Please note: Except for mechanism of uptake by kidneys, all other aspects are identical between DTPA and EC scans. Salient differences between DTPA and EC can be found in a dedicated section below.
Please note: An accompanying short animation video below explains the following concepts in a simplified manner.
The 2 kidneys form urine, which transiently pools in the funnel-shaped renal 'pelvis' (plural: pelves). A long and thin tubular structure called 'ureter' connects pelvis to the urinary bladder, which in turn stores and ejects urine through another short and thin tube called the 'urethra'.
Every healthy individual has 2 similar-sized kidneys. A 'nephron' is the functional unit of the kidneys. Each kidney has ~1 million (10 lakh) nephrons at birth. Each nephron consists of a C-shaped structure called the 'Bowman's capsule', which terminates in very slender tubular structures called 'renal tubules'. Urine forms for the first time in the Bowman's capsule. Journey of formed urine from blood is something like this: glomerulus → Bowman's capsule → renal tubule → collecting duct → calyx (plural: 'calyces') → pelvis → ureter → urinary bladder → urethra.
Arteries (called 'renal arteries') arising from the aorta progressively divide into smaller branches to form a tuft of very small arterioles called 'glomerulus' (plural: glomeruli). A very thin membrane called 'glomerular membrane' separates the space within the glomerulus from the space within the Bowman's capsule. An important feature of this glomerular membrane is that in healthy state it blocks the passage of cells and larger proteins, and only allows for unidirectional flow of filtered plasma and solutes (now technically considered to be 'urine').
A healthy human adult has about 5 litre of blood. Of this, ~45% consists of cells (called 'haematocrit'). Rest (~55%) is 'plasma'. In turn, plasma is mostly (~95%) water; large proteins and solutes (like glucose, minerals, amino acids and fatty acids) make up rest of the plasma. Incidentally, the entire volume (~5 litre) of blood circulates through the entire body in 1 min ('cardiac output').
Kidneys are commonly known to "filter blood", "remove waste from blood" and "produce urine". This is true, but this phrasing gives an impression that the kidneys 'suck out' impurities from the flowing blood and then eliminate only them leaving blood alone. However, this mechanism (called 'tubular secretion') is responsible for elimination of very few of the constituents of urine. Kidneys function in a counterintuitive manner. They filter out most of the plasma, and then reabsorb most of these constituents (including water and solutes) back, leaving out in the urine only small part of water and other constituents they 'want to' eliminate. This aspect of kidney function (reabsorbing what is needed as against eliminating only the unwanted) is very important to understand the principle of DTPA scan.
(For the purpose of this section, we will think of the 2 kidneys as a single functional unit.) Each minute ~5 L of blood circulates through the body, of which ~20% (1 L) reaches kidneys. Of this, 550 mL (~55%) is plasma ('renal plasma flow'), in turn one-fifth, of it is 'retained' by the kidneys following filtration ('glomerular filtration') across glomerular membrane into the glomeruli. Thus, each minute, ~110 mL of plasma is retained by the kidneys (the rate of production of this urine is known as glomerular filtration rate or 'GFR', and is measured in mL/min). This is nothing but 'urine'! In an entire day, the kidneys form ~160 L of urine. However, we do not pass that much urine, do we? In fact, we pass roughly just 1% of it everyday. And, that's because rest of the urine (including aforementioned solutes) is actually absorbed by the renal tubules (into the veins). So, in very simple words, out of 2,500 molecules of water that circulate in the blood, only 1 molecule of water is passed in the urine each minute.
DTPA is short for 'diethylenetriaminepentaacetic acid'. It is a molecule that is passively filtered across the glomerular membrane (just like water and other solutes). However, unlike most other substances in the urine, it is neither reabsorbed nor is it actively 'secreted'. In short, it tells the story of that single molecule of water (out of 2,500) that finally finds its way out through the urinary bladder.
Before injection, DTPA is combined with radioactive pertechnetate.
The passage of DTPA from blood to the urinary bladder is determined by 3 major factors: (a) blood supply, (b) kidneys' ability to form urine from plasma, and (c) ability of urine to reach from nephrons to the urinary bladder. So, DTPA scan can help us identify problems at all the 3 steps. The following video illustrates the normal flow of DTPA in plasma and urine.
DTPA scan gives us 4 important pieces of information:
The junction between the pelvis and ureter is knows as pelviureteric junction (PUJ) or ureteropelvic junction (UPJ). Narrowing, kinking, internal blockage or external compression at this site can cause obstruction in the flow of urine. This results in back pressure effect, i.e., the continuously forming urine compresses the kidney cortex against the fairly rigid capsule enclosing it.
Some of the structural manifestations of this back pressure include widening of the calyces and pelvis (called 'hydronephrosis') and apparent enlargement of the kidney. In more advanced cases, the cortex also becomes thinner. All these structural changes are termed 'obstructive uropathy'. Please note: while hydronephrosis is frequently an outcome of obstruction in the urinary tract, it can have other (possibly harmless) non-obstructive causes as well.
When (PUJ) obstruction has been severe or sufficiently long-standing, it causes permanent functional changes called 'obstructive nephropathy'. The uptake of blood (perfusion) of kidneys reduces first, which is then followed by reduced ability to form urine. Obstructive nephropathy is thought of as at least partly irreversible.
Please note: Obstruction in the urinary tract can be also at levels above (e.g., in a calyx because of kidney stone) the PUJ or below it (e.g., in the ureter). However, obstruction at such sites is less frequent.
Symptoms depend on severity and rate of development of obstruction. In fast-developing (acute) cases, the commonest symptom is pain in abdomen or back on the affected site. However, if the obstruction has been developing slowly (chronic), it could be without any accompanying symptoms, and is detected only incidentally (e.g., while undergoing some kind of abdominal scan).
Also, when PUJ obstruction is present only on one side (unilateral), the other unaffected kidney increases its function in compensatory fashion.
Most cases of PUJ obstruction are unilateral, so serum creatinine level does not rise because of compensation by the unaffected side.
PUJ obstruction can be likened to squeezing a pipe carrying water. If it is squeezed lightly, the flow of water might slow down, but does not completely stop; however, if it is squeezed very tightly, the water flow might completely stop.
There are no fixed definitions or criteria to definitively term an instance of PUJ obstruction as partial versus complete. In fact, PUJ obstruction is along a spectrum. The terms 'partial' and 'complete' are used to respectively convey mild versus severe degree of PUJ obstruction.
Partial obstruction implies that the lumen (elongated hollow space in a tubular structure) at the PUJ has become sufficiently narrow to slow down the flow of urine and/or to cause mild dilation of pelvis and calyces. Such structural changes can occur without true obstruction because of non-obstructive causes of hydronephrosis. In cases of 'partial' obstruction, we hope that either no treatment would be needed or that if any kind of treatment is attempted, it would result in complete normalization of the kidney function.
When a case is adjudged to be of complete obstruction, we fear that at least some portion of kidney function has been irreversibly lost. Complete obstruction is identified by reduced blood flow, reduced thickness of cortex, and reduced GFR and split function of the affected kidney. Treatment in such cases is aimed at partial recovery of kidney function, or to prevent its further decline through 'decompression' by widening the site of obstruction (e.g., by surgery or inserting hollow draining tubes into the pelvis). If the kidney function is damaged to a very severe degree, trying to relieve obstruction is considered futile, and in some instances the treating surgeon might consider its surgical removal ('nephrectomy').
Kindly note: Your treating doctor will likely use other techniques also to arrive at cause and severity of hydronephrosis / PUJ obstruction, and also to decide the next course of treatment.
'EC' stands for ethylene dicysteine. While DTPA enters into the renal tubules only through glomerular filtration, EC does so through tubular secretion as well (in addition to glomerular filtration). This results in much greater extraction of EC from blood (~50%) with each passage compared to DTPA (~20%). Clearance of injected drugs is conventionally calculated at the end of 3 min. So, for this calculation, ~50% of injected DTPA would be in the kidneys, whereas for EC this figure is ~87%.
Advantages of EC over DTPA
Advantages of DTPA over EC
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