The evaluation of renal function represents an important part of the investigations performed in nuclear medicine. The methods utilized, either based on blood sampling or camera based, are accurate and relatively simple to perform. There exists, however, a proper indication for nuclear medicine examinations in case of deterioration of renal function. Renal failure can be either an acute or a chronic impairment of the excretory function and, in an overwhelming number of cases, can be diagnosed by a specific renal scintigraphy (Fig. 7).

When the process develops slowly under a chronic cause, or simply as a consequence of physiological decline with age, renal failure can be difficult to reveal and its real gravity is easily underestimated. The routine examination is serum creatinine, but it requires an important loss of functioning parenchyma to become abnormal and can remain within the normal range despite an inulin clearance of 60–80 % below normal values (Levey et al. 1991).

In this context, renal scintigraphy has a proper indication in the quantification of chronic renal impairment, thanks to the more reliable proportion existing between clearance estimated by nuclear medicine methods and renal function. The strength of nuclear medicine in renal impairment is its ability to assess renal function even in case of very low levels of clearance.

Differently from chronic renal failure, whose reversibility is considered unlikely and therapeutic intervention is mainly limited to the prevention of further progression, in acute renal failure, there exists the real possibility of reversibility and, hence, recovery of the function.

Nuclear medicine has no utility in the immediate assessment of acute renal failure, as the priority in the initial phases is the rapid recognition of this life-threatening condition and recovery of the reversible factors.

Acute renal failure is often multifactorial and its causes are usually classified as prerenal, renal and post-renal. Imaging in acute renal failure has two potential roles, diagnostic and prognostic.

Prerenal causes of acute failure are usually dehydration or shock, which when prolonged in time may precipitate in acute tubular necrosis (ATN). It is not exactly a necrosis and represents temporary and reversible cessation of renal function. ATN presents an initial phase with oliguria, followed by a progressive improvement of renal function over 7–14 days manifested with a polyuric phase (Hilson 1994). As the perfusion is generally maintained in the initial phase, the typical nuclear medicine appearance in dynamic scintigraphy would be well-vascularized kidneys, but with absent or very poor tracer concentration or excretion. With 99mTc-DTPA, we can see a good vascular pool in the first pass, but no tracer retention and no visualization of the collecting system. With tubular excreted agents (MAG3), the feature changes and dynamic scintigraphy reveals a progressive tracer uptake in the kidneys, with no evidence of excretion.

A prognostic estimation can be done on the basis of the data derived from tubular excreted agents. If tubular function is preserved, typically seen as progressive accumulation of the tracer, the prognosis is generally good. But even if tracer retention is not seen, it does not mean that there will not be any recovery (Hilson 1994).

When the cause of renal failure is a parenchymal disease, nuclear medicine role is somehow limited, as despite giving a value to renal impairment, it has nothing to offer in identifying the specific renal pathology. Size and function criteria are not exclusive for one disease or another. In addition, some parenchymal pathologies, such as glomerulonephritis, diabetes, amyloidosis or myeloma, may present normal-sized or even enlarged kidneys, with globally reduced function. In these cases and in case of an acute deterioration in chronic failure, nuclear medicine finds a proper indication in a prognostic vision. A reasonably well-perfused kidney suggests potential recovery, although there will be a return to normal function only if before the onset of the acute failure the kidneys were normal.

The less frequent cause of acute renal failure is connected to post-renal problems, almost exclusively due to obstruction, which is one of the reversible causes of renal failure (O’Reilly et al. 2001). The drainage problem must be either bilateral or occurring in a solitary kidney; otherwise, it is unlikely to reach renal impairment by a simple obstruction, as the contralateral kidney usually compensates. Although obstruction is generally detected by a dynamic scintigraphy, this is not always the case, as for severe obstruction there can be a potential renal functional exclusion. In these cases, diagnosis must be a combination of different imaging techniques for the clinical manifestation.

In clinical practice, one of the principal requests for a nuclear imaging study concerns urinary tract dilatation. The main problem to investigate in urinary tract dilatation is the presence of obstruction, which when left untreated can lead to the loss of renal function. In adults, urinary tract dilatation must always be considered pathologic, as the cause, in an overwhelming number of cases, is an obstruction, or less frequently, a consequence of congenital abnormalities, left undiagnosed. In children, urinary tract dilatation can be related to many different pathological or non-pathological entities, which can not only include exclusively obstruction but either vesicoureteric reflux (VUR) or other paraphysiological situations, although the most worrisome condition is an obstructive system.

Presentation and diagnosis are different whether the patient is an adult or a child. In adults, obstruction is more frequently referred to a chronic condition, leading to flank pain, renal lithiasis, UTI or a progressive renal deterioration. As an acute event, it is usually a result of urinary stones or, more rarely, a consequence of iatrogenic cause (intervention, compressive masses, etc.). In paediatric population, obstruction is a complex clinical entity, mainly determined by congenital defects concerning potentially any part of the urinary tract (pelviureteric junction, vesicoureteric junction, urethra, etc.). One of the most frequent causes of obstruction in children is related to a unilateral or bilateral hydronephrosis. The pathology can be diagnosed either in the intrauterine period, by means of prenatal ultrasonography, or during childhood. In the later case, clinical presentation may be similar to the adult symptoms, but in some cases, the pathology can be completely silent.

Nuclear medicine can make an important contribution in discriminating between obstructed and non-obstructed system, as well as assessing the functional and urodynamic results of a corrective surgery (O’Reilly et al. 1978, 2001). The procedure of choice is diuretic scintigraphy which, compared to other imaging techniques, is essential in distinguishing patients who can benefit from surgery from those who can be treated conservatively (Boubaker et al. 2006; O’Reilly 2003).

In diuretic scintigraphy, the main drainage patterns to take into account can be classified as normal, dilated non-obstructed, equivocal and obstructed pattern.

In adults, referring parameters to assess obstruction are time to activity peak (T _max) and half-time or time to obtain a 50 % of the tracer washout (T1/2).

For the interpretation of data, either a visual analysis or a quantitative evaluation can be used. The visual analysis takes into account the aspect of the sequential dynamic images, produced by the summarized frames, but also the trend of the washout curves, rather than the rate of washout by measuring the T1/2 after furosemide. Prompt clearance of the radiopharmaceutical from the renal pelvis with a T1/2 of less than 10 min (up to 15 min) is considered a normal response, while a T1/2 greater than 20 min suggests obstruction (Figs. 8 and 9). Values between 15 and 20 min are considered indeterminate and should be interpreted with the context of the examination (O’Reilly et al. 1978). Approximately 10–17 % of the studies will be difficult to interpret because of an indeterminate response (Taylor and Nally 1995).

Usually, in concordance with the above-described features, the corresponding renograms show indicative shapes. The washout curve for a non-obstructed kidney is more likely of concave shape, whereas the washout curve for an obstructed kidney is usually a rising or convex curve. All the other shapes, not included in obstructed or non-obstructed cases, can be considered equivocal.

Some conditions, however, may lead to stepwise or irregular curves depending on other conditions, such as large fluctuations in blood flow, VUR, double moiety or intermittent hydronephrosis. Intermittent hydronephrosis may occur as a delayed double peak curve and can be considered as a possible fifth drainage pattern. It occurs usually 10–15 min post-diuretic injection and presents with an initially rapid tracer elimination followed by a sudden cessation of the effect or reversion to a rising curve. This type of response is known as Homsy’s sign (Homsy et al. 1988) and is an indicator to an intermittent hydronephrosis. To fully clarify the pattern, an F-15 study is indicated, as in selected cases it will lead to normalization of equivocal studies (Taylor and Nally 1995).