Radiation diagnostics in nephrology: methods and indications

Radiation diagnostics in nephrology addresses four fundamental objectives: quickly excluding life-threatening conditions, confirming or clarifying the diagnosis, providing quantitative parameters of function and blood flow, and assisting in the selection of treatment and monitoring strategies. The most commonly used methods include ultrasound with Doppler ultrasonography, computed tomography, magnetic resonance imaging, and radionuclide methods, which complement each other in sensitivity and specificity in various clinical scenarios. Current guidelines emphasize the transition from universal regimens to stratified routes based on clinical risk and the availability of techniques. [1]

Standards for selecting examinations are developed interdisciplinary. For example, a risk factor approach is used for hematuria; for renal colic, priority is given to rapid confirmation or exclusion of stones; for pyelonephritis, selection of patients who require imaging; and for chronic kidney disease, a focus on ultrasound signs of remodeling and the prevention of unnecessary contrast. These algorithms are reflected in documents of professional societies and are regularly updated. [2]

A key trend in recent years has been the expanding role of contrast-enhanced ultrasound and functional radionuclide techniques. Contrast-enhanced ultrasound clarifies the nature of lesions and graft perfusion, while diuretic renography and cortical scintigraphy provide objective indicators of drainage and distributed function, which are critical for obstruction and scarring. These technologies reduce dependence on ionizing radiation and contrast agents with their potential nephrotoxic risk. [3]

The safety of contrast agents in patients with impaired renal function has been reassessed based on new data. The risk of contrast-associated renal injury appears to be lower than previously thought, but prophylaxis protocols for patients with severely reduced glomerular filtration rate or acute kidney injury remain mandatory. For gadolinium-containing group II agents, the risk of systemic fibrosis is extremely low; however, the principle of reasonable adequacy applies. [4]

Table 1. Strengths and limitations of imaging techniques in nephrology

Method	Strengths	Main limitations	Typical tasks
Ultrasound with Doppler	Accessibility, absence of ionizing radiation, assessment of size, echogenicity, blood flow	Operator dependence, limitations in obesity and gases	Primary screening, chronic kidney disease, transplant, hydronephrosis, pre-intervention orientation. [5]
Contrast-enhanced ultrasound	Perfusion assessment without nephrotoxic contrast agents, clarification of foci	Not available everywhere, regulatory restrictions for children in some countries	Cystic and solid lesions, transplant, post-ablation assessment. [6]
Computed tomography	High accuracy in stones and acute conditions, uro-phase	Ionizing radiation, iodine contrast	Renal colic, trauma, high-risk hematuria, tumor staging. [7]
Magnetic resonance imaging	Soft tissue contrast, urography with and without contrast	Duration, motion artifacts, limitations with implants	Alternative for contraindications to iodine, staging, complex cysts. [8]
Radionuclide methods	Distributed function, drainage, scarring	Low spatial resolution	Diuretic renography, cortical scintigraphy, graft assessment. [9]

Indications and routes: hematuria

The approach to microhematuria is based on risk stratification, with ultrasound imaging and cystoscopy being preferred for low-risk patients when indicated, and computed tomography urography (CTU) being the preferred approach for high-risk patients. The 2025 update clarified the thresholds and role of urinary biomarkers, but the choice of method for the upper urinary tract remains risk-based. [10]

Computed tomography urography provides the best visualization of upper urothelial lesions, but requires weighing the benefits and risks, including ionizing radiation and iodine contrast. In cases of contraindications, alternatives such as magnetic resonance urography or a combined strategy of ultrasound and non-contrast computed tomography are appropriate. Guidelines from professional societies contain clear guidelines on this matter. [11]

In cases of macrohematuria, imaging is performed more frequently and quickly, with an emphasis on identifying urothelial pathology and stones. Anamnesis, exclusion of transient causes, and synchronization with endoscopic methods are important. Algorithms are aimed at excluding oncopathology without excessive radiation exposure. [12]

In pregnant women and patients with contrast limitations, the choice shifts toward ultrasound and, if necessary, magnetic resonance imaging without contrast. Such algorithm modifications are also present in current documents. [13]

Table 2. Hematuria

Clinical situation	Selection method	Alternative in case of contraindications
Low-risk microhematuria	Ultrasound of the kidneys and bladder	Delayed magnetic resonance urography according to indications. [14]
High-risk microhematuria	Computed tomography urography	Magnetic resonance urography or a sequence of ultrasound and non-contrast computed tomography. [15]
Macrohematuria	Comprehensive assessment: computed tomography urography plus cystoscopy	Magnetic resonance urography with contrast limitations. [16]

Renal colic and urolithiasis

Ultrasound is often used for initial evaluation to assess hydronephrosis and the distal ureter, but non-contrast-enhanced CT scanning is the "gold standard" for stone confirmation and density assessment. The 2025 European guidelines emphasize the sequence "ultrasound first, then CT scan if necessary," which allows for reduced radiation exposure without loss of accuracy. [17]

Low-dose CT protocols for colic have been validated and provide high sensitivity with significantly reduced doses. Additional capabilities, such as dual energy, help differentiate uric acid and calcium stones, which influences the choice of dissolution therapy. These solutions complement, rather than replace, clinical pain assessment and laboratory data. [18]

Post-treatment follow-up imaging depends on the stone removal method and clinical progression. Guidelines clarify the definition of "stone-free" and recommend using ultrasound and radiography of the kidneys and ureters for follow-up, with computed tomography (CT) scans as indicated. This approach minimizes cumulative dose while maintaining diagnostic confidence. [19]

In pregnant and young patients, ultrasound is preferred, while the decision to perform CT scanning is made on an individual basis. In cases of persistent pain, suspected complications, or atypical findings, the lower threshold for CT scanning is lowered. Strategies are outlined in tiered guidelines. [20]

Table 3. Acute nephrolithiasis colic: what and when to prescribe

Situation	1st line	2nd line
Typical colic, low risk	Ultrasound with Doppler	Low-dose non-contrast computed tomography when in doubt. [21]
Atypical pain, complications	Non-contrast computed tomography	Contrast phase as indicated, consultation with a urologist. [22]
Post-treatment follow-up	Ultrasound and radiography	Computed tomography as indicated. [23]

Infections: acute and complicated pyelonephritis

Most uncomplicated upper urinary tract infections do not require immediate imaging. It is indicated in severe cases, diabetes mellitus, immunodeficiency, pain unresponsive to therapy, and suspected abscess, obstruction, or emphysematous process. In these situations, computed tomography and, if contraindicated, magnetic resonance imaging are the methods of choice, while ultrasound is useful for the initial evaluation of hydronephrosis. [24]

Contrast-enhanced computed tomography (CT) reveals areas of hypoperfusion, abscesses, and complications, helping to decide on intervention. Magnetic resonance imaging (MRI) is useful in cases of limitations with iodinated contrast or in pregnant women, and contrast-enhanced ultrasound can serve as a functional alternative for assessing perfusion of the affected segment. The choice depends on the clinical objective and the availability of technology. [25]

In children, the strategy differs: an emphasis on dose reduction and assessment of the risk of scarring. Cortical scintigraphy with technetium-99m dimercaptosuccinic acid is performed for the diagnosis of acute pyelonephritis and late scar assessment under strict indications, while ultrasound remains the standard method. The relevant criteria are detailed in specialized documents. [26]

Monitoring of treatment efficacy is usually clinical, and repeat imaging is indicated if symptoms persist or abscess formation and obstruction are suspected. This pragmatic approach minimizes exposure and costs without compromising safety. [27]

Table 4. When imaging is needed for upper urinary tract infection

Clinical factor	Is the study indicated?	Preferred method
Severe course, sepsis, risk factor for complications	Yes, urgently	Contrast computed tomography, and if contraindicated, magnetic resonance imaging. [28]
The pain does not subside, abscess is suspected	Yes	Contrast-enhanced CT scan, drainage consideration. [29]
Pregnancy	Yes, if in doubt	Ultrasound, magnetic resonance imaging if indicated. [30]
Children with febrile infection	Yes, according to the indications	Ultrasound, cortical scintigraphy. [31]

Chronic kidney disease: what ultrasound reveals and why it's important

Ultrasound is the first-line diagnostic tool in chronic kidney disease: it measures cortex size and thickness, echogenicity, symmetry, the presence of cysts and dilatations of the renal pelvis, and blood flow parameters. These parameters correlate with the glomerular filtration rate and the stage of chronic kidney disease and help differentiate primary parenchymal disease from obstructive causes. Several recent studies have confirmed a significant association between echogenicity and cortex thickness and clinical renal function. [32]

Doppler ultrasonography is used for a preliminary assessment of vascular resistance and suspected renal artery stenosis or venous thrombosis, but definitive confirmation usually requires angiography with computed tomography or magnetic resonance imaging. When planning such studies, the risk of contrast is taken into account, and in complex cases, alternative non-contrast protocols are discussed. [33]

Magnetic resonance imaging and computed tomography are indicated when structure and complications need to be clarified, such as suspected tumors or complex cysts. In these situations, specific protocols and criteria are used to ensure clinically significant findings are not missed while avoiding unnecessary diagnostics. The approach depends on the clinical task and is complemented by laboratory data. [34]

In patients with chronic kidney disease, it is especially important to adhere to safety guidelines for contrast agents. Current guidelines describe glomerular filtration rate thresholds and preventive measures that allow for the safe use of modern agents when contrast is unavoidable. [35]

Table 5. Ultrasound markers of chronic kidney disease and clinical interpretation

Sign	Typical change	Clinical significance
Echogenicity of the parenchyma	Increases as function decreases	Correlates with glomerular filtration rate and stage of chronic kidney disease. [36]
Thickness of the bark	It's decreasing	Indicates chronic remodeling, helps in differentiating acute and chronic. [37]
Kidney sizes	They decrease and become asymmetrical	Supports the diagnosis of chronic kidney disease, helps identify scarring. [38]
Doppler resistance index	May increase	Non-specific, considered in conjunction with clinical symptoms. [39]

Kidney tumors and cysts: how to choose between ultrasound, computed tomography, magnetic resonance imaging, and contrast-enhanced ultrasound

Incidentally detected renal masses require a standardized approach. For unclear masses, computed tomography and magnetic resonance imaging with a multiphase program are considered the methods of choice, while contrast-enhanced ultrasound improves diagnostic accuracy in characterizing hypervascular lesions and in post-ablation follow-up. Updated guidelines emphasize that low-density homogeneous lesions can be interpreted as cysts without further escalation. [40]

For cystic lesions, the 2019 version of the Bosniak classification is used, which refines criteria and integrates magnetic resonance imaging data. This revision reduces overestimation of benign cases and helps more accurately identify lesions amenable to surgery, although interobserver variability remains. Validation studies in 2023–2025 confirmed improved stratification and a lower likelihood of aggressive progression in the IIF group. [41]

Contrast-enhanced ultrasound is useful for complex cysts and pretransplant evaluation of native kidneys, as well as for monitoring after local treatments. Recent reviews demonstrate comparable accuracy to contrast-enhanced computed tomography for a range of tasks, without nephrotoxicity or ionizing radiation. This is particularly important in patients with chronic kidney disease. [42]

The final choice of method is determined by the clinical question, the risk of contrast, and the availability of technology. When in doubt, a multidisciplinary discussion is preferable, taking into account potential treatment strategies and the need for biopsy. This approach minimizes unnecessary surgeries and repeat examinations. [43]

Table 6. Bosniak Classification version 2019 at a glance

Class	Main features	Orientation tactics
I-II	Simple cyst, thin walls, no significant enhancement	Observation without intervention. [44]
IIF	Minimal complicating features, no nodular enhancement	Dynamic observation, low risk of progression. [45]
III	Thickened partitions or reinforced wall	Consultation, usually surgery or ablation. [46]
IV	Nodular enhancement, high probability of malignancy	Surgical treatment or ablation as indicated. [47]

Kidney transplant: a fast and safe algorithm

Ultrasound with Doppler is the first-line method for graft dysfunction: it evaluates the size, perfusion, patency of vascular anastomoses and the ureter, and the presence of lymphoceles or hematomas. The resistive index is interpreted in the context of the clinical picture and time since transplantation. This noninvasive approach allows for the rapid identification of patients requiring further verification. [48]

Contrast-enhanced ultrasound is demonstrating a growing role in identifying perfusion abnormalities and early complications, and in questionable cases, it helps differentiate delayed function from acute rejection based on contrast dynamics. This is particularly valuable for patients at high risk of contrast-induced renal injury. [49]

Computed tomography and magnetic resonance imaging are used specifically to assess vascular complications, urological problems, and infections. The choice of contrast agent and protocol is based on a balance of safety and expected diagnostic value, as reflected in specialized recommendations. [50]

In the presence of ultrasound uncertainty and clinical suspicion, a low threshold for advanced imaging is justified, as early correction of the causes of reversible dysfunction directly impacts the prognosis of the transplant. Contemporary reviews emphasize this as the principle of triage. [51]

Table 7. Graft dysfunction: what to prescribe in the first 24-48 hours

Clinical task	Method	What are we looking for?
Quick triage	Ultrasound with Doppler	Vascular patency, perfusion, hydronephrosis, collections. [52]
Perfusion doubt	Contrast-enhanced ultrasound	Contrast arrival time in the cortical and medullary layers. [53]
Verification of vascular complications	Computed tomography angiography or magnetic resonance imaging angiography	Stenosis, thrombosis, urinary leakage. [54]

Radionuclide studies: when function is more important than morphology

Diuretic renography with technetium-99m mercaptoacetyltriglycine or diethylenetriaminepentaacetic acid is the standard for assessing obstruction and distributed function, particularly when the ultrasound image is ambiguous. A properly performed protocol yields surge-excretion curves and allows for quantitative assessment of the contribution of each kidney. Guidelines describe the choice of radiopharmaceutical and diuresis regimen. [55]

Cortical scintigraphy with technetium-99m dimercaptosuccinic acid is indicated for the verification of cortical defects and scarring following infections, particularly in pediatrics. In adults, it is prescribed based on individual indications when alternatives are less informative. This method complements, rather than replaces, ultrasound and tomography. [56]

The advantage of nuclear medicine is its low radiation exposure and high functional information content. This makes the methods invaluable in monitoring chronic conditions, where surgical decisions require objective quantitative criteria. Protocol selection and interpretation require standardized reporting. [57]

In children with suspected obstruction and reflux-associated scarring, specialized pediatric protocols are used, taking into account dosage, hydration, and sedation. Data from European guidelines systematize this approach. [58]

Table 8. Where radionuclide methods provide the greatest increase

Task	Method	Why him?
Obstruction and drainage	Diuretic renography	Quantitative drainage and contribution of each kidney. [59]
Bark scarring	Cortical scintigraphy	High sensitivity to cortical defects.[60]
Basic function before intervention	Diuretic renography	Measurement of relative function. [61]

Renovascular Conditions: When and How to Look for Stenosis

Renal artery stenosis is suspected in the presence of resistant hypertension, rapid loss of function, or size asymmetry. Doppler ultrasound is the screening method, but angiographic imaging techniques are crucial for planning intervention. The choice between computed tomography angiography and magnetic resonance imaging angiography depends on renal function, the presence of calcification, contraindications to contrast, and local examination. [62]

Computed tomography angiography provides high spatial resolution and better visualization of calcifications, while magnetic resonance angiography avoids iodine ionizing radiation and can be performed with or without contrast. In complex cases, digital subtraction angiography is considered as a diagnostic and therapeutic approach. The decision is made collaboratively. [63]

In patients with chronic kidney disease and diabetes mellitus, a careful assessment of the benefit-risk ratio of contrast agents is essential. In some cases, confirmation of the absence of hemodynamically significant stenosis based on Doppler and clinical examination is sufficient without escalation to angiography. This cautious approach is reflected in reviews of imaging in patients with renal failure. [64]

If there is a high clinical probability of renovascular hypertension, imaging is not delayed but is performed using risk-minimization protocols and careful hydration if necessary. This helps reduce the delay in making a decision about revascularization. [65]

Table 9. Renovascular hypertension: comparison of imaging options

Method	Pros	Cons	When is it preferable?
Doppler ultrasound	No contrast, accessibility	Operator dependence, difficulties with obesity and gases	Screening, dynamic observation. [66]
Computed tomography angiography	High resolution, sees calcification	Iodine contrast, ionizing radiation	Confirmation of stenosis, preoperative planning. [67]
Magnetic resonance imaging angiography	Without iodine, non-contrast techniques are possible	Implant limitations, sensitivity to movement	Alternative for reduced renal function. [68]

Contrast Safety: Current Thresholds and Practical Guidelines

Current guidelines on contrast media have reconsidered the risks of contrast-induced renal injury. Patients with severely reduced glomerular filtration rate or acute kidney injury are recommended to undergo preventive measures, primarily intravenous hydration with isotonic solutions, while the decision to administer iodine contrast is made on an individual basis. In many patients with moderate renal impairment, diagnostically necessary contrast is acceptable if protocols are followed. [69]

Differences in the risk of nephrogenic systemic fibrosis have been documented for gadolinium-containing drugs. Group II drugs are considered safe even with a significant reduction in glomerular filtration rate, subject to strict clinical selection, whereas the use of group I drugs should be avoided. Current international consensus documents from 2024 confirm these positions. [70]

Metformin is considered separately: in the case of a significant decrease in glomerular filtration rate, its temporary discontinuation and post-procedural assessment of renal function remain standard practice. For patients on dialysis, the timing of the study and dialysis is agreed upon individually. All these issues are regulated and should be taken into account when registering a patient. [71]

The risk of allergic-like reactions to contrast exists, but is manageable with trained personnel and first-line medications. Updated emergency care guidelines are available and recommended for use in all rooms where contrast-based examinations are performed. [72]

Table 10. Quick Contrast Safety Guidelines

Situation	Iodine contrast	Gadolinium-containing contrast
Glomerular filtration rate ≥ 45	Acceptable with standard measures	Let's say we prefer drugs from group II. [73]
Glomerular filtration rate 30-44	Individually, hydration, avoid repetitions	For strict indications, drugs of group II are allowed. [74]
Glomerular filtration rate < 30 or acute kidney injury	For vital indications with prophylaxis, consultation	Group II drug may be used for vital indications, discussion of risks and benefits. [75]

Organization of protocols and dose minimization

Low-dose protocols with iterative reconstruction and reduced tube voltage are used for computed tomography (CT) imaging of renal colic, significantly reducing the effective dose while maintaining high sensitivity for stones. This strategy is supported by current guidelines for urolithiasis. [76]

For magnetic resonance imaging, shortened protocols with rapid urography, non-contrast techniques for collecting system imaging, and motion suppression are preferred. When contrast is necessary, the safest gadolinium-containing agents are used at the lowest effective dose. This achieves a balance between informational content and safety. [77]

In ultrasound, quality depends significantly on preparation and standardization: patient positioning, proper transducer selection, depth and focus optimization, documentation of cortical size and thickness measurements, and maintaining standardized sections. For contrast-enhanced ultrasound, stable video loop recording and accurate analysis of contrast arrival time are essential. [78]

In radionuclide studies, success depends on proper hydration, precise timing of diuretic administration, and standardized post-processing with the generation of reproducible metrics. It is recommended that descriptive reports be replaced by structured templates. [79]

Table 11. Quick selection of a method for a clinical question

Clinical question	The first method	Escalation
Low-risk microhematuria	Ultrasound	Computed tomography urography in cases of increased risk. [80]
Acute colic	Ultrasound	Low-dose non-contrast computed tomography when in doubt. [81]
Suspected complicated pyelonephritis	Contrast-enhanced computed tomography	Magnetic resonance imaging with contrast limitations. [82]
Chronic kidney disease, without acute symptoms	Ultrasound	Magnetic resonance imaging or computed tomography as indicated. [83]
Incidental cyst	Bosniak computed tomography or magnetic resonance imaging	Contrast-enhanced ultrasound for clarification. [84]
Graft dysfunction	Ultrasound with Doppler	Contrast-enhanced ultrasound, angiography as indicated. [85]

Common pitfalls and how to avoid them

An ultrasound "pseudohydronephrotic" pattern is possible with an overfilled bladder and physiological dilation of the calyces, so repeat assessment after urination is essential. Similarly, distal stones may be masked by acoustic artifacts, requiring the addition of Doppler obstruction criteria to the examination. These simple techniques reduce the incidence of false positives. [86]

If a stone is suspected, it is important to be aware of pelvic phleboliths, which can mimic distal ureteral stones on radiographs and CT scans. The "comet tail" and perimeter calcification signs help differentiate phleboliths, and correlation with clinical and ultrasound findings reduces the risk of error. Practical reviews recommend systematic application of these criteria. [87]

When interpreting cysts, the 2019 criteria must be strictly followed: avoid vague terms, confirm true enhancement during subtraction, and avoid confusing calcifications with the nodular component. Validation studies show that adherence to the standard reduces unnecessary operations and improves reproducibility. [88]

When choosing a contrast agent, the most common mistake is refusing a clinically necessary study due to an overestimated risk. Current guidelines emphasize that, in most patients with moderately reduced glomerular filtration rate, contrast-enhanced studies are safe with appropriate prophylaxis, and this should be reflected in the informed consent. [89]

Conclusions

1. The choice of method in nephrology should be stratified by clinical scenario and risk, with ultrasound as the starting point and targeted escalation to computed tomography, magnetic resonance imaging, and radionuclide studies. This approach is enshrined in current guidelines. [90]
2. For upper urinary tract stones, the sequence "ultrasound followed by low-dose non-contrast CT if needed" optimizes the balance between accuracy and safety. [91]
3. For hematuria, the choice between ultrasound and computed tomography urography is determined by the level of risk, and the 2025 update clarified the stratification and role of biomarkers.[92]
4. Renal cysts are treated according to the 2019 Bosniak classification, using multiphase computed tomography or magnetic resonance imaging and contrast-enhanced ultrasound for complex cases. This reduces over- and undertreatment. [93]
5. Contrast safety has improved: Group II gadolinium preparations have an extremely low risk of nephrogenic systemic fibrosis, while individual assessment and hydration are key for iodine contrast agents. Do not delay clinically relevant examinations without compelling reasons. [94]