The inferior vena cava

branches can be easily seen. Where it passes posterior to loops of small intestine it is often obscured by gas. However, by turning the subject on to their right side the length of the aorta can almost always be visualized from the left side of the anterior abdomen (scanning behind the loops of bowel that tend to fall anteriorly and to the right).

The aortic diameter is 2-3 cm depending on age; it diminishes as the aorta progresses caudally. The aorta can be distinguished from the IVC on ultrasound of the upper abdomen by its course near the diaphragm. The aorta remains posteriorly placed to reach the aortic hiatus posterior to the crura of the diaphragm. The IVC, on the other hand, passes ventrally to pierce the central tendon of the diaphragm, hugging the posterior aspect of the liver.

Computed tomography With current CT angiography techniques, using a contrast bolus given at a high rate (eg. 4-5 mL/s) and rapid scan acquisition in the arterial phase, even tiny branches of the aorta can be identified. Axially acquired data can be displayed in angiographic format.

Magnetic resonance imaging MR angiography using flow-sensitive techniques can visualize the aorta and its branches without contrast medium. Anterior branches are visible on sagittal views and lateral branches are best seen in planes parallel to their course.

Angiography (see Figs 5.12, 5.21, 5.23 and 6.11) The anatomy, as described above, is best imaged by aortography or selective arteriography of its branches.

THE INFERIOR VENA CAVA

The inferior vena cava (IVC) is formed by the union of the right and left common iliac veins at L5 vertebral level behind the right common iliac artery. It passes on the right of the aorta as far as T12 level, where it is separated from the aorta by the right crus of the diaphragm. The IVC pierces the diaphragm at T8 level, passes through the pericardium and enters the right atrium. An incomplete semilunar valve is found at its entry to the atrium. Otherwise the IVC has no valves.

The IVC lies on the bodies of the lumbar vertebrae. Part of the right adrenal gland and the right inferior phrenic, right adrenal, right renal and right lumbar arteries pass posterior to it. It is related anteriorly from below upwards, to coils of small intestine and the root of the mesentery, the third part of the duodenum, the head of the pancreas and the common bile duct, the first part of the duodenum and the epiploic foramen, w i th the portal vein anterior to it. It then passes in a deep groove in the liver (sometimes a tunnel) before piercing the diaphragm and entering the heart.

Tributaries of the inferior vena cava

These are as follows: • Third and fourth lumbar veins (upper two to the azygos vein, the f i f th to the ileolumbar vein); • Right gonadal vein; • Right renal vein;

Right adrenal vein; • Small veins from right and caudate lobes of liver; • Right, middle and left hepatic veins; • Right inferior phrenic vein (left drains to left adrenal vein); and • Left renal vein (which has already received the left gonadal and adrenal veins).

Embryology and variants

The IVC arises embryologically by a complex series of fusion of primitive veins. As a result of this its lower tributaries lie posterior to the aorta and its upper tributaries lie anterior to the aorta. Variation in this development can give rise to several congenital abnormalities of the IVC. It may, for example, be double below the renal veins owing to persistence of a primitive vein on the left side (persistent leftsided IVC). This may be associated w i th a failure of union of the common iliac veins. Occasionally, the IVC is continuous w i th a much-enlarged azygos vein, which carries all of its blood to the superior vena cava (SVC). The IVC may be left-sided as far as the renal veins and then cross over (behind the aorta as a continuation of the left renal vein) and continue as a right-sided IVC through the liver to the heart.

Radiology of the inferior vena cava

Chest radiography The shadow of the inferior vena cava can be identified as it pierces the right hemidiaphragm and enters the heart. On a lateral chest radiograph it identifies a hemidiaphragm as being the right-sided one.

Cavography The IVC can be opacified by contrast medium that is introduced either simultaneously into the dorsal pedal veins of both feet or via both femoral veins, or by a catheter that is introduced into the vessel itself. Some contrast may be introduced into its tributaries by the performance of a Valsalva manoeuvre. During placement of a caval filter to prevent pulmonary emboli it must be established that there is not a persistence of a left-sided cava, which would require placement of a second filter in that vessel. This can be established by inducing contrast retrogradely into the left common iliac vein (Valsalva manoeuvre helps). The renal veins should likewise be identified, as the filter should be placed below these.