CT physics

Question 1

1st/2nd generation CT scanners:

1. What type/movement is a 1st generation?
2. What type/movement is a 2nd generation?

Answer 1

1. 1st: pencil beam; translate & rotating beam.
2. 2nd: fan beam; translate & rotating beam.

• 3rd generation: fan beam; rotating beam & detector.
• 4th generation: fan beam; rotating beam, fixed detector.
• 5th generation: electron beam CT.

Question 2

What does it mean when a CT scanner is 3rd generation?

Answer 2

Major point: the x-ray source and detectors rotate around the pt in synchrony & the linear attenuation coefficient of each pixel is calculated useing a reconstruction algorithm.

Minor point: fan beam geometry enabled one complete slice to be covered at one time.

• Originally 288 detectors used, now over 700 arranged in an arc.
• Scan duration: ~5 secs.
• 4th gen: >2000 detectors arranged in an outer ring which is fixed; beam is still fan-shaped; beam rotates, detector fixed; scan duration: a few seconds.

Question 3

What is helical/spiral CT acquisition?

Answer 3

• Previously, earlier CTs stopped and shot.
• With helical CT, the pt is moved through a rotating XR beam + detector set.
• The helical path results in a 3-D data set which can then be reconstructed into sequential images for a stack.
• Rad dose during a helical acquisition depends on the speed of the pt through the scanner, i.e., the pitch.

Question 4

Define multislice CT.

Answer 4

• Synonymous w/multi-detector-row CT (MDCT).
• Is a CT system w/multiple rows of CT detectors to create images of multiple sections.
• Conventional CT systems have only one row of CT detectors.
• Canon’s Aquilion ONE contains 320 detectors x 0.5mm.
• The number of detectors in the Z direction determines the # of slices that can be simultaneously acquired.

Question 5

What happens when:

• pitch < 1
• pitch = 1
• pitch > 1

1. What is the proportional relationship of pitch and dose?
2. What is a good example of a scan done with a pitch <1?

Answer 5

• pitch < 1: sometimes called “over-scanning”
  
  • Slower table.
  • Beam has overlap with each rotation.
  • Better image (resolution).
  • Higher dose.
• pitch = 1:
  
  • No overlap, no spaces b/w beams b/w rotations.
• pitch > 1:
  
  • Faster table.
  • Creates a gap: some anatomy missed w/spaces b/w rotations.
  • Poorer image (spatial resolution decreased).
  • Decreased dose.

1. They are inversely proportional: if the pitch is doubled, then the dose is 1/2; if the pitch is halved, then the dose is doubled.
2. Cardiac CT, pitch = 0.2; you really care about spatial resolution here.

https://www.youtube.com/watch?v=UNqv0GpigeY

Question 6

What target material is used in CT XR tubes?

Answer 6

Tungsten alloy (rotating) target placed on high speed rotating anodes.

Question 7

Draw the table that compares XR to CT re: current, kVP and focal spots:

Answer 7

XRAY

CT

Tube current

200-800 mA

Up to 1,000

kVp

50-120 kVp

80-120 kVp

Focal Tube Spot

1. 0-1.2mm
2. 6-1.2mm
   * So CTs are designed to run at reasonable voltages, but w/very high currents.

Question 8

DECT:

1. What is the basic philosophy behind DECT?
2. At what kVps are dual energy scans acquired?
3. What is actually recorded during these 2 scans?
4. Which 3 material decomposition images (removal) are there?

Answer 8

1. 2 different photon energy spectra are used to interrogate materials that have different attenuation properties at two different energies.
2. 80 & 140 kVp.
3. The HU of each pixel, so twice.
4. Iodine, calcium, uric acid.

Question 9

DECT: List some applications.

Answer 9

1. Determine gallstone & renal stone composition.
2. Can reduce metallic artifact.
3. Create virtual non-enhanced images, which reduces dose by eliminating a true non-enhanced acquisition.
4. Use increase the conspicuity of iodine in CEDECT to make free active extrav or endoleak more visible.

Question 10

How do newer-generation DECT scanners do relative to SECT (single-energy) re: radiation dose?

Answer 10

DECT scanners (newer generation) can deliver equal or smaller doses.

Question 11

1. What is the k-edge value of iodine?
2. Soft tissue structures?

Answer 11

1. 33.2 keV
2. WAY less–all of them contain carbon, etc., so <1 keV each.

Question 12

Which crystals (and related clinical condition) can DECT detect?

Answer 12

Monosodium urate crystals = gout

Question 13

Define k-edge.

1. How does k-edge relate to an element’s atomic number (Z)?

Answer 13

K-edge definition:

• It is the sudden increase in XR photoelectric absorption that occurs when the energy of the XRs is just above the binding energy of the innermost (k-shell) of electrons of any element.
• The attenuation value of a material hit by a photon beam is at its maximum close to the k-edge value.

1. The higher the atomic number, the higher the k-shell binding energy, thus the higher the photon energy at which the K-edge occurs.

Question 14

1. Which type of atomic number is more susceptible to photoelectric effect, high or low?
2. What are the atomic numbers of iodine, calcium and molybdenum?

Answer 14

1. High.
2. Iodine Z=53, K-edge 33.2 keV; Moly Z=42, K-edge = 20.0eV; Ca Z = 20, K-edge = 4 keV.

Question 15

How does peripheral vs. central CT dose compare in the head vs. body & why?

Answer 15

Head: central dose = peripheral, as the head diameter is relatively small.

Body: central dose < peripheral dose (about half), since the body diameter is much wider, so less radiation reaches the central body.

Question 16

1. Define CT Z-axis.
2. What is Z-axis variation?

Answer 16

1. Z-axis = the length of the scan, i.e., along the length of the patient.
2. The “tails” of radiation along the edge of the area being scanned.

Question 17

CT phantoms:

1. How large is a CT body phantom?
2. What happens to the dose if the pt is larger than the phantom?
3. Smaller than the phantom?

Answer 17

1. 32cm.
2. If the pt is larger then the dose is over-estimated, as there is more tissue in the larger person to absorb all the dose, so smaller Deff.
3. Smaller: dose is underestimated as there is less internal shielding in the pt, and tissue to absorb all the dose.

Question 18

1. Define CTDI.
   
   • What units are used?
2. Define CTDI100.
3. Define CTDIw.
4. Define CTDIvol.

Answer 18

1. CT dose index = a standardized measure of the radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners. It is the radiation dose normalized to beam width.

• It is measured as the average phantom dose for a single axial slice (one complete rotation without table motion), including scatter.
• mGy.
• It’s measured with 16cm and 32cm phantoms. The 16cm will always have a higher dose as it’s smaller.

1. CTDI100 = linear measure over a 100mm ionization chamber (mGy).
2. CTDIw (mGy) = 2/3 CTDI100 periphery + 1/3 CTDI100 center (so this is closer to the human dose profile).
3. CTDIvol (mGy) = CTDIw / pitch

• Should be less than reference values:
  
  • Adult head (16cm phantom): 75 mGy.
  • Adult abdo (32 cm phantom): 25 mGy.
  • Peds abdo (16 cm phantom): 20 mGy.

Question 19

1. Define DLP.
2. What does DLP effectively give you?
3. How do you estimate Deff (in mSv) from the DLP?

Answer 19

1. CTDIvol x the length of the scan in cm (measured in mGy*cm).
2. The overall dose output per scan. The DLP and CTDI give no information about actual absorbed or effective doses for any patient. This is the best estimate of radn risk from a CT.
3. Deff = DLP x organ weighting factor:

• So if DLP is 900 mGy*cm for an abdo CT, the radn exposure = (900 mGy*cm) x (0.017 mSv/mGy*cm) = 15.4 mSv.

Question 20

How does skin dose in XR differ from skin dose in CT?

Answer 20

XR skin dose: entrance skin dose is WAY higher than exit.

CT skin dose: b/c the scanner spins 360º, entrance and exit skin doses are similar. However, the center (in body CTs at least) receive less than the periphery (about half).

Question 21

1. Define effective dose for CT.
2. What units used?

Answer 21

1. D_eff = k x DLP where k is a constant for any given body part.
2. Because it actually measures dose in human tissue, it’s measured in Sv.

Question 22

List the typical Deff (mSv) ranges for common diagnostic exams:

1. XR
2. Fluoro
3. IR
4. CT

Answer 22

1. XR: 0.005 mSV (knee XR) up to 1.5 mSV (lumbar spine XR).
2. Fluoro: 6-8 mSV (UGIS to barium enema).
3. IR: 1-10 mSv (cerebral angio); 100 mSv (TIPS)
4. CT: 2-8 mSV:

• Head CT: 2 mSv
• Neck: 3 mSv
• Chest: 7 mSv
• Abdo: 8 mSv
• Pelvis: 6 mSv

Question 23

Average CT doses: List the average CTDI and effective doses for:

1. Adult head
2. Adult abdomen
3. Peds abdomen (5yo)

What are the ACR “reference doses” by definition?

List these for :

1. Adult head
2. Adult abdomen
3. Peds abdomen

Answer 23

1. Adult head: 58 mGy, D_eff 1-2 mSV
2. Adult abdomen: 18 mGy, D_eff 8-11 mSV
3. Peds abdomen (5yo): 15 mGy

ACR “reference doses” by definition: 75th %ile doses, above which should be investigated and reduced if posisble.

List these for :

1. Adult head: 75 mGy
2. Adult abdomen: 25 mGy
3. Peds abdomen: 20 mGy

Question 24

Effective mAs

• Where is this term used?
• Define.

Answer 24

• Helical scanning only.
• mAs / pitch
• As the pitch increases (all other settings constant), the # of XR photons contributing to the slice data will decrease (effective mAs).
• The effective mAs determines the dose to the slice (CTDI_vol) and SNR.

Question 25

List the risk of radiation induced cancer per dose in an adult, adult >50yrs and child.

Answer 25

Adult: 5% per Sv

Adult >50yrs: 0.5% per Sv

Child: up to 15% per Sv

Question 26

Name 3 pediatric CT considerations:

Answer 26

1. It’s recommended that you reduce the mAs.
2. Reduced CT techniques are possible b/c XR penetration is greater in children.
3. Dose reduction in peds head CTs are more modest than peds belly CTs.

Question 27

Name 3 strategies to reduce CT dose to the breast:

Answer 27

1. Do the scan at reduced mA (but the problem is that the images are terrible.
2. Use a mA modulation (adjust based on density), which is the preferred method.
3. Shield the breasts with bismuth: this will decrease dose by ~30% but will give artifact and a degraded image (beam hardening can falsely elevate HUs directly deep to the shield).

Question 28

Bismuth shielding:

• What is the downside to bismuth shielding?

Answer 28

• It can increase HU of tissues underneath it.
• Bismuth shields the pt but also prevents radn from exiting the pt, causing beam hardening, which elevates HU directly deep to the shield.

Question 29

The dose of 1 chest CT = how many PA/lat CXRs?

Answer 29

~100

Question 30

What is the approx effective dose of the extremities and why?

Answer 30

<1mSV

Because they don’t contain any radiosensitive organs/tissue.

Question 31

What is the dose to embryo in a CTAP?

Answer 31

~30mGy

Question 32

At what radiation level is occupational individual dose monitoring required?

Answer 32

>10% of the annual dose limit (500mrem).

Question 33

What happens to the CTDI_w in body and head phantoms if you increase the kVp?

Answer 33

Both, body and head CTDI_x will increase.

Question 34

CTDI_vol:

What does the parameter indicated by the arrow NOT account for?

Answer 34

• Scan length (that would be DLP).
• CTDI_vol = dose per slice as estimated with a 16 or 32cm phantom measured over 100mm length, taking into account mAs, kVp, pitch, and difference in dose at the periphery and center of the phantom.
• Equation:

CTDI_vol = [(1/3 x CTDI100_center + 2/3 x CTDI100_periphery) /pitch]

Question 35

Dose modulation:

1. Define.
2. What is not adjusted in this?

Answer 35

1. mA adjustment depending on the attenuation of the pt. The mA is varied as the tube rotates around the pt and along the long axis to provide best compromise b/w image quality and dose. New scanners can modulate during the rotation and along the Z-axis to decrease dose to radiosensitive organs.
2. kVP: this may be adjusted depending on body habitus, but it is not referred to as dose modulation.

Question 36

Linear attenuation coefficient:

1. Define this.
2. What happens to it with increasing kVP?
3. How is this affected by Z?

Answer 36

1. The LAC describes the fraction of photons that are attenuated per unit thickness of material. It inlcudes all possible interactions, including those removed directly by absorption by the PE effect or by Compton scatter.
2. The LAC decreases w/increasing photon energy except at k-edges.
3. As Z (atomic # increases), LAC increases as more photons are absorbed.

Question 37

Bow tie filters:

1. General purpose?
2. 3 specific purposes?
3. Material (thickness) used in the bowtie to filter?
4. Where do they filter most?
5. What is the positioning of the XR tube anode-cathode & why?

Answer 37

1. To remove low/high energy XRs that would only serve to increase dose.
2. a) compensate for uneven filtration; b) reduce scatter; c) reduce dose.
3. 6mm copper, aluminum, Teflon (any low Z material).
4. The periphery; they are more shelled out in the middle, just like a bowtie.
5. Perpendicular to the imaging plane to reduce heel effect.

Question 38

Collimators:

1. Where are they positioned in a CT?
2. 2 purposes?
3. What do they define?

Answer 38

1. At the XR tube and at the detector.
2. To shape the XR beam (you know this as in fluoro the collimators are adjustable diaphragms); to reduce some scatter.
3. The section thickness on a single slice.

Question 39

Define filtered back projection.

Answer 39

• Image data is sequentially collected from one object.
• Fancy math takes all that noisy data and removes the noise/sharpens the image.
• Then it back projects the sharpened image.
• This is being replaced by iterative reconstruction, which requires higher powered computing, that allows more noise and less dose.

Question 40

Filtered back projection:

• What does the term “filtered” refer to?

Answer 40

• Sharpening of the projection data prior to back projection.
• Blurring is removed during a deconvolution process.
• Without filtering, the images would be blurred and unsuitable for diagnostic imaging.
• This filtering does not remove artifacts, e.g., beam hardening.
• The application of different reconstruction kernels will determine how much sharpening occurs during the deconvolution process, resulting in less sharp (soft tissue kernel), or sharper (bone kernel) images.

Question 41

Iterative reconstruction:

• How does iterative reconstruction work?
• What does it improve compared to back filtering?

Answer 41

• It goes through repeated updates of the reconstructed data by comparing the forward data projections from each iteration to the actual measured projections to adjust the reconstruction.
• With each iteration, the difference b/w the estimated and real projections (the error) gets smaller.
• This improves the SNR.

Question 42

Define isotropic resolution, and what does it mean re: MDCT?

Answer 42

The spatial resolution in the transaxial (X-Y) plane is the same as that in the Z-plane. That is, the resolution is the same in all dimensions.

This is important as it allows non-axial reconstructions (coronal/sag) without stretching pixels.

It also helps to avoid artifacts: stair-step, and creates well-defined images.

Question 43

What CT factor determines minimal thickness slice (high yield per CtC)?

Answer 43

Detector element aperture width.

Question 44

In what 2 ways do new CT scanners adjust dose?

Answer 44

1. Scout: they use the scout data to estimate density & auto-adjust.
2. On the fly: as the scan is happening they use continuous modulation to adjust.

Question 45

CT:

Define ray, projection, sinogram.

Answer 45

Ray: a single line of data of XR attenuation from the source to a single detector.

Projection: all the rays at a given angle of the XR tube, aka all the rays that pass through the patient at the same orientation.

Sinogram: a collection of squiggly lines that represent the data from all of the projections of the tube angles.

Question 46

Fundamental CT trivia:

1. What kind of XRs are used w/CT?
2. What happens if you decrease the mAs?

Answer 46

1. Highly filtered, high kV (average energy 75 keV).
2. You increase the noise/get grainier images. (Think low-dose CTs which are super grainy.)

Question 47

Fundamental CT trivia:

1. What are the matrix dimensions for CT? How many pixels?
2. How many shades of gray? (How do you figure that out?)
3. How do you calculate pixel size?
4. Re: pixels, how do you improve spatial resolution?

Answer 47

1. 512 x 512 pixels = 262,144 pixels total.
2. 4,096 (12 bit, 2¹² = 4.096).
3. Pixel size = FOV / matrix size.
4. Make the pixels smaller, so decrease FOV or increase matrix size.

Question 48

Define pitch

Answer 48

distance of (table movement during a single tube revolution)

beam width

Question 49

Hounsfield Units:

1. What is used as the reference (zero point) for HUs?
2. Formula for calculating HUs?
3. What is the relationship b/w HU & XR attenuation?
4. By what %HUs do gray/white matter differ?

Answer 49

1. Water = 0.
2. HU = 1000 x (attenuation of material - attenuation of water) / (attenuation of water)
3. When HU increases by 10 HU, XRs are attenuated by 1% (which is to say that the material stopping the XRs is denser).
4. 0.5% HU.

Question 50

Hounsfield Units - 2

1. How does changing keV change HUs?
2. How does filtration/beam hardening change HUs?

Answer 50

1. Since the PE dominates at lower energy, low keV will create higher HU. The closer you approach the k-edge of a given substance, the more impressive the increase in attenuation (and higher HU). Example: IV contrast will have a HU of ~100HU at 140kV but will increase markedly to 400 HU at 80kV, where the average energy will land flush on its k-edge.
2. In beam hardening, the beam is filtered to increase the quality of the beam, i.e., increasing the kV. So if you remove lower energy photons to harden the beam, that will increase your average energy and lower your HUs. This is related to “cupping artifact”.

Question 51

1. What are the HU values of air, water, bone?
2. What is the HU of a material with twice the attenuation of water?

Answer 51

Air = -1,000

Water = 0

Bone = high, but variable, i.e., cortical bone can be 1,200-1,500 while the otic capsule can be >2,000.

2. 1,000 HU

Question 52

Window/Level:

1. What is window?
2. What is level?
3. Name some commonly used W/L values.

Answer 52

1. Window = the range of HUs that will be displayed, so the larger the window, the larger the range of HU #s that will be displayed.
2. Level: midpoint of the window grayscale that you’ve chosen. You want this to be at the level of the attenuation of the thing that you’re interested in.

Question 53

Axial vs. Helical CT data acquisition

Axial pros?

Helical pros?

Answer 53

Axial pros: stationary table; tube takes a picture, table moves up a slice, another full picture is taken.

• Better spatial resolution in Z-dimension since full image sets are taken per slice.
• No partial volume effect along the Z-axis (these are noticeable in helical CT especially along curved surfaces, e.g., skull).

Helical pros: tube is on the whole time & table moves at a constant speed.

• WAY faster.
• Lower probability of anatomic discontinuities b/w adjacent slices containing moving anatomy.

Question 54

kVp effect in helical CT

How does increased kVp affect:

1. Dose.
2. Image noise.
3. Conspicuity of iodinated contrast.

Answer 54

1. Dose: increase.
2. Image noise: decrease.
3. Conspicuity of iodinated contrast: decrease. Iodinated contrast will be more conspicuous at lower kVP (e.g., 80 kVP), as the average energy of the XR beam will be closer to the k-edge of iodine.

Question 55

Scanning beam width in helical CT

How does increased beam affect:

1. Scan time.
2. Motion artifact.
3. Partial volume.
4. Dose.

Answer 55

1. Scan time: reduced; larger coverage w/1 turn.
2. Motion artifact: reduced; less scan time.
3. Partial volume: increased; more divergent beam.
   
   • Partial volume defn: occurs when tissues of wildly different absorption (HUs) are captured in the same voxel, which shows a beam attenuation that is the average value of the 2 different tissues.
4. Dose: no change; mAs unchanged, and even though the scan time is less, a larger area of tissue is scanned at the same time.

Question 56

Iterative reconstruction vs. filtered back projection

1. Name 2 features of iterative reconstruction.
2. Name 2 limitations of FBP.

Answer 56

1. It can correct for noise, so lower doses may be used.
2. Noise & streak artifacts.

Question 57

Why does helical acquisition CT image reconstruction require interpolation prior to image reconstruction?

Answer 57

• B/c the acquired data from a 360° revolution does not lie on one plane.

Question 58

Cardiac CT

1. When during the cardiac cycle is this best performed?
2. Name the 2 main methods.
3. Name a pro and con of each.
4. What type of imaging approach is always used for prospective?

Answer 58

1. During diastole.
2. Prospective, retrospective.

Prospective: step & shoot during the R-R interval

• Pro: less dose b/c the scanner isn’t on the whole time.
• Con: No functional imaging.

Retrospective: scans the whole time then back-calculates.

• Pro: can do functional imaging.
• Con: more dose b/c on the whole time w/low pitch.

4. Prospective is always axial, never helical.

Question 59

Quantum mottle:

1. Define quantum mottle.
2. How does it work?
3. What type of distribution does the # of photons follow?
4. When is quantum mottle most prominent?

Answer 59

1. Mottle = noise.
2. It is the random fluctuation in the number of photons reaching the detector at any given point.
3. Poisson: the events occur independently of the previous events & the probabilities don’t change over time. The average # of photons reaching individual pixels on the detector = N.
4. In low-dose imaging, e.g., XR, fluoro.

Question 60

CT SNR

1. What does signal change directly in proportion to?
2. In what proportion does noise change?
3. So if the mA increase from 200 to 400, what is the SNR?
4. List 6 factors that increase SNR.

Answer 60

1. XR flux, i.e., mA; 1:1 proportion, so if mA doubles, so does signal.
2. sqrt(N), so if mA doubles, noise increases by sqrt(2) = 1.4.
3. 2/1.4 = 40%.
4. 6 factors:

• Higher mA
• Higher kVp
• Longer rotation time
• Smaller pitch (pt moves through the gantry slower)
• Larger slice thickness
• Larger pixel

Question 61

Contrast resolution

1. Define it.
2. What determines the ability for good contrast resolution in any imaging system?
3. How does CT rank for contrast resolution?
4. Why is CT contrast resolution so good? In what 2 ways is this accomplished?
5. What improves contrast resolution?

Answer 61

1. The ability to discriminate b/w structures w/similar attenuation characteristics, i.e., pixel values/colours.
2. The number of colours generated, i.e., pixel values. So CT and MR are excellent at generating colours, but XR is terrible.
3. CT is excellent.
4. B/c there is minimal scatter/noise reaching the detectors. 1) Very tight collimation (pre-patient & pre-detector); 2) windowing–a tight window maximizes contrast resolution.
5. Any factor that reduces noise: mAs, kVP, longer rotation time, decreased pitch, larger slice thickness, larger pixels.

Question 62

Spatial resolution

1. What imaging modalities have the best spatial resolution? Worst?
2. List, describe 5 factors that affect SR.

Answer 62

1. Best: XR & fluoro; worst: CT & MR.
2. 1. Focal spot size: smaller = better SR; a larger focal spot means the details are spread out over several detectors which degrades/blurs the image. Focal spot determines SR in the X-Y plane, so side to side.
3. Magnification: more mag blurs.
4. Detector aperture size: like focal spot size, smaller = better; this determines SR in the Z-plane; as the detector size is reduced, the z-axis increases; the x-y axis is not affected by aperture size.
5. # of projections: more projections, more data, better SR.
6. Reconstruction slice thickness: the thinner the element aperture, the better the spatial resolution in the Z-direction.

Question 63

Spatial resolution & pixel size/DFOV:

1. Define DFOV.
2. State the equation that relates the above 3 terms.
3. What happens to pixel size if you hold matrix size constant & decrease FOV?
4. If you increase FOV?

Answer 63

1. DFOV = the space defined by the user based on the anatomy size to be displayed.
2. pixel size = DFOV / matrix size.
3. As FOV decreases, so does pixel size, which increases SR but decreases contrast resolution.
4. As FOV increases, so does pixel size, which decreases SR but increases contrast resolution.

Question 64

Spatial resolution & pitch, motion & filters:

1. How does pitch affect SR?
2. How does pt motion affect SR?
3. How does a sharp/edge enhancement filter affect SR?
4. How does a smooth/noise reduction filter affect SR?

Answer 64

1. As pitch increases, the width of the slice sensitivity profile (SSP) increases, so the slice thickness increases, which increases contrast but decreases SR.
2. Pt motion blurs, which decreases SR.
3. Sharp = better SR, but more noise.
4. Smooth = worse SR, but less noise.

Question 65

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 65

Beam hardening: cupping.

• As the XR beam passes through an object the lower energy photons are removed preferentially, leaving a harder beam w/an increased average energy.
• The XRs passing through the middle of a uniform shape (like a head) are hardened more than those at the periphery.
• The harder the beam, the slower the rate of attenuation, so the center of the image appears darker.
• Fixes:
  
  • Modern CT scanners use filters to overcome beam hardening so it’s less often seen.
  • Filtration: pre-hardening of the beam to remove lower energy photons and/or bowtie filter.
  • Calibration correction: w/a phantom.
  • Software: use iterative correction algorithms.
  • Gantry tilt: tilt the gantry or position the pt to avoid areas that can cause hardening.

Question 66

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 66

Beam hardening: streak/dark bands.

• As the XR beam passes through an object the lower energy photons are removed preferentially, leaving a harder beam w/an increased average energy.
• Occurs in the setting of 2 dense objects. XRs that pass through one are less attenuated than those that pass through both. The result is dark bands or streaks b/w them.
• Classic location: bone, or when dense contrast is used.
• Fixes:
  
  • Modern CT scanners use filters to overcome beam hardening so it’s less often seen.
  • Filtration: pre-hardening of the beam to remove lower energy photons and/or bowtie filter.
  • Calibration correction: w/a phantom.
  • Software: use iterative correction algorithms.
  • Gantry tilt: tilt the gantry or position the pt to avoid areas that can cause hardening.

Question 67

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 67

Partial volume averaging:

• When 2 tissues of very different HUs/absorption inhabit the same voxel, resulting in a beam attenuation/HU that is the average value of the 2 tissues.
• Classic location: CT angiography looking for PEs, where the vessel looks like it has a contrast filling defect.
• Fixes:
  
  • Use thin section reconstructions to decrease voxel size.

Question 68

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 68

Photon starvation:

• A type of streak artifact seen in high attenuation areas, particularly behind metal implants.
• B/c of the high attenuation, insufficient photons reach the detector & during reconstruction, the noise is magnified leading to characteristic streaks.
• Classic location: shoulders or prosthesis site.
• Fixes:
  
  • Automatic tube current modulation.
  • Adaptive filtration to correct the attenuation profile & smooth the data.
  • Iterative reconstruction techniques.

Question 69

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 69

Ring artifact:

• A CT hardware artifact due to miscalibration or failure of one or more CT detector elements in a 3rd generation scanner–these only occur in 3rd generation scanners!
• It causes error in angular position.
• Classic location: close to the isocenter of the scan & usually visible on multiple slices at the same location. Common in cranial CTs.
• Fixes:
  
  • Recalibrate the scanner.
  • Replace the faulty detector element(s).
  • These cannot be corrected by the technologist at the scanner.

Question 70

Undersampling/aliasing artifact: why does this occur and how do you fix it?

Answer 70

Undersampling/aliasing:

• This is a physics-based artifact.
• When sampling the scanned data, the computer undersamples which causes the computer to process an inaccurate image, resulting in aliasing or misregistration artifacts.
• Fixes:
  
  • Acquire the largest number of projections per rotation–slow the rotation speed.
  • Use manufacturer-employed high resolution techniques.

Question 71

Name the artifact, why it occurs, & how to correct it.

Name the artifact

Answer 71

Stairstep artifact:

• Found in straight structures that are obliquely oriented w/respect to the table motion (z-motion).
• B/c of wide collimation of non-overlapping intervals.
• Classic location: on sagittal/coronal reformatted images, often 3Ds.
• Fixes:
  
  • Use thinner slices.
  • Less severe on a helical scanner where you can decrease the pitch to get overlap.

Question 72

1. Name the artifact.
2. How do you fix this (3 ways)?
3. What is the diff b/w over-scanning & over-ranging?

Answer 72

1. Motion.
2. Stabilize the pt; use a fast scanner; align the scanner in the direction of motion; decrease pitch (over-scan); use gating.
3. Over-scanning = decreasing pitch to create overlap; over-ranging: scanning above & below the target to collect additional data to add to the helical scan.

Question 73

1. Which type of metals re: Z have the most metallic artifacts?
2. 3 ways to fix this.

Answer 73

1. High Z metals have more metallic artifact, e.g., iron, platinum.

Lower Z have less, e.g., titanium.

2. Ask the pt to remove the metal; increase the kVp; use thinner slices.

Question 74

1. What is out of field or incomplete projection artifact?
2. How does this often happen?

Answer 74

1. When body parts hang outside the field but attenuate XRs which messes w/the computer’s math.
2. W/arms beside the pt or IV contrast on/near the pt.

Question 75

Why are head CTs still commonly done w/axial scanning vs. helical?

Answer 75

To decrease helical artifact in the axial plane:

• This often occurs near the top of the skull and is worse w/higher pitch scans.
• To minimize this, reduce variation in the z-direction, so decrease pitch, use thin sections.

Question 76

The dose profile from a sequential scan of a single axial slice is shown. What is the primary source of the dose to the tissue adjacent to the scanned slice, indicated by the arrow?

Answer 76

• Compton scatter.
• At energies used in routine CT, most of the scattered XR photos are due to Compton scatter, and a minority from Rayleigh scatter or characteristic radiation from the PE.
• Scatter in the pt is not reduced by post-pt collimation.
• Scattered XR photons from the primary beam contribute significantly to the overall pt dose.

Question 77

How does increased scan length affect dose?

Answer 77

• With all other parameters being the same, pt dose increases linearly w/scan length.
• Recall, scan length is the anatomy covered in a particular scan.
• DLP = CTDI_vol x (scan length)

Question 78

Multiple scan average dose.

• Define.

Answer 78

• It’s the average dose to a slice in the central portion of an entire scan, and includes the dose from scanning that slice as well as the dose to that slice by scatter from scanning adjacent slices.

Question 79

MIPs

• How are they made?
• What information do MIP images provide, and not provide?
• Which structures are preferentially shown?

Answer 79

• Only the voxel with the highest CT # along each ray, in every view, is displayed.
  
  • That is, for each XY coordinate, only the pixel with the highest HU along the Z-axis is displayed.
• MIP images show attenuation information, not depth information.
• Bone and contrast-containing structures.

Question 80

In the following, what does CTDIvol represent?

What is DLP?

Answer 80

• It’s the scanner-specific radiation output in this exam.
• DLP = CTDIvol x scan length, so it represents scanner output x scan coverage.

Question 82

Name 4 materials that may be displayed or removed using dual-energy CT?

Answer 82

• Iodine
• Urate: gout
• Bone
• Water