01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Renal imaging is a sequence of decisions rather than a hierarchy in which CT is automatically superior. Ultrasound rapidly answers structural and drainage questions without radiation or nephrotoxic contrast. CT provides greater spatial detail and can identify ureteric calculi, haemorrhage, trauma, gas, masses and surrounding pathology. MRI offers selected soft-tissue and vascular characterisation, while nuclear medicine measures physiology that conventional cross-sectional imaging cannot directly quantify.
The principal safety error is a mismatch between urgency and modality. Waiting for routine ultrasound in septic obstruction delays source control; requesting contrast CT for uncomplicated stone detection adds a drug exposure without the intended benefit; ordering non-contrast CT for a suspected enhancing mass may leave the question unanswered. State the suspected diagnosis, side, renal function, pregnancy possibility, relevant operation and exact desired information on the request.
Reports require translation into action. Mild pelvic fullness may reflect a full bladder rather than clinically important obstruction, and severe-looking chronic hydronephrosis may be stable. Conversely, obstruction can exist without dilatation early in its course, with retroperitoneal disease or in profound dehydration. Discordance between a concerning phenotype and apparently reassuring imaging should trigger radiology or urology discussion, not automatic closure.
Key points
- Name the question before naming the scan: retention, hydronephrosis, calculus, renal mass, urothelial lesion, vascular event, differential function and cortical scarring require different techniques.
- Ultrasound is radiation-free and shows kidney size, parenchymal appearance, gross masses, cysts, collecting-system dilatation and bladder residual, but a normal study does not exclude early or intermittent obstruction.
- For adults with suspected renal colic, NICE recommends urgent low-dose non-contrast CT within 24 hours; pregnancy changes the first-line investigation to ultrasound.
- In acute kidney injury with no identified cause or a risk of obstruction, NICE recommends urgent urinary-tract ultrasound within 24 hours; suspected pyonephrosis shortens that target to six hours.
- Non-contrast CT KUB is optimised for stones, whereas multiphase contrast CT or CT urography evaluates enhancement, masses, vessels and urothelium. One protocol cannot answer every renal question.
- Iodinated contrast is not simply 'forbidden' in kidney impairment. Balance diagnostic benefit against current AKI, eGFR, volume status and alternatives with radiology, and do not postpone life-saving imaging on the basis of an unexamined creatinine alone.
- MAG3 renography assesses perfusion, drainage and relative renal function; DMSA imaging maps functioning renal cortex and scarring. Neither substitutes for rapid anatomical imaging in an unstable patient.
- MRI avoids ionising radiation and can characterise selected masses or vascular and soft-tissue abnormalities, but gadolinium choice in advanced kidney disease requires the local radiology policy.
- Kidney length and echogenicity support chronicity but are not histological diagnoses. Diabetes, infiltrative disease and acute swelling can produce large kidneys despite impaired function.
- Always read the report alongside the images available, clinical time course and biochemical trend, then confirm who will act on incidental masses, complex cysts, hydronephrosis or an unexpected solitary kidney.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Fever or hypothermia, rigors, flank pain, pyuria and AKI with hydronephrosis suggest pyonephrosis or an infected obstructed kidney. Antibiotics do not penetrate or drain a pressurised system reliably; decompression is source control.
Sudden severe loin-to-groin pain, restlessness, nausea and haematuria supports ureteric calculus, but aortic, ovarian, testicular, gastrointestinal and musculoskeletal emergencies remain alternatives. Pain severity does not predict stone size or kidney function.
Hesitancy, weak stream, incomplete emptying, suprapubic discomfort, overflow incontinence or a catheter that has stopped draining points toward retention. A bedside bladder scan can redirect the pathway before formal upper-tract imaging.
Incidental solid lesions, enhancement, septation, nodularity or a mass found during haematuria work-up needs protocolled characterisation and urological ownership. A simple cyst has a different pathway from an indeterminate complex lesion.
Small echogenic kidneys, cortical thinning, scarring, asymmetry or multiple cysts can support a chronic process. Normal-sized kidneys do not exclude CKD, and morphology alone cannot quantify reversible activity.
Sudden flank pain with haematuria, embolic risk, severe hypertension, recent instrumentation or significant trauma can indicate infarction, arterial injury or collecting-system disruption and usually needs urgent contrast-enhanced specialist imaging.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Renal tract ultrasoundFirst step - Why
- Assess kidney size and symmetry, echogenicity, cysts, larger masses, collecting-system dilatation and bladder volume without ionising radiation.
- Interpretation and limitations
- Hydronephrosis shows dilatation, not automatically current functional obstruction. Correlate with ureteric jets where reported, bladder filling, symptoms and creatinine; early obstruction and retroperitoneal encasement may be non-dilated.
- 02
Low-dose non-contrast CT KUB - Why
- Detect and localise most ureteric calculi and identify alternative abdominal causes of adult renal-colic symptoms.
- Interpretation and limitations
- Report stone size, level and upstream dilatation, but combine them with infection, pain control and renal function. The absence of contrast limits mass, vascular and urothelial assessment.
- 03
Contrast-enhanced CT or CT urography - Why
- Characterise enhancement, renal masses, infection, trauma, vessels or urothelial disease using a question-specific phase protocol.
- Interpretation and limitations
- Review current AKI or eGFR, previous contrast reaction and pregnancy status with radiology. CT urography is not interchangeable with stone CT, and an urgent diagnostic benefit may outweigh a theoretical renal risk.
- 04
MR urography or renal MRI - Why
- Characterise selected indeterminate masses, venous involvement, congenital anatomy or collecting systems when MRI offers a diagnostic or radiation advantage.
- Interpretation and limitations
- Movement, implants and availability matter. In severe renal impairment, a radiologist selects whether contrast is needed and which gadolinium agent is acceptable under current policy.
- 05
MAG3 diuretic renogram - Why
- Measure relative renal uptake and time-dependent tracer drainage when anatomical dilatation leaves functional obstruction uncertain.
- Interpretation and limitations
- Drainage curves depend on hydration, bladder emptying, renal function and protocol. Poor global function can make the study equivocal, so the nuclear medicine conclusion must be read with anatomy.
- 06
DMSA cortical scan - Why
- Map functioning renal cortex, relative contribution and focal scars, especially in selected paediatric or recurrent-infection pathways.
- Interpretation and limitations
- A cortical defect supports scar or acute parenchymal change depending on timing; it does not show ureteric drainage and is not an emergency test for acute sepsis.
- 07
Measured GFR with an exogenous filtration marker - Why
- Provide a more accurate global filtration measurement when estimation is inadequate for kidney donation, selected chemotherapy or major clinical decisions.
- Interpretation and limitations
- This is a planned physiological measurement rather than anatomical imaging. Results still require the correct body-size convention and the requesting protocol's threshold.
04Clinical next stepsHow the result changes management or prompts escalation.
01Septic obstructionImage and drain without delayFirst stepSepsis or systemic illness with flank pain, AKI, anuria, stone history or suspected upper-tract blockage.+
- 1Begin sepsis assessment, cultures, renal-adjusted antimicrobials, lactate and haemodynamic care while alerting urology; do not wait for imaging to start resuscitation.
- 2Arrange immediate ultrasound within the NICE six-hour target when pyonephrosis is suspected, or the fastest locally agreed cross-sectional imaging if ultrasound will not answer the urgent question.
- 3When obstruction is present, obtain urgent source control by ureteric stent or percutaneous nephrostomy according to anatomy, local expertise and patient stability, then monitor post-obstructive output.
02Renal colicConfirm stone and risk-stratifyAdult with suspected uncomplicated ureteric calculus and no immediate resuscitation requirement.+
- 1Check observations, urinalysis, pregnancy possibility, renal function, analgesic safety and red flags including infection, solitary kidney, anuria and uncontrolled symptoms.
- 2Arrange low-dose non-contrast CT within 24 hours for adults under NICE NG118; use ultrasound first in pregnancy and follow local paediatric imaging protocols.
- 3EscalationEscalate infection or threatened function immediately; otherwise use stone size, position, symptoms and local urology guidance to choose observation, medical support or intervention.
03Unexplained AKIExclude obstruction appropriatelyAcute kidney injury without a clear cause or with clinical risk of urinary obstruction.+
- 1Check catheter and bladder drainage, examine for retention and review pelvic, prostate, stone, malignancy and neurogenic risk before requesting imaging.
- 2Obtain urgent urinary-tract ultrasound within 24 hours as recommended by NICE, shortening the timeframe when pyonephrosis is possible.
- 3If the scan is negative but anuria, persistent deterioration or retroperitoneal disease keeps suspicion high, discuss repeat or CT imaging directly with radiology and urology.
04Incidental lesionCharacterise and assign ownershipCyst, solid mass, asymmetry or unexpected abnormality reported on an unrelated study.+
- 1Read the full report and compare prior imaging, renal function, haematuria and malignancy history rather than copying an isolated phrase into the notes.
- 2DefinitiveAsk radiology which renal mass or cyst protocol is required; do not use non-contrast stone CT as definitive characterisation of an enhancing lesion.
- 3Make the indicated urology, nephrology or surveillance referral and document a named clinician or service responsible for results and follow-up.
05Procedure and medicine safetyRelevant preparation, treatment and contraindications.
Iodinated intravascular contrast medium
Use the radiologist-selected agent and protocol only when it materially answers the question; volume and route are examination-specific rather than a bedside prescription.Check current AKI or renal function, hydration, prior hypersensitivity and relevant interacting treatment under local radiology policy. Do not withhold a time-critical diagnostic scan reflexively; discuss risk mitigation and alternatives.
Furosemide during MAG3 renography
Administer only within the local nuclear-medicine diuretic renogram protocol at its specified timing and dose.Volume depletion, hypotension, poor renal function, bladder distension and protocol timing can distort the curve. This diagnostic use is not treatment of an acutely obstructed infected kidney.
Gadolinium-based MRI contrast agent
Radiology selects whether enhancement is essential and chooses the locally approved agent and dose for the examination.Advanced CKD, AKI, pregnancy and previous reaction require individual radiology assessment. Risk differs between agents, so a generic class instruction is unsafe.
06Risks, monitoring and follow-upComplications, safety checks and further assessment.
- After contrast imaging in a high-risk or acutely unwell patient, continue clinical fluid assessment and repeat renal biochemistry when the result will change care; routine testing is not a substitute for initial risk review.
- Following relief of bilateral or severe unilateral obstruction, chart urine output, weight, blood pressure, sodium, potassium, magnesium and creatinine for post-obstructive diuresis.
- For an observed stone, track fever, pain, vomiting, urine passage and renal function, and ensure a defined urology or imaging follow-up plan rather than open-ended reassurance.
- For an incidental mass or complex cyst, record the recommended modality and interval, responsible team and whether the examination was actually completed.
- When serial ultrasound is used in CKD, compare measured size, asymmetry and collecting-system findings with eGFR and ACR trajectory rather than treating minor wording differences as progression.
- After nuclear imaging, interpret differential function and drainage with the nuclear medicine report, ensuring that equivocal curves are not converted into an unsupported diagnosis.
07Special situationsVariants, exceptions and circumstances that change the usual approach.
Dilatation is not obstruction
A compliant collecting system may remain wide after a blockage resolves, while an early acute blockage may not yet be visibly dilated. Function, timing and symptoms decide significance.
Protocol determines the answer
A CT KUB, portal-venous abdominal CT, renal mass CT and CT urogram acquire different phases. Supplying the clinical question prevents a technically adequate but diagnostically inadequate scan.
Physiology needs nuclear medicine
Ultrasound can show two unequal kidneys but cannot quantify each kidney's contribution. MAG3 or DMSA can supply relative function for selected surgical and congenital decisions.
Normal imaging can coexist with disease
Glomerulonephritis, early diabetic kidney disease and drug-related interstitial injury often have no specific structural abnormality. Urine, serology and sometimes biopsy answer those questions.
Contrast decisions are comparative
The relevant comparison is the renal and systemic risk of the proposed agent against the harm of missed pulmonary embolism, infarction, abscess, trauma or malignancy and the value of a non-contrast alternative.
Incidental findings need a handover
A radiology recommendation is not a follow-up system. Explicit ownership prevents an indeterminate renal lesion from disappearing between acute and primary care.
08Common pitfallsFrequent interpretation and management errors.
- 01
Equating hydronephrosis with definite active obstruction without clinical or functional correlation.
- 02
Using a normal ultrasound to dismiss ongoing anuria or high clinical suspicion of retroperitoneal obstruction.
- 03
Ordering a contrast CT urogram for straightforward calculus detection when low-dose non-contrast CT is the intended adult test.
- 04
Delaying drainage of an infected obstructed kidney while waiting for antibiotic response.
- 05
Describing every small echogenic kidney as irreversible without checking prior imaging and the treatable clinical phenotype.
- 06
Treating a nuclear drainage half-time as self-interpreting despite poor function, dehydration or a full bladder.
- 07
Failing to assign responsibility for a complex cyst or renal mass found incidentally.