**7. Other imaging modalities**

### **7.1 MRI vs. CT**

CT cannot be recommended for the local staging of rectal cancer.

The decisive advantage of MRI over CT is that it displays much better the morphology of the tumor and its topographical relationship to the border lamella of the mesorectum and to neighboring structures (prostate, seminal vesicle, vagina, uterus, os sacrum and os coccygeum as well as bladder and sphincter apparatus). As we have shown above, the relationship of the tumor to the neighboring structures is just as important as the TNM classification scheme [17, 18]. In addition, the lymph node prediction accuracy of CT is lower than with MRI.

For the detection of distant metastases, however, contrast-enhanced CT (CECT) is currently the method of choice due to its high availability and supported by current guidelines [19]. In most cases, it consists of a combined examination of the thorax and abdomen, which is a routine protocol both preoperatively for staging and in follow-up.

### **7.2 MRI vs. PET-CT**

We do not routinely use PET-CT in our center for primary staging, nor for restaging after CRT, as complete remission can be evaluated much better with MRI. In fact, although PET-CT can address the question of tumor response, it cannot determine the presence of complete remission.

However, we do apply PET-CT in particular cases for metastasis detection and evaluation on the background of high CEA values.

### **7.3 MRI vs. EUS**

For the detection, characterization and staging of rectal tumors, MRI is being considered the imaging modality of choice alongside endoscopic ultrasound (EUS), which offers particular advantages for early tumor stages T1 and T2. Without radiation exposure, it enables excellent soft tissue imaging and offers the possibility of multiplanar image acquisition and reconstruction, which is the current standard for the preoperative imaging of rectal tumors [20].

Currently, MRI increasingly being replacing EUS in the local staging for rectal cancer. Both modalities are equivalent for assessment of tumor spread beyond the muscularis propria (i.e., T2 versus T3 status). However, MRI holds several advantages over EUS in case of locally advanced rectal cancers (LARC), because it allows to better characterize lesion size, morphology, tumor margin and other helpful details for surgical planning. In addition, this modality offers a precise characterization of important aspects that may impact therapeutic decisions, such as proximity of the tumor to the mesorectal fascia, presence of extramural vascular invasion (EMVI), presence of extramesorectal pelvic lymph nodes, and involvement of the peritoneum/anterior peritoneal reflection, as well as the assessment of the R0 resectability.

Many of these findings are either difficult to assess, or are beyond the scope of EUS. Because of these advantages, MRI has become the preferred modality in the initial staging of rectal cancer, particularly as part of an interdisciplinary approach [15].

### **7.4 Endorectal sonography (ERUS) and its evaluation in the MDT tumor conference**

At our clinic, our colleagues from gastroenterology apply ERUS routinely for the preoperative local diagnosis of rectal carcinoma and for restaging. The obtained images are then loaded together with colposcopy images into PACS, so that the obtained information is available to all involved personnel, including radiologists. The examination protocol is well defined and observed: clinical examination at first, followed by colposcopy with biopsy, and then by ERUS. After this series of examination, and after delivery of the histological finding, we do MRI. At the end, we discuss the results together with all involved departments in the MDT tumor conference.

ERUS is particularly well suited for the preoperative diagnosis of small tumors T1, T2, T3a, and b. However, ERUS has difficulties with large tumors, especially if they are high-set or stenosing carcinomas; likewise, the limited FOV (field of view) of large T3 and T4 tumors can push ERUS to its limits - MRI is superior here. Most of the misdiagnoses in MRI occur during differentiation between T1 and T2 tumors, mostly because of an inadequate representation of the submucosa. In conclusion, ERUS is slightly better suited for the preoperative diagnosis of small low-lying tumors than MRI.

The assessment of the mesorectal fascia (MRF) remains a domain of MRI; especially after neoadjuvant radiochemotherapy, endosonography can neither assess the distance of the tumor to the potential circumferential resection margin (CRM), nor does it offer sufficient sensitivity/specificity to assess the primarius.

**95**

*Imaging and Diagnosis for Planning the Surgical Procedure*

ERUS and MRI should not be considered as competing procedures, but rather as complementary imaging modalities. Additionally, we must consider that, especially for endorectal ultrasound, there is a steep learning curve, which possibly also contributes to the lower overall accuracy of ERUS in large multicenter studies. In the hands of an experienced investigator, however, ERUS has proven to be a cost-effective and reliable method for the preoperative diagnosis

At our Imaging Center we evaluated in the past 5 years (2015-2020) 135 patients

While restaging, MRI imager after nCRT are correlated with MRI images before nCRT in all elements evaluated in primary staging. This requires post therapeutic image acquisition under nearly identical protocol parameters and levels. Essential points at this stage are position, extent and signal intensity of the tumor. These features are compared in the MRI images before and after nCRT. Care is also taken to ensure that restaging or follow up is always performed with the same device, because of the decisive diffusion-weighted images. As already mentioned, different devices (e.g. Siemens vs. Philips) provide different diffusion values, which are not

Restaging is not for beginners and requires a long learning curve, similar to MRI of the mamma or MRI of the prostate. A minimum of 50 histologically confirmed

The difficulties of restaging are obvious: Neoadjuvant therapy leads to profound changes in tumor tissue and surrounding structures, such as excessive fibrosis, deep stoma aging, wall thickening, characteristic muscle remodeling, tumor necrosis, calcification and inflammatory infiltration. As a result, the diagnostic accuracy of the imaging procedures decreases significantly with respect to restaging.

In the first 2 years, we almost exclusively performed primary diagnoses, the question of restaging being very low. On the one hand, this was due to our surgeons, who did not want to reconsider their original operation planning after completing CRT; on the other hand, it was due to us, because we were still very busy delivering high quality MRI diagnoses. When our image diagnostic results matched the histology, we finally got an adequate appreciation. This required a long learning curve. Today, restaging is as obligatory in our institute as preoperative MRI diagnostics. Restaging is a very demanding examination and can only work if the primary staging is performed with constant high quality. At this point, at the latest, the

with rectal carcinoma using MRI (4 devices of 1.5 T, 2 devices of 3 T).

standardized examination protocol with DWI pays off.

cases/examinations are necessary to achieve a good performance.

Accordingly, the local tumor extent can be over- or underestimated.

These challenges can best handled using MRI with diffusion images. The accuracy of clinical examinations using endorectal ultrasound (EUS), computed tomography (CT) and 18F-FDG protrusion emission tomography with CT

(18F-FDG-PET/CT), is very low both for the assessment of mesorectal invasion and for the evaluation of lymph node metastases and is therefore not used at our clinic

Our restaging strategy includes digital rectal examination, endoscopy/EUS, and finally MRT (DWI). Care is always taken to ensure that this examination sequence is followed. This is where the multidisciplinary team meeting between surgeons, gastroenterologists and radiologists plays a special role (**Figure 12**). The decisive images are introduced into the PACS system and are available to everyone. When

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

**8. Imaging modalities for restaging**

of rectal cancer.

always comparable.

for restaging as the sole examination.

*Colorectal Cancer*

**7.3 MRI vs. EUS**

approach [15].

conference.

tumors than MRI.

**conference**

In fact, although PET-CT can address the question of tumor response, it cannot

However, we do apply PET-CT in particular cases for metastasis detection and

For the detection, characterization and staging of rectal tumors, MRI is being considered the imaging modality of choice alongside endoscopic ultrasound (EUS), which offers particular advantages for early tumor stages T1 and T2. Without radiation exposure, it enables excellent soft tissue imaging and offers the possibility of multiplanar image acquisition and reconstruction, which is the current standard for

Currently, MRI increasingly being replacing EUS in the local staging for rectal cancer. Both modalities are equivalent for assessment of tumor spread beyond the muscularis propria (i.e., T2 versus T3 status). However, MRI holds several advantages over EUS in case of locally advanced rectal cancers (LARC), because it allows to better characterize lesion size, morphology, tumor margin and other helpful details for surgical planning. In addition, this modality offers a precise characterization of important aspects that may impact therapeutic decisions, such as proximity of the tumor to the mesorectal fascia, presence of extramural vascular invasion (EMVI), presence of extramesorectal pelvic lymph nodes, and involvement of the peritoneum/anterior peritoneal reflection, as well as the assessment of the R0 resectability. Many of these findings are either difficult to assess, or are beyond the scope of EUS. Because of these advantages, MRI has become the preferred modality in the initial staging of rectal cancer, particularly as part of an interdisciplinary

**7.4 Endorectal sonography (ERUS) and its evaluation in the MDT tumor** 

At our clinic, our colleagues from gastroenterology apply ERUS routinely for the preoperative local diagnosis of rectal carcinoma and for restaging. The obtained images are then loaded together with colposcopy images into PACS, so that the obtained information is available to all involved personnel, including radiologists. The examination protocol is well defined and observed: clinical examination at first, followed by colposcopy with biopsy, and then by ERUS. After this series of examination, and after delivery of the histological finding, we do MRI. At the end, we discuss the results together with all involved departments in the MDT tumor

ERUS is particularly well suited for the preoperative diagnosis of small tumors T1, T2, T3a, and b. However, ERUS has difficulties with large tumors, especially if they are high-set or stenosing carcinomas; likewise, the limited FOV (field of view) of large T3 and T4 tumors can push ERUS to its limits - MRI is superior here. Most of the misdiagnoses in MRI occur during differentiation between T1 and T2 tumors, mostly because of an inadequate representation of the submucosa. In conclusion, ERUS is slightly better suited for the preoperative diagnosis of small low-lying

The assessment of the mesorectal fascia (MRF) remains a domain of MRI; especially after neoadjuvant radiochemotherapy, endosonography can neither assess the distance of the tumor to the potential circumferential resection margin (CRM), nor

does it offer sufficient sensitivity/specificity to assess the primarius.

determine the presence of complete remission.

the preoperative imaging of rectal tumors [20].

evaluation on the background of high CEA values.

**94**

ERUS and MRI should not be considered as competing procedures, but rather as complementary imaging modalities. Additionally, we must consider that, especially for endorectal ultrasound, there is a steep learning curve, which possibly also contributes to the lower overall accuracy of ERUS in large multicenter studies. In the hands of an experienced investigator, however, ERUS has proven to be a cost-effective and reliable method for the preoperative diagnosis of rectal cancer.
