## 3 Diagnostic Protocol  
In the clinical management of BC, the assessment of muscle invasion plays a crucial role in determining appropriate treatments. Currently, BC staging is based on clinical examination, TURBT specimens and radiological imaging findings. The evaluation of local bladder disease, nodal involvement, distant metastases and any concurrent upper tract disease utilises contrast-enhanced CT studies of the abdomen and pelvis. Suggested CT phases include pre-contrast, early arterial and delayed excretory phase [7; 8]. Multi-parametric MRI (mpMRI) plays a crucial role in assessing the detrusor muscle invasion due to its excellent contrast resolution [9]. It is strongly recommended that m mpMRI is performed before or at least two weeks after bladder intravascular treatment or procedures as reactive oedema or inflammation may result in local over-staging [6]. Furthermore, since free air in the bladder can cause distortion of diffusion weighted images (DWI) due to susceptibility artifacts, 2-3 days should elapse between the removal of a veiscal catheter or cystoscopy and MRI exam. Before starting the MRI scan, the intramuscular administration of an antispasmodic agent is suggested to avoid motion artifacts from bowel peristalsis. Adequate bladder distension is necessary to accurately assess BC and the possible invasion of the detrusor muscle. Patients are therefore advised to empty their bladder one or two hours before the exam or drink 500-1000 mL of water in the 30 min preceding imaging. The assessment of bladder distension with ultrasound may be useful to evaluate that the bladder is optimally full, with an approximate volume of 300 mL. An MRI scanner of 1.5 or 3.0 T should be preferred to achieve high spatial resolution and a high signal-to-noise ratio. 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. The mpMRI examination includes T2w images, DWI, and dynamic contrast-enhanced images (DCE-MRI) [6]. In detail, T2w images are usually acquired in at least two planes, using 2D fast-spin-echo (FSE) or turbo-spin-echo sequences. The location of the tumour influences the most appropriate plane to accurately evaluate the extent of the BC and its relationship with the bladder wall and perivesical fat. Minimizing any partial voluming effect is key to this process [10]; therefore, planes orthogonal to the tumour may be utilised. Additionally, thin slices of 3-4 mm are preferred to maximize spatial resolution, whilst needing to balance this against the signal-to-noise ratio. DWI, a functional MRI technique, measures the Brownian motion of water molecules within a voxel. This motion is dependent on tissue cellularity and therefore, in densely cellular tumour tissues which are associated with necrosis, fibrosis, haemorrhage and neo-angiogenesis, there is reduced Brownian motion [11]. The apparent diffusion coefficient (ADC) could be calculated from DWI sequences and is inversely proportional to the cell density. Two breathing-free spin-echo echo-planar imaging (EPI) sequences are recommended for the accurate staging of BC, with spectral fat saturation and a high b-value (800-1000 s/mm\({}^{2}\)). Regarding DCE images, a 3D T1w gradient echo (GRE) sequence with fat suppression is preferred due to its higher resolution than with 2D acquisition. Pre-contrast images are required and can be useful to identify haemorrhages and clots in the bladder. The gadolinium-based contrast agent is administered at a recommended dose of 0.1 mmol/kg at a rate of 1.5-2.0 mL/s followed by saline flush. Contrast images are acquired 30 s following the start of contrast injection and then followed by a series of four to six sequences every 30 s. This captures the enhancement pattern of tissues, with early enhancement of the inner layer of the bladder wall subsequently followed by the tumour.