**2.3 Examination protocol**

Rectal MRI can be performed routinely on a 1.5 T or 3 T system and takes about 25 minutes. However, our surgery department prefers 3 T systems because of their clearly higher resolution, shorter examination time, and the possibility of performing 3D imaging. A limited FOV ("field of view") is recommended, as it allows both accurate local tumor diagnosis, and excellent imaging of the mesorectum and adjacent organs.

We begin with a sagittal T2-weighted turbo spin echo (TSE) sequence, which serves as the planning sequence for the second axial thin-layer (3 mm) T2 TSE sequence and is the decisive sequence of the rectal protocol. Axial in this context always means perpendicular to the carcinoma, so that depending on the extent and location of the tumor, paraaxial, axial or paracorononary layers result!

The mandatory and most important measurements, done in mm, such as tumor infiltration depth into the mesorectum and the tumor distance to the mesorectal fascia, are performed based on these paraaxial images. If the radiological department performs accurately, then the measured values and the tumor staging correspond to

the histological results. Radiologists achieve this performance only after a relatively long learning curve. We always correlate our results with the pathology results during the tumor board.

Tips for the high resolution T2 axial sequence


As mentioned above, in deep carcinomas (lower third of the rectum) a **coronary** T2w TSE sequence is obligatory in order to detect or exclude infiltration of the muscle levator ani (T4 stage) or to diagnose infiltration of the anal canal. For deep carcinomas, we recommend to perform a Gd-enhanced T1 weighted axial and

### **Figure 1.**

*A. Paracoronal T2WI, no enhancement, shows a lower rectal T2 stage tumor without infiltration of the mesorectum, levator ani or anal canal. B. Gd enhanced paracoronal T1FS, anal canal. C. Gd-enhanced axial T1 FS. Markings: Yellow arrow: Levator ani; Yellow star: end of mesorectum; Green arrow: FMR = CRM; Red star: tumor; Red dashed arrow: muscularis propria; Red plain arrow: internal anal sphincter (IAS); Blue plain arrow: external anal sphincter (EAS)'Purple double arrow: anal canal; and Yellow dashed arrow: intersphincteric plane (ISP). Source: F. Bauer, Radiology Kaufbeuren.*

**81**

*Imaging and Diagnosis for Planning the Surgical Procedure*

coronal gradient echo sequence with fat saturation (GRE fs) as standard. These sequences depict the infiltration of the anal canal more accurately than with the

*Axial DWI-T2W image of a rectal cancer. Note the good demarcation of the tumor (red arrow) and of some irregular intramesorectal lymph nodes (yellow arrows) with the same signal intensity as the tumor. DWI is good in nodal detection, but has no value in assessing nodal malignancy. Source: F. Bauer, Radiology* 

Another important part of the MRI rectal protocol is the preparation of diffusionweighted sequences (DWI-MRI) including the quantitative measurement of ADC values (*apparent diffusion coefficient*), which in particular provides valuable additional information for the evaluation of therapy response after neoadjuvant radiochemotherapy. DWI is also very helpful for detection of lymph nodes (**Figure 2**), but it is not suitable for determining their benignity or malignancy, because in both cases the

Since about 12 months, we included the 3D volume measurement into the standard protocol, when using modern 3 T systems with newest hardware and software.

Rectum and anal canal emerge from the part of the endodermal intestinal tract known as the hindgut. At the ventrocaudal end (approx. 5th week of development) this has a sack-shaped dilatation, cloaca, which is closed to the outside by the cloacal membrane. The cloaca lined with endoderm provides not only the epithelial lining for the rectum and anal canal, but also for the bladder and urethra. Through growth or proliferation of the urorectal septum in the direction of the cloacal membrane (approx. 7th week of development), the cloaca is divided into the ventrally located urogenital sinus and the dorsally located anorectum. The cloacal membrane, which consists of epithelial cell clusters, disappears by apoptosis (rupture of the cloacal membrane), so that the urethral and anal canals are each open to the outside. The tip of the urorectal septum has now reached the body surface and forms the future perineum. Through the use of refined methods it has been disproved for decades that the cloacal membrane is the place where the endoderm and ectoderm meet.

Proliferating epithelial cell clusters, so-called anal membrane, temporarily displace

the anal opening. This lies at the level of the linea pectinata, which can already be detected at this point by the different immunohistochemical behavior of the surface epithelia. The epithelial closure disappears in the 8th week of development. In the following 9th week of development, the different epithelia proliferate and differentiate and the columnae and sinus anales are formed, thus not only the linea dentata is clearly

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

native T2 sequence alone, **Figure 1A** and **B**.

**Figure 2.**

*Kaufbeuren.*

lymph nodes have a high cellularity [2].

This supplementary measurement takes about 5 minutes.

**2.4 Clinically relevant embryology of rectum and anal canal**

*Imaging and Diagnosis for Planning the Surgical Procedure DOI: http://dx.doi.org/10.5772/intechopen.93873*

### **Figure 2.**

*Colorectal Cancer*

during the tumor board.

Tips for the high resolution T2 axial sequence

for optimal assessment in bulky tumor masses.

• A slice thickness of 3 mm or less is recommended.

angled perpendicularly to the tumor.

the histological results. Radiologists achieve this performance only after a relatively long learning curve. We always correlate our results with the pathology results

• Must be angled perpendicular to the tumor. The invasive center (the part of the tumor extending the most within the mesorectal fat) of the tumor must be detected on the sagittal plane. It is at this level where the sequence must be

• Sometimes, it may be necessary to obtain more than one sequence angulation

As mentioned above, in deep carcinomas (lower third of the rectum) a **coronary** T2w TSE sequence is obligatory in order to detect or exclude infiltration of the muscle levator ani (T4 stage) or to diagnose infiltration of the anal canal. For deep carcinomas, we recommend to perform a Gd-enhanced T1 weighted axial and

*A. Paracoronal T2WI, no enhancement, shows a lower rectal T2 stage tumor without infiltration of the mesorectum, levator ani or anal canal. B. Gd enhanced paracoronal T1FS, anal canal. C. Gd-enhanced axial T1 FS. Markings: Yellow arrow: Levator ani; Yellow star: end of mesorectum; Green arrow: FMR = CRM; Red star: tumor; Red dashed arrow: muscularis propria; Red plain arrow: internal anal sphincter (IAS); Blue plain arrow: external anal sphincter (EAS)'Purple double arrow: anal canal; and Yellow dashed arrow:* 

*intersphincteric plane (ISP). Source: F. Bauer, Radiology Kaufbeuren.*

**80**

**Figure 1.**

*Axial DWI-T2W image of a rectal cancer. Note the good demarcation of the tumor (red arrow) and of some irregular intramesorectal lymph nodes (yellow arrows) with the same signal intensity as the tumor. DWI is good in nodal detection, but has no value in assessing nodal malignancy. Source: F. Bauer, Radiology Kaufbeuren.*

coronal gradient echo sequence with fat saturation (GRE fs) as standard. These sequences depict the infiltration of the anal canal more accurately than with the native T2 sequence alone, **Figure 1A** and **B**.

Another important part of the MRI rectal protocol is the preparation of diffusionweighted sequences (DWI-MRI) including the quantitative measurement of ADC values (*apparent diffusion coefficient*), which in particular provides valuable additional information for the evaluation of therapy response after neoadjuvant radiochemotherapy. DWI is also very helpful for detection of lymph nodes (**Figure 2**), but it is not suitable for determining their benignity or malignancy, because in both cases the lymph nodes have a high cellularity [2].

Since about 12 months, we included the 3D volume measurement into the standard protocol, when using modern 3 T systems with newest hardware and software. This supplementary measurement takes about 5 minutes.

### **2.4 Clinically relevant embryology of rectum and anal canal**

Rectum and anal canal emerge from the part of the endodermal intestinal tract known as the hindgut. At the ventrocaudal end (approx. 5th week of development) this has a sack-shaped dilatation, cloaca, which is closed to the outside by the cloacal membrane. The cloaca lined with endoderm provides not only the epithelial lining for the rectum and anal canal, but also for the bladder and urethra. Through growth or proliferation of the urorectal septum in the direction of the cloacal membrane (approx. 7th week of development), the cloaca is divided into the ventrally located urogenital sinus and the dorsally located anorectum. The cloacal membrane, which consists of epithelial cell clusters, disappears by apoptosis (rupture of the cloacal membrane), so that the urethral and anal canals are each open to the outside. The tip of the urorectal septum has now reached the body surface and forms the future perineum. Through the use of refined methods it has been disproved for decades that the cloacal membrane is the place where the endoderm and ectoderm meet.

Proliferating epithelial cell clusters, so-called anal membrane, temporarily displace the anal opening. This lies at the level of the linea pectinata, which can already be detected at this point by the different immunohistochemical behavior of the surface epithelia. The epithelial closure disappears in the 8th week of development. In the following 9th week of development, the different epithelia proliferate and differentiate and the columnae and sinus anales are formed, thus not only the linea dentata is clearly 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 layer differentiates with a time delay in craniocaudal direction [3].

**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.
