CTMS in the study of post-EVAR endoleak in the treatment of aneurysmal dilations

Introduction

CTA is commonly used for the follow-up of patients treated with endovascular stents and stent grafts with the goal of evaluating patency and complications related to stent-graft.

The “endoleak” is the most common complication of the insertion of an endovascular prosthesis (EVAR).

Failure of the procedure creates persistent perigraft flux within an aneurysm sac [1-2], failure of diameter reduction, the persistence of a pulse, and/or the presence of “endotension”.

In fact, there are no standardized techniques for aortic CTA, so practices, type of scanners and acquisition protocols, contrast medium concentration (CM) and administered doses, delay times for contrast phases may vary.                                      

The two methods have been compared in order to evaluate which one is more performant.

The main difference among the two techniques is that the dose used in the split bolus is 70% lower than the one used with the biphasic method.

Materials and methods

In this work the study has been carried on 28 patients with endoprostheses for evaluation following EVAR. Exams were performed from January 2019 to February 2020. They were divided in two groups:

• GROUP A consists of 18 patients studied with biphasic technique.
• GROUP B consists of 10 patients studied with split-bolus technique.

These examinations were done with a TC GE Lightspeed 64 slice/rotation with this acquisition parameters: 120 kV, 240 mA, 512 512 pixels matrix, layer thickness 1 mm, reconstruction index 0.5, rotation time 0.5 seconds.

CTA protocol and image analysis

The radiographer positions the patient supine with upper limbs stretched over the head, to avoid artifact from hardening of the beam and folding of the intravenous cannula needle. Usually the patient enters the “head” in the TC gantry and he is centered on the bed by means of special lasers located in the gantry. The area of interest will correspond to the rotation isocenter, so as to have as little FOV as possible. [3]

Before the acquisition, a scanogram (or scout view) is performed from the lung apices to the pelvis, with respiration suspended whenever possible. It’s an image similar to a traditional X-ray. The tube is placed at 0, (above the patient) to see the thickness of the sections and the extension of the volume to be acquired, and at 90(laterally to the patient: lateral view) to see the thickness of the patient.

In modern TC scanners the scout AP and LL are used to modulate the dose during the subsequent acquisition. [3]

Then, the acquisitions start with the chosen technique.

The biphasic technique was used on group A but with a low dose acquisition without CM.

The biphasic technique provided for the injection of 120-140ml of contrast medium (IOPAMIRO 370 mgi/ml) at a flow of 4 ml/s and 40 ml of saline. The ROI has been positioned at the thoracic-abdominal passage and the appropriate delay time is determined by the intensity/time curve.

On group B patients the split-bolus technique was performed, by injecting 120-150 ml of CM (370mgl/ml) in two boluses.

The followed path was:

Injecting the first bolus [or zero time] (1.2ml/kg) at a flow of 2.0 ml/s followed by 20 ml of saline solution at the same flow rate, for venous phase (PV).

Second bolus (1.0 ml/kg) at a flow of 3.5 ml/s and ending with 20 ml of saline solution at the same flow rate, to achieve on a venous background the arterial phase (FA).

The acquisition was taken with the bolus-tracking, that involved the simultaneous injection of the normal bolus of CM and the continuous recording of the density in the circular ROI positioned at the thorax-abdominal passage, and the contrast scan began after a delay of 6 seconds from exceeding a density threshold.

With this technique an optimal and simultaneous visualization of both the venous phase and the arterial phase has been achieved, thanks respectively to the first and second bolus; in particular, the clear acquisition of the arterial phase was due to the high flow of the second bolus, thanks to which the bolus itself reached compactly the arterial system.

In this context, MPR, MIP, VR reconstructions are essential to provide specific and precise measures of the endoleak extension.

Results

Among the 18 patients in Group A n=6 had endoleak type I, n=2 endoleak type II, n=8 endoleak type III, n=2 endoleak type IV.

Among the 10 patients belonging to group B n=2 had endoleak type I, n= 4 endoleak type II, n=3 endoleak type III and n=1 endoleak type IV.

ENDOLEAK TYPE I

ENDOLEAK TYPE II

ENDOLEAK TYPE III

ENDOLEAK TYPE IV

Comparing the doses expected with the two techniques, has been highlighted that the total effective dose of the split bolus technique was 16.39mSv, which corresponds to about 820 rx thorax in AP, while the biphasic technique needed a total dose of 43,2mSv, corresponding to approximately 2162 rx thorax in AP.

The split-bolus technique has shown to reduce dramatically the radiation dose undergone to the patient (of 70 % approximately).

Limits

This study was done because a 64-slice TC single energy was available, and it was necessary to reduce the dose to the patient. Unfortunately, the split bolus technique has a limit: the artero-venous shunt is not recognizable with split-bolus method, it does not allow to see clearly the passage of the CM between artery and vein because they are both contrasted.

However, this limit can be easily overcome by the utilization of innovative equipment, such as TC 256 slice dual energy. This new CT modulates radiation erogation on the tissue involved.

Conclusions

With a TC 64 slice single energy, the biphasic technique is proved to be a very accurate method in the recognition of the various types of endoleak, which has the advantage to provide high resolution acquisitions (both spatial and contrast), reduction of acquisition times and reduction of artifacts but implies a high amount of radiation.

On the other hand, the split bolus technique can be proposed as an innovative and performant technique in order to optimize the study of endoleak obtaining results comparable to the biphasic technique both for image quality and for the accuracy in identifying endoleak, but with significantly reduced radiation dose.

References

1. Gorich J, Rilinger N, Sokiranski R, et al. Endoleaks after endovascular repair of aortic aneurysm: are they predictable? initial results. Radiology 2001; 218:477-480
2. Ohki T, Veith FJ, Shaw P, et al. Increasing incidence of midterm and long-term complications after endovascular graft repair of abdominal aortic aneurysms: a note of caution based on a 9-year experience. Ann Surg 2001; 234:323-335
3. Rita Golfieri, Rossella Trenti, Antonio Giovanni Maccione, Manuale di TC per TSRM, Milano, Poletto Editore 2019