Renal Cortical Imaging

7.5.1 Radiopharmaceutical

• [^99mTc]Tc-DMSA (di-mercapto-succinic acid) also known as Succimer.

7.5.2 Uptake mechanism / biology of the tracer

[^99mTc]Tc-DMSA binds mainly in the proximal tubule in the renal cortex for a prolonged time after injection and is suitable for static renal imaging to demonstrate renal mass or defects in the renal parenchyma. This agent is also called renal cortical agent. [^99mTc]Tc-DMSA retention in the renal parenchyma is about 50% of the injected amount. The uptake happens via both the blood pole and the urinary pole of the proximal tubular cells.

7.5.3 Indications

Static (planar or tomographic) renal imaging, so-called “Static renal scintigraphy” or “Cortical renal scintigraphy”, is indicated for the following purposes:

• morphological studies of renal cortex (abnormalities of size and shape, identification of reversible or irreversible damage due to renal infections, or, although very uncommon, renal infarction);
• individual kidney function (so-called Split or Differential Renal Function, SRF or DRF);
• location of ectopic kidney.

7.5.4 Contra-indications

Absolute:

• Hypersensitivity to the active substance (very rare), to any of the excipients (Stannous chloride dihydrate, Inositol, Ascorbic acid) or to any of the components of the labelled radiopharmaceutical.
• Pregnancy.

Relative:

• It is not recommended to interrupt breast feeding although an interruption of 4 h during which one meal is discarded can be advised to be on the safe side (1,2).
• Patients affected by tubular metabolic acidosis of Fanconi’s Syndrome have a lower [^99mTc]Tc-DMSA cortical retention.

Some chemical compounds or medicaments may affect the function of tested organs and influence the uptake of [^99mTc]Tc-DMSA.

• Ammonium chloride could substantially reduce renal uptake and increase hepatic uptake of [^99mTc]Tc-DMSA, and sodium bicarbonate as well as mannitol can reduce renal uptake of [^99mTc]Tc-DMSA. To avoid these influences, treatment with these chemical compounds should be interrupted where possible. Care should be taken to ensure the patient is adequately hydrated before scanning.
• Captopril: In patients with unilateral renal artery stenosis, uptake of [^99mTc]Tc-DMSA will be impaired in the affected kidney. This is usually reversible after discontinuation of captopril. This effect can be searched for in clinical setting.

7.5.5 Clinical performances

Images obtained with [^99mTc]Tc-DMSA  are clinically highly accurate in identifying small amounts of functioning renal tissue and for showing acquired renal damage, both reversible and reversible, with an overall accuracy slightly superior to CT or MRI and clearly superior to US [117–119].

The main clinical guidelines (published by America Academy of Pediatrics and by the Italian Society of Pediatric Nephrology) recommend its use for assessing the renal damage due to relapsing febrile Urinary Tract Infections [120].

7.5.6 Activities to administer

The suggested activity to administer is

• [^99mTc]Tc-DMSA: 70 MBq

In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 18.5 MBq.

7.5.7 Dosimetry

The effective dose for [^99mTc]Tc-DMSA is 8.8 µSv/MBq [3]. The organ with the highest absorbed dose is the kidneys: 180 µGy/MBq
The effective dose for [^99mTc]Tc-DMSA is 0.62 mSv per procedure

Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but  it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."

7.5.8 Interpretation criteria/major pitfalls

Normal images of the [^99mTc]Tc-DMSA show a high kidney/background contrast, with the renal pyramids hypoactive in respect to cortex and the hylar/pelvic region with no activity. The two kidneys normally have the upper pole slightly closer in respect to the lower pole. Abnormalities of shape can show renal tissue in many different and sometimes strangely looking fashion (Horseshoe kidney, S-shaped or L-shaped kidneys, superimposed, or flat-bread kidneys).

Reversible defects due to infection normally have a patchwork appearance and respect the renal outline. Irreversible defects frequently are triangle-shaped and alter the renal outline.

Presence of activity into the excretory system, due to high-grade dilatation, obstruction, reflux, poor hydration, or insufficient time elapsed between injection and scanning, could interfere with both visual and semi-quantitative analysis.

Low renal function could require a longer interval time between injection and scanning for obtaining a good kidney/background ratio and could hamper the calculation of SRF/DRF.

When the two kidneys tightly overlap or are fused, calculation of SRF/DRF can be considered useless.

7.5.9 Patient preparation

No special preparation. Fasting must be avoided, and care should be taken to ensure the patient is adequately hydrated before scanning.

7.5.10 Methods

Static images, in posterior, left and right posterior oblique 30°-35° projections are mandatory, 250 kcts each, 256x256 matrix (128x128 acceptable when zoom >2), field of view largely encompassing the two kidneys and reaching the bladder.

Anterior projection, with same parameters, is mandatory in case of abnormalities in number, shape and location, or whenever there is a suspicion or a different distance between each kidney and the posterior abdominal wall (because of space-occupying lesion or whichever cause).

SPECT can be performed; it is strongly recommended to do not increase neither the injected activity nor the sedation requests for this sole purpose. A typical SPECT protocol uses LEHRP collimator, a 128x128 matrix, 15 sec/projection, 360° elliptical orbit, 120 projections. Do not use SPECT/CT (the CT component increase the radiation burden and add nothing to the radionuclide image).

For calculating SRF/DRF, draw two tight ROIS around the kidneys and calculate the percentage of counts belonging to each kidney in respect to the sum of the two of them. Background subtraction is not necessary in patients with normal renal function.

In patients with a misplaced kidney, this calculation must be done using the geometric mean from anterior and posterior projection: