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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_605_Библиотеки_им_академика_М_И_Перельмана.pdf
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91
Scintillation detector s (Fig. 5.3) consist of a scintillation material that
converts X-rays into optical photons, an optical relay element to focus or
amplify them, and a photomultiplier or a photodiode to detect these particles and transfer them into electrical signals for further processing
[6,7].
Scintillation materials can be divided into organic scintillations or single
crystals of different chemical elements. The energy resolution of single
crystals is higher than the energy resolution of organic scintillations.
Moreover, a time resolution better than 1 ns and a count rate capability
up to 2 3 10
6
photons per second are achievable with semiconductor
materials. Scintillation detectors in conjunction with gas ionization detectors are called gas scintillation detectors. By combining the operation of
gas ionization chambers and photon detectors, overall performance can
be improved
[8].
Semiconductor detectors are solid-state devices that operate essentially
like ionization chambers, but offer higher detection efficiency and better
spectrometric resolution
[9]. In contrast to gas ionization detectors, the
charge carriers are not electronion pairs, but rather pairs of electrons
and holes. An electronhole pair is generated due to an electron that
moves from the valence band to the conduction band. In this process, a
free hole is created in the valence band. Under the influence of the electric field, the electrons and holes are swept away, and the proper electronics can collect the charge in a pulse
[10].
Figure 5.3 A sketch of a scintillation detector with the main constituent parts. From
MITI.
92 Biomedical Engineering in Gastrointestinal Surgery
In direct conversion flat panel detectors, X-rays are directly converted
to electron-hole pairs and the resulting charge is collected from individual
pixel electrodes. These detectors usually possess photoconductors made of
amorphous selenium and thin film transistors to read the charge signal.
The detectors enable a high spatial resolution and high X-ray absorption
efficiency at low energies
[2].
A conventional differential detector is a light-sensitive sensor for recording images, and consists of an integrated circuit containing an array of
linked or coupled capacitors. The X-ray energy is converted into light by a
scintillation material. The CCD then records the quantity of light emitted.
The light is converted into electrical charges
[11].Theavailableamountof
pixels goes up to 4096 3 4096 pixels, with pixel sizes of 12 3 12 µmand
readout times of less than 1 second. For high spatial resolution, a 520 µm
thick sapphire scintillation screen is optically coupled with a high-quality
microscope lens to give a spatial resolution of around 1 µm
[8].
Photon-counting detectors generate additional information by counting individual photons and measuring their energy. For computed tomography (CT), this facilitates the reconstruction of images free of spectral
artifacts and with identical quantum efficiency; it also reduces the image
noise in comparison with images obtained by energy integration
[12].
5.1.3 Projection Radiography
The possibility to use X-rays for diagnostic purposes hinges on the fact
that various body tissues have differences in their density. According to
which kind of tissue is examined, X-rays are absorbed at different intensities. The result of projection radiography is a so-called shadow image of
the internal structure that displays the variation of spatial intensity of the
radiation transmitted. Bones and foreign matter such as metallic devices
appear in white colors, and air-filled cavities are shown up in black.
These structures are well displayed in the image obtained because they
have either a higher or a lower density in contrast to the surrounding
softer tissue. Body organs are shown in shades of gray because of their
lower density and lower attenuation.
Projection radiography can be subcategorized into digital projection
radiography (DPR) and real-time imaging or fluoroscopy.
DPR uses digital detectors to generate a digital image, which is then
stored on a digital medium. This double-stage approach differs from
analog or screen-film radiography, in which the film combines detection
Diagnostic Procedures
Figure 5.4 (A) Historical and (B) current state-of-the-art viewing station: digital radiography. All from MITI.
93
and storage functions. DPR can be divided according to its readout process into computed radiography and direct radiography. The application
of DPR covers most body areas
[11].
In the past, each single image had to be handled physically. The
archiving required much space and a strict order to find them again in
case they were needed later on. X-ray archives were nicknamed “silver
mines” since they housed thousands of photographic images. Digital radiography significantly improves the handling, archiving, and reidentification of radiologic information.
Digital imaging consists of four separate steps: generation, processing,
archiving, and presentation of the image. After detecting the absorbed
energy and transforming it into electrical charges, they are recorded, digitized, and quantified. Postprocessing software is needed then to arrange
the raw data into a final and medically useful image, which is subsequently sent to a digitized storage system. The image can be either presented as a hard copy film or viewed on a computer workstation
[11]
(Fig. 5.4).
Compared to screen-film radiography, DPR is advantageous because
images can be stored digitally into a digital picture archiving and communication system (PACS). This offers a space-saving storage method and
allows the information to be accessed anytime.
Recent Developments and Current Research
Research on DPR involves the investigation of new storage phosphor s
and scanning systems for computed radiography and improvements of the
detective quantum efficiency and SNR of the detectors, which results in
further exposure reduction or higher image quality. An optimized
94 Biomedical Engineering in Gastrointestinal Surgery
architecture of the readout array could be achieved by reducing the size
of circuits and pixels
[11].
5.1.4 Real-Time Radiography
Real-time radiography or fluoroscopy gives a detailed view of the movement of a body part, of a medical instrument, or of a contrast agent moving through the body by displaying continuous X-ray images on a screen.
Real-time radiography is versatile for diagnostic and interventional
purposes such as angiographic examinations, catheter insertions, or the
manipulation and the placement of devices within the body.
The arrangement of tube, detector, and patient does not differ from
projection radiographic systems. The substantial element behind real-time
radiography is a fluoroscopic screen, which converts radiation to light.
The light signals can be observed directly, intensified, and/or converted
to a video signal which is presented on a screen.
In visceral surgery, fluoroscopy still has an important role.
Preoperatively, the highly dynamic motor responses of the upper GI
tract—in particular fast movements in the hypopharyngeal region—can
still be assessed best by high-speed fluoroscopy. Likewise, the dynamic
behavior of the small intestine can be examined reliably by means of a
Sellink’s procedure. The same holds true for the barium enema of the
colon (
are used. Barium is cheaper but should be avoided if a perforation/leakage
of the GI tract is suspected. In these cases, water soluble iodine-based
contrast media should be preferred.
system is visualized by direct injection of the contrast medium into an
artery. Visualization can even be combined with therapeutic approaches
(
However, radiation exposure, especially during longer examinations, is
higher compared to DPR.
medical imaging. Its simplicity and versatility, in addition to its low costs
compared to other imaging techniques, such as CT or MRI, mean that
DPR is expected to remain as relevant as it is today for the foreseeable
future.
Fig. 5.5).
For contrast enhancement, iodine-based or barium-sulfate compounds
Another important application is angiography. The arterial vascular
Fig. 5.6).
The wide field of application of real-time radiology is beneficial.
DPR is currently one of the most common diagnostic procedures in
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95
Figure 5.5 (A) High-speed fluoroscopy of the esophagus; (B) dynamic radiographic
examination (fluoroscopy) of the small intestine (so-called Sellink examination):
Initially, the duodenum and the first jejunal loops become visible (middle up); middle
center: after a few minutes, the loops of the jejunum are visible; middle bottom: the
last loops of the ileum appear. (C) Exploration of the rectum/descending colon using
a barium enema. All: Courtesy: Dr. K. Holzapfel, Klinikum rechts der Isar.
Figure 5.6 Direct angiography of intestinal vessels. The leakage of contrast medium
is clearly visible. A coil was positioned at the same session which stopped the bleeding immediately. Courtesy: Dr. A. Fingerle, Klinikum rechts der Isar.
96
Biomedical Engineering in Gastrointestinal Surgery
5.2 COMPUTED TOMOGRAPHY
CT (synonyms: X-ray CT or computerized axial tomography scan) is an
innovative tool to gain 3D data sets instead of 2D information as provided
by conventional radiology.
5.2.1 Principle of Computed Tomography
CT is an advancement of conventional projection radiography and overcomes one of its key problems: that certain features cannot be precisely
located because of overlapping parts or because features of interest are out
of the range of the plane. The solution offered by CT is to combine
information from a series of 2D X-ray absorption images, as the X-ray
source and the corresponding detector are rotated about a single axis.
Afterward, tomographic algorithms are used for reconstructing this
series of images to produce a 3D digital image. In this image, each voxel
(volume element or 3D pixel) represents the X-ray absorption at a specific
point. The 3D internal structure and the unique position of internal features can be inferred from the images due to the known relationship
between X-ray absorption and material density. De facto, 3D images are
represented as a series of 2D slices
In general, there are seven main topics of CT imaging modifications.
All CT applications are based on at least one acquisition system, which
[13] (Fig. 5.7).
Figure 5.7 Schematic illustration of X-ray CT acquisition and reconstruction processes. A volume data set is created by adding numerous levels of 2D data. From
Landis EN, Keane DT. X-ray microtomography. Materials Character 2010;61
(12):130516. Modified by D. Ostler.
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97
means one tube-detector pair. Multislice CT uses multiple rows of detectors and a widened X-ray beam to use the X-ray beam more effectively.
Cone beam CT allows a different image acquisition process due to the use
of a conical X-ray beam. Dual-energy and dual-source CT have two acquisition systems and can operate in different voltage settings. Phase-contrast
CT is a medical imaging technique that makes use of the phase shift,
which emerges when X-rays pass through different tissue. X-ray microtomo-
graphy is used to characterize tissue in its microstructure. Electron beam CT
detects calcium build-up in coronary arteries by using an electron emitter
to generate X-rays.
5.2.2 Multislice Computed Tomography
Multislice CT (MSCT) is an advancement of single-slice CT (SSCT).
The basic idea of MSCT is the use of multiple rows of detectors in conjunction with widening the X-ray beam in the z-direction (slice thickness) to use the X-ray beam more effectively. This indicates that the data
can be collected for more than one slice at a time
Real volumetric images are obtained in a shorter period of time.
However, radiation dose is higher. Even dynamic processes can be evaluated (
Fig. 5.8).
The major difference between SSCT and MSCT is in the design of
the detector arrays (
Fig. 5.9). The SSCT uses detector arrays that form a
1D array. In MSCT, each detector element is divided into several smaller
detector elements in the z-direction. These detector elements form a 2D
array. There are various types of rows of detector elements. Current
hospital systems have 64 rows or more of detector elements in order to
reach a very high resolution
[15].
[14].
Figure 5.8 Contemporary MSCT workplace. From MITI.
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Biomedical Engineering in Gastrointestinal Surgery
Figure 5.9 Single-slice computed tomography (left) versus multislice computed
tomography (right). From Goldman LW. Principles of CT: multislice CT. J Nucl Med
Technol 2008;36(2):5768. Modified by D. Ostler.
In MSCT, the slice thickness is not determined by the X-ray beam
collimation. Instead, it is determined by the detector configuration. This
length is often referred to as detector collimation due to the length each
individual detector has. There are several ways to combine detector
elements, as shown in
Fig. 5.10 [14].
The major advantages of MSCT are the shorter acquisition times, the
retrospective creation of thinner or thicker sections from the same raw data
set, and the improved 3D rendering. The possibilities of MSCT acquisition
are widespread: the scan of anatomical volumes with standard techniques at
significantly reduced scan times, scanning larger volumes previously not
accessible in practical scan times, or the scan of anatomical volumes with
high axial resolution
[16]. The disadvantages of MSCT are the high radia-
tion doses for the patients being subjected to an examination, and the high
costs of purchase and maintenance for such systems
[17].
Current developments and trends show systems with a larger number of
slices driven by clinical applications, which become possible through the
use of such detectors. Recent systems by Toshiba (Shimoishigami, Japan)
and Siemens (Erlangen, Germany) target these applications by introducing
Diagnostic Procedures
Figure 5.10 Examples of fixed array detectors (A, B) and adaptive array detectors (C, D)
for four-slice MSCT scanners. (A) Four 5-mm detectors built out of four linked 1.25-mm
elements. (B) Paired linking of the inner eight elements to act as four 2.5-mm detectors.
(C) Four 5-mm slices built with adaptive-array elements. (D) Four innermost elements
are paired to form 2.5-mm detectors which, along with the two 2.5-mm detectors, collect data for four 2.5-mm slices. From Goldman LW. Principles of CT: multislice CT. J
Nucl Med Technol 2008;36(2):5768. Modified by D. Ostler.
99
systems with 128 slices (Siemens) and 320 slices (Toshiba) using different
technological paths. These technological improvements offer the possibility
to acquire 4D images (3D plus time). There are prototype systems that use
a special flat-panel detector technology that was originally used for conventional catheter angiography. As the high radiation dose patients are subjected to is the key problem, the introduction of dynamic collimators will
eliminate the increasing problem of overradiation in spiral scans, which has
increased as a result of increasing detector width
[18].
5.2.3 Cone Beam Computed Tomography
Cone beam CT (CBCT) or digital volume CT is an advancement of
conventional CT that uses a divergent pyramidal or conical X-ray beam
instead of a fan-shaped beam
The geometrically different beam configuration of CBCT enables the
acquisition of sequential planar projection images of the field of view
(FOV) in a complete or sometimes partial arc. The resolution of CBCT
images is defined by the individual volume elements or voxels, which
[19].
100 Biomedical Engineering in Gastrointestinal Surgery
are produced from the volumetric data set. The voxel dimensions are primarily influenced by the pixel size on the area detector, whereas in conventional CT, the voxel dimensions depend on slice thickness
[19].
A CBCT system impresses with its simple operability and integration
into routine practices, shorter examination times—including higher
image sharpness and lower radiation dose—increased X-ray tube efficiency and lower image distortion because of movements of the patient.
The disadvantages of CBCT systems are based on the detection of large
amounts of scattered radiation during the acquisition, especially of larger
FOVs. This results in limitations in image quality related to noise and
contrast resolution
[19,20].
Recent Developments and Current Research
Future trends in CBCT imaging will probably lead to further reduction of scan time, improvements in image quality and accuracy—including soft-tissue contrast—and a further reduction of radiation dose
[19].
5.2.4 Dual-Energy Computed Tomography
Dual-energy CT (DECT) acquires two image data sets of the same anatomic body area with the help of a low-energy and a high-energy X-ray
spectrum. As a result, an analysis of energy-dependent changes in the
attenuation of different materials becomes possible. For the acquisition of
these different energy data sets, three DECT scanners are available: a
single-source dual-energy scanner with fast kilovoltage switching
(SSDESKS), a single-source dual-energy scanner with dual detector layers
(SSDESDDL) and a dual-source scanner with dual detector arrays. The
latter scanner type will be covered as a separate modification called dual-source CT (DSCT) further on.
A SSDESKS has a single radiation source and uses its ability to alternate rapidly between two kilovoltage settings (80 and 140 kVp) to generate the different spectra while the CT gantry makes a single rotation
To sustain the higher tube output at 140 kVp, the exposure time ratio is
varied between 80 and 140 kVp acquisitions to maximize the CNR. The
alternating high- and low-energy data are captured by a detector with a
fast response and a data system with a fast sampling ability
[22].
A SSDESDDL is based on a modified detector array with twoscintillation layers positioned one above the other to receive the separate
energy image data streams from a single X-ray source. The overlying
layer captures low-energy data, whereas the underlying detector captures
[21].