**2.2 Patient preparation**

*Colorectal Cancer*

**2.1 Useful MRI sequences**

spread of the disease.

In order to make good therapeutic decisions, an MRI must not only be carried out in a technically adequate manner, but must also be interpreted and presented accordingly. Moreover, the radiologist should also have a basic understanding of the various available therapy options. In particular, it is important to understand the surgery relevant aspects in order to have a target-oriented interdisciplinary discussion. Similarly, the treatment partners should also have basic knowledge of the findings and interpretation of MRI in

T1 weighted sequences (T1w) are highly sensitive to fat, marrow and gadolinium

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

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

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

take corresponding anatomical images using T2w sequences.

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

order to be able to understand the findings of the radiologist.

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

of the pelvic lymph nodes and the bone marrow.

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