MRI protocol
Timing
- 2 weeks after TURBT, bladder biopsy, or intravesical treatment [5];
- 2-3 day after cystoscopy or removal of Foley catheter [6].
Patient preparation
-intramuscular spasmodic agents to avoid motion artifacts;
-500-1000 ml of water per os within 30 min before examination to distend the bladder properly and to allow the visualization of all the layers of its wall [7-9].
Technical considerations
- 1.5 or 3.0 T MRI and multichannel phased array external surface coil are recommended [6].
Image acquisition
T2-weighted (T2W) imaging, DWI, and dynamic contrast-enhanced imaging (DCE) are fundamental components of mpMRI examination. In men, MRI scan should cover whole bladder, proximal urethra, pelvic nodes, and prostate. In women, adjacent pelvic viscera (uterus, ovaries, fallopian tubes, and vagina) should be included. A gadolinium based contrast agent is administered at a dose of 0.1 mmol/kg of body weight at a rate of 1.5–2.0 ml/s if standard contrast agent is used and followed by saline flush [10].
MRI appearances of anatomy
Anatomy of the bladder is shown in Fig.1. On T2 images, muscolaris propria (detrusor) appears as low signal intensity (SI) line, while the inner layer of urothelium and lamina propria is not visualized [11].
With DCE, the inner layer shows early enhancement, and it appears as high signal intensity line, while muscularis propria presents as a low signal intensity line that enhances slowly and progressively [10-11].
Fig.2 shows the appearance of bladder on these MRI sequences.
In case of inflammation and edema of urothelium and lamina propria, T2W images may show a thickened hyperintense line (ie, the edematous inner layer) that overlays the hypointense muscular layer [11].
Scoring and reporting of images
VI-RADS consists of a five-point MRI score based on the evaluation of BC size, morphology, growth and enhancement which allow to assess the probability of muscular invasion. Appearances on T2W imaging, DWI, and DCE can be used to calculate an overall risk of invasion [6].
T2W imaging
Muscle appears hypointense on T2W images. Interruption of the low SI muscular line may suggest muscle invasion, as shown in Fig.3.
VI-RADS stages on T2W categories are summarized in Fig.4:
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 [11-14].
DCE categories are summarized in Fig.5.
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 [14-15] and this allows categorization as shown in Fig.6.
Final scoring
A five-point VI-RADS score is generated using T2W, DWI, and DCE categories and suggests the probability of muscle invasion, as shown in Fig.7. Dominant sequences for risk evaluation are DWI (first) and DCE (second, mainly if DWI is suboptimal).
1. VI-RADS I (muscle invasion is highly unlikely): T2W, DCE, and DWI category 1.
2. VI-RADS II (muscle invasion is unlikely to be present): T2W, DCE, and DWI category 2; both DCE and DWI category 2 with T2W category 3.
3. VI-RADS III (the presence of muscle invasion is equivocal): T2W, DCE, and DWI category 3; SC category 3, CE or DW category 3, the remaining sequence category 2 .
4. VI-RADS IV (muscle invasion is likely): at least T2W and/or DWI and DCE category 4; the remaining category 3 or 4; T2W category 3 plus DWI and/or DCE category 4; T2W category 5 plus DWI and/or DCE category 4.
5. VI-RADS V (invasion of muscle and beyond the bladder is very likely): at least T2W plus DWIand/or DCE category 5; the remaining category 4 or 5 [6].
Post-operatoy assessment
Cystoscopy is the standard tool for post-treatment follow-up. However, as follow-up examination are needed, mpMRI has become a non invasive alternative [16-18], as it can be used to monitor therapeutic response [6].
Therefore, early mpMRI scanning may allow patient-tailored management, with early response supporting continued therapy or poor response supporting treatment switch, in order to avoid the useless patient exposure to the toxicity of chemotherapy. However, further data are needed before it can be used routinely.