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ANATOMY FOR DIAGNOSTIC IMAGING
generally be seen if it is at right-angles to the imaging plane in a subject with adequate fat. The renal substance is homogeneous on plain CT images. On MR, the intrinsic contrast between cortex and medulla is seen on T1- and T2-weighted images. On T1-weighted images the renal cortex has a slightly higher signal than the medulla. On T2-weighted images the renal cortex is slightly lower in signal than the medulla. Corticomedullary differentiation may be reduced in subjects who are dehydrated, as well as in renal disease. After intravenous contrast the cortex is first opacified and then the medulla and pyramids, making it possible to distinguish between them. On both CT and MRI three phases of enhancement can be appreciated: an arterial corticomedullary phase, where the cortex enhances strongly and contrast between cortex and medulla is greatest; a venous nephrographic phase, where the contrast is homogeneous throughout the kidney; and a delayed excretory phase, where contrast is seen in the collecting system. The pyramids are only seen from base to tip at the hilum on axial images and are cut at various degrees of obliquity in other slices. Intravenous contrast scans are best also to visualize the renal vessels. The arteries are best seen early in a contrast bolus (first 25 seconds); the veins are best seen after approximately 60 seconds. With MR, the renal arteries and veins can be imaged with or without intravenous contrast using flow-sensitive imaging sequences. Arteriography of the kidneys Direct arteriography allows assessment of vascular and other lesions of the kidneys, but is primarily used to facilitate interventional procedures such as renal artery angioplasty or stent placement. Aortography is performed prior to selective studies and identifies accessory renal arteries if present. These are found in more than 20% of arteriograms and are even commoner in horseshoe and ectopic kidneys. Aortography also establishes the presence and location of both kidneys. In selective studies, the upper pole is seen to be supplied by anterior and posterior branches of the renal artery, and the lower pole by an anterior branch. Because of the posterior angulation of the renal artery from the aorta, it is best seen by oblique views with the side of interest uppermost in the supine patient. Renal venography This is performed via the inferior vena cava. Although it is rarely used, it may be required to identify the location of a renin-producing tumour. The left adrenal and left gonad are also imaged via left renal venography because of the common drainage of veins from these organs on this side. The renal veins are seen to have valves. These are more common on the left side.
The right renal vein is multiple in 10% of venograms and receives the right gonadal vein in 6% of cases. The left renal vein is five times as long as the right. It is multiple in 14% of venograms, has tributaries which surround the aorta in 7%, and is retroaortic in 3.5%. Interventional procedures in the kidney These are performed via fluoroscopy, ultrasound, CT and angiography. Points of anatomical interest in these procedures include: • The posterior relationship of the diaphragm causes the kidney to move with respiration, and most punctures must be performed in suspended respiration. • The posterior relationship of the pleura makes an approach to the lower pole safer than to an upper pole. • A puncture of the kidney in its midlateral border is relatively bloodless because of the branching of the renal artery into three branches with separate areas of supply. Scintigraphy of the kidney This method is used primarily in the study of the physiology of the kidney. Technetium-labelled dimercaptosuccinic 99m acid ( Tc DMSA) static scans have some anatomical uses. These are used to establish how much of the fused part of horseshoe kidneys is functional renal tissue. Static scans are also useful in the evaluation of pseudotumours of the kidney due to hypertrophy of a column of Bertin. Technetiumlabelled diethylene triamine penta-acetic acid (DTPA) or mercaptoacetyltriglycine (MAG 3) are isotopes which are filtered into the urine and can be used to assess overall and split renal function as well as glomerular filtration rate, and also provide structural information regarding the collecting systems and bladder.
THE URETER
(see Fig. 5.49)
The ureters convey urine from the kidneys to the bladder. Each is 25-30 cm long and is described as having a pelvis and abdominal, pelvic and intravesical parts. The pelvis has been described with the kidney. The remainder of the ureter has a diameter of about 3 mm but is narrower at the following three sites: • The junction of the pelvis and ureter; • The pelvic brim; and • The intravesical ureter where it runs through the muscular bladder wall. The abdominal ureter passes on the medial edge of the psoas muscle, which separates it from the tips of the transverse processes. On the right side it is related to the second part of the duodenum and is crossed by the gonadal, the right colic and the ileocolic vessels and lies
