Kaura D R, Gray R R, Sadler D J, So C B, Saliken J C
Department of Diagnostic Imaging, Foothills Hospital, Calgary, Alta.
Can Assoc Radiol J. 1999 Oct;50(5):301-5.
Inferior vena cavae (IVC) can be of unusual geometry, often having odd shapes and being oriented (in long axes) away from the horizontal plane. However, after insertion of a filter, most IVC adopt a circular cross-section. The objective of this study was to determine if the IVC diameter estimated by frontal measurement (cavogram equivalent) reflects the true circular diameter of the infrarenal vena cava. Diameter estimation is clinically important in the correct selection of a filter, because mega cavae (diameter 28 mm or greater) require a particular filter.
The infrarenal IVC was measured on computed tomographic (CT) scans in 136 patients. The frontal diameter was recorded as that which would be obtained by a cavogram. Corrected circular diameter was obtained by mapping the circumference of each cross-section on CT to a straight line and calculating diameter from circumference.
The average frontal caval diameter was 20.5 (standard deviation 3.7) mm, whereas the average corrected circular diameter was 23.0 (standard deviation 3.4) mm. By frontal measurements, 6 IVC diameters were 28.0 mm or greater. Similarly, by corrected circular diameter, 6 IVC diameters were 28.0 mm or greater. However, of the 6 mega cavae extrapolated to cavograms, only 3 corresponded to mega cavae when corrected for true circular diameter. Yet, of the 6 mega cavae identified by corrected circular diameter measurement, 3 were not identified by frontal diameter assessment. Of the 6 patients with true mega cavae, 2 were being evaluated for right lower quadrant pain, 2 for lymphoma, 1 for a pelvic mass, and 1 for staging of a head and neck cancer.
Cavograms can over- or underestimate the true diameter of an IVC, and may thus lead to incorrect filter choice. It is recommended that a sonogram or CT scan be obtained to visualize the IVC in cases of suspected mega cava, and that true circular diameters be used for selection and placement of IVC filters.
下腔静脉（IVC）的形态可能异常，其形状通常怪异，长轴方向偏离水平面。然而，植入滤器后，大多数下腔静脉会呈现圆形横截面。本研究的目的是确定通过正面测量估计的下腔静脉直径（造影剂等效直径）是否反映肾下腔静脉的真实圆形直径。直径估计在正确选择滤器方面具有临床重要性，因为大腔静脉（直径28毫米或更大）需要特定的滤器。
对136例患者的计算机断层扫描（CT）图像上的肾下腔静脉进行测量。正面直径记录为造影检查所获得的直径。通过将CT上每个横截面的周长映射到一条直线上并根据周长计算直径来获得校正后的圆形直径。
下腔静脉的平均正面直径为20.5（标准差3.7）毫米，而平均校正后的圆形直径为23.0（标准差3.4）毫米。通过正面测量，有6个下腔静脉直径为28.0毫米或更大。同样，通过校正后的圆形直径测量，也有6个下腔静脉直径为大于28.0毫米。然而，在推断为造影检查的6个大腔静脉中，校正为真实圆形直径时只有3个对应于大腔静脉。然而，在通过校正后的圆形直径测量确定的6个大腔静脉中，有3个未通过正面直径评估识别出来。在6例真正的大腔静脉患者中，2例因右下腹疼痛接受评估，2例因淋巴瘤接受评估，1例因盆腔肿块接受评估，1例因头颈癌分期接受评估。
造影检查可能高估或低估下腔静脉的真实直径，从而可能导致滤器选择错误。建议在怀疑存在大腔静脉的情况下进行超声检查或CT扫描以观察下腔静脉，并使用真实的圆形直径来选择和放置下腔静脉滤器。
