**Figure 3.**

*Adjacent lymph nodes in mesorectal fat up to 4 mm in size. 4 mm extramesorectal lymph node.*

nodes decreases in size or increases in size after the practice of neo-adjuvant radio-chemotherapy [60, 61]. On the other hand, other authors consider as criteria for malignancy of the lymph nodes, based on MRI examination, the existence of extracapsular invasion or enlargement of the lymph nodes located on the walls of the pelvic cavity (extramesorectal), or changes in their morphology (presence or absence of heterogeneity) [62, 63].

However, the specificity of the diagnosis of malignant lymphadenopathy with MRI is around 70%, mainly due to fibrotic changes or mucinous degeneration of these lymph nodes, these results being due also to the fact that there are studies that have shown the existence of fibrotic changes also in case of benign lymphatic nodules [64]. There are also studies in the literature that recommend the practice of MRI with dynamic contrast-enhanced, in patients with rectal cancer, in order to increase the accuracy of MRI diagnosis of lymph node involvement. In the case of the administration of dynamic contrast, it is considered that, usually, the malignant lymph nodes, when examined in T2 sequences, have edges in hypersignal, and their center presents hyposignal [65].

#### **2.3 Detection of distant metastases using MRI in patients with rectal cancer**

Recently, the importance of MRI in the preoperative evaluation of rectal cancer patients has increased greatly despite the abdominal CT examination, especially due to the fact that diffusion-weighted MRI is much more effective in detecting small liver metastases compared to abdominal CT imaging [66]. There are also studies in the literature that have shown that the sensitivity and specificity of MRI in the detection of liver metastases is superior even to PET-CT [67, 68]. Therefore, there are authors who recommend performing a whole body MRI, in patients with rectal cancer [69]. On the other hand, the sensitivity and specificity of the detection of pulmonary metastases, in patients with rectal cancer, is lower in the case of MRI compared with chest CT scan [70].

Recently, in order to detect the existence of distant metastases, in patients with rectal cancer, PET-MRI is increasingly used. This method eliminates the lower sensitivity and specificity of MRI in the detection of lung metastases and brings in addition the increased sensitivity and specificity of MRI for the detection of liver metastases, compared to abdominal CT scan [71].

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

#### **2.4 Evaluation of the response to neo-adjuvant radiochemotherapy**

The prognosis of patients with rectal cancer has significantly improved, in recent years, on the one hand by introducing the neo-adjuvant radiochemotherapy in their treatment, as well as by improving imaging methods that allow a more accurate preoperative staging of these patients [72, 73]. Thus, it has been shown that the introduction of neo-adjuvant radiochemotherapy in patients with advanced loco-regional forms of rectal cancer has led to an improvement in their survival, decreased the risk of local recurrence and, in some cases, has even been recorded a complete pathological response, improving as well the postoperative morbidity and mortality of these patients. Also, for these patients it was found a better compliance to postoperative radio-chemotherapy [74–76].

With the initiation of neo-adjuvant radio-chemotherapy of particular clinical importance is the identification of patients with no response to this therapy, with incomplete clinical response or with a complete clinical response (the absence of residual tumor, the absence of neoplastic lymph nodes in the mesorectum). The identification of these groups of patients is very important given the principles of personalized medicine. It is also of crucial importance to identify patients who do not respond to radiochemotherapy, in which case it is beneficial for them to initiate the surgical treatment as soon as possible [77–80].

In this regard, in recent years there are authors who, in rectal cancer patients with neo-adjuvant radiochemotherapy to whom a complete clinical response is recorded, recommend either the practice of a resection surgery with preservation of the rectum or only the clinical follow-up of these patients, without the indication of a surgical treatment. In these situations, a complete clinical response is recorded in approximately 24% of cases [80, 81]. Some authors have shown that the usual MRI techniques (T2 weighted) cannot always correctly assess the clinical response to neo-adjuvant radiochemotherapy, recommending in these cases the use of functional MRI techniques (dynamic contrast-enhanced MRI - DCE-MRI and diffusionweighted imaging - DWI). These techniques have the advantage of providing much more accurate information about the existence of the residual tumor.

In this sense, in the case of the use of DWI-MRI, the so-called diffusion coefficient that evaluates the diffusion capacity of water at the tissue level is particularly important in evaluating the tumor response to neo-adjuvant radiochemotherapy. This coefficient is inversely proportional to tissue cellularity. Usually, viable tumor cells prevent the diffusion of water to the tissues, while necrotic tumor cells allow the diffusion of water at this level [82, 83]. The diffusion coefficient is also particularly useful in differentiating viable tumor tissue from inflamed areas, respectively necrosis areas. Thus, some authors consider that the value of this coefficient has predictive value in terms of response to neo-adjuvant radiochemotherapy of these patients [84].

DCE-MRI can provide important information about the vascularization of the tumor, the permeability of these vessels, as well as about the structure of the extracellular space. Also, this method has the possibility to identify the areas of hypoxia as well as the intensity of the microvascularization at the level of the tumor formation, both from a quantitative and a qualitative point of view. In this regard, there are studies in the literature that have shown that the existence of increased vascular permeability in the tumor before initiating neo-adjuvant radiochemotherapy is associated with a good therapeutic response, in these cases. Other authors have also shown that the existence of mucin at the level of the tumor formation is associated with a poor therapeutic response in these cases [84–87].

The major advantages of using MRI in evaluating the clinical response to neoadjuvant radiochemotherapy are represented on the one hand by highlighting the morphological changes that occur at the level of the rectal tumor (size, vascularity, structure) as well as the changes that occur in the pelvic lymph nodes. The limiting factor that may influence the accuracy of the method in these cases is the occurrence of local fibrosis after radiotherapy or post irradiation proctitis [88].
