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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_725_Библиотеки_им_академика_М_И_Перельмана
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Contraindicationstointravenouscontrastmedium:
Impairedrenalfunction
(consult your hospital protocol, remember the
importanceofpre-CThydration).
Historyofpriorallergicreaction
toiodinatedcontrastmedium.
Severeasthma
orcongestiveheartfailure.
Diabetic patient
on metformin (if renal function is normal you can use
intravenouscontrastbutmetforminshouldbestoppedfor2daysthereafter).
Multiplemyeloma
or
sickle-cellanemia
.
Pheochromocytoma
—i.v.contrastmayprecipitateahypertensivecrisis.
ReviewingtheabdominalCT
•
It is important to note the distance between two CT ‘slices’.
Usuallythetechniciansuse5mmintervalsbetweentheslicesbutit
issometimeshelpfultorequest3mmcutsoftheappendicealareain
aclinicallydifficultcase.Also,itisessentialtoensurethatyouhave
all the images by looking at the image numbers. Many hospitals
have done away with hard copies and introduced instead Picture
ArchivingandCommunicationSystems(PACS),whichmakeaccess
toimageseasier.Inthelattercase,scrollingthroughthescangives
informationthatismucheasiertointerpretthanifindividualfilmsare
examined.
•
Withindividualfilmswealwaysbeginwithagoodlookatthescout
film;itprovidessimilarinformationtoaflatplateoftheabdomenand
providesa‘globalview’.
•
The visualized portions of the lower lung fields should also be
looked at in both mediastinal and lung windows. Pulmonary
infiltratesandpleuraleffusionscanbeeasilyidentifiedandattimes
are a reflection of an acute sub-diaphragmatic process. An
unsuspectedpneumothoraxinatraumapatientwillalsobeobvious
inthelungwindows.
•
Whilst it iseasier to concentrateon the area of interest (e.g. the
right lower quadrant in a patient with suspected appendicitis) and
lookforfindingstosupportorexcludethediagnosis,itisessential
tolookattherestoftheabdomen.Oneneedstolookspecifically
forthe presence offreegas and freefluid,and to seeallthe solid
organs(liver,spleen,kidneys,pancreas),thehollowones(stomach,
small and large bowel), and blood vessels. One key point is to
follow the structure in questionin serialimages (stacking) to
obtainasmuchinformationaspossible.
•
It is important to visualize the abdominal contents both in the
transverse-axial (up-down) as well as the vertical-coronal plane
(front-back),astheimagestendtosupplementeachother.
•
Viewing the images with the PACS you can calculate the
HounsfieldUnits(HU)forthevariousstructuresyousee.Toremind
youtakealookat Table5.1below.
Afewadditionaltips:
•
Pneumoperitoneum. While an erect chest film can identify a
straightforwardcase of pneumoperitoneum, a CTscanisthe most
sensitive means available for its detection. On a CT scan, gas
collects beneath the two rectus muscles around the falciform
ligament( Figure5.6).Italsocollectsbetweentheliverandanterior
abdominalwallandwithinthe‘leaves’ofthemesentery.Thefindings
areattimesverysubtleandonlyafewbubblesofextraluminalgas
areallthatisrequiredtomakethediagnosisofpneumoperitoneum.
Thekeytotheidentificationofextraluminalgasisinspectionof
allthe scans oftheabdomenin lung windows. It is easier with
PACS as we can manipulate the window settings. Even if your
hospitalbelongstothedarkagesanddoesnothavePACS,theCT
scanstationwillhavetheabilitytodothis.
Figure 5.6. CT (lung window) demonstrating two pockets of extraluminal gas in the
epigastricregionoutlyingthefalciformligament(arrow).
•
Freefluid. Free fluidfromany source tends to accumulateinthe
most dependent parts of the peritoneal cavity — Morrison’s
hepatorenalpouchandthepelvis.Whenthereisalargeamountof
fluidthebowelloopsfloat to the midline. In addition to identifying
thepresenceoffluid,measurementofthefluiddensityofferssome
cluesregardingitsnature:lessthan15HUfortransudativeascites,
andmorethan30HUforexudativeascitesorblood.
•
Solidorgans.Whilesolidorganpathologyisararecauseofnon-
traumaticacuteabdominalconditions,CTisthemodalityofchoice
in the investigation of the hemodynamically stable victim of blunt
abdominaltrauma.Lacerationsofthesolidorgansappearaslinear
or branching low-attenuation areas. Subcapsular hematomas
appear as crescentic low-attenuation areas at the periphery.
Intraparenchymalhematomasappear as round or oval collections
ofbloodwithintheparenchyma.
•
Holloworgans. Theentiregastrointestinaltractfrom thestomach
torectumcanbetracedinserialsectionsandabnormalitiesshould
besought.Inthecaseofsmall bowelobstruction, thecause (e.g.
intussusception, tumor or inflammatory mass) and the site of
obstruction(thetransitionpoint)canbeidentified.Thepresenceof
pneumatosis can be identified more readily with CT than a plain
film and, if present, suggests intestinal ischemia. (Remember,
however, that there are benign causes of pneumatosis and, as
always,correlationwiththeclinicalpictureisessential.)CTisalso
sensitive for identifying inflammation, which is suggested by the
appearance of tissue infiltration or stranding. If i.v. contrast has
been administered, then reduced enhancement ofloops ofbowel
may signal ischemia. Similarly, the origins of the mesenteric
vesselsmaybeinspected—bestseeninthesagittalaxisslicesat
the abdominal aortic plane — to get some idea about patency.
Clots within the portal and/or mesenteric veins would provide
evidenceforthediagnosisofvenousmesentericischemia.
•
Acute appendicitis. The various CT scan findings that are
associatedwithacuteappendicitisareasfollows:
appendicealsigns:
-
appendix>6mmindiameter;
-
failureoftheappendixtofillwithoralcontrastorgastoitstip;
-
enhancementoftheappendixwithi.v.contrast;
-
appendicolith;
peri-appendicealsigns:
-
increased fat attenuation (stranding) in the right lower
quadrant;
-
cecalwallthickening;
-
phlegmonintherightlowerquadrant;
-
abscessorextraluminalgas;
-
fluidintherightlowerquadrantorpelvis;
Figure5.7.YoudonotneedaCTtickettoentertheoperatingroom!
Non-visualization of the appendix (in the absence of secondary
localinflammatorychanges),makesdiagnosisofacuteappendicitis
extremelyunlikely!
•
Colon.Similarly,strandingintheleftlowerquadrant,orthickeningof
the sigmoid colon suggests diverticulitis. Diffuse thickening of the
colon suggests an inflammatory process like colitis whether
infectiousorischemic.
•
Theretroperitoneum including the pancreasshould be looked at;
thepresenceofstrandingandfluidcollectionsaroundthepancreas
suggests pancreatitis ( Chapter 19). Retroperitoneal hematoma
nexttoanabdominalaorticaneurysmsuggestsaleak.
•
Pelvic organs. It is also important to look at the pelvic organs in
female patients. Particular attention should be paid to any large
cysticmassesintheadnexa,whichmaysuggestacomplicatedcyst,
ovariantorsionoratubo-ovarianabscess.
Yourpatientdoesn’trequireaCTtickettoentertheOR ( Figure
5.7), but manytimes CT willchange your operativeplans or even
canceltheneedfortheoperation.
Andnow,hereareafewwordsfromourradiologistfriend,HansUlrich
Elben.
HowtoreadandinterprettheabdominalCTforan‘acute
abdomen’
HowtoorderaCTexamination
Contrary to what you may think, some radiologists understand
somethingaboutmedicineandsurgery.Andafewofusknowsomething
about CT scans. We therefore respectfully request that you please
provide us with an accurate clinical picture and your tentative
diagnosiswhenrequestingascan.Youshouldtellusalso aboutany
relevant previous operations or injuries (like cholecystectomy,
appendectomy,hysterectomy).
Technicalstate-of-the-artCTexamination
A good CT examination is performed with a spiral CT after i.v.
administrationofacontrastmedium.Ifpossible,wealsoliketousean
oraldilutedGastrografin®medium.Thelattercanalsobegivenrectally
especially when suspecting an obstructing colonic lesion or colonic
trauma. In women with suspected gynecological pathology, you could
mark the position of the vagina with a normal vaginal tampon. An
importantexception:incasesofsuspecteduretericcolictheuseoforal
contrastisnotnecessary.Havingsaidthat,Iadmitthatinsomecenters,
inordertosavetime,oralcontrastisavoidedinacuteabdomenpatients.
Interpretation
Startwithascoutview,similartoaplain abdominalX-rayin asupine
patient.Lookatthedistributionofgasinthestomachandthesmalland
largeintestine.Aretheresignsoffreegasoutsidetheintestinallumen?It
isabsolutelynecessarytolookattheCTimagesinaspecialwindowfor
chestexamination(center—700HU,windowwidth2000HU)aswellas
inanormalwindow(center—40HU,windowwidth400HU).Thus,you
willrecognizefreegasoutsidetheintestinallumenmuchbetter.
Step-by-stepinterpretationofimagesbyorgans
Try to examine every organ from a cranial to caudal direction
completely.Especiallynotethelimitsandthestructuresofthetissues.
Liver
Lookatedgesoftheorgan,homogeneousenhancement,andluminal
contrast within the portal vein and its branches. Important diagnoses
include:blunttraumawithrupture ofthe liver,abscesses( Figure5.8),
andportalveinthrombosis.
Figure5.8.CT:liverabscess.
Gallbladderandbileducts
Theintrahepaticbileductsaccompanythebranchesoftheportalvein.
Normally they are hardly recognized unless dilated. If there is
cholangiectasis, follow the common bile duct down to the duodenal
papilla. Do you see any signs of tumor-associated obstruction or
choledocholithiasis?Normally,thewallofthegallbladderisthin(about23mm).Adistendedgallbladder,thickenedwall,apericholecysticlayerof
fluid,a‘halo’signandintramuralairarestrongindicationsofcholecystitis
( Figure 5.9). But, of course, when it comes to the gallbladder and
gallstones, ultrasound is more accurate. Indeed, CT can miss
gallstonescompletely!
Figure5.9.CT:acutecholecystitis. Note the gallstones,thickwallofthegallbladderand
surroundingfluid.
Spleen
Notice the size and form of the organ. Is there homogeneous
enhancement? Important diagnoses include traumatic or spontaneous
rupture with lack of contrast and fluidaround the spleen,and infarct of
thespleen withahypoperfused wedge-likearea. Look forextravasation
ofi.v.contrast.
Pancreas
The position of this organ 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 a uniform homogeneous
enhancement. In pancreatitis, the organ is enlarged diffusely. In
pancreaticnecrosis,partsoftheglanddonotlightupwithcontrast.The
surroundingfattytissueisnotdarkandinconspicuousbycomparisonbut
shows bright streaks. Fluid around the pancreas signifies inflammatory
exudate.
Kidneys,ureters,urinarybladderandurethra
Stonesyouwillseebestinanative(i.e.notcontrasted)scanwithinthe
renalpelvisoroneoftheureters.Theuretershavetobeexaminedalong
their entire course from the renal pelvis to the bladder. Any dilatation?
Any tissue reaction surrounding calcification(rim sign)? Irregularspotty
contrast of the renal tissue refers to nephritis, and wedge-shaped
absenceofcontrastimpliesarenalinfarct. Inrenalveinthrombosis,the
renal vein does not enhance with contrast. Streaky changes in the
perirenalfattytissuesuggestinflammation.
Organsofthepelvis
Inwomen,examinetheuterusandtheadnexapositionedlaterallytoit.
Do you see cystic structures (ovarian cysts)? Do you recognize
inflammatory signs in the surrounding fatty tissue or is there a fluid
concentrationwith enhancement ofitswall (tubo-ovarian abscess)? Are
theresignsofbleeding?Inmen,identifythebladder,prostateglandand
seminalvesicles.
Stomach,gutandperitonealcavity
Examine the whole intestinal tract starting with the stomach and
followingthesmallbowelfromduodenumtojejunum,ileumdowntothe
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
surroundingtissue( Figure5.10). Inthe right lower quadrant, look for
thececumand the vermiform appendix; signs of acute appendicitis are
well described in the previoussection. In active Crohn’s disease, you’ll
often recognize a considerably thickened wall of the terminal ileum (
Figure 5.11). In the descending and pelvic colon you should look for
diverticula and signs of inflammation — a thickened wall and streaky
thickened structures in the pericolic fat. Complicated diverticulitis is
suggested by extraluminal gas, leakage of contrast and an abscess.
Appendagitis is inflammation of an appendix epiploica and needs no
operation ( Figure 5.12). Colonic diverticula may perforatein the high
pressurezoneaboveanobstructingcarcinoma.CTisnotagoodtoolfor
distinguishingacolonicinflammatorymassfromamalignantone.
Figure5.10.CT:Meckel’sdiverticulum(arrow).
Figure5.11.CT:Crohn’sdisease.Notethethickenedwalloftheterminalileum(arrow).
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