Michael Zellerhoff

Low contrast 3D reconstruction from C-arm data

The integration of 3D-imaging functionality into C-arm systems combines advantages of interventional X-ray systems, e.g. good patient access and live fluoroscopy, with 3D imaging capabilities similar to those of a CT-scanner. To date 3D-imaging with a C-arm system has been mainly used to visualize high contrast objects. However, the advent of high quality flat panel detectors improves the low contrast imaging capabilities. We discuss the influence of scattered radiation, beam hardening, truncated projections, quantization and detector recording levels on the image quality. Subsequently, we present algorithms and methods to correct these effects in order to achieve low contrast resolution. The performance of our pre- and post-reconstructive correction procedures is demonstrated by first clinical cases.

3D Imaging with Flat-Detector C-Arm Systems

Three-dimensional (3D) C-arm computed tomography is a new and innovative imaging technique. It uses two-dimensional (2D) X-ray projections acquired with a flat-panel detector C-arm angiography system to generate CT-like images. To this end, the C-arm system performs a sweep around the patient, acquiring up to several hundred 2D views. They serve as input for 3D cone-beam reconstruction. Resulting voxel data sets can be visualized either as cross-sectional images or as 3D data sets using different volume rendering techniques. Initially targeted at 3D high-contrast neurovascular applications, 3D C-arm imaging has been continuously improved over the years and is now capable of providing CT-like soft-tissue image quality. In combination with 2D fluoroscopic or radiographic imaging, information provided by 3D C-arm imaging can be valuable for therapy planning, guidance, and outcome assessment all in the interventional suite.

Measurement of cerebral blood volume using angiographic C-arm systems

While perfusion imaging is a well established diagnostic imaging technique, until now, it could not be performed using angiographic equipment. The ability to assess information about tissue perfusion in the angiographic suite should help to optimize management of patients with neurovascular diseases. We present a technique to measure cerebral blood volume (CBV) for the entire brain using an angiographic C-arm system. Combining a rotational acquisition protocol similar to that used for standard three-dimensional rotational angiography (3D DSA) in conjunction with a modified injection protocol providing a steady state of tissue contrast during the acquisition the data necessary to calculate CBV is acquired. The three-dimensional (3D) CBV maps are generated using a special reconstruction scheme which includes the automated detection of an arterial input function and several correction steps. For evaluation we compared this technique with standard perfusion CT (PCT) measurements in five healthy canines. Qualitative comparison of the CBV maps as well as quantitative comparison using 12 ROIs for each map showed a good correlation between the new technique and traditional PCT. In addition we evaluated the technique in a stroke model in canines. The presented technique provides the first step toward providing information about tissue perfusion available during the treatment of neurovascular diseases in the angiographic suite.

Interventional 4-D C-Arm CT Perfusion Imaging Using Interleaved Scanning and Partial Reconstruction Interpolation

Tissue perfusion measurement during catheter-guided stroke treatment in the interventional suite is currently not possible. In this work, we present a novel approach that uses a C-arm angiography system capable of computed tomography (CT)-like imaging (C-arm CT) for this purpose. With C-arm CT one reconstructed volume can be obtained every 4-6 s which makes it challenging to measure the flow of an injected contrast bolus. We have developed an interleaved scanning (IS) protocol that uses several scan sequences to increase temporal sampling. Using a dedicated 4-D reconstruction approach based on partial reconstruction interpolation (PRI) we can optimally process our data. We evaluated our combined approach (IS-PRI) with simulations and a study in five healthy pigs. In our simulations, the cerebral blood flow values (unit: ml/100 g/min) were 60 (healthy tissue) and 20 (pathological tissue). For one scan sequence the values were estimated with standard deviations of 14.3 and 2.9, respectively. For two interleaved sequences the standard deviations decreased to 3.6 and 1.5, respectively. We used perfusion CT to validate the in vivo results. With two interleaved sequences we achieved promising correlations ranging from r=0.63 to r=0.94. The results suggest that C-arm CT tissue perfusion imaging is feasible with two interleaved scan sequences.

A dynamic reconstruction approach for cerebral blood flow quantification with an interventional C-arm CT

Tomographic perfusion imaging is a well accepted method for stroke diagnosis that is available with current CT and MRI scanners. A challenging new method, which is currently not available, is perfusion imaging with an interventional C-arm CT that can acquire 4-D images using a C-arm angiography system. This method may help to optimize the workflow during catheter-guided stroke treatment. The main challenge in perfusion C-arm CT is the comparably slow rotational speed of the C-arm (approximately 5 seconds) which decreases the overall temporal resolution. In this work we present a dynamic reconstruction approach optimized for perfusion C-arm CT based on temporal estimation of partially backprojected volumes. We use numerical simulations to validate the algorithm: For a typical configuration the relative error in estimated arterial peak enhancement decreases from 14.6% to 10.5% using the dynamic reconstruction. Furthermore we present initial results obtained with a clinical C-arm CT in a pig model.