Version May 2024
INTRODUCTION — Endoscopic ultrasonography (EUS) is a combination of endoscopy and ultrasonography. Conventional echoendoscopes operate across ultrasound frequencies ranging from 6 to 12 MHz, resulting in a spectrum of penetration depth and image resolution. Higher frequencies provide higher resolution but less penetration, while lower frequencies provide higher penetration but lower resolution.
Advances in technology have led to the development of small caliber ultrasound probes (commonly referred to as miniprobes or EUS miniprobes). Because miniprobes have a small outer diameter, they can be introduced through the accessory channel of a standard endoscope and can produce EUS imaging at anatomic locations (eg, ascending colon, cecum) that are not reachable with an echoendoscope. In addition, miniprobes scan at higher frequencies (12 to 20 MHz) than echoendoscopes, and this results in high resolution imaging with a penetration depth of approximately 20 mm.
This topic will review imaging principles, procedure technique, and clinical applications of EUS imaging with a miniprobe.
The indications, contraindications, and technical aspects related to performing conventional EUS (ie, echoendoscope-guided examination) are discussed separately. (See "Endoscopic ultrasound: Examination of the upper gastrointestinal tract".)
EUS-guided sampling technique and therapeutic interventions are also discussed separately:
●(See "Endoscopic ultrasound-guided fine needle aspiration in the gastrointestinal tract".)
●(See "Endoscopic ultrasound-guided fine needle biopsy in the gastrointestinal tract".)
●(See "Therapeutic endoscopic ultrasound".)
●(See "Endoscopic ultrasound-guided celiac plexus interventions for pain related to pancreatic disease".)
PROCEDURE
Patient preparation — The preprocedure preparation for patients undergoing an endoscopic procedure with EUS miniprobe examination is informed by the specific procedure, and these issues are discussed separately:
●Upper gastrointestinal endoscopy – (See "Overview of upper gastrointestinal endoscopy (esophagogastroduodenoscopy)", section on 'Preprocedure considerations'.)
●Lower gastrointestinal endoscopy (eg, sigmoidoscopy, colonoscopy) – (See "Overview of colonoscopy in adults", section on 'Patient preparation'.)
●Endoscopic retrograde cholangiopancreatography (ERCP) – (See "Overview of endoscopic retrograde cholangiopancreatography (ERCP) in adults", section on 'Patient preparation'.)
Imaging principles
Scanning frequency of miniprobes — EUS miniprobes scan at ultrasound frequencies ranging from 12 to 20 MHz, and they produce a high resolution image of 0.07 to 0.18 mm that is optimized when the ultrasound transducer is in close proximity to the gastrointestinal lesion and/or wall [1]. However, the limited depth of penetration (ie, 20 mm) prevents detailed examination of the surrounding tissue and more distant sites.
Endosonographic anatomy of the gastrointestinal wall — High frequency EUS miniprobes provide a detailed image of the gastrointestinal wall by displaying alternating hyperechoic (bright) and hypoechoic (dark) bands that correspond to the histologic layers [1-3]:
●The first layer (hyperechoic) and second layer (hypoechoic) correspond to the interface with the lumen and mucosal epithelium
●The third layer (hyperechoic) and fourth layer (hypoechoic) correspond to deep mucosa (lamina propria)
●The fifth layer (hyperechoic) and sixth layer (hypoechoic) correspond to the muscularis mucosa interface and muscularis mucosa
●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
In contrast, conventional echoendoscopes usually scan at frequencies ranging from 6 to 12 MHz, and they display the gastrointestinal wall as five alternating hyperechoic and hypoechoic bands that correspond to five histologic layers of the intestinal wall (image 1 and figure 1) [4].
Equipment — Commercially available systems for examining gastrointestinal lesions with high frequency ultrasound utilize mechanical miniprobes with the following features (picture 1) [1]:
●A single transducer mounted on the tip of a wire that allows the transducer to rotate and produce a 360-degree image, perpendicular to the longitudinal axis of the miniprobe
●A cable that excites the transducer and transfers signals to the image processor
●A flexible protective housing
●A working catheter length ranging from 170 to 270 cm with small outer diameter (ie, maximum diameter 3.4 mm)
Design variations exist, and some systems incorporate the use of a guidewire that facilitates examination of the bile and pancreatic ducts. Mechanical miniprobes with a guidewire port permit mechanical rotating sector scanning and linear scanning with three-dimensional reconstruction. (See "Intraductal ultrasound for evaluating the pancreaticobiliary ductal system".)
Technique
Initial steps — To perform an examination with the EUS miniprobe, the endoscopist advances a standard endoscope (eg, upper endoscope, colonoscope, duodenoscope) through the gastrointestinal tract until the target lesion or area is reached. Next, the endoscopist advances the miniprobe through the endoscope's accessory channel and positions the probe in close proximity to the target lesion to generate images and interpret them in real time.
Imaging the target lesion — Techniques to achieve acoustic coupling between the miniprobe and the target lesion include:
●Fluid immersion technique – We typically use an immersion technique that involves filling the gastrointestinal lumen with fluid (eg, sterile water) [5,6]. The steps of the fluid immersion technique are:
•Rinse the mucosal surface with sterile water to remove debris and bubbles. Flushing the mucosa with fluid minimizes imaging artifacts.
•Instill sterile water into the gastrointestinal lumen but avoid overfilling, especially in the upper gastrointestinal tract (eg, esophagus) to minimize the risk of aspiration. While the volume of fluid varies by anatomic site, the amount of fluid is adequate when the image has minimal air artifact with circumferential acoustic coupling to the gastrointestinal wall. As examples, the volume of fluid usually ranges from 10 to 30 mL in the esophagus and from 50 to 150 mL in more distal sites (eg, stomach).
•Insert the EUS miniprobe into the gastrointestinal lumen and begin imaging the target lesion.
●Balloon technique – The balloon technique involves inserting the miniprobe into a sheath with a distal balloon, advancing the miniprobe through the endoscope's accessory channel, and then filling the balloon with sterile water to facilitate acoustic coupling (picture 1). Although the balloon technique is simple, we reserve it for patients in whom we cannot achieve acoustic coupling with the fluid immersion technique. We do not typically use the balloon technique for gastrointestinal imaging because of its limitations. As an example, air pockets form between the balloon and intestinal wall, and it is challenging to suction the air pockets that contribute to suboptimal image quality. To address issues related to the balloon technique, some endoscopists use a larger caliber, double channel endoscope. The double channel endoscope allows the endoscopist to suction air pockets and inject small amounts of water into the lumen through the second accessory channel while advancing the EUS miniprobe through the primary channel [5].
Adverse events — EUS miniprobe examination during gastrointestinal endoscopy is generally a safe procedure. Adverse events related specifically to miniprobe use are rare but may include catheter fracture, perforation, and pancreatitis associated with duodenoscope-guided intraductal ultrasound. (See "Intraductal ultrasound for evaluating the pancreaticobiliary ductal system", section on 'Adverse events'.)
Some adverse events may be attributed to the effect of procedural sedation/anesthesia, while others are due to the endoscopy itself. These issues are addressed separately:
●(See "Adverse events related to procedural sedation for gastrointestinal endoscopy in adults".)
●(See "Overview of colonoscopy in adults", section on 'Adverse events'.)
Post-procedure care — After the procedure, patients are recovered from sedation or anesthesia. (See "Anesthesia for gastrointestinal endoscopy in adults", section on 'Post-anesthesia care'.)
CONTRAINDICATIONS — Contraindications to endoscopy with EUS miniprobe examination are similar to those for endoscopy alone and include (see "Overview of upper gastrointestinal endoscopy (esophagogastroduodenoscopy)", section on 'Contraindications' and "Overview of colonoscopy in adults", section on 'Contraindications' and "Overview of endoscopic retrograde cholangiopancreatography (ERCP) in adults", section on 'Contraindications'):
●Patients who cannot tolerate moderate sedation, monitored anesthesia care, or general anesthesia. (See "Anesthesia for gastrointestinal endoscopy in adults".)
●Patients who are hemodynamically unstable.
CLINICAL APPLICATIONS
Diagnostic role of EUS miniprobe — EUS using a high frequency ultrasound miniprobe can assist in the diagnostic evaluation of mucosal gastrointestinal lesions and of small, submucosal lesions before endoscopic resection of early-stage malignancies. Limitations of EUS miniprobe examination include that it is invasive, uses specialized equipment, requires expertise in advanced endoscopy, increases overall cost, and may not be widely available. In addition, miniprobes have a limited depth of penetration. Thus, for patients with malignant disease, we also perform an examination with a conventional echoendoscope to assess for vascular invasion and lymph node staging.
For patients with nonmalignant conditions, applications for EUS miniprobes include evaluating patients with achalasia, esophageal strictures, and gastroesophageal varices [7,8]. (See 'Esophageal disease' below.)
Esophageal disease
Superficial esophageal cancer — For patients with superficial esophageal cancers, the role of EUS miniprobe examination may include:
●Determining disease extent (T stage) for superficial tumors – High frequency miniprobes may distinguish subgroups of the T1 stage of an esophageal lesion (ie, tumor invasion into or through the muscularis mucosa), and such extension may preclude therapy with endoscopic resection alone [2]. (See "Management of superficial esophageal cancer", section on 'Pathologic subclassification and the risk of nodal metastases'.)
Although T1 lesions can be evaluated with echoendoscopes operating at 7.5 and 12 MHz, distinguishing subgroups of the T1 stage can be problematic with these instruments because the frequencies are too low to visualize the muscularis mucosa. However, we do use an echoendoscope to evaluate for regional lymph node involvement (image 2) [9]. Thus, assessment with a conventional echoendoscope is complementary to miniprobe examination. (See "Endoscopic ultrasound for evaluating patients with esophageal cancer", section on 'Preoperative staging'.)
The reported accuracy of miniprobes for determining tumor extension into the muscularis mucosa has ranged from 60 to 80 percent [10-14]. In a study of 321 patients with superficial esophageal squamous cell cancer who underwent either endoscopic or surgical resection, EUS miniprobe was performed in 86 patients with an overall T staging accuracy of 70 percent [14]. Most errors in T staging arose from understaging T1b lesions (52 percent). In another study of 143 patients with either esophageal adenocarcinoma or squamous cell cancer who underwent examination with EUS miniprobe, accuracy for T staging was 60 percent [11]. Notably, the accuracy for T staging improved over time with higher accuracy rates for examinations performed during the second half of the study compared with the first half (73 versus 41 percent). This observation may reflect a learning curve with EUS miniprobe technique.
The diagnosis, staging, and role of EUS in patients with esophageal cancer is discussed in more detail separately. (See "Clinical manifestations, diagnosis, and staging of esophageal cancer" and "Endoscopic ultrasound for evaluating patients with esophageal cancer".)
●Surveillance after chemoradiation therapy – We may use EUS miniprobes for assessing response after chemoradiation therapy (CRT) in patients with esophageal cancer, especially in those with post-treatment stricture [15]. In a study of 60 patients with esophageal squamous cell cancer who underwent EUS miniprobe examination and computed tomography (CT) after completing CRT, 24 patients (40 percent) had post-treatment esophageal strictures that were successfully traversed with the miniprobe. Patients with maximum esophageal wall thickness post-CRT of <8 mm had longer median survival compared with those with wall thickness ≥8 mm (26.7 versus 12.6 months). Although additional studies are needed, these findings may inform subsequent therapeutic options and surveillance intervals.
Nonmalignant esophageal strictures — Limited data suggested that miniprobes may be useful for assessing the severity of nonmalignant esophageal strictures and informing therapy [16,17]. In an observational study of 24 patients with nonmalignant esophageal strictures who underwent EUS miniprobe examination prior to stricture dilation, strictures involving the esophageal mucosa alone required fewer endoscopic dilations compared with those involving the submucosa or muscularis propria (1.8 versus 4.0 and 6.2 dilation sessions, respectively) [16]. Endoscopic interventions for esophageal strictures are discussed in detail separately. (See "Endoscopic interventions for nonmalignant esophageal strictures in adults", section on 'Endoscopic dilation'.)
Early gastric cancer — Studies have suggested that examination with EUS miniprobe may have a role in tumor staging for patients with early gastric cancer [18-20]. The depth of tumor invasion informs selection of therapy for early gastric cancers, and this is discussed separately. (See "Early gastric cancer: Epidemiology, clinical manifestations, diagnosis, and staging".)
In a meta-analysis of 19 studies including 3401 patients with early gastric cancer, the overall accuracy of EUS miniprobe for predicting the depth of tumor invasion was 84 percent [18]. In a study including 6084 patients with early gastric cancer, the overall accuracy for classifying tumors as T1a, T1b or ≥T2 was higher with EUS miniprobe compared with conventional echoendoscope examination (81 versus 62 percent) [20]. When stratified by tumor size, the T staging accuracy was higher with EUS miniprobe for tumor size ≤2 cm and tumor size between 2 and 3 cm (86 versus 62 percent and 75 versus 59 percent, respectively). In another study comparing EUS miniprobe plus diagnostic upper endoscopy with an examination using upper endoscopy alone, EUS miniprobe had higher accuracy for tumor invasion when tumors were >2 cm, irregularly depressed, or associated with an ulcer scar or altered gastric fold pattern [19].
Pancreatic and biliary diseases — Examination of the pancreaticobiliary ductal system with EUS miniprobe is commonly referred to as intraductal ultrasound (IDUS). To perform IDUS, the miniprobe is advanced through the accessory channel of a duodenoscope and into the pancreatic or bile duct. The miniprobe's wire-guided design, flexibility, and high resolution imaging facilitate the evaluation of biliary, pancreatic, and ampullary disorders (table 1). IDUS is discussed in more detail separately. (See "Intraductal ultrasound for evaluating the pancreaticobiliary ductal system".)
Colorectal cancer — Although we typically evaluate rectal cancers with EUS using a conventional echoendoscope, examination of more proximal colon tumors is challenging because of the echoendoscope's size and design that limit its passage through colonic flexures and/or malignant colonic strictures. (See "Endoscopic ultrasound for evaluating patients with rectal cancer" and "Pretreatment local staging evaluation for rectal cancer".)
The technical features of the EUS miniprobe (ie, flexible design, use of a colonoscope's accessory channel) permit reaching the target lesion in most patients [21].
EUS miniprobes may be used to assess the depth of tumor invasion (T staging) for selected patients with colorectal cancer (eg, those with early colon cancer prior to endoscopic resection, those with locally advanced tumors who may be candidates for neoadjuvant therapy) [21-25]. (See "Overview of colon polyps", section on 'High-grade dysplasia or cancer' and "Overview of the management of primary colon cancer", section on 'Management of localized disease'.)
Studies suggested that EUS miniprobe examination had good overall accuracy for determining the depth of invasion for colorectal tumors. In a systematic review of 10 studies including 642 patients with colorectal cancers, the pooled accuracy of EUS miniprobes for T staging based on histopathologic evaluation was 99 percent for T1 lesions, 95 percent for T2 lesions, and 97 percent for T3/4 lesions [21]. In a study of 90 patients with early colorectal cancer, the overall accuracy of EUS miniprobe for pretreatment T staging was 84 percent [22]. Lesion characteristics associated with lower accuracy of EUS miniprobes included larger tumors (≥2 cm) and submucosal lesions. In another study evaluating 40 patients with colon cancers proximal to the rectum who had histopathologic evaluation, the accuracy of EUS miniprobe for T staging was 88 percent and for N staging was 82 percent [23]. Limitations with EUS miniprobes included overstaging related to peritumoral inflammation in pathologically classified T2 lesions and understaging for larger tumors.
Other gastrointestinal applications — EUS miniprobe may be useful for evaluating small (<2 cm) gastrointestinal submucosal lesions (also referred to as subepithelial lesions) [6,26-28]. Miniprobe examination identifies the layer of lesion origin in the intestinal wall, and the miniprobe can access lesions in anatomic locations that are not easily reachable with an echoendoscope (eg, ascending colon). In a study of 662 patients who underwent EUS miniprobe examination followed by endoscopic resection of a gastrointestinal subepithelial lesion, the overall diagnostic accuracy of miniprobe for classifying tumor type was 80 percent [6]. The diagnostic accuracy of EUS miniprobe for pathologic diagnosis was 63 percent for gastrointestinal stromal tumor, 92 percent for leiomyoma, 97 percent for ectopic pancreas, and 97 percent for lipoma. The layer of origin was correctly predicted by EUS miniprobe in 88 percent of lesions.
SUMMARY AND RECOMMENDATIONS
●Background – Endoscopic ultrasonography (EUS) is a combination of endoscopy and ultrasonography. Advances in technology have led to the development of small caliber ultrasound probes (commonly referred to as miniprobes or EUS miniprobes). EUS miniprobes facilitate imaging at some anatomic locations (eg, cecum) that are not reachable with a conventional echoendoscope. The miniprobe's small caliber, flexibility, and high resolution imaging (ie, frequency range, 12 to 20 MHz) are beneficial for evaluating small, intraluminal gastrointestinal lesions and intraductal pancreaticobiliary lesions. However, the limited depth of penetration prevents detailed examination of extraluminal organs and lymph nodes. (See 'Introduction' above.)
●Procedure – EUS miniprobe examination is performed by inserting the ultrasound catheter through the accessory channel of a standard endoscope (eg, upper endoscope, duodenoscope, colonoscope) (picture 1). (See 'Procedure' above.)
To image the target lesion, we typically use an immersion technique that involves filling the gastrointestinal lumen with fluid (eg, sterile water) to achieve acoustic coupling.
●Pre- and post-procedure care – Pre- and post-procedure care is similar to that for patients who undergo diagnostic gastrointestinal endoscopy. (See 'Patient preparation' above and 'Post-procedure care' above.)
●Contraindications – There are few contraindications that are mainly related to endoscopy itself, including inability to tolerate anesthesia/sedation and/or hemodynamic instability. (See 'Contraindications' above.)
●Adverse events and limitations – EUS miniprobe examination is a safe procedure, and adverse events related specifically to use of the miniprobe are rare. (See 'Adverse events' above.)
Limitations of the miniprobe include that it is invasive, uses specialized equipment, requires expertise in advanced endoscopy, increases overall cost, and may not be widely available.
●Clinical applications – EUS using a high frequency miniprobe may facilitate the diagnostic evaluation of mucosal gastrointestinal lesions and of small, submucosal lesions. As an example, some centers use EUS miniprobe in the pretreatment assessment of early-stage malignancies. However, the EUS miniprobe has a limited depth of penetration. Thus, we also perform an examination with a conventional echoendoscope to assess for more advanced T stage and lymph node staging. (See 'Clinical applications' above.)
•Esophageal disease – EUS miniprobe may determine the extent of disease (T stage) for superficial esophageal cancers or evaluate esophageal strictures following chemoradiation (image 2). (See 'Esophageal disease' above and "Clinical manifestations, diagnosis, and staging of esophageal cancer".)
•Early gastric cancer – EUS miniprobe may have a role in tumor staging for early gastric cancers. (See 'Early gastric cancer' above and "Early gastric cancer: Epidemiology, clinical manifestations, diagnosis, and staging".)
•Pancreatic and biliary disease – The use of miniprobes allows for detailed examination of ductal and periductal tissue in patients with biliary, pancreatic, or ampullary disorders (table 1). (See "Intraductal ultrasound for evaluating the pancreaticobiliary ductal system".)
•Colon cancer – EUS miniprobe may determine the depth of tumor invasion for selected patients with colorectal cancer (eg, those with early colon cancer prior to endoscopic resection). (See 'Colorectal cancer' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Enrique Vazquez-Sequeiros, MD, PhD, who contributed to earlier versions of this topic review.
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