguous or overlapping structures.(262)
Differentiation of tumor from infection is problematic when the
standard uptake value is only minimally elevated as regional
lym-phocytes will metabolize an abundance of F-18 FDG Likewise,
colonic adenomas/polyps can demonstrate hypermetabolism
and be misinterpreted as a tumor Tumors that have a low cell
density, small size, or low metabolic activity (including carcinoid
and mucinous adenocarcinoma) have a higher likelihood of a
false-negative result.(261, 262)
Gastrointestinal Scintigraphy
Nuclear medicine scintigraphy is a useful tool for the colorectal surgeon A biologically significant substance (RBC, leukocyte)
is labeled with a radioactive isotope that will subsequently emit gamma radiation These gamma photons are detected by scin-tillation cameras and diagnostic images are generated Nuclear medicine scintigraphy is especially helpful in answering a specific question The evaluation for intraabdominal abscess, Meckel’s diverticulum, carcinoid tumor, biliary abnormality, pernicious anemia, and colonic transit time can be performed with radio-isotope labeled leukocytes, technetium, octreotide, iminodia-cetic acid, vitamin B12, and diethylene triamine pentaaiminodia-cetic acid (DTPA), respectively.(264)
With the expanding use of fused PET-CT imaging, traditional nuclear medicine scintigraphy has a limited role in the manage-ment of colorectal neoplasia In tumors that are known to have high false negative PET rates (i.e., mucinous adenocarcinoma), radioisotope labeled monoclonal antibodies may help in evaluat-ing for occult metastatic disease or recurrence.(264, 265) While multiple monoclonal antibodies have been approved by the FDA, none are currently in widespread clinical use.(266)
Tc-99m red blood cell scintigraphy is a frequently utilized examination for the evaluation of lower gastrointestinal bleed-ing The patient’s RBCs are labeled with the radioisotope tech-netium-99m (employing either an in-vivo or in-vitro method)
in an attempt to identify red blood cells within the lumen of the
GI tract, thereby localizing the source of bleeding Three criteria are needed to confirm a gastrointestinal bleed The radiotracer uptake pattern should conform to bowel anatomy, increase in intensity over time, and propagate in an antegrade or retrograde fashion (Figure 11.58) Multiple intraabominal abnormalities, including hepatic hemangiomas, accessory splenic tissue, or colonic angiodysplasia, can simulate a GI bleed but these abnor-malities will not change in location over time A false negative Tc-99m RBC scintigram can be secondary to a slow intesti-nal bleeding rate or an intermittent bleed.(266) The reported
Figure 11.57 PET-CT PET-CT images show a focal area of hypermetabolic activity in the presacral space, adjacent to the patient’s low anterior resection site for rectal
cancer, representing an area of recurrence Note that this lesion may have been overlooked on the noncontrast CT.
Trang 5Figure 11.58 A and 11.58B Lower Gastrointestinal Bleeding Figure 11.58A
shows a single image of a Tc-99m red blood cell scintigram with a GI bleed
originating in the transverse colon, near the hepatic flexure Figure 11.58B is taken
5 minutes later and shows the radiotracer uptake pattern conforming to bowel
and moving in an antegrade fashion towards the splenic flexure.
(a)
(b)
sensitivity and specificity of Tc-99m RBC imaging has been
reported as high as 93% and 95%, respectively.(264, 266) Tc-99m
RBC scintigraphy can detect GI bleeding rates as low as 0.2 cc/
minute (compared to 1.0 cc/minute for traditional angiography),
and is a sensitive tool that can help isolate the vascular territory
of a bleed and direct percutaneous or surgical intervention.(266, 267) In an unstable patient, a Tc-99m sulfur colloid can be used
to detect GI bleeding Sulfur colloid scintigraphy requires less time for patient preparation and image acquisition but has a lower sensitivity for detecting gastrointestinal bleeding
interVentionaL radioLogy Gastrointestinal (GI) Bleeding
The angiographic diagnosis of GI bleeding is based upon visual-ization of extravasation of contrast into the bowel lumen, and a high rate of bleeding (1 cc/min) is required to visualize extravasa-tion.(268) Angiograms are positive in only about 50% of patients, and a positive Tc-99m RBC scintigraphy scan within the first 5–9 minutes, makes angiography more likely to identify extravasa-tion.(269) The two techniques used for lower GI arterial bleeding are vasopressin infusion and embolization
Vasopressin (pitressin) infused into the proximal SMA or IMA causes both smooth muscle constriction and water reten-tion Vasopressin can control lower GI bleeding in up to 90% of cases, and half of the patients will never bleed again Vasopressin requires monitoring in an ICU Rare complications include car-diac or digital ischemia from vasoconstriction, or hyponatremia from water retention.(268–270)
Embolization controls GI bleeding by decreasing the arterial pressure and flow to the point that hemostasis can occur, with-out creating symptomatic ischemia Large particles, Gelfoam, or microcoils can be used Embolization is successful in over 90%
of cases, with few instances of bowel ischemia Rebleeding is reported to occur in 20% of patients Patients should be moni-tored for bowel ischemia Delayed ischemic colonic strictures have been reported.(268–270)
Percutaneous Abscess Drainage (PAD)
Percutaneous abscess drainage (PAD) has played a major role in decreasing the morbidity and mortality associated with surgical
Figure 11.59 Percutaneous Abscess Drainage Axial CT image demonstrates
needle placement into the large fluid/air filled abscess.
Trang 6limitations of colorectal imaging studies
exploration CT is the most appropriate modality in image guided
PAD (Figure 11.59).(271) PAD of an intraabdominal abscess is
effective with a single treatment in 70% of patients and increased
to 82% if a second drainage is performed.(272) The overall
findings from a large series of 2311 PADs report a success rate
of 80–85%.(273) Complication rates of PAD are between none
and 10% Vascular laceration may occur and, if the vessel is small,
the bleeding will usually stop spontaneously.(274) Percutaneous
abscess drainage may be complicated by bowel perforation from
the needle or catheter transversing the bowel If the patient
devel-ops signs of peritonitis after catheter penetration of bowel, then
surgical intervention may be required.(275)
Image-guided Percutaneous Biopsy
The majority of image-guided biopsies can be performed on an
outpatient basis All interventional procedures can result in
bleed-ing, but this complication can be reduced by correction of any
coagulopathy before the procedure.(276) US offers the advantage
of real-time needle visualization, low cost, portable, and no
ion-izing radiation (Figure 11.60) US guidance can be problematic
in obese patients because the echogenic needle can be hard to
visualize in echogenic fat Lesions located deep to bone or bowel
cannot be biopsied with US owing to lack of visualization of the
lesion CT can be used to guide biopsy needles to virtually any
area of the body CT provides excellent visualization of lesions
and allows accurate identification of organs between the skin and
the lesion.(277) Disadvantages of CT include increased cost,
ion-izing radiation, and longer procedure times Complications of
abdominal, liver, or lung biopsy include bleeding, introducing
infection, pneumothorax, and hemoptysis Postprocedure
pneu-mothorax may occasionally require chest tube placement and
observation in the hospital.(276, 277)
Radiofrequency ablation (RFA) and Chemoembolization
of Hepatic Metastasis
Radiofrequency ablations (RFA) of liver metastasis are
per-formed similar to image-guided needle biopsy, with the RF
probe taking the place of the needle The RF probe is placed in the hepatic tumor and vibrates at a high frequency, conduct-ing heat into and ablatconduct-ing the tumor.(278) Studies show that the overall 5-year survival rate for colorectal liver metastasis treated by RF ablation is similar to surgical series (25–40%) (279) There are no absolute contraindications, and relative con-traindications include low platelets and coagulopathy RFA of hepatic tumors is associated with very low complication rates, generally below 2% Complications include pain, pleural effu-sion, bleeding, and abscess formation.(278)
The treatment of certain tumors (metastatic hepatic lesions) with intravascular delivery of chemotherapeutic agents can be palliative and prolong life, but is not considered curative.(280)
A wide variety of chemotherapeutic regimens are used These chemotherapeutic medications are usually mixed with an embo-lic agent that slows flow and allows the drugs to remain in the organ Metastatic disease to the liver can also be embolized by Yttrium-loaded microspheres that emit beta-radiation Fulminant hepatic failure or liver abscess formation occurs in <1% of patients Gallbladder infarction due to chemoembolization is rare (280–282)
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