CT physics

Question 1

What is filtered back projection?

Answer 1

most commonly used algorithm - reverses the attenuation process and builds up the matrix where areas of high and low attenuation tend to reinforce eachother

Question 2

What are the disadvantages of 1st generation scanners?

Answer 2

• 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

Question 3

What are advantages and disadvantages of the third generation scanners?

Answer 3

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

Question 4

What are two major ways the 4th generation scanner differs from the 3rd generation?

Answer 4

• x-ray tube rotates around stationary detectors
• detector fan instead of a source fan

Question 5

What determines slice thickness in single detector arrays?

Answer 5

Collimation

Question 6

In single slice CT, what is the trade off between SNR and spatial resolution when increasing slice thickness?

Answer 6

• Number of photons detected increases linearly with slice thickness, thus higher SNR
• Thinner slices increase spatial resolution and reduce partial volume averaging

Question 7

For single slice detector arrays, the slice sensitivity profile is a consequence of what?

Answer 7

• detector width
• focal spot size and distribution
• collimator prenumbra
• combined influence of all the projection angles encircling the patient

Question 8

How do you calculate collimator pitch (single detector helical scanners)?

Answer 8

Collimator pitch = table movement (mm) per 360 gantry rotation ÷ collimator width (mm)

Question 9

WHat does a pitch of 1 imply?

Answer 9

the table movement per gantry rotation is equal to the collimator width

Question 10

What does a pitch < 1 imply

Answer 10

over scanning- higher image quality at higher dose to patient

Question 11

What does a pitch >1 imply?

Answer 11

partial scanning - reduces scan time, patient motion, patient exposure, but increases volume averaging

Question 12

What is binning?

Answer 12

electronic signals generated by adjacent detector elements are electronically summed

Question 13

WHat determines slice thickness in multiple detector array scanners?

Answer 13

determined by the width of the detectors in the slice tickness dimension (z-axis) and not by collimation

Question 14

How do you calculate detector pitch (multidetector scanners)?

Answer 14

detector pitch = table movement (mm) per 360 gantry rotation ÷ detector width (mm)

Question 15

How do you calculate collimator pitch of MDA scanners?

Answer 15

Collimator pitch = detector pitch ÷ N (number of detector arrays)

Question 16

What are some major advantages of MDA scanners?

Answer 16

• 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

Question 17

What are disadvantages of MDA?

Answer 17

• larger the number of detectors, the more cone beam artifacts
• more scatter radiation between adjacent slices
  
  • even with thin septa
• tube cooling capabilities

Question 18

What are advantages of Helical CT scanning?

Answer 18

• 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

Question 19

What are disadvantages of helical scanning?

Answer 19

• 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

Question 20

In axial aquisition - what is the Multiple Scan Average Dose?

Answer 20

The does to tissue that includes the dose attibutable to scattered radiation eminating from all adjacent slices

Question 21

How can you calculate/estimate the multiple scan average dose (MSAD)?

Answer 21

can be estimated with the CT dose Index (CTDI)

CTDI(mGy) = [reading (mGy) x length of chamber (mm)] ÷ beam width

Question 22

Compare CTDI_FDA to CTDI₁₀₀

Answer 22

• CTDI_FDA - 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

Question 23

How do you calculatre radiation dose with helical acquisition?

Answer 23

Helical dose = CTDI dose ÷ collimator pitch

• increasung the collimator pitch will decrease the patient dose
• Helical scanning usually uses less mAs

Question 24

What two variable affect image resolution in CT?

Answer 24

• Circumferential resolution
• Radial resolution

Question 25

How do circumferential and radial resolution differ?

Answer 25

* 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

Question 26

In CT interpolation algorithsm, how many full gantry rotations are required to reconstruct a slice?

Answer 26

2 full gantry rotations are required to reconstruct a slice

Question 27

What is the major difference between simple backprojection reconstruction and filtered back projection reconstruction?

Answer 27

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

Question 28

What is the difference between bone kernal and soft tissue kernals?

Answer 28

* 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

Question 29

What is the CT level?

Answer 29

CT number at the center of the window - all numbers above will be white, all below will be black

Question 30

What is Compton scatter dependent on?

Answer 30

Physical density (g/cm³) and electron density in tissue (p_e)

Question 31

How do you calculate CT number (HU)?

Answer 31

CT (x,y) = 1000 x [mu(x,y) - mu_water)] ÷ mu_water

Question 32

How does CT compare to radiography as far as spatial resolution and contrast resolution?

Answer 32

* 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

Question 33

What size of an object can a scanner resolve?

Answer 33

absolute size = reciprocal of spatial frequency (1/s)

Question 34

What is the mean equine pituitary height?

Answer 34

1.0 cm

Question 35

Which cheek tooth is most commonly involved in equine alveolitis/sinusitis and how does it appear on CT?

Answer 35

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

Question 36

How do 4th generation scanners eliminate rign artifact?

Answer 36

the reference beam is measured by the same detector used for transmission measurement

Question 37

What are disadvantages of helical scanners/acquisition?

Answer 37

* 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

Question 38

What causes ray aliasing and what dies it looks like?

Answer 38

* 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

Question 39

How do you correct ray aliasing?

Answer 39

* quarter detector off-set * dynamic focal spot

Question 40

When does view aliasing occur and how does it appear?

Answer 40

* cause by insufficient number of views per beam rotation * fine radial streaks in the image periphary

Question 41

How do you correct view aliasing?

Answer 41

* increase the tube rotation time (e.g. from 1s to 2s) * avoidance of high density objects (not really practical)

Question 42

What are examples of artifacts that occur due to movement?

Answer 42

ghosting, streaks, bluring and slice mismatch

Question 43

What causes windmill artifacts? How is it minimized?

Answer 43

* 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

Question 44

What is cone beam artifact? What does it look like?

Answer 44

* 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

Question 45

What does zebra artifact look like?

Answer 45

faint parallel stripes in the x-y dimension on multiplanar and 3D reformatted images (caused by helical image interpolation)

Question 46

How can partial volume averaging atrifact appear?

Answer 46

* blurring of object margins, erroneous attenuation measurements, pseudolesions, and streaks

Question 47

How does truncated view/out-of-field artifact appear? What causes it?

Answer 47

Characterized by a bright rim along the periphery and/or streaks originating from the area of structures outside the scan field of view

Question 48

What causes partial volume averaging?

Answer 48

occurs if and object is only partially intruding, traversing obliquely or is smaller than the scanning plane

Question 49

What causes rebound artifact?

Answer 49

edge enhancement

Question 50

Under what circumstances will point spread effect cause a object to appear thicker? Thinner?

Answer 50

* 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

Question 51

What is photon starvation? How do you correct it?

Answer 51

* 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

Question 52

What causes beam hardening?

Answer 52

selective resorption of low-energy photons from the polychromatic x-ray beam by a highly attenuating structure

Question 53

How can you reduce beam hardening?

Answer 53

* pre-patient filtering * bow-tie filter - minimize cupping

Question 54

How can you correct non-linear partial volume effect?

Answer 54

thick section reformating of thinly collimated slices