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11.5 Planar X-Ray Imaging and C-arm Systems
The original and still widely used method to acquire an X-ray image consists
of irradiating the object with X-rays from one side and capturing a single
transmission image with a suitable recording device on the other side of
the object. This technique produces a two-dimensional (2D) projection of
the 3D object without depth information along the direction of projection.
Traditional detectors consist of an image-intensifying phosphorescent layer
and a photographic or charge-coupled device (CCD) camera. These bulky
detectors have been replaced by much lighter and more compact flat-panel
detectors . Planar (projection) X-ray imaging is frequently used to display sin-
gle images of anatomical structures (bones, lungs), as well as for real-time
monitoring of interventional or surgical procedures ( fluoroscopy ). A com-
mon application for fluoroscopy is the visualization of blood vessels and
the perfusion of tissues through digital subtraction angiography (DSA). In
this procedure, first a catheter is inserted through a blood vessel (artery)
with the help of a guidewire. Projection X-ray images of 3-60 frames per
second (fps) at a low dose allow the doctor to monitor the correct placement
of the guidewire in real time. Next, a reference image with a higher dose
is recorded, immediately followed by the injection of a radiopaque contrast
agent through the catheter and the acquisition of a series of image frames.
The vessels filled with the contrast agent become clearly visible in the X-ray
images. To enhance the quality of the image, the reference image is automati-
cally subtracted for each frame. This masks obstructing anatomical struc-
tures like bones while leaving volumes filled with the contrast agent visible
( Figure 11.4 ) . A radiologist can then identify regions of reduced blood low
and their cause (e.g., stenoses).
C-arm systems ( Figure 11.5 ) acquire a series of planar images and are
often used for interventional procedures when real-time imaging is
required. Examples include angiography, needle biopsy, and transarte-
rial chemoembolization. Due to their high spatial resolution and real-time
image acquisition, C-arm systems are also well suited for monitoring the
delivery and tracking of stem cells. C-arm systems enable X-ray imaging
with large flexibility in spatial orientation. The X-ray source and detector
are mounted on opposite ends of a C-shaped support frame, which can
rotate around both its isocenter and its diameter. The C-arm can also be
moved along and rotated around the patient table. This allows for almost
any positioning of the imaging system around the patient and an optimal
choice of the projection angle. The open design of C-arm system offers easy
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