With the goal of providing improved treatment to patients with overactive bladder symptoms (OAB), the investigators seek to apply the new technology of time-resolved (cine) MRI combined with urodynamics (UDS) to understand the exact effect of intravesical botulinum toxin on bladder physiology. The investigators will specifically aim to assess the physiological mechanism by which intravesical botulinum alleviates urgency and urge incontinence symptoms in women with OAB refractory to medical therapy.