INTRODUCTION — Endoscopic ultrasound (EUS) has an important role in the evaluation of benign and malignant gastrointestinal diseases [1-8]. Echoendoscopes operate from 5 to 20 MHz, permitting a spectrum of depth of penetration and image resolution. Higher frequencies provide higher resolution but less penetration, while lower frequencies provide higher penetration but lower resolution.
Advancements in ultrasound technology have led to the development of small caliber ultrasound catheters (maximum diameter 2.6 mm) that can be passed through the biopsy channel of a standard endoscope [9]. These miniprobes operate across a range of frequencies (12 to 30 MHz), providing a greater image resolution (0.07 to 0.18 mm) than standard EUS scopes as well as improved accuracy for studying small or superficial lesions of the gastrointestinal tract [9,10].
This topic review describes the technical characteristics of the commercially available high-frequency ultrasound catheters and the experience with their use in a variety of conditions.
INSTRUMENTS — High-frequency ultrasound (HFUS) catheters can be broadly classified by their working mechanism into mechanical and electronic catheters.
Mechanical catheters — Mechanical catheters have a single ultrasound transducer located at the tip of the catheter, which is rotated by a cable and transmits the signal from the transducer to the ultrasound processor. A flexible housing protects the cable and the transducer from damage. When rotating, the ultrasound transducer produces a 360-degree image, perpendicular to the longitudinal axis of the HFUS catheter. Certain models of HFUS mechanical catheters can be inserted over a guidewire, facilitating examination of the bile and pancreas ducts. These probes have been modified so that linear imaging is also possible with subsequent three-dimensional reconstruction.
Electronic catheters — Electronic catheters consist of a probe that contains a number of fixed ultrasound transducers (ie, no rotating system) at their tips. These transducers transmit signals via microwires to the image processor. Electronic probes may be oriented radially (perpendicular to the longitudinal axis of the probe) or linearly (ultrasound plane oriented along the longitudinal axis of the probe). Most experience with these catheters has been in cardiovascular applications, although case series suggest that they also have promise in evaluating gastrointestinal disease [11,12].
EXAMINATION TECHNIQUES — A standard endoscope is negotiated through the gastrointestinal tract until the area of interest is reached. The high-frequency ultrasound (HFUS) catheter is advanced through the biopsy channel of the endoscope and placed in contact with the target lesion.
A number of techniques have been described to obtain adequate acoustic coupling between the HFUS catheter and the lesion of interest. The two methods most frequently used are the condom and the balloon techniques, both of which have advantages and disadvantages [10-15].
Condom technique — A latex condom is attached to the distal end of the endoscope. This obscures visualization and prevents air insufflation. As a result, endoscopy must first be performed without the condom. After advancing the endoscope to the region of interest, the condom is filled with water through the biopsy channel of the endoscope. The HFUS probe is inserted through the biopsy channel, and acoustic coupling between the probe and the area of interest is achieved with no or minimal compression artifacts. However, air pockets between the condom and the gut wall may still be present, resulting in ultrasound image degradation [10,13,14].
Balloon technique — The catheter is inserted into a latex sheath with a distal balloon that can be instilled with water to facilitate acoustic coupling. Although easy to work with, this balloon sheathed catheter still has limitations in obtaining adequate acoustic coupling. Once the HFUS catheter has been inserted through the biopsy channel of the scope, air pockets between the balloon and the gut wall are difficult to suction, leading to less than optimal image quality [15]. If the balloon sheathed HFUS catheter is used in conjunction with a double channel endoscope, one can suction air pockets and inject small pulses of water into the lumen through the second biopsy channel after the HFUS catheter has been inserted [16].
Immersion technique — The catheter with or without a balloon sheath can be inserted into the gastrointestinal lumen after instilling water into the lumen to permit acoustic coupling. As an alternative, a double channel endoscope can be used with the second channel serving to permit water instillation into the lumen. This technique can be used in any location where the water will not migrate (ie, the lesion is in a dependent position). When used in the stomach or duodenum, care must be taken to avoid overfilling resulting in potential aspiration [16]. Limited data have suggested that the water-immersion technique may be more accurate than the balloon technique [17].
NORMAL GASTROINTESTINAL WALL ANATOMY — Dedicated echoendoscopes, typically scanning at 7.5 to 12 MHz, provide a five-layer ultrasound image of the gastrointestinal wall: superficial (first layer: hyperechoic) and deep mucosa (second layer: hypoechoic), submucosa (third layer: hyperechoic), muscularis propria (fourth layer: hypoechoic) and adventitia/serosa (fifth layer: hyperechoic) [18]. Limited resolution precludes separate identification of the muscularis mucosa. In contrast, catheters operating at 20 to 30 MHz provide a more detailed ultrasound image of the gut wall (9 to 11 layers) [19-21].
In vitro and in vivo studies with high-frequency ultrasound probes have shown that [22]:
●The first (hyperechoic) and second layers (hypoechoic) correspond to the interface with the lumen and mucosal epithelium
●The third (hyperechoic) and fourth layers (hypoechoic) correspond to deep mucosa (lamina propria)
●The fifth (hyperechoic) and sixth layers (hypoechoic) correspond to the muscularis mucosae interface and muscularis mucosae
●The seventh layer (hyperechoic) is the submucosa
●The eighth layer (hypoechoic) is the inner layer of the muscularis propria
●The ninth layer (hyperechoic) corresponds to connective tissue and the interface between the muscle layers
●The tenth layer (hypoechoic) corresponds to the outer layer of the muscularis propria
●The eleventh layer (hyperechoic) is the serosa/adventitia
CLINICAL INDICATIONS — High-frequency miniprobes allow for the accurate evaluation of superficial neoplasms and small, submucosal lesions and thus are an important part of pretherapy assessment at centers that routinely perform endoscopic resection for early stage malignancies. However, the limited depth of penetration requires the use of dedicated echoendoscopes to provide lymph node staging and assessment of vascular invasion. High-frequency ultrasound (HFUS) catheters have been described in staging esophageal, gastric, pancreatic, biliary, and colorectal cancers.
Esophageal carcinoma — High-frequency miniprobes are best suited for determining tumor extension (T stage) with superficial tumors where they may guide therapy in patients for whom surgery in undesirable. The high-frequency probes delineate tumor extension into the muscularis mucosa, information that is critical since tumors that do not invade the muscularis mucosa can be cured by endoscopic mucosal resection alone [20] (see "Overview of endoscopic resection of gastrointestinal tumors").
The accuracy of high-frequency miniprobes for determining tumor extension into the muscularis mucosa is in the range of 80 percent, although there have been few detailed studies [17,19,21,23].
In contrast, dedicated endoscopic ultrasound scopes do not permit identification of the muscularis mucosae layer. On the other hand, they are much better suited for examining regional nodal involvement, making them complementary to examination with high frequency probes (image 1) [19].
HFUS probes can also assist in staging of stenotic esophageal tumors that cannot be traversed with the dedicated echoendoscope at all or without prior dilation (which is associated with up to a 25 percent perforation rate in some series). Although initial reports with this technique are promising, results have been questioned due to lack of histological confirmation of tumor stage [15,24,25]. Limited depth of imaging may also preclude complete tumor assessment.
Barrett's esophagus — High-frequency miniprobes do not appear to accurately predict the presence of invasive cancer in patients with Barrett's esophagus and high-grade dysplasia [26].
Gastric cancer — The accuracy of HFUS for delineation of tumor invasion into the muscularis mucosae layer in gastric cancer is approximately 70 percent [27,28]. Ulcer scars and dilated glands are responsible for the majority of staging mistakes. HFUS catheters (12 and 20 MHz: Olympus UM-3D2R and UM-3D3R), which provide three-dimensional imaging, have also been used for staging superficial carcinomas of the stomach, with a T-staging accuracy of approximately 90 percent in early cancers [29].
Colorectal cancer — One of the largest reports on HFUS in colorectal cancer focused on 67 rectal lesions that were imaged with a 20 MHz catheter [30]. Tumor staging accuracy (T stage) was 76 percent (overstaging in 16 percent and understaging in 8 percent of lesions). HFUS catheters were more accurate for studying small and flat lesions (100 percent accuracy for lesions less than 15 mm) than large or polypoid lesions (67 percent accuracy for lesions greater than 20 mm). In addition, HFUS catheters accurately determined if a tumor was invading the superficial third of the submucosa (SM1), middle third (SM2), or deep third (SM3) (accuracy: 83 percent to 100 percent). This distinction may be of value if endoscopic therapy is being considered. However, sessile colon polyps with invasive carcinoma are traditionally treated with surgery. In another report, HFUS was more accurate than high magnification chromoendoscopy for differentiating T1/2 disease [31].
Pancreatic and biliary diseases — Ultrasound catheters are available that may be passed over a guidewire into the bile and/or pancreatic ducts during endoscopic retrograde cholangiopancreatography, with a success rate approaching 100 percent. Intraductal ultrasound (IDUS) appears to be safe with less than 1 percent complication rate. IDUS has been studied in the evaluation of biliary stone disease, bile duct strictures, pancreatic disease, pancreatic duct strictures, and diagnosis of cholangiocarcinoma and ampullary tumors. (See "Intraductal ultrasound of the pancreaticobiliary ductal system".)
Submucosal lesions — HFUS catheters can be used to image small (<2 cm) submucosal lesions [32]. Enhanced delineation of wall layers may permit better assessment of the layer of origin. Additionally, the technique permits ultrasound imaging of lesions that may not be readily accessible by dedicated echoendoscopes (eg, right colon).
SUMMARY AND RECOMMENDATIONS
●Advancements in ultrasound technology have led to the development of small caliber ultrasound catheters (maximum diameter 2.6 mm) that can be passed through the biopsy channel of a standard endoscope. These miniprobes operate across a range of frequencies (12 to 30 MHz), providing a greater image resolution (0.07 to 0.18 mm) than standard endoscopic ultrasound scopes as well as improved accuracy for studying small or superficial lesions of the gastrointestinal tract. (See 'Introduction' above.)
●High-frequency miniprobes allow for the accurate evaluation of superficial neoplasms and small, submucosal lesions and thus are an important part of pretherapy assessment at centers that routinely perform endoscopic resection for early stage malignancies. However, the limited depth of penetration requires the use of dedicated echoendoscopes to provide lymph node staging and assessment of vascular invasion. (See 'Clinical indications' above.)
ACKNOWLEDGMENT — The author and UpToDate thank Dr. Enrique Vazquez-Sequeiros, who contributed to earlier versions of this topic review.
