CT Image Quality
What are some limiting factors to the in-plane spatial resolution?
- Focal Spot Size - CT Scanner Geometry - Detector Size - Reconstruction Algorithm
What are the geometric factors of the scanner that affect spatial resolution?
- Focal spot size - Detector aperture size - Focal spot to patient distance - Patient to detector distance
What parameters affect patient dose?
- Scan rotation time - The tube current - The peak tube voltage - The slice thickness - The table increment - The pitch - The anatomical coverage desired
What are some of the parameters that we directly control that affect spatial resolution?
- Slice thickness - Display FOV - Image Matrix - The Reconstruction Filter
What are some other tests performed on a less frequent basis by the service representative or site physicist? (11 items)
- Spatial resolution - Contrast resolution - Table accuracy - Dose - Accuracy of spatial measurements on the image - Consistency of CT numbers across the image. - Linearity of the gray scale of the image as it relates to the actual density of tissues measured. - Radiation scatter and leakage - Slice thickness accuracy - Fidelity of the video monitor - Film output
What does the Reconstruction or Display FOV affect?
- The apparent zoom level in the image - The appearance of noise - The in-plane resolution of the image
What is the CT number/ Hounsfield Unit for air?
-1000
What are the acceptable limits for the CT number calibration test?
-3 to +3
What is the CT number/ Hounsfield Unit for water?
0
What pixel level (color) is assigned to any CT numbers below the range specified by the window width?
0 (black)
Typically, tissues with differences in density of less than ____ % can be differentiated with CT.
0.5%
X-ray photons are attenuated via what two processes?
1. Absorption 2. Scattering
What are the two slice thickness parameters that must be specified on a MDCT scanner?
1. Acquired slice thickness 2. Reconstructed slice thickness or Effective slice thickness
What things are affected by slice thickness
1. Anatomical coverage 2. Image noise 3. Resolution along the direction perpendicular to the acquired image. 4. Patient dose
What four things does pitch affect?
1. Anatomical coverage 2. Scan time 3. Resolution 4. Dose
The amount of attenuation is dependent upon what four factors?
1. Atomic number 2. Density of electrons 3. Thickness 4. Photon energy
What are the two most frequently used standards for daily quality assurance assessment?
1. CT number calibration test 2. CT number standard deviation test
List the three image quality factors.
1. Contrast 2. Resolution 3. Noise
What are the four influences on parameter selection?
1. Contrast Resolution 2. Spatial Resolution 3. Image Noise 4. Patient Dose
What are the two factors that determine slice thickness on a single- row detector scanner?
1. Focal Spot Size 2. Pre-Patient Collimation
List four reasons to increase the pitch.
1. Greater anatomical coverage in less time 2. Faster scan for trauma 3. For appropriate contrast timing during a CTA study 4. Decrease the dose to the patient
What are two reasons that we might use a reduced pitch?
1. Improving the resolution along the slice direction. 2. Minimizing the appearance of noise in the image.
What are the two types of FOVs?
1. Scan Field of View (SFOV) 2. Reconstruction or Display Field of View (RFOV or DFOV
What defines the two dimensions of a voxel?
1. Slick thickness 2. In-Plane spacial resolution
What is the required difference in density in order for tissues to be differentiated with conventional radiography?
10%
Any measurements above ____ units generally indicates the presence of calcification.
100
Approximately where does blood fall on the CT number/ Hounsfield scale?
12 HU
What pixel level (color) is assigned to any CT numbers above the range specified by the window width?
255 (white)
How many shades of grey can an operators console display?
256
What is the most commonly used matrix size and is the value typically varied?
512x512; No, it is not typically varied.
Approximately what range does Congealed blood fall into?
56 to 76 HU
What do we call a single picture element of the image matrix?
A Pixel
In a spatial resolution test for CT, what is one hole and one space referred to as?
A line pair
What do we call a volume of element?
A voxel
how many shades of grey can the human eye distinguish?
About 20 shades of grey.
The ____ ____ ____ indicates which rows of detectors are being used to measure the photons that pass completely through the patients body.
Acquired slice thickness
What is the CT number/ Hounsfield Unit for fat?
Approximately -80
What is the CT number/ Hounsfield Unit range for soft tissue?
Approximately 30 to 80
Define attenuation.
Attenuation is the progressive reduction of the intensity of the x-ray beam as it passes through the patient's tissues.
When an x-ray beam enters tissue and loses lower energy photons to attenuation, there is an increase in the mean energy of the beam. This is referred to as what?
Beam hardening
Why must care be taken in choosing the scan FOV?
Because correction and calibration algorithms are often affected when selecting a scan FOV. For example: Choosing a head scan FOV of 25 cm is not equivalent to choosing a small body scan FOV of 25 cm, and will not produce identical image quality.
Why is spatial resolution and noise not affected by magnification?
Because the same amount of anatomy is represented by each pixel.
How do we optimize the in-plane spatial resolution?
By using a small reconstruction FOV.
The pixels in an image represent the ____ of a volume of tissue called a ____.
CT number, voxel
Define Contrast Resolution
Contrast resolution is the ability to differentiate small differences in density on the image.
What is another name for Reconstructed slice thickness?
Effective slice thickness
On a multi-row detector scanner in which four slices are collected simultaneously, an un-stretched continuous helix would correspond to what pitch?
Either 4 or 1
The number of photons measured by the detectors is reduced in an ____ fashion as the thickness of the tissue traversed is increased.
Exponential
T/F: The image matrix does not affect the apparent noise or resolution of the resulting image.
False
T/F: Increasing the number of photons increases the appearance of noise in the CT image.
False: Increasing the number of photons decreases the appearance of noise in the CT image.
T/F: In the range of energy levels used in CT, an increase in the kVp leads to a slight decrease in the radiation dose.
False: It leads to a slightly increase in the radiation dose.
T/F: The pitch affects the amount of gap or overlap between slices.
False: The pitch does not affect the amount of gap or overlap between slices! The pitch indicates how the data is going to be collected and the reconstruction interval handles the location of the reconstructed slices.
If the pitch is increased to more than 2 (i.e. in one rotation the patient table moves more than twice the total slice volume) ____ artifacts will occur.
Helical
The black stripes in a CT Spatial Resolution Phantom represent the ____.
Holes
Who produced the first clinically useful CT scanner for brain imaging and when did he/she do it?
Hounsfield, in 1967
What is the only reason a technologist might consider changing the kVp for a given exam?
If the patient is very large or very small.
CT scanners have many user- selectable parameters which can dramatically affect clinincal ____, ____ and ____.
Image quality, patient dose, and throughput
How is spatial resolution measured?
In line pairs per cm
What is the In-Plane Spatial Resolution Formula?
In-Plane Spatial Resolution= Reconstruction (Display) FOV/ # of pixels along each direction in the matrix Spatial Resolution= DFOV/The Matrix
What is the easiest way to increase the number of photons?
Increase the tube current or the scan time.
In all actuality, what does the in-plane spatial resolution formula determine?
It determines the size of an object that can be seen in the resulting image.
What does an acceptable standard deviation test indicate?
It is an indication of an increase in the image noise which could be caused by either a decrease in the delivered dose, resulting in a lower than expected signal, or an increase in image noise in the detector- amplifier chain.
On a single- row detector scanner, a pitch of 1 means what?
It means that the patient table moves a distance equal to the slice thickness during 1 rotation of the tube/ detector assembly.
What is the -3 to +3 acceptable limit for the CT calibration test referring to?
It refers to the fact that the CT number of water should be 0 (zero) but a variation of anywhere from -3 to -3 is acceptable.
How will increasing the window width change the representation of tissues on the image?
It will represent a wider range of tissues on the image.
What is the only way to enlarge the size of the image if the raw data is no longer available?
Magnification
What would we call an x-ray beam composed of photons all having the same energy level?
Monochromatic
What appears in a CT image as a point- to- point fluctuation in the CT number within a uniform material?
Noise
What do we call it when a CT image looks grainy or speckled?
Noise
When should increasing the dose to the patient be considered?
Only when the result of not doing so would otherwise be a non-diagnostic study.
Define out-of-field artifacts.
Out-of-field artifacts are characterized by image shading, streaks and incorrect assignment of CT numbers to the image pixels.
What do we call the effect that results from the mathematical averaging of several millimeters of varying density tissues, which is displayed as a single shade of gray on the resulting image.
Partial voluming
The anatomical coverage of a helical scan is determined by an important scan parameter called what?
Pitch
List the two possible formulas for determining pitch (scanner dependant).
Pitch=Movement of the patient table in one gantry rotation/ Total slice volume Pitch=Movement of the patient table in one gantry rotation/Thickness of a single slice
In reality, the x-ray photons generated by a CT x-ray tube have a variety of energies. This is referred to as being what?
Polychromatic
Define Reconstruction Field of View (DFOV).
RFOV is the region of interest illustrated in the resulting images and the FOV reconstructed from the complete set or raw data.
Reconstruction interval refers to how the helical slices are what?
Reconstructed
What does a resolution test consist of?
Scanning a water filled phantom and determining the smallest line pair that can be resolved.
Minimizing the noise with a large reconstruction FOV is attained only by compromising the image's ____ ____.
Spatial resolution
Define Spacial Resolution.
Spatial resolution is a measure of the size of the smallest object that can be visualized, or resolved, in an image.
What is the formula for determining spatial resolution of the set of lines with 8 line pairs per centimeter can just be resolved?
Spatial resolution= 10mm ÷ 16 lines (there are 2 lines per pair)
What do helical artifacts look like?
Swirls or windmills on the image.
The CT number scale is also referred to as what?
The Hounsfield Scale
In a helical scan, what determines whether the slices will be contiguous or if there will be a gap or an overlap of slices (images)?
The Reconstruction Interval
What determines the number of detectors collecting data for a particular scan?
The SFOV
How large should the SFOV be and why?
The SFOV should be large enough to cover all of the anatomy. If any part of the patient lies outside the SFOV, out-of-field artifacts will occur.
What controls how "contrasty" the image appears?
The Window Width
What are the two parameters evaluated during the CT number calibration test/ CT number standard deviation test?
The average value of the CT numbers and the standard deviation of the CT numbers within a region of interest.
What does the window level control?
The brightness of the image. Specifically, the window level sets the center CT number displayed on the monitor.
Define Patient Dose.
The dose is the amount of ionizing radiation absorbed bt the patient per unit mass.
What is the factor that is responsible for the amount of noise we see in the image?
The mAs
What is the definition of pitch on a single-row detector scanner?
The pitch equals the movement of the patient table in one gantry rotation divided by the slice thickness or pitch=movement of the patient table in one gantry rotation/slick thickness
How doe the slice thickness affect noise?
The pre-patient collimation allows more photons to reach the detectors when a thick slice is selected than when a thin slice is selected. So a thicker slice appears less noisy because there are more photons detected relative to the noise level in the thicker slices.
Define Quality Assurance.
The procedure of performing specified tests or measurements on a periodic basis in order to assure that a set level of quality, as specified by the system manufacturer, has not been compromised.
What do we call the thickness of the final image?
The reconstructed or effective slice thickness
What do we call the time required for the tube and detectors to rotate a complete 360°?
The rotation time
What determines the total area over which projection data is collected?
The scan field of view (SFOV)
What usually determines the amount of table incrementation selected?
The size of the structures that need to be visualized in the clinical study.
What are the acceptable limits for CT number standard deviation?
The standard deviation of water must not exceed a level specified by the manufacturer.
What do we call the voltage that is applied across the anode and the cathode?
The tube voltage
T/F: "Reconstruction Interval" is a scan parameter that is specific to helical scans only.
True
T/F: A finer reconstruction interval may be used for helical data than would be practical with conventional overlapping slices.
True
T/F: A large matrix increases the appearance of image noise.
True
T/F: A thinner acquired slice thickness means that smaller detectors are being selected.
True
T/F: As the slice thickness increases, the for the exam as a whole decreases.
True
T/F: Different manufacturers use one of two different formulas for determining the pitch of their scanners.
True
T/F: In general, the window level is set equal to the CT number of the tissue of interest.
True
T/F: Magnification does not affect the spatial resolution or affect the appearance of image noise.
True
T/F: Noise makes it difficult to see subtle contrast differences or fine details.
True
T/F: On a MDCT scanner, the reconstructed or effective slice thickness must be equal to or greater than the acquired slice thickness.
True
T/F: Overlapping slices may provide more thorough coverage of the anatomy than contiguous slices.
True
T/F: The attenuation of an x-ray beam is highly dependent upon the energy of the x-ray photons.
True
T/F: The mAs does not affect the image contrast or resolution.
True
T/F: The reconstruction or display FOV is always equal to or smaller than the scan FOV.
True
T/F: There are no parameters on the scanner that directly allow us to decrease the amount of noise.
True
T/F: Within the range determined by window width, the CT numbers are equally distributed from black to white.
True
T/F: kVp affects both the dose to the patient and the image noise.
True
The CT number scale is established relative to the attenuation of what?
Water
When is a wide window width generally used?
When viewing structures with a high degree of contrast, such as in the lungs. (Stark differences in density of tissues).
When is a narrow window width useful?
When visualizing structures exhibiting subtle inherent contrast, such as with the soft tissues of the brain. (Similar differences in density)
How do we accommodate the limitations of only being able to display 256 shades of grey and only being able to see about 20?
Window Width (WW) and Window Level (WL)
Define Window Width.
Window Width refers to the range of CT numbers displayed on an image and it adjusts how "contrasty" the image appears.
What are you doing when you magnify an image?
You are simply enlarging the individual pixels of the reconstructed image for display purposes.
The use of thicker slices reduces the resolution along the direction perpendicular to the ____ direction.
Z-direction
On a single-row detector scanners, the collimated x-ray beam thickness equals the ____ ____ ____, which also equals the final ____ ____.
acquired slice thickness, image thickness
On a single-row detector scanner, the slice thickness is determined by the ____ of the ____ ____.
collimation, x-ray beam
If the reconstruction interval equal to the slice thickness, the reconstructed slice will ____.
contiguous
Changing the kVp of the scan involves a tradeoff between ____, ____ and apparent ____.
contrast, dose, noise
By using thicker slices to cover the same anatomy, the radiation dose to the patient may be ____.
decreased
There is a ____ correlation between the apparent noise level and the ____ ____ ____ used in making the image.
direct, number of photons
On single- row detector scanners, as the pitch is increased, there is an unfortunate increase in the ____ ____ ____.
effective slice thicknes
To get contiguous slices, the table increment must be ____ to the slice thickness.
equal
Each pixel in the image is assigned a ____ ____ ____according to the CT number calculated by the ____ ____ ____.
gray scale number, filtered back projection
Higher CT numbers (i.e. dense structures) are assigned ____ shades of gray and lower CT numbers (i.e. structures which are not dense) are assigned ____ shades of gray.
lighter, darker
The ____ is the tube current. The ____ is the product of tube current and time.
mA, mAs
There is a tradeoff between ____ the image quality and ____ the patient dose.
maximizing, minimizing
The slice thickness must be chosen carefully to maintain an acceptable ____ ____ and appropriate ____ while minimizing ____ ____.
noise level, coverage, partial voluming
If the reconstruction interval is less than the slice thickness, the reconstructed slices will ____.
overlap
The use of thick slices produces an effect called ____ ____ or ____ ____ ____.
partial voluming, partial volume averaging
The pitch determines how ____ the x-ray path is during a helical scan.
stretched
On conventional or serial scans, the ____ determines if the slices will be contiguous or if there will be a ____ or ____.
table increment, gap, overlap