**3. Normal anorectal anatomy in MRI and the fasciae**

In MRI, we can divide the rectum into three sections based on its three lateral curvatures (in analogy to the Houston valves inside).

The upper two thirds are surrounded anterolaterally (upper third) and anteriorly (middle third) by the visceral peritoneum. This forms the anterior peritoneal fold approximately at the level of the middle curve (in the area of the so-called Kohlrausch's fold) and thus delimits the upper two thirds of the rectum from the extraperitoneally located wide lumen rectal ampulla.

The anterior peritoneal fold has a specific shape in axial stratification, which resembles the appearance of a seagull, hence the name "seagull sign" (**Figure 3C**).

As mentioned in 2.3, T2-weighted sequences optimally depict the individual wall layers of the rectum:


The mesorectal fascia (MRF) represents an important boundary structure for the description of the tumor extension and is well recognizable in T2-weighted sequences as a thin linear structure of low signal intensity.

The mesorectal fascia encases the perirectal (so-called mesorectal) fat including lymph nodes and vessels and represents an important natural barrier to tumor spread [4]. It corresponds to the so-called circumferential resection margin (CRM), which determines the extent of surgical resection in the context of total mesorectal excision (TME) [5], as seen in **Figure 3A** and **B**. At the level of the anterior peritoneal fold, the MRF fuses with the peritoneum. From this point on, the proportion of mesorectal fat decreases continuously until neither fat nor fascia are visible in imaging at the level of the anorectal transition. Inferiorly, the rectum fuses with the anal sphincter complex (Sphincter ani externus and internus).

The external sphincter consists of striated muscles, can be defined as a hypointense structure in all sequences in the MRI and only slightly accumulates contrast medium after gadolinium administration (a typical feature of striated muscles).

The boundary between the rectum and the anal canal can be easily recognized in MRI by the complex of the muscle levator ani at the upper end of the anal canal, which

**83**

**Figure 3.**

*Imaging and Diagnosis for Planning the Surgical Procedure*

fuses with the muscle layer of the inferior rectum [5]. The internal sphincter represents a sort of expansion of the circular muscle layer of the Muscularis propria of the rectum and consists of smooth muscle. In both T1-weighted and T2-weighted sequences it has an intermediate signal intensity. We use Gd-enhanced MRI with i.v. administration of

*Normal rectal wall in high-resolution MRI. A. Paraaxial T2WI depicts well layers of the rectal wall around the high-intensity intestinal lumen (filled with water): low intensity muscularis propria (red arrow) and high intensity mesorectal fat (yellow star) including lymph nodes and vessels. MRF (green arrow) is shown as a very thin line of low intensity surrounding the mesorectum. This line is crucial for surgery planning, as it represents the CRM (MRF=CRM). B. Paracoronal T2WI depicts additionally the low intensity mucosa (blue arrow), followed by the high intensity submucosa (yellow arrow) followed by again a low intensity structure, the muscularis propria. C. Paraaxial T2WI depicts the anterior peritoneal fold "Seagull sign" (yellow arrow).* 

By far the most common rectal adenocarcinoma, up to 90%, in MRI may appear as solid, polypoid or flat lesions within the intestinal wall, whereas the aspect of an annular or semiannular mass and growing with varying degrees of stenosis is the

the contrast agent to highlight the internal sphincter, **Figure 1B** and **C**.

**4. Tumor morphology with MRI**

*Source: F. Bauer, Radiology Kaufbeuren.*

most frequent image.

*DOI: http://dx.doi.org/10.5772/intechopen.93873*

### **Figure 3.**

*Colorectal Cancer*

layers of the rectum:

submucosa;

low signal intensity.

layer of high intensity.

marked, but the epithelial border between high-prismatic (cubic) epithelium and squamous epithelium becomes clear. In the mesenchyme around the anorectum, the smooth inner ring muscle layer differentiated, reaching with a thickened end in the 8th week of development to the level of the Linea pectinata. The outer longitudinal muscle

**Conclusion**: Only the rectum part above the linia anorectalis emerges from the endoderm, similarly to the colon. The anal canal emerges from the ectoderm, and for this reason, some authors do not consider it as belonging to the rectum.

In MRI, we can divide the rectum into three sections based on its three lateral

The upper two thirds are surrounded anterolaterally (upper third) and anteriorly (middle third) by the visceral peritoneum. This forms the anterior peritoneal fold approximately at the level of the middle curve (in the area of the so-called Kohlrausch's fold) and thus delimits the upper two thirds of the rectum from the

The anterior peritoneal fold has a specific shape in axial stratification, which resembles the appearance of a seagull, hence the name "seagull sign" (**Figure 3C**). As mentioned in 2.3, T2-weighted sequences optimally depict the individual wall

1.Submucosa, represented as an inner layer of high intensity. Appropriate examination parameters (see below), allow even to differentiate between mucosa and submucosa. In this case, the mucosa stands out as a fine low intensity line against both the positively contrasted intestinal lumen and the high intensity

2.Muscularis propria, represented as a further adjacent layer of intermediate to

3.Mesorectal fat, the natural barrier to tumor spread, represented as an outer

The mesorectal fascia (MRF) represents an important boundary structure for the description of the tumor extension and is well recognizable in T2-weighted

The mesorectal fascia encases the perirectal (so-called mesorectal) fat including lymph nodes and vessels and represents an important natural barrier to tumor spread [4]. It corresponds to the so-called circumferential resection margin (CRM), which determines the extent of surgical resection in the context of total mesorectal excision (TME) [5], as seen in **Figure 3A** and **B**. At the level of the anterior peritoneal fold, the MRF fuses with the peritoneum. From this point on, the proportion of mesorectal fat decreases continuously until neither fat nor fascia are visible in imaging at the level of the anorectal transition. Inferiorly, the rectum fuses with the

The external sphincter consists of striated muscles, can be defined as a hypointense structure in all sequences in the MRI and only slightly accumulates contrast medium after gadolinium administration (a typical feature of striated muscles). The boundary between the rectum and the anal canal can be easily recognized in MRI by the complex of the muscle levator ani at the upper end of the anal canal, which

layer differentiates with a time delay in craniocaudal direction [3].

**3. Normal anorectal anatomy in MRI and the fasciae**

curvatures (in analogy to the Houston valves inside).

extraperitoneally located wide lumen rectal ampulla.

sequences as a thin linear structure of low signal intensity.

anal sphincter complex (Sphincter ani externus and internus).

**82**

*Normal rectal wall in high-resolution MRI. A. Paraaxial T2WI depicts well layers of the rectal wall around the high-intensity intestinal lumen (filled with water): low intensity muscularis propria (red arrow) and high intensity mesorectal fat (yellow star) including lymph nodes and vessels. MRF (green arrow) is shown as a very thin line of low intensity surrounding the mesorectum. This line is crucial for surgery planning, as it represents the CRM (MRF=CRM). B. Paracoronal T2WI depicts additionally the low intensity mucosa (blue arrow), followed by the high intensity submucosa (yellow arrow) followed by again a low intensity structure, the muscularis propria. C. Paraaxial T2WI depicts the anterior peritoneal fold "Seagull sign" (yellow arrow). Source: F. Bauer, Radiology Kaufbeuren.*

fuses with the muscle layer of the inferior rectum [5]. The internal sphincter represents a sort of expansion of the circular muscle layer of the Muscularis propria of the rectum and consists of smooth muscle. In both T1-weighted and T2-weighted sequences it has an intermediate signal intensity. We use Gd-enhanced MRI with i.v. administration of the contrast agent to highlight the internal sphincter, **Figure 1B** and **C**.
