Babeh Edi

CT Instrumentation & Physics
Wilbur L. Reddinger, M.S., R.T.(R)(CT)
Copyright 1997 OutSource, Inc.
All rights reserved
Computed tomography (CT) is the science that creates two-dimensional crosssectional
images from three-dimensional body structures. Computed
tomography utilizes a mathematical technique called reconstruction to
accomplish this task. It is important for any individual studying the CT science
to recognize that CT is a mathematical process. In a basic sense, a CT image is
the result of "breaking apart" a three-dimensional structure and mathematically
putting it back together again and displaying it as a two-dimensional image on
a television screen. The primary goal of any CT system is to accurately
reproduce the internal structures of the body as two-dimensional cross-sectional
images. This goal is accomplished by computed tomography's superior ability
to overcome superimposition of structures and demonstrate slight differences in
tissue contrast. It is important to realize that collecting many projections of an
object and heavy filtration of the x-ray beam play important roles in CT image
formation. Each component of a CT system plays a major role in the accurate
formation of each CT image it produces.
CT Gantry
The first major component of a CT system is referred to as the scan or imaging
system. The imaging system primarily includes the gantry and patient table or
couch. The gantry is a moveable frame that contains the x-ray tube including
collimators and filters, detectors, data acquisition system (DAS), rotational
components including slip ring systems and all associated electronics such as
gantry angulation motors and positioning laser lights. In older CT systems a
small generator supplied power to the x-ray tube and the rotational components
via cables for operation. This type of generator was mounted on the rotational
component of the CT system and rotated with the x-ray tube. Some generators
remain mounted inside the gantry wall. Some newer scanner designs utilize a
generator that is located outside the gantry. Slip ring technology eliminated the
need for cables and allows continuous rotation of the gantry components. The
inclusion of slip ring technology into a CT system allows for continuous
scanning without interference of cables. A CT gantry can be angled up to 30
degrees toward a forward or backward position. Gantry angulation is
determined by the manufacturer and varies among CT systems. Gantry
angulation allows the operator to align pertinent anatomy with the scanning
plane. The opening through which a patient passes is referred to as the gantry
aperture. Gantry aperture diameters generally range from 50-85 cm. Generally,
larger gantry aperture diameters, 70-85 cm, are necessary for CT departments
that do a large volume of biopsy procedures. The larger gantry aperture allows
for easier manipulation of biopsy equipment and reduces the risk of injury
when scanning the patient and the placement of the biopsy needle
simultaneously. The diameter of the gantry aperture is different for the diameter
of the scanning circle or scan field of view. If a CT system has a gantry
aperture of 70 cm diameter it does not mean that you can acquire patient data
utilizing a 70 cm diameter. Generally, the scanning diameter in which patient or
projection data is acquired is less than the size of the gantry aperture. Lasers or
high intensity lights are included within or mounted on the gantry. The lasers or
high intensity lights serve as anatomical positioning guides that reference the
center of the axial, coronal, and sagittal planes.
X-ray Tube, Collimation, Filtration
CT procedures facilitate the use of large exposure factors, (high mA and KvP
values) and short exposure times. The development of spiral/helical CT allows
continuous scanning while the patient table or couch moves through the gantry
aperture. A typical spiral/helical CT scan of the abdomen may require the
continuous production of x-rays for a 30 to 40 second period. The stress caused
by the constant build up of heat can lead to a rapid decrease of tube life. When
an x-ray tube reaches a maximum heat value it simply will not operate until it
cools down to an acceptable level. CT systems produce x-radiation
continuously or in short millisecond bursts or pulses at high mA and KvP.
values. CT x-ray tubes must possess a high heat capacity which is the amount
of heat that a tube can store without operational damage to the tube. The x-ray
tube must be designed to absorb high heat levels generated from the high speed
rotation of the anode and the bombardment of electrons upon the anode surface.
An x-ray tubes heat capacity is expressed in heat units. Modern CT systems
utilize x-ray tubes that have a heat capacity of approximately 3.5 to 5 million
heat units(MHU). A CT x-ray tube must possess a high heat dissipation rate.
Many CT x-ray tubes utilize a combination of oil and air cooling systems to
eliminate heat and maintain continuous operational capabilities. A CT x-ray
tube anode has a large diameter with a graphite backing. The large diameter
backed with graphite allows the anode to absorb and dissipate large amounts of
heat.
The focal spot size of an x-ray tube is determined by the size of the filament
and cathode which is determined by the manufacturer. Most x-ray tubes have
more than one focal spot size. The use of a small focal spot increases detail but
it concentrates heat onto a smaller portion of the anode therefore, more heat is
generated. As previously described, when heat is building up faster than the
tube can dissipate it the x-ray tube will not produce x-rays until it has
sufficiently cooled. CT tubes utilize a bigger filament than conventional
radiography x-ray tubes. The use of a bigger filament increases the size of the
effective focal spot. Decreasing the anode or target angle decreases the size of
the effective focal spot. Generally, the anode angle of a conventional
radiography tube is between 12 and 17 degrees. CT tubes employ a target angle
approximately between 7 and 10 degrees. The decreased anode or target angle
also helps eleviate some of the effects caused by the heel effect . CT can
compensate any loss of resolution due the use of larger focal spot sizes by
employing resolution enhancement algorithms such as bone or sharp
algorithms, targeting techniques, and decreasing section thickness.
In CT collimation of the x-ray beam includes tube collimators, a set of prepatient
collimators and post-patient or pre-detector collimators . Some CT
systems utilize this type of collimation system while other do not. The tube or
source collimators are located in the x-ray tube and determine the section
thickness that will be utilized for a particular CT scanning procedure. When the
CT technologist selects a section thickness he or she is determining tube
collimation by narrowing or widening the beam. A second set of collimators
located directly below the tube collimators maintain the width of the beam as it
travels toward the patient. A final set of collimators called post-patient or predetector
collimators are located below the patient and above the detector. The
primary responsibilities of this set of collimators are to insure proper beam
width at the detector and reduce the number of scattered photons that may enter
a detector.
There are two types of filtration utilized in CT. Mathematical filters such as
bone or soft tissue algorithms are included into the CT reconstruction process
to enhance resolution of a particular anatomical region of interest. Inherent tube
filtration and filters made of aluminum or Teflon are utilized in CT to shape the
beam intensity by filtering out low energy photons that contribute to the
production of scatter. Special filters called "bow-tie" filters absorb low energy
photons before reaching the patient. X-ray beams are polychromatic in nature
which means an x-ray beam contains photons of many different energies.
Ideally, the x-ray beam should be monochromatic or composed of photons
having the same energy. Heavy filtration of the x-ray beam results in a more
uniform beam. The more uniform the beam, the more accurate the attenuation
values or CT numbers are for the scanned anatomical region.
Detectors
When the x-ray beam travels through the patient, it is attenuated by the
anatomical structures it passes through. In conventional radiography we utilize
a film-screen system as the primary image receptor to collect the attenuated
information. The image receptors that are utilized in CT are referred to as
detectors. The CT process essentially relies on collecting attenuated photon
energy and converting it to an electrical signal, which will then be converted to
a digital signal for computer reconstruction. A detector is a crystal or ionizing
gas that when struck by an x-ray photon produces light or electrical energy. The
two types of detectors utilized in CT systems are scintillation or solid state and
xenon gas detectors. Scintillation detectors utilize a crystal that fluoresces when
struck by an x-ray photon which produces light energy. A photodiode is
attached to the scintillation portion of the detector. The photodiode transforms
the light energy into electrical or analog energy. The strength of the detector
signal is proportional to the number of attenuated photons that are successfully
converted to light energy and then to an electrical or analog signal. The most
frequently used scintillation crystals are made of Bismuth Germinate
(Bi4Ge3012) and Cadmium Tungstate (CdWO4). Earlier designs utilized
Sodium and Cesium Iodide as the light producing agent. One of the problems
associated with these element was that at times it would fluoresce more than
necessary. The after glow problems associated with Sodium and Cesium Iodide
altered the strength of the detector signal which could cause inaccuracies
during computer reconstruction.
The second type of detector utilized for CT imaging system is a gas detector.
The gas detector is usually constructed utilizing a chamber made of a ceramic
material with long thin ionization plates usually made of Tungsten submersed
in Xenon gas. The long thin tungsten plates act as electron collection plates.
When attenuated photons interact with the charged plates and the xenon gas
ionization occurs. The ionization of ions produces an electrical current. Xenon
gas is the element of choice because of it's ability to remain stable under
extreme amounts of pressure. Utilizing more gas in a detector increases the
number of molecules that can be ionized therefore, the strength of the detector
signal or response is increased. The long thin tungsten plates of the gas detector
are highly directional. Ionization of the plates and the resultant detector signal
rely on attenuated photons entering the chamber and ionizing the gas. If the
xenon gas detectors are not positioned properly there is a chance that the ability
of the detector to produce an accurate signal is compromised because the
photons may miss the chamber. The xenon gas detectors are generally fixed
with the position of the x-ray tube which occurs with 3rd generation scanner
geometry designs.
The term detector refers to a single element or a single type of detector used in
a CT system. The term detector array is used to describe the total number of
detectors that a CT system utilizes for collecting attenuated information. 3 rd
generation CT imaging systems employ 800-1000 detectors while 4th
generation scanners include 4000-5000 individual detectors in a detector array.
Overview
The path that an x-ray beam travels from the tube to a single detector is referred
to as a ray. After the x-ray beam passes through the object being scanned, the
detector samples the beams intensity. The detector reads each ray and measures
the resultant beam attenuation. The attenuation measurement of each ray is
termed a ray sum. A complete set of ray sums is referred to as a view or
projection. It takes many views to create a computed tomography image.
Obtaining a single view does not give the entire perspective of the object being
scanned. Therefore, we can say that the detector is "seeing" an insufficient
amount of information. The attenuation properties of each ray sum are
accounted for and correlated with the position of each ray. At this point, the
detector has "collected" the projection or raw data. The more photons collected,
the stronger and more accurate the detector signal. This is essential for accurate
image reconstruction. The detector accomplishes this task by adding together
all the photon energy it has received. The detector receives all the projection
data and subsequently generates an electrical or analog signal. The signal
represents an absorption or attenuation profile. An attenuation profile is
obtained for each view or projection. Every detector in the detector array is
responsible for this task.
Detector efficiency describes the percent of incoming photons that a detector
converts to a useable electrical signal. The two primary factors that determine
how well a detector can capture photons relative to efficiency is the width and
the distance between each detector. It is important that detectors are placed as
close to one another as possible. Scintillation detectors convert 99-100 percent
of the attenuated photons into a useable electrical signal. Xenon gas detectors
are less efficient, converting 60-90 percent of the photons that enter the
chambers. The efficiency of the xenon gas detector is compromised by the
absorption of some of the photons by the ionization plates. Additionally,
photons may pass through the chamber without interacting with the gas
molecules. However, one advantage to this situation may be that some of the
photons absorbed by the plates were scattered photons. As in conventional
radiography scatter also adversely effects the CT image. Therefore, it is
reasonable to conclude that the gas detectors have low scatter acceptability.
Scintillation detectors convert almost all the information it receives including
scattered photons therefore, the detectors have high scatter acceptability.
The dynamic range describes how many levels of information a detector can
detect. The dynamic range determines the ability of a detector to detect and
differentiate a wide range of x-ray intensities. "Dynamic range of a detector
describes the range of x-ray exposures at the detector to which the system can
respond without saturation and produce satisfactory gray-scale images
(Morgan, 1983)." Current CT systems have an approximate dynamic range of
1,000,000 to 1 and 1,100 views or projections a second. CT systems have the
ability to respond to 1,000,000 x-ray intensities at approximately 1,100 views
per second. Unfortunately, display systems and human visual perception limits
the full use of this massive amount of data.
Data Acquisition System (DAS)
Once the detector generates the analog or electrical signal it is directed to the
data acquisition system (DAS). The analog signal generated by the detector is a
weak signal and must be amplified to further be analyzed. Amplifying the
electrical signal is one of the tasks performed by the data acquisition system
(DAS) (Seeram, 1994). The DAS is located in the gantry right after or above
the detector system. In some modern CT scanning systems the signal
amplification occurs within the detector itself. Before the projection or raw
data, which is currently in the form of an electrical or analog signal, goes to the
computer it must be converted to digital information. The computer does not
"understand" analog signals therefore, the information must be converted to
digital information. This task is accomplished by an analog to digital converter
which is an essential component of the DAS. The digital signal is transferred to
an array processor. The array processor solves the statistical information using
algorithmic calculations essential for mathematical reconstruction of a CT
image. An array processor is a specialized high speed computer designed to
execute mathematical algorithms for the purpose of reconstruction (Berland,
1987). The array processor solves reconstruction mathematics faster than a
standard microprocessor. It is important to note that special algorithms may
require several seconds to several minutes for a standard microprocessor to
compute. Recently, processors that compute CT reconstruction mathematics
faster than an array processors have been utilized to solve reconstruction
mathematics essential to the development of CT fluoroscopy. The term image
or reconstruction generator is used to describe this type of computer.
Further discussion of the CT system computer and image reconstruction will be
provided in a future module titled CT Instrumentation and Physics Part 2.
CT Patient Table or Couch
The final component of the scan or imaging system is the patient table or
couch. CT tables or couches should be made with a material that will not cause
artifacts when scanned. Many CT tables or couches are made of a carbon fiber
material. The movement of the table or couch is referred to as incrementation
or indexing. Helical/spiral CT table incrementation or indexing is quantified in
millimeters per second mm/sec because the table is moving for the entire scan.
All table or couch designs have weight limits that if exceeded may compromise
incrementation or indexing accuracy. Various attachments are available for
different types of scanning procedures. Attachments for direct coronal scanning
and therapy planning are commonly used in many CT departments.
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