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123 Cards in this Set

  • Front
  • Back
What do 2D picture elements correspond to?
3D voxel elements
What does each pixel in a CT image display?
Average xray attenutation properties of the tissue in the corresponding voxel
What is a ray?
Single transmission measurement through the patient made by a single detector at a given moment in time
What is a projection or view?
Series of rays that pass through the patient in the same orientation
What type of geometry to most CT scanners use?
fan beam geometry for the acquisition and reconstruction process
What is the purpose of a CT scanner?
To aquire a large number of transmission measurements through the patient at different positions
What is the formula for the xray intesity of the atteunated xray beam?
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)
What is used to produce CT images?
CT reconstuction algorithm; filtered backprojection is most common
How does backprojection work?
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
Describe data aquistion and process of computing a CT image:
Make transmission measurements through the object at multiple angles-->reverse aqusition geometry and backproject on a digital matrix
Describe the first generation of CT scanners:
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
Advantages and disadvantages of pencil beam technology of first generation CT scanners
Advantages: efficient scatter reduction, b/c scattered beam deflected away from the pencil beam is not measured by a detector
Describe second generation CT scanners:
-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
What kind of geometry does projection radiography use?
Open beam geometry
Describe 3rd generation CT scanners:
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
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
Disadvantge of third generation CT scanners:
Ring artifacts
what causes ring artifacts of 3rd generation scanners?
-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
How do 4th generation CT scanners overcome problems with ring artifact of 3rd generation scanners?
Rotate/stationary system:
Detectors are in a stationary 360 ring around the patient-->requires more detectors
Describe the difference b/w source fan of 3rd generation CT scan and detector fan of 4th generation CT scan:
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
Describe the difference in data acquistion b/w 3rd and 4th generation CT scanners:
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
In 4th generation CT scanners what is the significance of each dtector acting as its own refence detector?
Each detector has its own gain; g cancels out in the equation and eliminates ring artifact
Describe the 5th generation CT scanner:
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
In the 1990's what did the design of the 3rd and 4th generation scanners incorporate?
slip ring technology so gantry can rotate continously w/o tethering from wires
Describe 6th generation CT scanners:
-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
Describe 7th generation CT scans:
-Multiple detector array
-WIDER collimator spacing
-more xrays produced by xray tube are used to produce an image
What does the use of multiple detector array overcome due to opening up the collimator
-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
Describe xenon detectors:
High pressure xenon gas in long thin cells b/w two metal plates
How do you overcome detector -inefficiency of a gas detector compared to a solid one?
-Make it very thick (>6 cm) to compensate for low density
-Make metallic septa very thin to reduce dead space--> ↑ geometric efficiency
What is geometric efficiency?
fraction of primary xrays exiting a patient that strike an active detector
Why must xenon detectors be positioned in a fixed orientation wrt xray source?
Long, thin ionization plates of xenon detecor are highly directional
Why can xenon detectors not be used in fourth generation CT scanners?
xenon detectors must be positioned in a fixed orientation wrt to x-ray source; so only used in third generation systems
How do xenon detectors work?
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
Describe the physical makeup of a solid state detector
scintillator coupled tightly to a photodetector (photodiode)
How does a solid state detector work?
scintillator emits visible light when stuck by xrays-->light reaches the photodiode
What type of scintillators are used in solid state CT detectors?
CdWO4, yttrium, gadolinium
Why do solid state CT detectors have better xray absorption than xenon detectors?
Increased atomic number
What somewhat reduces geometric efficiency of detector elements?
Small gaps b/w detector elements put there to reduce "crosstalk" b/w detector elements
What makes solid state CT detectors capable of xray detection over a wide range og angles?
Top surface of solid-state CT detectors are essentially flat (unlike xenon detectors)
What type of scanners are solid state detectors used in?
4th and higher generation 3rd
What is a multiple detector array?
Assembly of multiple solid-state detector array modules
How do you practically adjust the slice width with a multidetector array system?
Grouping one or more detector units together; to combine the signal-->detectors are wired together
Why do multiple detector array scanners make use of third generation geometry?
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
What determines slice thickens in a single detector array CT system?
Physical collimation of xray beam with two lead jaws; as gap b/w lead jaws widens, slice thickness increases
What places an UPPER LIMIT on slice thickness in a single detector array system?
Width of detectors in the single detector array
What are the advantages of increasing slice thickness?
1. # of detected photons increases linearly with increased slice thickness
2. Increases SNR (contrast resolution)
What are the disadvantages of increasing slice thickness?
Decreases spatial resolution; thin slices improve spatial resolution and decrease partial volume averaging
What will partially compensate for loss of xray photons due to thin slices?
Increasing the mA
What is the slice sensitivity profile?
Contrast decreases as object moves to the edge of a a slice;
In single detector array, what is the shape of slice sensitivity profile due to?
-finite width of the xray focal spot
-penumbra of the collimator
-projection angle imaging
Why does helical scanning have a slightly broader slice sensitivity profile?
due to translation of the patient during the scan
In a multidetector array scheme, what is slice thickness determine by?
Width of detectors in the slice thickness dimension
How do you change the width of detectors in multidetector array scheme?
binning different numbers of detector elements together (ie summing electronic signals of adjacent detector elements)
In conventional 4 slice axial scanning (ie not helical) describe what happens to 2 the slices at the edge of the scan?
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
In helical imaging, what is te difference in detector array contribution?
-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
When is pitch used?
helical scan protocols
What is the formula for pitch for a single detector array?
Collimator pitch=table movement (mm) per 360 degree rotation of the gantry/collimator width (mm) at the isocenterW
What does pitch influence?
-radiation dose to the patient
-image quality
-scan time
What does a pitch of 1 mean?
number of CT views acquired when averaged over the long axis of the patient is comparable to the number acquired with continguous axial CT
What does a pitch of <1 mean?
slight improvmenet in image qulaity and higher radiation dose to the patient;
What are common pitches used?
pitches up to 1.5 commonly used-->manufacturers spend time trying to develop pitch >1
What is the significance of a pitch >1?
-Some degree of partial scanning of the patient
-faster scan time, less motion, less radiation
-sometimes less radiation
What is the minimum requirement to produce an adequate CT image?
180 degrees + fan angle
What is the formula for detector pitch and collimator pitch for multiple detector array?
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
What is a sinogram?
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)
What would a bad detector in a third-generation scaneer show up as on a sinogram?
Vertical line
How do you calculate the number of data point?
Rays/view x view
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)
What happens to images if you reduce the ray sampling?
low resolution, blurred images
How do you compensate for decreased number of rays?
Increase detector aperture size-->blurs images
Artifact created by too few veiws?
View aliasing-->exacerbated by objects with sharp edges
What happens to data scquired by a CT scanner before reconstruction?
preprocessing:
1. calibration data from air scans
2. Geometric efficiencies correction
What dies the reconstructed value in each pixel correspond to?
liner attenutation coefficient for the corresponding voxel
What does helical imaging allow compared to conventional axial scanning?
-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)
What is the significance of interleaved images?
-allows placement of additional images along the patient so that the exam is almost uniformly sensitive to subtle abnormalities.
-but increases reconstruction time
What should you not confuse with the ability to reconstruct CT images at short intervels (with helical CT) compared to axial resolution
Just b/c 5 mm images can be reconstructed every 1mm, does not mean 1 mm spatial resolution is achieved
What is simple backprojection
Mathematical process based on trig designed to emulated the acquisiton process in reverse; each ray in each view represents an individual measurement of u
Describe the process of simple backprojection:
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
What is a limitation of simple backprojection
Characteristic 1/r blurring as a by product
How do you solve the characteristic blurring of simple backprojection?
filtering step called filtered backprojection
What is filtered backprojection?
mathematically reverses image blurring and restores image to an accurate representation of the object that was scanned
What does the mathematical filtered step involve?
Convolving the projection data with a convolution kernal
What is a convolution kernal?
shape of the filter function in the spatial domain (where the filtered step occurs); different kernal used in soft tissue and bone imaging etc
What do you use to convert a function expressed in the spatial domain (mm) into the spatial frequency domain (cycles/mm)
fourier transformation
What type of filter compensates for the 1/r blurring function?
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
When does the Lak (ramp) filter work the best?
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
What filter is used to reduce high requency noise in a final CT image?
Shepp-Logan filter-->incorporates some roll-off at higher frequencies and decreases high frequency noise
What has even better high frequency noise suppression from the Shepp-logan filger?
Hamming filter has even more pronounced high-frequency roll-off
If you reduce noise (greater roll-off at high frequencies) what do you sacrifice?
decreased spatial resolution, decreased noise-->used to image soft tissues (liver)
What does kind of kernal bone imaging use?
Less high frequency roll off-->increased noise, increased spatial solution
what is the formula to convert each number in a CT pixel CT (x,y)?
CT (x,y)=[1000 * u (x,y)]-uwater/uwater
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
What is the value of uwater for xray energies usually used in CT?
0.195 (almost o)
What do CT numbers and CT images derive contrast form?
-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
What physical properties does compton scatter depend on?
-Density (linearly related to u)-->plays the dominant role
-electron density (depends on Z/A) and hydrogenous tissues are visualized better
What are some ways to postprocess CT images?
1. widow/level
2. Multiplaner reconstruction (ex sagittal and coronal reformats)
Why do you have to window and level?
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
What does the window width mean?
Shades of gray

Determines the contrast-->narrower window=greater contrast
What is the level?
Center of the shades of gray
How do you make sagittal and coronal multiplaner reformatted images?
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
How can you improve blur in the z direction of MPR reformatted images?
thinner slice thickness down to 1 mm
What are the 2 types of 3D reconstructions?
-Volume rendering
-Reprojection
What does volume rendering require?
segmentation-->ID of specific 2D structrues before 3D reconstruction occurs
When is segmentation for 3D reformating easiest to perform?
When there is a large difference in CT number between the target structures and adjacent anatomy.
Describe the process of volume rendering:
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
What limits volume rendering in real life?
Imperfect SNR of CT images
How does reprojection work? and why is it used over volume rendering?
-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
Compare radiation dose in CT with projection radiography:
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)
What is the principle interaction in CT and what does this mean in terms of dose?
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
What is the standard for determining radiation in CT?
MSAD (multiple scan average dose)-->includes dose attributable to scattered radiation emanating from adjacent slices
What is a good approximation for MSAD?
CT dose index-radiation dose to any point in a patient+scattered radiation from 7 adjacent slices in both directions (14 slices)
What are 2 ways to measure CT dose index?
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)
How does CTDI compare to MSAD?
underestimates MSAD for smaller slice thicknesses (2mm) b/c a significant amount of rdiation is scatterend beyond the 7 slice thicknesses
What is the relationship b/w dose and mAs in CT?
Proportional-->doubling mAs, doubles dose
How do you calculate dose in helical CT?
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
What are the kV and mA typical CT scan?
120 kV and 200 mA verses PA chest radiograph is 120 and 5
How do you reduce patient dose in CT fluroscopy?
Decrease the mA per slice
What is the advantage of modulating mA in CT?
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
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
How does beam hardening artifact occur?
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
How do you correct for beam hardening?
beam hardening correction algorithms
What is partial volume averging?
-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
How can you compensate for partial volume averaging?
Thinner slices