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123 Cards in this Set
- Front
- Back
What do 2D picture elements correspond to?
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3D voxel elements
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What does each pixel in a CT image display?
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Average xray attenutation properties of the tissue in the corresponding voxel
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What is a ray?
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Single transmission measurement through the patient made by a single detector at a given moment in time
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What is a projection or view?
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Series of rays that pass through the patient in the same orientation
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What type of geometry to most CT scanners use?
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fan beam geometry for the acquisition and reconstruction process
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What is the purpose of a CT scanner?
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To aquire a large number of transmission measurements through the patient at different positions
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What is the formula for the xray intesity of the atteunated xray beam?
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It=Ioe^(-ut)
ln (Io/It)=ut With a preprocessing step, t cancels out and this reduces the dependency of CT image on machine dep parameters (Io and It) |
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What is used to produce CT images?
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CT reconstuction algorithm; filtered backprojection is most common
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How does backprojection work?
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the u value for each ray is smeared along the same path in the image of the patient-->areas of high/low attenuation reinforce each other-->builds up image in the computer
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Describe data aquistion and process of computing a CT image:
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Make transmission measurements through the object at multiple angles-->reverse aqusition geometry and backproject on a digital matrix
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Describe the first generation of CT scanners:
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Rotate/translate pencil beam system (only 2 detectors measure xrays through 2 different slices): Xray tube and single detector (per slice) translates across the FOV (160 rays across 24 cm)-->produces a series of PARALLEL rays-->system rotates slightly-->tranlates back across at a slightly different angle-->repeat at 1 degree intervels across 180 degrees
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Advantages and disadvantages of pencil beam technology of first generation CT scanners
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Advantages: efficient scatter reduction, b/c scattered beam deflected away from the pencil beam is not measured by a detector
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Describe second generation CT scanners:
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-Fan beam technology with an array of 30 detectors
-increases scatter compared to pencil beam technology -Narrow fan angle of 10 degrees -In theory, 30x reduction in scan time expected-->not realized b/c ↑ data point acquistion-->↑ image quality |
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What kind of geometry does projection radiography use?
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Open beam geometry
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Describe 3rd generation CT scanners:
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Rotate/Rotate system:
-↑ to >800 detectors -↑ angle of fan beam arc to interrogate entire span the patient-->eliminates translational motion -mech. joined xray tube and detector array rotate together around patient w/o translation -↓ scan times |
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Compare scan times:
1. 1st gen 2. 2nd gen 3. 3rd gen 4. 4th gen |
1. 270 sec
2. 18 sec 3. 5 sec 4. 1/2 sec |
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Disadvantge of third generation CT scanners:
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Ring artifacts
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what causes ring artifacts of 3rd generation scanners?
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-In rotate/rotate geometry-->each detector corresponds to a "ring" on an image
-central detectors provide info about small diameter rings -peripheral detectors about large diameter rings -vertex of "fan" is xray focal spot -if detector is miscalibrated (electronic drift), tainted data leads to ring artifact b/c there are differences in gain b/w detectors so (g1 should = g2 but if miscalibration then g1≠g2 -even slight imabalances affect the ut that is backprojected-->ring artifact |
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How do 4th generation CT scanners overcome problems with ring artifact of 3rd generation scanners?
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Rotate/stationary system:
Detectors are in a stationary 360 ring around the patient-->requires more detectors |
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Describe the difference b/w source fan of 3rd generation CT scan and detector fan of 4th generation CT scan:
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Source fan-xray tube is the apex of the fan (source fan)
Detector fan-normalize the data so the apex of a fan is at the individual detector |
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Describe the difference in data acquistion b/w 3rd and 4th generation CT scanners:
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3rd generation: fan data acquired by detector array simultaneously
4th generation: fan beam data acquired over time needed for xray tube to rotate through the arc angle of the fan each detector acts as its own reference detector |
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In 4th generation CT scanners what is the significance of each dtector acting as its own refence detector?
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Each detector has its own gain; g cancels out in the equation and eliminates ring artifact
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Describe the 5th generation CT scanner:
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Stationary/Stationay:
-used in cardiac cine -large tungsten arc encircles the patient-->lies opp. the detector ring -high energy e- ring strikes tungsten-->produces xrays -50msec scan times |
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In the 1990's what did the design of the 3rd and 4th generation scanners incorporate?
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slip ring technology so gantry can rotate continously w/o tethering from wires
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Describe 6th generation CT scanners:
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-helical CT
-gantry does not have to be stopped after each slice is acquired b/c table moves continously -sometimes entire scan can be done in one breath hold |
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Describe 7th generation CT scans:
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-Multiple detector array
-WIDER collimator spacing -more xrays produced by xray tube are used to produce an image |
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What does the use of multiple detector array overcome due to opening up the collimator
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-In conventional, single detector array ↑ collimator, ↑ slice thickness-->↑ utilization of xray beam but ↓ spatial resolution
-with multiple detector array-->slice thickness determined by detector size and not by collimator |
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Describe xenon detectors:
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High pressure xenon gas in long thin cells b/w two metal plates
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How do you overcome detector -inefficiency of a gas detector compared to a solid one?
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-Make it very thick (>6 cm) to compensate for low density
-Make metallic septa very thin to reduce dead space--> ↑ geometric efficiency |
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What is geometric efficiency?
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fraction of primary xrays exiting a patient that strike an active detector
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Why must xenon detectors be positioned in a fixed orientation wrt xray source?
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Long, thin ionization plates of xenon detecor are highly directional
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Why can xenon detectors not be used in fourth generation CT scanners?
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xenon detectors must be positioned in a fixed orientation wrt to x-ray source; so only used in third generation systems
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How do xenon detectors work?
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voltage is applied across electrodes; xrays interact with the xenon atoms and cause ionization-->ions move to the electrodes-->electronic signal is amplified and digitized-->numerical value is directly proportional to x-ray intensity striking the detector
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Describe the physical makeup of a solid state detector
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scintillator coupled tightly to a photodetector (photodiode)
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How does a solid state detector work?
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scintillator emits visible light when stuck by xrays-->light reaches the photodiode
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What type of scintillators are used in solid state CT detectors?
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CdWO4, yttrium, gadolinium
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Why do solid state CT detectors have better xray absorption than xenon detectors?
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Increased atomic number
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What somewhat reduces geometric efficiency of detector elements?
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Small gaps b/w detector elements put there to reduce "crosstalk" b/w detector elements
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What makes solid state CT detectors capable of xray detection over a wide range og angles?
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Top surface of solid-state CT detectors are essentially flat (unlike xenon detectors)
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What type of scanners are solid state detectors used in?
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4th and higher generation 3rd
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What is a multiple detector array?
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Assembly of multiple solid-state detector array modules
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How do you practically adjust the slice width with a multidetector array system?
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Grouping one or more detector units together; to combine the signal-->detectors are wired together
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Why do multiple detector array scanners make use of third generation geometry?
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Fan angle in 3rd gen CT scanners is 60 degrees and in 4th gen. is 360 degrees so would need a lot more (60x) as many detectors as in a third generation system
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What determines slice thickens in a single detector array CT system?
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Physical collimation of xray beam with two lead jaws; as gap b/w lead jaws widens, slice thickness increases
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What places an UPPER LIMIT on slice thickness in a single detector array system?
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Width of detectors in the single detector array
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What are the advantages of increasing slice thickness?
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1. # of detected photons increases linearly with increased slice thickness
2. Increases SNR (contrast resolution) |
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What are the disadvantages of increasing slice thickness?
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Decreases spatial resolution; thin slices improve spatial resolution and decrease partial volume averaging
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What will partially compensate for loss of xray photons due to thin slices?
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Increasing the mA
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What is the slice sensitivity profile?
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Contrast decreases as object moves to the edge of a a slice;
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In single detector array, what is the shape of slice sensitivity profile due to?
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-finite width of the xray focal spot
-penumbra of the collimator -projection angle imaging |
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Why does helical scanning have a slightly broader slice sensitivity profile?
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due to translation of the patient during the scan
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In a multidetector array scheme, what is slice thickness determine by?
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Width of detectors in the slice thickness dimension
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How do you change the width of detectors in multidetector array scheme?
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binning different numbers of detector elements together (ie summing electronic signals of adjacent detector elements)
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In conventional 4 slice axial scanning (ie not helical) describe what happens to 2 the slices at the edge of the scan?
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inner edge of each slice is determined by the inner edge of the detector; outer edge determined by the collimator penumbra or the outer edge of the detector depending on the adjustment of the collimator
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In helical imaging, what is te difference in detector array contribution?
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-Each detector array contributes to the image reconstruction
-slice sensitivity profile for each detector array needs to be similar to prevent artifact -do this by adjusting collimation so that focal spot-collimator blade penumbra falls outside the edge of the detectors-->leads to higher radiation with but reduces artifact |
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When is pitch used?
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helical scan protocols
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What is the formula for pitch for a single detector array?
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Collimator pitch=table movement (mm) per 360 degree rotation of the gantry/collimator width (mm) at the isocenterW
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What does pitch influence?
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-radiation dose to the patient
-image quality -scan time |
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What does a pitch of 1 mean?
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number of CT views acquired when averaged over the long axis of the patient is comparable to the number acquired with continguous axial CT
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What does a pitch of <1 mean?
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slight improvmenet in image qulaity and higher radiation dose to the patient;
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What are common pitches used?
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pitches up to 1.5 commonly used-->manufacturers spend time trying to develop pitch >1
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What is the significance of a pitch >1?
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-Some degree of partial scanning of the patient
-faster scan time, less motion, less radiation -sometimes less radiation |
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What is the minimum requirement to produce an adequate CT image?
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180 degrees + fan angle
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What is the formula for detector pitch and collimator pitch for multiple detector array?
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Detector pitch=table movement (mm) per 360 degree rotation of the gantry/detector wideth (mm)
Collimatory pitch=Detector pitch/N where N is number of detector arrays |
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What is a sinogram?
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way data acquired for one CT slice can be displayed before reconstruction
Y axis: views (each projection angle), X axis: rays (different rays in each projection) |
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What would a bad detector in a third-generation scaneer show up as on a sinogram?
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Vertical line
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How do you calculate the number of data point?
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Rays/view x view
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What does each affect?
a. number of rays used to reconstruct a CT image b. number of views |
a. radial component of spatial resolution (goverened mostly by spacing and wideth of ray data)
b. circumferential component of spatial resolution (number of views used to reconstruct the image) |
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What happens to images if you reduce the ray sampling?
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low resolution, blurred images
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How do you compensate for decreased number of rays?
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Increase detector aperture size-->blurs images
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Artifact created by too few veiws?
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View aliasing-->exacerbated by objects with sharp edges
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What happens to data scquired by a CT scanner before reconstruction?
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preprocessing:
1. calibration data from air scans 2. Geometric efficiencies correction |
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What dies the reconstructed value in each pixel correspond to?
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liner attenutation coefficient for the corresponding voxel
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What does helical imaging allow compared to conventional axial scanning?
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-CT images can be reconstructed at any position along the length of the scan to w/i 1/2 pitch (slice thickness of each edge of the scanned volume)
-production of additional overlapping images with no additional dose to the patient (interleaved images) |
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What is the significance of interleaved images?
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-allows placement of additional images along the patient so that the exam is almost uniformly sensitive to subtle abnormalities.
-but increases reconstruction time |
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What should you not confuse with the ability to reconstruct CT images at short intervels (with helical CT) compared to axial resolution
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Just b/c 5 mm images can be reconstructed every 1mm, does not mean 1 mm spatial resolution is achieved
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What is simple backprojection
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Mathematical process based on trig designed to emulated the acquisiton process in reverse; each ray in each view represents an individual measurement of u
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Describe the process of simple backprojection:
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Empty image matrix (all pixels set to zero)-->u value from each ray in ALL views smeared or backprojected onto image matrix-->so u is added to each pixel in a line through the image corresponding to the ray's paths
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What is a limitation of simple backprojection
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Characteristic 1/r blurring as a by product
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How do you solve the characteristic blurring of simple backprojection?
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filtering step called filtered backprojection
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What is filtered backprojection?
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mathematically reverses image blurring and restores image to an accurate representation of the object that was scanned
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What does the mathematical filtered step involve?
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Convolving the projection data with a convolution kernal
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What is a convolution kernal?
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shape of the filter function in the spatial domain (where the filtered step occurs); different kernal used in soft tissue and bone imaging etc
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What do you use to convert a function expressed in the spatial domain (mm) into the spatial frequency domain (cycles/mm)
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fourier transformation
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What type of filter compensates for the 1/r blurring function?
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Lak filter or ranmp filter b/c 1/r blurring function in spatial domain becomes 1/f blurring function in the freq. domain and L(f)=f therefore 1/f x f=1
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When does the Lak (ramp) filter work the best?
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When there is no noise in the data but there is always xray quantum noise so if you use the Lak filter image will be very noisy
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What filter is used to reduce high requency noise in a final CT image?
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Shepp-Logan filter-->incorporates some roll-off at higher frequencies and decreases high frequency noise
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What has even better high frequency noise suppression from the Shepp-logan filger?
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Hamming filter has even more pronounced high-frequency roll-off
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If you reduce noise (greater roll-off at high frequencies) what do you sacrifice?
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decreased spatial resolution, decreased noise-->used to image soft tissues (liver)
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What does kind of kernal bone imaging use?
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Less high frequency roll off-->increased noise, increased spatial solution
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what is the formula to convert each number in a CT pixel CT (x,y)?
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CT (x,y)=[1000 * u (x,y)]-uwater/uwater
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What does each correspond to:
1. u (x,y) 2. uwater 3. CT (x,y) |
1. floating point number of the (x,y) pixel before conversion
2. attenutation coefficient of water 3. Hounsfield unit |
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What is the value of uwater for xray energies usually used in CT?
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0.195 (almost o)
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What do CT numbers and CT images derive contrast form?
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-physical properites of tissues that influence Compton scatter
-Ex Ct images made with highly filtered, high KV energy beam 75keV: for muscle 91%, fat 94%, bone 74% interactions are compton scattering |
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What physical properties does compton scatter depend on?
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-Density (linearly related to u)-->plays the dominant role
-electron density (depends on Z/A) and hydrogenous tissues are visualized better |
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What are some ways to postprocess CT images?
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1. widow/level
2. Multiplaner reconstruction (ex sagittal and coronal reformats) |
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Why do you have to window and level?
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CT images possess 12 bits of gray scale (4096 shades of gray) and laser imagers used for filming CT studies has 8 bits--> window/level b/c 12 bit images must be reduced to 8 bits
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What does the window width mean?
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Shades of gray
Determines the contrast-->narrower window=greater contrast |
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What is the level?
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Center of the shades of gray
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How do you make sagittal and coronal multiplaner reformatted images?
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combine the x and y axis data along the z axis-->mismatch in spatial resolution occurs-->compensates for this by interpolation-->MPR images maitain proper aspect ratio-->but cannot improve image resolution in the z direction-->leads to blur on this axis
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How can you improve blur in the z direction of MPR reformatted images?
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thinner slice thickness down to 1 mm
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What are the 2 types of 3D reconstructions?
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-Volume rendering
-Reprojection |
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What does volume rendering require?
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segmentation-->ID of specific 2D structrues before 3D reconstruction occurs
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When is segmentation for 3D reformating easiest to perform?
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When there is a large difference in CT number between the target structures and adjacent anatomy.
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Describe the process of volume rendering:
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Segmentation of CT converts image to one-bit representation-->a 0 if target structure is not in the pixel and a 1 if target is in the pixel-0->once target structures is appropriately segmented-->software calculates a number of surfaces from this data (surface rendering)-->surfaces closest to the viewer obscure surfaces at greater depth
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What limits volume rendering in real life?
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Imperfect SNR of CT images
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How does reprojection work? and why is it used over volume rendering?
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-uses ray-tracing software through the volume data set from a specified viewing angle-->software displays the noramlized sum of CT numbers from all voxels through which the ray has passed
-saves time compared to volume rendering |
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Compare radiation dose in CT with projection radiography:
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1. single Ct image acquired in a highly collimated way-->volume of tissue irradiated by a primary beam is less
2. Volume of tissue irrdiated exposed to raditation from all angles-->more evenly distributes the radiation (in chest radiograph for ex, tissue irridated by the entrance beam has > dose than tissue near exit surface) 3. Need a high SNR, so radiation does to the slice volume is higher (use higher kV and mAs) |
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What is the principle interaction in CT and what does this mean in terms of dose?
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Radiation dose attributable to scattered radiation is significant-->can be higher than the dose from the primary beam-->also get scattered dose from adjacent tissue slices
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What is the standard for determining radiation in CT?
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MSAD (multiple scan average dose)-->includes dose attributable to scattered radiation emanating from adjacent slices
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What is a good approximation for MSAD?
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CT dose index-radiation dose to any point in a patient+scattered radiation from 7 adjacent slices in both directions (14 slices)
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What are 2 ways to measure CT dose index?
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1. TLDs (small thermoluminescent dosimeters) placed in holes along 14 slice thickness increments
2. Pencil ionization chamber-long enough to span the width of 14 7mm contingous slices measures the CTDI of a single slice at the center of the inonization pencil chamber and calculates it by CTDI=fXT*L (measures air kerma and not dose directly) |
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How does CTDI compare to MSAD?
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underestimates MSAD for smaller slice thicknesses (2mm) b/c a significant amount of rdiation is scatterend beyond the 7 slice thicknesses
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What is the relationship b/w dose and mAs in CT?
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Proportional-->doubling mAs, doubles dose
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How do you calculate dose in helical CT?
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Use collimator pitch verses detector pitch (in axial scanning)
Dose (helical)=Dose (axial) x 1/collimator pitch If pitch <1, increases dose; if pitch>1, decreases dose |
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What are the kV and mA typical CT scan?
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120 kV and 200 mA verses PA chest radiograph is 120 and 5
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How do you reduce patient dose in CT fluroscopy?
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Decrease the mA per slice
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What is the advantage of modulating mA in CT?
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fewer xray photons needed to penetrate thinner tissues and more to penetrate thicker tissues-->with constant mA the inc. noise in thicker tissues cancels out the inc. SNR in thinner tissues-->so to reduce patient dose, reduce mA
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For a given scenario, give the change in contrast resolution and SNR:
1. Contrast resolution is linked to what parameter? 2. Doubling the mA 3. Dose 4. Pixel size (FOV) |
1. SNR
2. Inc. SNR by sq. root of 2-->contrast increases 3. dose inc. linearly with the mA-->inc. SNR and contrast res. 4 |
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How does beam hardening artifact occur?
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Lower energy beam xray ray photons attenutated to a greater degree than higher energy xray photons-->"harder" or higher energy beam as it exists the patient
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How do you correct for beam hardening?
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beam hardening correction algorithms
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What is partial volume averging?
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-some voxels contain densities of different tissues in one voxel
-usually most pronounced for softly rounded structures that are almost parallel to the CT slice -Ex near top of head where cranium shares voxels with brain tissue-->details of brain parenchyma are lost b/w large u of bone dominates q |
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How can you compensate for partial volume averaging?
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Thinner slices
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