Ch 3 Ct Image Quality Flashcards
Influences on user parameter selection p 4
Contrast resolution Spatial resolution Temporal imaging Image noise Patient dose
Tissues with difference of _____% in density can be differentiated in ct
O.5%
Contrast resolution p 4
ability to,differentiate small differences in density on the image
Spatial resolution p 4
Measure of size of the smallest on object that can be visualized in an image
In line pairs
Spatial resolution affected by geometric factors of the scanner p 4
Focal spot size
Detector Aperture size
Focal spot to patient distance
Patient to detector difference
Spatial resolution factors that control it p 4
Slice thickness
Display for
Image matrix
Reconstruction filters
Temporal resolution p 5
Precision of measurement with respect to time
2 effects Increases temporal resolution p 5
Quicker scans
Increase rows of detectors
Noise p 6
Grainy image that makes it difficult to see subtle contrast differences or fine details
Dose p 7
Amount of ionizing radiation absorbed by patient per unit of mass
To maximize image quality sometimes there is increase in______ p 8
Patient dose
Attenuation p 9
Progressive reduction of intensity of the X-ray beam as it passes through patient tissue
Amount attenuation is dependent on p 9
Anatomic number
Density of electrons
Thickness
Photon energy
Number photons measure by detectors is reduced as thickness tissue ______ p 9
Increases
When X-ray beam passes through patient , number of photons in beam is reduced while mean energy of photons _____ p 10
Increase
Ct number p 11
Gray scale number assigned to each pixel in Ct image
Hounsfield number p 11
Aka Ct number named after inventor of Ct in 1967 aka HU
Water Ct number
0
Air Ct number
-1000
Fat Ct number
-80
Soft tissue Ct number range
30-80
Ct number shows calcification
Above 100
What calculates ct number p11
Filtered back projection
Higher ct numbers assigned to shades of grey?
Lighter shades of grey for dense structures
Lower ct numbers assigned to shades of grey? P 11
Darker shades for non dense structures
Ma p 13
Tube current
Mas p 13
Ma x time
Increasing the mas _____ image noise but _____ patient dose
Decrease
Increase
What doesn’t affect image contrast or spatial resolution ?
Mas
Scan times should be short to minimize _____ and maximize _____
Motion
Patient throughput
Kv p 15
Tube voltage
Kvp
Kilovoltage peak or max energy of photons
_____ kvp increases the energy of photons and _____ ability to penetrate patient
Increase
Increase
Kvp controls ? P 15
Contrast
Kvp range in ct
100-140
Slice thickness p 16
Number of millimeters of anatomy intersected by thickness of xray beam which is represented by Ct image
Slice thickness aka (2)
Image thickness
Section thickness
Single row detector scanner slic thickness p 16
Defined by size focal spot and collimators
Single row detector scanner thinner slice results from collimation?
Collimators close together
Single row detector thicker slice results in collimators?
Farther apart
Single row scanner , collimated xray beam thickness ____ the acquired slice thickness , which equals final image thickness p 17
Equals
On single row detector scanner , the slice thickness is determined by _____ of xray beam
Collimation
Acquired slice thickness p 17
On mult row detector scanner , indicates which rows of detectors are being used to measure the photons that pass completely throw the patients body
Multi row detector a thinner slice is specified by ____ detectors are used
Smaller
Multi row detector thicker acquired slice thickness is specified by ____ detectors or combo of ____ detectors are used
Larger
Smaller
On multi row detector scanner , slice thickness is determined by the the data collected from one or several adjacent _____arrays
Detector
Reconstructed slice thickness p18
Aka effective slice thickness
Thickness of final image
On mdct, final slice thickness must ____ or be ____than acquired slice thickness p 18
Equal or greater
Effects of slice thickness p 18
Anatomical coverage Number of images Image noise Spatial resolution Dose
Noise of image is affected by slice thickness by pre patient _____which allows more photons to reach detectors when thick slice selected than a thin one
Collimation
Partial voluming or partial volume averaging p 19
Mathematical averaging of varying density
If we scan a single slice , as slice thickness increases, the volume of irradiated tissue increases and patient dose _____ p 20
Increases
Multi slice studies, slice thickness increases, the dose of the total volume ____
Decreases
On conventional or serial , the _________ determines if the slices will be contigious or if there will be a gap or overlap
Table increment
To get contiguous slices , table increment must equal to ______
Slice thickness
If table increment is less than slice thickness the slices will _____
Overlap
If table increment is greater than slice thickness, there will be _____between slices
Gaps
Using larger table increments , will _____anatomical coverage that you can image with pre determined slice thickness and number slices .
Increase
Larger table increment , allows use fewer slices thus ______ dose
Decreasing
Table increment affects visualization of fine detail by larger table increments than slice thickness and gaps occur some anatomy will be skipped
P 22
Overlapping slices increase number photons passing through patients body thus ___ dose
Increasing
Pitch p 23
AntomicAl coverage of a helical scan
Single row detector pitch p 23
Pitch equals movement of patient table in one gantry rotation divided by slice thickness
Pitch= movement of patient table in one gantry rotation
——————————————————
Slice thickness
Pitch determines ____ p 23
How stretched xray path is during helical scan
Multi row detector pitch
Pitch equals movement of table in on e gantry rotation dived by total slice volume
Or
Pitch equals movement of patient table in one gantry rotation divided by thickness of single slice
Effects of pitch on shape of helix p 24
Helix is unstretched
Pitch increased the helix stretched
Reasons increase pitch p 24
Greater anatomical coverage in less time
Faster scan for trauma
Appropriate contrast timing during cta study
Decrease dose to patient
Pitch affects p 25
Anatomical coverage
Scan time ( temporal resolution)
Spatial resolution
Dose
Reconstruction interval p 27
When helical scan is compete , mathematical calculation can divide the helical data into planar samples of raw data at the right slice location
Reconstruction interval can be modified before the scan is run or as retrospective reconstruction
P 27
In helical scans , reconstruction interval determines if slice are (2)?
Contiguous
Overlap or gap
There is no additional dose to patient during reconstruction interval
P 27
Effect of overlapping slices with reconstruction interval
If object falls exactly halfway between two contiguous slices, it may not be visualized well
If slices are overlapped by proper selection of reconstruction interval the object maybe more completely contained in one of the slices
Pixel p 29
A single picture element in the image
Voxel p 29
Three dimensional volume element
Pixels in ct image reperesent the ________ of a volume of tissue at the position in the slice
Ct number
One dimension of video?
Other two dimensions ?
P 29
One- slice thickness
Two- in plane spatial resolution
Scan field of view ( fov) p 29
Parameter which is adjusted for the size of the anatomy . The number of detectors collecting data in a scan
Two fov
Scan fov
Reconstruction fov
Out field artifacts characterized by
P 30
Image shading
Streaks
Incorrect assignment of ct numbers to image pixels
Reconstruction fov p 30
Region interest illustrated in resulting image and fov reconstructed from the complete set of raw data
Aka display fov
Reconstruction fov ( display fov) is equal to
Equal or smaller
Two reconstruction fov
Small
Large
Small/ narrow reconstruction fov p 30
Restricting area of interest and displaying image larger like zoom lens
Large/ wide reconstruction fov p 30
Anatomy appears small
Thicker slice means ____ noise p 31
Less
Wider reconstruction fov mean _______ noise
Less noise
Reconstruction fov affects p 31
Spatial resolution
Image noise
In plane spatial resolution p 31
Reconstruction fov / number of pixels along direction in image matrix
Matrix p 33
Grid pixels that form image
Resolution of image expressed as reconstruction fov formula
Reconstruction fov/ matrix
Increase matrix mean ______ pixels and allows ____ detail
Smaller
Smaller
Small matrix means ____ pixel size
Large
Large matrix means ______ pixel size
Small
Larger image matrix means ____ noise
More
Matrix affects
Spatial resolution
Appearance of image noise
Common matrix size p 33
512x512
Magnification p 34
Enlarges individual pixels of reconstructed image for display purposes
Magnification does not affect (2) p 34
Spatial resolution
Image noise
Smaller reconstruction fov means _____size of displayed image
Increase
Decreasing reconstruction fov will _____ spatial resolution and noise level
Increase
Magnification is post processing technique applied to _____ data
Image
Magnification is only way to enlarge image data if ____ data no longer available p 35
Raw
Window width p 36
How much contrast appears in image
Ct number -1000
Air
+1000 ct number
Dense bone
Operator console displays ____ shades grey
P 36
256
Human eye detects ____ shades of grey
20
Ct number above range are assigned pixel level ?
255 white
Any ct number below this range are assigned a pixel value of?
0( black)
Increase window width ____
Higher contrast
Lungs studies have ____ contrast
High
Larger window width mean ____ contrast
Less
Narrow window width is ____ contrast
More
Window level p 37
Adjusts brightness of the tissues in the image
Ct number higher than 250 appears_____
White
Ct number lower than -250 will appear _____
P 38
Black
Low window level makes image _____
Brighter
High window level makes image _____
Darker
Quality assurance p 39
Concept that comprises all the oversight and management practices developed by ct imaging team led by supervising physician, to ensure that every imaging procedure is necessary and approximate, the acquisition parameters are appropriate for clinical situation , image generated are sufficient to solve clinical problem , images are correctly interpreted, and that exam generates lowest risk to patient while meeting clinical objective
Quality control p 39
Procedure performing specified tests or measurements on periodic basis in order to assure that set level of quality has not been compromised
Two daily qa done
Ct number calibration test
Ct number standard deviation test
Ct number calibration test acceptable limits
Ct number water must measure between -3 and + 3 HU
Ct number standard deviation test p 40
Standard deviation of water must not exceddd a specific level
Less frequent tests p 40
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