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Original Research |
1 Department of Radiology, Institute of Radiation Medicine, Seoul National
University College of Medicine, Seoul National University Medical Research
Center, Clinical Research Institute, Seoul National University Hospital, 28
Yongon-dong, Chongno-gu, Seoul 110-744, Republic of Korea.
2 Department of General Surgery, Seoul National University College of Medicine,
Seoul National University Medical Research Center, Clinical Research
Institute, Seoul National University Hospital, Seoul, Korea.
Received November 28, 2007;
accepted after revision April 20, 2008.
Address correspondence to J. W. Chung
(chungjw{at}radcom.snu.ac.kr).
Abstract
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SUBJECTS AND METHODS. One hundred consecutively registered patients with varicose veins underwent CT venography. The image quality of overall 3D volume rendering was rated, and the absolute attenuation of each component at the level of the knee was measured. Factors that affected visualization of varicose veins were identified. For comparison analysis, 50 of the 100 patients also underwent Doppler sonography, and saphenous vein size and morphologic features on CT were compared with the functional information from Doppler sonography.
RESULTS. The overall quality of 3D volume-rendered images in the visualization of varicose veins was excellent in 76% of patients, fair in 21%, and poor in 3%. The entire length of the great saphenous vein (GSV) was visualized with CT venography in 99.5% of 200 GSVs. The quality of 3D volume-rendered images was better when a thick subcutaneous layer, no skin changes, and no subcutaneous edema were present. Size of the GSV determined whether findings at CT venography and Doppler sonography correlated well, the linear regression coefficient being 0.72. At CT venography, the mean diameter of GSVs exhibiting insufficiency on duplex sonography was 7.0 mm, and the mean diameter of GSVs exhibiting competence on duplex sonography was 4.9 mm (p < 0.001). Prediction of GSV insufficiency with CT venography had a sensitivity of 98.2% and a specificity of 83.3%.
CONCLUSION. CT venography has adequate image quality for evaluation of the venous system of the lower extremities.
Keywords: 3D image • CT • CT venography • varicose veins
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CT of the entire lower extremity vascular system and volume-rendered reconstruction have become feasible [8]. In addition, MDCT has sufficient spatial resolution to show superficial veins 2 mm in diameter and smaller, thus 3D visualization of the entire superficial venous system is possible. Three-dimensional CT venographic images also can be used as complementary road maps for preoperative mapping of varicose veins. In this study, we evaluated the imaging quality of CT venography for clinical evaluation of the lower extremity superficial venous system. We also compared CT and duplex sonographic findings about varicose veins.
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MDCT Venography Technique
Immediately before CT, patients were asked to remove their underwear,
stockings, and socks, which might have compressed superficial veins. On the CT
table, extra support was provided for the buttocks and heels to prevent direct
contact between the lower extremities and the CT table. CT examinations were
performed with an 8-MDCT (LightSpeed Ultra, GE Healthcare) or 16-MDCT
(Sensation 16, Siemens Medical Solutions) scanner. An 18- or 20-gauge catheter
was placed into an antecubital vein, and 2 mL/kg of nonionic contrast material
(iopromide, Ultravist 370, Bayer HealthCare) was injected with a power
injector (Envision CT, Medrad) at a rate of 2.5 mL/s. We used a scan delay
time of 3 minutes after initiating injection of the contrast material for CT
veno graphy to enhance deep and superficial veins, including varicose veins
and perforating veins.
The scanning parameters for CT venography were as follows. For the 8-MDCT scanner, the values were collimation, 1.25 x 8 mm; pitch, 1.35; slice thickness, 1.25 mm; reconstruction interval, 1 mm; x-ray tube voltage, 120 kVp; effective tube current, 150 mAs. For the 16-MDCT scanner, the values were detector collimation, 0.75 x 16 mm; pitch, 1.5; slice thickness, 1 mm; reconstruction interval, 0.7 mm; x-ray tube voltage, 120 kVp; effective tube current, 150 mAs. The scan range was the iliac crest to the end of the feet.
Postprocessing of Volume Data
All thin-section axial images were transferred to a workstation running
PC-based 3D reconstruction software (Rapidia, Infinitt). Individual volume
data were loaded into the 3D program, and two experienced radiologists
performed the 3D reconstruction (Fig.
1A,
1B,
1C). The techniques used for 3D
reconstruction of the superficial venous system were smoothing of axial images
for noise reduction and interactive volume rendering to display surface venous
channels and varicose veins with muscle planes (Fig.
2A,
2B). For the perforator survey,
we used interactively rotating volume-rendered images and corresponding axial,
sagittal, and coronal images. To evaluate the saphenofemoral and
saphenopopliteal junctions, we used axial images and maximum intensity
projection and multiplanar reformatted images in the interactive image
plane.
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Image Analysis
Evaluation of overall image quality was followed by analysis of the factors
believed to affect image quality.
Assessment of overall quality of 3D volume-rendered images—Two radiologists evaluated the quality of volume-rendered images using a three-level grading system (excellent, fair, or poor), and grades were reached by consensus. Excellent meant volume-rendered images vividly showed all varicose vein channels and perforators. Fair meant some of the channels and perforators were not clear on volume-rendered images but that the quality of volume-rendered images was sufficient for detection of channels and perforators. Poor quality meant the volume-rendered images were unsuitable for detection of channels and perforators and that axial images were needed to conduct the evaluation.
Enhancement of deep and superficial veins and varicose veins—To quantify vein enhancement and attenuation differences in relation to sur rounding tissue, we measured the mean attenuation of each compo nent at the level of the knee. The mean attenua tion of the great saphenous vein (GSV), small saphenous vein, popliteal vein, and varicose veins was measured within a circular region of interest. The attenuation of muscle and the popliteal artery was measured for comparative purposes. Venous en hancement was assessed for homogeneity (homo geneous vs heterogeneous), layering, throm bi, obliteration, filling defect, and absence of seg mental opacification.
Factors affecting visualization of varicose veins—An analysis was performed to identify factors affecting image quality. The factors examined included severity of varicosity and presence of abnormal skin thickening, subcutaneous edema, and thickening of the subcutaneous layer. The severity of a varicose vein was divided into three categories. Varicosities involving the calf and thigh were ranked severe; varicosities involving more than one half of the surface of the calf were ranked moderate; and varicosities involving less than one half of the surface of the calf were ranked mild. Abnormal skin thickening was defined as skin thickening of more than 1 mm. Subcutaneous edema was defined as subcutaneous attenuation greater than that of the normal subcutaneous fat layer. Thickening of the subcutaneous layer was measured on axial CT images at the level of the knee at the medial aspect of the calf.
Other factors used for statistical analysis were mean attenuation of the GSV, homogeneous or inhomogeneous enhancement pattern of the GSV, mean attenuation of the popliteal artery, mean attenuation of the popliteal vein, mean attenuation of muscle, mean attenuation of the varicose vein, and the difference between the maximum and minimum attenuations of the varicose vein. Multiple regression testing was performed with the GraphPad InStat program (version 3.05, GraphPad Software).
Correlation Between CT Venography and Duplex Sonography
Among 100 patients who underwent consecutive CT venographic examinations,
50 patients (22 men, 28 women; mean age, 56.5 years; range, 17–83 years)
also underwent Doppler sonography. Doppler sonography was requested by
vascular surgeons for preoperative evaluation of varicose veins. These 50
patients were included in the comparative analysis. One radiologist performed
all duplex sonographic studies using a color Doppler sonograph (Accuvix XG,
Medison) and a 7.5-MHz linear probe. The average time between CT venography
and Doppler sonography was 0.6 days. Instead of adopting the classic standing
position, patients were positioned on a tilt table 10–20° from
vertical and were asked to support the upper body on the table but were
requested to rest the leg being examined on the table. The GSV was measured at
the most proximal segment of the GSV that was tubular under minimum pressure
to avoid underestimation of the diameter due to compression by the ultrasound
probe. The GSV and small saphenous vein were measured with CT at the site
evaluated with duplex sonography. Sizes measured with CT venography and duplex
sonography were compared, and the sizes of insufficient and competent
saphenous veins were analyzed.
The presence of valvar insufficiency in GSVs and small saphenous veins was evaluated primarily. Reflux flow for more than 500 milliseconds after the Valsalva maneuver or distal manual compression and release was considered to indicate the presence of saphenofemoral or saphenopopliteal insufficiency [9]. All CT findings, such as focal ectasia, diffuse dilatation more than 6 mm, asymmetry, and tortuosity, directly connect ed to varicosity of GSVs and small saphenous veins that were confirmed insufficient at Doppler sonography were analyzed on volume-rendered images. The incidence of these CT finding in cases of GSV insufficiency was calculated. Using these CT findings, two radiologists separately predicted the presence of GSV or small saphenous vein insufficiency. With duplex sonography as the reference standard, the sensitivity and specificity of CT venography in the prediction of insufficiency of the GSV and small saphenous vein were calculated.
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Enhancement of Deep and Superficial Veins and Varicose Veins
The mean attenuation of vessels and muscle is summarized in
Table 1; enhancement
homogeneities are summarized in Table
2. In the 151 limbs with varicose veins, 111 varicose veins had
homogeneous enhancement throughout varicosities, 14 had heterogeneously
enhanced segments, 11 exhibited layering of opacified and unopacified blood in
dilated varicose veins, four had a filling defect with varicose veins, and 11
had no segmental opacification.
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Factors Affecting Varicose Vein Visualization
Varicosity was ranked severe in 17 limbs, moderate in 63, and mild in 68;
the other 51 limbs did not have varicosities. Abnormal skin thickening was
found in 18 limbs and subcutaneous edema in 18 limbs. Three-dimensional
volume-rendered images of excellent overall quality showed that limbs with
varicose veins had a thicker subcutaneous layer (p < 0.0036), no
skin changes (p < 0.026), and no subcutaneous edema (p
< 0.0001) compared with limbs with varicose veins depicted on 3D
volume-rendered images of fair or poor overall quality. Severity of
varicosity, degree of enhancement, and homogeneity were found to have no
significant correlation with overall image quality.
Correlation with Doppler Sonographic Findings
CT venography was performed with the patient in the supine position, and
sonography was performed with the patient in a 10–20° from vertical
semierect position. Nevertheless, the sizes of the GSV determined with the two
techniques correlated well, having a linear regression coefficient of 0.72
without any statistically significant discrepancy
(Fig. 3). On CT venography, the
mean diameter of GSVs found insufficient at Doppler sonography was 7.0 mm and
the mean diameter of GSVs found competent was 4.9 mm (p <
0.001).
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There were 15 insufficient small saphenous veins. Focal ectasia (62.3%) and diffuse dilatation (54.1%) were found in insufficient small saphenous veins, but tortuosity was not, probably because of the smaller interfascial spaces in which these veins are located. Seven of the 15 insufficient small saphenous veins had normal morphologic features. CT venography had a sensitivity of 53.3% (eight of 15 cases) and a specificity of 94.9% (56 of 59 cases) in prediction of insufficiency of the small saphenous vein.
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The degree of venous enhancement on CT venography is an important consideration. The bolus-tracking method is preferred for CT during peak enhancement of the arterial system. In the case of the venous system, however, determining the period to peak enhancement is difficult owing to the different circulation times in the venous system. Venous return of contrast medium from some veins may not have started before return from other veins is effectively complete. On occasion, direct CT venography has been performed with direct injection of diluted contrast medium into a targeted vein [12]. This direct CT venographic method may work for central veins, but we have found that in many cases not all venous channels in the lower extremity are filled, especially varicose veins.
Yankelevitz et al. [13] drew time–density curves of the common femoral veins of 20 patients after CT pulmonary angiography and reported that peak enhancement of the common femoral vein occurred a mean of approximately 2 minutes after administration of the contrast medium. In our study, for CT venography we used a 3-minute delay to obtain homogeneous enhancement of deep and varicose veins. We found that at 3 minutes, the circulation was in equilibrium and all veins were almost equally enhanced. However, degrees of enhancement were not as high as those achieved at CT arteriography, for which attenuation is usually more than 300 HU. Nevertheless, at CT venography, the average attenuation of enhanced veins in the lower extremity was 140–150 HU. This attenuation was sufficient to allow us to differentiate targeted veins from surrounding muscle and subcutaneous fatty tissue, which have an attenuation of 70 HU and less than 0 HU, respectively. In the case of a patient with severely dilated varicose veins, some segments of the varicose vein remained in an unopacified state 3 minutes after contrast injection (Table 3). In cases such as this, compression on or rubbing of the varicose vein after contrast injection and before the start of CT acquisition is helpful for opacifying the entire varicose vein.
Varicose veins are located in the subcutaneous layer, and surrounding fatty tissue contrasts against blood-filled veins. Three-dimensional volume-rendered images obtained without contrast enhancement can be used to visualize varicose veins [14]; the deep venous system and perforators can be visualized on CT only after contrast enhancement. Even though evaluation of the deep venous system was not one of the objectives of this study, CT venography is an excellent means of evaluating this system. Calf veins especially have many tributaries at both superficial and deep locations. Imaging of a deep calf vein with Doppler sonography is not easy because of the deep location and small caliber of the vein and the sheer number of vessels. Ascending venography can show both the superficial and deep venous systems in the calf, but it is difficult to differentiate the vessels and to detect abnormal veins among abundant dilated varicose veins, especially in severe cases of varicose veins, because ascending venography has the intrinsic limitation of projectional imaging.
Contrast-enhanced CT venography can show the calf veins with clear anatomic landmarks such as muscle and bone, and use of this technique facilitates detection of abnormalities of the deep calf veins. The presence of deep venous thrombosis is an important consideration during the preoperative evaluation of patients with varicose veins, and the diagnostic accuracy of CT in the detection of deep venous thrombosis has been established [11, 15]. CT venography is sensitive in the depiction of deep venous thrombosis, and one of its strong points is that it can show chronic changes. For example, after reorganization and recanalization in cases of deep venous thrombosis, affected veins are narrowed and contain fibrotic bands and organized thrombi, which disturb venous flow and cause development of superficial collateral vessels with and without varicose vein formation [16].
The development of duplex sonography has made possible the precise functional evaluation of varicose veins and has led to determination of the primary cause of varicose veins. Nevertheless, surveying all significant perforators during the evaluation of varicose veins remains difficult and time-consuming. The survey should be thoroughly conducted, however, because recurrence rates largely depend on the management of insufficient perforators [7, 17, 18]. Although it has good resolution and is useful for evaluating venous insufficiency, duplex sonography does not show an overview of the situation and is operator dependent. Because of these short-comings, duplex sonography is limited for finding all perforators. In contrast, CT venography can show all perforators but does not yield functional information. Thus CT venography and duplex sonography complement each other. CT venography also can be used to obtain an excellent road map of perforator vessels. Three-dimensional imaging of varicose veins not only yields a road map of the superficial venous system for the sonographic evaluation of functional anatomy but also improves the performance of Doppler sonography. The improvement is achieved through shortening of the examination time and increasing the rate of detection of perforators because perforator veins can be identified on 2D CT images and marked on 3D images. In addition, contrast-enhanced CT depicts both the deep venous and superficial venous systems better than does Doppler sonography,
Varicose vein evaluations with Doppler sonography usually are performed with the patient standing or sitting. In this study, we used a tilt table and performed Doppler sonography with the patient in a 70–80° semierect position. The advantage of using a tilt table is that patients can lean on the table and thus feel more comfortable during examinations, which can take 40–60 minutes. CT was performed with the patient in the supine position; thus hydrostatic dilation due to hydrostatic pressure in the lower-extremity veins was not an issue. Nevertheless, GSV diameters on CT images correlated well with diameters measured at sonography. Veins collapsed easily during sonographic examinations in the supine position, and diameters measured in the standing and supine positions at sonography differed. However, no significant difference was observed between diameters determined with CT and sonography when the techniques were performed in the supine and semierect positions, respectively. We presume that the relaxed leg position and minimal pressure applied to the superficial venous system during this study explains how we obtained good correlation between venous diameters measured with CT in the supine position and those measured with sonography in the semierect position.
CT images are an excellent 3D overview of varicose veins but contain no functional information on reflux or valvar insufficiency. Nevertheless, functional information can be derived from the morphologic information. In our study, CT-based predictions of the primary causes of varicose veins agreed well with the duplex sonographic findings. Determining the primary cause of venous insufficiency is the most important aspect of planning the management of varicose veins. In particular, GSV insufficiency, which is the main cause of varicose veins, can be predicted with high accuracy on the basis of CT venographic findings alone.
CT venography has disadvantages in relation to duplex sonography: the need for contrast medium and ionizing radiation and the lack of utility in the study of venous valve function. The radiation dose administered to the lower extremities during CT venography is in the range of 1.6–3.9 mSv, determined by direct measurement and calculation [19], and most of this dose is delivered to the pelvic region. Pelvic scanning yields information about the pelvic vasculature, such as the status of pelvic varices and the iliac vein. However, if the purpose of CT venography is superficial venous mapping, the range of acquisition can be decreased from the femoral head or great trochanter to the feet, substantially reducing radiation exposure. In terms of radiation dose, MR venography can be considered an alternative technique, but there have been few studies of MR venography in the evaluation of varicose veins.
CT venography was found to have sufficient image quality for evaluation of varicose veins. In addition, CT venographic evaluation of the superficial venous system yields comprehensive anatomic information, leading to avoidance of anatomic pitfalls and increasing surgeon confidence in the surgical approach. Three-dimensional images can be considered a complementary road map for surgical planning,
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