During renal stone treatment planning, non-contrast computerized tomography (NCCT) is a highly recommended examination, used for assessing the stone size, number, and location. Non-excretory phases do not provide precise insight into detailed stone distribution inside the renal collecting system. With the use of an additional excretory phase scan, the urologist can study in-depth the relationship between the stone and the occupied pelvicalyceal system.

European Association of Urology (EAU) Urolithiasis Guidelines recommend percutaneous nephrolithotomy (PCNL) as the first-line treatment of large stones, more than 2 cm (1). Urologists usually define a renal stone as being complex based on the complicated and difficult pelvicalyceal anatomy, branching of the stone into the pelvicalyceal system, or the existence of multiple stones in various calyces. PCNL treatment of complex renal stones frequently require several access tracts and clear out the stone completely (2–4). However, the overall complication rate, as well as the transfusion rate, is higher when using multiple tracts (5–7). The key point for successful complex stone removal with minimal risk of complications is to establish the most appropriate access route (8). That means that the urologist should select and puncture that calyx(s) which will lead to complete stone extraction, while they will not increase perioperative complication rate.

Understanding the three-dimensional (3D) relationship of kidney stones and renal collecting systems is crucial for planning and creating an optimal access route. In recent years the use of advanced imaging and 3D reconstruction of CT data in the treatment of kidney stones is increasing (8). Advanced imaging systems have been shown to provide the 3D perception of target organs, surrounding anatomic structures, depth perception, and spatial orientation during endoscopic and minimally invasive surgery (9, 10).

There is a continuous search for new and improved methods that could help and refine the course of the PCNL procedure by achieving a more accurate approach to the renal collecting system and stone treatment while decreasing the risk of complications (11).

For the purpose of this study an elective semi-automated instrument was created for a more exact and quantitative evaluation of the renal collecting system and the stone by using a 3D volume segmentation tool. This could help for preoperative comprehensive PCNL access route planning but is also viable for real-time use in the operating room. The aim of this study was to assess the impact of 3D software in getting the desired access.

In this pilot case study, we enrolled 5 patients, which were selected with kidney stones, planned for PCNL procedure. CT scans were performed using a 64 multi-detector row CT scanner (General Electric) with 0.625 mm axial plane reconstruction. The scan was performed before and after contrast administration and both phases (plain and excretory) were used for the segmentation process and fusion of the 3D images into a single semi-transparent model demonstrating the stone distribution in the collecting system. While NCCT was used for stone segmentation, the excretory phase was used for collecting system segmentation. The scan delay time for excretory phase after i.v. contrast administration was 10–15 min, depending on the kidney excretion and degree of obstruction if present. The data set was routinely analyzed in Advantage Workstation Software V 4.6. The contrast medium used was Ultravist 370 mg J/mL, approximately 70–100 mL depending on the patient’s weight.

After NCCT acquisition DICOM images were used for segmentation and 3D reconstruction of the renal stones using the 3D Gastro CT Ex tool. The same software was used for segmentation and 3D reconstruction of pelvicalyceal system images obtained at the excretory phase scan. The reconstructed 3D model represented the interrelation between the pelvicalyceal system and stone with the greater anatomical details. Visualization is achieved by adjusting the scanned object transparency. Segmentations were performed to obtain stone volume (SV) and pelvicalyceal system volume (PSV). After volume rendering of the collecting system and renal stone, the stone volume (SV) was analyzed and compared with the pelvicalyceal system (PS) volume. The volumetric ratio of the stone and collecting system (SV/PS) was also analyzed.

All patients were completely investigated by conventional protocols proposed by guidelines. After confirming the indication for surgery, an additional puncture strategy was made after 3D reconstruction using the 3D Gastro CT Ex tool. Images from conventional CT scanners were compared with new images and an optimal axis for percutaneous puncture was made upon case characteristics. Due to the 3D reconstructed semi-transparent images, it was possible to analyze volumetric datasets in both anterior and posterior view (Figures 2 and 3). Patients from the study group had complex renal stones (Guy stone score 3 and 4) (23). All cases were treated by using a single puncture (Figure 4). The use of flexible nephroscope was indicated by case characteristics and surgery flow. Stone-free status was achieved in all cases, confirmed by follow up imaging. No perioperative complication occurred. In our series there was no unsuccessful outcome of PCNL procedures performed regardless the use of the 3D software for puncture planning.

The pelvicalyceal system with a complex stone represents a complex 3D structure. Thus, for accurate access route planning, the semi-transparent 3D model representing the correlation between the collecting system and stone seems to be very useful.

Rendering of the CT images with the ability to adjust the transparency of different tissue layers allowed the operator to see the stone through the renal collecting system, locate the stone and navigate the puncture needle. This semi-transparent 3D model could provide additional information during endourological procedures and therefore raise the possibility of achieving one-stage clearance of complex renal stones. The greatest advantage of using a 3D software is in creating a clear preoperative plan and selecting a target access route. Once having a clear preoperative puncture plan further steps of puncture guidance and control can be performed very precisely, usually with less fluoroscopy use. In this way the total fluoroscopy time can be reduced. As the total fluoroscopy time is a surrogate for radiation exposure, we assume that preoperative planning is a good way to also achieve the goal of reducing the use of fluoroscopy to a minimum in accordance with ALARA principles.

It has been proven that the surgeon is more comfortable with the initial puncture due to the 3D preoperative planning (24, 25). PCNL is routinely performed under 2D visualization with ultrasound and/or fluoroscopy guidance. In our technique the use of flexible nephroscope is not mandatory but flexible nephroscope should always be available during PCNL in aim to clear all stones from the pelvicalyceal system but also upper ureter.

By using a 3D model of CT images in planning the route and controlling the needle path, we would be able to use all the advantages of multidetector CT (MDCT) without raising the risk of radiation dose as it would be the case with CT fluoroscopy or Cone-beam CT guidance. 89% of success rate for percutaneous kidney access has been reported when using Cone-beam CT for guidance, but it comes with a higher radiation dose than conventional fluoroscopy guidance (26).

An accurate preoperatively defined puncture trajectory may also contribute to decreasing total fluoroscopy time to the lowest possible level. Once the operator has a clear preoperative puncture plan during fluoroscopy and ultrasound-guided PCNL procedures, fluoroscopy can be used only for access tract formation and wire position confirmation. Shortening of fluoroscopy time during PCNL is an important technical requirement to ensure safety and efficiency in PCNL while also reducing the radiation exposure of the patient and surgical team (11).

Our puncture technique involves ultrasound guidance with the assistance of pulsed fluoroscopy. Compared with our initial cases which were performed by ultrasound and continuous fluoroscopy we have already achieved significant reduction in fluoroscopy time from 155.4 s (median value) to 76.8 s (27).

If we compare the total fluoroscopy time values of PCNL procedures performed without the use of 3D software already reported in our larger series (27) with the total fluoroscopy time values of the PCNL procedures performed in this study (after creating the semi-transparent 3D model of pyelocaliceal system and stone) a significant reduction in the total fluoroscopy time is noted when 3D software was used (76.8 s vs 27.7 s).

Bearing in mind that in this study the results obtained on a small sample of five patients are presented, we cannot speak from the relevance of the statistically obtained data. This reduction can also be a result of an increased experience and skills of surgeon (OD), as his volume increased during the years.

To this study, CT images were segmented by focusing on renal stone, collecting system, and bone structures serving as a landmark for translation of measurements on the patient’s skin. Contrast is not absolutely mandatory for MDCT examination, especially in cases with damaged renal function. As the majority of patients have normal renal function the use of contrast offers a huge benefit in term of detail determination and distinction between stone and pelvicalyceal system.

Measurements of puncture angles and distances from the referent points were collected from a 3D model and translated in the operating room with accuracy. Vascular structures’ segmentation was not taken into consideration because the initial puncture of the renal collecting system was performed by an experienced urologist with real-time ultrasound guidance. The vessel segmentation process could take additional time before the procedure starting point without any substantial decrease in the rate of vascular injuries.

From the point of PCNL safety, avoidance of unnecessary puncture(s) is crucial. A greater rate of complications has been reported during calyceal stones treatment in comparison with the treatment of pelvic stones (28). When planning treatment with rigid instruments, the access route should be as straight as possible from the skin through the calyx in the axial line of the calyx through the infundibulum of the certain calyx and into the renal pelvis. Any additional intraoperative angulation of the access sheath would increase the risk of renal injury and bleeding. Every percutaneous kidney puncture is associated with a risk of bleeding. With the need for tract formation and dilatation, this risk is increased. The transfusion rate is higher in cases where multiple tracts were used during PCNL. Therefore, a decrease in access tracts is one of the goals of endourologists. The use of flexible nephroscope can decrease the need for additional puncture. In our opinion, even with flexible nephroscope the importance of optimal access axis remains of huge importance. Optimal access offers two biggest benefits: avoidance of torqueing and in that way less possibility of bleeding and better visibility during the procedure, especially in complex cases with stone distribution in different calices. Another benefit of detailed preoperative planning and selection of an optimal access route is in possibility to easily approach to all other calices that contain stones (such as in parallel calices, that sometimes can not easily be approached, even with flexible nephroscope from the punctured calyx) or might be a possible place of stone migration (usually upper calyx).

Stone burden and volume distribution in the renal collecting system is proven to be a significant determinant of the stone-free rate (SFR) (24), which was defined as no residual fragments larger than 3 mm in diameter (29, 30). In our initial experience and case series, the use of this tool helped significantly in preoperative planning and surgical strategy, allowing access to secondary calices from the main axis tract more easily.

There were no additional costs for this kind of evaluation, the software is free for use and its utilize was simple and intuitive. Collaboration with radiologists and software engineers was necessary, especially as the program offers the opportunity of rotation of the image and creating a picture in all positions used for PCNL, whether prone or supine or Valdivia modified used for retrograde intrarenal surgery (RIRS).

The limitation of the study was a shift and change in the position of the stone that can happen with the positioning of the patient. This was especially the case with smaller stones in the dilated collecting system. After 3D model calculation and taking measurements in a few cases the stone had changed its position most likely due to patient positioning. The CT scan was routinely performed while the patient was in the supine position and the PCNL procedure was performed in a prone position.

Having a clear and precise preoperative puncture plan is the key point to ensure efficiency in PCNL. Optimal access according to stone distribution and renal collecting system anatomy may contribute to both PCNL procedure efficacy and safety. This semi-automated segmentation tool for 3D visualization of the collecting system and stone interrelation is proven to be very useful for preoperative PCNL access planning. Currently, this might be a time-consuming process, but in the future, it should be a completely automated process with widespread use and adoption for all complex stone procedures.