**2. The utility of magnetic resonance imaging in patients with rectal cancer**

In recent years, abdominal and pelvic magnetic resonance imaging (MRI) has established itself as a gold standard method in the evaluation of patients with rectal cancer because of its crucial role in identifying non-responsive patients to neodjuvant radio chemotherapy [5, 6]. However, a particularly important role in the preoperative and postoperative clinical evaluation of these patients is played by accurate images of the anatomical structures of the pelvis, rectal tumor and their relationship with the surrounding anatomical structures [7].

In this sense, the most used MRI sequence in the preoperative evaluation of these patients for visualization of the rectum, tumor, and its relationship with surrounding tissues is High-spatial-resolution T2-weighted imaging [7]. On the other hand, one of the major advantages of rectal MRI scanning in T2 sequences is that 3 layers of the rectal wall can be differentiated. The inner layer is represented by the mucosa and submucosa, the middle layer is represented by the muscularis propria, and the outer layer is represented by the mesorectal fat. This allows for a much more accurate understanding of tumor invasion of the rectal wall and surrounding structures compared to other imaging studies [8, 9].

One of the disadvantages of MRI is the rather long time required to perform this investigation and therefore it is recommended that patients be positioned in a supine position for maximum comfort. But despite this inconvenience the benefit of this imaging method is major [10]. Current studies debate the optimal MRI resolution in the evaluation of patients with rectal cancer (1.5 T or 3 T). While 3 T cameras provide much better spatial resolution, they also have a higher susceptibility to artifacts during diffusion-weighted imaging (DWI) [10–12].

#### **2.1 Local staging in rectal cancer using MRI**

Newer studies have shown that MRI can identify patients who are at increased risk of local recurrence. In this sense, it has been shown that patients with tumors that invade only the rectal mucosa have a good long-term prognosis, while patients with invasion of the mesorectal fascia and pelvic organs in the vicinity of the rectum have a particularly high risk of recurrence [13–15]. In this respect MRI has a special utility for the detection of extramural tumor invasion as well as mucin deposits at this level [16]. On the other hand, more and more studies have shown that, in the case of superficial rectal tumors, EUS (endorectal ultrasound) has a special value in the identification of tumors and invasion of surrounding structures, while EUS is less useful in the case of tumors that penetrate the mesorectal fascia, respectively the anatomical structures in the vicinity of the rectum [17, 18].

Regarding the technique of performing MRI in these patients, in order to obtain good quality anatomical images, most authors recommend that the scan plane be perpendicular to the rectal wall at the level of the tumor with a slice thickness of maximum 3 mm. The sections are made in coronal, sagittal and axial plane [19]. On the other hand, there are debates in the literature regarding the use of intravenous contrast in these patients. Most authors do not recommend the routine use of

#### *The Utility of Magnetic Resonance Imaging in the Multidisciplinary Treatment of Patients… DOI: http://dx.doi.org/10.5772/intechopen.99580*

intravenous contrast [16]. However, there are authors who consider that the use of gadolinum contrast increases the accuracy of detecting transmural tumor invasion as well as vascular invasion [9, 20, 21].

There is further controversy in regards to patient preparation for MRI. Some authors recommend the administration of spasmolytic drugs prior to imaging studies especially in patients with upper rectal tumors and if 3 T devices are used. Other authors recommend that the use of diffusion-weighted MRI be preceded by endorectal filling. But in these cases, dilation of the rectum can affect the measurement of the distance between the mesorectal fascia and the tumor [22, 23]. To eliminate this inconvenience, some authors recommend that a maximum of 60 ml of gel be used for endorectal filling [24].

One of the major advantages of performing MRI in patients with rectal cancer is that it is possible to accurately identify both the circumferential invasion of the tumor in the rectal mucosa and its transmural invasion. This fact is especially important because newer studies have shown that one of the main factors that can lead to local recurrence is incomplete resection, especially in the lateral aspect of the resection specimen [25]. At the same time, pelvic MRI has the ability to accurately detect the macroscopic type of rectal tumor (polypoid, ulcerative) and the presence or absence of mucin at this level [9].

When performing rectal MRI in T2 sequences, the rectal mucosa appears hypointense, the submucosa hyperintense, and the muscularis propria appears as a circumferential hypersignal. Precise identification of the layers of the rectal wall thus allows a precise location of the tumor at the level of the rectal wall [26]. According to the TNM classification of rectal cancer, in stage T1, the tumor is limited to the mucosa and submucosa, in stage T2, the tumor does not extend beyond the muscularis propria, in stage T3, the tumor exceeds muscularis propria and in stage T4, the tumor extends beyond the rectal wall [19] (**Figure 1**).

If the tumor invades the mesorectal fat it is considered to be stage T3 and if it invades the peritoneum of the pelvic cavity, it is interpreted as stage T4. The invasion of intersphincter space is considered a T3 stage. and the invasion of the external anal sphincter is considered a T4 stage [27–29] (**Figure 2**).

A limiting factor in these cases is the existence of fibrous tissue in the rectal wall or in the tissues around the tumor. The existence of fibrosis at this level can make it difficult properly stage the patient, especially by over staging [30]. In this respect, there are studies in the literature which have shown that it is sometimes difficult to differentiate by MRI, peritumoral fibrosis from residual tumor deposits, especially in patients who have undergone neoadjuvant radiochemotherapy. Therefore, most

**Figure 1.** *MRI image, axial view, the tumor invades the mesorectal fascia.*

#### **Figure 2.**

*MRI image, sagital view – tumor recurrence invasive in the bladder and prostate.*

authors in the literature recommend that, in patients who have undergone neoadjuvant radio-chemotherapy, MRI examination should be performed by physicians experienced in this type of pathology [31].

Further MRI findings regard the relationship of the tumor with the anal sphincter as well as the distance between the tumor and the anocutaneous line. Tumors located less than 6 cm are considered low rectal tumors, tumors whose lower edge is located 7–11 cm from the ano-cutaneous line are considered medium rectal tumors, and tumors whose lower limit is located more than 11 cm from the ano-cutaneous line are considered superior rectal tumors [29]. The precise location of the tumor and its relationship to the anal sphincters are particularly important in determining the type of surgery to be performed in these patients (abdomino-perianal resection, abdominal resection) and the extent of the surgery to be performed.

Given the importance of precise localization of the rectal tumor relative to the anocutaneous line in determining the subsequent therapeutic decision in these patients, there are many studies that have investigated the specificity and sensitivity of MRI compared to colonoscopy in establishing the exact distance between the lower edge of the tumor and ano-cutaneous line.

In this regard, there are studies that have shown that MRI cannot rule out performing colonoscopy in these patients, especially because of the fact that colonoscopy offers the possibility of collecting biopsies for histopathological examination. But in many cases the assessment of the distance between the lower edge of the rectal tumor and the anocutaneous line during colonoscopy is subjective, both due to local anatomical details and the experience of the person performing colonoscopy, so many authors conclude that pelvic MRI it is much more useful in establishing the distance between the lower edge of the tumor and the ano-cutaneous line [32–35].

In patients with rectal cancer, a particularly important factor that determines their long-term prognosis, both in terms of the occurrence of local recurrence and survival is represented by extramural vascular invasion (EMT). Recent studies have shown that, T2-weighted MRI was able to identify EMT in 80–90% of cases. EMT is manifested by the existence of morphological changes in the blood vessels adjacent to the tumor [29, 36–38].

#### *The Utility of Magnetic Resonance Imaging in the Multidisciplinary Treatment of Patients… DOI: http://dx.doi.org/10.5772/intechopen.99580*

Another particularly important prognostic factor that can be identified in these patients using MRI and is represented by the distance between the tumor margin and the mesorectal fascia. Thus, it has been shown that in patients in whom the distance between the rectal tumor and the mesorectal fascia is less than 1 mm, the risk of local recurrence is approximately 22%; if the distance is greater than 1 mm, the risk of local recurrence is only 5% [39, 40]. Regarding the assessment of the distance between the tumor edge and the mesorectal fascia, a factor that may limit the effectiveness of MRI is the existence of a low layer of mesorectal fat between the anterior wall of the rectum and the seminal vesicles in men, respectively the posterior wall of the vagina in women. In these cases, it has been shown that MRI sensitivity and specificity may be affected [41].

Regarding the accuracy of MRI, in the correct evaluation of the T descriptor of the TNM classification of rectal cancer, a very important role is played by the experience of the radiologist performing the investigation [42, 43]. Thus, population studies have shown that the sensitivity of MRI in the correct evaluation of the T descriptor varies between 29 and 57% and the specificity varies between 50 and 83% [43–47]. These results are due, in part, to the experience of the examining physician and, on the other hand, to the difficulty of differentiating in some cases a stage T1 tumor from a stage T2 tumor. In some cases, the desmoplastic reaction of the tumor makes a tumor look like T3 stage on MRI when in fact, following surgical specimen examining the surgical resection piece is actually a T2 stage [48].

Last but not least, the knowledge of local anatomical details, of the relations of the rectal tumor formation with the surrounding structures, allows the surgical team an adequate programming of the resection surgery, thus diminishing the possible intraoperative surprises regarding local invasion of the rectal tumor. In this way, the morbidity and postoperative mortality of these patients can be significantly reduced.

### **2.2 Detection of lymph node metastases using MRI in patients with rectal cancer**

The existence of loco-regional lymph node metastases at the time of diagnosis is a poor prognostic factor in patients with rectal cancer, the first lymph nodes affected being those located in the mesorectum. In the case of rectal cancer, locoregional lymph nodes are considered to be the obturator lymph nodes, internal iliac lymph nodes and the ones located in the mesorectum [49–51]. Therefore, the correct assessment of the existence of lymph node metastases in patients with rectal cancer is of particular importance in the preoperative assessment of these patients, the experience of the examining physician having a particularly important role in these cases [52, 53] (**Figure 3**).

It is often difficult to assess the status of loco-regional lymph nodes using MRI and it has been found that in about 25% of cases loco-regional lymphadenopathy which were considered as lymph node metastases were not confirmed positive on the histology report [54, 55]. However, some studies have shown that the use of high-resolution T2-weighted sequences can improve the sensitivity and specificity of MRI in the detection of lymph node metastases. These results are due to the fact that, especially in the case of patients undergoing neo-adjuvant radiochemotherapy, local fibrosis makes it difficult to correctly assess the status of loco-regional lymph nodes [56–58].

A much debated topic in the literature is the diagnostic criteria for lymph node metastases based on MRI examination. Thus, there are authors who consider that lymph node adenopathy with a diameter larger than 5 mm represents malignant lymphadenopathy, and those with a diameter below 5 mm are benign [59]. On the other hand, other authors consider that the most faithful sign of suspicion for malignancy is represented by the fact that the diameter of the loco-regional lymph
