Tracing the flow
Radiotracers are both a specific and accurate way of visualising the movement of urine within our body in real time.
IT is fairly easy to record things that move around us, but what about things that move inside of us?
Take urine, for instance – how can we visualise the flow of urine inside us in real time?
You might be asking: why is it necessary to do this in the first place?
Well, that is because it is one of the ways doctors assess how well the kidney is functioning.
Poor urine output might simply be due to poor fluid intake, but it may also be caused by kidneys that are not working well.
Our urinary system (aka “water works”) consist of the kidneys, ureter, urinary bladder and urethra.
The kidneys help the body excrete waste products and regulate the levels of necessary minerals within the body.
Meanwhile, the ureters and urethra, and the urinary bladder, function similarly to the pipes and holding tank of a water supply system respectively.
Most of us have two functioning kidneys.
A small minority might have only a single functioning kidney without even realising it, as we can live just as any other healthy person with just a single functioning kidney.
Having two functioning kidneys is like having a back-up in the case of possible failure.
This is why healthy people with two functioning kidneys can donate one to another person in need without much harm to themselves.
Kidney failure can occur in any age group, from newly-born babies up to the elderly.
In adults, the common causes of kidney failure are type 2 diabetes and hypertension (high blood pressure), whereas in children, congenital anomalies such as polycystic kidney and urinary obstruction are common.
Three techniques
We can image the kidneys using ultrasound, allowing us to see the size, shape and any structural anomalies of these organs.
However, this does not allow us to visualise how well the kidneys are functioning.
Two common imaging methods that allow us to do so are an intravenous urogram (IVU) and a CT (computed tomography) urogram.
In an IVU, a contrast agent is injected into the patient’s veins and its excretion is imaged using a series of sequential X-rays.
This allows doctors to determine if the kidneys are able to excrete the contrast agent and if there are any obstructions along the contrast agent’s path to the bladder.
In a CT urogram, the CT scanner is used instead of the X-ray machine.
There is one other method that has been used in visualising kidney function for decades, and is in fact widely utilised in both developed, as well as developing countries like Bangladesh and the Philippines.
Unfortunately, this method is neither widely known nor widely available in Malaysia.
It involves using radiotracers such as Tc-99m DTPA, Tc-99m MAG3, Tc-99m DMSA and Cr-57 EDTA, which are detected with a special camera.
The advantage of studying kidney function using radiotracers is that they specifically target the urinary system.
Doctors would be able to determine the function of each kidney quantitatively, as well as evaluate the flow of the urine dynamically.
The latter means that the imaging would be like a continuous video recording, compared to a series of snapshots, which is what IVUS and CT urograms provide.
This form of imaging has high sensitivity, and most importantly, does not harm the kidneys.
This is particularly important in patients already known to have some form of kidney problem.
Unlike the risk of potentially worsening kidney function that comes with using contrast agents in conventional imaging, radiotracers like Tc-99m DTPA are excreted almost entirely at the end of the process.
If one is worried about radiation from these nuclear medicine kidney studies, then it should be noted that patients are also exposed to a certain amount of radiation during a CT urogram.
In any case, it is a common misperception that all radioactive substances are harmful to the body.
In general, different radioactive substances emit different rays of varying energies, be it X-rays, gamma rays, beta rays or alpha rays.
Whether we like it or not, there is radiation everywhere, including in our own bodies!
The emitted radiation from radiotracers, i.e. gamma rays, is basically used for imaging purposes.
On the other hand, radioactive substances that emit beta and alpha rays, or high energy X-rays, are used mainly for treatment purposes.
It is important to understand that only low doses of radioactive substances are used as radiotracers in imaging, and the emitted radiation is of correspondingly low energy.
As a matter of fact, nuclear medicine studies can be performed for newborn babies, and in some cases, even pregnant women, without causing harm to the fetus.
In children
Obstruction of the urinary system can be due to various causes, including kidney stones, strictures and masses within the abdomen.
Early intervention is needed to prevent kidney problems from reaching irreversible kidney failure.
The timing of intervention is crucial and this is where nuclear medicine studies play an important role as they are not only safe, but also sensitive and accurate.
Some children incidentally found on ultrasound to have a single kidney at birth can be further investigated using Tc-99m DMSA for confirmation.
Although children can grow up healthily with a single kidney, it is important to ensure that they do indeed just have one kidney.
This is as some of these children may actually have two kidneys, with the “missing” one being an ectopic (malpositioned) kidney.
If left undiagnosed, this ectopic kidney could be mistaken for a tumour at a later time.
As Tc-99m DMSA is specific for the kidney, it will distribute to both kidneys, regardless of their position, thus confirming the misplaced mass as an ectopic kidney.
Another condition where nuclear medicine kidney studies are of use is in children with frequent urinary tract infections due to reflux at the insertion of the ureter to the urinary bladder (vesicoureteric reflux).
Such patients are prone to developing scarring in the kidney, which can lead to loss of kidney function.
Hence, identifying these children is important to initiate the appropriate treatment at the right time.
In cancer
In patients undergoing treatment for cancer, it is important that they have good kidney function before they start treatment as some of the chemotherapy drugs are nephrotoxic (toxic to the kidneys).
If the oncologist finds that the patient’s kidney function is becoming progressively worse, the chemotherapy drugs can be either reduced or stopped altogether.
This can be checked by either using Tc-99m DTPA, Tc-99m MAG3, or even Cr-57 EDTA, which uses serial blood test quantification.
The accuracy of such studies is important as doctors do not want to treat one disease while causing another!
Kidney failure would mean having to undergo haemodialysis or peritoneal dialysis for the rest of their life, unless the patient is fortunate enough to find a suitable donor for a kidney transplant.
In summary, nuclear medicine has an important role to play in the imaging of the urinary system.
Do not let fear of the unknown deter its use. Knowledge is empowering, and as the saying goes: “Action without knowledge is useless. Knowledge without action is futile!”
Dr Alex Khoo Cheen Hoe is a consultant nuclear medicine physician, Dr Teoh Boon Wei is a consultant urologist and Dr Tan Pek Yong is a consultant paediatrician. For more information, email starhealth@thestar.com. my. The information provided is for educational purposes only and should not be considered as medical advice. The Star does not give any warranty on accuracy, completeness, functionality, usefulness or other assurances as to the content appearing in this article. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.