7.2.5

Flat Detector Technology in Neuroangiography

(Figs. 7.50–7.57)

Flat detector (FD) technology was introduced in the early 1990s, almost 100 years after the discovery of X-rays by Conrad Roentgen. Initial medical applications were for thorax and skeletal X-rays, followed by digital mammography. Although it has only recently become possible to implement FDs in angiographic systems, image intensifier systems are rapidly being replaced in cardiac, general and neuroangiography. The main reason for this development is a clearly improved image quality of 2D DSA imaging combined with other technological advantages.

The currently dominating FD technology is based on an indirect X-ray conversion process, using a cesium iodide (CsI) scintillator and an amorphous silicon active pixel matrix (Figs. 7.52–7.54). Cesium iodide can be grown as needle-shaped crystals measuring 5–10 μm in diameter, and ensuring that light reaches the photodiode with only little scatter, while limiting its lateral diffusion. This scintillator also has very good X-ray absorption properties (Spahn 2005). The detector used in the system described here is 30 cm×43 cm (A-plane) and 20 cm×20 cm (B- plane) with a pixel size of 154×154 and 184×184 μm respectively. This pixel size is optimized for high resolution required in diagnostic radiology and provides a square matrix of about 9 million pixels on the large detector.

FDs are smaller and add less weight to the C-arm allowing for faster rotations, providing more stability and fewer distortions. While image intensifier systems (II systems) used a multi-step conversion from X-ray beams into a digital image signal that causes increase of noise, this conversion is only a two-step process in FD technology (Figs. 7.50 and 7.52). Resulting advantages over II systems are more image formats, less geometric distortion, more homogenous exposure, excellent coarse contrast and high X-ray sensitivity. One of the most important features of FDs is their wide dynamic range (14 bit vs 12 bit in II systems). It allows one to cover a wider dose range without risking wrong exposure, which is particularly advantageous in angiography where broader dose ranges need to be covered from low levels of about 10 nGy to much higher system dose levels of about 5 μGy (Spahn 2005).

The high dynamic range provides a better contrast resolution (16,384 different grey scale values available per pixel as opposed to 4096) allowing for soft tis-

sue imaging using DynaCT (see below). In addition, an increase in image matrix from 1024×1024 (1 k) to 2480×1920 (2 k), allows for better identification of small arteries such as dural branches of the cavernous portion of the ICA. Figure 7.55 shows an example that demonstrates the improved image quality of 2DDSA.

7.2.6

Rotational Angiography and 3D-DSA

(Figs. 7.58, 7.68 and 7.71–7.74)

Early experimental work on rotational angiography was performed in the 1970s and was quickly followed by clinical applications (Cornelis et al. 1972; Voigt et al. 1975). Thron and Voigt (1983) were able to demonstrate that cerebral rotational angiography added value to the existing angiotomography or magnification angiography (Wende and Schindler 1970), techniques used at that time to improve visualization of cerebral aneurysms and AVMs. The authors used a 70-mm camera, mounted onto a C-arm that acquired one image every 5° during a 5–6 s sweep. Although digital subtraction was not possible at that time, the set up provided relatively good visualization of size, shape and orientation of cerebral aneurysms.

The next step was the development of the so-called 3D-morphometer by French investigators (Heautot et al. 1998). The morphometer consisted of a CT gantry in which two X-ray tubes plus the image intensifier systems were integrated. This rather complex technology provided angiographic 3D images of aneurysms and AVMs of satisfactory quality.

Three-dimensional images of intracranial vasculature acquired with a rotating C-arm were presented for the first time by Picard et al. (1997). Since then, a continuous improvement of 3D imaging using rotating C-arms has resulted in widely accepted clinical use of 3D angiography in neuroendovascular treatment.

Because the main focus in 3D angiography was initially on diagnosis of cerebral aneurysms and AVMs, bony structures adjacent to this vasculature compromised its visualization and were commonly removed during the image post processing (Siemens systems).

Advantages of simultaneous reconstructions of osseous and vascular structures have been demonstrated only to a limited degree (Gailloud et al. 2004), but are obvious in areas with a complex ar-

Image Intensifier

Image

signal

Fig. 7.50. Conversion process in an traditional image intensifier (II) uses multiple steps to convert an X-ray beam into a digital image signal (X-rays-light-electrons-light-electrons). This can cause increased background noise, while the spatial resolution is determined by the camera’s resolution, which is usally 1K (1024×1024)

X-Rays

Scintilator

Conversion of x-Rays into light

Photo Diode

Conversion of the light photons into electric charges

Read out Matrix

(TFT)

Fig. 7.51. C-arm mounted image intensifier of its last generation. This system has been used for rotational angiography and 3D-DSA since its introduction in the late 1990s (see examples in Figs. 7.70–7.73, 7.81, 7.83, 7.85)

Fig. 7.52. As opposed to image intensifiers, the FD uses only a two-step (indirect conversion) conversion process (X-rays- light-electric signal)

X-Rays

CSL-Scintilator

a-Si Pixel matrix

Fig. 7.53. New technologies employ flat detector (FD) with fast-imaging capability. The FD used in Siemens and Philips systems is based on Cesium Iodide (CsI), combined with an amorphous silicon active matrix array. This provides excellent quantum efficiency and a good resolution due to the needle-shaped or pillar-like crystal structure, limiting the lateral light diffusion. The matrix can be increased to 2K (2480×1920), although monitor systems for full 2K resolution are not commercially availabe yet. As an alternative, new medical grade displays (56 inch, 3840×2160 pixel) are recently available

Fig. 7.54. The CsI crystals can be grown in needle-shaped form, measuring 5–10 μm. The size of one detector element (154 μm) defines (among other factors) the spatial resolution in an acquisition mode without binning (pixel binning). In order to reduce the size of data sets to be reconstructed, programs with various binning modes (1×1, 2×2 detector elements), faster rotations and fewer projections are used

Fig. 7.56. Flat detector used in the current biplane systems Axiom Artis dBA.

Area: 30×40 cm

Pixel size: 154×154 μm (detector element size)

Frame rates: Up to 30 fps (monoplane), 15 fps biplane. No binning: 7.5 fps

Binning: 10 nGy–3.5 μGy

Analog to digital conversion: 14 bit.

a

b

Fig. 7.55a,b. 2D-DSA demonstrating the improved image quality of cerebral angiograms using recent FD technology.

Two arteriograms of the left ICA, same views in the same patient. a June 2004, b August 2006. Although the overall filling of the ICA in a appears better, opacification of the cavernous carotid branches, MHT (short arrow) and ILT (arrow), as well as of the mandibular artery (thin arrow) is clearly superior (not only sharper, but also reveals more branches). Note that in theory, the spatial resolution of an II system using the maximum zoom format is 6 LP/mm, while the FD reaches approximately 3 LP/mm. Various other factors including size of focal spot (0.3 mm), dynamic range (14 vs. 12 bit) and improved image post processing also play a role.

rangement of these components such as the skull base. For quite some time, investigators have been focusing on improving anatomic understanding using colored plastic casting of vessels in cadaver specimens as the main tool to investigate the minute and vascular anatomy imbedded in bony canals and foramina, especially in the middle cranial fossa and parasellar region (Lasjaunias 1984; San Millan Ruiz 1999; Parkinson 1963, 1984; Rhoton et al. 1979, 1984). The use of 3D vascular imaging of the cavernous sinus and its related structures employing 3D angiography and modern reconstruction techniques, documented in the literature only scarcely (Nishio et al. 2004; Lasjaunias et al. 2001, Mitsuhashi et al. 2007; Hiu et al. 2009), has been of major interest for the author (Benndorf 2002). Initial results of such 3D reconstructions (Figs. 7.70– 7.73, 7.81 and 7.83–7.85), using non-subtracted rotational angiography, obtained with an II system (Neurostar, Siemens), which already encouraging. Using simultaneous reconstructions of the osseous and vascular structures, it has been possible to visualize small dural arteries and their course through skull base foramina. One can follow the artery of the foramen rotundum through its foramen and canal, the course of the AMA through the foramen ovale or anastomotic vessels through the foramen lacerum. Using this type of angiographic 3D imaging, identification of arteries feeding a DCSF becomes easier and is possible without using traditional 2D landmarks (Benndorf 2002, 2008; Hiu et al. 2009). Commonly, high-contrast tissue like bone causes reconstruction artifacts. Due to isotropic resolution of cone-beam reconstructions, as used in 3D-DSA, these artifacts become of minor importance (Fahrig et al. 1997). Each voxel has the same size in the X, Y and Z directions, leading to a reduction of the nonlinear partial volume effect. Visualization of vessels and bone in maximum intensity projections (MIPs) is possible because vascular density is increased up to 8000 HU, compared to 2000–2500 HU, typical for bone. The combination of high contrast and spatial resolution provides higher accuracy in displaying anatomic details, not obtainable using other imaging technology, including modern multi-slice CT scanners. Using an II system (C-arm mounted image intensifier), el Sheik et al. (2001) showed that highresolution multiplanar reconstructions of osseous spongiosa can be obtained (rotational osteography). The usefulness of 3D reconstructions after cervical myelograms, revealing the relationship between contrast-filled thecal sac and osteophytes of the cer-

vical spine has also been demonstrated (Kufeld et al. 2003).

Image quality of angiographic 3D reconstructions has been significantly improved by the recent introduction of FD technology, as demonstrated in examples displaying small vessel reconstruction using the older II system (Neurostar, Siemens Medical) and the latest FD system (Axiom Artis, dBA, Siemens Medical) in Fig. 7.55. The current spatial resolution of the FD is defined by the size of its detector elements: 154 μm providing 0.13 mm which correspond to 4.1 Lp/mm (Nyquist frequency) in the unbinnend mode, while the maximum resolution of a Somatom 64 is 0.36 mm, corresponding to 1.4 LP/mm.

The available computing power can use a 20-s rotation with 543 projections only when two detector elements are combined (binned), which doubles the pixel size from 154 to 308 μm. Special programs, capable of using the full detector resolution, providing so-called 2 k datasets, are currently under evaluation.

Nonetheless, visibility of vascular and bony details is superior to conventional (II based) 3D-DSA or CT/CTA. This high spatial resolution is of importance for imaging small arteries such as the dural branches of the cavernous ICA and its communications with the ECA. The artery of foramen rotundum for example has a normal diameter of approximately 135 μm (Lang 1979).

Different from the arterial network within and around the CS, the venous anatomy appeared to be of rather minor interest in the medical literature (Braun and Tournade 1977; Braun et al. 1976). Hence, veins and sinuses in the middle cranial fossa and the CS region and its communications with the extracranial veins and venous plexus of the skull base are neither well studied nor fully understood. More recently, several investigators have pointed out the importance of these veins for anatomy and physiology (Gailloud et al. 2000; Schreiber et al. 2003; Doepp et al. 2004; Takahashi et al. 2005) of the cerebral blood circulation. Angiographic computed tomography (ACT, see below)-based MIPs, obtained using intravenous or intra-arterial contrast injections, can reveal even the smallest veins and sinuses in this area (Benndorf 2002; Nishio 2004;

Mitsuhashi 2007).

Because of its superior spatial and temporal resolution, intra-arterial DSA is the imaging tool of choice for cerebral vascular lesions, primarily for arteriovenous shunting diseases. Analysis of radiology data is mostly done by reading 2D images. Interventional neuroradiologists and other endovascular neurosur-