X-Ray Imaging and Computed Tomography

Interventional CT

Figure 1
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Interventional radiology comprises minimally-invasive procedures such as stenting and aneurysm coiling which are based on the insertion of interventional materials like guide wires, stents or coils directly into the vasculature of a patient. Today’s interventions are usually guided using conventional projective fluoroscopy (2D + t) in single- or bi-plane systems. Due to the projective nature of this guidance the images show only a superposition of the patient’s anatomy (figure 1) and therefore the visualization of more complex structures and their spatial relationship is often ambiguous. Alternatively tomographic images can be taken with modern C-arm systems, but those are not in a fluoroscopic mode (figure 1). Continuous acquisition and conventional reconstruction of CT data sets enables 4D (3D + t) intervention guidance but, unfortunately, such fluoroscopic tomography exceeds acceptable X-ray dose levels.

In intervention guidance, multiple repetitive scans of the same body region are performed. This enables the option to incorporate prior information into reconstructions for 4D intervention guidance. The dose for 4D intervention guidance can be considerably reduced by combining a high quality prior image without interventional materials with the special features of 4D intervention guidance:

  • imaging high contrast structures is of the foremost interest,
  • differences of consecutive data sets (time frames) are sparse,
  • certain artifacts in the volume remain tolerable.

Our research group, therefore, developed the reconstruction method Prior Image Dynamic Interventional CT (PrIDICT), complying with the special features of interventional guidance and combining undersampled data sets during the intervention with high quality prior data sets acquired before intervention.

This algorithm takes into account that difference images are ad hoc sparse in image domain and do not necessarily require additional sparsifying transforms. Therefore, difference images of the forward projected prior image and the undersampled projection data can be reconstructed and only the voxels with the highest absolute intensity (significant voxels) contribute to the reconstruction of the time frame, a combination of the prior image and the significant voxels.

Using this reconstruction technique, undersampled projection data (e.g. 10 to 20 projections per half rotation) acquired at a very low dose, are sufficient to reconstruct high quality time frames based on the prior images and therefore realize 4D intervention guidance. Figure 2 shows an in in-vivo animal experiment imaged with an experimental cone-beam CT setup  with dose levels in the same range as those of the projective fluoroscopy could be reached.

This approach, which we call tomographic fluoroscopy, may enable real-time 4D guidance for clinical applications. Furthermore, new intervention guidance processes may be achieved and the invasiveness of image-guided procedures minimized with the ultimate goal to make interventional radiology safer and faster for the patients' benefit.

To make tomographic fluoroscopy more robust, our group further developed the running prior technique as an extension to the PrIDICT algorithm. This method corrects for motion of the patient during the intervention by continuously updating the prior image. A non-rigid registration combined with the replacement of outdated projections by the information of just acquired ones accounts for changes in the patient’s anatomy without the need of extra projections. That means the dose level remains at the level comparable to projective fluoroscopy.

Figure 2
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