CT physics Flashcards
What is filtered back projection?
most commonly used algorithm - reverses the attenuation process and builds up the matrix where areas of high and low attenuation tend to reinforce eachother
What are the disadvantages of 1st generation scanners?
- poor spatial resolution
- after glow (NaI detector) occured secondary to slow decay
- large change in x-ray flux between the patient’s head and exterior to the head
- slow spead
What are advantages and disadvantages of the third generation scanners?
Advantages
- scan time is significantly reduced
- there is better scatter rejection than 4th generation scanners
Disadvantages
- suffers from ring artifacts and aliasing
- need perfect alignment of the tube and detectors
- scanning velocity limited by mechanical factors
What are two major ways the 4th generation scanner differs from the 3rd generation?
- x-ray tube rotates around stationary detectors
- detector fan instead of a source fan
What determines slice thickness in single detector arrays?
Collimation
In single slice CT, what is the trade off between SNR and spatial resolution when increasing slice thickness?
- Number of photons detected increases linearly with slice thickness, thus higher SNR
- Thinner slices increase spatial resolution and reduce partial volume averaging
For single slice detector arrays, the slice sensitivity profile is a consequence of what?
- detector width
- focal spot size and distribution
- collimator prenumbra
- combined influence of all the projection angles encircling the patient
How do you calculate collimator pitch (single detector helical scanners)?
Collimator pitch = table movement (mm) per 360 gantry rotation ÷ collimator width (mm)
WHat does a pitch of 1 imply?
the table movement per gantry rotation is equal to the collimator width
What does a pitch < 1 imply
over scanning- higher image quality at higher dose to patient
What does a pitch >1 imply?
partial scanning - reduces scan time, patient motion, patient exposure, but increases volume averaging
What is binning?
electronic signals generated by adjacent detector elements are electronically summed
WHat determines slice thickness in multiple detector array scanners?
determined by the width of the detectors in the slice tickness dimension (z-axis) and not by collimation
How do you calculate detector pitch (multidetector scanners)?
detector pitch = table movement (mm) per 360 gantry rotation ÷ detector width (mm)
How do you calculate collimator pitch of MDA scanners?
Collimator pitch = detector pitch ÷ N (number of detector arrays)
What are some major advantages of MDA scanners?
- True isotropic spatial resolution
- cubic voxels can be realized - image equally sharp in any plane traversing the scanned volume
- Improved temporal resolution - less motion artifact
- Increased SNR (decreased noise)
- higher tube current used because more patient length is scanned
- Increased speed - 1 rotation/0.5 seconds
- CTA advantage - higher contrast rate for more separation of phases
- less unusable penumbra radiation
- higher umbra:penumbra ratio
What are disadvantages of MDA?
- larger the number of detectors, the more cone beam artifacts
- more scatter radiation between adjacent slices
- even with thin septa
- tube cooling capabilities
What are advantages of Helical CT scanning?
- faster
- sampling along the z-axis is uniform
- images can be reconstructed at arbitrarily detremined positions and intervals along the z-axis
- optimizes enhancement effect of contrast
- image quality is equivalent to that of conventional CT
What are disadvantages of helical scanning?
- inherent inconsitency in the acquired projections because of the constant parient translation leading to artifacts
- Increased heat loading
- increased processing time due to z-interpolation
- High pitch - decreased z-axis resolution
In axial aquisition - what is the Multiple Scan Average Dose?
The does to tissue that includes the dose attibutable to scattered radiation eminating from all adjacent slices
How can you calculate/estimate the multiple scan average dose (MSAD)?
can be estimated with the CT dose Index (CTDI)
CTDI(mGy) = [reading (mGy) x length of chamber (mm)] ÷ beam width
Compare CTDIFDA to CTDI100
- CTDIFDA - the radiation dose to any point in the patient including the scattered radiation from 7 CT slices in both directions
- significantly underestimates the MSAD for small slice thickness
- CTDI100 - provides a better estimation of MSAD for thin slices
How do you calculatre radiation dose with helical acquisition?
Helical dose = CTDI dose ÷ collimator pitch
- increasung the collimator pitch will decrease the patient dose
- Helical scanning usually uses less mAs
What two variable affect image resolution in CT?
- Circumferential resolution
- Radial resolution
How do circumferential and radial resolution differ?
- Circumferential resolution is affected by number of views
- reducing the number of views creates aliasing
- Radial resolution is affected by the number of rays
- reducing ray sampling results in low resolution, blurred images
In CT interpolation algorithsm, how many full gantry rotations are required to reconstruct a slice?
2 full gantry rotations are required to reconstruct a slice
What is the major difference between simple backprojection reconstruction and filtered back projection reconstruction?
In filtered, the raw data are mathmatically filtered before being back projected onto the image matrix, reversing blurring.
This is not done in simple back projection so there is blurring, 1/r
What is the difference between bone kernal and soft tissue kernals?
- bone kernals - less high frequency roll-off
- higher spatial resolution, decreased SNR
- Soft tissue kernals - more high frequncy roll off
- use when high contrast more importal than spatial resolution (e.g. looking for mets)
- increased SNR, lower spatial resolution
What is the CT level?
CT number at the center of the window - all numbers above will be white, all below will be black
What is Compton scatter dependent on?
Physical density (g/cm3) and electron density in tissue (pe)
How do you calculate CT number (HU)?
CT (x,y) = 1000 x [mu(x,y) - muwater)] ÷ muwater
How does CT compare to radiography as far as spatial resolution and contrast resolution?
- CT has significantly worse spatial resolution
- Comparting MTF for radiography and CT
- Screen-film: 7 lp/mm
- Digital radiography: 5 lp/mm
- CT: 1 lp/mm
What size of an object can a scanner resolve?
absolute size = reciprocal of spatial frequency (1/s)
What is the mean equine pituitary height?
1.0 cm
Which cheek tooth is most commonly involved in equine alveolitis/sinusitis and how does it appear on CT?
The first molar -
- hypoattenuation of the cementum, destruction of the enamel and filling of the infundibular cavity with gas
- gas bubble in the root area or fragmentation of the root
- see page 41 of CT notes for more changes
How do 4th generation scanners eliminate rign artifact?
the reference beam is measured by the same detector used for transmission measurement
What are disadvantages of helical scanners/acquisition?
- motion artifacts
- increased chance of partial volume averaging
- reduced resolution in the z-direction when using large pitch factors
- increase in processing time due to z-interpolation required
What causes ray aliasing and what dies it looks like?
- objects in the image with sharp, well-defined edges that contain strong, high-frequency components exceeding the Nyquist limit
- looks like pattern of radial dak and bright streaks emanating directly from a high frequency object
How do you correct ray aliasing?
- quarter detector off-set
- dynamic focal spot
When does view aliasing occur and how does it appear?
- cause by insufficient number of views per beam rotation
- fine radial streaks in the image periphary
How do you correct view aliasing?
- increase the tube rotation time (e.g. from 1s to 2s)
- avoidance of high density objects (not really practical)
What are examples of artifacts that occur due to movement?
ghosting, streaks, bluring and slice mismatch
What causes windmill artifacts? How is it minimized?
- Due to the complex interpolation of image data from helical multislice CT
- minimized by applying different types of image interpolation (z-filter interpolation) or using non-integer pitches in relation to detector width
What is cone beam artifact? What does it look like?
- Due to the relative small focal spot and relatively larger detector width, x-ray beam is more cone/wedge shaped
- small objects in periphery may be detected in one scan field but not the opposite, leading to conflicting density information
- leads to streak formation
What does zebra artifact look like?
faint parallel stripes in the x-y dimension on multiplanar and 3D reformatted images (caused by helical image interpolation)
How can partial volume averaging atrifact appear?
- blurring of object margins, erroneous attenuation measurements, pseudolesions, and streaks
How does truncated view/out-of-field artifact appear? What causes it?
Characterized by a bright rim along the periphery and/or streaks originating from the area of structures outside the scan field of view
What causes partial volume averaging?
occurs if and object is only partially intruding, traversing obliquely or is smaller than the scanning plane
What causes rebound artifact?
edge enhancement
Under what circumstances will point spread effect cause a object to appear thicker? Thinner?
- Thicker:
- widest with a low freq algorithm protocol and low and narrow settings (e.g. brain)
- when inherent object contrast is reduced (e.g. fluid filled bulla or sinus)
- Thinner:
- high frequency algorithm, viewed with high wide setting
What is photon starvation? How do you correct it?
- Streaking artifacts that occur in highly attenuating body parts such as the shoulders or thighs (insufficient photons reach the detector)
- increase current or increase slice thickness
What causes beam hardening?
selective resorption of low-energy photons from the polychromatic x-ray beam by a highly attenuating structure
How can you reduce beam hardening?
- pre-patient filtering
- bow-tie filter - minimize cupping
How can you correct non-linear partial volume effect?
thick section reformating of thinly collimated slices
