Aplicatiile US in Chirurgie

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Ultrasonography: Surgical Applications

Grace S. Rozycki, M.D.

ACS Surgery 2003. 2003 WebMD Inc.All rights reserved.

Posted 06/30/2003

Introduction

Although the scientific principles underlying ultrasonography first began to be elucidated in the 19th century, itwas not until the second half of the 20th century that this technology could be effectively applied to medicine. Inparticular, surgeons in the United States have now embraced ultrasonography as a key diagnostic tool in manyareas of clinical practice. Because ultrasonography is noninvasive, portable, rapid, and easily repeatable, it isespecially well suited to surgical practice. In addition, computer-enhanced high-resolution imaging andmultifrequency specialized transducers have made ultrasonography increasingly user friendly, enhancing itsapplicability to a variety of surgical settings.

Physics and Instrumentation

Before the application of ultrasound devices to patient evaluation is addressed, it is worthwhile to briefly reviewcertain basic physical principles and terminology associated with ultrasonography (seeTable 1,Table 2, andTable 3).[1-5] Nowhere in diagnostic imaging is the understanding of wave physics more important than inultrasound diagnostic imaging, because ultrasonography is highly operator dependent. To perform an ultrasoundexamination correctly, a surgeon must be able to interpret echo patterns, determine artifacts, and adjust themachine appropriately so as to obtain the best images.

In diagnostic ultrasonography, the transducer or probe interconverts electrical and acoustic energy (see Figure1).[6] To accomplish this interconversion, the transducer contains the following essential components:

1. An active element. Electrical energy is applied to the piezoelectric crystals within the transducer, and anultrasound pulse is thereby generated via the piezoelectric effect. The pulse distorts the crystals, and anelectrical signal is produced. This signal causes an ultrasound image to form on the screen via thereverse piezoelectric effect.

2. Damping or backing material. An epoxy resin absorbs the vibrations and reduces the number of cyclesin a pulse, thereby improving the resolution of the ultrasound image.

3. A matching layer. This substance reduces the reflection that occurs at the transducer-tissue interface.The great difference in density (i.e., the impedance mismatch) between the soft tissue and thetransducer results in reflection of the ultrasound waves. The matching material decreases this reflectionand facilitates the transit of the ultrasound waves through the body and into the target organ.
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Figure 1. Components of ultrasound transducer. Shown are the basic components of an ultrasound transducer.

Transducers are classified according to (1) the arrangement of the active elements (array) contained within thetransducer and (2) the frequency of the ultrasound wave produced. Transducer arrays contain closely packedpiezoelectric elements, each with its own electrical connection to the ultrasound instrument.[7] These elementscan be excited individually or in groups to produce the ultrasound beam. There are four main transducer arrays:(1) the rectangular linear array, which yields a rectangular image, (2) the curved array, which yields a

trapezoidal image, (3) the phased array, a small transducer in which the sound pulses are generated byactivating all of the elements in the array, and (4) the annular array, in which the elements are arranged in acircular fashion. The advantage of transducer arrays is that the ultrasound beam can be electronically steeredwithout any moving mechanical parts (except for the annular array) and focused.[7,8] In the clinical setting, thisarrangement allows the operator to adjust the focal zone so that he or she can accurately image a large organ(e.g., the liver) while still being able to obtain fine details of a lesion.

The frequency of the transducer is determined by the thickness of the piezoelectric elements within thetransducer: the thinner the piezoelectric elements, the higher the frequency.[7,8] Although diagnosticultrasonography makes use of transducer frequencies ranging from 1 MHz to 20 MHz, the most commonly usedfrequencies for medical diagnostic imaging are those between 2.5 and 10 MHz (seeTable 4). Ultrasound beamsof different frequencies have different characteristics: higher frequencies penetrate tissue poorly but yieldexcellent resolution, whereas lower frequencies penetrate well but at the cost of compromised resolution.

Accordingly, transducers are generally chosen on the basis of the depth of the structure to be imaged.[9] Forexample, a 7.5 MHz transducer is a suitable choice for imaging a superficial organ such as the thyroid, but a 3.5MHz transducer would be preferable for imaging a deep structure such as the abdominal aorta.

Ultrasound machines vary in complexity, but each has the following essential components:

1. A monitor (for displaying the ultrasound image).

2. A keyboard (for labeling the image and making adjustments to produce a quality image).

3. A transducer (for interconverting electrical and acoustic energy).
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4. An image recorder (for producing copies of the ultrasound images).

Finally, there are three scanning modes, A, B, and M; these modes evolved over several years.[10] A mode(amplitude modulation), the most basic form of diagnostic ultrasonography, yields a one-dimensional image thatdisplays the amplitude or strength of the wave along the vertical axis and the time along the horizontal axis.Therefore, the greater the signal returning to the transducer, the higher the "spike." B mode (brightnessmodulation), the mode most commonly used today, relates the brightness of the image to the amplitude of theultrasound wave. Thus, denser structures appear brighter (i.e., whiter, more echogenic) on the image becausethey reflect the ultrasound waves better. M mode relates the amplitude of the ultrasound wave to the imaging of

moving structures, such as cardiac muscle. Before real-time imaging became available, M-mode scanningformed the basis for echocardiography. [10,11]

Clinical Applications of Ultrasonography in Surgical Practice

As an extension of the physical examination, ultrasonography is a valuable adjunct to surgical practice in theoffice, the emergency department, the OR, and the SICU. Once surgeons have learned the essential principlesof ultrasonography, they can readily build on this experience and extend the use of this technology to variousspecific aspects of surgery. In what follows, I list and briefly describe several clinical areas in which surgeon-performed ultrasonography has proved to be an effective diagnostic and interventional tool.

Breast

Ultrasound-directed biopsy of breast lesions is now a common office procedure for general surgeons. Theincrease in the number of screening mammograms performed since the late 1970s has led to the detection ofmore nonpalpable breast lesions. The traditional choice for further evaluation of such masses has been opensurgical excision, but the yield of malignancies with this approach has been only about 20%. [12-14] Advances inultrasound technology, including automated biopsy needles, high-resolution transducers,[15] and computer-aideddiagnosis programs,[16] have prompted a surge of interest in fine-needle and core biopsy tissue sampling as analternative to open biopsy. Such procedures are appealing because they are minimally invasive, are about asaccurate as open biopsy,[17] and can be performed by the surgeon in the office setting.[18] Essentially, surgeonsuse ultrasound to evaluate the breast for the presence of a solid or cystic lesion and to identify thosecharacteristics of a lesion that suggest whether it is benign or malignant.

Current indications for breast ultrasonography include (1) evaluation of mammographically detected

microcalcifications or nonpalpable, new, or growing masses, (2) evaluation of duct size in the presence of nippledischarge, (3) assessment of a dense breast or a vaguely palpable mass, (4) differentiation between a solidpalpable mass and a cystic one, and (5) guidance of percutaneous drainage of an abscess.[19-24] Additional usesinclude postoperative follow-up for hematomas, seromas, and prostheses.

Ultrasound-guided interventions now in clinical use include cyst aspiration, biopsy of solid lesions, preoperativeneedle localization, axillary lymph node fine-needle aspiration (FNA), and peritumoral injection for sentinellymph node biopsy (see V:3 Lymphatic Mapping and Sentinel Lymph Node Biopsy).[25] Reports suggest thathigh-resolution ultrasonography can accurately detect intraductal spread of tumors and delineate their multiplefoci. Ongoing developments in imaging technology and contrast agents have given perfusion studies anenhanced contrast resolution that increases the sensitivity of ultrasonography for small nodal metastases.Accordingly, the use of breast ultrasonography in the office setting has become considerably more sophisticatedand sensitive, allowing more patients to be screened for microdisease.[18]

Gastrointestinal Tract

Endoscopic and endorectal ultrasonography have added a new dimension to the preoperative assessment andtreatment of many GI lesions. Endoscopic ultrasonography (EUS) involves the visualization of the GI tract via ahigh-frequency (12 to 20 MHz) ultrasound transducer placed through an endoscope. With the transducer nearthe target organ, images of the gut wall and the surrounding parenchymal organs can be obtained that aredetailed enough to define the depth of tumor penetration with precision and to detect the presence of involvedlymph nodes as small as 2 mm. When done preoperatively, EUS is 80% to 90% accurate at predicting the stageof the tumor; if an endoscopically directed biopsy attachment is used, the diagnostic potential is even higher.[26]


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Indications for EUS include (1) preoperative staging of GI malignancies, (2) preoperative localization ofpancreatic endocrine tumors, particularly insulinomas, (3) evaluation of submucosal lesions of the GI tract, and(4) guidance of imaging during interventional procedures (e.g., tissue sampling and drainage of a pancreaticpseudocyst).[27-30] Currently, EUS is being used in conjunction with FNA biopsy to evaluate submucosal lesions ofthe GI tract as well as lesions of the pancreas. This combination is especially useful for pancreatic lesions: EUS-guided FNA accurately detects neoplastic pancreatic cysts and therefore may be helpful in determining whethermedical or surgical treatment is indicated.[31,32]

Endorectal ultrasonography is used in the evaluation of patients with benign and malignant rectal conditions.[33-41]

It is commonly performed with an axial 7.0 or 10.0 MHz rotating transducer that produces a 360 horizontalcross-sectional view of the rectal wall. This special transducer is 24 cm long and is covered with a water-filledlatex balloon. After the transducer is advanced above the rectal lesion, the balloon that surrounds the transduceris filled with degassed water to create an acoustic window for ultrasound imaging. The transducer is graduallywithdrawn while the examiner views the layers of the rectal wall (see Figure 2) by means of real-time imaging.[42,43] These layers are important landmarks in ultrasonographic staging, just as they are in postoperativepathologic staging. For example, if the middle white line (i.e., the submucosa) is intact, a benign lesion may beremoved via a submucosal resection. A classification of preoperative tumor staging called uTNM has beenproposed that is analogous to the TNM classification for tumor staging.[44] This classification is based onultrasonographic determination of the infiltrative tumor depth (the prefix ustands for ultrasonography).

Figure 2. Five-layer model of rectal wall anatomy. Depicted is the five-layer model of rectal wall anatomy asdelineated by endorectal ultrasonography.[110]

The sensitivity of ultrasonography in determining the depth of tumor invasion is about 85% to 90%; however, itcan sometimes overestimate the extent of invasion in the presence of tissue inflammation and edema.[35] Furtherresearch is needed to assess the accuracy of ultrasonography in detecting recurrent cancer after surgery.[45]

Errors in staging are likely to occur with tumors that invade the lamina muscularis mucosae or are associatedwith inflammation of the lamina propria mucosae.[46] In addition, lesions characterized by ultramicroscopic

invasion of the submucosa may be misstaged because the technology currently available cannot provide thefine resolution necessary to assess such invasion.[35,47] Flexible 360 rotating transducers are now available forthe evaluation of rectal lesions. Investigators from Madigan Army Medical Center found that whereas rigidendoscopic transducers were slightly more sensitive than flexible transducers in detecting lesions, the flexibledevices were highly accurate (77%) in staging rectal cancers; learning curves were comparable for the twotypes of transducers.[48]

Endoanal ultrasonography is an important part of the evaluation of anal incontinence because it is capable ofdetecting defects in the internal and external sphincters.[49-53] It is done in much the same way as endorectalultrasonography, except that the 10 MHz transducer is covered with a sonolucent hard plastic cone instead of awater-filled balloon. Although endoanal ultrasonography does not measure sphincter function, ultrasound-detected sphincter disruption correlates well with pressure measurements[54,55] and operative findings.[53,56]


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Additional indications for endoanal ultrasonography include evaluation of patients with an exophytic distal rectaltumor (e.g., a villous adenoma) and assessment of patients who have a perianal abscess, fistula in ano, apresacral cyst, or a rectal ulcer.

Acute Conditions

Traumatic. The FAST (Focused Assessment for the Sonographic examination of the Trauma patient) is a rapiddiagnostic test developed for the evaluation of patients with potential truncal injuries. Historically, itsdevelopment is rooted in several fundamental studies that demonstrated the high sensitivity of ultrasonographyin detecting small degrees of ascites,[57] splenic injury,[58] and hemoperitoneum in the hepatorenal space and thepelvis.[59] The FAST determines the presence or absence of blood in the pericardial sac and three dependentabdominal regions, including Morison's pouch, the splenorenal recess, and the pelvis.

Ultrasonography may also be used in traumatic settings to detect hemothorax, sternal fracture, andpneumothorax.[60-62]

Nontraumatic. In the acute nontraumatic setting, surgeons are currently using ultrasonography for the followingpurposes:

1. Assessment for multiple loculations and drainage of a soft tissue abscess. [63,64]

2. Early diagnosis of wound dehiscence through visualization of the fascial defect (see Figure 3).

3. Detection of a foreign body in soft tissue.[65-67]

4. Evaluation of a patient with abdominal pain (e.g., from gallstones).[63,64,68,69]

5. Confirmation of the reduction of an incarcerated hernia through identification of the fascial defect and

observation of the reduction occurring with real-time imaging (see Figure 4).[70]

6. Identification of an abdominal aortic aneurysm in a patient who presents with back pain andhypotension. Intramural calcification and intraluminal thrombus are common findings (see Figure 5). Ifthe aortic aneurysm ruptures into the peritoneal cavity, the FAST can detect the presence ofhemoperitoneum.


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Figure 3. Ultrasound of midline abdominal wound dehiscence. Ultrasound image shows midline abdominalwound dehiscence. Transducer orientation is sagittal with respect to long axis of wound. Interruption inhorizontal white line (arrows) represents separation of fascia.


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Figure 4. Sagittal ultrasound showing ventral hernia. Sagittal ultrasound image shows ventral hernia with fascialdefect (arrow).

Figure 5. Transverse ultrasound showing abdominal aortic aneurysm. Transverse ultrasound image showsabdominal aortic aneurysm with intraluminal thrombus.

Laparoscopy and Intraoperative Use


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Examination with intraoperative or laparoscopic ultrasonography is an integral part of many hepatic, biliary, andpancreatic surgical procedures. With this tool, surgeons can detect previously undiagnosed lesions or bile ductstones,[71] avoid unnecessary dissection of vessels or ducts, clarify tumor margins, and perform biopsy andcryoablation procedures.[72] Compared with preoperative imaging modalities, intraoperative ultrasonography ismuch more sensitive in detecting malignant or benign lesions.[73] The precision with which intraoperativeultrasonography can delineate small lesions (5 mm) and define their relationship to other structures facilitatesresection, reduces operative time, and frequently alters the surgeon's operative strategy.[73-76]

Intraoperative ultrasonography makes use of both contact scanning and so-called standoff scanning for imaging.[77] In contact scanning, the transducer is directly applied to the organ so that the deepest part of the organ isaccurately depicted. This technique is most often used for imaging large organs (e.g., the liver). In standoffscanning, the transducer is placed about 1 to 2 cm away from the structure in a pool of sterile saline solutionthat permits the transmission of ultrasound waves. This technique is often used to image blood vessels, bileducts, or the spinal cord; it allows good visualization of the structure without compression by the transducer. Thesize, shape, and type of ultrasound transducer used for intraoperative scanning depend on the anatomicstructure to be examined. For example, a pencillike 7.5 MHz transducer is used for scanning the common bileduct, whereas a side-viewing T-shaped 5 MHz transducer is preferable for imaging a cirrhotic liver.Intraoperative ultrasound examinations are conducted systematically to ensure that no subtle pathology ismissed and that the examination is reproducible. For example, the liver is imaged sequentially according to asystem based on Couinaud's anatomic segments (see V:31 Hepatic Resection).[78]

Similar principles apply to laparoscopic ultrasonography, except that the transducers are made to adapt to the

laparoscopic equipment.[79,80] Indications for this modality include detection of common bile duct stones, stagingof pancreatic cancer to prevent unnecessary celiotomy, and resection or cryoablation of hepatic metastases.[80]

Vascular System

Color flow duplex imaging and endoluminal ultrasonography have significantly expanded the diagnostic andtherapeutic aspects of vascular imaging. Vascular diagnostic imaging is commonly used for diagnosing arterialdisease or deep vein thrombosis (DVT); however, it is also helpful for diagnosing other disorders, such asRaynaud disease and thoracic outlet syndrome. In the office setting, surgeons use ultrasonography to screen forabdominal aortic aneurysm or to follow patients with a diagnosed aneurysm, because it is capable of detectingchange in aortic diameter as small as a few millimeters.[81] In patients who have undergone repair of anabdominal aortic aneurysm, color flow duplex imaging is highly specific for the diagnosis of anastomotic falseaneurysms. In one study, this modality was compared with B-mode ultrasonography, CT, digital subtraction

arteriography, and magnetic resonance imaging and emerged as the diagnostic test of choice when theaccuracy, cost, safety, and availability of each method were assessed.[82]

Color flow duplex scanning is also used to examine the patency and size of the portal vein and the hepaticartery in patients who have undergone liver transplantation, to assess the resectability of pancreatic tumors, todiagnose superior mesenteric artery occlusion, and to diagnose a pseudoaneurysm or an arteriovenous fistulaafter percutaneous arterial catheterization.[83,84] In the acute setting, several investigators have found color flowduplex imaging to be a reliable, time-saving, noninvasive alternative to arteriography for the detection of arterialinjury.[85-89]

Duplex imaging of the lower extremity is used to assess the patency of the deep venous system and is capableof detecting DVT reliably.[90] The addition of color flow imaging facilitates the examination by making the arteryand its associated vein easier to identify. By performing serial duplex venous ultrasound imaging to detect DVT,one group of investigators was able to identify a subgroup of injured patients who were at highest risk forpulmonary embolism; they suggested that these patients be given DVT prophylaxis and undergo closesurveillance with duplex imaging.[90]

Intraoperative duplex imaging can be used to detect technical errors in vascular anastomoses as well asabnormalities in flow.[91] Arteriography assesses the patency of an anastomosis and measures distal arterialrunoff, but it is invasive. Intraoperative duplex imaging, on the other hand, permits rapid visualization of theanatomic and hemodynamic aspects of a vascular reconstruction, and it is noninvasive, easily repeatable, andless time-consuming than arteriography.

Surgical Intensive Care Unit


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Indications for surgeon-performed ultrasonography in the SICU include localization of a central vein or an arteryfor hemodynamic monitoring[92] and detection of a pleural effusion (see Figure 6). Not only are fewer lateraldecubitus x-rays ordered when ultrasonography is done in the SICU, but the safety of thoracentesis is alsoenhanced when it is performed under ultrasound guidance.[93,94]

Figure 6. Sagittal ultrasound: pleural effusion. Sagittal ultrasound image demonstrates pleural effusion.

General Considerations for Diagnostic Ultrasound Examinations

Instrumentation

Before an ultrasound examination is performed, the following three steps should be observed:

1. The correct ultrasound machine and transducer should be chosen for the specific type of examination tobe done. For example, if a vascular study is to be performed, the machine should have Dopplercapability and, ideally, color flow capability as well.

2. The transducer should be chosen according to the structure or organ to be imaged. It must provide bothsufficient depth of penetration to image the entire organ and sufficient resolution to allow the examinerto distinguish the details of lesions.

3. Although many machines have preset controls for power and gain, a standard image should be obtainedto confirm that the settings are correct for the specific examination being done. For example, the FAST

begins with an image of the heart so that blood can be identified and the gain controls adjusted (ifnecessary) to permit accurate detection of hemoperitoneum.

Patient Positioning

The patient should be positioned so that all of the images required for a particular examination can be readilyobtained. The surgeon should take time to review the scanning planes (see Figure 7) and understand theorientation of the patient on the monitor screen in relation to the transducer. It is also important to followconventional scanning protocols so that when the images are reviewed, a lesion can be accurately located andthe scan can be reproduced even by another ultrasonographer. An example of such a protocol is the radial-


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scanning technique recommended for examination of the breast (see Technique for Selected SurgicalApplications of Ultrasonography, Breast Examination).

Figure 7. Scanning planes. Depicted are scanning planes used in ultrasonography.

Documentation

The machine's annotation keys are used to record the patient's name and identification number, the area ofinterest, and the scanning plane. Most machines have function keys that automate the recording of these data.Furthermore, the internal clock automatically labels each image with the date and time (accurate to 0.01second).

Any hard copies of the ultrasound images that may be required should be printed, saved, and reviewed. Ideally,the ultrasound images should be videotaped, because the dynamic real-time image provides more informationthan still images, thereby increasing the confidence level associated with each observation.

Continuous Performance Improvement

As part of the performance improvement process, ultrasound images should be routinely reviewed, with specialattention paid to false positive or false negative examinations. The goal of this process is to help identify anycorrectable factors associated with such examinations and thereby minimize or prevent their recurrence. Somestudies have noted the presence of a pronounced learning curve, as a result of which the sensitivity andspecificity initially achieved by new surgeon-ultrasonographers have been relatively low[68,95-97]; however, there is


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evidence that surgeons' performance may be improved with the help of an ultrasound training course thatfocuses on those pitfalls of imaging that were found to be problems in the clinical setting. For example, in onestudy, surgeons learned both to perform examinations correctly and to interpret positive results accurately inpatients with minimal as well as pronounced ascites; as a result, they were better able to distinguish relativelysubtle differences within the spectrum of positive FAST results.[97] Other suggestions for improving performanceare (1) to perform the ultrasound examination initially on normal tissue (as in evaluation of a breast mass) and(2) to perform the examinations on patients with known disease (e.g., a palpable breast mass, ascites,gallstones, or benign pericardial effusion). The rationale for the latter suggestion is that it should help thesurgeon learn more rapidly how to recognize lesions with varying degrees of pathology.

Technical Tips

The following general technical tips should prove useful in a wide range of ultrasonographic applications:

1. The ultrasound machine should be inspected according to the guidelines of the institution's departmentof biomedical engineering to ensure that it is functioning properly.

2. The patient's orientation on the monitor or screen relative to the position of the transducer should bechecked by applying gel to the transducer's footprint (i.e., the part of the transducer that is in contactwith the patient's skin) and then rubbing the footprint with a finger near the indicator line of thetransducer. Motion on the left side of the screen indicates that the transducer is properly oriented.

3. Liberal amounts of gel should be applied to the area being examined. The gel acts as an acousticcoupler, helping to transmit the ultrasound waves and reduce their reflection. If not enough gel has beenapplied, the waves will not be transmitted properly, and a dark area will appear on the ultrasound image.

4. The transducer should be manipulated with small movements (not wide sweeps), and gentle pressureshould be applied initially. This second point is especially important in imaging the breast or the thyroid:the tissues are superficial, and too much pressure can easily compress them and distort the ultrasoundimage.

5. The gain and time-gain compensation settings should be rechecked for each new examination. Forexample, after completing a breast examination, the sonographer should not begin an examination ofthe carotid vessels without confirming that these settings are correct.

6. Normal tissue should be examined ultrasonographically before the sonographer turns to the area ofinterest. For example, if the goal is to assess an abscess or DVT in one extremity, the first step shouldbe to inspect the other extremity to see what the corresponding normal tissue looks like. This helps tosensitize the examiner to subtle pathologic changes in the abnormal tissue.

7. The patient should be asked to take a deep breath so that the motion of the diaphragm and the organscan be observed. If the motion of these structures is impaired, inflammation or an abscess may bepresent.

8. If the left upper quadrant is difficult to examine (as is sometimes the case in the FAST), a nasogastrictube should be inserted to decompress the stomach and minimize the presence of air so that it does notinterfere with the transmission of the ultrasound waves.

9. Although B-mode ultrasound is usually sufficient to identify blood vessels, it sometimes is unable todistinguish the artery from the vein because of pulsations transmitted from the artery. In such cases, useof the Doppler mode, compression of the vessel (veins compress very easily), or having the patientperform the Valsalva maneuver can help differentiate arterial from venous anatomy. In addition, thevena cava is more readily identified as the patient completes inspiration.

10. A full bladder is needed for pelvic ultrasound examinations: it acts as an acoustic window, facilitatingvisualization of the pelvic structures. It should not, however, be so full that it is overdistended. If thebladder is not full enough, the urinary catheter can be clamped to allow it to fill; if it is too full, thecatheter can be unclamped to allow it to drain. In this way, hematomas in the pelvis can be more easilydetected.


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Technique for Selected Surgical Applications of Ultrasonography

Focused Assessment for the Sonographic Examination of the Trauma Patient

The FAST is performed during the Advanced Trauma Life Support secondary survey while the patient is in thesupine position (see I:2 Trauma Resuscitation). With the thoracoabdominal area exposed, warmedhypoallergenic, water-soluble ultrasound transmission gel is applied to the abdomen in four specific areas. Afocused, limited examination for the detection of blood in these four regions is conducted in sequence as

follows: (1) the pericardial area, (2) the right upper abdominal quadrant, (3) the left upper abdominal quadrant,and (4) the pouch of Douglas (see Figure 8).

Figure 8. Transducer positions used in FAST. FAST. Shown are four transducer positions used in FAST: (1)pericardial area, (2) right upper quadrant, (3) left upper quadrant, and (4) pelvis.[95]

The transducer is oriented for sagittal sections and placed in the subxiphoid region. The heart is then identified,with the density of blood used as a standard. The subxiphoid approach through the longitudinal axis is taken toenable the examiner to identify the heart and to look for blood in the pericardial region (see Figures 9a and 9b).


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Figure 9a. FAST of normal heart. FAST. Sagittal ultrasound image of heart shows pericardium as singleechogenic (white) line; normal findings.

Figure 9b. FAST: Blood in pericardium. Sagittal ultrasound image of heart shows separation of pericardiallayers by blood.

The transducer is then placed in the right midaxillary line region between the 11th and 12th ribs to enable theexaminer to identify the liver, the kidney, and the diaphragm and to look for blood in Morison's pouch (seeFigures 10a and 10b).


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Figure 10a. FAST: Normal liver, kidney, and diaphragm. FAST. Sagittal ultrasound image of liver, kidney, anddiaphragm yields normal findings.

Figure 10b. FAST: Blood in right upper quadrant. Sagittal ultrasound image of right upper quadrant shows bloodbetween liver and kidney and between liver and diaphragm.

Next, the transducer is positioned on the left posterior axillary line between the 10th and 11th ribs to enable theexaminer to visualize the spleen and the kidney and to look for blood in the space between these organs andposterior to the spleen (see Figures 11a and 11b).


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Figure 11a. FAST: Normal spleen and kidney. FAST. Sagittal ultrasound image of spleen and kidney yieldsnormal findings.

Figure 11b. FAST: Blood in upper left quadrant. Sagittal ultrasound image of left upper quadrant shows bloodbetween spleen and kidney.

The transducer is then oriented for transverse sections and placed in the midline approximately 4 cm superior tothe symphysis pubis to determine whether there is blood around the full bladder (see Figures 12a and 12b).


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Figure 12a. FAST of normal pelvis. FAST. Coronal ultrasound image of pelvis shows full bladder; normalfindings.

Figure 12b. FAST: Bladder surrounded by blood. Coronal ultrasound image of pelvis shows full bladdersurrounded by blood.

An analysis of 1,540 injured patients undergoing FAST examinations performed by surgeon-ultrasonographersreached the following conclusions[97]:


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1. Ultrasonography should be the initial diagnostic adjunct for the evaluation of patients with precordialwounds and blunt truncal injuries because it is rapid and accurate and augments the surgeon'sdiagnostic capabilities.

2. Surgeon-performed FAST is most accurate when used for the evaluation of patients with precordial ortransthoracic wounds and a possible hemopericardium and for the evaluation of hypotensive patientswith blunt torso trauma.

3. Because of the high sensitivity and specificity of ultrasonography when it is used for the evaluation of

patients with precordial or transthoracic wounds and hypotensive patients with blunt torso trauma,immediate operative intervention is justified in these patients when the ultrasound examination ispositive (see Figures 13 and 14).

Figure 13. Evaluation of patients with penetrating precordial wounds. FAST. Shown is an algorithm for use ofultrasonography in evaluation of patients with penetrating precordial wounds. [111]

Figure 14. Evalutaion of patients with blunt abdominal trauma. FAST. Shown is an algorithm for use ofultrasonography in evaluation of patients with blunt abdominal trauma.[110]


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Although the FAST accurately detects the presence or absence of hemoperitoneum in patients with blunttrauma, it does not readily identify intraparenchymal or retroperitoneal injuries. Therefore, a computedtomographic scan of the abdomen may be needed to complement the FAST and reduce the incidence of missedinjuries.[95,97-100] There is some evidence that false negative results are more common in patients with pelvic ringfractures, which suggests that CT of the abdomen is routinely indicated in such patients.[101]

The increase in surgeon-performed ultrasound examinations has led to decreased performance of diagnosticperitoneal lavage and CT scanning in the trauma setting. It has become apparent that the FAST can replacecentral venous pressure monitoring in the diagnosis of hemopericardium and can replace diagnostic peritoneal

lavage in the detection of hemoperitoneum in many injured patients. Although CT scanning remains a valuablediagnostic test, the indications for its use in the evaluation of injured patients are now narrower than they oncewere.

Breast Examination

The surgeon must be thoroughly familiar with the ultrasonographic anatomy of normal breast tissue to be able torecognize a mass, discern its ultrasonographic characteristics, and determine whether it is likely to be benign(see Figure 15) or malignant (see Figure 16).[102,103] Analytic criteria for the interpretation of focal lesions detectedon breast ultrasound examinations have been well described and depicted elsewhere (see Figure 17).[102]

Figure 15. Breast examination: simple cyst. Breast examination. Ultrasound image shows simple cyst (arrow) ofbreast characterized by sharp, smooth margins and homogeneous, anechoic interior.


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Figure 16. Breast examination: malignancy. Breast examination. Ultrasound image shows malignant breastlesion (arrow) with indistinct, jagged margins, few internal echoes, and slight posterior shadowing.


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Figure 17. Criteria for interpretation of breast sonograms. Breast examination. Shown is a schematicrepresentation of analytic criteria for the interpretation of breast sonograms.[102]

As noted, breast examination should be done according to a specific scanning protocol. The recommendedapproach is the radial-scanning technique reported by Teboul.[104] A 7.5 MHz linear-array transducer is used, and

the patient is placed in the supine position with the ipsilateral arm behind the head. The transducer is placed atthe 6 o'clock position; the breast tissue is scanned, and the transducer is then advanced toward the peripherybeyond the breast tissue. Next, the 5 o'clock region is evaluated in the same manner. Each sector (or "hour") ofthe breast is then scanned in a sequential counterclockwise fashion until the process is completed. Someexperts recommend that the nipple be used as a visual pivot point during scanning, remaining in the upper leftcorner of the monitor throughout the ultrasound examination.[105]

To image the nipple-areola complex, the transducer is placed next to the nipple and angled toward theretroareolar area. Several transverse scans are performed to assess the uniformity of the ligamentous structuresand to detect any small tumor that may be present between these structures. Finally, the axilla is scanned withtransverse and longitudinal sweeps of the transducer to inspect for lymph nodes.[105]


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An important principle in the performance of breast ultrasonography is that the examination must be performedin a consistent and methodical manner so that findings can be accurately described and reproduced. If thisprinciple is followed, a trained examiner can probably identify 80% to 90% of mammographically detectednonpalpable breast masses.[106] One important drawback to remember, however, is that ultrasonographygenerally will not reveal lesions less than 5 mm in diameter or lesions with an isoechoic appearance.[107]

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Tables

Table 1. Ultrasound Physics Terminology Relevant to Ultrasonographic Imaging[4-6]

Term Definition Significance

Ultrasound High-frequency (> 20 KHz) mechanicalradiant energy transmitted through a medium

Frequency Number of cycles/sec (106 cycles/sec = 1MHz)

Increasing frequency improves resolution

Diagnostic ultrasound: 1-20 MHz

Wavelength Distance traveled by wave per cycle: asfrequency becomes higher, wavelengthbecomes smaller

Wavelength is related to spatial resolution ofobject: shorter wavelengths yield better resolutionbut poorer penetration

Amplitude Strength or height of wave Amplitude and intensity are reduced (attenuated)as waves travel through tissue; time-gaincompensation circuit compensates for thisattenuation

Attenuation Decrease in amplitude and intensity of waveas it travels through a medium; attenuation isaffected by absorption, scattering, andreflection

Absorption Conversion of sound energy into heat

Scattering Redirection of wave as it strikes a rough orsmall boundary

Reflection Return of wave toward transducer

Propagationspeed

Speed with which wave travels through softtissue (1,540 m/sec)

Propagation speed (determined by density andstiffness of medium) is greater in solids than inliquids and greater in liquids than in gases

Table 2. Essential Principles of Ultrasound

Principle Explanation

Piezoelectriceffect

Piezoelectric crystals expand and contract to interconvert electrical and mechanical energy

Pulse-echoprinciple

When ultrasound wave contacts tissue, some of signal is reflected while some is transmitted intotissue; these waves are then reflected to crystals within transducer, generating electrical impulsecomparable to strength of returning wave

Acoustic Acoustic impedance = density of tissue x speed of sound in tissue
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impedance Strength of returning echo depends on difference in density between two structures imaged:structures of different acoustic impedance (e.g., bile and gallstone) are relatively easy todistinguish from one another, whereas those of similar acoustic impedance (e.g., spleen andkidney) are more difficult to distinguish

Table 3. Terminology Used in Assessment of Ultrasonograms[3,108]

Term Definition

Echogenicity Degree to which tissue echoes ultrasonic waves (generally reflected in ultrasound image as degreeof brightness)

Anechoic Showing no internal echoes, appearing dark or black

Isoechoic Having appearance similar to that of surrounding tissue

Hypoechoic Less echoic or darker than surrounding tissue

Hyperechoic More echoic or whiter than surrounding tissue

Resolution Ability to distinguish between two different structures; spatial resolution improves as frequency

increases

Lateral Resolution transverse to ultrasound wave; relates to width of structure

Axial Resolution parallel to ultrasound wave; relates to depth of structure

Table 4. Clinical Applications of Selected Transducer Frequencies

Frequency Applications

2.5-3.5 MHz Renal

Aortic

General abdominal

5.0 MHz Transvaginal

Pediatric abdominal

Testicular

7.5 MHz Vascular

Foreign body in soft tissue

Thyroid

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Grace S. Rozycki, M.D., Associate Professor, Department of Surgery, Emory University School of Medicine,and Director of Trauma/Surgical Critical Care, Grady Memorial Hospital

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