Editor: Ian Penman
2. EUS equipment and technique
Anne Marie Lennon & Koji Matsuda
This chapter describes the wide range of equipment available for EUSprocessors, echoendoscopes, and accessoriesand discusses their roles and limitations in EUS procedures.
Endoscopic ultrasound (EUS) was initially developed in the early 1980s primarily to overcome the limitations of transabdominal
ultrasound in imaging the pancreas. EUS combines endoscopy with high-frequency ultrasound by incorporating a small ultrasonic
transducer at or close to the tip of a modified endoscope. EUS transducers can be either single element, where the transducer
must be mechanically rotated to produce an image, or phased array, where multiple piezoceramic crystals are stimulated by
electronic pulses. Phased arrays can incorporate Doppler ultrasound, which can be used to identify active blood flow. Single-element
transducers were initially the mainstay of EUS; however, there is an increasing move towards phased arrays in the newest generation
of instruments being developed.
Radial and linear endosonographic probes
Early echoendoscopes used mechanical radial scanning, where the transducer is orientated perpendicular to the shaft of the
endoscope and generates a radial image of 360°(Figs 13). Some, although not all authors, feel that this type of imaging gives a better overview of the GI wall and paramural structures
. More recently, electronic radial instruments have been developed (Fig. 4). At the same time as mechanical radial endoscopes were being developed, curved linear array (CLA) echoendoscopes were also
produced. These generate ultrasound images along the long axis of the echoendoscope(Fig. 5), allowing real-time visualization of fine-needle aspiration (FNA) biopsy of lesions. Very few studies have compared the
accuracy of these two types of imaging; however, those that have found similar accuracies for pancreatic and esophageal cancer
staging [2,3]. Figure 6 lists commonly used echoendoscopes, but a more detailed list of all available instruments can be found in Rosch .
Objects moving towards an US beam augment reflection from their surface, which increases the frequency of the returning wave.
Conversely, if the motion is away from the receiver, the received wave has a lower frequency compared with its incident frequency.
The importance of this equation is that the velocity of the object is directly related to the change in frequency of the US
beam. This is called the Doppler shift effect and can be used to determine if an anechoic structure is a vessel with active
blood flow. It is only possible to measure the Doppler effect with phased array transducers.
Contrast-enhanced sonography was initially described using carbon dioxide microbubbles which were infused into the celiac
or superior mesenteric artery [4,5]. More recently Levovist, a sonography contrast agent, has been used in Doppler ultrasound studies. However, several limitations
of this technique have been found, including blooming artifacts, poor spatial resolution, and low sensitivity to slow flow
. Coded phase inversion harmonic imaging is a newly available sonographic technique which is based on a combination of phase
inversion harmonics and coded technology . With the use of a microbubble contrast agent (a suspension of monosaccharide microparticles in sterile water), it depicts
signals from microbubbles in very slow flow without Doppler related artifacts, and enables visualization of slow flow in microscopic
vessels. A recent study by Kitano et al.  identified tumor vessels in 67% of pancreatic ductal carcinomas using contrast-enhanced ultrasonography, although most were
relatively hypovascular compared with the surrounding pancreatic tissue. Interestingly, although contrast-enhanced ultrasonography
was clearly superior in its sensitivity in identifying pancreatic tumors of 2 cm or less in size compared with contrast-enhanced computed tomography (95% vs. 68%), it was no better than routine endosonography
(95% sensitivity). This is an interesting new technique; however, further studies are required to delineate its exact role
Catheter-based EUS probes (miniprobes)
EUS is typically performed using an echoendoscope. This often entails two procedures: an initial endoscopy to identify the
lesion or area of interest, followed by EUS to evaluate it further. EUS probes are larger than regular endoscopes and are
not always able to cross tight strictures. Standard echoendoscopes may also have difficulty visualizing small or early lesions
due to their relatively low imaging frequency and oblique forward-viewing optics.
Catheter- or 'mini' probes have been developed, which overcome some of these problems. They consist of a cable with a mechanical transducer at
its end (Fig. 7). The majority of miniprobes use a radial transducer; however, dual-plane reconstruction probes (UM-DP-12-25R, 12 MHz and
UM-DP20-25R, 20 MHz; Olympus) are also available and these allow the user to scan simultaneously in both linear and radial planes. The miniprobes
come in a variety of diameters and lengths with probes of over 2 m being used for colonic and pancreatico-biliary imaging
(Fig. 8). They are also available in a variety of frequencies. The higher the frequency, the greater the image resolution, with up
to nine gut-wall layers visualized with high-frequency probes [19,20]. However, this higher resolution comes at the expense of limited depth of penetration, with an average of 1 cm penetration with a 30-MHz probe compared with 2.9 cm for a 12-MHz probe.
The miniprobe is inserted down the working channel of a regular endoscope. Air is removed from the lumen, which is then filled
with water to allow transmission of the ultrasound. Particular care should be taken in imaging the esophagus, where the head
of the bed should be raised to at least 30° to prevent aspiration. A catheter balloon sheath (UM-BS20-26R; Olympus), which consists of a small water-filled balloon that
inflates around the transducer tip, can be used in combination with the miniprobe. This improves acoustic coupling and has
been shown to improve the clarity of the image and the depth of penetration for esophageal imaging [21,22]. In pancreatico-biliary imaging a catheter balloon sheath is not required as bile and pancreatic juice provide a fluid-filled
Intraductal ultrasound (IDUS) can be performed either through a percutaneous or retrograde transpapillary route. In the case
of a retrograde procedure, an ERCP is performed first and a guidewire is passed proximal to the lesion. A wire-guided probe
(Fig. 9) is then advanced under fluoroscopic guidance. Alternatively, a non-wire-guided intraductal ultrasound probe may be used;
however, up to 20% of patients will require a sphincterotomy using this technique . The IDUS probe can also be inserted into the pancreatic duct. This can be a more difficult procedure, particularly if the
duct is tortuous or not dilated, with intubation rates falling from between 94% and 97% in the head to 89% to 90% in the body,
and only 45% in the tail [24,25].
Miniprobes in cancer
Miniprobes have been used to stage several cancers, including esophageal, gastric, duodenal, colorectal, ampullary, and pancreatico-biliary.
The accuracy of miniprobes for staging cancers varies depending on the organ. Miniprobes provide better visualization and
higher T-staging for esophageal cancer than standard EUS probes . For gastric cancer, miniprobes have again been shown to be superior in overall staging accuracy compare with standard EUS
(72% vs. 65%) . However, miniprobes are less reliable for advanced cancer staging due to limited depth of ultrasound penetration . They have also been shown to be superior to standard EUS in the visualization and diagnosis of ampullary tumors (100% vs.
59.3%)  with nodal staging accuracy varying from 66.7% to 93% [32,33].
Other uses of miniprobes
Miniprobes have also been used to examine Barrett's esophagus [34,35], achalasia , esophageal varices , MALToma, Menetrier's disease, linitis plastica , and inflammatory bowel disease .
Although miniprobes offer high resolution of small lesions in a single procedure, they do have some disadvantages. These include
the limited penetration, which can make lymph node staging difficult. Unless a balloon sheath is used, the lumen needs to
be filled with water, which may increase the risk of aspiration. Although the probes are reusable, they have a limited lifetime
and undergo image deterioration after a certain number of cases.
Needles and accessories for EUS
The ability to sample tissue using EUS has greatly increased the sensitivity for diagnosing malignant lesions. It also overcomes
some of the limitations of transcutaneous US- and CT-guided needle aspiration, which are limited when the lesions are small
or no safe 'skin to lesion' route for the needle can be found. EUS-FNA is now part of the investigative algorithms for patients with pancreatic, esophageal,
rectal, and lung cancer , and also for patients with submucosal tumors.
Different types of needles
An EUS-FNA needle (Fig. 10) is a stainless steel echogenic needle with a bevelled edge. A stainless steel or nitinol stylet is inserted inside the needle,
which prevents tissue or blood from clogging the needle while it is being advanced into the lesion (Figs 1114). These stylets can be either bevelled to fit flush with the needle's tip or they can protrude a short distance from the
needle tip with a sharp or a blunt tip. The needles come in a variety of diameters from 18 to 25 G, with 22 G being the most
commonly used. The use of larger needles (18 or 19 G) is helpful if viscous fluid or large volumes are being aspirated. Spring-loaded
needle designs are also available to help penetrate difficult-to-pierce lesions. There are as yet no studies comparing spring-loaded
needles with routine EUS-FNA needles.
EUS-FNA is performed using a linear echoendoscope, which allows visualization of the entire needle in real time. The needle
is inserted through the instrument channel of the echoendoscope and into the lesion, and the stylet is removed and the needle
moved through the lesion with a rapid inward thrust and slower backwards motion. Suction is usually used except when initial
aspirates are bloody. In this case the use of suction may decrease the sensitivity  of FNA and tissue should be aspirated without suction. Up to 10 back-and-forth movements are usually used ; suction is released, the tip of the needle is withdrawn back into the needle sheath, the needle is removed, and the contents
are spread thinly on slides for examination by a cytopathologist. The exact number of needle passes required depends on whether
a cytopathologist is available at the procedure, and the type of lesion being biopsied. If a cytopathologist is present the
number of passes needed will be determined by the adequacy of sample obtained. If a cytopathologist is not available most
series suggest that 34 passes  are sufficient to ensure a cellular sample, although up to 10 passes may be required in well-differentiated tumors or solid
pancreatic masses .
Accuracy and safety
The accuracy of EUS-FNA is dependent on the technique used [55,56], the experience of the endosonographer [48,52], and the cytopathologist . Even in the most challenging scenario of hard pancreatic masses, EUS-FNA can provide a cytological diagnosis in 8094% of pancreatic lesions .
EUS-FNA has a low complication rate of 12% [50,53,6265], similar to that reported for CT- or US-guided FNA or biopsy . The major complications are infection in cystic lesions , bleeding , pancreatitis [53,70,71], and duodenal perforation . Clinically significant bacteremia after EUS-FNA is rare but, because of the small but documented risk of infecting cystic
lesions at EUS-FNA, intravenous broad-spectrum antibiotics are often used with EUS-FNA of cystic lesions  and puncture of any lesion through the colonic wall . To date two deaths have been reported with EUS-FNA [62,76]. Needle-tract seeding is a potential complication; however, the risk is minimal in EUS-FNA, with only one case reported to
date , as the needle travels from the gut lumen to the lesion, a pathway that usually does not cross peritoneal or pleural surfaces.
In addition, the complete needle tract is usually included should the lesion subsequently undergo resection.
Core tissue biopsies
One of the disadvantages of FNA is that tissue architecture is not preserved. This is particularly important for the diagnosis
of stromal tumors and lymphomas [78,79]. Core biopsies allow tissue to be obtained with preservation of underlying architecture. A modified Trucut biopsy needle
has been developed, consisting of a 20-mm tissue tray, with a 19 G outer cutting sheath and a spring-loaded mechanism built
into the handle that automates collection of specimens (Fig. 15). The biopsy needle is prepared before introduction into the endoscope by pulling back the spring handle until it clicks
into the 'firing' position. This action draws the outer cutting sheath back and allows the inner needle tissue tray to be advanced. The inner
needle remains in the withdrawn position until the specimen is obtained. The sheath assembly is advanced through the echoendoscope
and then securely screwed into the accessory port channel with the Luer-lock adapter. The needle assembly is advanced into
the lesion under EUS visualization. Once the lesion is entered, the tissue tray is further advanced into the tissue. Once
the tray has exited the cutting sheath, the spring is released, which causes the cutting sheath to deploy rapidly over the
tissue tray. The needle is withdrawn into the sheath and the entire assembly is removed from the endoscope. Samples up to
20 mm long can often be obtained with this device (Fig. 16).
Accuracy and safety
The overall accuracy of EUS core biopsies is 8594% . However, there is substantial variation in the accuracy, depending on whether biopsies are performed through the stomach
or through the duodenum. In a study by Gines  the diagnostic yield was 57% with a transduodenal approach, compared with 89100% for transesophageal or transgastric biopsies, which is comparable with findings from other studies [83,84]. The reason for higher failure rate with transduodenal biopsies has not been fully investigated but may be due to the increased
angulation of the echoendoscope in the setting of a stiffer 19 G TCB needle. EUS-guided core biopsy is a relatively safe procedure
with a complication rate of 2%, which is similar to that of EUS-FNA .
Outstanding issues and future trends
Several new imaging techniques are being developed. A curved linear array echoendoscope for endobronchial ultrasound-FNA (EBUS) (Fig. 17) and 360° electronic radial echoendoscopes have been launched recently. Three-dimensional EUS probes and software are being developed
to evaluate esophageal, colorectal, and pancreatico-biliary lesions [86,87]. Preliminary results show that this is feasible for pancreatic evaluation and may enhance the evaluation of pancreatic lesions,
especially in the setting of established chronic pancreatitis . In animal studies, unresectable pancreatic tumors have been ablated using a modification of the EUS needle as a radiofrequency
catheter . EUS can also provide minimally invasive targeted delivery of specific therapies to tumors. EUS-guided FNA has been used
to deliver a local immunotherapy (Cytoimplant) in patients with advanced pancreatic carcinoma with no procedure-related complications
, and has also been used to inject a modified adenovirus into pancreatic adenocarcinomas . Case reports of EUS-guided photodynamic therapy have been reported and studies of EUS-guided brachytherapy are awaited with
interest. Finally, a specially designed applicator permeable to sound waves has been developed to apply radiation directly
by needles under ultrasound guidance for anal cancer . In a study of 18 patients with anal cancer EUS-guided chemotherapy produced complete initial remission in all patients with
13.9% recurrence at mean follow-up of 44 months . More studies are required to assess these technologies further but they point the way forward for EUS, and hopefully the
next few years will see further growth in the field of EUS-guided therapy in a range of gastrointestinal and mediastinal diseases.
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