Download computed tomography

COMPUTED
TOMOGRAPHY
CHAPTER 33
FUNDAMENTALS
Creating a cross-sectional tomographic
plane of any body part
A patient is scanned by an x-ray tube
rotating around the body
A detector assembly measures the
radiation exiting the patient
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FUNDAMENTALS
Exiting radiation: Primary data
Primary data is collected by detectors
The computer compiles and calculates the
data based on preselected alogorithm and
an image is
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PRESELECTED ALGORITHM
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IMAGE
Each image is displayed in an axial form
(usually)
The images are displayed on a cathode
ray tube
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CT
Conventional Radiographs: Frequently
body structures are superimposed
In CT: A tightly collimated x-ray beam is
directed thought the patient from different
angles – “cross sectional image”
Essentially eliminating superimposition of
body structures
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CT
Claim to fame:
Exceptional Contrast Resolution
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Contrast resolution = differentiation of
densities, capable of differentiating among
tissues with similar densities
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CT
Due to the reduction in amount of
scattered radiation

Reducing over lapping structures and 2
collimators
Digitized image: because of this numerous
image manipulation techniques can be
used to enhance and optimize the
diagnostic information.
Window/Level, Axial, Sagittal, Coronal
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Third Generation
Fan-shaped x-ray beam

960 detectors opposite the x-ray tube
Complete 360 degree rotation

Rotate/Rotate movement
One rotation = one slice
Second data acquisition could be made as
the tube and detectors move in the
opposite direction.
Time reduced to 1 sec per slice
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3rd generation configuration
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Fourth Generation
Developed in 1980’s

Fixed ring of as many as 4800 detectors,
completely surrounding the patient, Rotate
only movement
Rotating x-ray tube provides short bursts
of radiation

Detectors collect the remnant radiation to
reconstruct into an image
1 minute for multiple slices
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4th generation
configuration
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Modern Scanners
No longer categorized into Generations
Contemporary CT scanners are either
third or fourth generation designs

Scanners are categorized by tube and
detector movement
Slip Ring Technology: connects generator
with tube (no cables)
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Technical Aspects
Optimum imaging: patient/area of interest
and gantry are perpendicular to each other
Tube rotates around the patient, irradiating
the area of interest.
Detectors measure the transmitted x-ray
values, covert them in to an electric signal,
and relay the signal to the computer.
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Raw Data
The remnant radiation that is converted
into an electrical signal values are called
projections, scan profiles or raw data

Raw data is collected and digitized
This process assigns a whole number to
each signal.

The value assigned is directly proportional to
the strength of the signal.
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Map Indirect vs. Direct Digital
Capture
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Digital Image
Array of numbers arranged in a grid of
rows and columns called a matrix.
Single square, or picture element, with in
the matrix is called a pixel.
Slice thickness gives the pixel and added
dimension called the volume element, or
voxel
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Voxel
Each pixel in the image corresponds to the
volume of tissue in the body section being
imaged.
The voxel volume is a product of the pixel
area and slice thickness
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Hounsfield units
Each pixel within the matrix is assigned a
number that is related to the linear
attenuation coefficient of the tissue within
each voxel
These are CT numbers or Hounsfield
units.
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Hounsfield units
Defined: A relative comparison of x-ray
attenuation of a voxel of tissue to an equal
volume of water.
Water is used because it is in abundance
in the body and has a uniform density
Water is assigned an arbitrary value of 0
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CT numbers
Tissue denser than water are given
positive CT numbers
Tissue with less density than water are
assigned negative CT numbers
The scale of CT numbers ranges from
-1000 for air to +3,000 for dense bone
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Displaying the image
On the CRT, each pixel within the image is
assigned a level of gray
The gray level assigned to each pixel
corresponds to the CT number or
Hounsfield units for that pixel
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System Components
Computer, Gantry &Table & Operator’s
Console
Computer – provides the link between the
CT technologist and the other components
of the imaging system
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Computer
The computer has four basic functions:
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Control of data acquisition
Image reconstruction
Storage of image data
Image display
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Array Processor
Raw Data is sent to the array processor
Array processor only performs algorithm
calculations.

Applies desired filters to raw data.
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Gantry
Gantry is a circular device that houses the
Data Acquisition system (DAS)
Tube
 Detectors
 Filters
 Collimators
 Analog-to –Digital
Converter (ADC)

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Gantry
Can be tilted forward or backward up to 30
degrees to compensate for body part
angulation.
The opening within the center of the gantry
is termed the aperture
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CT Tubes
X-ray tube for CT is similar in design to the
conventional radiography tube,

but is specially designed to handle and
dissipate excessive heat units – much higher
heat loading
Ceramic target backing

Decreases tube
weight
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Detectors
Function as image receptors for remnant
radiation

then converts the measurement into an
electrical signal proportional to the radiation
intensity.
Two basic detector types are used:
Scintillation (solid state) and Ionization
(xenon gas) detectors.
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Detectors
Used to record photon activity
Materials include: cadmium tungstate,
cesium iodide, gadolinium or yttrium
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Table
Automated device linked to the computer
and gantry

Designed to move in increments after every
scan according to the technologists scan
program
Accurate and reliable table movements is
vital to image quality and accuracy

Has weight limits
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Operator’s Console
Where the technologist controls the
scanner

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Keyboard
graphic monitor
Mouse
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Display Monitor
For the CT image to be displayed monitor
in a recognizable form, the digital CT data
must be converted into a gray-scale image
Each digital CT number is the matrix is
converted into an analog voltage
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Image Display
The brightness value of the gray-scale
image correspond to the pixels and CT
numbers of the digital data they represent
Because the image is digital image
manipulation can be performed
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FOV
The field of view determines the
amount of data to be displayed on
the monitor
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Image manipulation
Most common: windowing or gray-level
mapping
This technique allows the technologist to
alter the contrast of the displayed image

by adjusting the window width and window
level.
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Windowing
Window width: is the range of CT numbers
that are used to map signals into shades
of gray

Wide/Narrow or Long/Short
Window level: determines the midpoint of
the range of gray levels to be displayed

Darker or Lighter
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Window Width
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Window Level
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Image manipulation
Multiplanar reconstruction or MPR
Ability to reconstruct axial images into
coronal, sagittal or oblique body planes
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Diagnostic Applications
Most common procedures: head, chest,
abdomen, pelvis
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CT is the exam of choice for head trauma
CT can evaluate the CNS for infarctions,
hemorrhage, disk herniation, craniofacial and
spinal fractures, tumors and other cancers
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Biopsy & Abscess Drainages
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CT examinations
CT demonstrates abnormalities such as
metastatic lesions, aneurysms, abscesses,
and fluid collections from infection or
trauma
Interventional procedures: abscess
drainage, tissue biopsy, cyst aspiration
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CT Angiography
CTA: Uses three dimensional (3D)
imaging techniques to evaluate the
vascular system
Advantages over conventional angio:
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Image reconstruction without the use of more
patient exposure to radiation or IV contrast
Overlying structures can be eliminated (post
processing)
Does not require an arterial puncture
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Contrast Media
Is used in most CT imaging to distinguish
normal anatomy from pathology
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Iodine based IV contrast, similar to the IVU
Oral/Rectal contrast
2% Barium mixture is used
Iodinated oral contrast can be used
(Hypaque) Gastrographen must be at low
concentration to prevent contrast artifacts
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What are factors that affect
image quality?
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Factors Affecting Image Quality
Four main factors contributing to image
quality are: Spatial resolution, Contrast
resolution, Noise, and Artifacts
Diagnostic imaging has superior spatial
resolution compared to CT
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How do we measure spatial resolution?
What is the range for plane films?
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Technologist Determines Image
Quality Factors
The technologist choices will effect the
resultant image quality”
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slice thickness
focal spot size
display FOV
technique selection
pitch
reconstruction algorithm
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DYNAMIC SCANNING
Based on the principal that after contrast
administration different structures enhance
at different rates
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Spiral Scanning
Table and Tube moving throughout
exposure
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Helical Scanning
only the tube moves during the exposure
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SPIRAL/HELICAL CT
Spiral CT = Continuous Tube and Table
movement
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Pitch
.5 = more information more radiation
2 = less information less radiation
Helical CT = Table moves in
predetermined increments
 Post processing is not possible
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Pitch = 2
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Pitch = 0
How does the Pitch affect patient dose?
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MULTI-SLICE SPIRAL/HELICAL
Multi-slice Helical CT (MSHCT)
Multiple aligned row of detectors
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4, 16, 32, 64
Improved Spatial resolution
Decreased scan times
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3D IMPROVES RAD THERAPY
This method helps the dosimetrist to plan
treatment so that the radiation dose to the
target (tumor) is maximized and the dose
to the normal tissue is minimized.
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MRI vs CT
With the introduction of MRI the profession
speculated if CT would become obsolete
Advantages of CT : Metal is not affected,
Claustrophobia, uncooperative patiens,
obese patients, fast scan times, trauma,
more cost effective
Both modalities are used to provide as
much diagnostic information as possible
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CT in the Future
CT technologist has an increased
responsibility to understand scanning
dynamics
This imaging modality will continue to be a
highly respected diagnostic tool
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Review/Questions
What does the window width control for
image display?
Pitch how does this affect image quality
and patient dose?
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