COMPUTED TOMOGRAPHY

Instructor Gabriel Leo Hizon │ MIDTERM Reviewer │ 2nd Semester

BALBARADA, Zhane A.

GANTRY
 Is the ring-shaped part of the CT Scanner.
 Often referred by some patients as the Doughnuts.
 Critical component that rotates around the patient to capture cross-sectional images of
the body.
 It houses many of the components necessary to produce and detect x-rays.
 Data Acquisition System (DAS) - converts analog signal to digital signals.
 Diameter of the opening or aperture varies in size, as does the gantry.
 The aperture range size is usually 70-90 cm.
 The gantry can be tilted forward or backward as needed to accommodate a variety of
patients and examination protocols, usually 15-30 degrees.
 On either side of the gantry you’ll find control panels used by radiologic technologists to
control the alignment lights, gantry tilt and table movement.
 The gantry also includes a laser light that is used to position the patient within the
scanner.
 There is also a microphone in the gantry that allows communication between the
radiologic technologist and the patient throughout the scanning procedure.

GANTRY EXTERNAL VIEW

1. Gantry Aperture
2. Microphone
3. Sagittal Laser Light Alignment
4. Patient Guide Table
5. X-Ray Exposure Indicator Light
6. Emergency Stop Button
7. Gantry Control Pannel
8. External Laser Alignment Light
9. Patient Couch
10. ECG Gating Monitor

GANTRY INTERNAL VIEW

1. X-Ray Tube
2. Filter, Collimator
3. Internal Projector
4. Oil Cooler
5. High Voltage Generator (0-75kV)
6. Direct Drive Gantry Motor
7. Rotation Control Unit
8. DAS
9. Detector
10. Slip Ring
11. Detector Control Unit
12. High Voltage Generator (75-150kV)
13. Power Unit
14. Line Noise Filter

SLIP RINGS
 Early CT scanners used recoiling system cables to rotate the gantry frame. This design
limited the scan method to the step-and-shoot mode and considerably limited the gantry
rotation times.
  Current systems use electromechanical devices called slip rings.
 Slip rings permit the gantry frame to rotate continuously, eliminating the need to
straighten twisted system cables.
 By replacing traditional cables, slip rings reduce wear and tear, improve reliability,
and enable faster scanning speeds.
 It consist of two main components: metal ring and graphite or metal brush contact.
 The rings are mounted on the rotor and it is rotating. And brushes are fixed and
mounted on the brush house.
 As the rings rotates, the electrical current is passed through the brushes. Therefore, it
makes a continuous connection between the rings (rotating system) and brushes (fixed
system).

GENERATOR
 Modern CT scanners use high-frequency generators, which are small enough to fit inside the gantry.
 In the past, CT scanners used three-phase generators, which were very stable but had to be placed
near the gantry and connected with cables. Because of this, they are now outdated and no longer
used.
 Generators function is to create high voltage and send it to the X-ray tube.
 The power of the generator is measured in kilowatts (kW).
 The power capacity of the generator determines the range of exposure techniques (kV and mA
settings) available on a particular system.
 CT generators usually produce high kV (around 120-140 kV) to make the X-ray beam more penetrating
and helps reduce the radiation dose for the patient.
 Using a higher kV setting also allows the scanner to use a lower mA setting, which reduces the
heat produced by the X-ray tube.
 Less heat means the X-ray tube lasts longer, saving time and money on replacements.
HIGH VOLTAGE GENERATORS
 CT scanners use high voltages to power the x-ray tube and high currents (hundreds of
milliamperes) to produce enough x-rays for imaging. Each scan is very quick, taking only
0.5 to 2 seconds.
 CT scanners use high-frequency generators to make sure the x-ray tube gets a steady
and stable supply of electricity. This helps create consistent and clear images.
 Large focal spot (1 mm): Used when the scanner needs to produce a lot of x-rays
(high power, up to 60 kW).
 Small focal spot (0.6 mm): Used when the scanner needs to produce sharper, more
detailed images (low power, below 25 kW).
 CT x-ray tubes generate a lot of heat because they work at high power. To handle this,
the tubes are built with high heat capacity, meaning they can store a lot of heat without
getting damaged.
 Modern CT x-ray tubes can store more than 3 MJ (megajoules) of heat and can release
heat at a rate of 10 kW (kilowatts). This allows them to work for longer periods without
overheating.
 The x-ray tube is positioned perpendicular to the area being scanned. This helps reduce
the anode heel effect, where one side of the x-ray beam is weaker than the other,
ensuring more even images.
 Copper or aluminum filters are used to remove low-energy x-rays from the beam. This
reduces beam hardening, which happens when the x-ray beam becomes more intense as
it passes through the body, potentially distorting the images.

TYPES OF X-RAY TUBES USED IN CT SCANNERS

 CT scanners use specialized X-ray tubes to produce detailed cross-sectional images.
 The most common type is Standard CT (Single-Source CT), which uses one X-ray tube.
 Dual-Source CT uses two X-ray tubes for faster scanning and better tissue differentiation.
 The choice of X-ray tube affects image quality, speed, and scanning capabilities

1. STANDARD CT (SINGLE-SOURCE CT) X-RAY TUBES

 These scanners use one X-ray tube and one detector, making them the most widely used
CT systems.

Standard Rotating Anode X-ray Tube

 The most common tube in general CT scanners.
 Uses a tungsten-rhenium rotating anode to prevent overheating.
 Found in routine diagnostic imaging in hospitals and clinics

Straton X-ray Tube (Siemens)

  A more advanced rotating anode tube with an integrated cooling system.
 Allows for faster scanning speeds, reducing motion artifacts.
 Used in higher-end Standard CT scanners for cardiac and highspeed imaging

2. DUAL-SOURCE CT X-RAY TUBES

 Dual-Source CT scanners use two X-ray tubes and two detectors, improving speed and
material differentiation

Dual-Energy X-ray Tubes

 Two tubes operate at different energy levels (low and high kVp).
 Helps differentiate tissues (e.g., distinguishing iodine contrast from calcium).
 Used in cardiac imaging, oncology, and kidney stone analysis.

Two Rotating Anode X-ray Tubes

 Each tube has its own rotating anode, reducing the burden on a single tube.
 Enables ultrafast imaging, useful in heart and trauma scans

3. ADVANCED AND FUTURE X-RAY TUBES IN CT

 These tubes enhance CT imaging beyond standard and dual-source systems.

Flying Focal Spot Tubes

 Uses electron beam deflection to create two focal spots.
 Improves spatial resolution by reducing blur.
 Useful in vascular studies and high-detail imaging

Stationary X-ray Tube Arrays (Photon Counting CT)

 A new technology using multiple stationary tubes instead of a rotating anode.
 Improves image quality while reducing mechanical wear.
 Expected to play a key role in next-generation CT scanners

COOLING SYSTEMS IN CT SCANNERS

 CT scanners generate significant heat due to the continuous operation of X-ray tubes.
 Efficient cooling is necessary to prevent overheating, ensure consistent image quality, and
extend tube lifespan.
 Two main cooling systems are used: Air-Cooled and Water-Cooled systems.

1. Air-Cooled CT Systems
Cooling Mechanism:
 Uses fans and heat exchangers to dissipate heat from the X-ray tube.
 Air circulates around the components, carrying heat away
Advantages:
 Simple and cost-effective design.
 Requires less maintenance than water-cooled systems.
 Suitable for standard CT scanners in hospitals and clinics.
Disadvantages:
 Limited cooling capacity—may not be sufficient for high-performance or continuous-use CT
scanners.
 Heat dissipation depends on room ventilation, which can affect performance
Common Usage:
 Found in general-purpose CT scanners that do not require high-speed, high-volume
imaging

2. Water-Cooled CT Systems
Cooling Mechanism:
 Uses a closed-loop water circulation system to absorb heat from the X-ray tube.
 The heated water is cooled down in a heat exchanger before recirculating.
Advantages:
 Superior cooling efficiency, making it ideal for high-performance CT scanners.
 Allows for longer and faster scanning sessions without overheating.
 Less heat released into the room, maintaining a stable environment.

Disadvantages:
 More complex and expensive to install and maintain.
 Requires regular water circulation system checks to prevent malfunctions.

FILTRATION AND COLLIMATION IN CT SCANNERS

 In CT scanning, filtration and collimation play essential roles in optimizing image quality
while reducing radiation exposure.
  Filtration removes low-energy X-rays that do not contribute to imaging, while collimation
controls the X-ray beam shape and size to minimize unnecessary exposure

1. Filtration in CT Scanners
Purpose:
 Eliminates low-energy X-rays that would be absorbed by the patient without contributing
to image formation.
 Reduces patient dose while improving image contrast.

TYPES OF FILTERS:
INHERENT FILTRATION
 Built into the X-ray tube housing.
 Includes the glass envelope, cooling oil, and tube exit window.
 Provides basic filtration but may not be sufficient for high-performance imaging
ADDED FILTRATION
 External filters made of aluminum or copper placed in the X-ray beam path.
 Further removes low-energy X-rays, reducing radiation dose to the patient.
 Helps optimize beam quality for specific types of imaging.
BOWTIE FILTERS
 Specially shaped filters that adjust X-ray intensity based on body part thickness.
 Reduces unnecessary exposure to thinner areas (e.g., the edges of the body).
 Improves image uniformity and reduces scatter radiation

2. Collimation in CT Scanners
Purpose:
 Restricts the X-ray beam to the desired slice thickness, preventing excess exposure.
 Reduces scatter radiation, improving image contrast and resolution.

TYPES OF COLLIMATION:
PRE-PATIENT COLLIMATION
 Located before the patient at the X-ray tube.
 Shapes the X-ray beam width to match the detector row coverage.
 Controls slice thickness, affecting spatial resolution.
POST-PATIENT COLLIMATION
 Positioned after the patient, just before the detector array.
 Further refines the beam, reducing scatter radiation from reaching the detectors.
 Improves image contrast and clarity

DETECTORS
 Detectors are devices used in imaging systems (such as CT scanners) to capture X-rays
that pass through a patient and convert them into electrical signals.

DETECTOR ASSEMBLY
 The assembly moves in a circular motion along with the X-ray tube during the scan,
ensuring a full 360-degree image capture. In a CT scanner, the detector assembly is
positioned opposite the X-ray tube to capture X-rays after they pass through the body.
Detector size used in CT is measured in millimeters 44 inches or 110 cm.

Capture Efficiency:
 Measures how well the detector receives X-ray photons from the patient.
Absorption Efficiency:
 Determines how effectively the detector converts incoming X-ray photons into usable
signals.

Conversion Efficiency:
 Represents how efficiently the detector converts absorbed photon energy into digital
signals for the CT computer system. Higher conversion efficiency means better image
quality with lower radiation doses.
Stability:
 Refers to how often the detectors need recalibration to maintain accuracy. A stable
detector requires less frequent adjustments.
Response Time:
 The speed at which the detector recognizes an X-ray photon and recovers for the next
one. Faster response times improve image sharpness.
Dynamic Range:
 The ratio between the strongest and weakest signals a detector can measure. A higher
dynamic range ensures better image contrast and clarity. Modern CT scanners can handle
 a dynamic range of 1,000,000-1 meaning they can detect both very weak and very strong
signals effectively.

XENON GAS DETECTORS

 Xenon detectors are known for their stability and ability to handle high-speed scanning,
but they are less sensitive compared to newer detector types. Xenon gas detectors absorb
60% to 87% of incoming X-ray photons, making them effective but less sensitive
compared to solid-state detectors.

SOLID-STATE CRYSTAL DETECTORS

 Solid-state crystal detectors (also called scintillation detectors) use a crystal material, such
as cadmium tungstate or gadolinium oxysulfide, to convert X-rays into visible light. This
light is then detected by a photodiode, which converts it into an electrical signal.

PATIENT COUCH
 Must be constructed of material with low atomic number such as carbon fiber.
 Should be smooth and power driven to allow for precise patient positioning.
 Should be capable of automatic indexing.
 Maximum patient limit of 300-600 lbs.
 Exceeding the patient limit can cause:
1. Inaccurate indexing
2. Damage to the table motor
3. Breakage of the table top

COMPUTER
 Is the unique subsystem of the CT scanner.
 Provides the link between the CT technologist and other components of the imaging
system.
 The microprocessor and the primary memory determines the time between the end of
scanning and the appearance of an image - reconstruction time.
 Many CT scanners uses an array processor instead of a microprocessor. It is more faster
and can be reconstructed in less than 1 second.
 Data Acquisition - the method in which the patient is scanned.

OPERATING CONSOLE
 Point from which the CT technologist controls the scanner.
 Many CT scans are equipped with 2-3 consoles.
1. Console to operate the scanner.
2. Console to process the image for filming.
3. Console for the physician to view the image and manipulate it' s contrast, size and
general appearance
 The console controls the technique factors.
1. 120 kVp above
2. 100 mA - usual tube current
3. 1-5 seconds - scanning time
 The slice thickness can also be adjusted in the console panel.
 Normal thickness are 1-10 mm
 0.5 mm for high resolution
 It has usually 2 TV monitors (Scan Monitor and Display Monitor)
1. Patient data
2. For viewing of resulting scan prior to transferring to either hardcopy or the physician
viewing console
IMAGE STORAGE
 Current CT scanners store images on either magnetic tapes or disc.
 Each tape generally accommodates 150 scans
 CT scans are usually recorded on film (14 x 17) in a laser camera
 Commonly used filming device in CT:
1. Matrix camera – once was the standard imaging
2. Laser printer – preferred device now for imaging.

PRINTER
 Printer is an output device. It gives output on the paper. The printed output is called hard
copy.
 The most common types of printers are laser, ink-jet and dot matrix printers.
Dot Matrix Printer
 Dot matrix printer prints by arranging dots on a paper.
 The quality is not very good but the printing cost is less.
 It produces a noise during printing.
Ink-Jet Printer
 An ink-jet Printer has a print head that sprays tiny ink drops on a paper.
 It is better for smaller jobs and colour images.
 These printers popular for home usage
Laser Printer
 The laser printer is especially suited for printing large number of pages.
 It is quick, silent and produces a high-quality print.

IMAGE CHARACTERISTICS
 The image characteristics of a CT scan comprising CT numbers, spatial resolution, contrast
resolution, noise, artifacts, and the reconstruction process collectively determine the
clarity and diagnostic power of the images produced. Each characteristic plays a vital role
in ensuring that the images not only accurately represent the patient’s anatomy but also
provide the detailed information necessary for effective diagnosis and treatment planning.
 In a CT scan, the image is represented as a grid, called a matrix, which is made up of tiny
squares known as pixels. Each pixel corresponds to a specific spot in the scanned area of
the body.
 The CT machine assigns a number to each pixel based on how much the X-rays were
absorbed by the tissue at that location. This number, called a Hounsfield Unit (HU), reflects
the tissue's density (e.g., air has a low value, bone has a high value). These numbers are
then converted into shades of gray to create the image.
 The intensities are visually represented in grayscale.
 For example: Low-density tissues (like air) appear darker. High-density tissues (like bone)
appear lighter. Intermediate densities (like muscles or organs) appear as varying shades
of gray.

CT NUMBERS
 CT numbers, or Hounsfield Units (HU), are numerical values (Number) that measure how
tissues absorb X-rays in a CT scan. These numbers represent tissue density. Each pixel in
the CT image is assigned a CT number, creating a detailed map of the body’s internal
structures. By differentiating between tissue types, CT numbers help radiologists identify
abnormalities and make accurate diagnoses. They are a critical tool for understanding the
body’s composition and guiding medical decisions.

IMAGE RECONSTRUCTION
 Image reconstruction is the process of turning raw data from the X-ray detectors into
clear, understandable pictures.
 The image is reconstructed from these projections by a process called filtered back
projection
 CT image reconstruction requires the solution of over 250 000 simultaneous equations.
 The CT machine collects information about how X-rays pass through your body from
different angles.
 This information is sent to a computer, which uses special math (called algorithms) to
figure out what your body looks like inside.
 The result is a series of pictures, shown as slices, where each little square (a pixel)
represents a small part of your body.

IMAGE QUALITY
Conventional Radiographs:
1. Spatial Resolution
 The ability of the imaging system to display small details clearly.
 Importance: High spatial resolution is necessary to visualize fine structures, such as
fractures or small calcifications.
2. Contrast Resolution
 The ability to distinguish between tissues with slight differences in density or composition.
 Importance: Conventional radiographs typically have lower contrast resolution than CT
scans, making it harder to differentiate between soft tissues.
3. Noise
 Random variations that appear as graininess in the image, reducing clarity.
 Sources: Often caused by low radiation dose or detector limitations.
 Management: Balancing radiation dose and image acquisition parameters reduces noise
without overexposing the patient.

CT Image Quality:
1. Spatial Resolution
  Similar to conventional radiographs, spatial resolution in CT defines the scanner’s ability to
display small details
2. Contrast Resolution
 CT has superior contrast resolution compared to conventional radiographs.
 It enables clear differentiation between tissues with similar densities, which is particularly
useful for soft tissue imaging.
3. Noise
 Noise in CT images refers to random variations that can obscure fine details. Advanced
reconstruction techniques like iterative reconstruction (modern technique used to process
raw data from the scanner into high-quality images) can reduce noise while maintaining
image quality.
4. Linearity
 Linearity in CT ensures that CT numbers (Hounsfield Units) are consistent and accurately
represent tissue densities.
 Importance: Accurate linearity is critical for diagnostic reliability and tissue
characterization.
5. Uniformity
 Uniformity ensures that the same tissue in different parts of the image appears with the
same density value.
 Importance: This consistency is essential for diagnostic accuracy, preventing
misinterpretation of varying densities.

 Conventional Radiographs prioritize spatial resolution and simplicity, while CT

Image Quality adds more advanced factors like linearity and uniformity
alongside superior spatial and contrast resolution, enabling more detailed and
reliable imaging of tissues.

SPATIAL RESOLUTION
 The ability of the scanner to distinguish and display small details or structures that are
close together. It determines how sharp and clear the image is and how well tiny objects
can be visualized separately.
 Spatial resolution can be measured using a phantom made of lucite. Embedded in the
lucite are closely metal strips.
 Edge-Response Function (ERF) is the ability of the CT scanner to reproduce a high contrast
edge with accuracy and is expressed mathematically.
 Modulation Transfer Function (MTF) is a mathematical procedure for measuring spatial
resolution in CT scan. It is also the ratio of the image to the object. The most common
method to of describing spatial resolution.
 Spatial resolution may also be described by the terms highcontrast resolution, blur, and
modulation transfer function (MTF)
 Low Spatial Frequency: 1. Represents large objects. 2. Better Contrast Resolution
 High Spatial Frequency: 1. Represents small objects. 2. Better Spatial Resolution

FACTORS AFFECTING SPATIAL RESOLUTION

Detector Size
 Smaller detector elements capture more precise information, enhancing spatial resolution.
Patient Positioning
 Incorrect positioning can lead to misaligned image planes and reduced spatial resolution.
Proper alignment ensures optimal detail capture.
Rotation Speed of the Gantry
 Faster gantry rotation speeds can affect resolution if motion blurring occurs. Slower, more
stable rotations allow for better detail capture.
Detector Technology
 Advanced detectors, like solid-state detectors, provide higher precision and better spatial
resolution compared to older gas-based detectors.
Scatter Radiation
 Scatter reduces image clarity, indirectly affecting resolution. Proper collimation and
shielding reduce scatter and improve overall image quality.

CONTRAST RESOLUTION
 The ability to distinguish between tissues with similar densities. It ensures that subtle
differences in tissues, like detecting soft tissue lesions or small abnormalities, can be
clearly visualized. High contrast resolution is enhanced by adjusting X-ray settings (mA
and kVp), using contrast agents, and employing advanced reconstruction techniques. It is
essential for accurate imaging, especially in soft tissue analysis.
FACTORS AFFECTING CONTRAST RESOLUTION
Tube Voltage (kVp)
 Lower kVp improves contrast because lower-energy X-rays enhance the differences in
attenuation between tissues with similar densities. This is especially useful for detecting
soft tissue abnormalities or enhancing the visibility of contrast agents.
Detector Sensitivity
 High-quality detectors with better sensitivity can capture subtle differences in X-ray
attenuation, improving contrast resolution.
Use of Contrast Agents
 Injected or oral contrast agents enhance the differences in attenuation between tissues,
significantly improving contrast resolution for specific regions or structures.
Patient Size
 Larger patients cause more X-ray scatter, which can reduce contrast resolution. Adjusting
scan parameters for patient size helps mitigate this.

LINEARITY
 Refers to the relationship between CT numbers and linear attenuation values.
 It requires frequent calibration to ensure consistent representation of tissues, and
deviations from linearity indicate scanner malfunction or misalignment.
 It is a crucial quality control parameter that ensures the accuracy of CT images.
 It is measured using phantoms with different materials and the coefficient of
determination (R2) to assess the strength of the linear correlation.
 Linearity is important for accurate diagnostics, as higher density objects appear brighter in
images.

LINEARITY TEST
 CT QC Phantom Pin Layer- Shows the arrangement of various materials (air, Teflon,
polyethylene, water, and resin) and pins within the phantom, used for evaluating different
aspects of image quality such as spatial resolution and contrast.
 The CT numbers (HU) of these materials are measured and plotted against their known
densities.
 A linear relationship should be observed: as density increases, HU values should increase
proportionally.

CORRECTING NON-LINEARITY
 Beam Hardening Correction: Software algorithms can correct for beam hardening effects.
 Regular Calibration: Ensures the CT scanner maintains linearity over time.
 Scatter Correction: Reduces the impact of scatter radiation on CT numbers.

FIVE-PIN PERFORMANCE
 Refers to a quality assurance test using a phantom with five different material pins, each
with a distinct density, to assess the scanner's ability to accurately measure and
differentiate between different tissue densities, primarily evaluating its "contrast
resolution" and "linearity" performance by comparing the CT numbers assigned to each
pin.
 5 metal pins are used to assess image sharpness, contrast, and alignment

UNIFORMITY
 Uniformity in CT imaging refers to the consistency of pixel values (CT numbers) across the
entire image.
 In an ideal scenario, a uniform phantom (a object with known properties used to test
imaging systems) should produce an image where the pixel values are constant
throughout.
 However, due to various factors such as beam hardening, scatter radiation, and detector
sensitivity, some variation in pixel values is expected.
 Non-uniformity can lead to incorrect diagnoses (e.g., tissues appearing denser or less
dense than they really are).
 It’s a key quality control measure in CT imaging.
UNIFORMITY TEST
 To test spatial uniformity, a uniform phantom (typically a water-filled cylinder) is scanned.
The resulting image is then analyzed to ensure that the pixel values are consistent across
the image.
 This analysis can be done using internal software packages that allow for the plotting of
CT numbers along any axis of the image. The data can be represented as a histogram or a
line graph.

ACCEPTABLE UNIFORMITY CRITERIA

 Pixel values should be within 2 standard deviations (σ) of the mean (μ).
 Formula: μ ± 2σ.
 If 95% of values fall in this range, the system has acceptable uniformity.

FACTORS AFFECTING UNIFORMITY

1. Beam Hardening: Higher-energy photons dominate, causing artifacts.
2. Scatter Radiation: Adds noise to the image.
3. Detector Sensitivity: Uneven detector responses.
4. Calibration Errors: Incorrect system settings.

NON-UNIFORMITY CAN BE PREVENTED BY:

 Regularly calibrate the CT system.
 Use beam hardening correction algorithms.
 Apply scatter correction techniques.
 Ensure detector calibration for even sensitivity.

WINDOWING/GREY LEVEL MAPPING

 Windowing allows us to focus on a specific range of HU values and map them to shades of
gray, making it easier to see details in specific tissues.
 Windowing, also known as grey-level mapping, contrast stretching, histogram
modification, or contrast enhancement is the process in which the CT scan greyscale
component of an image is manipulated via the CT numbers (Hounsfield Units).
 Window width and level optimize the appearance of CT images by determining the
contrast and brightness levels assigned to the CT image data.
 CT images typically possess 12 bits of gray scale, for a total of 4,096 shades of gray.
 X-ray CT images are often described as “density distributions” because they provide a
gray-scale display of linear attenuation coefficients that are closely related to the physical
density of tissues.

HOUNSFIELD UNITS
 Hounsfield Units (HU) are a dimensionless unit (CT) scanning to express CT numbers in a
standardized and convenient form.
 CT images are made up of pixel values called Hounsfield Units (HU), which range from -
1000 HU (air) to +3000 HU (dense bone).
 However, the human eye can only distinguish about 50 shades of gray.
 It is universally used in computed tomography.

CT VALUES

Window Width (WW)

 Refers to the number of different HU units that will be represented by different shades of
gray.
 Any HU value that falls below the lower value of the window width will show up as black on
the scan while any HU value that is above the upper value of the window width will be
white.
Window Level (WL)
 The Window Level (WL) refers to the window centre or midpoint HU value that is
represented on the window setting.

Window Settings
 Determine how the calculated tissue attenuation values are displayed.

WINDOWING SETTINGS
Adjusting the Window Width
 Hence, increasing the window width
will decrease the contrast of the
image.
 Hence, decreasing the window width
will increase the contrast of the
image.
Adjusting the Window Level
 Hence, increasing the window level
will decrease the brightness of the
image.
 Hence, decreasing the window level
will increase the brightness of the image.