The staging of bladder lesions relies on evaluating muscle invasion. On T2-weighted imaging (T2WI), the detrusor muscle appears as a low-signal intensity line along the bladder wall, showing intermediate signal intensity on diffusion-weighted imaging (DWI) without early enhancement on dynamic contrast-enhanced imaging (DCE). An interruption in this hypointense line is a key indicator of muscle invasion. Additionally, DWI can detect areas of diffusion restriction, which are typical of urothelial carcinoma, appearing hyperintense on DWI with corresponding hypointensity on the apparent diffusion coefficient (ADC) map.

PATIENT PREPARATION AND TIMING

• It is highly recommended to perform mpMRI either before or at least two weeks after bladder intravesical treatments or procedures, as reactive edema or inflammation may lead to local over-staging.
• The presence of free air in the bladder can distort diffusion-weighted images (DWI) due to susceptibility artifacts. For this reason, it is preferable to wait 2–3 days between the removal of a vesical catheter or a cystoscopy and the MRI examination.
• Prior to starting the MRI scan, the administration of an antispasmodic agent is suggested to minimize motion artifacts caused by bowel peristalsis.
• Proper bladder distension is essential for accurately evaluating bladder cancer (BC) and assessing potential detrusor muscle invasion.
• Patients should be advised to empty their bladder one to two hours before the exam or consume 500 to 1000 mL of water approximately 30 minutes prior to imaging.

MRI SEQUENCES

The mpMRI examination includes T2W, diffusion-weighted (DWI) and dynamic contraste enhanced (DCE) sequences.

• At least 2 planes of multiplanar T2W turbo-spin-echo.
• 2 planes, including axial, of DWI sequences. High b value (800-1000) needed.
• Pre-contrast image and DCE.

Thin slices of 3 to 4 mm are preferred to optimize spatial resolution

The field of view (FOV) of all images should include the whole bladder, proximal urethra, pelvic lymph nodes, prostate in male patients, uterus, ovaries, fallopian tubes, and vagina in female patients

VI-RADS SCORE SYSTEM

Scoring and reporting of images

• According to the VI-RADS system, each of the three sequences (T2WI, DWI and DCE) is scored on a 5-point scale.
• Appearances on T2W imaging, DWI, and DCE can be used to calculate an overall risk of invasion.

• T2W imaging

Muscle appears hypointense on T2W images. Interruption of the low SI muscular line may suggest muscle invasion.

• DCE imaging

On DCE imaging, tumor and inner layer enhance early and could enhance at the same time and grade. Muscularis propria should maintain no enhancement in the early phase, and it is visible as a low SI line under the tumor.

• DWI imaging

Tumor is hyperintense on DWI. Muscularis propria may present intermediate signal intensity, while the stalk and inner layer have low signal intensity on DWI

• Then, all appearances on T2W imaging, DWI, and DCE are combined to determine the overall VI-RADS score, classifying the likelihood of MIBC into five categories.

• VI-RADS 1: muscle invasion is highly unlikely to be present
• VI-RADS 2: muscle invasion is unlikely to be present
• VI-RADS 3: muscle invasion is equivocal
• VI-RADS 4: muscle invasion is likely to be present
• VI-RADS 5: invasion of muscle and perivesical tumor extension