Endoluminal Ultrasound (ELUS) is an imaging technique that can be applied in a variety of luminal structures. ELUS is based on detecting an echo time delay of high frequency sound waves, which are backscattered by structures in tissue. Plotting of the reflecting ultrasound amplitude vs depth composes endoluminal images. The most common examples of endoluminal ultrasound are intravascular ultrasound, transvaginal ultrasound and transrectal ultrasound. Advances of this technique has led to small catheter-based ultrasound probes allowing visualization of a variety of other luminal structures, such as bile ducts, fallopian tubes, small bowel, oesophagus and blood vessels [14, 15]. Current ultrasound probes consist of flexible 2.4 mm diameter (7.2 F) catheters containing a 12.5 or 20 MHz transducer (Olympus, Tokyo, Japan) [16]. ELUS probes designed for use in the upper urinary tract have a hole through the end for placement over a Terumo guide-wire. The ultrasound probes are re-usable and can be sterilized in the same way as standard endoscopy equipment. The ultrasound transducer is attached to a motor that continually rotates providing real-time 360° cross-sectional images of the ureteric wall and surrounding structures and a pullback allowing 3D imaging of a UUT segment. Imaging penetration depth is dependent on the frequency of the ultrasound transducer. The 12.5 MHz transducer enables axial imaging with a penetration depth of maximum 40 mm around the ureter. The 20 MHz transducer enables axial imaging with a penetration depth of maximal 20 mm from the centre of the transducer and provides an axial resolution of 100 μm (Olympus, Tokyo, Japan) [16].

The transmission of sound waves needs a conducting medium. This restricts imaging to fluid filled structures, such as the lumen of the upper urinary tract. However, air bubbles introduced accidentally in the irrigation water due to the procedure create large back scattering, disturbing signals. An advantage of endoluminal ultrasound is its deeper imaging depth compared to other endoluminal imaging techniques (Table 12.1) which can be helpful to determine the relation of specific pathologies with its direct surroundings, like increased lymph nodes in malignancies [16]. In the ureter, endoluminal ultrasound has mainly been used for the treatment of UPJ stenosis, when an endoluminal incision was considered and the location of crossing vessels had to be determined. By using ELUS, the exact location of crossing vessels can be determined and therefore the best incision plane can be chosen [17]. Other possible indications for ELUS are identification and staging of upper urinary tract tumours, submucosal calculi, endometriosis and inflammation [16, 18] (Fig. 12.2).

However, improvement of CT imaging has led to a decrease of the use of ELUS in the field of urology, because it is less invasive and considerable costs are involved.

Optical Coherence Tomography (OCT) is a high resolution imaging technology originally applied in ophthalmology [19]. OCT is analogous to ultrasonography, using back-scattered light instead of back-scattered sound waves to produce micrometer-scale resolution, cross-sectional images (Fig. 12.3). “State of the art” OCT research investigates the position of this relative novel technology in the diagnostic workup of several epithelial cancers [20, 21]. The use of optical fibers allows OCT to be applied endoluminally through the working channel of small calibre rigid and flexible endoscopes. The commercially available OCT imaging probe currently at use for upper urinary tract imaging is originally designed for cardiovascular applications. For this reason, the 0.9 mm (2.7 Fr) fiber has to be interfaced with an intravascular imaging system. This automatic pullback system scans a longitudinal trajectory of 54 mm in approximately 5.4 s, producing a 540-frame dataset at 20 μm axial resolution. In OCT images of the urinary bladder wall, layered tissue anatomy can be distinguished, e.g., the basement membrane, which integrity is indicative of low stage in case a bladder tumour is identified [22]. Additionally, the light scattering causes a decrease of OCT signal magnitude over depth, and limits the imaging range to approximately 2 mm depth. This rate of OCT signal decrease with depth is quantified by the attenuation coefficient (μ_oct) that allows in-vivo differentiation between different tissue types [21, 23–25]. This distinction results from differences in intra- and extracellular organization of the tissue, which is reflected in the light scattering properties. Measurement of μ_oct could therefore discriminate differences in organization that could be associated with different grades of the lesion. This has theoretically been described and tested in an animal study by Xie et al. [26], where they showed differences in scattering properties between normal and malignant bladder urothelium.

The potential of OCT for staging and diagnosis of UUT-UC has been investigated ex-vivo in the porcine ureter and these investigations demonstrated that OCT can clearly distinguish the ureteral wall layers, particularly the urothelium and lamina propria [27, 28]. When compared to endoluminal ultrasonography, OCT significantly better distinguishes the wall layers of ex-vivo porcine ureter [29]. An in-vivo human pilot study showed that normal appearing urothelium including basal membrane, CIS and visible protrusions could be visualized on OCT images (Fig. 12.4). Additionally, OCT is able to visually differentiate between non-invasive and invasive tumours (Fig. 12.5) and it can differentiate between low- and high-grade lesions by quantifying μ_oct [30]. However, current OCT analyses cannot obtain a reliable μ_oct from normal appearing urothelium or CIS due to the limited thickness of these layers (around 50 μm). Improvement by increasing the resolution of OCT could solve this limitation. Recently published novel attenuation analysis methods for thin layers could solve this limitation, but these methods do require additional research since it is still in an early stage of development [31].

Several features of OCT are appealing for intraluminal diagnostics. First, flexible OCT probes are compatible with modern endoscopes and easy to apply in the ureter, pyelum and calyces, enabling OCT imaging at all sites within the urinary tract and not interfering with deflecting properties and rinsing during the procedure. Second, OCT measurement duration per patient adds only maximal five minutes operating time during URS, as the probe is easy to use and a single measurement takes 5.4 s per region under investigation. Third, OCT does not require a conducting medium or direct contact, which makes it easily applicable in the ureter. Finally, the OCT system is compact and portable, which results in an easy to use system in the operating theatre. In conclusion, the advantages of OCT are the non-invasive, real-time and high-resolution 3D imaging that allows visual staging and extraction of the optical attenuation coefficient that allows grading. However, current commercial available OCT systems are limited to image a lumen with a maximal diameter of 10 mm, compromising visualization of the pyelum and bladder as a whole. Furthermore, if tumour thickness transcends scattering-limited imaging depth in tissue (~2 mm), invasiveness cannot be assessed. A recent study quantified the learning curve and inter-observer variance of the μ_oct. It was concluded that routine μ_oct determination for tissue classification does not require extensive training and that OCT naïve people only require three trainings to acquire the same results as experienced OCT investigators. This increases the clinical potential of OCT [32].