Computed tomography (CT) utilizes ionizing radiation to produce cross-sectional images with high spatial resolution. In general, computed tomography can be performed in the preoperative assessment of potential recipients and postoperatively for symptoms and signs outside of the renal transplant, e.g., fever, leukocytosis, or gastrointestinal complaints. An optimal CT examination requires iodinated contrast administered orally and intravenously. However, iodinated intravascular contrast may cause contrast-induced nephropathy (CIN) in patients with preexisting renal impairment and diabetes, and its use should be judicious in renal transplant recipients [10, 11]. Elevation in serum creatinine after the use of contrast agents within the early postoperative period is likely multifactorial, with contributory causes including ATN, rejection, cyclosporine toxicity, and dehydration. Prehydration prior to the CT study decreases the risk of contrast nephrotoxicity but does not eliminate it [10]. Although never specifically or adequately evaluated in the renal transplant population, other prophylactic measures such as administration of the antioxidant acetylcysteine are employed by many clinicians in hopes of preventing or reducing the nephrotoxicity of contrast agents.

In the immediate postoperative period, restoration of renal function may be delayed and it is therefore our practice to withhold intravenous contrast until renal function is adequate, or the use of vascular contrast considered absolutely necessary. A stable serum creatinine below 1.5–2.0 mmol/mL [1.5–2.0 mg/dL] or eGFR above 60 mL/min is a general cutoff for administering intravenous contrast media unless clinical urgency dictates the need for contrast-enhanced studies. When indicated, a reduced dose of iodinated contrast may be used. For most postoperative indications such as fever, pain, and diarrhea, an adequate CT study can be obtained without intravenous contrast but preferably with oral contrast. Alternatively, a magnetic resonance study may be substituted, as is discussed later.

CT is excellent for detection and anatomic localization of fluid collections and for guidance of interventional procedures. Transplant biopsies may also be performed using CT guidance instead of ultrasound for deep or less accessible allografts. A non-contrast CT scan may be useful in evaluating for stones in the native kidney or renal allograft and ureters.

Multidetector CT cystography has become the standard technique for evaluation of traumatic bladder injuries and is now standard at our institution for evaluation of bladder leaks after renal transplantation [12–14]. After a pre-contrast image acquisition, approximately 250–300 mL of diluted iodinated contrast is infused through a Foley catheter and another series of images is obtained when the bladder is distended. CT cystography is highly sensitive to small leaks and 3D multiplanar reformations increase diagnostic confidence. In parallel, CT nephrostography is extremely useful for the diagnosis of ureteral leaks and strictures but requires the prior placement of a percutaneous nephrostomy tube.

Magnetic resonance imaging (MRI) is a very versatile modality. MRI can be performed in multiple planes, does not use ionizing radiation, and has high contrast resolution, making it more sensitive than CT to pathologic change within tissues. The use of varying pulse sequences enables tissue characterization (such as fluid, fat, and hematoma) and differentiation of fluid collections. Masses in the allograft and native kidneys or other sites are readily evaluated. MR can detect flowing blood in major vessels without the use of iodinated contrast. In the past, gadolinium-based contrast agents (GBCAs) were considered preferential to iodinated intravascular contrast material in the evaluation of patients with renal failure. GBCAs increased image resolution and improved detection of small vessels allowing the acquisition of high quality angiographic images noninvasively, with a lack of nephrotoxicity [15]. However, current awareness of the association of GBCAs with nephrogenic systemic fibrosis (NSF) in patients with reduced renal function has significantly impacted the use of GBCAs, requiring careful screening of patients at risk and tailoring of examinations to maximize useful information while minimizing exposure.

Non-contrast-dependent angiographic techniques based on fast spin-echo, gradient-echo, phase-contrast, and inversion-recovery principles have been developed or maximized to assess the renal transplant vasculature, including ECG-gated 3D non-enhanced magnetic resonance angiography (MRA) performed with selective inversion-prepared fast imaging with steady state free-precession (TrueFISP). This has been shown to depict the renal transplant arteries comparable to contrast-enhanced MRA [16, 17], and is used most commonly at our institution. When contrast-enhanced MRI is unavoidable, the dose and type of GBCA should be carefully selected to minimize risk, as recent studies have shown that when restrictive GBCA administration guidelines based on glomerular filtration rate (GFR) are followed, the incidence of developing NSF significantly decreases [18].

Functional MRI of renal allografts is a field of active research. MR techniques such as contrast-enhanced perfusion imaging, diffusion-weighted imaging, BOLD (blood oxygen level-dependent) imaging, and arterial spin labeling are available to measure perfusion, blood oxygen level, and GFR [19–22]. This has great potential in the noninvasive diagnosis of transplant dysfunction. Newer techniques such as MR elastography which measures tissue stiffness may in future play a role in the detection and quantification of fibrosis [23]. Diffusion-weighted imaging has become increasingly useful in the detection and staging of cancer with future potential in tissue characterization and grading of neoplasms and lymphadenopathy [24].

MRI is contraindicated in patients with most pacemakers and some metallic implants such as intracranial aneurysm clips and heart valves. Patients with claustrophobia may be unable to tolerate MRI despite sedation. Other disadvantages of MRI include the cost, availability, and relative length of the studies. Many of the newer sequences are breath held and require patient cooperation. Artifacts from surgical clips and stents may degrade image quality and cause spurious stenoses [25, 26].

Nuclear scintigraphic renal studies are of value for the assessment of renal allograft function. For many years, technetium 99m pentetate (DTPA) was the most frequently used radiopharmaceutical for evaluation of renal allografts. In the last decade, this agent has been replaced by technetium 99m mertiatide (MAG3). Technetium 99m MAG3 is an agent that undergoes predominantly tubular secretion, and is a more efficient imaging agent than technetium DTPA. Images are obtained over the kidney immediately after the intravenous injection of technetium MAG3 to assess perfusion of the transplant, with development of perfusion parameters for quantitative evaluation [27]. Normal, decreased, or absent allograft renal blood flow may be calculated by comparison of the intensity of radiotracer activity over the kidney with the adjacent aorta or iliac blood vessels. GFR and effective renal plasma flow (ERPF) may be calculated. However, serial studies are often necessary in the determination of abnormal renal perfusion and ideally, a baseline MAG3 study is obtained within several days of the transplantation. Study quality may be compromised by poor hydration, and evaluation of the transplant may also be limited by uptake and secretion of MAG3 within the native kidneys if these remain partially functional.

Renal transplant vascular complications may be identified during the perfusion phase of technetium MAG3 studies although these abnormalities are more optimally evaluated and detected with Doppler ultrasound techniques.

Delayed scintigraphic imaging of the renal transplant is obtained after 30 min. The secretion of MAG3 into the renal collecting system, ureter, and bladder is measured and imaged. MAG3 studies remain very helpful for the detection of urinary leak and ureteral obstruction that most often occur at the anastomosis of the transplant ureter to the recipient bladder. MAG3 studies may also be helpful in the detection of vesico-ureteric reflux. This complication is frequently seen in transplanted kidneys due to the absence of a sphincter at the ureterovesical junction. Radiotracer intensity over the ureter and kidney may alternate over time due to periodic reflux of radiotracer-labeled urine from the bladder into the ureter and renal pelvis.

Isotope renography can be used to investigate renal dysfunction. In ATN, perfusion is normal but excretion is delayed and decreased. With rejection both perfusion and excretion are abnormal. However since other causes of graft dysfunction may have a similar picture, isotope renography is not of major value for the detection of rejection in transplant kidneys.

While conventional catheter-based angiography remains the gold standard for the diagnosis of arteriovenous fistulae, pseudoaneurysms, and renal artery or vein stenosis, in practice, vascular abnormalities are usually initially detected by Doppler ultrasound and may be confirmed by MR or CT angiography. Instead, conventional angiography with digital subtraction techniques is reserved for confirmation of suspected abnormalities immediately prior to percutaneous transcatheter interventions such as angioplasty and stent placement in transplant artery stenosis or embolization of a fistula [28, 29]. Allograft arterial and venous thrombosis is usually diagnosed by ultrasound and the role of interventional radiology is limited to mechanical thrombectomy and catheter-directed thrombolysis in select cases [30, 31].

The limitations of conventional angiography include the relative invasiveness and the required use of nephrotoxic iodinated intravascular contrast media. To limit the nephrotoxicity of administered iodinated contrast, catheter-based angiography can be performed using low doses of low- or iso-osmolar contrast material [32], possibly in conjunction with carbon dioxide gas, which has been used as a sole intravascular agent but has poor image contrast [33, 34]. Although GBCAs have historically been used as intra-arterial angiographic contrast agents, given the emergence of NSF and the relatively high doses needed for equivalent radio-opacity, they are no longer acceptable in patients with renal dysfunction [32].

Interventional radiologists play an invaluable role in the postoperative management of transplant-related complications by performing endovascular treatment, percutaneous urinary intervention, and abscess or fluid drainage [32, 35–37], as will be addressed later under the specific complication.

There is a limited role for abdominal radiography, limited to evaluation of stents, foreign bodies, renal calculi, and bowel complications. Chest radiography is widely used postoperatively.

Patients with long-standing renal disease often have many comorbidities which can affect graft and patient survival. Given the high demand for, and shortage of, donated kidneys, pre-transplant screening plays an important role in detecting coexisting illnesses and to evaluate feasibility of transplantation. This evaluation typically includes complete history and physical examination, with appropriate laboratory testing and up-to-date preventative health measures including colonoscopy and/or mammograms [38]. Basic radiologic imaging, including chest radiograph and abdominal ultrasound, is usually performed with more advanced imaging dictated by historical or clinical factors.

In many centers, preoperative contrast-enhanced CT arteriography (CTA) of the abdomen and pelvis is obtained in potential recipients. Selection criteria include age >50, chronic renal disease caused by diabetes and hypertension, known history of atherosclerosis or identification of atherosclerotic calcifications on radiography, or prior transplantation [39]. Important considerations include potential need for pre-transplant nephrectomy; evaluation for preexisting renal cell carcinoma or other malignancy, particularly in the dialysis population; and evaluation for sufficient atherosclerotic plaque-free patent vasculature for vascular anastomoses. In patients already on dialysis, normal dose contrast-enhanced CTA is performed. For patients who are predialysis, the risk of CIN can be lessened by hydration, before and after contrast administration [11, 40–42]. A recent study using 50–60 % of the standard dose of contrast for CTA with pre- and post-procedural hydration did not result in development of CIN in patients not yet on dialysis [43]. Additionally, at our institution, we are evaluating the potential of low kV (80 kV) CT scanning to potentiate lower doses of intravenous contrast (less than half the standard dose of 100 cc). Oral contrast is not administered as the high density interferes with volume rendering and creation of 3D reconstructions used for evaluation and presurgical mapping of vascular calcification.

Additionally, using CT, the native urinary tract can be assessed to determine need for concurrent nephrectomy (i.e., polycystic kidneys) or the presence of malignancy, given increased risk of renal cell carcinoma in the long-term dialysis-dependent patient [44].

The use of routine postoperative imaging is institution dependent. Common indications for imaging are absent or decreased urine output, delayed graft function, rising or persistently elevated creatinine, fever, pain over the allograft, or drop in hematocrit. Hypertension or hematuria may also prompt imaging evaluation. Patients with delayed graft function may have baseline imaging studies prior to discharge, which can be useful for comparison when follow-up studies are obtained. Ultrasound is the most commonly performed modality due to its unique advantages previously described. However CT, scintigraphy, and MR are complementary modalities in the radiologic armamentarium.

Immediate complications, occurring in the first week, are mostly related to the surgical procedure and include renal artery or vein thrombosis, hemorrhage, and ureteral edema. Nonsurgical complications consist mainly of ATN and acute rejection. Early complications occur between 1 week and 1 month and include acute rejection, urinary leak, infection, fluid collections, and vascular thrombosis. After 1 month, lymphoceles, acute or chronic rejection, ureteral strictures, renal artery stenosis, infection, cyclosporine toxicity, recurrence of renal pathology, and neoplasms form the bulk of problems likely to be encountered. Overall, the most common complications are perinephric fluid collections which occur in up to 50 % of recipients [45, 46]. General complications relating to abdominal surgery are also encountered and include postoperative ileus, bowel obstruction, venous thromboembolic disease, and infections (systemic, pulmonary, renal, or bowel in origin). CT is the most useful imaging study when searching for infection or in patients with nonspecific chest or abdominal symptoms. Ultrasound is the study of choice for extremity deep venous thrombosis but is limited in the upper mediastinum or pelvis. For suspected thoracic, pelvic, or inferior vena cava thrombus, CT with contrast or MRI is the preferred modality. Multidetector CT pulmonary arteriography is the study of choice for pulmonary embolism.