11 The Urinary Tract Philip S. Cosgriff 11.1 Introduction 11.2 Anatomy Despite recent technical advances in computed to- The urinary tract comprises two kidneys, individ- mography, magnetic resonance, and ultrasound ually connected to the urinary bladder by ureters, imaging, nuclear medicine (NM) has main- and a urethra that connects the bladder to the ex- tained its crucial role in the functional assess- ternal genitalia. The kidneys are situated in the ment of the urinary tract, particularly the kidn- lumbar region at a depth of about 6 cm from the eys. surface of the back. They are positioned symmet- Indeed, nuclear medicine techniques maintain rically about the vertebral column, their upper “gold standard” status in the diagnosis of up- and lower poles lying between the 12th thoracic per urinary tract obstruction and pyelonephritic vertebra and the 3rd lumbar vertebra respectively. scarring secondary to urinary tract infection Each kidney is about 12 cm long, 6 cm wide, 3 cm (UTI). Importantly, all NM renal imaging tech- thick and weighs approximately 150 grams. niques also provide an estimate of relative A cross-section through the long axis of the renal function. Absolute renal function (e.g. kidney reveals that the renal parenchyma con- glomerular filtration rate (GFR) in ml/min) can sists of a pale outer region, the cortex, and an also be measured by blood sample-based ra- inner darker region, the medulla. Unlike the cor- dionuclide methods that are superior in ac- tex, which has a relatively homogeneous appear- curacy to routinely used indicators of renal ance, the medulla consists of radially striated cones function (e.g. serum creatinine). Finally, reno- called renal pyramids, the apices of which form graphic techniques also play important roles in papillae which project into the renal sinus and the diagnosis of renovascular hypertension, re- interface with a calyx (Figure 11.1). As the re- nal transplant complications, and some lower nal artery enters at the hilum it divides into sev- urinary tract disorders such as vesico-ureteric eral interlobar arteries which themselves branch reflux. to form arcuate (“bow-shaped”) arteries which The main imaging techniques in the inves- run along the boundary between medulla and tigation of the urinary tract are renography, cortex. Smaller interlobular arteries branch off which has numerous variants, and static DMSA at right angles from the arcuate arteries heading imaging. The techniques and indications for these outwards into the cortex. Finally, the branches tests will be considered, along with their strengths from the interlobular arteries, called afferent ar- and weaknesses. A test selection guide is given in terioles, supply blood to the glomerular capillar- Table 11.1. ies. 205 206 PRACTICAL NUCLEAR MEDICINE Table 11.1. A test selection guide for the urologist/nephrologist. See text for details. Prerequisite: renal ultrasound. When MAG3 is mentioned it is the clearly preferred radiopharmaceutical. However, 99mTc-DTPA can be used as an alternative if renal function is good Clinical question(s) Procedure to request Comment Need accurate estimate of DMSA renal scan In situations where accuracy is paramount (e.g. prior to relative renal function planned nephrectomy) or where renographic estimate is likely to be technically difficult (e.g. in certain infants). No information is provided on the status of the outflow tract Suspected renal scarring DMSA renal scan Estimate of relative renal function will be routinely provided. Suspected PUJ (and/or Diuresis MAG3 renogram Estimate of relative renal function will be routinely ureteric) obstruction provided Suspected renal scarring Diuresis MAG3 renogram Scarring may be apparent on the early MAG3 images. If and suspected upper not proceed to a DMSA scan tract obstruction Suspected VUR (or UUR in Basic MAG3 renogram followed by Estimate of relative renal function will be routinely duplicated systems) Indirect radionuclide provided cystogram Suspected VUR and Diuresis MAG3 renogram followed If indirect cystogram negative for reflux, repeat the test suspected upper tract by indirect cystogram without furosemide obstruction Need accurate estimate of GFR measurement (51Cr-EDTA) Could also use 99mTc-DTPA absolute GFR in ml/min Need estimate of both Basic DTPA renogram with GFR In principle, both can be measured by a single injection relative and absolute of 99mTc-DTPA. However, in children (or adults with GFR compromised renal function) it is preferable to inject 99mTc-MAG3 and 51Cr-EDTA simultaneously. Absolute GFR is measured by blood sampling PUJ, pelvi-ureteric junction; VUR, vesico-ureteric reflux; UUR, ureter-to-ureter reflux. The functional unit of the kidney, the nephron, the cortex and are known as cortical nephrons. consists of a glomerulus and its attached tubule In contrast, the juxtamedullary nephrons have (Figure 11.2). There are approximately 1 million glomeruli situated in the inner third of the cor- nephrons in each kidney. The glomerulus is a tight tex. These two populations of nephrons also dif- cluster of specialized blood capillaries which serve fer with respect to the length of Henle’s loop, to filter potential urinary excreta into the tubule, juxtamedullary nephrons having relatively long thereby forming a fluid called the glomerular fil- loopswhichextenddeepintothemedulla,whereas trate. Under normal circumstances proteins and cortical nephrons have short loops which ex- other large molecules are too large to pass through tend only a short distance into the medulla the filter and return to the systemic circulation (Figure 11.2). via the efferent arteriole, perirenal capillaries, re- nal venules and, finally, the renal vein. The tubule originates as a blind sac, known as Bowman’s cap- 11.3 Physiology sule, which leads in turn to the proximal convo- luted tubule, Henle’s loop, the distal convoluted The main function of the kidneys is to conserve tubule and, finally, the collecting tubule (or duct). substancesthatareessentialtolife,andtheyshould For functional reasons, the proximal convoluted therefore be regarded as regulatory organs that tubule and Henle’s loop are sometimes collectively help maintain the constancy of the extracellular referred to as the proximal tubule and the distal fluid (ECF), in terms of both volume and com- convoluted tubule and collecting duct as the distal position. The importance of this lies in the fact tubule (or distal nephron). that most body cells will only function properly if The majority of nephrons (approximately 85%) the concentration of solutes in the tissue fluid sur- have glomeruli situated in the outer two-thirds of rounding them is kept within quite narrow limits. Figure 11.1. Gross anatomy of the kidney. Figure 11.2. Anatomy of the nephron, showing main components (glomerulus and tubule) and associated blood vessels. The nephron illustrated has its glomerulus in the inner third of the cortex and is known as a juxtamedullary nephron, having a relatively long loop of Henle (see text). 207 208 PRACTICAL NUCLEAR MEDICINE The importance of the kidneys in human phys- After the blood has passed through the iology can be gauged from the staggering fact that glomerulus, it enters the efferent arteriole which a pair of organs accounting for about 3% of body leads to a second capillary network woven around weight consume about 20% of all the oxygen used the tubule (see Figure 11.2), an arrangement by the body at rest. Compared to other organs, a unique to the kidney. This provides the oppor- relatively high renal blood flow (RBF) of around tunity for selected materials (particularly salt and 1100ml/min(i.e.20%ofcardiacoutput)isthusre- water) to be reabsorbed from the glomerular fil- quired to (a) provide energy for the “blood cleans- trate into the blood via the tubular cells, a pro- ing” process, (b) provide basic oxygen and nutri- cess referred to as tubular reabsorption. The main ents for the renal cells, and (c) maintain a net pos- site of all reabsorption is the proximal convo- itive glomerular perfusion pressure (GPP) for fil- luted tubule, which accounts for about 90% of tration.Collectively,theglomerulihaveahugesur- filtered sodium and 75% of filtered water. In addi- face area, so only a modest GPP (about 8 mmHg) tion to being partially filtered by the glomerulus, is required for filtration. However, a drop in GPP certain substances are also transported from the of only 15% stops filtration altogether. The main- post-glomerular blood capillaries into the lumen tenance of renal blood flow (RBF) is therefore of the proximal tubule. This process, which can be critical and there is a unique mechanism to au- considered the reverse of tubular reabsorption, is toregulate it. Moreover, a “back-up” mechanism called tubular secretion. is triggered when a change occurs that cannot be In summary, the functions of the kidney can be corrected by autoregulation (see Section 11.5.7). explained by studying the functions of an individ- Renal plasma flow (RPF), referring to the compo- ual nephron, consisting of high pressure plasma nent of blood available for filtration, is around 600 filter (the glomerulus) and fine control device (the ml/min ((1 − hematocrit) × RBF). tubule). The first stage of the urine production pro- cess is glomerular filtration, whereby plasma water and its non-protein constituents (crystal- 11.4 Radiopharmaceuticals loids) are passively separated from blood cells and protein macromolecules (colloids). Although the The site of uptake of the main renal radiopharma- glomerulus is actually considerably more complex ceuticals is shown in Figure 11.3. than a simple sieve, it behaves as if it were a filter- The pharmokinetics of agents used in nuclear ing membrane containing pores of 7–10 nm di- medicine are largely determined by their degree of ameter, excluding to a large extent any substance protein binding in plasma (Table 11.2). The min- with a molecular weight of greater than about imal protein binding of 99mTc-DTPA and 51Cr- 60 000.