The ureter

generally be seen if it is at right-angles to the imaging plane in a subject w i th 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 w i th 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 w i th 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 w i th 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 w i th 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 acid (99mTc 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 k i dney 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 w i th 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

Fig. 5.49 Intravenous urogram. The right kidney has three major calyces. The left kidney has t wo major calyces, and a bifid pelvis. Note the course of the lower ureters. The distal part of the ureter passes behind the bladder.

1. Right upper-pole (major) calyx 2. Right middle (major) calyx 3. Right lower-pole (major) calyx 4. Left upper-pole (major) calyx 5. Left lower-pole (major) calyx 6. Minor calyx (infundibulum of) 7. Papilla 8. Infundibulum 9. Fornix 10. Bifid left renal pelvis 11. Right renal pelvis 12. Right ureter 13. Left ureter: vascular impression 14. Upper pole right kidney 15. Right psoas outline 16. Gas in body of stomach 17. Gas in transverse colon 18. Intravesical ureter

lateral to the IVC. On the left side it is crossed by the gonadal and left colic vessels and has jejunal loops anterior to it, and is crossed at the pelvic brim on the left side by the mesentery of the sigmoid colon and is posterior to this part of the colon.

The ureter enters the pelvis at the bifurcation of the common iliac artery anterior to the sacroiliac joint. It then lies on the lateral wall of the pelvis in front' of the iliac artery to a point just anterior to the ischial spine, where it turns forwards and medially to enter the bladder.

Close to the bladder in the male, the ureter passes above the seminal vesicle and is crossed by the vas deferens. In the female this part of the ureter is close to the lateral fornix of the vagina and 2.5 cm lateral to the cervix. It passes under the uterine artery in the base of the broad ligament.