Radiopharmaceuticals have two components:
- Radionuclide – atom with excess nuclear energy – emitted as gamma radiation which is detected by imaging (e.g Tc99, Ga68)
- Compound with desired physiological or pharmacological properties which provide useful information (e.g DMSA, MAG3, PSMA)
For renal functional imaging the compounds are treated differently by the kidney, but all bound to technetium-99 as the radionuclide (half-life technetium-99m is 6 hours)
DTPA
Diethylene triamine pentaacetic acid
Excreted by glomerular filtration – therefore measures GFR
Not very useful in renal failure as little glomerular filtration
Also provides information on obstruction
DMSA
Dimercaptosuccinic acid
Cleared by both filtration and secretion, and then bound in the tubules
Localises to renal cortex – little accumulation in papilla or medulla
Therefore great for identifying cortical scars or defects, and ectopic or aberrant kidneys
Can distinguish pseudotumour (normal uptake) from non-functioning malignant tumour
Gives split function, but no information on obstruction or drainage
MAG3
Mercaptoacetyl triglycine
Excreted by tubular secretion (90 – 95 % proximal tubule secretion)
Therefore, not influenced by GFR (and does not measure GFR)
Remaining 10 % or so excreted hepatobiliary
Logistics of MAG3:
- Well hydrated patient
- Consider placing a catheter if any reflux, stent, pelvic kidney, significant outlet obstruction
- Renal failure will cause some limitations
Phases of MAG3:
1.Renal perfusion
- First 60 seconds
- Immediate uptake of radionuclide by kidney – indicative of blood flow to kidney
2. Extraction / tubular concentration / parenchymal
- 10 seconds – 5 minutes
- Reflects renal uptake of tracer and parenchymal function
- Measurement of split function
3. Excretory phase
- 5 minutes +
- Excretion of tracer into the collecting system (tubular secretion)
- Usually tracer is in collecting system by 5 mins and bladder by 10 minutes
Renogram
Usually time to peak activity is 3 – 5 minutes.
T ½ is the time taken for half the tracer to leave collecting system
- < 10 minutes unobstructed
- > 20 minutes obstruction
- 10 – 20 minutes equivocal
Frusemide to encourage renal diuresis is classically given at t + 20 minutes – 20 minutes after the injection of MAG3.
Maximal effect of Lasix occurs 18 minutes after administration.
Therefore to maximise effect – can give Lasix at t – 15 minutes to have it working maximally during scan.
- Reduces equivocal results from 15 to 7 %.
For practical purposes – consider giving Lasix at same time as tracer t = 0.
Normal renogram
- Perfusion phase – rapid uptake
- Parenchymal/tubular phase – slower uptake, peaking at 3-5 minutes
- Excretory phase – loss of tracer as it is excreted
O’Reilly curves
1 – normal – rapid rise, peak, and rapid washout
2 – obstructed – rising with no fall, no response to Lasix
3a – hypotonic / non obstructed but dilated – rises, but responds quickly to diuretic
3b – equivocal – initial rise, plateaus and flattens with diuretic
4 – delayed compensation (Homsy) – rises, then responds to diuretic for a bit, but then continues to rise – “intermittent obstruction affected at high flow rates”
