What is a dynamic renogram scan?
Water-soluble substances are predominantly cleared from the body via the kidneys. The rate at which waste products are removed depends on kidney function.
Many conditions can affect this function. Without clearing waste products, the patient would deteriorate and eventually, one organ system after another begins to fail. We must be able to assess renal clearance or function to intervene and act in a timeous fashion.
Patients with renal failure also allow us to see when dialysis will become necessary. Patients with kidney transplants will know when the transplant starts to deteriorate and often tell us what is causing the deterioration.
The path of the tracer through the kidney also excludes any obstruction that can be severely painful and cause rapid renal deterioration if not corrected, e.g., kidney stone.
We use chemicals almost exclusively cleared by the kidneys in nuclear medicine and attach a radioactive isotope that emits gamma rays (similar to Xrays). These rays allow us to image the clearance process using the detectors of a particular machine called a gamma camera.
We can also measure the number of gamma-ray counts over time and plot the uptake, excretion, and clearance data on a graph to form an objective analysis.
But injecting different chemical tracers that are excreted in different ways by the kidney, we can infer information about various aspects of kidney function. DTPA is filtered and used to assess filtration function. MAG3 is secreted by the kidney tubular cells and used to evaluate tubular function (and indirectly assess filtration function). OIH combines both and allows us to determine the effective renal plasma flow parameter. These parameters are all vital for your doctor.
What can I expect to happen?
Many things can affect kidney function that can give spurious results. These include:
- A dehydrated state
- A full bladder
- Immature kidneys, e.g. in new-borns
- Poor injection of the tracer
- Loss of tracer into other body spaces
- Not following the same protocol between patients
- Not following the same protocol between successive studies in the same patient
As such, the dynamic renogram protocol in our unit has been standardised. The results cannot be compared to that of another unit unless they perform their studies in the same way. This often requires them to use the same camera and processing software.
Before the study, we will require you to drink plenty of water – at least 2-3 glasses.
An IV line may be placed before the study.
You will be asked to empty your bladder before imaging.
A bolus injection of the tracer will be given with you already lying on the imaging bed. Immediately we start acquiring a series of quick images to assess the blood flow to the kidney and then to assess the tracer progression through the kidney.
After 20 minutes, if excretion and clearance aren’t visually demonstrated from the kidney reservoirs, a diuretic will be injected, causing these reservoirs to contract and empty. This allows us to assess for obstruction – obstruction prevents the reservoirs from draining. Another run of images is acquired to image this process.
During this time, the bladder may fill again. Often this may prevent the reservoirs from emptying, thus simulating obstruction. Therefore, you will be asked to empty the bladder, and before and after images will be taken. If reflux of urine back into the kidneys is suspected, a set of images will be taken to visualise the effect of pushing the urine out on the bladder and whether that increased pressure causes reflux. This is mainly done in children.
A specialised test requires two separate renograms where the kidney blood vessels are suspected of being the cause of a patient’s high blood pressure. One scan acts as a baseline. The second requires us to see if there is a change in function if we inject a blood pressure agent that “knocks out” the effect causing the high blood pressure. Ultimately this vascular response is required to maintain blood flow to the kidneys. This condition is called renovascular hypertension.
How long will the whole scan take?
Usually about 1-3 hours (including reporting time). Repeat images may be required, especially where renovascular hypertension is suspected.
