Overview of Transabdominal Ultrasound
Ultrasound provides a unique role in evaluating internal anatomy. The ability to visualize cross-sectional and sagittal anatomy and to clearly differentiate cystic structures from solid masses makes it a critical tool in assessing the pathologic changes in the abdomen prior to operative intervention. Ultrasonography is a safe, portable, and noninvasive technique without deleterious side effects.
Diagnostic ultrasound uses repetitive pulses of sound with high frequencies, which penetrate soft tissues once they are concentrated into a narrow, focused column. As these waves traverse the abdomen, they are partially reflected at tissue interfaces. The sectional anatomy that is displayed needs to be accurately conceptualized to interpret the images meaningfully. There are two standard orientations: longitudinal (eg, imaging the aorta along its length, so that it appears as two parallel lines) and transverse (eg, imaging the aorta across its width, so that it is visualized as a circle) both must be acquired to properly perform a transabdominal ultrasound.
Understanding Gray Scale Imaging
An ultrasound image comprises thousands of pixels. Each pixel is a particular shade of gray depending on the strength of the returning ultrasound signal. Tissues that are identified as strong reflectors will return ultrasound images that range from brilliant gray to white. Weak reflecting tissues will be seen as a dark shade of gray to black. Ultrasound examination is reported in terms of six categories of this gray scale imaging (Table 6-1).
Table 6-1 Categories of Gray-Scale Imaging on Ultrasound |Favorite Table|Download (.pdf)
Table 6-1 Categories of Gray-Scale Imaging on Ultrasound
|Hyperechoic||Brighter than surrounding structures/bright white (eg, renal sinus compared to cortex)|
|Hypoechoic||Darker than surrounding structures|
|Anechoic||Black in color, eg, a simple cyst|
|Echogenic||Bright white appearance against a black background|
|Homogenous||Uniform shade of gray throughout the organ denotes normal appearance of the organ|
|Heterogenous||Nonuniform shades of gray throughout the organ, denotes abnormal appearance of the organ|
On an ultrasound examination a cyst characteristically appears to be anechoic with thin walls. A complex cyst will appear anechoic with thickened wall, and may contain debris.
Posterior shadowing is associated with calculi. This refers to acoustic impedance between the calculus and the surrounding tissue or fluid (in the case of bile).
It is important to remember that solid masses may present on ultrasonography as a range of gray shades. Hepatomas may appear heterogeneous, while metastatic tumors may appear hypoechoic or hyperechoic.
General Instrumentation and Technique
Transabdominal ultrasound should be performed with a multifrequency sector, curvilinear array or linear array transducer.1 Adequate penetration and resolution can be obtained using a 3.5-MHz transducer for the intra-abdominal cavity while a 5-MHz curved array transducer is optimal for evaluating the bowel and its mesentery. A higher-frequency probe (7.5 MHz) will provide higher-resolution images, but should be reserved for more superficial lesions of the abdominal wall given its lower depth of penetration.
With patients in the supine position, the entire abdominal cavity needs to be explored and evaluated for other pathology or concomitant disease. The dependent portions of the abdomen should be examined carefully for fluid, which might suggest a perforated viscus. The transducer should first be placed in the subxiphoid region and tilted toward the pericardium. It is then placed in the right midaxillary line and swept subcostally to inspect the liver, right kidney, diaphragm, subhepatic region and Morison pouch. Following this, the left kidney, spleen, and diaphragm are evaluated from the left anterior, middle, and posterior axillary lines. With the patient in reverse Trendelenburg position and the transducer oriented longitudinally, the epigastric region and right paramedian region are evaluated for pneumoperitoneum, which would appear as a hyperechoic line inferior to the abdominal wall. Free air might also be seen between the liver and abdominal wall with the patient in the left lateral decubitus position. Following a systematic assessment of all parts of the abdomen, attention should be focused on the area where the pain is most severe. If the view of the bowel is obscured by air, the technique of graded compression can be utilized to displace the interposing material. Applying continuous pressure to the patient’s abdominal wall reduces artifact and improves the overall image.1
Indications for Transabdominal Ultrasound
Due to its ease of use, low risk, and relatively low cost, ultrasound has become a safe, accessible alternative to CT, MRI, and other minimally invasive approaches to the evaluation of patients with abdominal pain. Transabdominal ultrasound findings often complement the physical examination and can help guide further diagnostic studies or operative interventions. Hernias, acute cholecystitis, appendicitis, bowel obstruction, intestinal ischemia, and diverticulitis are among the many conditions presenting with acute abdominal pain that can be evaluated constructively utilizing ultrasonography.1
Ultrasonography in the setting of an acute abdomen is conducted with the patient supine. The technique is no different to standard ultrasonography. However, the objective is not to delineate the areas of suspicion or confirm diagnosis; rather it is to evaluate the abdominal cavity for other pathologic changes and thus assist in establishing a differential diagnosis. The survey commences with visualization of the liver and gallbladder, followed by the common bile duct. The retroperitoneum, right and left kidneys, the stomach, duodenum and pancreas, and upper abdominal organs are imaged in an orderly fashion.
The evaluation is completed by imaging the pelvis and searching for the presence of intra-abdominal fluid in the pouch of Douglas. The next steps focus on examining the area of reported pain and are therefore of major interest. In situations where a perforated viscus is suspected, it is important to turn the patient into the left decubitus position and thus be able to identify intraperitoneal air between the liver and the abdominal wall. A diagnostic tap with aspiration under ultrasound guidance is recommended in all cases of suspected perforation with evidence of intra-abdominal fluid.
In 1986, Puylaert first described the use of ultrasound in the diagnosis of acute appendicitis.2 He utilized a technique referred to as graded compression that starts by placing the probe over the area of interest and then gradually increasing the pressure applied, thus compressing the cecum and bowel to better visualize the appendix. An inflamed appendix is imaged more easily than the normal appendix because of its larger size and its distension with fluid. It appears as a blind-ended tubular structure with a laminated wall arising from the base of the cecum; it is notably noncompressible and aperistaltic with a diameter greater than 6 mm.3 The “target sign” is a characteristic feature in ultrasonography (Figure 6-1). Puylaert reported a sensitivity of 89% and specificity of 100% for this technique.2
Appendicoliths appear as bright echogenic foci with distal acoustic shadowing. Lim and Quillin describe the use of color Doppler in detecting an inflamed appendix.4,5 The inflamed, thick-walled, noncompressible appendix, which is fixed in position by compression with the transducer, will be visualized as circumferential, colored rings, in contrast to the normal intestine which is thin walled and compliant with frequent peristalsis and therefore transmits minimum or no signals. Doppler signals disappear when gangrene or perforation is present. Although CT scanning has been purported as the imaging modality of choice for patients with possible acute appendicitis, critical analysis of CT utilization for this purpose has not been proven to have a clear and absolute advantage over ultrasonography in the acute setting of appendicitis.6,7,8
Ultrasonography has been reported in one study as being more accurate in supporting the diagnosis of an ileus or of intestinal obstruction than plain flat and upright radiographic films.9 Examination for possible intestinal obstruction begins with systematic longitudinal and transverse scans of the abdominal cavity. In the longitudinal view, the bowel appears as a multilayered sandwich-like structure with a serpentine array of bowel loops. The lumen is between 3 and 5 mm and is easily compressible during graded compression. The sonographic hallmark for diagnostic purposes is seen in the transverse view with the “target” appearance of the bowel and enlarged transverse diameter of the small bowel between 10 and 20 mm, while that of the colon is 30 mm.
An obstructed bowel appears distended, fluid-filled with thickened walls; there is an absence of peristalsis. Computed tomography is superior to ultrasonography at delineating the cause and location of an intestinal obstruction. Ultrasonography, however, is considered the method of choice for diagnosing intussusception. The findings on imagining are characterized by segmental pathologic target phenomena consisting of multiple concentric rings with demonstration of mechanical obstruction (Figure 6-2).10,11
The value of ultrasonography in the difficult diagnoses of mesenteric infarction is unclear. The images demonstrate hyperperistaltic bowel with mucosal edema, which are nonspecific findings. As the ischemia progresses, the bowel wall may appear thicker than 6 mm. Phillips and colleagues have reported the successful use of color Doppler imaging in certain cases of small bowel infarction of the proximal superior mesenteric artery or thrombosis of the superior mesenteric vein.12
Hernias and Abdominal Wall Masses
Transabdominal ultrasound can be used to identify and distinguish between hernias, seromas, abscesses, and other abdominal wall masses. Moreover, it can be used to guide aspiration or biopsy of such lesions with high accuracy and minimal morbidity or risk to the patient.
Abdominal Aortic Aneurysm
Abdominal aortic aneurysm should be considered in any patient who presents with abdominal pain of unknown etiology. In patients who are unstable, CT scan is too time-consuming and may delay definitive treatment. Bedside transabdominal ultrasound can rapidly and accurately detect the presence and size of a ruptured or nonruptured AAA with a sensitivity that approaches 100%.1
With the patient lying supine, a 3.5 MHz convex transducer should be used to identify the aorta below the xiphoid. The normal aorta should be an anechoic tubular structure with reflective walls that follows the curve of the lumbar spine and tapers off slightly at the inferior portion. The transducer should then be swept caudally toward the umbilicus in a transverse orientation to obtain measurements above and below the takeoff of the renal arteries. By orienting the transducer longitudinally, anteroposterior (AP) measurements can be obtained and the aorta assessed for the presence of a mural thrombus (Figure 6-3).
Ultrasonography can also be used to detect the presence of an aortic dissection. In the presence of an aortic dissection, a channel of blood will track down inside the tunica media and appear as a thin echogenic septum within the true lumen. An intimal flap can sometimes be seen on longitudinal view and the vascular channels delineated using color Doppler flow.
Intra-Abdominal Sepsis and Ascites
In patients that present with signs of sepsis, it is often difficult to elicit a complete history or obtain a reliable physical examination. In this situation, ultrasound can be used to examine the dependent portions of the abdomen for free fluid and bowel edema that would suggest peritonitis (Figure 6-4). It can also be used to look for reverberation artifacts in the epigastric region and right upper quadrant, which might indicate presence of pneumoperitoneum.
Ultrasound has become an indispensable tool in the emergency room setting and care for the patient with acute trauma. First introduced into practice in the 1970s in European trauma rooms, it was adopted into routine use by North American emergency teams in the 1990s. The focused assessment with sonography for trauma (FAST) is a limited ultrasound assessment with the sole objective of identifying the presence of free intraperitoneal or pericardial fluid. It is quick, can be performed at the bedside, and offers the option of repeat serial scans that may aid in the follow-up of a trauma patient who is managed nonoperatively. Ultrasound is less sensitive than DPL in detecting intraperitoneal blood and in estimating the blood volume that is free in the peritoneal cavity, but it does not involve any invasive intervention, which is required for DPL.
The accepted practice in emergency rooms is to screen all patients with blunt trauma and possible solid organ injury or hemoperitoneum with rapid ultrasound assessment (FAST). The patient may be monitored with repeat scans or taken for emergency surgical exploration depending on the findings and the clinical status of the patient.
Gallbladder and Biliary Tract Pathology
Transabdominal ultrasound is the initial diagnostic method of choice in cases of suspected cholecystitis and choledocholithiasis as the finding of cholelithiasis and bile duct stones will help guide further management. The normal gallbladder is typically an anechoic, oval-shaped, thin-walled structure that lies just under the right lobe of the liver, lateral to the portal vein. Occasionally it is enveloped within the liver parenchyma or is located more inferiorly toward the right iliac fossa. It is best visualized when the patient has been fasting. In the sagittal orientation, the probe should be swept horizontally from the right edge of the liver along its inferior surface toward the portal vein until the gallbladder comes into view. Gallstones should produce acoustic shadows that differ from shadows caused by loops of bowel by their lack of echogenic streaks.
The cystic duct can be seen running posteromedially from the medial side of the gallbladder neck toward the common bile duct. The corkscrew-shaped spiral valve of Heister is located at the most superior aspect of the duct. The common bile duct can be seen anterolateral to the portal vein. Unlike the branches of the portal vein, the intrahepatic bile ducts arising from the CBD have brightly echogenic walls. The CBD should be imaged in the transverse plane and traced through the head of the pancreas to the medial aspect of the duodenum. A curvilinear transducer with large footprint can help to minimize shadowing from overlying bowel gas and help to better visualize the duct.
The use of ultrasound in assisting in the diagnosis of gallbladder disease is well established. The patient is usually required to fast and is maintained in the supine position. The gallbladder is approached with the probe in the right lateral longitudinal and intercostal planes. Gallstones will appear as round, mobile echogenic foci within the lumen of the gallbladder. Stones larger than 1 mm will often cast a posterior shadow. The typical findings of acute cholecystitis include gallbladder wall thickening greater than 4 mm, hypoechogenic thickening of the tissues around the gallbladder, pericholecystic fluid, and localized pain while imaging the gallbladder with mild compression which is referred to as a “sonographic Murphy sign” (Figure 6-5)13,14 The presence of gallbladder wall stranding or layering echogenic gas within the lumen are suggestive of gangrenous and emphysematous cholecystitis, respectively. Chronic cholecystitis can be distinguished from acute cholecystitis by the pattern of mural calcification on ultrasonography (porcelain gallbladder) with or without obliteration of the gallbladder lumen.
Ultrasound can also be utilized to detect other pathologic findings in the gallbladder and biliary tree, including adenomyomatosis, polyps, malignant neoplasms, and choledocholithiasis with biliary obstruction. Adenomyomatosis is a benign condition due to hyperplasia and invagination of the epithelium and muscular layers of the gallbladder. It most typically presents as small echogenic intramural diverticula or as a polypoid lesion located in the gallbladder fundus. Gallbladder carcinoma can appear as diffuse wall thickening (especially if greater than 1 cm) or a heterogeneous, vascular mass that obscures the normal plane between the gallbladder and liver.
Ultrasonography has a sensitivity of 75% for the detection of stones within the common bile duct.15 The normal extrahepatic bile duct should be less than 6 mm at the level of the crossing right hepatic artery. A dilated duct in the presence of obstructive symptoms is highly suggestive of choledocholithiasis and can be confirmed by the presence of echogenic stones with distal acoustic shadows on ultrasonography. Dilated common bile ducts within the porta hepatis with tapering of the CBD and surrounding mass are suggestive of a cholangiocarcinoma. This intra- or extrahepatic malignancy can either spread outward from the CBD (exophytic), be contained within the wall of the duct (infiltrative), or appear as a polyp-like mass within the duct (polypoid) (Figure 6-6).
The normal liver has a homogeneous pattern that is more echogenic than the renal cortex, but less echogenic than the spleen. It is anatomically divided into eight segments based on the branching of the portal structures and hepatic veins that appear as sonolucent tubular structures within the liver parenchyma. The three hepatic veins typically run in an oblique direction from the upper end of the IVC to the right, middle, and left portions of the liver.
The liver is best evaluated with the patients turned slightly on their left side, which allows the liver to slide downward into a better view. The patient should hold his or her breath, which depresses the diaphragm. The probe is placed in a sagittal orientation in the right anterior axillary line and swept horizontally from right to left until the far edge of the left lobe is visualized. The probe is then rotated and oriented axially under the right ribs and angled upward until the top of the liver comes into view. It is then swept vertically from the cranial to caudal direction. If the liver is obscured by the lower ribs, the probe can be rotated until it lies along the intercostal space. In order to visualize the hepatic veins, the probe should be oriented axially under the xiphoid and tilted cranially to bring the junction of the IVC with the right atrium into view. The hepatic veins should then be visualized running into the IVC.
The portal vein runs in a horizontal orientation from the inferior surface of the liver along the right lobe. The portal vein has a more reflective wall than the hepatic veins and is therefore more echogenic. It is best visualized by placing the probe in the midclavicular line, where it is rotated counter-clockwise from the sagittal orientation until it points toward the right shoulder, and is then tilted medially toward the aorta. The portal vein can then be followed down to where it joins the superior mesenteric and splenic veins.
Transabdominal ultrasound is often used to detect and localize benign or malignant neoplasms, metastases, cysts, hemangiomas and abscesses within the liver (Figure 6-7). Hepatic adenomas typically appear as well-demarcated, isolated lesions with variable echogenicity on ultrasound. Focal nodular hyperplasia is difficult to distinguish from an adenoma, but is more often isoechoic with characteristic “spoke wheel” or “stellate” vascularity on color Doppler. Hepatic metastases have a variable appearance depending on their origin and there are no features that routinely distinguish them from primary hepatocellular carcinoma. Tumors arising from the gastrointestinal tract are more likely to be multifocal, hyperechoic, and have a hypoechoic rim creating a “bull’s-eye” or “target” appearance on ultrasound. Hepatocellular carcinoma is more often hypoechoic in relation to the surrounding liver parenchyma, with posterior echo enhancement or “halo” effect. Portal or hepatic vein invasion is also more suggestive of hepatocellular carcinoma than metastatic disease.
Ultrasound can localize benign and malignant lesions based on the liver segments involved and the distance of the lesion from the hepatic and portal veins. The proximity to the vessels also helps determine resectability and to plan the operative approach. Transabdominal ultrasound is also useful for following focal hepatic lesions over time and for monitoring patients’ response to therapy. Ultrasound with Doppler capability can be used to assess hepatic arterial and portal venous flow after hepatectomy or liver transplantation. When neoplasms are deemed unresectable, ultrasound is a useful adjunct to alternative ablative procedures such as radiofrequency thermal ablation or cryoablation.
Unlike children in whom the liver appears brighter than the pancreas on ultrasound, in adults, the pancreas is brighter than the liver and lies posterior to the antrum of the stomach. Occasionally it is obscured by the transverse colon. With the patient lying supine, a 3.5 MHz transducer should be placed over the xiphoid and aimed downward. Images should be obtained in both longitudinal and transverse planes in order to best identify the pancreas in relation to the surrounding vasculature. The pancreas will appear as an inverted “U” inferior to the splenic vein. The head and neck of the pancreas should be seen wrapping around the echogenic fat pad at the origin of the SMA, while the tail runs superiorly toward the left axilla. The tail can best be seen by rotating the probe counterclockwise until it points toward the left axilla or by turning the patient on his/her right side and visualizing it through the spleen.
Transabdominal ultrasound is often the first imaging study done for suspected pancreatitis and can identify peripancreatic edema, fluid collections and pseudocysts in advanced disease (Figure 6-8). The first sign of acute pancreatitis is often an increase in pancreatic volume on ultrasound; as the disease progresses, pancreatic necrosis and edema will give the pancreas a nonhomogenous and hypoechoic appearance. Pancreatic pseudocysts are usually round, well-defined anechoic structures with acoustic enhancement on ultrasound, while hemorrhagic or infected pseudocysts may have a more complex appearance with septations and internal echoes.
Transabdominal ultrasound can also be used to identify pancreatic malignancies, especially in the head of the pancreas. Even in the absence of a discrete mass on imaging, the finding of common bile duct and pancreatic duct dilatation due to obstruction is highly suggestive of a malignancy. This characteristic finding is called the “double duct” sign (Figure 6-9).
The spleen is often obscured by the lower ribs in the left mid-axillary line. It will appear kidney shaped with a homogeneous gray echo-texture. The small acoustic window between the ribs often limits the scope of view. By flattening out the swept gain control, the spleen may appear more uniform. The patient should be turned onto his/her right side and the probe aligned along the 9th-11th intercostal spaces. By having the patient hold his or her breath, the spleen may be pushed down into a better view. The splenic artery and vein run from the middle of the deep surface of the spleen.
Transabdominal ultrasound can be used to identify cysts, abscesses, and infarcts as well as to distinguish benign from malignant mass lesions of the spleen (Figure 6-10). A simple splenic cyst will usually appear as a thin-walled, anechoic, homogeneous structure within the parenchyma. More complex cysts or those with an infectious etiology can have septations, echogenic wall calcifications, or irregular wall thickening. There is often mixed echogenicity within the parenchyma due to tribeculation or hemorrhage. A common benign lesion found within the spleen is the hemangioma. These are most often small, solitary vascular lesions with a variety of ultrasound patterns depending on the type of hemangioma. Capillary hemangiomas are usually hyperechoic while the cavernous type is hypoechoic with a heterogeneous pattern and calcifications.
An adult kidney is between 8 and 13 cm long and 5 cm wide, surrounded by a renal capsule that appears as a dense linear echo at the periphery. The gray echogenic parenchyma of the cortex surrounds the hypoechoic renal pyramids in the medulla. The renal calyces and renal pelvis that comprise the renal sinus appear as central dense hyperechoic complexes (Figure 6-11).
The patient should be positioned in the anterior oblique or lateral position. Coronal images are obtained by placing the probe at the flank and sliding it posteriorly until the kidney comes into view. The liver can be used as an acoustic window on the right and the spleen on the left, moving the probe intercostally until the entire kidney is visualized. Kidney measurements can best be obtained with the patients lying prone. A cross-section of the kidney is obtained at the hilum to determine the width and depth. The dimension from the hilum to the posterior aspect of the kidney is obtained and the maximum dimension is determined at right angles to this.
Normal ureters are small and in a retroperitoneal location making them difficult to visualize using ultrasonography. Nondilated distal ureters can sometimes be seen as two small projections in the trigone, on either side of the midline of the posterior wall of the bladder. The lumen of the bladder is best seen when the bladder is distended, and should be scanned longitudinally and transversely by rocking the probe side to side to visualize the dome, the base, and the sidewalls. The bladder wall in adults is typically 3 mm thick when distended versus 5 mm when empty. The volume of the bladder can be estimated as can the degree of emptying by measuring the residual volume in the bladder.
Ultrasound techniques can be used to identify a number of pathologic conditions involving the urinary tract, including renal cysts, benign tumors of the bladder and kidney, renal and transitional cell carcinoma, nephrocalcinosis and nephrolithiasis (Figure 6-12). Kidneys that are obstructed by a mass or stone will have echo-free areas within the white, hyperechoic center, compressed renal parenchyma, and thinned renal pyramids. Renal or ureteral stones will appear as bright objects that cast shadows within the kidney or dilated ureter. Renal cysts will often be single or multiple, smooth, well circumscribed, hypoechoic regions usually found at the periphery of the kidney (which differentiates them from hydronephrosis).
Advances in Transabdominal Ultrasound
Despite the considerable advances in ultrasound technique over the past decade, conventional B-mode ultrasound has a limited ability to detect blood flow at the tissue level. Over the past several years, contrast-enhanced ultrasound (CEUS) has emerged as a specialized imaging technique that enables dynamic, real-time evaluation of tissue perfusion. Utilizing microbubble contrast agents that enhance the Doppler signal within the bloodstream, CEUS can detect solid and hollow organ lesions that would otherwise be missed using conventional ultrasound.
CEUS has been most widely used throughout Europe and Asia for characterizing liver lesions and is shown to have 92% sensitivity and 86.7% specificity for detecting hepatocellular carcinoma.16 CEUS can evaluate lesions in the arterial, portal-venous and late contrast phases and is comparable to CT or MRI for detecting isolated hepatic metastases (Figure 6-13). It can detect smaller masses than those typically seen on conventional ultrasound and can therefore provide a more accurate and timely diagnosis of metastatic disease.
Other advantages of CEUS include its ease of use and its relative safety compared to other imaging techniques. It is noninvasive, utilizes no ionizing radiation, and can be safely used in pregnant women, children, and patients with renal insufficiency. The use of CEUS will undoubtedly continue to expand and improve our accuracy for detecting pancreatic, kidney, splenic, breast, and lymph node pathology in the near future.
Advantages and Limitations to Transabdominal Ultrasound
- A disease process that causes only microscopic changes in an organ structure will not be visible on ultrasound.
- Diagnostic accuracy can be affected by incorrect gain settings or image processing.
- False negative or false positive findings may be found when the patient is inadequately positioned or poor technique is employed.
Overview of Intraoperative Ultrasound
The first attempt at intraoperative ultrasound for identifying bile duct calculi occurred in the early 1960s utilizing A-mode ultrasound. Due to the difficulty in obtaining and interpreting the images, the technique did not gain widespread acceptance until the late 1970s when IOUS with real-time B-mode imaging was developed.17 In 1977, the initial use of electronic linear array transducers for intraoperative ultrasound examination of the liver and pancreas, together with real-time B-mode sector and linear IOUS, were introduced for a wide range of surgical procedures.17 Hepatobiliary and pancreatic surgery in particular have benefitted from the introduction of IOUS, as it is one of the few ways to assess the liver parenchyma during surgery. This development has significantly influenced the operative decisions and therefore has had a major impact on the outcomes of surgery. Color Doppler imaging and laparoscopic ultrasound were incorporated into IOUS in the 1990s and further helped to guide intraoperative decision-making.
Intraoperative ultrasound provides real-time images without subjecting patients to unnecessary radiation. It quickly distinguishes between cystic and solid lesions and accurately assesses blood flow within solid organs. IOUS is also useful for obtaining biopsies of suspicious lesions by facilitating the placement of biopsy needles in the correct and safe plane. IOUS continues to be an indispensable method for diagnosing and staging of malignancies. It is gradually being introduced into the training curriculum of surgical residents.
General Instrumentation and Technique
Intraoperative ultrasound is best performed using B-mode ultrasound with high frequency linear-array or sector transducers. Transducer frequencies for IOUS range from 5 to 10 MHz, with 7.5 MHz being the most common.18 A 5–10 MHz transducer penetrates less deeply than transducers at lower frequencies, but obtains greater resolution images, and can detect calculi and vascular lesions as small as 1mm.18 Unlike the transabdominal approach, IOUS is applied directly to the surface of the organ being evaluated and does not need to penetrate the abdominal wall. Therefore, a higher frequency transducer with a depth of penetration of 6–8 cm is adequate during surgery.
For side-viewing of large, flat organs such as liver and pancreas, a flat T- or I-shaped probe is often used. End-viewing is best done using a small cylindrical probe that can scan small vessels and ductal structures. Intra-abdominal organs should be examined from both longitudinal and transverse planes, thereby eliminating the potential artifact from refraction and differentiating the organ of interest from nearby surfaces.
In order to examine deeper lesions within the parenchyma of an organ, the probe should be applied directly on the surface of the organ being evaluated (contact scanning) in a dry field.1 A small amount of saline can be added for acoustic coupling between the probe and the tissue. For more superficial lesions or organ surfaces, the probe can be completely immersed in saline and positioned several centimeters away from the structure being examined (probe-standoff technique), placing the structure of interest at the focal distance of the transducer.18 Compression scanning is another technique employed in IOUS whereby the probe is used to compress the tissue being examined in order to eliminate any air between the tissue and the transducer. This technique can be used when gas in the bowel obscures the view of other structures, or when it is necessary to distinguish between veins and arteries.
The area of interest should be scanned systematically, and a combination of longitudinal, transverse, and oblique views of the target lesion must be obtained. The longitudinal view is parallel to the long axis of any given structure, while the transverse view is at a right angle to the long axis. Lateral movement, rotation and angulation of the probe are various scanning maneuvers used to obtain real-time three-dimensional images.1
Indications for Intraoperative Ultrasound
Intraoperative ultrasound is a safe and rapid way to obtain information that cannot otherwise be obtained using standard preoperative imaging techniques or routine exploration. It can aid in localizing suspected lesions that are not visible on the CT scan or on the MRI; it is therefore useful for detecting occult metastases and determining the extent of tumor invasion. Intraoperative ultrasound is also a useful adjunct to intraoperative angiography during biliary and vascular surgery, with even higher accuracy for detecting ductal calculi, intimal flaps, or thrombi. IOUS can also be used to confirm complete resection of tumors or lesions and to assess the patency of vascular reconstructions or anastomoses. In addition, IOUS can help guide diagnostic and therapeutic procedures such as biopsies, fine needle aspirations, and cannula placements for ablative procedures.
Typical Appearance on Intraoperative Ultrasound
Although tumor appearance varies widely depending on its histology, size, and vascularity, tumors, in general, can have a hypechoic, hyperechoic, or mixed appearance. Even though intraoperative ultrasound cannot reliably distinguish between benign and malignant lesions, smaller malignancies are more often hypoechoic compared to the surrounding parenchyma and they can sometimes appear as target lesions, especially in solid organs such as liver.
Calculous lesions are characteristically echogenic with an anechoic posterior shadow caused by a difference in acoustic impedance from surrounding bile. Gallbladder calculi are usually mobile, while pancreatic and renal stones are often fixed. Simple cystic lesions are usually thin walled, anechoic, and demonstrate posterior enhancement due to an artifact from surrounding tissue.19 A simple cyst with these features is most likely benign. Complex cysts can be associated with abscesses, hemorrhage or tumors, and typically appear septated, with irregular, thickened walls. They may or may not contain debris and should be aspirated or evaluated with other imaging modalities to rule out malignancy.
Evaluation of Specific Organ Systems
Gastrointestinal tumors such as adenocarcinoma and lymphoma should be evaluated by high-frequency IOUS that demonstrates a layered appearance of the abdominal wall and can accurately detect the depth and lateral extent of tumor invasion. Surrounding structures including regional lymph nodes can be assessed for metastatic involvement and this information is used to help determine resectability and to guide in further management. When a splenic lesion is known or suspected based on preoperative imaging, IOUS can be used to determine whether it is solid or cystic, and to guide aspiration or biopsy of the lesion. When splenectomy is performed for malignancy, IOUS provides useful staging information and can help localize accessory spleens that are not otherwise visible or palpable intraoperatively.
Makuuchi et al was the first to describe the use of intraoperative ultrasound for determining the resectability of hepatic tumors.18 Unlike other diagnostic modalities such as CT scan and MRI, IOUS can detect smaller lesions within a cirrhotic liver, both prior to surgery and, most importantly, intraoperatively. It can further accurately determine the extent of tumor involvement and can therefore help define the extent of anatomic resection and provide the information on the best chance for negative margins. Detection of location and of tumor metastases is routinely practiced prior to resection or cryoablation to identify intrahepatic tumor deposits and to delineate the extent of resection or ablation that is required. Even though IOUS is less sensitive for detection of superficial lesions smaller than 1 cm in diameter, IOUS can assist surgeons to detect 35% more lesions than is achieved by liver palpation alone. When IOUS is combined with palpation, it further improves the accuracy for detection of hepatic lesions.18
More than 12% of patients with colorectal cancer that metastasized to the liver have lesions that are so small that they are only detectable on intraoperative ultrasound.20,21 Although not used routinely in all colorectal cancer patients, IOUS is an excellent screening test, since it has a 90% sensitivity and specificity for detecting colorectal metastases.20 The use of IOUS is currently limited to patients with locally advanced tumors, lymph node involvement, or recurrent colorectal cancer.
Intraoperative ultrasound of the liver is best performed using a T-shaped transducer that can access the lateral segments. Starting with the probe in a transverse orientation, each lobe is scanned carefully beginning with the left medial segment, looking for any intraparenchymal lesions or abnormalities. The normal parenchyma appears homogenous with echolucent holes representing the hepatic veins that join the vena cava. Colorectal, pancreatic, and gastric carcinomas metastatic to the liver often appear as well-defined hyperechoic lesions with a hypoechoic periphery that lacks the homogeneous ground-glass appearance of the liver parenchyma. The major hepatic veins and two major branches of the portal vein should also be visualized in order to define the lobar anatomy and to detect any tumor thrombus within the lumen. Submerging the porta hepatis in saline helps visualization of the biliary tree, including the common bile duct.
Biopsies or aspirations of intraparenchymal liver lesions can also be accomplished using intraoperative ultrasound. Ultrasound-guided radiofrequency ablation is increasingly being used in patients with unresectable primary, metastatic, or recurrent liver tumors to provide local control.22 It is less invasive, safer, and has fewer complications than either surgical resection or cryoablation techniques.
Evaluation of the biliary tree during laparoscopic cholecystectomy was among the first uses of laparoscopic ultrasound. Even though this is the most widely studied application of IOUS, it can also be used to determine the extent of biliary or gallbladder tumor invasion into the liver or surrounding vascular structures, as well as to guide biopsies of these structures to help determine resectability. End-viewing transducers are best used for visualization of the biliary tree due to the minimal contact required with ductal structures to obtain high-resolution images.
IOUS can accurately diagnose small biliary calculi with a sensitivity of 90–95% and a specificity of 98–99%, making it comparable to intraoperative cholangiography.23 This is clearly an adequate and almost required screening method for choledocholithiasis.18,23 The gallbladder is first imaged to visualize cholelithiasis and to adjust the ultrasound settings as necessary. The transducer is then positioned to obtain a transverse view of the portal triad at the level of the cystic duct. Ducts and vessels in this region have a distinct ultrasound appearance and anatomic relationship, enabling IOUS to identify aberrant or obscured ductal anatomy, guide dissection, and help to avoid iatrogenic injury to the ducts and surrounding structures during surgery.
Within the porta hepatis, the normal common bile duct appears as a thin walled structure with anechoic lumen that is located in an anterolateral position. The portal vein is posteromedial and displays a venous signal on Doppler flow detection. The hepatic artery is often interposed between the vein and common duct in an anteromedial position and appears pulsatile and noncompressible. Stones within the bile duct have an echogenic surface with posterior shadowing that point away from the ultrasound transducer. A common duct that contains stones may have a thickened wall and exhibit distal tapering before it enters the duodenum.
Disadvantages of IOUS include its inability to display the entire biliary tree at once. It is also less sensitive for evaluating bile duct strictures, fistulae, and injuries. In the setting of more complex biliary disease or equivocal IOUS results, intraoperative cholangiogram should also be performed in conjunction with IOUS.
Intraoperative ultrasound can provide helpful information during the evaluation of a complicated pancreatitis, as well as localization of pancreatic neoplasm and of endocrine nodules such as islet cell tumors and can, thus, guide pancreatic procedures. Patients with pancreatitis can progress to develop pseudocysts, dilated pancreatic ducts, pancreatic abscesses, and splenic vein thrombosis. The diagnosis of these complications is facilitated by the use of IOUS, which can detect small lesions or subtle abnormalities, otherwise missed on preoperative imaging. IOUS can also be used to guide needle placement for cyst drainage procedures or biopsies, or for opening of small pseudocysts.
IOUS is particularly useful in assessing the extent of tumor invasion to the liver, portal system, and lymphatics, and for determining pancreatic tumor resectability during laparotomy.18 In fact, IOUS can detect nonpalpable liver metastases and is more accurate than CT for detecting portal vein involvement.18 The pancreas can be evaluated either transgastrically (if the stomach is decompressed), or directly by opening the lesser sac and placing the ultrasound probe directly on the pancreas. Any lesions within the parenchyma as well as the major vessels that course beneath it can be visualized.
Intraoperative ultrasound can also help localize small primary endocrine neoplasms that are often not seen on preoperative CT scan or MRI. Combining open exploration and palpation with intraoperative ultrasonography allows exact localization of tumors that have been regionalized preoperatively by intra-arterial calcium stimulation with hepatic venous sampling. Islet cell tumors appear hypoechoic on IOUS, making it possible to detect lesions as small as 3 mm that are not palpable or visible to the naked eye. In fact, IOUS has been found to detect and localize insulinomas and intrapancreatic gastrinomas with a sensitivity of 83–100%, and 95%, respectively, which is superior to that of preoperative angiography.18,24 This is especially true for insulinomas located in the body and tail of the pancreas.
Advantages and Limitations
- IOUS should be accomplished using a small flat or cylindrical probe with high-frequency linear array or convex transducers.
- IOUS provides multiplanar images from multiple locations providing more imaging information than conventional radiography.
- IOUS is more accurate than other preoperative imaging studies such as CT and MRI in detecting locoregional metastases and deep, nonpalpable lesions.
- IOUS can be accomplished rapidly with greater accuracy than conventional radiography and yields immediate results.
- IOUS can be used to guide diagnostic and therapeutic procedures and therefore has a wider applicability than intraoperative contrast radiography.
- The use of color or power Doppler imaging with IOUS facilitates image interpretation by clarifying vascular anatomy in relation to the lesion of interest.
- IOUS is less reliable for smaller lesions and will not detect lesions smaller than 3–5 mm.
- IOUS cannot detect tumors that are isoechoic to surrounding tissue.16
- IOUS is unable to display an entire ductal system or vascular tree in one image.
- IOUS should not be used in lieu of other intraoperative techniques, but as an adjunct to surgical exploration, palpation, and other appropriate radiographic modalities.
Future Developments in Intraoperative Ultrasound
With ongoing advances in technology, the clinical applications of IOUS will continue to expand. Improved resolution and tissue penetration of IOUS will support its routine use in a wider range of surgical procedures. Despite ongoing improvements in the sensitivity of ultrasound, Doppler-based techniques cannot yet accurately detect low-velocity blood flow in smaller vessels. The future use of harmonic imaging with contrast agents will hopefully improve the use of color or power Doppler imaging to detect arterial enhancement of metastases. In fact, studies already show that intraoperative ultrasound with contrast is the most sensitive imaging modality for detection of deeper lesions as well as those that are smaller than 1 cm and are not visualized on CT or MRI.25 Harmonic imaging with contrast is already being used to provide angiography-like images of intracranial vascular pathology.
Further advances in three-dimensional (3D) imaging, including the use of contrast, will facilitate IOUS-guided tumor ablation and resection by providing better anatomical information. 3D imaging with contrast can further guide needle placement for radiofrequency ablation procedures and for biopsies, and improve the visualization of active lesions.
Overview of Laparoscopic Ultrasound
Laparoscopic ultrasound (LUS) using an A-mode rigid probe to define the extent of a liver tumor was first reported in 1958.26 Fukada et al first documented the use of real-time two-dimensional imaging for localizing intrahepatic lesions in 1984.26 But it wasn’t until laparoscopic cholecystectomy became popular in the early 1990s that laparoscopic ultrasonography began to evolve and be accepted. At the time, it was most commonly used for the evaluation of the biliary tract for stones and for masses during laparoscopic cholecystectomy. Its use has now been expanded to include staging of hepatic and pancreatic tumors during diagnostic laparoscopy and multiple other applications in gastric and hepatopancreaticobiliary surgery. The development of laparoscopic color Doppler imaging has also made it possible to rapidly differentiate between bile ducts and vascular structures and to determine the extent of vascular invasion by tumor, or the presence of tumor thrombus within a vessel.
General Instrumentation and Technique
Most current laparoscopic ultrasound devices consist of linear array 6–10 MHz transducers that are mounted on flexible or rigid probes and are connected to compact, real-time B-mode scanners. The probes must measure less than 10 mm to fit through a laparoscopic port and must range from 35 to 50 cm in length. Flexible probes can be bi- or quad-directional and are most often used for visualizing the liver, pancreas, and other retroperitoneal structures, while rigid probes are best used for examining the bile ducts. Rigid probes can also be used for transgastric imaging of the pancreas and for ultrasound-guided biopsies that require parallel alignment of the aspirating needle with the beam of the ultrasound waves.
During LUS, the transducer is apposed directly to the structure being imaged and the scanner can usually be controlled from the sterile field. Higher-frequency probes (10 MHz) are used to image structures closer to the transducer, while lower-frequency probes (7.5 MHz) are ideal for evaluating the liver, kidney, and other solid organs. Instruments that are capable of Doppler flow are ideal and usually include audible flow signals or flow wave patterns displayed on a screen. Color Doppler imaging was introduced in the 1990s and provides color-coded images of blood flow within vascular structures.
Indications for Laparoscopic Ultrasound
Although best known for its use in diagnosing choledocholithiasis, laparoscopic ultrasound can also be used to identify anatomic variations in the biliary tree and to help guide dissection during laparoscopic cholecystectomy. It can also aid in the detection of gastrointestinal tumors, guide biopsies of lesions, and help determine the extent of local neoplastic spread and invasion.
Evaluation of Specific Organ Systems
Laparoscopic ultrasound is currently part of the routine staging of upper GI malignancies. It is more accurate than CT or endoscopic ultrasound for assessing tumor stage, and for detecting carcinomatosis and metastatic lesions in the liver. It has been found to be useful in determining resection margins for primary gastric cancer, although it is less sensitive when recurrent cancer or gastric lymphomas are involved. Laparoscopic ultrasound can accurately detect metastatic disease and nodal involvement, and prevent unnecessary laparotomy in a significant number of patients, especially when combined with diagnostic laparoscopy. However, it can also downstage more advanced tumors by excluding the presence of direct invasion. Apart from adding no additional risk to diagnostic laparoscopy, it can be done quickly and does not increase patient morbidity.
The placement of three ports is generally required for adequate assessment of gastric malignancies, including a 10-mm port at the umbilicus, a 10- to 12-mm port in the right upper quadrant and a 5-mm port in the left upper quadrant. The anterior gastric wall is usually examined by placing the probe directly on the stomach and instilling saline for acoustic coupling. By compressing the stomach or filling the stomach with saline, the posterior wall can be visualized and assessed for masses or for posterior invasion of the pancreas. Tumors usually appear as hypoechoic masses that arise from the mucosa. A probe with a flexible tip is used to examine the liver parenchyma, celiac axis area, and hepatoduodenal ligament for metastases. It is also used to evaluate the presence of perigastric nodes, including those around the celiac trunk, splenic hilum, and aorta, to determine local invasion and to confirm resectability.
Laparoscopic ultrasound is primarily used for staging of hepatocellular and metastatic colon cancer. It has also been found to effectively detect gallbladder carcinoma, cystic diseases, and other benign tumors of the liver, as well as tumors of the proximal common bile duct. Studies have also found LUS to be equivalent or superior to intraoperative cholangiography for diagnosing choledocholithiasis with a sensitivity of 90–96% and a specificity of 100%.27,28 Compared to laparoscopic intraoperative cholangiography (IOC), laparoscopic ultrasound is less sensitive for detecting intrapancreatic common bile duct stones and identifying abnormal ductal anatomy. However, it can be done more rapidly and requires less dissection than is needed for IOC.
When laparoscopic ultrasonography and diagnostic laparoscopy are combined, the sensitivity and specificity for staging of hepatocellular carcinoma are reported to be 80–100% and 75–90%, respectively; this combination identifies almost 50% more liver lesions when compared to preoperative CT scan alone.1,29,30 Even without diagnostic laparoscopy, laparoscopic ultrasound can provide additional staging information and thereby alter the management of liver tumors in more than 40% of cases.30
Laparoscopic ultrasound of the liver is best accomplished with a 6 MHz flexible probe introduced through ports at the umbilicus and right abdominal wall, in order to gain access to the anterior and superior surfaces of the liver.1 The left lateral segment can be visualized via an additional left-sided port or by placing the transducer obliquely against the falciform ligament. The appearance of the liver and any other lesions is similar to that seen on open intraoperative ultrasound.
For laparoscopic ultrasound of the biliary tree the port placement is similar to that used during laparoscopic cholecystectomy. This usually requires placement of a 10-mm port below the xiphoid or subcostally between the mid- and anterior axillary line in order to obtain transverse imaging of the porta hepatis. The junction of the cystic duct and common duct can be seen by sweeping the probe along the lateral edge of the hepatoduodenal ligament. The transducer can also be placed directly over the bile duct following retraction of the liver, superiorly. Orienting the probe transversely in the middle of the CBD produces a cross-sectional image of the hepatic artery, portal vein, and common duct that has a very characteristic appearance (Figure 6-14).31 The portal vein is the largest, most posteriorly located structure within the portal triad. The hepatic artery lies anterior and to the left of the portal vein, while on cross-sectional imaging, the common bile duct lies anterior to the portal vein and to the left of the proper hepatic artery.
Schematic view of the hepatoduodenal ligament showing characteristic appearance of the portal triad during laparoscopic ultrasound (CBD = common hepatic duct; HA = hepatic artery; PV = portal vein). (Adapted from Holzheimer RG, Mannick JA. Surgical Treatment: Evidence Based and Problem-Oriented. W. Zuckschwerdt Verlag GmbH; 2001.)
To obtain longitudinal images the flexible probe can also placed through an umbilical port. This permits scanning of the liver and visualization of the common bile duct medial to the gallbladder. This view also facilitates imaging of the proximal intrahepatic ducts and of the porta hepatis from a posterior position.
Despite extensive diagnostic workup, the majority of patients with pancreatic cancer are found to have unresectable disease at the time of surgery. Even with advanced preoperative imaging techniques, 20–70% of patients will undergo nontherapeutic laparotomy.32 Diagnostic laparoscopy has therefore become an important tool in the accurate staging of pancreatic cancer. Laparoscopic ultrasound can improve the results of staging by compensating for the lack of tactile information that laparoscopy alone can provide. A recent meta-analysis demonstrated that the routine combined use of laparoscopy with laparoscopic ultrasound can prevent up to 50% of unnecessary laparotomies, thereby expediting the commencement of alternative treatments following a low risk, minimally invasive procedure.32
Pancreatic cancer is considered unresectable when there is metastatic tumor in the liver, or if there is involvement of celiac lymph nodes or invasion of the celiac, hepatic, or superior mesenteric arteries. With the use of color Doppler signals, laparoscopic ultrasound can identify occult metastatic disease and predict tumor resectability with an accuracy of 98%.33 LUS has also been found to identify unresectable disease in up to 28% of patients whose lesions were missed on diagnostic laparoscopy alone, with less than 10% false negative rate.30 One report found that diagnostic laparoscopy combined with laparoscopic ultrasound is more sensitive than many other diagnostic modalities for the detection of liver and peritoneal metastases.34 Compared to diagnostic laparoscopy, laparoscopic ultrasound is better for assessing tumor size and for detecting small liver metastases that are missed on inspection and other imaging studies alone.34
Laparoscopic ultrasound assessment of the pancreas usually requires a single port placement in the umbilical region or right lateral abdominal wall. The body and tail of the pancreas can be visualized by placing the probe on the anterior wall of the stomach. The head of the pancreas is best seen directly via the lesser sac by opening the gastrocolic ligament and placing the probe on the surface of the pancreas. Alternatively, the head can be seen by compressing the walls of the duodenum with the probe. The normal pancreas appears homogeneous with a ground-glass appearance, while most pancreatic lesions appear as ill-defined hypoechoic masses.1 If a thorough inspection reveals no metastatic disease, a more detailed examination is performed to inspect the liver parenchyma, portal vein, mesenteric vessels, celiac trunk, hepatic artery, peripancreatic, and periportal lymph nodes. For intraoperative localization of pancreatic insulinomas thought to be located at the head or neck of the pancreas, a laparoscopic Kocher maneuver can be performed allowing positioning of the probe posterior to the uncinate process.
The morbidity of LUS for the evaluation of pancreatic cancer ranges from 0% to 4% and is primarily related to the risk of wound infection and port-site bleeding. This is often balanced by the shorter hospital length of stay associated with laparoscopic staging and LUS compared to open exploration.30
Laparoscopic ultrasound can help localize and guide the intraoperative drainage and decortication of renal cysts as well as lymphoceles after kidney transplantation. It has also been used to help guide radiofrequency ablation of renal tumors. LUS can identify deeper structures not seen on inspection or laparoscopy alone and thereby decrease the incidence of iatrogenic injury to the kidney and surrounding structures. It can lead to a decrease in length of hospital stay and local infections of the fluid collections compared to open drainage of lymphoceles and renal cysts, as well as a decrease in postoperative pain.
Laparoscopic ultrasound can also be used to localize the adrenal gland (which can be difficult to visualize in the adipose tissue of the retroperitoneum) and surrounding vasculature during laparoscopic adrenalectomy. The use of laparoscopic ultrasound during adrenal surgery requires a 12-mm camera port to allow passage of the flexible ultrasound probe. Retracting the liver anteriorly and dividing the posterior peritoneal attachments of the liver exposes the superior portion of the right adrenal gland, while the left adrenal can be visualized after medial rotation of the spleen. Neoplastic lesions of the adrenal gland appear hypoechoic relative to other structures and they have a hyperechoic rim.
Advantages and Limitations
- LUS combined with staging laparoscopy has a higher success in diagnosing patients with locally advanced pancreatic cancer compared to other types of periampullary tumors.
- The advantages of laparoscopic ultrasound include the ability to accurately determine the extent of tumor invasion and to plan the operative approach on the basis of this information.
- Even though laparoscopic ultrasound can help guide appropriate treatment for locally advanced disease, false negative studies may lead to unnecessary laparotomies and therefore increased morbidity and cost to the patient.
- The diagnostic yield of LUS is operator-dependent and has a steep learning curve; it is also influenced by tumor histology, size, and location.