**2. Magnetic resonance imaging (MRI) basics**

### **2.1 Useful MRI sequences**

T1 weighted sequences (T1w) are highly sensitive to fat, marrow and gadolinium contrast media, which are able to detect a high intensity signal, so they appear light in T1w images. On the other hand, water retrieves a low intensity signal, so it appears dark. Therefore, you recognize T1w images by the dark gray representation of water, e.g. urine in the bladder, and by the almost white representation of fat, like in the bone marrow. T1w sequences are important particularly for the examination of the pelvic lymph nodes and the bone marrow.

T2 weighted sequences (T2w) are the most important sequences for MRI pelvic imaging. They provide high-resolution anatomical images that allow an accurate representation of the rectal carcinoma and its relationship to surrounding structures. It depicts the mesorectal fascia (CRM) as a thin line of low T2-signal intensity. You may easily recognize the T2w-images by their parameters: the signal-rich representation of water, e.g. urine in the bladder, and the signal-rich representation of fat. T2-weighted (T2w) sequences are water-sensitive, so water is signal-rich (light), whereas fibrotic tissue with low water content is signal-poor (dark). Paradoxically, fat also appears signal-rich (light) in T2w-images, which makes it difficult to distinguish from water in individual cases. A specific suppression of the fat signal might help here. However, since fat suppression techniques (FS) all lead to a loss of signal and thus either to increased image noise or to limited spatial resolution, they are not recommended for pelvic imaging - only after intravenous administration of gadolinium chelates contrast agent. In addition, FS techniques reduce the contrast between the low-signal rectal carcinoma in the T2 weighting and the signal-rich mesorectal fat tissue, which has a negative impact on the exact spread of the disease.

Diffusion weighted imaging (DWI) is achieved using diffusion-sensitive gradients in fast T2w sequences. DWI, in contrast to T1 and T2 measurement (excellent for morphological properties) is an **in vivo measurement** and shows the mobility of water molecules in different tissues (normal, tumorous, or fibrotic tissue). The limited diffusion in malignant tissue leads to a higher signal (bright) on the DW images. Tumors therefore usually appear bright on DWI images, while their lower diffusivity then leads to a low signal on ADC maps. DW sequences have the lowest spatial resolution of all sequences used in routine clinical protocols due to limitations in the signal-to-noise ratio, and they are susceptible to artifacts and distortion. These distortions are particularly pronounced at air-tissue boundaries (e.g. intestine) and OP clips. Moreover, the DWI technique necessarily requires fat saturation, so that fat presents itself with little or no signal. For spatial orientation, we always take corresponding anatomical images using T2w sequences.

Diffusion-weighted sequences are optional for the primary staging of rectal carcinoma, but we strongly recommend including them into the standard MRI protocol, as they can significantly facilitate the localization of the tumor and lymph

**79**

*Imaging and Diagnosis for Planning the Surgical Procedure*

*2.2.1 Bad diagnosis always begins with bad patient preparation*

nodes, and later restaging. MRI examinations for restaging of the rectal carcinoma after neoadjuvant therapy should contain a DWI sequence in order to be able to

Contrary to what is still being claimed, preparation of the bowel by means of enema (clyster or micro clyster) immediately prior to the examination is extremely important. We want to perform a high-precision examination similar to colonoscopy in a clean medium and not in a contaminated organ. The patient will always be informed in detail about the exact procedure of the examination, and this ensures active cooperation in most cases. This way, we minimize restlessness and movement artifacts. After flushing with Microlax Rectal Solution, the rectum is filled with warm tap water. Water is an excellent contrast medium without risking distension of the intestinal wall. In our department, we only use ultrasound gel for MRI defaecography, but not for tumor diagnosis, as the expansion of the rectum due to compression may restrict the assessment of the mesorectal space. Water as contrast medium allows an exact detection of even small flat lesions, which may be the case after RCT. Another advantage is the elimination of air besides stool residues. This procedure also creates perfect conditions for high quality DWI, which plays a particularly important role in restaging. Air is an enemy and real falsificator of DWI measurements! Last but not least, we prepare an infusion for administration of butylscopolamine to reduce

intestinal motility, and for administration of contrast agent, if necessary.

Having this done, specially trained medical-technical staff accompanied by doctors trained in rectal MRI perform the actual MRI examination. They always follow a standardized protocol with particular attention to angulation. The main axial layers must always be orthogonal to the tumor. Only in this way can the MRI results correspond with histology in terms of local tumor staging, and measurement of infiltration depth and distance to the mesorectal fascia. Furthermore, it is important to ensure that the restaging examination is always performed with the same equipment as the primary staging examination was done. Our experience has shown that different devices (e.g. Siemens vs. Phillips) deliver different DWI, which can make precise restaging difficult. These organizational challenges can only be overcome if we are all aware of them.

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

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

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

location of the tumor, paraaxial, axial or paracorononary layers result!

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

detect or exclude vital tumor remnants.

**2.2 Patient preparation**

**2.3 Examination protocol**

adjacent organs.

nodes, and later restaging. MRI examinations for restaging of the rectal carcinoma after neoadjuvant therapy should contain a DWI sequence in order to be able to detect or exclude vital tumor remnants.
