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What are the 5 key influences on ct parameter selection?
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____- resolution is the ability to differentiate small differences in density on the image
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ct image quality
| Question | Answer |
|---|---|
| What are the 5 key influences on ct parameter selection? | contrast resolution, spatial resolution, temporal resolution, image noise, patient dose |
| ____- resolution is the ability to differentiate small differences in density on the image | contrast |
| Typically, tissues with differences in densities of less than ___% can be differentiated with ct | 0.5% |
| _____ resolution is a measure of the size of the smallest object that can be visualized in an image | spatial resolution |
| 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 |
| What are some factors that we directly control which affect spatial resolution? | Slice thickness, reconstruction factors which display FOV, image matrix and reconstruction filter |
| Spatial resolution is often expressed in terms of ______ | line pairs |
| _____ resolution refers to the precision of a measurement with respect to time | Temporal resolution |
| There is a relation between temporal resolution and ___ resolution | Spatial resolution |
| ___ 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 |
| True/fasle, noise makes it difficult to see subtle contrast difference or fine detail | true |
| An image will appear more noisy, or grainy when there are ____ photons contributing to the pixels | more |
| There is an indirect/direct correlation between the apparent noise level and the number of photons used in making the image | Direct |
| Increasing the number of photons _____ the appearance of noise | decreases |
| 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 |
| This refers to the amount of ionizing radiation absorbed by the patient per unit mass | dose |
| When you increase the number of photons to increase the quality of the image, you are increasing what as well | pt dose |
| 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 |
| ____ is the progressive reduction of the intensity of the xray beam as it passes through the patients tissues | Attenuation |
| Xray photons are attenuated via two processes: | Absorption and scattering |
| Amount of attenuation is dependent upon: | Atomic number, density of electrons, thickness, photon energy |
| Number of photons ___ as thickness of pt ____ | Decreases, increases |
| The attenuation of an xray beam is highly dependent upon the ____ of the xray photons | energy |
| Xray photons generated by a ct xray tube are ____ | Polychromatic |
| 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 |
| Loss of low energy photons ____ the mean energy of the beam, and the beam is said to become ____ | Increases, harder |
| Each ___ in a ct image is assigned a ____ ____ ____ which is called the ct number of the represented tissue | pixel, Gray scale number |
| The ct number is established relative to the attenuation of ___ | water |
| The gray scale number, or ct number is known as | The Hounsfield scale |
| Hounsfield scale number for air | -1000 |
| Hounsfield scale number for water | 0 |
| Hounsfield scale number for fat | -80 |
| Hounsfield scale number for bone/calcification | +1000 |
| Each pixel in the image is assigned a gray scale number according to the ct number calculated by what projection | Filtered back projection |
| Higher ct numbers are assigned shades of ____ and lower ct numbers are assigned ___ shades of gray | gray , darker |
| Tube current = ___ | mA |
| the ___ is the tube current flowing from the cathode filament to the anode in the xray tube | mA |
| the parameter mAs is the mathematical product of ___- and the ______ | mA & rotation time |
| increasing the mAs, ____ the appearance of image nose at the expense of patient dose | decreases |
| 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 |
| When you increase mAs, you do what to the photons | Increase number of photons that will hit the detector |
| Xrays requires a high/low voltage to be applied across the anode and cathode of the xray tube | high |
| The tube ____ will casue a current to flow, heating the cathode filament and releasing electrons | Current |
| The energy of the electrons that are boiled off of the filament is determined by what to get their energy | kVp |
| Increasing kvp _____ the energy of the photons which ____ their ability to penetrate the patient | Increasing, increasing |
| 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 |
| ___ is the parameter that controls the contrast in the ct image | kVp |
| 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 |
| on a single-row detector scanners, the slice thickness is partially defined by the size of the ___ in the ct xray tube | focal spot |
| on a single row detector scanner, a thinner slice results when the collimators are … | closer together |
| on a single row detector scanner, a thicker slice results when the collimators are … | farther apart |
| on a single – row detector scanner, the slice thickness is determined by the _____ of the xray beam | collimation |
| in multi-row detector scanners, what are the 2 slice thickness parameters that much be specified? | Acquired slice thickness & reconstructive/effective slice thickness |
| The ___ slice thickness indicated which rows of detectors are being used to measure the photons that pass through the patients body | Acquired slice thickness |
| A thinner acquired slice thickness means that what is being selected? | Smaller detectors |
| A thicker acquired slice thickness means that what is being selected? | Larger detectors or a combination of smaller detectors are used |
| On a muti row detector scanner, the slice thickness is determined by the | Data collected from one or several adjacent detector arrays |
| The ____ slice thickness is the thickness of the final image | Reconstructive, effective slice thickness |
| slice thickness affects what 5 things | anatomical coverage, number of images, image noise, spatial resolution and dose |
| 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 |
| true/false, thicker slice thickness increases resolution | false - it dose not |
| to get contiguous slices, the table increment must be ___ to the slice thickness | equal |
| what happens when the table increment is greater than the slice thickness | there will be gaps between slices |
| what happens when the table increment is less than the slice thickness | the slices will overlap |
| 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 |
| 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 |
| ___ is the relationship between collimation and table movement per gantry rotation | pitch |
| the ___ determines how stretched the xray path is during a helical scan | pitch |
| as the pitch is increased, the helix is ______ | stretched |
| 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 |
| what happens when you reduce pitch | improving resolution & minimzing appearance of noise in the image. this increases pt dose |
| pitch affects (4)... | anatomical coverage, scan time (temporal resolution), spatial resolution, dose |
| while pitch determines how the data is collected, the ________ _____ determines how the data is calculated into images | reconstruction interval |
| another helical scan parameter and the parameter that indicates the amount of overlap between adjacent helical slices is the ______ | reconstruction interval |
| if the reconstruction interval is equal to the reconstructed slice thickness, then slices will be _____ | continuous |
| in helical scans, the reconstruction interval determines if the slices are ____ or if there will be an _____ or ____ | contiguous, overlap, gap |
| 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 |
| when you decreases the reconstruction interval, you thereby ______ the slice overlap | increase |
| a single picture element of the image matrix is called a _____ | pixel |
| a volume element is known as | a voxel |
| refers to the volume of tissue represented by a pixel in the matrix used to display a particular shade of gray | voxel |
| a parameter which is adjusted for the size of the anatomy is the ____ ____ ____ | scan field of view FOV |
| the scan FOV determines the total area over which projection ____ is collected | data |
| the scan FOV determines the number of _____ that are collecting data for a scan | detectors |
| what are the 2 types of FOV | scan FOV, reconstruction FOV |
| 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 |
| the reconstruction FOV may also be called | display FOV |
| when you have a small, or narrow reconstruction FOV the anatomy is displayed | larger, acts like a zoom lens of camera |
| when you have a wide, or large reconstruction FOV the anatomy is displayed | smaller |
| reconstruction FOV affects what 2 things | spatial resolution and image noise pg 31 |
| this is the grid of pixels that form the image | image matrix |
| the image matrix affects the apparent _____ and ______ of the resulting image | noise, resolution |
| the matrix affects what 2 things | spatial resolution, appearance of image noise |
| when you increase matrix, you ______ to pixels | increase smaller pixel size |
| _____ permits the viewer to enlarge part of the reconstructed image on the operators console monitor and on the film | magnification |
| 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 |
| magnification is a post processing technique applied to the ____ data and not the ____ data | image, raw |
| most operators consoles can only display ____ shades of gray while the human eye can only distinguish about ____ shades of gray | 256, 20 |
| ct numbers above this range are assigned a pixel level of 255 are _____ | white |
| ct numbers above this range are assigned a pixel level of 0 are _____ | black |
| window _____ adjusts how contrasty the image appears | width - determines the range of ct numbers displayed on an image |
| increasing the window width of an image will display ____ contrast between tissues with similar densities | less , black and white |
| a wide window width is generally used when viewing structures such as | lungs - high degree contrast |
| a narrow window width is useful for visualizing structures exhibiting ______ inherent contrast | subtle |
| a narrow window width is generally used when ? | contrast between tissues of similar density and allows for contrast differentiation between them |
| examples of images that would need a narrow window width | soft tissues of the brain |
| the window _____ adjusts the brightness of the tissues in the image | level |
| quality _____ = the measurement of the scanners performance through quality testing procedures and evaluation of the test results | assurance |
| quality ____= the implementation of corrective actions to improve any identified performance inadequacies of the ct system | control - performed by the tech |
| what are the 2 most common quality control tests | ct number calibration test and ct number standard deviation test - both need water phantoms |
| while performing a ct number calibration test (qc), the ct number of water must measure between ____ and ___ HU | -3 and +3 |
| true of false, standard deviation of water must not exceed a specific level | true |
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