Portable Ultrasound

Ultrasound imaging.

Portable ultrasound units are becoming standard equipment in many large animal clinics, affording easy access to this imaging modality. Many units used for reproductive practice are equipped with a linear, 5- to 7.5-MHz transducer. This type of machine can provide reasonably good-quality images of the thorax and adjacent soft tissues. When available, a curvilinear probe provides superior image quality, but certainly is not required for diagnostic use. Appropriate patient preparation is paramount for obtaining a good-quality image. Wool or hair over the site of interest should be clipped, although the use of coupling agents (e.g., gel, vegetable oil, alcohol) can be helpful in some instances. Owing to the nature of the functioning, gas-filled lung, ultrasonography of the respiratory tract is more limited than that of other body systems. For example, ultrasound examination of the pharyngeal region may provide an easy means of identifying retropharyngeal abscesses when they are suspected from findings on palpation. In such cases, the probe should be placed parallel to the lateral aspect of the trachea and directed dorsomedially towards the opposite ear. Abscesses typically have a hyperechoic wall, with variable echotexture of the contents.

Ultrasound imaging also can provide useful information in evaluation of the thorax. The clinician should become familiar with the appearance of normal aerated lung, allowing rapid identification of areas that lack the normal appearance. Normal lung is recognizable by the bright hyperechoic line of the visceral pleura above a classic reverberation artifact induced by the aerated lung. The reverberation artifact is typical of ultrasound waves hitting a gas interface and consists of sequential hyperechoic lines spaced at regular intervals. It is important to realize that any images appearing on the screen deep to the start of the reverberation artifact are indeed artifacts and not images of the lung parenchyma (Figure 7.3). Once an appreciation for the normal appearance of lung has been achieved, the thoracic exam can be systematically performed. With use of a linear or a curvilinear probe, the probe should be oriented parallel to the ribs in the intercostal space. We prefer to start at the most dorsal aspect of each intercostal space and slowly move downwards to the ventral thorax observing the lung surface along the path. This is repeated in each intercostal space moving caudally. The image quality is maximized by following the natural “lay” of the wool or hair (in a dorsal-ventral direction). As the exam progresses caudally, the diaphragm comes into the image while moving ventrally, often with the adjacent liver filling the space below. With use of this method, the extent of the thoracic lung field can be determined. Focused examination of the cranioventral lung fields consistently identifies lesions associated with bronchopneumonia; this exam can be performed in the nonhaired axillary region without requiring clipping fleece and is facilitated by “flipping” the sheep and restraining them on their rump. The three primary lesions that may be observed are parenchymal masses in the lung that are adjacent to the visceral pleura, lung consolidation, and the characteristic “comet tail” lesions associated with pleural thickening and inflammation. The first of these lesions is readily identified by the observation of echo-dense masses interrupting the normal reverberation artifact of the lung. Such masses can be measured to allow for sequential ultrasonographic examination as a means of assessing treatment success or resolution of the lesion. In our experience, these lesions most commonly are associated with parenchymal abscesses. Consolidated lung is recognized on deeper imaging, beyond the normal lung reverberation. In many instances, the consolidated lung may have an appearance similar to that of liver (“hepatized lung”) or may be seen to contain scattered gas shadows associated with presence of gas in the larger airways or in abscesses. “Comet tails” are recognizable as small, hyperechoic spots with a comet tail-shaped artifact located deep to the spot. These lesions are non-specific but often are associated with thickening or inflammation of the pleura.

If pleural fluid is present, it is imaged as an anechoic or hypoechoic area in the ventral thorax, with normal lung reverberation noted at the lung-fluid interface. Because the mediastinum is not always easily imaged, radiographs remain the preferred imaging modality for identification of mediastinal masses.

E Ultrasound Evaluation of the Urinary Tract

PREOPERATIVE ASSESSMENT

Use of Ultrasound

Recently there has been great interest in the use of ultrasound-guided peripheral nerve blocks in children.¹²⁷ The availability of high-resolution portable ultrasound machines has become increasingly commonplace and will likely soon become the new standard of care for many peripheral nerve blocks. Although this requires sophisticated expensive equipment, it may have a larger role in pediatric regional blockade because most blocks are performed while the child is anesthetized. Several manufacturers make ultrasound machines that are about the size of a laptop computer and have been designed for ease of use by the anesthesiologist. The cost-effectiveness of acquiring these devices is justified because they serve a dual purpose for placing invasive central lines. Direct visualization of the nerve may facilitate correct placement of the local anesthetic and may also help reduce the total dose of local anesthetic needed for successful blockade. It is imperative to use an ultrasound machine that is capable of scanning superficially because most of the nerves in children are usually less than a few millimeters from the skin. A more in-depth discussion of ultrasound guidance for peripheral nerve blocks can be found in Chapter 43.

Use of Ultrasound

Recently there has been great interest in the use of ultrasound-guided peripheral nerve blocks in children.¹⁶² The availability of high-resolution portable ultrasound machines has become increasingly commonplace and has revolutionized the practice of regional anesthesia in children. Many argue that it is already the new standard of care for most peripheral nerve blocks.¹⁹ Emerging evidence suggests ultrasound guidance increased success rates, reduced pain scores, prolonged block duration, and reduced time of block performance and the number of needle passes, particularly in younger children.¹⁹ Although this practice requires sophisticated expensive equipment and the acquisition of new skills, it is likely to have an essential role in pediatric regional blockade because most blocks are performed while the child is anesthetized. The cost-effectiveness of acquiring these devices is justified because they serve a dual purpose for placing invasive central lines, peripheral and arterial catheters. Direct visualization of the nerve may facilitate correct placement of the local anesthetic and may also help reduce the total dose of local anesthetic needed for successful blockade. It is imperative to use an ultrasound machine that is capable of scanning superficially because most of the nerves in children are usually less than a few millimeters from the skin. A more in-depth discussion of ultrasound guidance for peripheral nerve blocks can be found in Chapter 43. Because ultrasound might still not be available to every practitioner, a complete discussion of landmark and nerve stimulator–guided peripheral blockade is included in this chapter.

Ultrasonography

The introduction of harmonic and compound imaging, advances in high-resolution transducers, and improvements in color Doppler evaluation have all combined to enhance the diagnostic capabilities of portable ultrasound. In general, ultrasound is useful for imaging solid organs and fluid-filled structures, but it is unable to penetrate gas-filled structures. For example, overlying bowel gas often precludes a complete sonographic evaluation of the pancreas. Ultrasound is a relatively versatile imaging technique, in that it can be performed via many different routes, including transabdominal, endoscopic (as part of EGD), transrectal, intravascular, and endovaginal approaches. In addition, it is excellent for many image-guided interventions because of its real-time evaluation.

With regard to GI pathology, ultrasonography is used most frequently to evaluate the liver and biliary system. Suspected acute cholecystitis (Chapter 158) is a common indication for right upper quadrant sonography; classic findings include cholelithiasis, gallbladder wall thickening, and a sonographic Murphy's sign (reproducible pain when the transducer is pressed over the gallbladder) (Fig. 135-3). The sensitivity for detecting gallstones with ultrasonography exceeds 95%. Acalculous cholelithiasis can be a more challenging diagnosis because the findings overlap with nonspecific gallbladder wall thickening in critically ill patients. Ultrasound is typically the first imaging test obtained in patients with new-onset jaundice or cholestatic laboratory findings because it offers a rapid, noninvasive evaluation of the biliary tree to differentiate obstruction from other causes. If biliary ductal dilation is present, the level and cause of the obstruction can sometimes be demonstrated on ultrasound; common causes include choledocholithiasis and pancreatic head masses. In most cases of biliary obstruction, additional imaging tests will be necessary, consisting of CT, MR cholangiopancreatography (MRCP), endoscopic retrograde cholangiopancreatography (ERCP), or percutaneous transhepatic cholangiography (PTC), depending on the specific circumstances.

Ultrasonography can be used to detect or further characterize focal liver lesions (Chapter 154), although it is typically less sensitive and specific than CT or MRI. Ultrasound is quite capable of distinguishing cystic from solid lesions. Although not approved for use in the United States, intravenous contrast agents for ultrasound have been studied fairly extensively in other countries and appear to offer similar advantages seen with CT and MRI contrast agents.

In diffuse disease, ultrasound is being used with increased frequency to screen patients with viral hepatitis for cirrhosis and hepatocellular carcinoma (Chapters 156 and 202Chapter 156Chapter 202). Sonographic findings in cirrhosis include a heterogeneously coarsened parenchymal echotexture, nodular surface contour, predominantly right-sided volume loss, and evidence of portal hypertension, including ascites, splenomegaly, and portosystemic collaterals. Focal hepatic lesions in the setting of cirrhosis are concerning for hepatocellular carcinoma, but they may also represent regenerative or dysplastic nodules. In noncirrhotic patients with elevated liver enzymes, ultrasonography can often suggest the diagnosis of hepatic steatosis (fatty liver; Chapter 155) when the parenchyma demonstrates increased echogenicity and decreased penetration of the sound beam. The findings of steatosis can be focal, multifocal, or diffuse; MRI is more specific and can confirm the diagnosis.

Color and power Doppler evaluation allows the noninvasive sonographic assessment of vascular patency. Doppler evaluation of the liver is commonly performed in patients with end-stage liver disease (Chapter 157) to evaluate the portal system and search for portosystemic collaterals. Abnormal portal vein findings include hepatofugal flow and thrombosis (Fig. 135-4). Doppler ultrasound is also used for the evaluation of transjugular intrahepatic portosystemic shunts (TIPS), both before and after stent placement. In orthotopic liver transplant recipients, Doppler evaluation is frequently performed to assess the hepatic vasculature, with particular attention to the hepatic arterial supply.

Specific Considerations for Diagnosis

Physical examination, single-shot angiography, and vascular exposure were previously used for timely diagnosis. Currently, extensive imaging capabilities have appeared in more forward hospitals. Hand-held Doppler, portable ultrasound and, very recently, formal angiography using a mobile c-arm are liberally used for vascular injury diagnosis. Nonetheless, computed tomography is not freely available at role 2E. Modified Kornilov’s classification of acute limb ischemia (primarily found in 1967 and released in 1971²) is now used for limb evaluation and decision-making concerning vascular treatment strategy according to new interventional capabilities (Table 32.1).

Insertion of a Central Venous Catheter

The Seldinger technique is commonly used to insert a central venous catheter. Figure 10–5 shows the steps to catheterization of the right internal jugular vein with this technique.

To reduce the number of cannulation attempts and the risk of inadvertent arterial puncture, a portable ultrasound vessel-imaging device is often used to visualize the vascular structures as shown in Figure 10–6.

There are several different kinds of catheters that can be inserted centrally, including single-lumen catheters, multi-lumen catheters, and introducer sheaths. Often a multilumen catheter will be inserted if the catheter is primarily intended for administration of medications and/or CVP monitoring. Introducer sheaths are used when there is the need for a larger catheter, such as for rapid volume administration or for the insertion of a pulmonary artery catheter. When placing an introducer sheath, the use of a tapered-tip, stiff dilator should be placed into the introducer to facilitate passage of this large cannula over the guidewire from the skin, through the subcutaneous tissues, and into the vein. Figure 10–7 shows examples of a multilumen catheter and an introducer catheter.

Radiographs

Plain radiography of the abdomen may reveal the “eggshell” appearance of a calcified AAA, but it is not helpful in excluding AAA from the differential diagnosis in patients who are at risk. Multiple studies have demonstrated that ultrasound is 100% sensitive in the detection of AAA and that this can be done at the bedside in the emergency department (ED) by emergency physicians using portable ultrasound devices with the same accuracy. ED bedside ultrasound for AAA has been demonstrated to significantly decrease time to diagnosis and disposition. Obesity and bowel gas may limit visualization of the aorta in some patients, making diagnosis difficult. All patients who are at risk for AAA should have bedside ultrasound scanning performed in the ED. Stable patients can undergo a formal ultrasound or CT scan. The advantages of CT over ultrasound are that CT can better define the extent of disease, identify retroperitoneal hemorrhage or rupture, or identify alternative diagnoses in the setting of a normal aorta. The major disadvantage is that the patient must leave the ED to go to receive the CT scan, and resuscitation is often difficult in that setting. Aortography is helpful for preoperative planning in elective repairs but is not indicated for diagnosis in the ED.

Conclusions

In the third millennium, medical students and, in particular, cardiologists will learn heart anatomy in the form of medical imaging. Imaging technology in cardiology (and in general in medicine) has advanced enormously in the last decade and will likely become less expensive and widely available in the next few years. Small and light as a cell phone, new portable ultrasound machines will substitute for the stethoscope, showing heart and valves in three dimensions. Fundamentally, does not the word stethoscope derive from the ancient Greek words στηθóυς (stetheos), which means “chest,” and σκοπεω (skopeo), which means “look into”? New CT machines with high resolution power and ultralow radiation exposure are able to depict the anatomy of the heart with an unbelievable precision and fidelity to anatomic specimens. CMR has finally met the expectation of cardiologists offering in one examination superior anatomic imaging of heart and vessels, chamber structures (in particular myocardial necrosis and fibrosis), and tissue perfusion. We strongly believe that the above-mentioned imaging techniques will be extensively used in medical schools and universities to supplement and reinforce (if not substitute) the classic anatomy based on cadavers and anatomic specimens. We hope that this chapter will also serve this aim.