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| REVIEW ARTICLE |
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| Year : 2021 | Volume
: 2
| Issue : 1 | Page : 1-5 |
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Role of magnetic resonance imaging in defining rectal anatomical landmarks and primary rectal cancer evaluation
Kenis Shedrack Felangu1, Akims Shattah2, Ubaka Edwin Akanegbu3, Omofuma Ighodalo Oisenele4, Ibinaiye Philip Oluleke5
1 Division of Radiology, Echolab Radiology and Laboratory Services, Abuja, Nigeria
2 Department of Surgery, Federal Medical Centre Keffi, Nasarawa Hospital, Keffi, Nigeria
3 Department of Radiology, Nisa Hospital, Abuja, Nigeria
4 Department of Surgery, Nisa Hospital, Abuja, Nigeria
5 Department of Radiololy, Ahmadu Bello University Teaching Hospital, Zaria, Nigeria
| Date of Submission | 12-Jan-2021 |
| Date of Decision | 18-Apr-2021 |
| Date of Acceptance | 21-May-2021 |
| Date of Web Publication | 22-Jun-2021 |
Correspondence Address:
Kenis Shedrack Felangu
Division of Radiology, Echolab Radiological and Laboratory Services, Garki, Abuja
Nigeria
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/jrmt.jrmt_1_21
In contemporary practice, magnetic resonance imaging (MRI) has become a useful tool in evaluating rectal and perirectal anatomy. It encourages accurate local staging of rectal cancer and has overall improved presurgical evaluation and planning, leading to reduce local recurrence and more sphincter conserving procedures. This article aims at providing a concise review of rectal MRI technique and anatomy and to relate this to local staging and surgical management. Locally generated images in our practice were reviewed and images that best depict different anatomical landmarks and pathologies were selected and presented. Also, PubMed database was searched for relevant and contemporary existing literature on the subject.
Keywords: Mesorectal fascia, mesorectal fat, rectal cancer, total mesorectal excision
How to cite this article:
Felangu KS, Shattah A, Akanegbu UE, Oisenele OI, Oluleke IP. Role of magnetic resonance imaging in defining rectal anatomical landmarks and primary rectal cancer evaluation. J Radiat Med Trop 2021;2:1-5 |
How to cite this URL:
Felangu KS, Shattah A, Akanegbu UE, Oisenele OI, Oluleke IP. Role of magnetic resonance imaging in defining rectal anatomical landmarks and primary rectal cancer evaluation. J Radiat Med Trop [serial online] 2021 [cited 2023 Mar 26];2:1-5. Available from: http://www.jrmt.org/text.asp?2021/2/1/1/319107 |
| Introduction | |  |
Richard John Heald introduced the technique of total mesorectal excision (TME) in 1978,[1] since then, TME with tumor-free margin remains the best surgical treatment of rectal cancer. In this procedure, the rectum, mesorectal fat (MRF), perirectal nodes, and the enveloping mesorectal fascia are completely removed.[1] TME is now included as a standard technique in low anterior resection and abdominoperineal resection (APR) which are all surgical approaches in the treatment of rectal cancer.
For primary TME to be undertaken, the enveloping mesorectal fascia also called the circumferential resection margin (CRM) has to be tumor free.[2] That is to say, no tumor should be within 10 mm of CRM.[3] Once the CRM is involved, down staging with chemoradiotherapy is necessary before surgery to reduce the rate of local recurrence.[4] The CRM is only completely circumferential in the low rectum but mildly and largely deficient anteriorly in the mid and upper rectum, respectively [Figure 1]. The areas deficient in CRM are covered by peritoneum. Therefore, when evaluating this “peritonised” segment of the rectum, it is important to clearly define the part of peritoneum in contact with the rectum to know if it is involved or not, as this will impact a higher T staging (T4a) in the event of the former. Furthermore, because of the proximity of the lower rectum to the anal sphincter complex, often low rectal cancer can affect this sphincter complex and it is therefore imperative to determine whether or not the sphincter complex is involved prior to any surgery for low rectal cancer as this will determine the resectability and the type of sphincter conserving surgery that can be undertaken.[5] | Figure 1: (a) Schematic diagram illustrating classification of the rectum as low, mid, and upper rectum. (b) Is transverse section depicting the extent of peritoneal covering of the rectum in the lower, mid, and upper rectum
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From the foregoing, an appropriate imaging of the rectum in rectal cancer should be able to define clearly the MRF, the outline of the CRM, anterior peritoneal reflection, and the anal sphincter complex. Magnetic resonance imaging (MRI) is best positioned for demonstrating these anatomies and is the imaging modality of choice for local staging of rectal cancer. In addition, MRI is able to detect and characterize perirectal nodes and determine the presence of macroscopic extramural vascular invasion (EMVI), which are all important factors for treatment and prognostication.[6],[7]
This article aims to provide a concise review of rectal MRI technique, MRI anatomy of the rectum, and to illustrate how this information can be applied in the setting of rectal cancer as well as the implication on staging and treatment.
Overview of the Technical Aspects of the Magnetic Resonance Imaging of the Rectum
To be able to clearly define rectal anatomical landmarks and accurately relate them to the tumor, strict attention must be given to obtaining good quality images. At present, acceptable spatial resolution and signal-to-noise ratio for rectal evaluation are obtained using a 1.5T or 3.0T machine, similar accuracy for local staging has been demonstrated with these machines.[8] A pelvic phased-array surface coil is able to cover all the structures of the pelvis if they extend from the aortic bifurcation to the anal verge.
High-resolution T2-weighted imaging is the workhorse of the MRI evaluation of the anatomical landmarks of the rectum and primary rectal cancer. This sequence should generally be obtained in a small field of view with thin slice thickness (<3-mm).[9] In our practice, image acquisition is started with the sagittal series, which is determined by the longitudinal tumor axis. From these, the axial and coronal planes can be planned. For the axial planes, images should be obtained perpendicular to the tumor or region of interest, as incorrect angulation leads to volume averaging with the resultant false impression of CRM involvement and consequently incorrect T staging [Figure 2]. The coronal view is particularly important for low rectal tumors. In order to accurately evaluate invasion into the adjacent anal sphincter, images are acquired parallel to the anal canal. | Figure 2: Wrongly angulated axial images (yellow lines not perpendicular to the rectal wall) resulted in false foreshortening of Mesorectal fat (yellow arrow)
with proper technique the MRF is clearly evident (red line and red arrow)
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We routinely acquire other complimentary sequences as part of our protocol. For instance, high B value DWI is not a generally recommended sequence in primary staging; however, we acquire this sequence to help in lymph node detection. In addition, a large field of view FSE T2-weighted MRI in axial and sagittal plane of the entire pelvis allows for evaluation of distant lymph node chains and localization of the primary tumor to the midline structures, such as the anal verge [Figure 3]. It is known that contrast-enhanced T1-weighted imaging does not improve the diagnostic accuracy of local staging of rectal cancer.[10] Therefore, we do not recommend or use Gadolinium. | Figure 3: Sagittal magnetic resonance imaging image illustrating tumor localization. The white curve arrow is pointing at the inferior pole of a tumor, located more than 5 cm above the anorectal junction, making the tumor in this image mid and upper rectal tumo
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| Anatomical Magnetic Resonance Imaging Features and Land Marks | | |
Understanding key anatomic landmarks of the rectum is important if the accurate local staging of rectal cancer is to be provided and there are designated imaging planes best suited for the identification of each specific landmark or anatomical structure. For instance, MRF and CRM are best depicted on the axial plane [Figure 4] and the inferior peritoneal reflection is easily seen on the sagittal and axial axis [Figure 5], while the anal sphincter complexes are best appreciated on the coronal plane [Figure 6]. | Figure 4: An axial T1W magnetic resonance imaging image indicating the complete enveloping circumferential mesorectal facia (black arrow), which surrounds the MRF (red arrow)
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 | Figure 5: Sagittal and axial magnetic resonance imaging images showing the anterior peritoneal reflection which appears as a linear thin hypointense line on sagittal view (arrowhead), and as V shape line (seagull sign) on axial view (arrow)
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 | Figure 6: Coronal magnetic resonance imaging image showing the external anal sphincter (short arrow), which continues superiorly with levator muscle. The internal anal sphincter (arrowhead) and the intersphincteric plane (long arrow) are demonstrated as well
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On MRI, MRF has high signal intensity on both T1 and T2 weighted images. On the axial scan, the MRF is located just anterior to the sacrum and is seen encasing the rectum [Figure 4]. It extends to the prostate and seminal vesicles in males and uterus and upper vagina in females. On this high-intensity background of the MRF, other structures are clearly defined. First, the CRM is seen as a fine low signal intensity line surrounding the MRF. Second, the rectal wall layers are seen on T2 axial as inner hyperintense (submucosa) and outer thick hypointense layer (muscularis propria), as in [Figure 7]. Sometimes, the mucosa can be appreciated as an innermost thin hypointense area. Finally, in the anterior aspect of the superior most part of the MRF, a V-shaped hypointense thin line (seagull sign) is seen in the axial T2 representing the anterior–inferior peritoneal reflection [[Figure 5] left panel]. This can also be appreciated on the sagittal plane above the seminal vesicle in men or utero cervical junction in women as a hypointense linear structure [[Figure 5] right panel]. | Figure 7: Axial magnetic resonance imaging image illustrating the components of the rectal wall that are discernable on magnetic resonance imaging examination. The submucosa is seen as an inner hyperintense corrugated layer (arrow) and the muscularis propria as outer thick circumferential hypointense layer (arrowhead)
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The anal sphincter is best evaluated by a coronal plane [Figure 6] and this is particularly important when there is a low rectal cancer. The sphincter complex consists of internal and external anal sphincter as well as the intersphincteric plane. The internal sphincter is made up of smooth muscle and is essentially a continuation of the circular muscle layer of the rectum, whereas the external sphincter complex is the inferior extension of the levator ani and the puborectalis sling. In between the internal and external sphincter is the interphincteric plane that is filled with fat.
| Implication of Tumor Relationship with Anatomical Landmark | | |
The first structure to be evaluated for tumor extent is the rectal wall. While T1 and T2 tumors are limited to the wall [Figure 8] and [Figure 9], T3 tumor extends beyond it. T4 are tumors which involve the adjacent peritoneum or organs. Relating the tumors to the anatomy discussed above, T1 tumors are limited to the hyperintense submucosa [Figure 7], T2 tumors extend into but not beyond the outer hypointense layer (the muscularis propria), and T3 tumors are those which extend beyond this layer. Any tumor that involves the peritoneum or adjacent organ is designated as T4. T2 tumors and early T3 tumors may be quite challenging to distinguish on MRI. This is because, on one hand, very low-volume extramural spread may not be adequately resolved, leading to under staging, while, on the other hand, fibrosis from desmoplastic tumor reaction may be misinterpreted as tumor infiltration causing over staging. Fortunately, the treatment option of both T2 and T3a is the same – TME. Only if there is a preference for local excision through transanal endoscopic microsurgery (TEM-procedure), this differentiation is crucial, but these situations are extremely rare. | Figure 8: A patient referred for magnetic resonance imaging after a biopsy specimen during colonoscopy diagnosed rectal adenocarcinoma. The T2 tumor is seen as an eccentric thickening that is limited by muscularis propria (arrow)
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 | Figure 9: Schematic diagram illustrating the different T staging of rectal cancer
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Once the tumor grows through the rectal wall into the surrounding MRF, it is designated as T3. T3 tumors generally require further differentiation on MRI, as this relates to further management options. The factors that determine further classifications are the distance the tumor extends beyond muscularis propria and the distance between the tumor and mesorectal fascia (or CRM). Based on this distances, tumors are classified as either T3a, T3b, or T3c and positive (+ve) CRM or negative (−ve) CRM [Figure 9]. Tumor deposits which are not necessarily outgrowths from the primary tumor or involve node that are within 10 mm of the CRM make the CRM to be designated +ve [Figure 10]. When evaluating involvement of CRM, particularly for tumors in the anterior quadrants, it is important to remember that for mid and high rectal cancers the peritoneum sits anteriorly and when involved, the tumor is designated T4a rather than a T3 with + ve CRM. | Figure 10: A T3 rectal tumor with extramural extension in the 9 o'clock position (curve arrow). This extension does not threaten the CRM. However, a deposit is seen on the CRM around the 4 o'clock position (straight arrow), the presence of this deposit makes this tumor to be labeled as CRM +ve
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For low rectal cancers, which are proximal to anal sphincter, the presence and level of sphincteric involvement will determine the extent of sphincter conservation during surgery.[5] This ranges from complete sphincter sparing surgery like ultra-low anterior resection to more extensive surgeries like standard APR and extralevator APR [Figure 11]. Standard APR is indicated for tumors that infiltrate the anal canal and external sphincter. Extralevator APR is similar to Standard APR but here, there is a broader dissection of the sphincter complex to involve the levator ani. Both APR procedures are characterized by the entire resection of the sphincter complex, resulting in a permanent colostomy. In between these procedures of complete sphincter conservation and total sphincter, removal is intersphincteric resection in which the external sphincter is preserved and hence patient does not require a colostomy. This procedure is considered in cases where the intersphincteric plane is not infiltrated by the tumor. | Figure 11: Surgical techniques depending on the extent of disease. Dotted lines = structures removed. Orange area = tumor. (a) An ultra-low anterior resection and TME with complete sphincter preservation. In (c and d) there is resection of the internal and external sphincter with provision of a permanent colostomy. The resection plane is wider for (d), extra levator APR. (b) shows an intersphincteric APR, when the intersphincteric plane is preserved, here the external anal sphincter is not resected
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| Additional Role of Magnetic Resonance Imaging of the Rectum | | |
It is important to determine the number and level of lymph node involvement as this is an important factor in prognostication and treatment. To do this, a combination of morphologic and size criteria is used to increase the diagnostic accuracy of positive or negative lymph nodes. As a general rule, nodes larger than 9 mm are regarded as involved but smaller lymph nodes will need to have additional malignant characteristics such as being rounded, having indistinct border or heterogeneous internal architecture [[Figure 12] right panel]. Also, the presence of expanded and irregular tubular structure in the vicinity of the tumor [Figure 12 left panel] may suggest EMVI and is a known risk factor for recurrent and metastatic disease. | Figure 12: Image of a patient with rectal cancer illustrating an expanded tubular structure (black arrow) contiguous to the tumor signifying EMVI. The image of the right panel is of a different patient with an involve node which appears rounded and enlarged (arrow)
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A large field of view on coronal and sagittal plane is able to evaluate the relationship between tumor and the anorectal junction, helping in classification of rectal cancer into low, mid, and high rectal cancer if within 5 cm, 10 cm, or 15 cm of the junction respectively [Figure 1] and [Figure 3].
Other pelvic vessels and surrounding nodes could also be evaluated as this may impact higher staging.
| Conclusion | | |
MRI is well positioned as the imaging modality of choice in the evaluation of rectal cancer. It is capable of predicting with a high degree of accuracy risk factors of local recurrence and is able to triage patients for more conservative treatment from those who must be treated aggressively. To be able to obtain relevant clinical information, radiologists and surgeons must arm themselves with adequate information of rectal and perirectal anatomy.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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| 10. | Gollub MJ, Lakhman Y, McGinty K, Weiser MR, Sohn M, Zheng J, et al. Does gadolinium-based contrast material improve diagnostic accuracy of local invasion in rectal cancer MRI? A multireader study. AJR Am J Roentgenol 2015;204:W160-7. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]
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