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Flashcards in Week 6 Readings Deck (82):

mA and scan time together are referred to as mAs and defines the _____ of the x-ray energy



Total x-ray beam exposure in CT is dependent on a combination of ____, ______ and ____

mA setting, scan time, and kVp setting


Small filament size

-concentrates focal spot, reducing penumbra ( geometric unsharpness)
-which positively affects image quality
-do not tolerate heat well


In SDCT scan time is

The time the X-ray beam is on for the collection of data for each slice
-most often it is the time it takes the gantry to make a complete 360 degree rortation


In MDCT the scan time is

The time it takes the X-ray tube to make a 360 degree rotation, even though many slices may be produced


____mA allows for _____ scan times to be used. A _____ scan time is critical in avoiding image degradation as a result of patient motion

Higher, Shorter, short


The thicker and denser the part being examined, the more ____ that is required to produce an adequate image



True/false: it is more common practice to manipulate the mAs rather than the kVp

-mA is more flexible (larger range 20-800 mA)
-the effect on image quality is more straightforward and predictable


Uncoupling effect

Using digital technology, the image quality is not directly linked to the dose, so even when an mA or kVp setting that is too high is used, a good image results
-this makes it difficult to identify when a dose that is higher than necessary is used
-does not play a role when the mA or kVp setting is too low because quantum noise will result and provide evidence of the inadequate exposure settings


Automatic tube current modulation

Software that automatically adjusts the tube current (mAs) to fit specific anatomic regions
-this software adjusts mAs during each gantry rotation to compensate for large variations in x-ray attenuation, such as when scans move from the shoulders to the rest of the thorax



Determines the area within the gantry, for which raw data are acquired
-scan data are always acquired around the gantrys isocenter
-the DFOV determines how much, and what section, of the collected raw data are used to create an image


Filter functions are only applied to _____ data



Reconstruction algorithms vs WW and WL

-changing the window setting merely changes the way the image is viewed
-changing the reconstruction algorithm will change the way the raw data are manipulated to reconstruct the image



Relationship between slice thickness and table travel per rotation during a helical scan acquisition


Detail (high contrast resolution) aka spatial resolution

The level of detail that is visible on the image
-for ex if two thin wires lie close together in an object, will they be seen as two separate lines on the image?
-the ability to resolve (as separate objects) small, high-contrast objects
-the systems ability to resolve, as separate forms, small objects that are very close together


Contrast detectability (low contrast resolution) aka contrast resolution

The ability of the system to differentiate between objects with similar densities
-for ex consider an object that has nearly the same density as its background. Will this object be distinguishable on the CT image


How frequently an object will fit into a given space is its

Spatial frequency


A large object will have a _____ spatial frequency



Small objects have a _____ spatial frequency




The ratio of the accuracy of the image compared with the actual object scanned
-MTF indicates image fidelity
-scale from 0 to 1
-if the image were blank and contained no information about the object, the MTF would be 0
-if the image reproduced the object exactly, the MTF of the system would have a value of 1


True/false: As objects become smaller (higher spatial frequency) they will not be as accurately depicted on the CT image



As the size of the object increases, the MTF ____

Also increases


True/false: An object twice the size of another object may not necessarily possess twice the image fidelity

-not a linear relationship


An MTF curve extending to the right indicates a system with _____ spatial resolution capabilities



what affects the quality of spatial resolution

-quality of the raw data
-reconstruction method


Matrix size and DFOV selection determine _____

Pixel size


The greater the total pixels present in the image, the ____ each individual pixel




-determines how much raw data will be used to reconstruct the image
-works like the zoom on a camera and can be used to show the entire area or to display a specific region of interest in greater detail
-increasing DFOV increases the size of each pixel in the image
-a larger pixel will include more patient data


_____ pixel size will increase spatial resolution



Thinner slices produce _____ images



True/false: when the imaging voxel is equal in size in all dimensions there is no loss of information when data are reformatted in a different plane



Sampling theorem aka nyquist theorem

Because an object may not lie entirely within a pixel, the pixel dimension should be half the size of the object to increase the likelihood of that object being resolved on the image
-by reducing the size of the pixel we can increase our chance of accurately representing a small object


Larger focal spots cause more what?

-geometric unsharpness in the image
-reduce spatial resolution


Increasing the pitch, _____ resolution



Increasing the pitch, _____ resolution in the image



Contrast resolution

-the ability to differentiate a structure that varies only slightly in density from its surrounding


The ability to distinguish an object that is nearly the same density as its background is referred to as

Low-contrast detectability


Image noise

Undesirable fluctuation of pixel values in an image of homogenous material
-grainy appearance or "salt and pepper" look on an underexposed image
-caused by a combination of factors, most prevalent being quantum noise, or quantum mottle


Quantum mottle

Occurs when there are an insufficient number of photons detected
-it is inversely related to the number of photons used to form the image
-as the number of x-ray photons used to create an image decreases, noise increases


In CT, the number of x-ray photons detected per pixel is also referred to as

Signal to noise ratio (SNR)


Pixel size

Keeping all other scan parameters the same
-as pixel size decreases, the number of detected x-ray photons per pixel will decrease
-fewer photons per pixel results in an increase in noise and subsequent decease in contrast resolution


Bone algorithms

-produce lower contrast resolution
-but better spatial resolution


Soft tissue algorithms

-improve contrast resolution at the expense of spatial resolution


Temporal resolution

Refers to how rapidly data are acquired
-it is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals



Anything appearing on the image that is not present in the object scanned


Beam hardening

-as an x-ray beam passes through an object, lower-energy photons are preferentially absorbed, creating a "harder" beam
-individual rays are hardened to differing degrees, and this variation cannot be adjusted for by the reconstruction algorithm
-the degree of beam hardening is depend on the composition of the part being examined and the extent the beam must travel through various tissues
-the beam is hardened by more dense objects (ex more by bone and less by fat)
-two types of artifacts can result from this effect: cupping artifacts (the periphery of the image is lighter) and the appearance of dark bands or streaks between dense objects in the image


CT systems use what three features to minimize beam hardening?

1. Filtration
2. Calibration correction
3. Beam hardening correction software


What is the best strategy available to the operator to avoid beam hardening?

Select the appropriate SFOV to ensure the correct filtration, calibration, and beam-hardening correction software is used


Partial volume effect

-partial volume effect occurs when more than one type of tissue is contained within a voxel


What is the best method of reducing partial volume artifacts?

Use thinner slices


Image reconstruction

Used when raw data are manipulated to create pixels that are then used to create an image


Image reformation

Used when image data are assembled to produce images in different planes, or to produce 3D images


Retrospective reconstruction

-reconstructing raw data to create raw images can only be done at the operators console
-the images that result are always in the same plane (ex coronal, axial) and the same orientation as were the original images
-provided the raw data are available they can be reused to create new images
-DFOV, image center, and reconstruction algorithm, can be changed on both axial and helical data
-could use this if one parameter (such as DFOV) varied among the prospective images in a study, preventing their reformation into a single image


Overlapping reconstruction

-on helical data from either SDCT or MDCT systems, image incrementation can be changed
-this is often done to produce overlapping images that are then used in multiplanar or 3D reformations
-when the slice thickness is wider or the FOV is larger, it is common practice to reconstruct images with an overlap of approximately 50% whenever multiplanar or 3D post-processing is expected to occur
-when scans are generated with isotopic (or near isotropic) voxels, not much is gained from overlapping reconstructions
-the more the slice thickness exceeds pixel size (ex the voxel is a rectangular solid rather than a cube), the greater the benefit of overlapping reconstructions for source data in multiplanar reformation (MPR) and 3D reformation


Retrospectively changing image thickness

The goal of using a thin slice for scanning and reconstructing thicker slices for viewing and storing is to maintain the advantage of high-resolution imaging but also create image files that are manageable and more easily reviewed by radiologists


Image reformation

To reformat a CT study all the source images must have an identical DFOV, image center, and gantry tilt, and they must be contiguous
-even a small amount of motion on source images can seriously degrade the reformation
-image reformation uses only image data to generate images in a plane or orientation different from the prospective image
-they are used to better display anatomic relationships


Multiplanar reconstruction (MPR)

Reformation that is done to show anatomy in various planes
-2D in nature
-unlike 3D displays, 2D image displays always represent the original CT attenuation values
-can be created in transverse, coronal, Sagittal, or oblique planes
-the advent of MDCT has expanded the use of multiplanar images in diagnosis
-this is because the volumetric image data sets that result from MDCT methods produce very high quality multiplanar images, and reformations are generated quickly and easily
-when voxels are isotropic, any oblique plane can be created with virtually no loss of image quality
-reformatted image is virtually identical in quality to the original axial reconstruction
-if the voxels are not isotropic, image quality can be improved by overlapping source images


Three dimensional reformation

Seeks to represent the entire scan volume in only one image
-3D techniques manipulate or combine CT values to display an image; the original CT value information is not included
-available only on independent workstations
-the thinner the original CT slices, the better the final 3D image
-all slices must be contiguous
-ex surface rendering, volume rendering


Surface rendering (SR) aka shaded surface display (SSD)

Similar to taking a photograph of the surface of the structure in that the voxels located on the edge of a structure are used to show the outline or outside shell of the structure
-images are created by comparing the intensity of each voxel in the data set to some predetermined threshold CT value
-the software will include or exclude the voxel depending on whether its CT number is above or below the threshold and use this info to create a surface of the object
-remaining voxels in the image are usually invisible
-useful for examining tubular structures ex inside surfaces of airways, colon, blood vessels
-setting the appropriate threshold CT values is critical
-an advantage is it only uses a small portion (approx 10%) of the data available, images can be created quickly, even on less powerful computers
-most uses of SR have been replaced by VR
-still remains useful for orthopedic imaging as it excels at bone surfaces


Projection display MIP

3D technique
-examines each voxel along a line from the viewers eye through the data set and selects only the voxel with the highest value for inclusion in the displayed image
-everything else is ignored
-tends to display bone and contrast-filled structures
-can be generated from the entire data set or from only a selected portion of it which can be useful when other structures are present that may obscure the area of interest
-lower-attenuation structures are not well visualized


Projection display MinIp

3D technique
-involves selecting the voxel with the minimum value from the line for display
-useful to display the bronchial tree
-can be generated from the entire data set or from only a selected portion of it which can be useful when other structures are present that may obscure the area of interest


Volume rendering (VR)

3D semitransparent representation of the imaged structure
-favorite 3D imaging technique
-an advantage of VR compared with other 3D techniques is all the voxels contribute to the image
-this allows VR to display multiple tissues and show their relationships to one another
-VR techniques sum the contributions of each voxel along the line
-each voxel is assigned an opacity value based on its HU
-this opacity value determines the degree to which it will contribute, along with other voxels along the same line, to the final image
-this process is repeated for the voxels along each line, with each line producing one voxel in the VR image
-the pixels in the final VR image can be assigned a color, brightness, and degree of opacity and user can also change window settings


Endoluminal imaging

A form of VR designed to reveal the inside of the lumen of a structure
-this technique is also called virtual endoscopy, virtual bronchoscopy, and virtual colonoscopy
-visualizes a structure as if it were hollow and the viewer were inside of it. Once inside the viewer can "fly through", which provides the impression of virtual flight through the selected body region equivalent to directing an endoscope into the lumen
-software can also correlate findings on the 3D endoluminal imaging with the 2D cross-sectional source images to allow better characterization and localization of abnormalities


Region of interest editing

Is done to remove obscuring structures from the 3D image
-3D software allows this editing to be in a manual, automatic, or semiautomatic fashion


Manual segmentation

Process by which a user identifies and selects data to be saved or removed
-can be done on either 2D or 3D images
-the user traces an outline around anatomy to be saved or discarded on a series of axial or MPR images, or a 3D model


Semiautomatic segmentation

Combine many of the benefits of manual and automatic segmentation techniques
-by supplying initial information about the region of interest, the user may guide an otherwise automatic segmentation process


Segmentation errors

Errors in the reformatted image can be introduced when important vessels or other structures are inadvertently edited out of the data set
-some errors are obvious and some are not especially when the missing structure is unexpected like an accessory artery


True or false: MPR images are not as degraded by Image noise



Noisy images can be reduced by

-High mAs
-Lower pitch setting
-slower rotation speed
-in obese patients, a larger display field of view and thicker slices will also help reduce image noise
-in these patients it is also helpful to reconstruct thicker slices to use as a source data for reformation and review



Artifacts on source data will also degrade the reformatted images
-most common artifacts are a result of motion or from high density material (surgical clips) that produce streaks
-stair-step artifacts can occur when voxels are not isotropic


When the slice thickness exceeds pixel dimensions _____ appear on the reformatted images



Spatial resolution vs low contrast resolution

Spatial resolution: The scanners ability to resolve closely placed objects that are significantly different from their background
-that is, the contrast of these objects to their background is high
Low contrast resolution: where the CT number difference between the objects and background is small


Scatter radiation has been controlled effectively in the past by placing a _______ collimator in front of the detector to reject any photons that do not follow the straight line path between the x-ray source and the detector cell

Postpatient collimator


With introduction of multislice and volumetric CT, scatter-induced artifacts can be corrected with _____ by carefully measuring or estimating the scatter distribution in the projection. The estimated scatter can then be removed from the measured intensities to arrive at the true signals that represent the line integrals of the scanned object



Which of the following determines image quality
1. Spatial and contrast resolution
2. Noise
3. Artifacts
4. Radiation dose
A) 1 only
B) 1 and 2 only
C) 1,2 and 3 only
D) 1, 2, 3, and 4

D! 1,2,3 and 4


The unit for spatial resolution of a CT scanner is:
A) line pairs per cm (lp/cm)
B) line pairs per inch (lp/inch)
C) lp/mm
D) lp/meter

A! Lp/cm


The capability of the CT scanner to discriminate small differences in tissue contrast is called:
A) spatial resolution
B) contrast resolution
C) noise
D) cross-field uniformity

B) contrast resolution


The pixel size in CT is equal to?

FOV divided by matrix size


CT number fluctuation from pixel to pixel in the image for a scan of water phantom is referred to as
A) noise
B) linearity
C) spatial resolution
D) cross-field uniformity

A) noise


To reduce noise (standard deviation) by a factor of 2, the dose must be increased by a factor of:
A) 2
B) 4
C) 6
D) 8

B) 4


Which of the following produces streak artifact
1. Aliasing
2. Metal objects
3. Motion of patient
A) 1 only
B) 2 only
C) 1 and 2 only
D) 1, 2, and 3

D) 1,2,3


Ring artifacts in CT arise from
A) beam hardening
B) the patients necklace
C) one or more detector cells
D) a depression in the table

C) one or more bad detector cells