ct image quality

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Answer

What are the 5 key influences on ct parameter selection?

contrast resolution, spatial resolution, temporal resolution, image noise, patient dose

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____- resolution is the ability to differentiate small differences in density on the image

contrast

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Typically, tissues with differences in densities of less than ___% can be differentiated with ct

0.5%

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_____ resolution is a measure of the size of the smallest object that can be visualized in an image

spatial resolution

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What is spatial resolution affected by?

Geometric factors such as focal spot size, detector aperture size, focal spot to patient distance & patient to detector distance

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What are some factors that we directly control which affect spatial resolution?

Slice thickness, reconstruction factors which display FOV, image matrix and reconstruction filter

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Spatial resolution is often expressed in terms of ______

line pairs

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_____ resolution refers to the precision of a measurement with respect to time

Temporal resolution

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There is a relation between temporal resolution and ___ resolution

Spatial resolution

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___ appears in a CT image as a point to point fluctuation in the ct number within a uniform material, or more simply, it makes the image look grainy or speckled

noise

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True/fasle, noise makes it difficult to see subtle contrast difference or fine detail

true

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An image will appear more noisy, or grainy when there are ____ photons contributing to the pixels

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There is an indirect/direct correlation between the apparent noise level and the number of photons used in making the image

Direct

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Increasing the number of photons _____ the appearance of noise

decreases

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What is the easiest way of increasing the number of photons that contribute to the pixels in the images to make it less noisy

Increase tube current (mA) or scan time – doing this increases pt dose

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This refers to the amount of ionizing radiation absorbed by the patient per unit mass

dose

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When you increase the number of photons to increase the quality of the image, you are increasing what as well

pt dose

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Patient dose in ct is affect by what parameters

Rotation time, tube current, the peak tube voltage, slice thickness and table increment, the pitch & anatomical coverage desired

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____ is the progressive reduction of the intensity of the xray beam as it passes through the patients tissues

Attenuation

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Xray photons are attenuated via two processes:

Absorption and scattering

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Amount of attenuation is dependent upon:

Atomic number, density of electrons, thickness, photon energy

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Number of photons ___ as thickness of pt ____

Decreases, increases

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The attenuation of an xray beam is highly dependent upon the ____ of the xray photons

energy

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Xray photons generated by a ct xray tube are ____

Polychromatic

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When an xray beam passes through the patient, the number of photons in the beam is ____ while the mean energy of the photons is _____

Reduced, increased

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Loss of low energy photons ____ the mean energy of the beam, and the beam is said to become ____

Increases, harder

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Each ___ in a ct image is assigned a ____ ____ ____ which is called the ct number of the represented tissue

pixel, Gray scale number

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The ct number is established relative to the attenuation of ___

water

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The gray scale number, or ct number is known as

The Hounsfield scale

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Hounsfield scale number for air

-1000

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Hounsfield scale number for water

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Hounsfield scale number for fat

-80

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Hounsfield scale number for bone/calcification

+1000

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Each pixel in the image is assigned a gray scale number according to the ct number calculated by what projection

Filtered back projection

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Higher ct numbers are assigned shades of ____ and lower ct numbers are assigned ___ shades of gray

gray , darker

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Tube current = ___

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the ___ is the tube current flowing from the cathode filament to the anode in the xray tube

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the parameter mAs is the mathematical product of ___- and the ______

mA & rotation time

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increasing the mAs, ____ the appearance of image nose at the expense of patient dose

decreases

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true/false, mAs DOES affect the image contrast or spatial resolution

FALSE – it does not! mAs is the key factor in determining the amount of noise on the image

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When you increase mAs, you do what to the photons

Increase number of photons that will hit the detector

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Xrays requires a high/low voltage to be applied across the anode and cathode of the xray tube

high

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The tube ____ will casue a current to flow, heating the cathode filament and releasing electrons

Current

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The energy of the electrons that are boiled off of the filament is determined by what to get their energy

kVp

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Increasing kvp _____ the energy of the photons which ____ their ability to penetrate the patient

Increasing, increasing

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How do you achieve the best contrast in a ct image?

Kvp should allow the denser structures to attenuate the photons while the photons pass through the structures having lower densities

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___ is the parameter that controls the contrast in the ct image

kVp

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the ____ is the number of millimeters of anatomy intersected by the thickness of the xray beam, which is represented in the ct image

slice thickness

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on a single-row detector scanners, the slice thickness is partially defined by the size of the ___ in the ct xray tube

focal spot

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on a single row detector scanner, a thinner slice results when the collimators are …

closer together

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on a single row detector scanner, a thicker slice results when the collimators are …

farther apart

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on a single – row detector scanner, the slice thickness is determined by the _____ of the xray beam

collimation

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in multi-row detector scanners, what are the 2 slice thickness parameters that much be specified?

Acquired slice thickness & reconstructive/effective slice thickness

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The ___ slice thickness indicated which rows of detectors are being used to measure the photons that pass through the patients body

Acquired slice thickness

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A thinner acquired slice thickness means that what is being selected?

Smaller detectors

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A thicker acquired slice thickness means that what is being selected?

Larger detectors or a combination of smaller detectors are used

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On a muti row detector scanner, the slice thickness is determined by the

Data collected from one or several adjacent detector arrays

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The ____ slice thickness is the thickness of the final image

Reconstructive, effective slice thickness

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slice thickness affects what 5 things

anatomical coverage, number of images, image noise, spatial resolution and dose

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true/fasle, thicker slice thickness makes the image have less noise?

true - more photons will be reaching the detector, although, having thicker slices reduces the resolution

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true/false, thicker slice thickness increases resolution

false - it dose not

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to get contiguous slices, the table increment must be ___ to the slice thickness

equal

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what happens when the table increment is greater than the slice thickness

there will be gaps between slices

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what happens when the table increment is less than the slice thickness

the slices will overlap

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what is the disadvantage of having overlapping slices

increases the number of photons passing through a specific volume of the patients body, increasing pt dose

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what is an advantage of overlapping slices

if a lesion falls between slices it may be hard to visualize, but if slices overlap it will be better visualized

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___ is the relationship between collimation and table movement per gantry rotation

pitch

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the ___ determines how stretched the xray path is during a helical scan

pitch

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as the pitch is increased, the helix is ______

stretched

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what are some reasons to increase (stretch out helix) (4)

greater anatomical coverage in less time, faster scans for traumas, for appropriate contrast timing on cta, decrease dose to pt

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what happens when you reduce pitch

improving resolution & minimzing appearance of noise in the image. this increases pt dose

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pitch affects (4)...

anatomical coverage, scan time (temporal resolution), spatial resolution, dose

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while pitch determines how the data is collected, the ________ _____ determines how the data is calculated into images

reconstruction interval

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another helical scan parameter and the parameter that indicates the amount of overlap between adjacent helical slices is the ______

reconstruction interval

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if the reconstruction interval is equal to the reconstructed slice thickness, then slices will be _____

continuous

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in helical scans, the reconstruction interval determines if the slices are ____ or if there will be an _____ or ____

contiguous, overlap, gap

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true/false, there is no additional dose to the patient when making the helical slices overlap via the reconstruction interval

true - no more additional radiation . go over pg 23 - 27. just remember - reconstruction interval does not give additional rad bc its just a reconstruciton

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when you decreases the reconstruction interval, you thereby ______ the slice overlap

increase

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a single picture element of the image matrix is called a _____

pixel

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a volume element is known as

a voxel

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refers to the volume of tissue represented by a pixel in the matrix used to display a particular shade of gray

voxel

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a parameter which is adjusted for the size of the anatomy is the ____ ____ ____

scan field of view FOV

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the scan FOV determines the total area over which projection ____ is collected

data

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the scan FOV determines the number of _____ that are collecting data for a scan

detectors

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what are the 2 types of FOV

scan FOV, reconstruction FOV

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what happens when a patient lies outside the scan FOV?

out-of-field artifacts will occur. - image shading, streaks and incorrect assignment of ct numbers to image pixels - usually when a pt is obese

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the reconstruction FOV may also be called

display FOV

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when you have a small, or narrow reconstruction FOV the anatomy is displayed

larger, acts like a zoom lens of camera

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when you have a wide, or large reconstruction FOV the anatomy is displayed

smaller

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reconstruction FOV affects what 2 things

spatial resolution and image noise pg 31

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this is the grid of pixels that form the image

image matrix

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the image matrix affects the apparent _____ and ______ of the resulting image

noise, resolution

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the matrix affects what 2 things

spatial resolution, appearance of image noise

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when you increase matrix, you ______ to pixels

increase smaller pixel size

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_____ permits the viewer to enlarge part of the reconstructed image on the operators console monitor and on the film

magnification

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true/false, magnification does not affect the spatial resolution or affect the spatial resolution or affect the appearance of image noise

true - mag just simply enlarges the individual pixels of the reconstructed image for display purpose

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magnification is a post processing technique applied to the ____ data and not the ____ data

image, raw

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most operators consoles can only display ____ shades of gray while the human eye can only distinguish about ____ shades of gray

256, 20

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ct numbers above this range are assigned a pixel level of 255 are _____

white

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ct numbers above this range are assigned a pixel level of 0 are _____

black

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window _____ adjusts how contrasty the image appears

width - determines the range of ct numbers displayed on an image

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increasing the window width of an image will display ____ contrast between tissues with similar densities

less , black and white

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a wide window width is generally used when viewing structures such as

lungs - high degree contrast

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a narrow window width is useful for visualizing structures exhibiting ______ inherent contrast

subtle

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a narrow window width is generally used when ?

contrast between tissues of similar density and allows for contrast differentiation between them

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examples of images that would need a narrow window width

soft tissues of the brain

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the window _____ adjusts the brightness of the tissues in the image

level

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quality _____ = the measurement of the scanners performance through quality testing procedures and evaluation of the test results

assurance

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quality ____= the implementation of corrective actions to improve any identified performance inadequacies of the ct system

control - performed by the tech

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what are the 2 most common quality control tests

ct number calibration test and ct number standard deviation test - both need water phantoms

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while performing a ct number calibration test (qc), the ct number of water must measure between ____ and ___ HU

-3 and +3

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true of false, standard deviation of water must not exceed a specific level

true

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Created by: 2024767030873700