Kidney transplantation is the therapeutic method of choice in end-stage kidney disease. Living donor transplantation is a better option than cadaver transplantation due to a longer period of preoperative preparation, shorter duration of ischemia and longer graft survival. Preoperative preparation involves numerous examinations, including blood and urine tests, renal function tests, electrocardiogram, prostate-specific antigen analysis, or mammography, as well as a complete radiological evaluation. Radiologists are responsible for adequate imaging evaluation, which consists of reporting on renal anatomy, vasculature, the collecting system, as well as on focal or diffuse renal diseases. Careful and accurate reporting enables more successful surgical procedures and reduces the number of potential complications. In the preoperative evaluation of the kidney, the imaging modality of first choice is computed tomography (CT) and it is still the gold standard, despite the theoretical risks, which we will discuss later in this paper ,,.
In our center, potential living kidney donors are evaluated with a 64-row multidetector CT scanner (Siemens Medical Solution) with 5 mm collimation and 1 mm retroreconstruction slices in post-processing. Our protocol includes a native phase of the examination for possible detection of nephrolithiasis, upon which 2 ml/kg of intravenous contrast agent is administered at a rate of 4 ml/s. Acquisition in all patients is done in at least three postcontrast phases (arterial, venous and excretory phase) using the bolus-tracking technique with the region of interest (ROI) at the level of the thoracoabdominal aorta and a threshold of 150 HU, without prior oral preparation. In postprocessing, multiplanar reconstructions and volume-rendering 3D techniques were used.
Apart from its role in the detection of calculi, the native phase also serves as a baseline for the subsequent enhancement of the lesions. The arterial phase is used to depict the vascular anatomy, both arterial and venous, due to their different opacification . The venous phase is ideal for evaluating the renal parenchyma and can adequately depict small veins such as the adrenal and gonadal veins. The excretory phase is usually obtained between four and eight minutes after contrast administration (6 minutes in our institution), and evaluation of the renal collecting system as well as evaluation of urothelial pathology can be performed in this phase .
Our school of pretransplantation and posttransplantation imaging is under the jurisdiction of the Croatian transplantation school, so the training of our specialists in medical centers in Zagreb contributed to the adoption of this CT protocol in our institution.
In some transplant centers, CT protocols have been modified to reduce the radiation dose. In their protocol, Kawamoto et al. do not use the native scanning technique, rather the detection of nephrolithiasis is done in the arterial phase of the examination with modifications in postprocessing . In some protocols, scanning in the native phase is done with a reduced voltage (less than 100 kV) while the excretory phase is replaced by a topogram, which significantly reduces the radiation dose and does not affect the diagnostic accuracy of the anatomy of the pelvicalyceal system .
COMPUTED TOMOGRAPHY VERSUS MAGNETIC RESONANCE IMAGING
Both CT and magnetic resonance (MR) imaging can show vascular and pelvicalyceal anatomy, but MRI enables the evaluation without ionizing radiation and the risk of contrast-induced nephropathy. However, CT has higher spatial resolution and speed, unlike MRI which is more susceptible to motion artifacts. Vascular calcifications and nephrolithiasis are better detected with CT, and some studies have shown this to also be true of accessory renal arteries. The risk of contrast-induced nephropathy is negligible in patients with normal renal function. Therefore, CT is still the gold standard despite these theoretical risks ,.
EVALUATION OF RENAL PARENCHYMA
Radiological assessment of the renal parenchyma includes information such as the number of kidneys, their length, position, volume and anatomical variants, as well as anomalies and diseases of the donor kidney and its vasculature ,. These data have prognostic significance and are related to renal function in the recipient, up to 36 months after transplantation . Unilateral agenesis, horseshoe kidney, cortical atrophy, polycystic disease, medullary sponge kidney and renal papillary necrosis exclude the possibility of donation .
Kidney length is defined as the maximum longitudinal diameter on a coronal CT section, and a two-centimeter or greater difference in length between two kidneys generally requires further testing of renal function (split function radioisotope scan) .
Kidney volume can be measured manually or using semi-automated and automated segmentation techniques, but all of them are based on measuring the renal cortex in a larger number of sections .
Kidneys with unilateral parenchymal scars and normal renograms are suitable for transplantation . Also, kidneys with cysts of moderate size (less than 5 cm) and number, and without elevated echogenicity, can be considered good grafts for transplantation (Figure 1) . Cyst characteristics should be evaluated to rule out the presence of a solid mass. Also, kidneys with small angiomyolipomas can be safely transplanted, given their slow growth and the absence of the risk of morbidity .
RENAL VASCULARIZATION ASSESSMENT
After the evaluation of the renal parenchyma, the second mandatory step is the evaluation of the vascular structures. This is perhaps the most important step, because it requires a broad knowledge, not only of radiology, but also of surgical approaches, in order to make the entire transplantation procedure successful.
In most individuals, the renal artery originates at the level between the upper margin of the L1 and the lower margin of the L2 vertebrae, and the right renal artery usually originates above the left. Variations of renal arteries can be divided into two groups: (1) early branching and (2) extrarenal arteries ,. In the right kidney, early segmental arterial branching means branching behind the inferior vena cava (IVC), i.e., retrocaval branching, which occurs 1 cm from the right IVC margin. This can create difficulties for surgeons due to the greater possibility of injuring large blood vessels when working behind the IVC. In the left kidney, early segmental arterial branching is branching within 1.5 cm of the origin of the left renal artery ,,. There are three types of extrarenal arteries: (1) hilar, (2) polar, and (3) capsular. Hilar (accessory) arteries enter the kidney at the level of the hilus, with the main renal artery (Figure 2) ,,. The largest number of accessory arteries originate from the aorta, but they can also originate from another blood vessel, such as the iliac, gonadal arteries, the mesenteric vasculature, etc. . Polar (aberrant) arteries enter the kidney at the renal poles, directly through the renal capsule. Finally, capsular arteries are thin blood vessels that surround the kidney and perfuse the renal capsule ,,. The presence of more than two accessory renal arteries can be considered a contraindication for transplantation, due to a high risk of thrombosis and longer operative time . Accordingly, accessory arteries and their diameter must be mentioned in the report, in order to prevent not only thrombosis, but also unnecessary operative bleeding . Small arteries (diameter of less than 2 mm) are acceptable, because, in that case, the volume of infarcted kidney tissue is less than 10% .
There are three mandatory measurements of the renal arteries: (1) the distance between the origin of the right renal artery and the first segmental branching, (2) the distance between the right margin of the inferior vena cava and the first segmental branching of the right renal artery, and (3) the distance between the origin of the left renal artery and its first segmental branching ,.
The most important diseases of the renal arteries that must be detected are atherosclerosis, fibromuscular dysplasia, aneurysms, arteriovenous malformations, dissection, and thrombosis (Figures 3 and 4) . Renal artery stenosis most commonly has an atherosclerotic etiology and it affects the origin and the proximal segments of the renal arteries, in elderly patients. The presence of calcified plaques is a contraindication for transplantation, because the blood vessel cannot be adequately clamped, which can subsequently lead to laceration of the intima of the renal artery and aorta and to life-threatening bleeding ,. Fibromuscular dysplasia (FMD) is the second most common cause of renal artery stenosis and is a non-atherosclerotic, non-inflammatory vascular disease of unknown etiology that affects medium and large arteries, more precisely the middle and distal segments of the renal arteries. On CT, it typically presents as the ‘string of beads’ sign, but it can also present as a focal stenosis or as aneurysmal changes ,,,. If a unilateral segment of fibromuscular dysplasia is found, it can be replaced with a graft (biological or synthetic) and the kidney can be used as a graft. However, bilateral FMD excludes the possibility of donation ,.
Venous evaluation is extremely important, because venous bleeding is the most common reason for the decision to switch from laparoscopic to open surgery, but also because anatomical variations of renal veins are more common than variations of renal arteries . The number, course, and length of the trunk of the main renal veins and their tributaries should be reported on . Late segmental confluence of the right renal vein is defined as segmental confluence at a distance of less than 1 – 2 cm from the inferior vena cava. Usually, a short right renal vein is the reason why the left kidney is preferred for transplantation. Late segmental confluence of the left renal vein occurs 1.5 – 2 cm from the left aortic margin . The most common variations of the left renal vein are supernumerary (accessory) and circumaortic renal veins. Large systemic tributaries of the left renal vein, such as the gonadal, adrenal, lumbar, and retroperitoneal veins, should be carefully described . In most cases, the right renal vein has no tributaries, or these tributaries can be the right gonadal vein and the retroperitoneal veins . The presence of multiple renal veins is associated with a higher incidence of posttransplantation thrombosis (Figure 5) . During CT evaluation, the most important measurements that must be made are: (1) the distance between the segmental confluence of the right renal vein and the IVC, (2) the distance between the segmental confluence of the left renal vein and the IVC, and (3) the distance between the segmental confluence of the left renal vein and the left margin of the aorta .
ASSESSMENT OF THE UPPER URINARY COLLECTING SYSTEM
Nephrolithiasis is the first among the possible pathologies that should be mentioned, due to its high frequency. Small calculi (smaller than 4 mm) are not an obstacle for donation, but multiple calculi or single calculi larger than 5 mm preclude donation until they are removed, or until metabolic testing is performed . Hydronephrosis, papillary necrosis, medullary sponge kidney, as well as transitional cell tumors, indicate exclusion from kidney donation . Any anatomical variation of the collecting system, such as duplication of the ureter, ureter fissus, extrarenal pelvis, bifid renal pelvis, obstruction of the ureteropelvic junction, etc., must be reported ,. It should also be noted that neither complete nor partial duplication of the ureter represent absolute contraindications for donation, however, they must be carefully considered .
Transplantation remains the best therapeutic method in patients with end-stage renal disease, but it requires a precise multidisciplinary approach. Radiological evaluation of potential living kidney donors is crucial for successful transplantation, and CT plays a crucial role in this process. The key to a good radiology report is knowing the surgical techniques and the difficulties that surgeons may face during the kidney transplant process.