Nuclear Medicine
CLINICAL DECISION SUPPORT
Betiatide (MAG3) is a compound that converts into mertiatide during labelling with Technetium-99m. The process requires heating. Mertiatide (Nephromag/Technescan MAG3) is also available and can be labelled directly, thereby obviating the need for heating. In either case, the same radiopharmaceutical ([99mTc]Tc-mertiatide) is injected.
MAG3 is a small molecule (263 g.mol-1) that binds strongly, albeit reversibly, to plasma proteins (80-90%). This reversible binding explains why only 5% is filtered in the glomeruli, the major portion (50%) is secreted in the proximal part of tubules by the organic anion transporters. This leads to a first-pass extraction rate of circa 55%, which is much higher than that of [99mTc]Tc-DTPA.
It also has a small hepatobiliary elimination (10%).
[99mTc]Tc-MAG3, like other tubular agents ([123I]OIH and [99mTc]Tc-bisicate), has a high extraction rate which leads to high image quality. In most cases, the tubular and glomerular functions are parallel, so the split renal function can be assessed with [99mTc]Tc-MAG3 as well as with [99mTc]Tc-DTPA. Exceptions are:
In either case, [99mTc]Tc-MAG3 secretion can be preserved even when glomerular filtration is severely decreased.
When renal function is low, the high extraction makes [99mTc]Tc-MAG3 even more efficient than [99mTc]Tc-DTPA to assess split function.
The suggested activity to administer is:
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 15 MBq.
The effective dose for [99mTc]Tc-MAG3is 7.0 µSv/MBq in patients with normal renal function, for patients with abnormal renal function: 6.1 µSv/MBq [3]. In both cases the organ with the highest absorbed dose is the urinary bladder: 110 µGy/MBq.
The effective dose for [99mTc]Tc-MAG3is: 0.53 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."
After basic, visual assessment, the relative renal function should be measured by tracing renal ROIs and background ROIs (ideally perirenal rings) to get a corrected renogram. The best quantifying method is the Rutland-Patlak graph which requires placing the heart into the field of view. Alternatively, the area method (taken between 1- and 2-min post-injection, and in any case well before the peak activity has been reached) can be used.
In patients with low renal function, the liver activity must be considered when assessing the right kidney function.
Care should be taken to ensure the patient is adequately hydrated before scanning
If diuresis renography is considered, prior hydration should be ensured, per os if possible, with circa 7 mL/kg body weight. When i.v. infusion must be used, a sodium-low solute must be used to ensure rapid diuresis, such as 5% glucose solution In infants, however, pure glucose solution is dangerous, and mixed saline-glucose solution is recommended [112].
Probenecid is taken by the same organic ion transporters with possible competition. Discontinuation should be considered.
Further information can be found in in the published literature [118,119].