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(solution) In Computational Tomography (CT scan), from the viewpoint of


In Computational Tomography (CT scan), from the viewpoint of minimizing centrifugal force, it seems logical to place the x-ray tube and detector at the same distance from the isocenter.  What practical design issues may prevent us from doing so? (Question 6-12 from below attachment)


Chapter 6 Major Components of the

 

CT Scanner

 

6.1 System Overview

 

Before giving a detailed analysis and description of major components in a CT

 

scanner, this chapter will present a system overview to explain how different

 

components work together to produce CT images. Figure 6.1 presents a

 

generic block diagram of a CT system. The actual system architecture for

 

different commercial scanners may deviate from this diagram, but the general

 

functionalities of all CT scanners are more or less the same.

 

For a typical CT operation, an operator positions a patient on the CT

 

table and prescribes a scanogram or scout view. The purpose of this scan is to

 

determine the patient?s anatomical landmarks and the exact location and

 

range of CT scans. In this scan mode, both the x-ray tube and the detector

 

remain stationary while the patient table travels at a constant speed. The scan

 

is similar to a conventional x ray taken either at an A-P position (with the tube

 

located in the 6 or 12 o?clock position) or a lateral position (with the

 

tube located in the 3 or 9 o?clock position). Once such a scan is initiated,

 

an operational control computer instructs the gantry to rotate to the

 

desired orientation as prescribed by the operator. The computer then sends

 

instructions to the patient table, the x-ray generation system, the x-ray

 

detection system, and the image generation system to perform a scan. The

 

table subsequently reaches the starting scan location and maintains a constant

 

speed during the entire scanning process. The high-voltage generator quickly

 

reaches the desired voltage and keeps both the voltage and the current to the

 

x-ray tube at the prescribed level during the scan. The x-ray tube produces

 

x-ray flux, and the x-ray photons are detected by an x-ray detector to produce

 

electrical signals. At the same time, the data acquisition system samples the

 

detector outputs at a uniform sampling rate and converts analog signals to

 

digital signals. The sampled data are then sent to the image generation system

 

for processing. Typically, the system contains high-speed computers and

 

digital signal processing (DSP) chips. The acquired data are preprocessed and

 

211 Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Chapter 6 212 Figure 6.1 Block diagram of a CT system. enhanced before being sent to the display device for operator viewing and to

 

the data storage device for archiving.

 

Once the precise location and range are determined (based on the

 

scanogram image), the operator prescribes CT scans based either on preset

 

sets of protocols or newly created protocols. These protocols determine the

 

collimator aperture, detector aperture, x-ray tube voltage and current, scan

 

mode, table index speed, gantry speed, reconstruction FOV and kernel, and

 

many other parameters. With the selected scanning protocol, the operational

 

control computer sends a series of commands to the gantry, the x-ray

 

generation system, the table, the x-ray detection system, and the image

 

generation systems in a manner similar to that outlined for the scanogram

 

operation. The major difference between the processes is that the CT gantry is

 

no longer stationary. It must reach and maintain a constant rotational speed

 

during the entire operation. Since a CT gantry typically weighs more than

 

several hundred kilograms, it takes time for the gantry to reach stability.

 

Therefore, the gantry is usually one of the first components to respond to the

 

scan command. All of the other operating sequences are similar to the ones

 

described for the scanogram operation.

 

In many clinical applications, the operational sequence may differ from the

 

one described above. For example, in interventional procedures, the x-ray

 

generation may be triggered by a footpaddle rather than a computer. In contrastenhanced CT scans, the injection of the contrast agent must be synchronized

 

with the scan, which may require the integration of a power injector with the CT

 

scan protocols. In other operations, the generated x-ray images are sent directly

 

to filming devices to produce hard copies or to a PACS (picture archiving and

 

communication system) for viewing. These deviations, however, should not

 

impact our general understanding of the CT operation mechanism. Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Major Components of the CT Scanner 213 6.2 The X-ray Tube and High-Voltage Generator

 

The x-ray tube is one of the most important components of a CT system.

 

Indeed, x-ray tubes supply the necessary x-ray photons to perform the scan.

 

In the early days of CT, pulsed x-ray tubes were generally used.1 In the

 

pulsed mode, x-ray tubes produced x-ray photons in short-duration pulses.

 

The pulse time varied between 1 and 4 ms. The nonoperating period was

 

typically 12 to 15 ms during which no x-ray photons were emitted because

 

x-ray detectors could not take measurements while the signals were

 

sampled. Some benefits of the pulsed x-ray tube include the elimination of a

 

large number of signal integrators, the ability to reset the electronics

 

between pulses, the ability to adjust pulse length (and therefore photon

 

flux) based on patient size, and the potential reduction of azimuthal

 

blurring.

 

With the advances in electronics and tube technology, the advantages

 

of the pulsed x-ray tube have diminished (although its use in micro-CT

 

continues). In fact, for high-speed CT scanning, pulsed x-ray tubes

 

are disadvantageous since the duty cycle of the tube is less than 100%.

 

The duty cycle is defined as the fraction of the x-ray emitting time

 

over the total operating time. For high-speed scanning, it is necessary

 

to have x-ray tubes constantly producing x-rays to provide sufficient x-ray

 

flux.

 

Although the size and appearance of the x-ray tube have changed

 

significantly since its invention by Roentgen in 1895, the fundamental

 

principles of x-ray generation have not changed.2 The basic components of an

 

x-ray tube are a cathode and an anode. The cathode supplies electrons and the

 

anode provides the target. As discussed in Chapter 2, x-ray photons are

 

produced when a target is bombarded with high-speed electrons. The intensity

 

of the produced x rays is proportional to the atomic number of the target

 

material and to the number of electrons bombarding the target. The energy of

 

the generated x-ray photons depends on the electric potential difference

 

between the cathode and the anode.

 

Most of the x-ray tubes used in CT scanners today employ the heated

 

cathode design, which dates back to 1913 when Coolidge built the first highvoltage tube.2 Figure 6.2 is a photograph of a glass envelope x-ray tube. The

 

glass frame provides the housing and support to the anode and cathode

 

assemblies and sustains a vacuum of 5  10?7 Torr (the vacuum level in the

 

early operating life tends to be in the range of 10?6 to 10?7 Torr, and the

 

internal pressure decreases over the tube life). The glass frame is a composite

 

of several types of glass. The main section is a borosilicate glass with good

 

thermal and electrical insulation properties. The thickness of the glass is

 

typically between 0.18 and 0.30 mm. The glass seals at both ends of the tube

 

are made with splice rings of various grades of glass to match the thermal

 

expansion coefficients between the metal and the borosilicate glass. In more

 

advanced tube designs, the glass frames are replaced with metal frames, Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Chapter 6 214 Figure 6.2 Photograph of an early vintage glass envelope x-ray tube. Figure 6.3 Photograph of a metal frame tube with a cutout to show the anode and cathode

 

assemblies. as depicted in Fig. 6.3. The metal frame has the advantage of being able to

 

operate at or near ground potential to improve the efficiency of the motor

 

that drives the anode assembly. Another advantage of the metal frame is

 

the reduced spacing between the frame and anode, which accommodates a Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Chapter 6 216 Figure 6.4 Illustration of a target assembly. The target rotates at a very high speed so that

 

the heat generated by the electron bombardment is distributed over a large area (light gray

 

band on the target). In addition, the target surface is at a shallow angle (a ¼ 7 deg) with

 

respect to the CT scan plane to increase the exposure area while maintaining a small

 

projected focal spot length. Figure 6.5 Illustration of a target?s cross-section and anode temperatures of an older

 

vintage tube. Triangles represent focal spot temperature, squares represent focal track

 

temperature, and diamonds represent target bulk temperature. inside the vacuum so that the anode, shaft, and rotor can rotate freely inside

 

the tube envelope. A stator is placed outside the envelope to provide an

 

alternating magnetic field that causes the rotator assembly to rotate.

 

To further increase the impact area, the focal track is at a shallow angle a

 

with respect to the scanning plane of the CT (see Fig. 6.4). a is often called the

 

target angle. The projected focal spot length h is related to the actual focal

 

spot length L by the following equation:

 

h ¼ L sinðaÞ: (6.2) Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Chapter 6 216 Figure 6.4 Illustration of a target assembly. The target rotates at a very high speed so that

 

the heat generated by the electron bombardment is distributed over a large area (light gray

 

band on the target). In addition, the target surface is at a shallow angle (a ¼ 7 deg) with

 

respect to the CT scan plane to increase the exposure area while maintaining a small

 

projected focal spot length. Figure 6.5 Illustration of a target?s cross-section and anode temperatures of an older

 

vintage tube. Triangles represent focal spot temperature, squares represent focal track

 

temperature, and diamonds represent target bulk temperature. inside the vacuum so that the anode, shaft, and rotor can rotate freely inside

 

the tube envelope. A stator is placed outside the envelope to provide an

 

alternating magnetic field that causes the rotator assembly to rotate.

 

To further increase the impact area, the focal track is at a shallow angle a

 

with respect to the scanning plane of the CT (see Fig. 6.4). a is often called the

 

target angle. The projected focal spot length h is related to the actual focal

 

spot length L by the following equation:

 

h ¼ L sinðaÞ: (6.2) Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Major Components of the CT Scanner 217 In a typical x-ray tube design, a is selected roughly at 7 deg. As a result,

 

the actual focal spot length can be more than a factor of 8 larger than the

 

projected focal spot. This is often called the line focus principle. Although this

 

approach has the advantage of increased exposure area, it has two minor

 

problems. The first problem is that the focal spot size and shape become

 

location dependent. Although the focal length described by Eq. (6.2) is valid

 

when viewed at a location perpendicular to the focal line, a significant error is

 

present when the viewing location is farther away. The location-dependent

 

focal spot and shape is one factor that contributes to the spatially variant

 

resolution in CT. This phenomenon will be described in detail in Chapter 8. A

 

second problem is the heel effect, which describes a phenomenon in which the

 

x-ray intensity is not constant along the direction perpendicular to the CT

 

gantry plane. This effect can be explained by the fact that on average, x rays

 

are emitted a certain depth from the anode surface, as shown in Fig. 6.6.

 

Because of the shallow angle of the target surface, the x-ray path length

 

through the tungsten changes significantly. The tungsten target itself serves as

 

an x-ray filter, and a longer path length through tungsten reduces the x-ray

 

intensity, as illustrated in the figure. In general, the x-ray intensity declines as

 

we move from the cathode side of the tube to the anode side. For CT scanners

 

with a small z coverage, this effect can be safely ignored because the coverage

 

along the patient long axis (perpendicular to the CT gantry plane) is relatively

 

smaller. As the volume coverage increases, however, this issue can no longer

 

be ignored and must be properly addressed.

 

Figure 6.7 shows normalized x-ray output intensities measured by a flatpanel detector on an x-ray radiographic system. As we move toward the anode

 

side, the x-ray intensity drops. Because of the tube target geometry, the intensity

 

drop is nonlinear. For this particular system, the x-ray flux peak-to-valley ratio is Figure 6.6 Illustration of the heel effect. X-ray photons generated inside the tungsten

 

target travel different path lengths before exiting the anode. The filtration of the tungsten

 

produces variation in the x-ray intensities and spectrums. This effect is negligible for

 

commercial CT scanners with smaller coverage but cannot be ignored when the coverage is

 

large. Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx 218 Chapter 6 Figure 6.7 Example of the heel effect on an x-ray radiographic tube. The normalized x-ray

 

flux is plotted as a function of the detector channel (from the cathode side to the anode side)

 

of a flat-panel system. The peak-to-valley flux ratio is slightly over 2. slightly over 2. That is, near the edge of the FOV, the x-ray flux is less than half

 

that of the central region. The impact of the heel effect is not limited to the

 

intensity drop across the FOV in z; the x-ray spectrum also changes with respect

 

to the imaging location. In general, the x-ray spectrum becomes harder (higher

 

average energy) for x-ray beams toward the anode side of the tube and softer

 

(lower average energy) toward the cathode side. The change in x-ray spectral

 

intensity across the z coverage not only brings complexities to the calibration

 

process, but also introduces variation in the LCD across the z FOV.

 

One method of reducing the heel effect is to increase the tube target angle. As

 

the target angle increases, the difference between the shortest and the longest

 

path length in Fig. 6.6 is reduced. Since the major cause of the heel effect is

 

differential path length, the overall heel effect should be reduced. On the other

 

hand, a reduced target angle decreases the effectiveness of the line focus principle.

 

Because of the enormous heat deposited on the target, special attention

 

must be paid to target design. Traditionally, the tube target is made of a

 

molybdenum alloy and the focal track consists of a layer of tungsten-rhenium.

 

The advantage of this design is the quick heat transfer from the focal track to

 

the bulk of the target. With the introduction of helical/spiral CT and faster

 

scan speeds, this design no longer offers the required heat capacity. The newer

 

design uses brazed graphite in which the metal target (similar to the

 

traditional design) is brazed to a graphite body to increase the heat storage per

 

unit weight. This design combines the good focal track characteristics of the

 

metal tube with the heat storage capability of graphite.2 An alternative design

 

uses chemical vapor deposition to deposit a thin layer of tungsten-rhenium on

 

a high-purity graphite target. The advantages of this approach are the light

 

target weight and high heat storage capacity. The disadvantages are higher

 

cost, increased potential for particles, and limited focal spot loading. Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Major Components of the CT Scanner 219 Since one of the most important issues that impacts x-ray tube

 

performance is heat management, x-ray tubes are often rated in terms of

 

their heat capacity. Tube heat management specification is described in heat

 

units (HU):

 

1 heat unit ¼ 0.74 J: (6.3) For example, a 30-s clinical protocol operating at 120 kVp and 300 mA

 

deposits a total of 1080 kJ of energy (30  120  300). This corresponds to

 

1459 KHU. Typical tube specifications include anode heat storage capacity

 

(in MHU), maximum anode cooling rate (in MHU/min), casing heat capacity

 

(MHU), and average casing cooling rate (in KHU/min).

 

Because heat management is an important and complicated issue, many

 

commercial CT scanners employ computer algorithms (often called tubecooling algorithms) to estimate the different conditions under which clinical

 

protocols can be used to prevent premature damage to the tube. For example,

 

a protocol may be safely run when the x-ray tube is cold, but the same

 

protocol could potentially cause damage to the tube when it is hot. Under this

 

condition, the algorithm will recommend either a reduced technique (reduced

 

tube current or scan time) or a cooling period before starting a scan. In

 

essence, the tube-cooling algorithms derive a compromised solution between

 

performance and tube life.

 

With the introduction of multislice CT (see Chapter 10), tube utilization

 

has become highly efficient. Since most CT protocols can be completed in a few

 

seconds, the scanners are capable of executing nearly all clinical protocols

 

without encountering any tube cooling. Therefore, tube heat transfer and the

 

handling of tube cooling have become less important. A more significant

 

performance parameter now relates to the maximum x-ray tube power that can

 

be delivered in a short period of time. As will be discussed in Chapter 12, one of

 

today?s most demanding clinical protocols is coronary artery imaging of the

 

heart. To avoid heart motion, scan speeds of less than 0.4 s are typically used in

 

conjunction with a half-scan (about 60% of the full-scan acquisition time) and

 

thin-slice acquisition. To deliver sufficient x-ray flux to the study and ensure

 

good signal-to-noise ratio, the maximum tube power must be substantially

 

increased. Note that for a given kVp setting, the total x-ray flux received at a

 

detector cell is proportional to the product of the tube current (mA), the

 

detector aperture (mm), and the acquisition time (s). When the detector

 

aperture and acquisition time are both substantially reduced, a significantly

 

higher tube current is required to maintain the same flux and the x-ray tube

 

target must be able to handle a huge heat load in a short period of time. This is

 

a significantly different requirement from the older vintage scanners in which a

 

moderate amount of heat had to be controlled over an extended period of time.

 

Other parameters are also important to x-ray tube performance. A good

 

example is the focal spot width and focal spot length shown in Fig. 6.4. These Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Major Components of the CT Scanner 223 Figure 6.11 Illustration of voltage waveform for fast kV switching. and finally to rectify and smooth the power supply for the x-ray tube.

 

Although the ripple produced by such a power supply is about 5%, it has a

 

lower cost and reduced complexity due to the use of digital technology.

 

Interested readers can refer to Ref. 3 for additional information.

 

A hot topic in recent CT research is dual energy for beam-hardening

 

correction, material separation, and tissue characterization. Although

 

Chapter 12 will discuss this topic in detail, we will mention the impact of

 

one dual-energy CT approach to high-voltage generator design: the fast kVp

 

switching technology used to acquire two x-ray energies nearly simultaneously. During data acquisition, the input x-ray tube voltage is quickly

 

switched between two different energy settings (e.g., 80 and 140 kVp) on a

 

view-by-view basis. This requires the high-voltage generator to be able to

 

provide a voltage waveform that has a short rise and fall time, as shown in

 

Fig. 6.11. In general, the shorter the rise and fall time in the voltage

 

waveform, the less contamination occurs between signals of adjacent views,

 

and the more effective is the energy separation of the collected signals.

 

Because the voltage is switched between views that last less than a millisecond,

 

new technologies must be used in the voltage generator design. 6.3 The X-Ray Detector and Data-Acquisition Electronics

 

The x-ray detector is as important as the x-ray tube to the performance of a

 

CT scanner. Like x-ray tube technology, detector technology has experienced

 

tremendous growth over the past 40 years. Figure 6.12 shows a collection of

 

CT detectors used on various third-generation scanners, arranged chronologically from top to bottom and then from left to right. The earlier vintage

 

detectors, such as the GE 7800, had smaller FOVs used mainly to perform

 

head scans. The later vintage detectors, such as the GE 9800 HiLight?

 

detector and GE LightSpeed? detector, had a much larger FOV to handle

 

entire bodies. More recent development will be discussed later.

 

Detectors of the third-generation scanners use either a high-pressure inert

 

gas (usually xenon) or solid state scintillators coupled with photodiodes.

 

The operating principle of the xenon detector is illustrated in Fig. 6.13. Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Major Components of the CT Scanner 221 delivered to a particular tube, similar to the full odometer reading on a car

 

(although each scan is measured in mAs, the accumulative reading over the

 

tube life is more conveniently measured in kAs). This metric provides an

 

indication of the average tube power usage but not the real stress on a tube.

 

Note that a 600-mA cardiac scanned over 0.3 s carries the same mAs as an

 

180-mA routine chest scanned over 1.0 s. The stress on the tube, however, is

 

higher for the cardiac exam. The third metric is the total actual scan time in

 

seconds over the life of the tube. Although each metric taken on its own does

 

not provide sufficient information to evaluate the tube reliability, taking all

 

metrics together provides a good overview of the tube life.

 

One component that impacts tube life is the anode assembly. As was

 

discussed previously, the anode is rotated at a very high speed, typically

 

between 8,000 and 10,000 rpm, to prevent the target from melting. In the

 

traditional design, the anode shaft is held in place by a pair of ball bearings, as

 

shown in Fig. 6.9(a). Since the shaft and the bearings are connected only via

 

point contact, the stress at the contacting points is very high. Considering the

 

high-temperature environment, the wear-and-tear of the ball bearing is often

 

the cause of tube failures. To overcome this shortcoming, the new design

 

employs a technology called spiral groove in which the bearing assembly is

 

removed. The entire shaft surface is in direct contact with the housing instead

 

of point contacts to significantly reduce the stress. To minimize friction

 

between the shaft and housing, liquid metal is used as a lubricant. The surface

 

of the anode shaft is also carved with a specially designed groove pattern that

 

allows a constant flow of the liquid metal to the shaft?housing interface, as

 

shown in Fig. 6.9(b). In addition, special surface chemistry must be used to

 

ensure that the liquid metal is spread uniformly onto the shaft surface.

 

The other important component in the x-ray generation system is the

 

high-voltage generator. To produce and maintain the desired x-ray flux Figure 6.9 X-ray tube shafts that support the rotating anode: (a) older vintage tube design

 

where ball bearings are used to support the shaft; (b) new spiral grove design where the

 

shaft is supported without the ball bearing and liquid metal material is used for lubrication. Downloaded From: http://ebooks.spiedigitallibrary.org/ on 09/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Chapter 6 222 output, the voltage and current to the x-ray tube must be kept at a constant or

 

desired level. Unfortunately, the voltage of the power supply provided by the

 

power line fluctuates sinusoidally from negative to positive relative to the

 

ground. This is undesirable for two reasons. First, as previously discussed, if

 

the cathode voltage becomes positive relative to the anode, the electrons

 

emitted from the hot anode will accelerate toward the cathode, causing

 

premature aging of the cathode filament and generating undesired x-ray

 

photons (located significantly off the x-ray focal spot). Second, considerable

 

fluctuation of the voltage causes significant difficulties in the calibration and

 

data conditioning of CT systems. The energy spectrum of the x-ray flux is

 

closely linked to the voltage pote...

 


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