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Module 2
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CT Basics: Equipment and Instrumentation
Module 2
1.
Title Screen
2.
License Agreement
3.
Objectives
After completing this module, you will be able to:
1)
Explain the major components of the computed tomography computer system.
2)
Trace the sequence of events in CT scanning from the application of the electrical
current to the radiographic tube to image display.
3)
Explain how adjusting operator console parameters affects CT image data.
4)
Discuss the elements of a digital image.
4.
Major CT Scanner Components
Modern CT scanners are an elegant blend of function and technology. To the untrained eye, a CT
scanner appears to be the model of simplicity — a patient lies on the table, the table moves through the
scanner and an image appears. On further inspection, however, the true complexity of the CT systems
becomes apparent.
The observer is astonished at the intricacy of design and the need for all pieces of CT equipment
to work perfectly in unison to produce the highest quality diagnostic image with the least possible dose
to the patient.
Regardless of CT vendor, each computed tomography room contains three major pieces of
equipment: an imaging system that consists of a gantry and patient table, a computer capable of
processing the CT image data, and an operator’s console that controls the entire imaging process and
displays the final image.
These three components work together to create and manipulate the x-rays that are
transformed into a digital image. This digital image then can be reconstructed in various ways to
diagnose disease processes within a patient’s body.
5.
Patient Table
The patient table, sometimes referred to as the patient couch, is made of a material that will
absorb the least amount of radiation possible while still supporting the weight of the patient. Carbon
fiber is the most common material used in CT tables. The table does much more than simply transport
the patient into the scanner; its movements determine which part of a patient’s anatomy is scanned and
the thickness of the image sections.
The table even keeps the patient safe during the examination. Because the table must move
precisely and protect the patient, it is important to know your table’s weight limit. Most modern
scanners can handle patients weighing up to 450 pounds, but you should check the specifications of
your equipment before imaging.
6.
X, Y and Z Axes
For the CT scanner, direction is based on the patient and the table the patient is lying on. Three
coordinates define direction: the x, y and z axes. The x-axis is referred to as the sagittal plane because it
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CT Basics: Module 2
divides a patient lying on the table into a left side and a right side. The y-axis, or the coronal plane,
divides the patient’s body into anterior and posterior sections.
The z-axis is called the axial plane because it divides the body into superior and inferior parts. In
a multislice CT scan, the gap between each slice is called the z-gap. The z-gap is determined by the pitch.
If the pitch increases, the z-gap decreases and image quality improves.
7.
Gantry
The largest piece of CT equipment is the large, circular apparatus known as the gantry. The
gantry houses most of the functional equipment parts needed to acquire image data. The x-ray tube,
detectors and even most generators are found inside the gantry. The gantry also contains a cooling
system that allows the x-ray tube to operate at increased speed for a longer time.
All of these components rotate within the gantry as the patient moves through a large hole
known as the aperture. To line up pertinent anatomy during the scan, the gantry can tilt forward or
backward between 12° and 30°.
8.
X-ray Tube
The x-ray tube is mounted inside the gantry and rotates continuously around the patient. The
introduction of multislice CT scanners increased heating and cooling demands on the x-ray tube. Some
manufacturers replace the tube on an annual basis to avoid tube arcing. Modern CT tubes can last for
150,000 to 200,000 slices, which in a busy department equals approximately one year of scanning.
The tube consists of two major components: a cathode and an anode. The electron beam travels
from the cathode and strikes a target on the anode. The tube then generates high-energy photons from
the anode. The cathode contains compact tungsten filaments that set the current of the electrons
flowing to the anode. The temperature of the tungsten filaments affects the current of these electrons.
The anode assembly consists of a rotor, hub and a bearing unit that permit rapid rotating speeds
of 3,600 to 10,000 rotations per minute. The rotating anode usually is composed of an alloy of tungsten,
molybdenum and rhenium, and the target area of the anode is made of tungsten. Tungsten is an ideal
metal for use in multislice CT scanners because of its high heat tolerance and high melting point of
3,400° C.
Tungsten also dissipates heat quickly so that the target area can cool rapidly and be ready for
the next bombardment of electrons. The target is fixed at an angle of approximately 11° to 12°. The
cathode and anode are enclosed in a metal tube. Glass tubes have been used in the past, but they form
tungsten deposits that can result in quicker tube degradation. Metal envelopes are able to withstand
higher tube currents. Click the button to see an x-ray tube in operation.
The CT technologist can change the tube voltage, kilovoltage (kV), and tube current,
milliamperes (mA), that move the electron beam from the cathode to the anode. Changing the mA
changes the cathode filament temperature so that the cathode produces the desired number of
electrons. The technologist can control the energy level of these electrons by adjusting the kV. Altering
the kV affects the penetrating power of the electrons that pass through the patient’s body. Use the
slider bars on this animation to see how changing the mAs or the kVp will affect the output from the xray tube.
9.
Generator
The generator is responsible for the high voltage needed to create x-rays. It produces voltages
from 90 to 140 kV, with a typical CT scan using 120 kV. Some generators are found outside the gantry in
a fixed location within the examination room, while others rotate next to the x-ray tube. Most modern
generators are so compact and efficient that the unit can be located inside the gantry.
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CT Basics: Module 2
Generators convert the low-voltage alternating current to a high-voltage direct current that
powers the x-ray tube with constant energy. The incoming power supply of 60 hertz (Hz) is transformed
into a high-voltage, high-frequency current of 500 to 25,000 Hz. The power demands on a multislice CT
unit are enormous, typically 20 to 100 kilowatts (kW). A 60-kW generator produces enough voltage to
provide 80 to 120 kV and 20 to 500 mA.
10.
Detectors
The detectors, which measure the patient’s x-ray attenuation data, are located opposite the xray tube. Detectors are very sensitive. They recognize the ionizing radiation that has passed through the
patient, capture the signal and then transport the signal to the digitizer. X-rays produce an analog signal
that must be converted into a digital signal so that the computer can read the information and produce
the final CT image.
The detector geometry is the relationship of the tube, the beam shape and the detectors. As you
can see on this page, current CT scanners use hundreds of detectors that are arranged in a curved array
and aligned with the x-ray tube. Both units rotate simultaneously around the patient.
Detector efficiency determines how accurately the CT image is reproduced every time and with
every patient. Different terms describe the detector’s efficiency. Capture efficiency is the measurement
of how efficiently the detectors gather the photons coming from the patient. Absorption efficiency
describes how efficiently the photons are captured by the detectors.
Stability is the measurement of how consistently the detectors respond. The response time is
how fast the detectors record the photons and how quickly they recover for the next event. Dynamic
range refers to the accuracy of the detector’s response to both high-energy and low-energy radiation.
Finally, reproducibility describes how consistently the detectors respond to similar transmitted radiation
events.
11.
Detectors
Two types of detectors currently are used in multislice CT scanners: gas ionization detectors and
scintillation detectors. Gas ionization detectors convert the x-rays directly into an electrical signal.
Scintillation detectors first change the x-rays into light, and then the light is transformed into an
electrical signal. Scintillation detectors are the industry standard because they are more sensitive and
they need less frequent calibration than gas-filled detectors.
Scintillation detectors use a solid-state scintillation crystal mounted next to a photomultiplier
tube. The crystal absorbs the x-ray photons and then emits flashes of light that are directly proportional
to the energy of the collected photons. The photomultiplier amplifies the light and converts the light
into a digital signal for computer processing. Scintillation detectors are associated with lower patient
dose and reduced image noise.
12.
Collimators
The goal of the collimators is to provide a consistent beam width, which is defined by slice
thickness. The beam width is measured in the z-axis at the center of the rotation for a single-row
detector array. Collimation limits the amount of x-ray exposure to the patient by reducing scatter
radiation and improves image contrast. CT scanners contain both prepatient and postpatient
collimators.
Prepatient collimators are located just outside the x-ray tube where the beam leaves the tube.
These collimators, which are made of thick metal plates, define beam width and restrict the shape of the
x-ray beam before it ever reaches the patient. In single-slice CT scanning, the collimators define the
thickness of the cross-sectional slice.
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CT Basics: Module 2
Prepatient collimators also define the thickness of the x-ray beam in multislice CT scanning,
which spreads the beam over the entire detector array, or multiple rows of detectors. In multidetector
CT scanning, however, image reconstruction rather than prepatient collimation determines the slice
thickness. Postpatient collimators are positioned just above the detector array. These collimators
improve image quality and axial resolution.
Postpatient collimation also works in conjunction with prepatient collimation to help define slice
thickness. If postpatient collimation is reduced, the slice thickness decreases. Thin collimation results in
better resolution, but it takes longer to scan a particular area of anatomy. Wider collimation results in
lower resolution, but it provides better volume coverage speed.
13.
Practice Question
14.
Practice Question
15.
Scanner Configurations
From the development of CT in the 1970s until the present time, all CT scanners have used a
fairly similar construction. A gantry containing an x-ray tube and set of detectors rotates around the
patient and collects image data that are processed by a computer.
Significant developments in modern CT scanner design have led to increased image quality,
faster scans times and decreased patient radiation exposure. Helical scanning is a major leap forward in
CT scanner construction. Let’s briefly look at its development and the unique properties that make
helical scanning possible.
16.
Historical Development of the Helical Scanner
The development of computed tomography required advances in digital computing and special
mathematics. The first generation CT scanner developed by Sir Godfrey Hounsfield used an x-ray source
and detector to collect data for a single slice. The scanning motion of the first generation equipment was
called translate and rotate because the scanner moved across the patient, rotated slightly and then
made the next translation.
The second generation scanner was a big improvement in design because it had a larger fan
beam, but the equipment still used the translate and rotate motion. Third generation CT scanners used
an even larger x-ray beam that was capable of covering the entire patient cross-section, and the tube
and assembly both rotated around the patient.
Current CT scanners have a similar configuration in which the tube and the detectors rotate;
however, modern scanners contain multiple detector rows. For each gantry rotation, the scanner
acquires multiple sections. Similar to single-slice configurations, the scan can be taken in either a stepand-shoot or helical mode. The biggest advantage to this type of configuration is that large areas of
anatomy can be covered more quickly. Modern scanners also have better contrast resolution and less
patient motion artifacts than their predecessors.
17.
Conventional Axial Scanning
Axial scanning is characterized by the start-and-stop rotation of the x-ray tube around the
patient. After the data are acquired during a single revolution, the table is indexed to the next position
and the process is repeated. This process continues for the duration of the scan until the prescribed
anatomy is covered. Click on the button to see a CT scanner in a conventional axial scan mode.
Often referred to as step-and-shoot, axial scanning is time consuming, but the method is still
used today for brain imaging because it provides better image quality than helical scanning. Most
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CT Basics: Module 2
neurologists request axial scanning of the brain because they believe it’s superior to other types of CT
scanning.
18.
Multidetector Computed Tomography (MDCT)
Mulitdetector-row CT, or MDCT, scanners are similar to single detector-row scanners in that
they obtain images in one rotation of the x-ray tube. The difference is that MDCT scanners cover more
anatomy in one rotation because the detector array acquires multiple, parallel slices. For example, for a
16-slice detector, the CT scanner obtains 16 slices per rotation, and then the table is indexed to the next
position to obtain the next 16 slices.
MDCT is much faster than single-row CT, but not as fast as helical scanning. The disadvantages
of MDCT include the need for more contrast to complete a scan, the possibility of misalignment
between scans that can affect postprocessing and the chance that anatomy can be missed if patient
breathing is not consistent.
19.
Volumetric Data Acquisition
Volumetric data acquisition involves continuous movement of the x-ray tube as the table moves
through the gantry. The beam traces a helical, or spiral, path around the patient, allowing much faster
scan times compared to axial scanning. Click on the button to see a CT scanner in a helical scan mode.
You can see the x-ray tube rotate continuously around the patient in a helical path as the table moves
through the gantry.
Spiral and helical are interchangeable terms when describing this type of scanning. Interscan
delay refers to the time period between the end of one scan and the beginning of the next scan. The
purpose of the delay is to allow time for tube cooling. Interscan delays are less noticeable with multislice
CT, and delays don’t occur in helical scanning because the scan is continuous.
20.
Volumetric Data Acquisition
One of the major advantages of helical scanning is that it allows much faster imaging because
large volumes of data can be collected during each scan. Helical scan times are now seconds rather than
minutes or hours, and scans usually can be completed in a single breath hold. For example, a helical scan
from the chest through the pelvis can be obtained in a single breath hold, whereas an axial scan would
require many breath holds for that amount of anatomy.
A single breath hold reduces the likelihood of patient motion during the scan, as well as
problems that arise when patients must hold their breath multiple times. If the patient has to take
several breath holds throughout the scan, each breath hold might be different, leading to
misregistration artifacts that occurs when the images are reconstructed and the patient’s anatomy does
not line up properly.
Faster scans often require less contrast material. For example, the contrast injection rate during
a CT scan for suspected pulmonary embolism is 4 to 5 cc per second. If the area of anatomy is covered
quickly, the scan can be stopped before the entire amount of contrast is used.
Another advantage of helical scanning is that the images can be reconstructed at a smaller slice
width than the slices of the original scan. For instance, if the protocol calls for 3-mm cuts through the
abdomen and pelvis, the scan often can be reconstructed using 2-mm slices. The reconstruction has a
smoother appearance that no longer shows the “stair step” artifact found in some reconstructed
images. During a helical scan, the x-ray tube must be able to rotate around the patient without stopping.
The ability of the tube to rotate continuously was made possible by the development of slip-ring
technology.
21.
Slip Rings
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CT Basics: Module 2
Before the introduction of slip-ring technology, cables connected the gantry components. The
gantry rotated and acquired the image, and then the cables had to unwind before the next acquisition
to avoid tangling the tube and detectors. Now, slip rings allow the gantry components to be coupled
without cables. The x-ray tube rotates continuously around the gantry without hanging up the electronic
mechanisms. The technology eliminates the time-consuming, start-and-stop process of earlier CT
scanners and permits data acquisition to begin very quickly. Faster scan times led to the development of
continuous acquisition exams such as computed tomography angiography. Slip-ring technology made
helical CT scanning a reality.
Slip rings are composed of electrical conductive rings and brushes. The slip ring transmits an
electrical current across the rotating surface. Slip rings supply the electrical power to the x-ray tube and
help transfer the signals from the detectors to the computer for image reconstruction. High-voltage slip
rings provide greater voltage capacity, typically more than 600 volts.
The slip-ring power supply uses either a disk or cylinder design.The disk design consists of
conductive rings located within the plane of the rotation of the disk. The brushes transfer electrical
power by sliding across and coming in contact with the grooves on the slip ring. Two types of brush
designs are common: wire brushes and composite brushes. Wire brushes use conductive wire such as
copper to make contact with the ring. The composite brush is a conductive block, often a silver graphite
alloy, that acts as a sliding contact. Several different configurations are available to maintain contact
between the brushes and the ring. One example is an arrangement of compressions and springs that
push the brushes forward onto the stationary slip ring to provide contact.
22.
Slip Rings
CT scanners use both high-voltage and low-voltage slip rings; the main difference between the
two is how each component is positioned within the gantry. Low-voltage slip-ring design uses
alternating current (AC). The x-ray signals are transmitted to the slip rings by passing a low-voltage
current to the conductive brushes.These brushes are in contact with the stationary ring, or back plate.
The slip ring provides power to the high-voltage transformer that powers the x-ray tube. The
components are arranged in the following sequence: AC main power > Slip ring > High-voltage generator
> X-ray tube.
High-voltage slip-ring design is similar to the low-voltage design except the high-voltage
generator does not rotate with the x-ray tube. The components are arranged in the following order: AC
main power > High-voltage generator > Slip ring > X-ray tube.
23.
Practice Question
24.
Practice Question
25.
Operator Console and Data Acquisition
The CT technologist enters instructions into the system console. The system is responsible for
controlling the high-voltage generator, the gantry operations and the patient table. The console consists
of a liquid crystal display, or LCD, monitor and keyboard. The technologist interacts with the CT scanner
by typing on a keyboard, using a mouse or using touch screen sensors.
The patient’s information, sequence selection and sequence parameters are entered into the
console before the scan starts. The console also is used for postprocessing tasks and data transfer to a
picture archiving and communication system, or PACS. All this information is processed through the host
computer and passed on to the system controller.
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CT Basics: Module 2
The operator console contains all of the controls necessary for completing a CT scan. Decisions
made on the operator’s console have a direct impact on image quality that, in many cases, cannot be
reversed. Let’s briefly look at some of these controls and see how they affect the CT image.
26.
Pitch
The pitch is the distance the table travels during one revolution of the x-ray tube. Pitch is
expressed as the ratio of distance the table travels per rotation of the total collimated x-ray beam width.
If that distance equals the slice thickness (in other words, the thickness of the collimated beam) the
pitch is said to be 1:1. A pitch of 1 provides the best image quality because data are collected on all
anatomy. Click on the different pitch setting buttons to begin the animation and see the difference
between a pitch of 1 and a pitch setting greater than 1.
As the pitch increases, the patient moves through the gantry at a faster rate. This is because the
helical path is stretched, and so more anatomy can be covered in a shorter period of time. Therefore,
when the pitch increases, patient dose decreases because of shorter exposure to the x-ray beam. If the
pitch is increased by a factor of 2, the patient dose decreases by one-half. This relationship is only valid if
all other parameters remain the same.
27.
Scan Field of View (SFOV)
The scan field of view, or SFOV, is the actual area of interest selected by the CT technologist
before the scan begins. The transmission measurements of this region are recorded during the duration
of the scan. The scan field of view determines the number of detectors needed to collect data for a
particular scan.
The scan field of view, or SFOV, must be larger than the area of interest because any anatomy
that falls outside the scan box won’t be recorded by the detectors and therefore can’t be used in image
reconstruction. The result is out-of-field artifacts. The manufacturer often preselects SFOVs with the
idea that the technologist will align the scan box to each specified body part.
28.
Display Field of View (DFOV)
The display field of view, orDFOV, is also called the reconstruction field of view. The DFOV is the
reconstructed area seen on the image monitor, and it can be equal to or less than the scan field of view.
The DFOV is taken from the original scan and can be manipulated with reconstruction algorithms, or
particular areas can be zoomed or panned.
Using a narrow display field of view shows a smaller area than the scan field of view and can be
useful in certain diagnoses. The display field of view affects the resolution and noise of the image. Using
a wider DFOV increases the number of photons from which the original data were collected, so noise is
reduced but spatial resolution also decreases. As the field of view decreases, spatial resolution improves
but image noise increases.
To compensate for increased noise, the mAs can be increased, but this also increases the
radiation dose to the patient. Anatomy also appears larger in a smaller field of view because the area of
interest is enlarged to fill the displayed image.
29.
Annotation
Annotation is the process of marking an image with relevant information. Annotations are used
to include additional information regarding the study that might be beneficial to the radiologist. CT
technologists can annotate a scan by selecting a predetermined note or using the comments section to
make multiple notations. For example, information such as “precontrast,” “postcontrast” or “5-minute
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CT Basics: Module 2
delay” can be included. Annotations also can be made after the image has been sent to a PACS. The
technologist can note important information by using different font sizes and colors.
30.
Scout
A scout film is essential for every CT scan. Other terms for the scout image include scanogram,
pilot, topogram and survey. It is basically a plain-film radiograph used to set the scan box, or area of
anatomy to be scanned. The film is taken using a stationary tube and translating the table through the
gantry. Both AP and lateral scouts can be taken, with the main difference being the position of the x-ray
tube.
If an AP scout image is required, the x-ray tube is positioned above the patient. The lateral scout
is taken with the x-ray tube positioned to the left or right of the patient, or 90° from the AP scout.
Scouts of different lengths are programmed into each protocol, but the CT technologist can choose the
scout length. Common scout lengths are 128, 256, 512, 768, 1,024 and 1,536 mm, depending on the
examination and anatomy to be scanned.
A typical scout length for a CT abdomen/pelvis is 512 mm. The technologist also can stop the
scout after it has adequately covered the area of anatomy, so that the patient is not irradiated
unnecessarily. Click on the buttons on the animation to view an AP and lateral scout scan.
31.
Region of Interest
Region of interest, or ROI, is a tool used in CT scanning to “circle” an area on the image and
obtain a CT number. The tool helps the radiologist identify a specific tissue and obtain certain statistical
information. ROIs can be placed and dragged to select a larger area of coverage.
32.
CT/Hounsfield Numbers
Before we discuss the next two controls on the operator console, window width and window
level, let’s look at the concept of CT/Hounsfield numbers. Individual anatomy on the CT image is
represented by different shades of gray. The mathematical unit that describes the shade of gray is called
the Hounsfield unit (HU) or the CT number. The CT number represents the gray scale value seen on the
final image and is related to the linear coefficients of the tissue within a CT image section.
The information found in each digital image unit is assigned a CT number. Water is always
assigned zero and serves as the basis from which other CT numbers are calculated. All other values
represent various shades of gray. Atoms that are dense are assigned the highest CT numbers. Bone, with
a CT number near +1000, appears white on a CT image. Air is associated with the lowest CT number at 1000 and is displayed as black on the CT image.
33.
Window Width
The window width, abbreviated WW, is the range of CT numbers displayed in the CT image and
is represented by a gray scale. Window width determines the maximum number of shades of gray that
can be displayed on the CT monitor. It also determines how much contrast appears in the image.
Increasing the window width provides a wider range of tissue information in the image. Consequently, a
wide window width displays less variation between tissues with similar densities and often is used for CT
studies that have a great deal of subject contrast, such as a CT scan of the lung.
A narrow window width is used for anatomy that has minimal inherent contrast between
structures, such as the brain. The white matter and gray matter of the brain only differ by 5 to 10
Hounsfield units, so CT scans of the brain benefit from a narrow window width because it increases the
contrast differences between the tissues. A narrow window width enhances image contrast and ensures
that the transition from black to white takes place over a relatively few CT numbers.
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CT Basics: Module 2
Although a CT scanner is capable of producing approximately 2,000 shades of gray, the
LCD monitor can only display 256 shades and the human eye can only distinguish 20.
34.
Window Level
The window level, abbreviated WL, designates the center or midpoint of the range of CT
numbers. It can be positioned anywhere within the window width and determines how bright the image
appears. A low WL makes the image appear brighter, and a high WL makes the image appear darker.
The window level should be set to the CT number of the anatomy being imaged. For instance, for a CT
scan of the brain, the technologist should set the window level at approximately 40 Houndsfield units.
35.
WW/WL Example
Let’s look at an example of window width and window level. Suppose that you set the WW to
600 and the WL to 200 for the CT scan you want to perform. These settings would represent a gray scale
of +300 and -300, with a WL setting of 200. Therefore, 300 + 200 would equal an upper setting of 500
and -300 + 200 would equal a lower setting of -100. So the range is 500 to -100.
Tissues with CT numbers falling below -100 would appear black on the scan and anatomy with
CT numbers above 500 would appear white on the image. This page shows some typical window widths
and window levels. Remember that the range of CT numbers encompasses the CT number of the tissue
of interest.
36.
Practice Question
37.
Practice Question
38.
Computer
The image data acquired by the scintillation detectors and converted into electrical signals are
eventually sent to the computer for processing. The analog signals must be amplified and then digitized
by the analog-to-digital converter (ADC). The raw data are stored until converted into the final image by
the array processor. The image then is transferred to the host computer where it is displayed on the LCD
monitor.
After the image is amplified, it’s sent to the sample/hold unit, which is located between the
amplifier and the ADC. The sample/hold unit determines the relative attenuation of the x-ray beam by
the patient’s tissues and assigns various shades of gray to the image.
The array processor is responsible for applying algorithms to the attenuation data to produce
the final CT image. Its computing capacity can perform several mathematical calculations at lightning
speed on an enormous volume of projection data, so that the CT technologist is able to view an image
after only a fraction of a second delay. The array processor also performs retrospective reconstructions.
In addition to image data, the main computer stores patient information so that it can be
retrieved for postprocessing. The host computer is linked to other hospital systems and is DICOM
compatible. DICOM stands for “digital imaging and communications in medicine” and is the industry
standard for distributing and viewing any kind of medical image data regardless of how the image was
created and stored.
39.
Digital Imaging
The final, processed CT image data are a type of digital image, just like cassette-based and
cassetteless digital images currently replacing film in many radiography departments. In fact, CT scans
were some of the first digital images found in radiology. Digital imaging is a complex process that
includes data acquisition, image processing, image display, image communication and storage. At each
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CT Basics: Module 2
step, from collecting the initial data to viewing the final digital image, computers are an essential part of
manipulating and managing the image data.
Data acquisition involves collecting the image information from the patient. Collection
devices include the x-ray tube and the detectors that record the information. Data acquired from the
patient are in the form of linear attenuation coefficients that are based on the type of tissue scanned.
The information is an electrical signal that must be converted to a digital signal by the
analog-to-digital converter. Image processing consists of the steps needed to create a CT image. The raw
data set produced by the ADC is converted back into an analog signal for image display. Various image
processing techniques such as filters and algorithms are applied to improve image quality, reduce noise,
increase image sharpness or enhance detail.
Image processing also allows the CT technologist to view an image at the same time the scan is
taking place. The final image is the result of multiple computer operations and decisions made by the
technologist. The process takes an amazing amount of computing power to process enormous amounts
of information. The digital-to-analog converter (DAC), makes image display possible by converting the
digital data to an analog signal for the LCD monitor.
After the image is ready for display, it is stored electronically by the PACS. PACS are
capable of archiving vast quantities of radiology images without taking up a large amount of physical
storage space. Another advantage of a PACS is accessibility. Physicians at multiple facilities or remote
sites are able to retrieve diagnostic images easily and quickly.
40.
Pixel
Digital image data are acquired as extremely small, individual pieces of information, just a
fraction the size of a millimeter. These individual units are known as pixels, a term that is short for
“picture elements.” A pixel is a two-dimensional element that is assigned a gray value in the form of a CT
number.
The CT number is displayed on an image matrix according to the mean attenuation value of the
corresponding anatomy. The pixel information directly corresponds to the attenuation coefficient of the
object. So, the pixel number is related to the atomic number and the mass of the scanned tissue. The
field of view and the matrix determine the size of an individual pixel. To calculate pixel size, we use the
equation: pixel = field of view/matrix.
So, for example, let’s suppose the CT technologist selects a 24-cm field of view with an image
matrix of 1024 x 1024. We first convert the field of view from centimeters to millimeters and then divide
by the matrix size. In this case, the field of view is 240 mm; 240 divided by 1024 is 0.232. Therefore, the
size of an individual pixel would be 0.232 mm. Use the slider bar on this page to see how the pixels in
the resulting image change as the field of view changes.
41.
Matrix
The image matrix is a two-dimensional array of numbers made up of multiple pixels arranged in
columns and rows. Typical image matrices in CT are 256 x 256, 512 x 512 or 1024 x 1024. As the matrix
number gets higher, the image resolution increases. For instance, a 1024 x 1024 matrix has a better
resolution than a 512 x 512 matrix.
To calculate how many pixels are in an image matrix, simply multiply the columns by the rows.
So for a 512 x 512 matrix, there are 262,144 pixels. The CT technologist selects the matrix when the field
of view is selected.
42.
Voxel
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CT Basics: Module 2
A voxel is a volumetric, or three-dimensional, picture element that represents a volume of tissue
in the reconstructed CT image. Voxel size depends on three factors: slice thickness, matrix size and the
field of view. The voxel size is determined by multiplying the pixel size by the slice thickness.
Voxel reconstruction is classified as isotropic and anisotropic. When the x, y and z plane (that is,
the length, width and height) are equal, the volume data set is isotropic. For CT, isotropic display means
that the size of each voxel in a given volume data set is equal. In other words, an isotropic voxel
represents a perfect cube. This relationship is achieved by making the slice thickness equal to the pixel
size. Isotropic imaging increases spatial resolution in all imaging planes.
The more a multislice scanner is capable of increasing the number of slices obtained per rotation
of the x-ray tube, the better the scanner is at scanning isotropically. Isotropic scanning is important
because it enhances 3-D and multiplanar reconstructions. Detector configuration also affects isotropic
imaging. As the scanner increases the number of detector rows, isotropic scanning becomes more
effective. The benefits of isotropic imaging in CT include improved image quality in all three dimensions,
reduction of the stair step artifact and the ability to achieve a resolution of less than 0.4 mm.
Anisotropic imaging, on the other hand, occurs when the slice thickness is not equal to the pixel
size. In this case, an anisotropic voxel displays as a rectangular shape rather than a cube.
43.
Digital Image Characteristics in CT
Decisions made at the operator’s console or during patient positioning can affect the display
characteristics of the digital image. Some display characteristics are inherent in the image data set based
on patient size, anatomical structure or pathology.
44.
Sampling and Aliasing
The very first step in CT data collection is sampling. Sampling occurs at the CT detectors where
the incoming x-ray beam is recorded (or sampled) to ensure that enough signal is present to produce an
image. The detector will assign a grayscale quantity based on the amount of radiation absorbed by the
detector; this grayscale quantity is the CT number or Hounsfield unit.
The sampling process must follow a specific set of rules to avoid potential artifacts such as
streaking. To avoid sampling artifacts thinner slices may be acquired or more compressed detectors can
be used. Some vendors have redesigned the x-ray tube geometry to reduce the occurrence of sampling
errors. Sampling data are different for each of the different beam geometries, such as parallel beam, fan
beam and cone beam.
Aliasing refers to a sampling problem that arises when structures and spaces cannot be
distinguished. These artifacts show up as streaks and are caused by an insufficient number of samples
available for image reconstruction. Aliasing often is referred to as a sampling error.
45.
Spatial Resolution
Spatial resolution describes the degree of blurring in an image and is a measure of how small an
object can be imaged. It is the ability to discriminate objects of varying density a small distance apart
and against a uniform background. CT is a superior imaging modality with respect to spatial resolution
because it’s able to distinguish tissue with density differences of less than 0.5%. In comparison,
conventional radiography can only differentiate densities as low as to 10%.
A high-contrast area such as a bone-soft tissue interface is more difficult to image than a lowcontrast area such as the liver-spleen boundary. Spatial resolution in CT can be improved by using
thinner slice thicknesses, adjusting the image matrix to ensure a smaller pixel size, using a
reconstruction filter such as a high-frequency convolution filter or using a small detector size.
46.
Contrast Resolution
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CT Basics: Module 2
Contrast resolution describes the caability to image adjacent tissues that have similar mass
density and effective atomic density. In other words, contrast resolution is the ability to distinguish
between similar tissues. Examples of areas that have similar tissue densities are the liver/spleen
interface and the gray and white matter of the brain. CT has superior contrast resolution because
narrow x-ray beam collimation reduces scatter radiation.
47.
Temporal Resolution
Another type of scanning should be briefly mentioned here. Half-scanning techniques are, in
some ways, a return to the early days of CT when only a portion of the 360° arc was used to create the
data set. Although the scanner components rotate around the patient in a complete 360° circle, only a
portion of the image data is used to create the image.
Special algorithms are able to create a diagnostic quality image from a smaller data set. Halfscanning techniques are used to reduce the appearance of motion in a CT scan from involuntary
movement such as a beating heart. The ability to resolve or view an object in motion is termed
“temporal resolution.” The half-scan mode is beneficial simply because it shortens the scan time
considerably and improves temporal resolution.
48.
Noise
Noise, which is also referred to as quantum mottle, is the primary factor that affects lowcontrast resolution. Noise appears as a grainy texture throughout the image and presents a nonuniform
image. Factors that contribute to noise include patient size and density, detector size, patient dose, slice
thickness, image matrix and the display field of view.
Increasing mAs reduces the noise in a CT image because more x-ray photons hit the detectors.
The matrix size also affects the noise seen on an image. A smaller matrix results in larger pixels and a
decrease in noise, but at the expense of spatial resolution.
49.
Partial Volume Averaging
Partial volume averaging occurs when the CT numbers are misrepresented because the anatomy
extends into the adjacent slice thickness. CT numbers are based on the linear attenuation coefficients
for a volume of tissue. If the anatomy contains only one tissue type, then the voxel will accurately
represent the CT number.
However, if the anatomy is divided between two slices, an average CT number is assigned to the
tissue. For example, in the human brain, white matter with a CT number of 46 and gray matter with a CT
number of 43 can be represented on the same voxel. Therefore, the voxel can be assigned a single CT
number that is the average for the tissues, in this case a CT number of 44 or 45.
Partial volume averaging can lead to misdiagnosis because the assigned average CT number
doesn’t accurately represent the tissue. Partial volume averaging can be improved by selecting thinner
slices, which allows the anatomy to be evenly distributed over each pixel and assigned a correct CT
number.
50.
Practice Question
51.
Practice Question
52.
Conclusion
©2010 ASRT. All rights reserved.
CT Basics: Module 2
We’ve come to the end of Module 2: Equipment and Instrumentation. In this module, we looked
at the major components of a modern CT scanner, including the data acquisition equipment, operator’s
console and some of the basic functions the computer uses to create a digital image. You can get a more
in-depth look at the functions of the CT computer in Module 4 of this series: Image Processing and
Reconstruction.
As you’ve seen, a CT scanner may look uncomplicated on the outside, but it’s capable of carrying
out a complex set of procedures. The only major component of the modern CT scanner not discussed in
this module is you, the CT operator. Having the knowledge, skill and patience to correctly use the
various components of the CT equipment will not only create a quality data set for image interpretation,
but also will avoid many of the artifacts inherent in digital imaging. Your ability to understand and
manage all the factors related to CT equipment is critical to ensure a quality exam for your patient.
53.
Acknowledgements
54.
Bibliography
Papp J. Quality Management in the Imaging Sciences. 3rd ed. St. Louis, MO: Mosby; 2006.
Seeram E. Computed Tomography: Physical Principles, Clinical Applications, and Quality
Control. 3rd ed. St. Louis, MO: Saunders; 2009.
55.
Objectives
This marks the completion of this learning module. Please review the learning objectives before
taking the end of module assessment. Once this window closes click on the "assessment" button to take
the end of module quiz.
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CT Basics: Module 2