Over the past several decades there have been tremendous improvements in the outcomes of patients with rectal cancer. Imaging modalities such as endorectal ultrasonography (ERUS), magnetic resonance imaging (MRI), and computed tomography scanning play a pivotal role in evaluating rectal cancer preoperatively, planning surgical procedures, and selecting patients for neoadjuvant therapy. This article provides an overview of rectal anatomy and pelvic structures relevant to preoperative assessment and surgical treatment. The technical aspects, applications, and limitations of the imaging tools currently in use are examined, focusing on MRI and ERUS.
Key points
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Endorectal ultrasonography (ERUS) provides excellent visualization of the layers of the bowel wall; magnetic resonance imaging (MRI) does not.
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ERUS provides better visualization of superficial tumors; MRI provides better visualization of locally advanced or stenotic tumors.
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MRI and ERUS are complementary tools in evaluation of lymph nodes.
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MRI is best at visualizing the circumferential resection margin.
Introduction
In the last decades, we have seen dramatic improvements in the outcomes of patients with rectal cancer. The rate of local recurrence has decreased, the probability of survival has increased, and the quality of life has improved. Advances in surgical pathology, which have added to our understanding of the causes of locoregional recurrences, refinements in surgical techniques, and the widespread use of neoadjuvant therapy, have all contributed to these improvements. However, advances in imaging have also played a pivotal role in identifying those at risk for recurrence, helping in planning surgical procedures and selecting patients for neoadjuvant therapy. Adequate imaging is a fundamental component of rectal cancer management.
Preoperative evaluation of the patient with rectal cancer goes beyond determination of tumor stage. Treatment planning requires not only defining the depth of tumor invasion in relation to the bowel wall, and the presence of metastatic regional lymph nodes, but also the precise relationship of tumor to other pelvic structures such as the mesorectal fascia, the levator muscle, the anal sphincters, and adjacent organs. In addition, assessment of the pelvic nodal basins outside the mesorectal fascia, and assessment of the retroperitoneal nodes, liver, and lungs, can provide useful information that may affect the treatment strategy.
Magnetic resonance imaging (MRI), endorectal ultrasonography (ERUS), and computed tomography (CT) are the imaging modalities most commonly used in evaluation of the patient with rectal cancer. Each has unique strengths and limitations. Rather than being used in the locoregional staging of rectal cancer, CT scanning is mainly used to exclude distant metastasis, and will not be reviewed here.
MRI of the rectum can be performed with either an endorectal coil or a phased-array surface coil. Endorectal coil MRI yields high-resolution images of the layers of the rectal wall, although clear differentiation between the mucosa and submucosa remains difficult. However, endorectal coil MRI provides only limited views of the rectum, cannot be used in the setting of stenotic or high tumors, and is poorly tolerated by patients. Therefore, endorectal coil MRI is not commonly used in the evaluation of rectal cancer and is not discussed in this review. Rectal MRI with a phased-array surface coil does not depict the layers of the bowel wall; however, it yields large field, high-resolution images of the rectum and other pelvic structures and is now an essential tool in the preoperative evaluation of rectal cancer. ERUS is a simple, widely available office procedure that provides high-resolution images of the rectal wall and surrounding tissues, within a short focal range. Because ERUS depicts the different layers of the bowel wall, it is useful in staging early rectal cancer; however, given its short focal range, it is not accurate in assessing the relationship of more advanced tumors to important anatomic structures, such as the mesorectal fascia. Thus, MRI, ERUS, and CT should be seen as complementary rather than competing tools, each providing unique and important information in the evaluation of the patient with rectal cancer.
In this article, an overview is provided of the anatomy of the rectum and other pelvic structures relevant to the preoperative evaluation and surgical treatment of rectal cancer. We then review the technical aspects, applications, and limitations of MRI and ERUS, the imaging modalities commonly used in preoperative assessment.
Introduction
In the last decades, we have seen dramatic improvements in the outcomes of patients with rectal cancer. The rate of local recurrence has decreased, the probability of survival has increased, and the quality of life has improved. Advances in surgical pathology, which have added to our understanding of the causes of locoregional recurrences, refinements in surgical techniques, and the widespread use of neoadjuvant therapy, have all contributed to these improvements. However, advances in imaging have also played a pivotal role in identifying those at risk for recurrence, helping in planning surgical procedures and selecting patients for neoadjuvant therapy. Adequate imaging is a fundamental component of rectal cancer management.
Preoperative evaluation of the patient with rectal cancer goes beyond determination of tumor stage. Treatment planning requires not only defining the depth of tumor invasion in relation to the bowel wall, and the presence of metastatic regional lymph nodes, but also the precise relationship of tumor to other pelvic structures such as the mesorectal fascia, the levator muscle, the anal sphincters, and adjacent organs. In addition, assessment of the pelvic nodal basins outside the mesorectal fascia, and assessment of the retroperitoneal nodes, liver, and lungs, can provide useful information that may affect the treatment strategy.
Magnetic resonance imaging (MRI), endorectal ultrasonography (ERUS), and computed tomography (CT) are the imaging modalities most commonly used in evaluation of the patient with rectal cancer. Each has unique strengths and limitations. Rather than being used in the locoregional staging of rectal cancer, CT scanning is mainly used to exclude distant metastasis, and will not be reviewed here.
MRI of the rectum can be performed with either an endorectal coil or a phased-array surface coil. Endorectal coil MRI yields high-resolution images of the layers of the rectal wall, although clear differentiation between the mucosa and submucosa remains difficult. However, endorectal coil MRI provides only limited views of the rectum, cannot be used in the setting of stenotic or high tumors, and is poorly tolerated by patients. Therefore, endorectal coil MRI is not commonly used in the evaluation of rectal cancer and is not discussed in this review. Rectal MRI with a phased-array surface coil does not depict the layers of the bowel wall; however, it yields large field, high-resolution images of the rectum and other pelvic structures and is now an essential tool in the preoperative evaluation of rectal cancer. ERUS is a simple, widely available office procedure that provides high-resolution images of the rectal wall and surrounding tissues, within a short focal range. Because ERUS depicts the different layers of the bowel wall, it is useful in staging early rectal cancer; however, given its short focal range, it is not accurate in assessing the relationship of more advanced tumors to important anatomic structures, such as the mesorectal fascia. Thus, MRI, ERUS, and CT should be seen as complementary rather than competing tools, each providing unique and important information in the evaluation of the patient with rectal cancer.
In this article, an overview is provided of the anatomy of the rectum and other pelvic structures relevant to the preoperative evaluation and surgical treatment of rectal cancer. We then review the technical aspects, applications, and limitations of MRI and ERUS, the imaging modalities commonly used in preoperative assessment.
Relevant anatomy
The rectum corresponds to the distal portion of the large bowel, but neither its beginning nor end is sharply defined by specific anatomic landmarks. Proximally, the sigmoid colon transitions into the rectum at the rectosigmoid junction, located approximately 12 to 15 cm from the anal verge. The relationship of the large bowel with the promontory, often considered the beginning of the rectum, is variable, because it depends on the laxity of the mesorectum. Distally, the rectum transitions into the anal canal at the level of the anorectal ring, a palpable anatomic landmark corresponding to the impromptu of the puborectalis on the bowel wall. The anal canal extends from the anorectal ring to the anal verge, the palpable groove between the distal edge of the internal sphincter and the subcutaneous portion of the external sphincter. The location of a rectal tumor is best determined by measuring the distance of the lower edge of tumor from the anal verge, using a rigid proctoscope. This permits simultaneous viewing of the tumor, the anal verge, and the measuring marks on the instrument. However, measurements obtained from high quality sagittal images can also provide a good estimate of the distance of the tumor from the anal verge.
Similar to other segments of the gastrointestinal tract, the rectal wall is composed of a mucosa, a, submucosa, and a muscularis layer. The muscularis propria of the rectum has an inner circular layer, which, close to the anus, becomes the internal sphincter, and an outer longitudinal layer, which, at the level of the anal canal, forms the conjoined longitudinal muscle between the internal and external sphincter. In the anterior aspect of the upper and mid rectum, the muscularis is covered by peritoneum. In the remainder of the rectum, the muscularis is in contact with the perirectal-mesorectal fat. The different layers of the bowel wall are best visualized on ERUS ( Fig. 1 ).
The upper portion of the rectum is located above the anterior peritoneal reflection and is covered with peritoneum in the front and on both sides. Posteriorly, the upper rectum is attached to the concavity of the sacrum by a mesentery, the mesorectum, which is a continuation of the mesentery of the sigmoid colon. Below the peritoneal reflection, the rectum is completely extraperitoneal and fully surrounded by mesorectum.
The mesorectum, the visceral mesentery of the rectum derived from the dorsal mesentery of the hindgut, contains the terminal branches of the superior rectal vessels and the lymphatic drainage of the rectum. The mesorectum is covered by a thin, glistening membrane called the mesorectal fascia. The mesorectum extends posteriorly from the promontory to Waldeyer’s fascia, a condensation of connective tissue spanning the area from the fourth sacral vertebra to the anorectal ring ( Fig. 2 ). Posteriorly, the mesorectum is separated from the presacral fascia by an avascular plane of loose areolar tissue. The plane between the mesorectal fascia and the presacral fascia is the natural plane of dissection during a total mesorectal excision (TME).
The mesorectum is thick posteriorly, where it has a characteristic bilobular appearance. However, anteriorly, it is either absent (in the upper intraperitoneal portion of the rectum) or reduced to a thin layer of areolar tissue (in the mid and distal rectum). The mesorectal fascia is not as well defined anteriorly as it is posteriorly ( Fig. 3 ). In the front, the thin mesorectum is separated from the urogenital organs by a remnant of the fusion of 2 layers of the embryologic peritoneal cul-de-sac known as Denonvillier’s fascia, which is not well visualized in imaging studies. MRI provides excellent visualization of the seminal vesicles and prostate. The relationship of the rectal wall with seminal vesicles, prostate, and vagina is also well visualized on ERUS ( Fig. 4 ).
The mesorectum tapers off distally as the rectum funnels down toward the anorectal ring, where the longitudinal layer of the muscularis propria of the rectum is in direct contact with the levator muscle. The proximity of the rectal wall to the levator muscle must be taken into consideration when deciding between an abdominoperineal excision or a sphincter-sparing procedure, in the setting of transmural rectal cancers located at or below the level of the anorectal ring. Below the peritoneal reflection, the mesorectum intermingles on both sides of the pelvis with a condensation of connective tissue surrounding the autonomic nerves that pass from the pelvic plexus to the rectum. These bilateral condensations of the endopelvic fascia are known as lateral ligaments, and connect the pelvic sidewall with the mesorectum. In some individuals, the lateral ligaments contain accessory middle rectal vessels.
The blood supply of the rectum comes primarily from the superior rectal artery, which is the continuation of the inferior mesenteric artery after it gives off the left colic artery. The superior rectal artery gives several sigmoidal branches before diving into the mesorectum, where it gives multiple branches to the rectum. The superior rectal vein has a parallel course to its homonymous artery, on its way to join the left colic vein to form the inferior mesenteric vein draining into the splenic vein. The lower portion of the rectum also receives blood supply from the internal iliac vessels. The middle rectal artery, an inconsistent branch of the inferior vesical artery, is usually located deep in the pelvis, running over the levator muscle toward the distal wall of the rectum. The inferior rectal artery is a branch of the pudendal artery and provides blood supply to the anal canal and anal sphincter. The middle and inferior rectal vessels anastomose with the upper rectal vessels to supply enough blood to the entire rectum. As in other locations, the middle and inferior rectal veins follow the course of the homonymous arteries and drain into systemic circulation through the internal iliac veins. Metastatic lymph nodes in patients with rectal cancer tend to be located in the mesorectum and along the superior rectal vessels, but in some cases, they can be located along the internal iliac vessels. Both MRI and ERUS can show the presence of metastatic lymph nodes, with varying degrees of accuracy ( Fig. 5 ).
The autonomic pelvic nerves are rarely visible on imaging studies, but an understanding of their relationship to the anatomical landmarks that are commonly visualized helps in planning the surgical procedure and anticipating postoperative function in patients with locally advanced rectal cancer. The sympathetic plexus is at risk of injury near the origin of the inferior mesenteric artery and at the level of the aortic bifurcation during the dissection and ligation of the superior rectal vessels. The parasympathetic nerves are most at risk during the posterolateral dissection of the mesorectum, close to the lateral ligaments. The neurovascular bundle is most at risk close to the junction of the seminal vesicles, with the lateral aspects of the prostate.
The pelvic floor, also known as the pelvic diaphragm, comprises the levator ani and coccygeus muscles. The levator muscle is a thin, broad layer of muscle that inserts in the inner wall of the pelvis and unites with the muscle of the opposite side to form the greater part of the pelvic diaphragm. The anterior portion inserts in the posterior aspect of the pubic bones lateral to the symphysis, running posteriorly and obliquely to join the perineal body. This muscle is known as the levator prostate (in males) or the pubovaginalis (in females). The next muscular fascicle, known as the puborectalis, extends from the posterior aspect of the pubic bone and loops around the back of the rectum, becoming continuous with the muscle of the opposite side ( Fig. 6 ). The lower fibers of the puborectalis intermingle with the fibers of the upper portion of the external sphincter. The ileococcygeus and pubococcygeus insert in the pubis and in the tendinous arch of the obturator fascia, extending obliquely downwards and backwards from/toward the perineal body, the anal sphincter, the anococcygeal ligament, and the coccyx. The levator muscles create a funnel-shaped diaphragm with a central opening delineated by the puborectalis sling and the symphysis of the pubis, which allows passage of the urethra and the rectum in both males and females, and the vagina in females. The coccygeus muscle extends from the spine of the ischium and the sacrospinous ligament and inserts in the coccyx.
Techniques and image interpretation
MRI
MRI imaging using multiple sequences, with and without contrast, provides excellent anatomical and tissue resolution. The addition of advanced functional MRI sequences such as diffusion-weighted imaging (DWI) and dynamic contrast-enhancement (DCE, or “perfusion”) allow for the quantification of tumor biologic processes such as microcirculation, vascular permeability, and tissue cellularity. These images are potentially useful for early assessment of rectal cancer response to neoadjuvant therapy, but are still considered experimental.
For efficient planning of the pulse sequences to be employed in MRI, the radiologist should obtain information about the approximate location and size of the tumor, as well as any history of previous surgery or disease of the pelvic organs. Some protocols require cleansing of the rectum with an enema, to eliminate stool residue that might otherwise cause image misinterpretation. However, rectal cleansing is not routinely used in some centers. There is also wide variation on the use of rectal solutions as negative contrast agents against the normal rectal wall on T2-weigted images. Some of the agents commonly used to reduce bowel motion artifacts are glucagon, hyoscyamine, or scopolamine. Rectal MRI is typically performed on a 1.5- or 3-T MRI scanner using a surface pelvic phased-array coil with 4–32 channels. The exam is performed with the patient supine. Anatomic coverage for the localizing sequences extends from the lower level of the kidneys to the perineum. For normal sequences, anatomic coverage extends from the aortic bifurcation to the symphysis of the pubis. For high resolution rectal images, the field of view is centered over the area of the rectum containing the tumor. MRI in most rectal cancer protocols typically include multiplanar T2-weighted (axial, coronal, sagittal, oblique axial, oblique coronal); and multiple axial single b-value diffusion-weighted imaging. Images are obtained with a non-breath-hold turbo spin-echo pulse sequence, featuring a high-resolution matrix, thin section (3 to 5 mm) imaging, and a small (18 to 24 cm) field of view, yielding an in-plane resolution of 0.8 to 1 mm.
MR imaging has excellent anatomical and tissue resolution, but it is unable to distinguish the mucosa from the submucosa of the bowel; these two components are seen as an inner hyperintense layer on T2-weighted imaging. The muscularis propria is visualized as an intermediate hypointense layer, while the mesorectal fat appears hyperintense on T2-weighted imaging. The mesorectal fascia is identified as a thin, low signal intensity layer enveloping the mesorectum and the surrounding perirectal fat (see Figs. 2 and 3 ). Rectal tumors are characterized by intermediate signal intensity—between the high signal intensity of the fat tissue and low signal intensity of the muscular layer—but higher than that of the mucosal and submucosal layers (see Fig. 5 ). Rectal cancers typically demonstrate avid enhancement after contrast administration. Adenomatous polyps and superficial T1 tumors are not well visualized by MRI because they do not modify the muscularis propria. Therefore, MRI cannot accurately stage early rectal cancer that does not reach the muscularis propria. T2 tumors are characterized by involvement of the muscular layer, with loss of the interface between the muscle layer and the submucosa. The muscular layer is partially reduced in thickness, although the outer border between the muscularis propria and perirectal fat remains intact. Because of the limitations of MRI in distinguishing between deep T1 and T2 tumors, these are often grouped together as tumors confined to the bowel wall, with smooth margins. The crucial criterion in distinguishing T2 from T3 tumors is extramural spread of the tumor into the perirectal fat, which appears as a rounded or nodular advancing margin or a blurry interface between the muscular layer and the perirectal fat ( Fig. 7 ). Extramural spread into the mesorectal fat, measured with MRI imaging, correlates well to histopathological measurements.
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