geons are forced to mentally combine multiple views to create a 3D model, adding further complexity to what is already a difficult task. This complex mental process constitutes a universal problem to be solved not only for surgeons using 2D information when operating vascular lesions in a 3D space, but also for the endovascular operator forced to use similar mental processing to translate 2D images. In order to monitor and control the catheter positions and the delivery of embolic devices and agents in a complex vascular structure such as the CS, complete understanding of 3D anatomy can become crucial.

Four main steps compose 3D angiography (Siemens system):

Data acquisition:

(Fig. 7.59) C-arm rotations between 5 and 20 s acquiring images in a projection matrix of 960×1240 (1 k) or 2480×1920 (2 k)

Data transfer:

The data are transferred using a fast 1:1 connection (100 Mb/s)

Data reconstruction:

Using the modified cone beam method of Feldkamp, axial slice images are calculated in a matrix of 256×256 or 512×512

Data processing:

Image post processing is performed using a dedicated software package (Syngo DynaCT, Leonardo, Siemens) in VRTs (In Space) or as MPRs or MIPs for cross-sectional imaging

The acquisition is obtained using a rotating C-arm with various program settings, depending on the clinical and diagnostic question. A standard 3D-DSA is performed using 5- or 10-s rotations, obtaining opacified 130 or 273 projections. After positioning the patient’s head in both planes into the iso-center, the C-arm moves to two predetermined positions before starting the actual rotation. A 3D-DSA consists of an initial mask run and a second filling run. Projections are acquired in identical positions of the C-arm and angle triggered, allowing for a precise projection-an- gle determination for the reconstruction.

The total angle per rotation is typically 200° or 220°. During the filling run, contrast medium is injected either manually or using a power injector according to the length of rotation. A typical injection protocol for a 10-s rotation is 2.5 cc/s, a total of 28 cc. Images are then transferred to a workstation for post processing and automatically reconstructed using either a 256×256 or a 512×512 matrix, either as a subtracted or non-subtracted dataset. Second-

ary reconstructions can be performed using various matrix sizes, kernels and volumes of interest (VOIs). The steps necessary to obtain a 3D reconstruction may slightly differ among various manufacturers. The Allura system from Philips for example acquires usually a filling run only. For a subtracted reconstruction, the mask run has to be performed separately as a second step which causes delay and possible additional motion artifacts.

7.2.6.1

Dual Volume Technique (DVT) (Fig. 7.60)

Current software for angiographic systems (Axiom Artis dBA, Siemens Medical Solutions) allows for separately reconstructing the two components of a DSA rotational angiogram. The mask run is reconstructed to provide information on radiopaque objects within the projection field like bony structures, coils, clips or stents. Then, a second reconstruction is performed using the subtracted filling run, which contains only information on vascular structures filled during the rotation. In a 5-s rotation mainly arteries will be filleds, but during a 10-s rotation cerebral veins will also be filled. Both volumes can be loaded and interactively displayed on the Leonardo (Siemens Medical Solution) workstation. The user can freely choose the extent to which each volume is shown. This tool, initially developed to better separate coils or clips from residual or recanalized aneurysms, allows excellent imaging of the vascular-osseous relationships in complex anatomical regions.

DVT permits visualization of vascular and osseous anatomy simultaneously and allows for identifying their precise anatomical relationship without interfering with each other. DVT is further helpful for demonstrating a coil mass, clip or stent in relation to an aneurysm neck. Consequently, aneurysm regrowth, misclipping and stent malapposition can be detected more easily than on 2D-DSA images.

7.2.6.2

Angiographic Computed Tomography (ACT), DynaCT (Siemens), C-arm Flat Detector CT (FD-CT), Flat Panel CT (FP-CT) or Cone Beam CT (Figs. 7.58, 7.59, 7.61–7.69)

The integration of flat detectors into rotating C-arms in combination with a higher number of projections per rotation has led to another new modality that allows cross-sectional “CT-like” images to be obtained. The use of multiplanar reconstructions, based on rotational angiograms or radiograms, is per se not new

Fig. 7.58. Cone beam CT is based on the measurement of an entire volume (cone) in one single orbit, obtained with either image intensifier or flat panel detector systems (from SUETENS 2002)

Fig. 7.59a–c. The rotating C-arm is capable of aquiring up to 543 projections (20 s, increment 0.4) that are mostly used to reconstruct a 3D dataset within less than a minute. This 3D volume can be viewed in a cross sectional imaging mode either as multiplanar reconstructions (MPRs), maximum intensity projections (MIPs) or in volume rendering technique (VRT)

a

b

c

Fig. 7.60. Dual volume technique (DVT). Both rotational sweeps are used for reconstructing two separate data sets. The filling run provides the vascular anatomy, while the mask contains the relevant background information, usually consisting of high-contrast objects, in this case the platinum coils. Both volumes are simultaneously viewed, while the amount of information being displayed can be freely chosen

Fig. 7.61. DynaCT (543 projections in 20 s, 512×512 “bone normal”), 1 mm MPR. Intraprocedural bleeding that occurred during AVM embolization. Note, although the contrast resolution is lower than in conventional CT, hyperdensities caused by intraparenchymal blood (here mixed with some contrast after vessel perforation) become visible. In this case, no significant mass effect and no hydrocephalus was noted so that the procedure could be continued

Fig. 7.62. DynaCT (543 projections in 20 sec, 512×512 “bone sharp”) 20 mm MIP, shows postembolization cast of Onyx® after endovascular occlusion of a DAVF of the cribiforme plate (inset). The resolution of high-contrast objects, such as a cast of liquid embolic material is more detailed than achievable by digital radiography or conventional CT. This data set was obtained immediately postembolization, while the patient was still under general anesthesia on the angio table

and has been reported in a number of clinical applications. Its clinical use, however, has been limited to multiplanar reformatted images for visualization of high-contrast objects, such as contrast filled vessels or bony structures (3D myelogram, etc.).

In the past, cross-sectional imaging in neuroradiology has been mainly performed using conventional CT and MRI. The use of rotating C-arms in the angiographic suite was initially focused on 3D imaging of intracranial vascular lesions such as aneurysms and AVMs, employing mainly volumerendering techniques.

Conventional CT or X-ray computed tomography produces cross-sectional images, based on X-ray attenuation. The word tomography stems from the Greek words tomos (slice) and graphe (write). Angiographic computed tomography (ACT, when contrast is used to opacify vascular structures) or DynaCT (Siemens) is a similar cross-sectional imaging modality based on the cone beam algorithm from Feldkamp, while in conventional CT parallel beam geometry, or in recent CT generations fan beam geometry, is usually employed. The terms flat detector CT, flat panel CT or C-arm CT are meanwhile synonymously used for what is technically cone beam CT.

The cone beam CT technology using C-arms was pioneered by Siemens but is meanwhile implemented by other vendors too (X-perCT by Philips). The description of technological details in the following chapter is based on the use of the first installed angiographic bi-plane FD system at the Methodist Hospital Houston, Texas. This system has been improved and continuously upgraded since September 2004.

The cone beam reconstruction algorithm is the basis for rotational 3D angiography, where the 3D datasets are reconstructed from a series of 2D projections. The method is derived from the standard fan beam formula. The density in one voxel is obtained as the sum of contributions from all projections through the voxel while the angle between two projections (increment) and the distance between the X-ray tube and the detector are taken into account (Suetens 2002).

With the introduction of FD technology into modern angiographic systems, cross sectional imaging in the angiographic suite has gained importance providing so-called low-contrast imaging. In particular, the improved contrast resolution due to the wider dynamic range (see above) currently allows for “soft tissue imaging” in the angiographic room. Using a 20-s single rotation acquiring 543 non-subtracted projections (Axiom Artis, Siemens), a contrast reso-

lution of <10 HU can be achieved, allowing for visualizing low contrast structures such as grey and white matter and the delineation of brain tissue from the intracranial ventricular system. In other words, this cross-sectional imaging allows for visualization of hyperdense intracranial lesions such as subarachnoid hemorrhage (SAH), hematoma caused by vessel perforation during an endovascular treatment or a developing hydrocephalus. Early recognition and monitoring of intraprocedural complications during neuroendovascular treatment has become possible and is of great value for interventional neuroradiologists and endovascular neurosurgeons (Heran et al. 2006). DynaCT contributes to the overall safety of EVT (Figs. 7.61 and 7.62), which is of particular importance for the often time-consuming transvenous embolizations of DAVFs or DCSFs.

7.2.6.3

Image Post-Processing (Figs. 7.63–7.65)

Various software tools for post-processing and analyzing angiographic volume data sets are in use today. Similar to CTA, they employ dedicated display modes, such as MPRs, MIPs or VRTs. Because not every interventional neuroradiologist or endovascular neurosurgeon may be familiar with 3D imaging some of the basic principles are explained in the following.

1. MPR: Multiplanar Reformatting – technique to display up to three orthogonal cut planes using the averaged voxel values along one beam, which can be correlated with a true 3D image.

2. MIP: Maximum Intensity Projection – technique to display up to three orthogonal cut planes using the maximum voxel values along one beam.

3. VRT: Volume-Rendering Technique – technique to visualize the structure of volume datasets.

4. SSD: Surface Shaded Display – technique to visualize a surface that corresponds to an isovalue in the dataset.

1. Multiplanar Reformatting (MPR) is one of the oldest visualization techniques used for viewing 3D medical images, providing the possibility for an arbitrarily positioned and oriented 2D plane to be placed in a 3D data set so that the projection of the data on that plane may be viewed. In modern software packages there are usually three planes simultaneously shown, each of them corresponding to one of the major axes, sagittal, coronal and axial (Fig. 7.63). This allows for precise orientation and localization of any object in the 3D data set. The

aSagittal MPR

bCoronal MPR