15 Three-Dimensional Evaluation of the Venous System in Varicose Limbs by Multidetector Spiral CT

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15 Three-Dimensional Evaluation of the Venous

System in Varicose Limbs by Multidetector Spiral CT

Jean-François Uhl and Alberto Caggiati

J-F. Uhl, MD

Varicose Vein Surgical Center, 113 av. Charles De Gaulle, 92200 Neuilly, France; and Laboratoire d’Anatomie, Uni- versité Paris V – Necker, 75006 Paris, France

A. Caggiati, MD

Department of Anatomy, University of Rome “La Sapienza“, Via Borrelli, 50, 00161 Rome, Italy

CONTENTS

15.1 Introduction 199

15.2 Principles of Spiral CT 199 15.2.1 Methods 199

15.2.1.1 Data Acquisition 199 15.2.1.2 Data Reconstruction 200 15.2.1.3 Postprocessing of Data 200 15.2.1.4 Contrast-Medium Injection 202 15.2.1.5 Venous Puncture 202

15.2.1.6 Injection Technique 202 15.2.1.7 Contrast-less 3D Venography 202

15.2.1.8 Transmission of 3D Images to the Specialist 202 15.2.1.9 Limits, Pitfalls, and Drawbacks 202

15.2.2 Clinical Applications 203 15.2.2.1 Preoperative Assessment of CVI on Varicose Patients 203

15.2.2.2 Investigation of Pelvic Varix 203

15.2.2.3 Investigation of a Varix of the Sheath of the Sciatic Nerve 206

15.3 Conclusion 206 References 206

15.1 Introduction

The anatomy of the venous system of the lower ex- tremities is extremely complex and variable, espe- cially in varicose and/or postthrombotic limbs [2].

There is a need for research into even more global and morphologically accurate techniques for exami- nation of the vascular tree.

Traditional venography lost its title of gold stan- dard for the morphofunctional examination of the venous tree of the lower limbs because, in the major- ity of cases, Duplex ultrasonography (US) furnishes a more accurate imaging and more complete hemo-

dynamic evaluation with a less traumatic, expensive and time-consuming technique.

Recent advances in computer techniques have brought an innovative technique for investigation of venous disease based upon the use of spiral CT (veno-CT or VCT). Veno-CT furnishes an accurate three-dimensional (3D) representation of the whole venous system of the lower limb, demonstrating, in same cases, hemodynamic patterns which are not available from Duplex US.

15.2 Principles of Spiral CT

The spiral or helical CT scan is the result of two different moves combined: Firstly, the rotation of an X-ray tube attached with detectors rotating around the patient’s bed; Secondly, a continuous linear translation of the same bed. This enables the ac- quisition of volume data, with different results: 3D reconstructed images and slices.

15.2.1 Methods

The three steps of the VCT investigation are data acquisition, reconstruction, and postprocessing.

After treatment, reconstructed images are trans- mitted by intranet and can be seen on a PC by the angiologist (Fig. 15.1).

15.2.1.1

Data Acquisition

A multislice and multidetector CT scan (Siemens Somatom sensation 16) is used, producing 400–600 slices by series during 25–40 s. The protocol details are shown in Table 15.1, for 8- and 16-detector spiral CT.

The patient lies on his/her back (feet-first into the scanner), with no contact points with the table

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200 J-F. Uhl and A. Caggiati

except for the buttocks and heels: It is important to avoid any compression of the calf and posterior thigh during acquisition time. The patient has to keep perfectly still during this short time and is of- ten asked to make a Valsalva maneuver.

Fig. 15.1 A reconstructed image after data transmission

Table 15.1 Multislice and multidetector spiral CT protocols

Protocols Acquisition Reconstruction Postprocessing Contrast injection

8-Detector CT:

400 slices in 30 s

120 kV 130 mAs slice collimation:

8×2.5 mm field 512

rotation time 0.5-s feed/

rotation 15 mm FOV 380 mm

Slice width 3 mm Slice increment 2 mm filter B20

Matrix 512×512 Zoom factor 1.7

1994–1997 Surface-shading rendered (with manual segmentation)

Medrad MCT injector system

Uniphasic injection 20 ml of iodine contrast medium in 180 ml of serum Puncture of a vein of the dorsal foot or rarely the varices of the thigh

16-Detector CT:

600 slices in 25 s

120 kV 150 mAs slice collimation:

16×1.5 mm field 512 FOV 380 mm

Slice width 2 mm slice increment 1.5 mm filter B30

matrix 512×512 zoom factor 1.7

1998-2002 Volume rendering Fast & automatic with tissue transparency 15.2.1.2

Data Reconstruction

Raw data are processed to perform a slice recon- struction. We use a slice width of 2 mm with a slice increment of 1.5 mm, a filter B30, and a zoom factor of 1.7 (Table 15.1)

15.2.1.3

Postprocessing of Data

To obtain 3D reconstruction of the venous system, the data are sent by intranet on a dedicated worksta- tion for postprocessing using dedicated 3D recon- struction software.

Surface-rendering technique. (Also called “surface- shading rendering” – SSR). Huge progress has been made since 1994 in 3D image reconstruction. At the beginning, a manual segmentation of the image was necessary to obtain reconstructed 3D images by a SSR 3D model. A pixel extraction had to be done by the observer using appropriate windowing: the maximal minimal-density threshold was chosen manually in order to select the voxels correspond- ing to an anatomical structure. Although performed on Sun or Silicon graphics workstations, this tech- nique was time-consuming and used only some of the data. In turn, it is possible to achieve manual reconstruction of some other structure of interest as nerves (Fig. 15.2).

Volume-rendering techniques (VRT). Today, beautiful 3D images of the venous system (Fig. 15.3) are quickly produced by reconstruction with VRT, with easy-to-

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use, built-in protocols. This saves time and takes full advantage of all the 3D information of the dataset. New 3D dedicated software running on a PC with 1 gigabyte of RAM are now available for this purpose.

Main functions of VRT are: rotation, tilt, pan, zoom, and use of different transparencies of the tissues in

real time. Automatic presets are available to directly visualize skin, muscles, or vessels. Quick and easy images are captured or animated. Output functions are useful to transmit resulting images to a PC.

The VRT software we use is Plug & View 3D (VOXAR Inc.; Tiani-Medgraph).

Fig. 15.2 Surface rendering with sciatic nerve reconstruction

Fig. 15.3 Volume rendering:

different transparencies of the tissues

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15.2.1.4

Contrast-Medium Injection

An injection of diluted contrast is to be preferred in order to enhance the contrast of the venous network, allowing display of it alone, without the surrounding tissues with the VRT software (by using automated preset protocols).This is also true for the perfora- tors: their course and precise connection with the deep veins are to be checked before surgery. Lastly, it is mandatory to investigate the detailed morphology of the deep system.

15.2.1.5

Venous Puncture

A dorsal foot vein puncture is performed in most cases. In particular cases, the cannulation of a varix of the leg or thigh could be necessary, to opacify a varicose network of the root of the limb, a pelvic varix, or a pelvic origin of the reflux, or to show an excluded varicose region.

15.2.1.6

Injection Technique

We use an automated Medrad MCT injector sys- tem. A uniphasic injection of 20 ml of iodine con- trast medium is done in 180 ml of serum, at the rate of 2-3 ml/s. The duration of injection (usu- ally 1 min) is synchronized with acquisition time, starting 30 s before and lasting until the end of acquisition.

Associated techniques can be used to enhance the contrast: a Valsalva maneuver is frequently asked of the patient. A tourniquet is put at the root of the thigh in case of investigation of the popliteal area. A balloon placed in the suprailiac area is inflated dur- ing acquisition for better evaluation of the inguinal veins.

15.2.1.7

Contrast-less 3D Venography

3D imaging of the venous tree can be obtained also without contrast-medium injection [3–5].

Technique. Parameters of acquisition: 125 kVp;

120 mA; collimation 2; pitch 4; 1 reconstruction.

Images were reformatted at the CT console and transferred in a DICOM format to a dedicated work- station equipped with dedicated software such as Voxar (www.voxar.com).

Contrast-less veno-CT visualizes the external face of the veins, not their lumen. As a consequence, contrast injection is necessary when the goal of the examination is to evaluate the patency of a venous segment.

15.2.1.8

Transmission of 3D Images to the Specialist

The slices and a selection of 3D reconstructed images can be saved on a CD, but the best way to achieve the data transmission between the radiologist and the angiologist is to export the movie files of a ro- tating 3D model using different transparencies of the tissues: skin, muscle, vessels. (Fig. 15.3). These dynamic data are easily built and exported by the new VRT software.

One of the best ways is to use the QTVR software (Quicktime virtual reality) from Apple. It achieves a true interaction with a rotation of the 3D model according to the horizontal move of the mouse, and modifies tissue transparency according to the verti- cal move of the mouse.

In that way, the surgeon can have all of the in- formation about the veno-CT available on his laptop computer in the operating room, together with the data of paper cartography [5] and of the US skin- mapping [6]. They can be used together as a road- map for surgery.

By an exquisite depiction of the venous course, VCT may avoid the anatomical pitfalls of venous surgery. This makes possible a better surgical choice of technique, and a more accurate and limited skin approach. Accordingly, it improves the aesthetic result as well as the efficacy, reducing the varicose vein recurrence rate.

15.2.1.9

Limits, Pitfalls, and Drawbacks

Technical problems. Venous puncture was rarely impossible (3-4% of the limbs). However, a VCT without injection is possible, providing detailed anatomical information, but restricted to the su- perficial network [3].

Limits. The main drawback is that VCT provides no hemodynamic data, and it can only be performed in a prone position. It means that the Duplex US exami- nation (in a standing position) is always necessary.

It provides a complete map of the superficial venous network and perforators with anatomical and hemo- dynamic data [7]. In addition, a preoperative skin-

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mapping is performed [8], to be use as a guideline by the surgeon during surgery.

Pitfalls. The possible lack of injection of some veins means a careful differentiation from a venous thrombosis has to be made. It may be due to com- petent valves, venous congenital agenesis or hypo- plasia, or compression point(s), usually calf, by the table. There is an exclusion area in case of huge and/or high-located varicose veins. The region is not filled by the foot injection: the solution is to make a direct puncture of the varix to obtain good visualization, so there is a case for pelvic varix vi- sualization. A direct puncture of a perineal vein is the best way to get a fine injection of these com- plex networks. (Fig. 15.4). The main reason is an improper bolus timing of contrast-material injec- tion, usually a too-short injection time: the contrast material had no time to reach the anatomical region of interest. In order to avoid this problem, one can use a tourniquet and/or a Valsalva maneuver. This is particularly true for popliteal fossa investigation:

here the tourniquet of the root of the limb increases the quality of contrast and avoids an examination failure. In case of difficulty, we have to keep in mind that 3D reconstructions are not fully reliable. These clever postprocessing routines create images that do not exist. Hence, to avoid a misunderstanding or a false diagnosis by the use of the 3D images, do not hesitate to go back to the original slices and also to compare with other investigations (US).

Drawbacks. X-ray exposure is the main criticism of CT venography. A pure venogram can be obtained by other “radiation-free” and less-invasive tech- niques, but they provide a lower quality of images:

MR venography without gadolinium injection using protocols that incorporate 2D time-of-flight acquisi- tion. Recently, F

^RASER

et al. [6] proposed a new MR venography technique called VESPA (venous en- hanced subtracted peak arterial) with gadolinium injection, using spatial subtraction which elimi- nates the need to cannulate a foot vein.

15.2.2 Clinical Applications

15.2.2.1

Preoperative Assessment of CVI on Varicose Patients

On the basis of the experience derived from a large series of investigations, VCT is usefully recom- mended in about 15% of patients undergoing sur- gery for varicose veins. [9]. It is particularly useful in the following cases:

• Postoperative recurrences, especially at the pop- liteal fossa. (Fig. 15.5)

• High termination (Fig. 15.6) or dystrophic termi- nation of the short saphenous vein (Fig. 15.7)

• Duplication of the saphenous popliteal junction (Fig. 15.8)

• Varicose veins of the long saphenous region fed by an ascending flux of the Giacomini vein via a saphenous popliteal reflux (Fig. 15.9)

• Large and complex varicose networks, to improve information furnished by clinical and US map- ping (Fig. 15.10; skin mapping, 1; VCT skin level, 2; and VCT muscle level, 3)

• Patients with large perforators of the thigh In all these cases, the association with color-coded Duplex of the venous network is mandatory, because VCT does not provide hemodynamic data.

15.2.2.2

Investigation of Pelvic Varix

The pelvic origin of varicose veins is not rare, and is responsible for specific symptoms. Color Duplex using a vaginal probe is useful to study hemody- namics.

VCT investigates the reflux route and anatomical connections of the pelvic network: a possible cause of reflux is an incompetent lumbo-ovarian vein com-

Fig. 15.4 Puncture of the varix of the thigh to enhance con- trast

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Fig. 15.5 Popliteal fossa cavernoma following short saphenous vein surgery

Fig. 15.6 High termination of the short saphe- nous vein

Fig. 15.7 Dystrophic short saphenous vein and saphenous popliteal junction

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Fig. 15.8 Duplication of the saphenous popliteal junction

Fig. 15.9 Long saphenous vein varix fed by the Giacomini vein

Fig. 15.10 Improved information about complex network

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ing from the inferior vena cava, feeding the puden- dal network by the inguinal canal. In such a case, interventional radiology using a coil associated to foam with sclerotic agent is the best choice to treat the origin of the reflux, associated with removal of the varix of the limb by phlebectomies.

15.2.2.3

Investigation of a Varix of the Sheath of the Sciatic Nerve

According to neurovenous embryogenesis, varicosis of the sciatic nerve follows the route of the nerves:

sciatic nerve at the thigh level, fibular nerve at the knee level, and lateral saphenous nerve below knee level [7]. VCT usually shows the deep route at the thigh along the sciatic nerve: on the 3D reconstruc- tions as well as on the slices, the dystrophic venous network infiltrates the sheath of the nerve. It is as- sociated here with a stasis in the muscular arcades of the semimembranosus muscle (Fig. 15.12; network, 1; femoral vein, 2; dilatation of the semimembrano- sus arcades, 3).

15.3 Conclusion

The aim of spiral VCT (veno-CT) is to provide a pre- cise 3D anatomical depiction of the venous network.

A multislice and multidetector spiral CT acquisition of the lower limb with contrast injection produces about 400 to 600 slices in 30 s. Dedicated volume- rendering software compute interactive 3D images of the venous system.

Always associated with color-coded Duplex, which provides hemodynamic data, VCT is a pow- erful new tool to investigate patients with varicose veins, particularly in the case of a recurrences or

for complex networks of the popliteal fossa; it truly provides a 3D roadmap for surgical planning. This role seems to be extremely important in selected cases, such as in the presence of pelvic venous con- gestion with reflux transmitted to the veins of the leg. Beside the patient’s assessment, the VCT make us enter into the virtual reality world. Hence, it is a tool of the highest potential for a better understand- ing of venous disease, to learn venous anatomy, and for research.

References

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graphy of the saphenous venous system. Circulation 102:

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7. Lemasle P, Lefebvre-Vilardebo M, Uhl JF, et al. (2000) La cartographie veineuse superficielle. Considérations pratiques. Phlébologie 53:363–373

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9. Uhl JF, Verdeille S, Martin-Bouyer Y (2003) Interêt du phléboscan hélicoïdal avec reconstruction 3D dans le bilan pré-opératoire des varices. Phlébologie 56:11–16