Co

mm

on

Sym

pto

ms,

Sig

ns, a

nd C

ond

itio

ns

4

Imaging Anatomy and Overview

OverviewWithin the past 20 years, the various imaging tools used toevaluate patients with abdominal symptoms have changeddramatically. In this era with relatively easy access to cross-sectional imaging and endoscopy, plain radiography andfluoroscopic ("barium") exams have lost much of their utility,though not all. Ultrasonography (US), computed tomography(CT), and magnetic resonance (MR) are now the dominantmodalities in common use.

Many surgeons and other physicians caring for patients withhepatobiliary, pancreatic, or gastrointestinal (GI) disorders arelikely to have received relatively little formal instruction incross-sectional imaging anatomy or its appearance on imagingstudies during medical school or residency. Moreover, thecontinuing rapid evolution of imaging equipment and scanprotocols may make it difficult for radiologists, and nearlyimpossible for other physicians, to keep abreast of newer andimportant developments. With this introductory chapter andthe rest of this book, we hope to make our readers moreknowledgeable and comfortable in understanding the properrole and interpretation of various imaging tools for evaluationof abdominal disorders.

We will start with a brief overview of individual imagingmodalities and an assessment of their strengths andlimitations, acknowledging that there are legitimate reasonsfor considerable variability in practice patterns based on thetraining of physicians and the availability of specificequipment.

Imaging Modalities and EquipmentRadiographyPlain radiography has undergone the least technicaladvancement and the most drastic decline in use over the past2 decades. Like other imaging techniques, "plain films" arenow acquired and displayed digitally, which offers advantagesin image quality, lower radiation does, and ease of distributionthroughout the medical environment. No longer do physiciansneed to physically visit the radiology department to view orcheck out an abdominal film of their patient. Film reporting isnow done by voice recognition, allowing almost immediateavailability of the formal interpretation, obviating the need tohear the interpretation directly from a radiologist or waiting(hours or even days) for the paper report to make it to thepatient's chart. On balance, these changes are good, but all ofus "senior" radiologists miss the opportunity to discussindividual patients with referring physicians. Without question,the most ideal setting is one in which the referring physicianselects the ideal imaging modality based on the patient'sclinical presentation, and the radiologist interprets the studywith a full understanding of the concerns and questions to beaddressed in his or her report. Regrettably, this is rarely thecase today.

The most common indication for an abdominal plain filmtoday is to check the position of a feeding tube or otherdevice. For this, a supine portable film is sufficient andaccurate. A term sometimes used as a synonym for a plain filmis a KUB (meaning kidneys, ureters, bladder), and this reflectsthe prior belief that such films were quite accurate indetecting calculi within the renal collecting system. A similarbelief that plain films reliably depicted free intraperitoneal air(bowel perforation) and bowel obstruction persisted until CTcame into widespread use, causing us to conclude that we hadbeen missing a lot of disease on plain films.

If plain films are to be used to detect any abdominal disorderother than tube malposition, the study must include supineand either upright (not sitting) or decubitus views. Thelatter are essential in order to detect free intraperitoneal air orair-fluid levels.

FluoroscopyThe other major "victim" of the modern era has been thefluoroscopic study of the GI tract. During the senior author'straining and earlier practice, air-contrast barium studies of theentire GI tract were commonly employed to diagnose almostthe whole spectrum of inflammatory, infectious, ischemic, andneoplastic disorders. Today, in most industrialized countries,most fluoroscopic studies are performed to evaluate patientsbefore and after surgical procedures. Among the morecommon of these are antireflux (GERD), such asfundoplication, and bariatric surgical procedures, such assleeve gastrectomy and Roux-en-Y gastric bypass. For patientswho have had all or part of the colon resected, some form ofcontrast enema is generally performed to ensure there is noanastomotic stricture or leak.

Fluoroscopy has retained a primary role in evaluation ofpatients with dysphagia or symptoms of GERD and iscomplementary to various endoscopic and manometricprocedures. These protocols and disorders are discussed atlength in the Esophagus section of this book.

UltrasonographyUS has become the most widely utilized imaging study in theworld, owing to its portability, affordability, and lack ofionizing radiation. Some physicians consider US an extensionof the physical exam and encourage its use by practitioners ofall sorts. This trend has been accelerated by the availability ofsmall, inexpensive US units that can be used in the office,clinic, or emergency department. At the same time, somecaution is warranted. Although some conditions and disordersmay be readily apparent to even the infrequent USpractitioner, such as large amounts of fluid in the peritonealcavity, other conditions require considerable experience andexpertise in US for accurate assessment.

US has at least 2 inherent limitations: A rather small field ofview and its inability to image structures "hidden" by overlyingbone or air. Although these can, to some extent, be overcomeby repositioning the patient, US imaging of the GI tract willalways be technically challenging. US has a primary role inevaluation of hepatobiliary disorders, as discussed at length insubsequent chapters.

Endoscopic US has attained an important role in evaluation ofhepatobiliary and pancreatic disorders. A high-frequency UStransducer placed near the tip of an endoscope providesdirect visualization and high-resolution images of the bileducts and pancreas, along with the ability to biopsy orotherwise retrieve tissue or fluid samples for laboratoryanalysis.

US equipment and protocols have evolved along 2 somewhatdiverging paths: The simple but cheap and the complex butmore expensive. The latter includes developments such aspower Doppler, harmonic imaging, and even US contrastagents. Recently approved for use in the USA as well as inEurope and Asia are US "bubble" contrast agents. Real-time USimaging of the abdomen following IV bolus injection of one ofthese agents has shown great promise in evaluation of thevascularity and etiology of many neoplastic and otherdisorders. For more comprehensive and sophisticated

Co

mm

on Sym

pto

ms, Sig

ns, and C

ond

itions

5

Imaging Anatomy and Overview

applications of US, considerable expertise and experience aredemanded for both its performance and interpretation.

Computed TomographyCT has assumed a dominant role in evaluation of abdominaldisorders of almost any etiology. With current CT scanners,high-resolution images can be obtained through the abdomenand pelvis in much less time than a minute with almostinstantaneous "reconstruction" of the images available in anydesired plane, such as coronal or sagittal (multiplanar CT). Weroutinely review all CT studies of the abdomen and pelvis in atleast the axial and coronal planes. Curved planar reformationscan be extremely instructive in evaluation of structures, suchas the biliary and pancreatic ducts and blood vessels, as will beillustrated in multiple chapters. The ability of modern scannersto quickly acquire thousands of submillimeter-thick axialsections, coupled with powerful computers, has also led tothe development of specific applications, such as CTenterography, colonography, and angiography, each ofwhich has important uses in abdominal imaging.

The newer generation of CT scanners also provides CThardware and software that has markedly decreased thedose of ionizing radiation to patients with many CT protocolsnow resulting in an effective dose that is less than yearlybackground radiation. Many educational and decision-supporttools are available on the internet that can help guidereferring physicians in ordering the most appropriate imagingstudies, including those offered by the American College ofRadiology and the American Board of Internal Medicine(through its Choosing Wisely campaign). Referring physiciansshould always weigh the risk:reward ratio in evaluation of anydiagnostic or therapeutic procedure, and consultation withradiologists in a particular case is very useful in choosing themost appropriate imaging test and protocol.

Unlike other imaging studies, CT need not be limited toevaluation of a single organ or region, such as the right upperquadrant, and it is equally effective in depicting the spectrumof inflammatory, infectious, vascular, and neoplastic disordersthat may affect any of the thoracic, abdominal, pelvic, orretroperitoneal organs. For almost all CT protocols, with theexception of suspected renal calculi, abdominal CT scans aremost informative when performed during the rapid bolusinfusion of iodinated contrast medium. Newer contrastagents can be administered safely, even to patients with renalinsufficiency, utilizing smaller volumes of contrast media,newer CT scan protocols (e.g., lower kilovoltage tube current),and adequate hydration of the patient before and after thestudy. The prevalence of contrast-induced nephropathy(CIN) from IV administration of contrast for CT is extremelylow, especially in adequately hydrated patients. There havebeen numerous, well-designed studies of CIN and itsrelationship to CT scanning, and there is broad agreementthat the risk of CIN from CT has been greatly exaggerated. Weagree with the recommendations of the Mayo Clinic and theAmerican College of Radiology that IV contrast media shouldnot be withheld when it is deemed necessary for accurateCT diagnosis.

Another technical advance in CT has been the fasteracquisition and display of images, allowing selective imagingthrough large structures or regions (e.g., the entire liver orabdomen) in the arterial, venous, &/or delayed phase ofcontrast passage and uptake. Such multiphasic contrast-enhanced CT protocols have greatly improved our ability todetect disorders, such as GI bleeding, as well as hypervascular

tumors and their metastases. We will discuss and illustratespecific uses of multiphasic CT in subsequent chapters.

Magnetic ResonanceNo imaging modality has undergone more rapid and varieddevelopment over the last decade than MR. MR is inherentlymore sensitive to slight differences in tissue contrast than CTor US and can directly depict the body in any plane of section.Individual MR sequences and protocols can be utilized todisplay and quantitate, for example, the fat or iron content ofthe liver, the presence of hemorrhage, or many othercharacteristic features that allow more accurate diagnoses ofabdominal disorders. Owing to the length of most abdominalMR protocols (> 30 minutes), MR is less optimal for imagingclinically unstable patients or larger anatomic regions (e.g.,chest, abdomen, and pelvis). Because of its expense and morelimited distribution, MR may be reserved for more detailedanalysis of a disorder 1st suggested by CT or US, but it isassuming a more prominent role as the 1st-line imaging studyin many cases, as discussed throughout this book.

As with CT and US, MR protocols have evolved to include theuse of novel contrast agents. For abdominal MR protocols, themost important of these is gadoxetate disodium,commercially known in North America as Eovist andelsewhere as Primovist. Gadoxetate differs from most othergadolinium-based contrast agents in having substantial (~50%) hepatobiliary excretion. Gadoxetate-enhanced MRprotocols are of proven value in allowing improved quality MRcholangiography and in helping to characterize certain hepaticdisorders and masses, as addressed in subsequent chapters.

Nuclear MedicineRadionuclide scintigraphy is inherently limited in its spatialresolution but offers unique physiologic properties that allowit to retain an important role in abdominal imaging. Foremostamong these is the pairing of the physiologic display ofglucose metabolism (radiolabeled FDG) and the anatomicdisplay of CT into combined ("fused") PET/CT studies. PET/CThas achieved a critical role in oncologic imaging and is uniquelyvaluable in the initial staging, evaluation of response totreatment, and ongoing surveillance of GI malignancies.Unique PET tracers have been designed for specific targets,such as somatostatin analogues. Gallium 68-DOTATATEPET/CT is of proven superiority in detection and staging ofneuroendocrine tumors, as discussed in other chapters.PET/MR is already entering clinical practice and will find itsplace in abdominal imaging as well.

Selected References1. McDonald RJ et al: Intravenous contrast material-induced nephropathy:

causal or coincident phenomenon? Radiology. 278(1):306, 20162. Parakh A et al: CT radiation dose management: a comprehensive

optimization process for improving patient safety. Radiology. 280(3):663-73,2016

3. McDonald RJ et al: Controversies in contrast material-induced acute kidneyinjury: closing in on the truth? Radiology. 277(3):627-32, 2015

4. Edinger B: Statistical sins of CIN (contrast material-induced nephropathy).Radiology. 270(3):938, 2014

5. Mayo-Smith WW et al: How I do it: managing radiation dose in CT. Radiology.273(3):657-72, 2014

6. Newhouse JH et al: Quantitating contrast medium-induced nephropathy:controlling the controls. Radiology. 267(1):4-8, 2013

7. Balemans CE et al: Epidemiology of contrast material-induced nephropathyin the era of hydration. Radiology. 263(3):706-13, 2012

8. Nakaura T et al: Abdominal dynamic CT in patients with renal dysfunction:contrast agent dose reduction with low tube voltage and high tube current-time product settings at 256-detector row CT. Radiology. 261(2):467-76,2011

Co

mm

on

Sym

pto

ms,

Sig

ns, a

nd C

ond

itio

ns

6

Imaging Anatomy and Overview

(Left) US demonstrates thenormal appearance of thegallbladder (GB). Note the thinGB wall st (which shouldmeasure < 3 mm) and theclear, anechoic bile stdistending its lumen. (Right)The GB fundus st is its distaltip, whereas the neck strepresents the proximalportion of the GB, whichtapers toward the junction ofthe GB with the cystic duct.The cystic duct is notroutinely visualized well on USbut is seen here as a tubular,tortuous structure.

(Left) Sagittal US sectionshows the normal look andrelationships of collecting bileduct (CBD) and portal vein(PV) st. The hepatic artery stlies between the CBD and PV.The CBD is normally ~ 40% thediameter of the accompanyingbranch of the PV. (Right)Sagittal US through the livershows its homogeneousechogenicity with interspersedvessels st and smallintrahepatic bile ducts. Theright kidney st and liver havesimilar echogenicity, whichhelps determine whether theliver is abnormally echogenic,suggesting steatosis.

(Left) US shows the confluenceof the hepatic veins st withthe inferior vena cava (IVC)st. Note the uniform, low-level echogenicity of thenormal hepatic parenchyma.The diaphragm is marked .(Right) Transverse USdemonstrates the normal rightst and left PVs, both ofwhich have the thickechogenic wall that ischaracteristic of the portalvenous system (and allows PVsto be easily differentiatedfrom hepatic veins).

Co

mm

on Sym

pto

ms, Sig

ns, and C

ond

itions

7

Imaging Anatomy and Overview

(Left) First of 6 axial CECTimages through a normal livershows the middle st and right hepatic veins joining theIVC st. (Right) More caudal CTsection shows the lateral stand medial segments ofthe left PV st. The hepaticveins travel in a morecephalocaudal direction andare seen in short axis on thisaxial image.

(Left) More caudal sectionshows the anterior branch stof the right PV and theundivided left PV st. Themiddle and right hepatic veins are seen in crosssection. The hepatic veins tendto have a more vertical course,unlike the more horizontalcourse of the PVs. (Right)More caudal section shows theposterior branch st of theright PV as well as the righthepatic artery . Thecommon hepatic artery branches from the celiac trunkst along with the splenic andleft gastric arteries.

(Left) More caudal sectionshows the falciform ligamentcleft st, separating the medialand lateral segments of theleft lobe. Also noted are theGB and superiormesenteric artery st and vein. (Right) More caudalsection shows the inferiorsegments of the right hepaticlobe and GB . Thepancreatic head liesbetween the superiormesenteric vein and the2nd portion of the duodenumst.

Co

mm

on

Sym

pto

ms,

Sig

ns, a

nd C

ond

itio

ns

8

Imaging Anatomy and Overview

(Left) First of 3 arterial-phasecontrast-enhanced T1 MRsections shows thehomogeneously enhancinghepatic parenchyma and theenhanced left PV st. Themiddle and right hepatic veins are not yet enhanced onthis early phase of imagingand appear dark. (Right) Morecaudal section shows theanterior branch st of the rightPV as well as the nonenhancedhepatic veins . The falciformligament cleft st is also seen.

(Left) More caudal sectionshows the posterior branch ofthe right PV st. Also note theceliac trunk and briskenhancement of thepancreatic body . (Right)First of 3 axial fat-suppressednonenhanced T2 MR sectionsshows normal, low-intensityliver parenchyma and very lowsignal (flow void) withinvessels, such as the righthepatic vein st, IVC , andaorta . Note bright signalfrom static fluid within theintrahepatic bile ducts st andcerebrospinal fluid withinthe spinal canal.

(Left) More caudal MR sectionshows bright signal within theright hepatic duct st, whichlies immediately anterior tothe right PV st. Theintrahepatic bile ducts are thebranching, high-intensity(bright) foci within the liver.(Right) More caudal MRsection shows bright signalfrom bile within the neck ofthe GB st and the righthepatic duct st. Low-signalflow voids mark the hepaticartery , PV , and celiactrunk .

Co

mm

on Sym

pto

ms, Sig

ns, and C

ond

itions

9

Imaging Anatomy and Overview

(Left) Coronal MRCP, obtainedwithout the use of contrastmedia, shows bright signalfrom fluid within the GB st,common duct st, andintrahepatic bile ducts . Aportion of the pancreatic duct and fluid-filled duodenum are also seen on this planeof section. (Right) Sectionfrom an MRCP study showsthe GB, cystic duct st, andcommon hepatic st andintrahepatic ducts as wellas a portion of the pancreaticduct , which was betterseen on other planes ofsection.

(Left) CT angiogram showsenhancing arteries, includingthe common hepatic st,gastroduodenal st, splenic, and superior mesentericarteries . CT and MRangiography usually makecatheter angiographyunnecessary for diagnosis,reserving this forinterventions. (Right) Venous-phase, contrast-enhanced MRangiogram shows brightenhancement of venousstructures, including the PVsst and hepatic veins .Portions of the splenic andsuperior mesenteric vein are included in this plane.

(Left) In this 1st axial CECT,the normal pancreas is seen asa homogeneous, soft tissuedensity viscus lying just ventralto the splenic vein st, whichlies in a groove along thedorsal surface of thepancreatic body st. Thepancreatic tail lies on a morecephalic plane and inserts intothe hilum of the spleen .The stomach lies justventral to the pancreas.(Right) More caudal sectionshows most of the pancreas,including the body, neck st,head st, and uncinate process, which lies dorsal to thesuperior mesenteric vein .