Radial-scanning flexible EUS of the anorectum and pelvis

Abstract

Standard upper gastrointestinal flexible radial EUS probes are well suited for imaging of anorectum and pelvic pathologies. They offer multiple advantages over conventional rigid rectal probes. The current transducers allow imaging at variable frequencies and are Doppler capable. The flexible shaft of the endoscope and optics allow easy probe insertion to upper sigmoid. Flexible radial EUS probes allow evaluation of anal sphincter complex, rectosigmoid mural pathologies, and paraluminal pelvic disorders. A thorough understanding of pelvic anatomy and image orientation is the key to appropriate image interpretation. In this review, we describe the principles and methodology for anorectal EUS imaging using a flexible radial EUS probe.

Keywords: Anal canal; EUS; anal sphincters; aorta; iliac vessels; inferior vena cava; pelvis; prostate; rectum.

Figure 1

(a) Cross-sectional images of computed tomographic and magnetic resonance imaging scans are oriented as if the patient is being imaged from below, with the right side of the patient toward the left of the image. Note the direction marks on the computed tomographic image in this patient (arrows). (b) Display direction of EUS images. Note that the endoscopic images and EUS images have an upside-down relation, but right and left directions correspond. These relationships are important to localize and describe lesions. (1) Display direction of EUS image in “NORMAL” mode; (2) Display direction of EUS image in “INVERSE” or reverse mode; (3) Endoscope image “up” direction; 4: EUS image “up” direction. (c) Radial EUS imaging in the upper GIT is traditionally done in the “NORMAL” orientation and corresponds to the computed tomographic images. Note that the aortic arch in this transesophageal study is depicted on the left side, similar to the computed tomographic orientation. (d) “INVERSE” or reverse orientation would flip the images, with the aortic arch now seen on the right side. The reverse orientation is seldom used in the upper gastrointestinal tract. (e) When imaging the anorectum in the “NORMAL” mode, radial EUS images are the reverse depiction of computed tomographic orientation. This is because the direction of the probe is from below upward. In the NORMAL mode, the patient's right side is depicted on the right side of the image. The pelvic organs are placed superiorly in the image, which now corresponds to the patient's anterior side. (f) To display images similar in orientation to the cross-sectional radiological images, use the “INVERSE” mode for anorectal imaging. In Figures 1e and f, note the yellow arrow on a small pararectal lymph node; the red arrow points out the image orientation indicator. For ease of understanding, the images in this review are created in the “NORMAL” mode

Figure 2

The flexible EUS probe can usually be advanced to above the pelvic brim. Cross-sections of the aorta and inferior vena cava are seen. The abdominal aorta ends at the left side of the body of 4^th lumbar (L4) vertebra. The inferior vena cava is formed by the union of the common iliac veins on the right side of 5^th lumbar (L5) vertebra. (1) Aorta; (2) Inferior vena cava; (3) Vertebra

Figure 3

(a) The aorta is seen to divide into the left and right iliac arteries, seen in cross-section here. The two common iliac veins terminate into the inferior vena cava behind the right common iliac artery. The left common iliac vein lies medially and behind the corresponding artery. The right common iliac vein is behind and to the right of the corresponding artery. (1) Left common iliac artery; (2) Right common iliac artery; (3) Left common iliac vein; (4) Right common iliac vein. (b) As the probe is withdrawn, the right-sided iliac vessels can be traced; the left iliac vessels move away from the field of view. The right common iliac artery and vein are seen in long axis along the left and bottom of the screen, as the probe is slowly withdrawn. The artery is closer to the probe. (c) The right iliac vein is behind and lateral to the corresponding artery. The ala of the sacrum lies behind the common iliac vessels. (d) The right common iliac artery divides between the 5^th lumbar (L5) vertebra and the sacrum, into the right external iliac and internal iliac (hypogastric) arteries. Here, we see the “fork-like” division of the right common iliac artery into its internal and external divisions. The external iliac artery is larger, anterior, and lateral, and is positioned closer of the two branches to the EUS probe. The internal iliac artery is smaller, medial, and posterior. Note that the internal iliac artery further divides into anterior and posterior divisions. The posterior branch leads to uterine, vesical, and rectal arteries. However, these individual branches cannot be defined and traced further. The body of ilium bone and the iliac muscle lies deep to these branches of the common iliac vessels. The ureter is related to the common iliac artery at its point of division, although it usually cannot be seen here. (1) Right external iliac artery; (2) Right internal iliac artery; (3) Anterior branch of right internal iliac artery; (4) Posterior branch of right internal iliac artery. (e) A different perspective of the right common iliac artery and vein divisions. LIV: Left internal iliac vein; RIIA: Right internal iliac artery; REIA: Right external iliac artery

Figure 4

(a) Coronal T1-weighted magnetic resonance image of the pelvis showing the pelvic fascial compartments. Mesorectal fascia is marked by (★). The levator ani muscle separates the extraperitoneal pelvis into a supralevator or pelvirectal space above, and an ischiorectal space below. The obturator internus muscle corresponds to the lateral pelvic wall. (1) Mesorectum; (2) Levator ani; (3) Ischioanal/ischiorectal space; (4) Obturator internus. (b) The lower-third “extra-peritoneal rectum” is encircled by a variably thick fibro-fatty sheath of mesorectum containing perirectal lymph nodes, superior hemorrhoidal vessels, and fibrous tissue. The mesorectum and its external margin (fascia recti/propria) is clearly visible in patients when it is sufficiently fatty. In this axial T2-weighted magnetic resonance image of the pelvis, the rectum, perirectal fat, and mesorectal fascia are clearly seen. T2-weighted magnetic resonance image shows the mesorectal fascia as a low signal intensity layer (★) enveloping the high signal intensity mesorectal fat. (c) On EUS, a faint demarcation of the mesorectal fascia can be made out (★)

Figure 5

(a) The pelvic organs are placed superiorly in the image by electronic image rotation. Here, we see the bladder as a large anechoic structure, with the hyperechoic reflection of the pubic bones deep to it. The ureters may be seen entering the lateral aspect of the bladder. The seminal vesicles are seen behind and slightly below the bladder. (1) Pubis symphysis; (2) Bladder; (3) Left ureter; (4) Seminal vesicles. (b) Hyperechoic reflection of the symphysis pubis is seen anteriorly. The hyperechoic Denonvilliers fascia can be seen between the anterior rectal wall and the prostate and seminal vesicles (★). The obturator internus muscle which contributes to the lateral wall of the pelvic cavity is seen on both sides. The prostate is a walnut-shaped anterior organ below the bladder, with central and peripheral zones. The prostatic urethra can be made out in the center (arrow). (1) Prostate (peripheral zone); (2) Obturator internus muscle; (3) Symphysis pubis. (c): The penile or bulbar urethra turns anteriorly as an anechoic structure, with the corpus cavernosa on either side. (1) Penile urethra; (2) Prostate; (3) Obturator internus muscle; (4) Hyperechoic reflection of ischium

Figure 6

(a) The uterus is depicted across the rectum because of its asymmetric lie, and the lateral position of the patient. The uterine fundus is toward the left of the screen in this patient. The bladder is seen further away on the top. The peritoneum is reflected anteriorly from the rectum onto the posterior fornix of the vagina and the uterus, and then forward on to the posterosuperior surface of the bladder. (b) The upper vagina (★) is seen as a dark horizontal ellipse, with central air stripe. It is separated from the rectum by the rectovaginal septum

Figure 7

(a) The puborectalis portion of the levator ani muscles originates from the pubis and slings around the rectum. It is one of the three muscles which make up the levator-ani complex. (b) On EUS, the puborectalis muscle is seen as a U-shaped mixed echogenic band that slings around the rectoanal junction. This muscular sling maintains the anorectal angle. (1) Mucosa-submucosa complex of upper anal canal; (2) Internal anal sphincter; (3) Puborectalis sling. The conjoined longitudinal ligament is marked by (★)

Figure 8

Schematic representation and EUS images of the anal canal. The anal canal can be divided into upper, middle, and lower-thirds. (a and b) The upper third of the anal canal is identified by a complete ring of hypoechoic internal anal sphincter, a hyperechoic sling of puborectalis posteriorly and laterally, and absence of the external anal sphincter in the midline anteriorly. (c and d) The puborectalis is continuous with the striated external anal sphincter, and this ring of external anal sphincter closes anteriorly in the middle third of the anal canal. In addition, the internal anal sphincter is thickest in this part. (e and f) In the lower third of the anal canal, the internal anal sphincter is not seen, and the hyperechoic external anal sphincter is the only circular muscle around the anal canal. The mixed echogenicity fibers of the median conjoint ligament are difficult to demarcate clearly from the hyperechoic external anal sphincter. They thin out distally in the lower third of the anal canal. The space around the anal canal is the ischioanal space. (1) Mucosa-submucosa complex; (2) Internal anal sphincter; (3) Striated muscle of puborectalis and external anal sphincter; (4) Median conjoint ligament