DU STUDIES ILIOCAVAL STENT
DU STUDIES ILIOCAVAL STENT
Duplex Ultrasound improves treatment of femoro-iliocaval occlusive venous disease
By Jan M. Sloves, RVT
and Jose I Almeida, MD
The last two decades have witnessed a great increase in stent treatment for femoro-iliocaval occlusive venous disease. We know from previous reports that the major causes of acute stent occlusions are missed iliocaval or common femoral venous lesions; and/or poor inflow due to post-thrombotic disease in the femoral or profunda femoris veins.1
Characteristics of stent occlusion were recently studied in Jackson, Mississippi, and published by Jayaraj et al.2 They found that stent occlusions predominantly occur after recanalization of chronic total occlusions, and the majority of the stent occlusions (69 percent) were >30 days after placement. Multivariable logistic regression did not reveal in-stent restenosis as a predictor for stent occlusion. Occlusions in stented patients with non-thrombotic (May-Thurner) lesions are known to be rare.3
Iliocaval duplex imaging is an area in evolution since the advent of new ultrasound technology that now allows for better penetration and image resolution of the deeper abdominal and pelvic structures.
We recently published a protocol using duplex ultrasound (DU) image optimization techniques to enhance the diagnostic efficiency of detecting obstructive abnormalities in the native iliocaval venous outflow tract.4
Herein, we present a similar protocol for iliocaval stent surveillance using DU from the experience of two busy vascular labs (New York City and Miami). Challenges remain for treating clinicians because often the abnormalities seen with duplex surveillance are present in asymptomatic patients (Table 1). It is important to note that the following protocol has not been validated in a clinical trial.
STENT ABNORMALITIES SEEN WITH DUPLEX SURVEILLANCE
Stent separation (shelving)
In-stent restenosis (intraluminal acute or chronic thrombus)
Missed lesions in native veins proximal or distal to the stent
THE ISR QUAGMIRE
In-stent restenosis (ISR) is a term that has evolved, used to represent the intraluminal material often seen within the stent lumen during surveillance. David Williams, MD, professor in the Department of Radiology at the University of Michigan, recently reported at the VEITHsymposium on biopsy findings of these ISR lesions. He noted that they fall into two general categories: thrombus and fibrin. He suggested treatment with anticoagulation in the thrombus group and perhaps balloon angioplasty of the organized fibrous lesions.
Currently, our labs in New York and Miami report ISR as mild, moderate or severe based on the amount of luminal reduction seen. The echogenicity of ISR lesions can range from echogenic to echolucent. On gray scale imaging echolucent defects can be more difficult to visualize within the stent because they possess the same echogenicity as blood. Currently we cannot stratify lesions (thrombus versus fibrin) based on echogenicity alone. Stents can appear normal until color flow or power Doppler is placed on the stent, thus demonstrating a diminished flow channel. ISR lesions can be mild to severe and should be thoroughly interrogated with color flow and pulse wave Doppler techniques to quantitate severity.
We are currently investigating these lesions with duplex measurement of intraluminal area – similar to what we currently do with intravascular ultrasound (IVUS) planimetry.
In our experience the results have been mixed regarding the reliability of the peak vein velocity with significant ISR lesions. Typically, normal stents demonstrate peak vein velocities of ranging from 15-45cm/s. We have noted some significant ISR lesions with elevated peak vein velocities up to 150cm/s. However, many significant ISR lesions do not produce an increase in velocity; the velocities often remain within normal limits despite significant in-stent disease.
Furthermore, this finding maybe confounded by poor inflow or outflow. Because there is a lack of sensitivity in velocity, and lesions are often poorly demarcated with gray scale imaging, an alternate method of quantifying the degree of narrowing within the stent is with the use of color-coded percent area reduction method.
The stent is evaluated with color flow and/or power Doppler in the longitudinal and transverse axis plane for comparison. The color flow preset is optimized with a small sector width and color box, color scale is between 3-10cm/s and the gain is set to 30-50 percent.
Next, the operator acquires a clear color-coded transverse image demonstrating the stent struts and residual flow lumen. There are two steps to this measurement. Area 1, the ellipse is placed on the stent struts; Area 2, the ellipse is placed on the color-coded residual flow lumen. The ultrasound system calculates the percentage of area reduction (Fig. 1). This measurement is simple to perform and coincides with the IVUS concept.
Patients fast overnight and take two tablets of simethicone the night before and the morning of the examination to reduce bowel gas. In the supine position, with the patient’s head elevated 10 to 15 degrees, the scan is begun with an anterior approach. Undergarments are removed, and privacy draping is placed. If excessive bowel gas is noted, the curved array transducer is used to massage the gas out of view, or the patient is rotated into the right or left lateral decubitus position.
COMMON FEMORAL VEIN
The exam is begun with a linear transducer at the CFV level. Stents extending caudally into the CFV are evaluated in relation to the confluence of the femoral or profunda femoris veins (profunda-femoral confluence). The stent is examined for adequate vein wall apposition and for the presence of any acute or chronic thrombus.
The CFV stent diameter is measured along with the profunda-femoral veins with calipers (wall to wall) in the longitudinal and transverse axis planes. Color flow and pulsed Doppler are used to assess patency. Pulse Doppler waveforms are obtained utilizing a 60-degree angle of insonation to examine the symmetry of venous flow.
Asymmetric CFV flow patterns suggest a proximal obstruction; however, symmetric flow patterns do not rule out the presence of a proximal obstruction. Detection of nonphasic flow or a continuous CFV flow pattern suggests post-thrombotic disease.
ILIAC VEIN STENT EVALUATION
Next, a curved array transducer is placed at the inguinal ligament-femoral head. The gray-scale image is optimized with chroma tint and the appropriate depth of penetration is set using time gain compensation (TGC). Overall gain and TGC are adjusted to clearly depict the arterial and venous walls. In the absence of a stent crossing the inguinal ligament-femur head level, the EIV is examined for PTS scar and compression by the overlying artery.
When a stent is present, the EIV stent is evaluated for potential ISR, external stent compression and fracture. The transducer is then advanced cephalad along the stent towards the EIV-IIV confluence and then to the CIV-caval confluence.
Stents should be carefully delineated in both the longitudinal and transverse planes for appropriate vein wall apposition and luminal shape-size. Stent diameters in the proximal, mid and distal segments are measured with calipers in the longitudinal and transverse axis planes. Multiple stents (stent stacks) should have 1-2cm stent-to-stent overlap without shelving.
Venous stents are interrogated for ISR, occlusion, external compression, migration, shelving and separation (Fig. 2). The iliocaval confluence examination should look for jailing of the contralateral CIV by the ipsilateral stent.
Color flow settings are optimized, and color is then advanced through the length of the stent to identify any abnormality in the longitudinal and transverse planes. Stents identified with ISR require meticulous assessment with color flow and/or power Doppler. It is paramount to isolate the narrowed residual flow lumen in the longitudinal and transverse plane for comparison.
Next, in transverse the stent struts and residual flow lumen are well delineated and measured with percent area reduction method. Pulsed Doppler waveforms are assessed for respiratory phasicity, asymmetry and any notable changes in peak velocity within the lumen.
All non-stented iliocaval veins are thoroughly evaluated for potential missed or skipped lesions. Vein segments with PTS scar will frequently display higher color flow velocities with significant color aliasing and continuous flow patterns. These lesions require diameter measurement with complete color flow-pulse Doppler evaluation (Fig. 3).
INFERIOR VENA CAVA STENT EVALUATION
The curved array transducer is placed at the umbilicus for distal inferior vena cava (IVC) evaluation in the gray-scale image mode. The transducer is advanced cephalad following the course of the IVC to the right side of the heart in the longitudinal and transverse axis planes. In the absence of stents in the iliocaval confluence and IVC, acute thrombosis, PTS scar, stenosis, and extrinsic compression must be excluded. Diameters of the native IVC are measured both distally and proximally.
Stent evaluation at the iliocaval confluence and IVC will require the use of a small sector width, orange chroma tint with the use of multiple harmonic frequencies depending on the body habitus. When assessing infra or supra renal and bilateral iliac stents extending cranially into the cava, luminal shape, wall apposition and the proximal and distal end points of the stents must be documented. Double barrel stents are well visualized in the longitudinal and transverse plane (See Figure 4).
Diameters of the stent and native IVC segments are measured wall-to-wall. Color flow and pulsed wave Doppler techniques are optimized and used in conjunction to determine patency. Normal flow in the IVC is pulsatile because of the reflected right atrial pulsations with reversal of flow during atrial systole. If a continuous flow pattern is noted within the IVC, it is consistent with an obstruction. Congenital anomalies of the IVC (i.e., caval duplication or agenesis, renal collar) are uncommon but must not be overlooked.
FOR ILIOCAVAL STENTS
The orange chroma tinted feature enhances image resolution and should be used in conjunction with the appropriate harmonic frequency and depth that best depicts the patient’s body habitus. Overall gain should be set to reflect a full range of signals that are high to low in their amplitude.
The iliac veins and IVC vary in depth throughout their length. Precise gain adjustments to the TGC from the near field to the far field are needed to eliminate reverberation artifact, thus defining the native and stent walls for accurate measurement. The focal zone must be set at the area of interest or just below for optimal image detail. Decreasing the sector width size will improve image resolution and better demarcate the areas of stent malfunction, vein compression or scar. Color settings are optimized for low flow. Stents are angle dependent and require color scale parameters to be set between 3 to 10 cm/s and color gain to 30 percent to 50 percent, wall filter is set low to medium and color persistence is set high.
Color gain that bleeds into the surrounding soft tissues and covers portions of the stent should be eliminated as it can mask the disease. In some instances, the transducer and sector width can be steered to help create a more appropriate angle of incidence, which will necessitate better color filling. Disease free stents should display wall-wall color filling.
When encountering ISR it is paramount to isolate the narrowed residual flow lumen. The optimized longitudinal and transverse image should underscore the stent struts and residual flow lumen for measurement. Color flow and pulsed-wave Doppler techniques are combined to interrogate the entire length of the stent in a step-by-step manner (See Figure 5).
Velocities are acquired from the distal, mid and proximal portions of the stent with an emphasis on the narrowed residual flow lumen. Pulsed Doppler waveforms should be acquired in the same direction from baseline. The stent is interrogated in the longitudinal axis with a 60-degree angle parallel to the walls.
DU is very useful for surveillance of iliocaval venous stents. As labs develop more experience with image optimization, treatment strategies will be developed to best manage some of the abnormalities often seen. ISR is particularly puzzling and with improved duplex guided quantification of luminal area (and flow) we will be in a better position to create guidelines for treatment. VTN
1Neglen P, Hollis KC, Olivier J, Raju S. Stenting of the venous outflow in chronic venous disease: long-term stent-related outcome, clinical, and hemodynamic result. J Vasc Surg 2007;46:979-90.
2 Jayaraj A, Crim W, Knight A, Raju S. Characteristics and outcomes of stent occlusion after iliocaval stenting. J Vasc Surg Venous Lymphat Disord. 2019 Jan;7(1):56-64.
3Raju S, Tackett P Jr, Neglen P. Reinterventions for nonocclusive iliofemoral venous stent malfunctions. J Vasc Surg 2009;49:511-8.
4Sloves J, Almeida JI. Venous duplex ultrasound protocol for iliocaval disease. J Vasc Surg Venous Lymphat Disord. 2018 Nov;6(6):748-757.
Jan Sloves, RVT RCS FASE, is a pioneer in the field of vascular imaging and president of Vascular Imaging Professionals LLC in New York. He has honed his imaging skills as a vascular imaging expert through academic and office-based practices spanning more than 20 years. He is also a professional advisor to Philips Health Care ultrasound division, serving as a primary investigator within the research and development team to optimize the performance of ultrasound systems and transducers.
Jose I. Almeida, MD, FACS, RPVI, RVT is a veteran academic vascular surgeon who practices endovascular venous surgery from his Miami Vein Clinic. He is a voluntary professor of surgery at the University of Miami School of Medicine, and he has authored numerous chapters for textbooks and peer-reviewed journal articles, as well as author of his own textbook on endovascular venous surgery. Dr. Almeida is on the board of the American Venous Forum and is chairman of the Society for Vascular Surgery Patient Safety Organization Venous Quality Committee.