Renal Scan: DMSA

Static renal scintigraphy with technetium-99m dimercaptosuccinic acid (DMSA) images the functioning renal cortex. DMSA binds to the proximal tubular cells and about 40 to 50% of the dose is held in the cortex at 6 hours, with little in the urine.

The image shows the shape, size, position and function of the cortex of each kidney, and the relative function.

Adults receive 40 to 100 MBq by intravenous injection, and children a weight based dose with a minimum of about 15 to 20 MBq. The kit is used fresh, because old preparations give poor kidney uptake and high liver and bone uptake.

Images are taken 2 to 4 hours after injection (up to 24 hours in poor function). Posterior, posterior oblique and anterior views are collected, with at least 300 000 counts in each view, and SPECT of the kidneys is added in many centres.

The effective dose is about 0.7 to 1 mSv, with the kidney receiving most. The study takes 20 to 30 minutes of imaging and needs a wait after injection, during which the patient drinks fluids and empties the bladder.

Urinary tract infection affects 5 to 8% of febrile infants, and renal scarring develops in 10 to 15% after a first pyelonephritis, more with reflux, delay in treatment and recurrent infection.

Reflux nephropathy is a leading cause of hypertension and end stage kidney disease in young people. DMSA has a sensitivity of about 85 to 90% for acute pyelonephritis and is the most sensitive test for cortical scars.

Indications.

• Acute pyelonephritis in children, when the diagnosis is uncertain or the result changes management (scan within the first week).
• Renal scarring after urinary tract infection or reflux (scan 4 to 6 months after the infection).
• Follow up of children with vesicoureteric reflux and recurrent infection.
• Relative function of each kidney, as a check on a dynamic study or when the dynamic study is not suitable.
• Congenital anomalies: ectopic, horseshoe, crossed, duplex, dysplastic and multicystic kidneys.
• Renal masses and pseudotumours: a hypertrophied column has normal uptake, while a tumour or cyst shows no uptake.
• Renal trauma, infarction and segmental perfusion loss.
• Hypertension in children, to find scars.
• Localisation of functioning tissue before surgery, for example in an ectopic or small kidney.

Contraindications and precautions.

• Pregnancy: avoid unless essential. Breast feeding: a brief interruption as the local rule requires.
• Allergy is very rare.
• Drugs that change cortical uptake include ammonium chloride, sodium bicarbonate, acetazolamide, mannitol and recent iodinated contrast. Urine pH and hydration matter.
• Fresh kit: oxidised preparations give poor uptake. Check the quality and the time since preparation.
• Children under 4 to 5 years usually need swaddling or light sedation for the SPECT or static images.
• Hydration and bladder emptying before images reduce pelvic activity.
• Acute pyelonephritis must be treated. The scan should not delay antibiotics.

Preparation and technique.

No fasting is needed. After injection, the patient drinks fluid and passes urine often. A full bladder or a dilated pelvis with retained urine may hide the lower pole, and a repeat image after voiding or standing is taken.

Images are taken with a high resolution parallel hole collimator, or a pinhole collimator in infants for fine detail. Posterior and both posterior oblique views show the cortex, and an anterior view is added for ectopic, horseshoe or transplant kidneys. SPECT improves sensitivity for small defects.

Relative function is calculated from the counts in each kidney on the posterior view, with background subtraction and the geometric mean of anterior and posterior counts when depth differs. Scans are reported with the kidney length, outline, uptake pattern and relative function.

Normal appearances.

Each kidney is bean shaped with smooth outlines and uniform cortical uptake, a central area of lower activity (medulla and sinus), and no focal defects. The right kidney often lies slightly lower than the left. Foetal lobulation causes shallow indentations without a cortical defect.

Relative function is 45 to 55% for each kidney. Kidney length is in the normal range for age.

Interpretation.

• Acute pyelonephritis: one or more focal or diffuse areas of reduced uptake with a preserved outline and no loss of volume. Defects resolve in most patients over months, and those that persist become scars.
• Renal scars: focal loss of cortex with contour indentation and volume loss, most often at the poles. Generalised thinning gives a small kidney with reduced uptake. Scars are mild, moderate or severe by number and extent.
• Reflux nephropathy: polar scars with a clubbed calyx, often bilateral, and a small kidney in severe disease.
• Relative function: under 45% is abnormal, and under 40% is a significant loss. A fall on repeat studies shows progressive damage.
• Dysplasia or hypoplasia: a small kidney with low uptake and a smooth outline. Multicystic kidney: absent uptake.
• Duplex kidney: lower uptake of an obstructed or refluxing moiety, often the upper pole in obstruction and the lower pole in reflux scarring.
• Ectopic and horseshoe kidneys: abnormal position, with an isthmus joining the lower poles in the horseshoe form.
• Renal infarct: wedge shaped defect, or a global loss with a rim of cortex.
• Mass: a rounded defect. Tumours, cysts and abscesses have no uptake, while a hypertrophied column or a lump of normal tissue has normal uptake.
• Trauma: laceration defects, or absent uptake in a devascularised kidney.
• Chronic kidney disease: small kidneys with patchy uptake.

Critical values and findings needing immediate action.

• Acute pyelonephritis in a neonate or an infant with sepsis: intravenous antibiotics and renal ultrasound for obstruction.
• Pyonephrosis or infected obstructed kidney: urgent decompression.
• Absent function in a solitary kidney: urgent nephrology and urology review.
• Renal infarction or devascularisation after trauma: vascular and urology review.
• Bilateral scarring with raised blood pressure or falling renal function: nephrology review and treatment.

Limitations and artefacts.

• A scan taken too early after infection (under 4 to 6 months) overcalls scars, because defects from acute inflammation have not resolved.
• A single scan does not separate acute inflammation from old scars, so timing and clinical history matter.
• Poor kit quality, dehydration and drugs reduce kidney uptake and mimic reduced function.
• A dilated pelvis or calyx gives a photopenic area that mimics a cortical defect. Repeat after voiding or use SPECT.
• Overlap of the spleen, liver or stomach with the upper pole, and tilted kidneys, change counts and relative function.
• Foetal lobulation, a dromedary hump and a hypertrophied column look like defects.
• Static images give no information on drainage, so dynamic imaging is needed for obstruction.
• Relative function depends on depth and attenuation, and SPECT or the geometric mean helps.
• Ultrasound is less sensitive for scars and for acute pyelonephritis.
• Poor function (anuria, severe failure) gives an image of low quality.

Special groups.

• Infants and young children: the main group. The dose is weight based, and sedation is rarely needed with swaddling. A pinhole collimator or SPECT shows fine detail.
• Children with recurrent infection or reflux: scans at intervals to check for new scars.
• Adults with recurrent infection or chronic pyelonephritis: scars and unequal function.
• Kidney transplant: rarely used, for a focal lesion.
• Pregnant patients: avoid.
• Regions: urinary infection, schistosomiasis and tuberculosis cause scarring, and children present late. Cortical scanning is available in few centres, so ultrasound, urine culture, blood pressure and creatinine are used.

Follow up tests.

• Renal and bladder ultrasound.
• Micturating cystourethrogram or direct radionuclide cystography for reflux.
• Dynamic scan with a diuretic for obstruction.
• Urinalysis, urine culture, creatinine, cystatin C and estimated glomerular filtration rate.
• Blood pressure monitoring and urine protein measurement.
• CT or MRI urography for complex anatomy.
• Referral to paediatric nephrology, paediatric urology or nephrology.