CT physics Flashcards
What is the definition of pitch?
Pitch= table movement/ beam width
Pitch 1= no overlap between slices
Pitch>1= table moved faster than the beam and you have a gap between slices
Pitch<1= table moved slow and your slices overlapped… dose is high but you have better spatial resolution (Trade off between spatial resolution and dose)
What is the tradeoff of a mathematical filter (kernel)?
Spatial resolution versus noise
Sharp kernel versus smooth kernel?
- Sharp kernel= images with high spatial resolution but increased noise (i.e. bone)
- Smooth kernel= low noise but reduced spatial resolution (i.e. brain kernel)
Benefit of axial CT acquisition over helical? Benefit of helical over axial?
Axial-
- Less partial volume artifact
- Better spatial resolution in the z dimension
Helical-
- Way faster!
- Less motion breathing artifact
- Less stair step artifact and better localization (because overlapping)
What type of solid state detector is used for CT?
Scintillator detector “good geometric efficiency” Way better than gas-filled detectors which aren’t used anymore
What defines the slice thickness on single slice CT? Multi slice CT?
- Single slice= collimation
- Multiple slice= width of the detector rows
Purpose of a pre-patient collimator?
- Shapes beam
- Lowers dose
Purpose of a pre-detector collimator?
- Shapes beam
- Adjusts the width of the attenuated beam
- Removes scatter radiation
HVL is assumed to be for Xray?
for CT?
for mammography?
- 3 cm
- 3-4 cm
- 1 cm (because lower energy beam quality)
Note a highly filterd beam can have a HVL of up to 10 mm Al (i.e. 120 kV - HVL 9 mm, 140 kV- HVL 11 mm)
4 benefits of a Filter
- Reduces Noise
- High Average Energy
- More uniform (monochromatic) beam
- **Less beam hardening within the patient***
What type of filter is used for CT?
- Copper or Aluminum (6 mm)
- Note anode cathode axis is perpendicular to reduce heel effect
CT xray tube characteristics?
Tungsten alloy with high speed rotating anodes
What are typical mAs for CT?
High up to 1000 (versus xray 200-800 mAs)
What are typical kVps for CT?
80-120 kVp **120 kVp is the workhorse** (versus x ray 50-120) Remember you use a highly filtered, high KV (average 75 keV) for CT
What focal spots are used for CT?
0.6-1.2 mm (vs xray 1.0 - 1.2 mm)
Two benefits of iterative reconstruction?
can better correct for noise… so you can use a lower dose
Formula for HU
hu= 1000 x (attenuation of material- attenuation of water)/attenuation of water
What is the relationship between HU and xray attenuation?
When HU increases by 10 HU, xray attenuation increases by 1%
How does keV change HU?
In general, lower keV will create a higher hU The closer you get to the k-edge of a given substance, the more impressive increase in attenuation (and HU) there will be. Ex. IV contrast will have a HU of 100 at 140 kV but will be 400 HU at 80 kV (where it lands on its k edge)
As photon energy decreases how do HU changes?
Positive HU will become more positive Negative HU will become more negative
Can filtration alter HU measurements?
Yes… ex. Cupping artifact
What is a typical window and level for the Brain?
W 80, L + 40
What is a typical window and level for lung?
W 1500, L -400
What is typical window level for the abdomen?
W 400, L +50
What is typical window level for bone?
W 1600, L +500
When do you increase the kVp?
Obese patients (ex. problem here is scatter… causing decreased contrast resolution) Reduce beam hardening/metal artifacts Battling a thick skull (ex. increase to 140kVp in the posterior fossa due to the skull)
What has a bigger impact on dose kVp or mAs?
kVp
Does automated exposure increase radiation dose to the patient in CT?
Yes. This is unlike conventional radiology. It works by altering the mAs and not kVp
What is the effect on noise and contrast with increasing kVp?
Image noise decreases Lesion contrast decreases
How does dose increase relative to increases in mAs versus kVp?
mAs= proportional increase kVp= square of kV
When would you decrease the kVp?
Children (has a bigger impact on dose than mAs) - mLimiting factor is penetration power (esp for bigger kids) - but lower the kVp will often improve the contrast resolution for thin children - Often 80kVp is good for peds
General kVp levels in children?
<10 kg= 80kV 10-20 kg= 100 kV >20 kg = 120 kV
What is the k-edge of barium? What is the k-edge of iodine?
Barium 37 keV Iodine 33 keV
What kVp would you pick if using barium or iodine to take advantage of the k-edge?
65-90 kV… Because the average energy is going to be between 1/3 and 1/2 the maximum energy
What type of Z material makes a more pronounced change in HU?
Higher Z material will have a greater change in HU with changing kV
What kVp would you use for a CT angio study?
80-100
mAseff- Effective mAs definition?
mAs/pitch
This is how the automatic exposure control works
You reduced the pitch, but the CTDIvol did not change. What happened?
The scanner adjusted the mAs to maintain the effective mAs (= mAs/pitch)
Strategies to reduce breast dose (3)
Do the scan at a lower mAs (bad image quality) Use milliampere modulation (preferred) Shield the breasts with bismuth (artifact and a degraded image– beam hardening also elevates the HU deep to the shield)
mA versus mAs?
S- time per rotation of the gantry (the s in mAs) problems with increasing the time per rotation - longer scan = more motion - more mAs= higher dose in a linear fashion
Solutions for scanning a larger individual (4 but only 1 is usually used)
- increase kVP (dose increases in a non-linear fashion but often increased to 150kVp for penetration power) - increase mA (increase the dose in a linear fashion and you have to increase the mA by a factor of four to double your SNR) - mAs (increase time per rotation of the gantry) but this increases dose in a linear fashion and increases movement due to increased scan table rotation time - decrease pitch (doesn’t work well and scan time and dose increase)
Increasing the beam width does what to the scan time?
Reduces it- Pro
Increasing the beam width does what to the motion artifact?
Reduces motion artifact (less scan time)- Pro
Increasing the beam width does what to the partial volume artifact?
Increases the partial volume artifact (more divergent beam)- Con
Decreasing the beam width does what to the patient dose?
Increases the dose - Con
What improves overbeaming?
The more detectors you have and the wider your beam
How does pitch affect overranging?
More pitch= more overraning
6 ways to improve SNR?
higher mAs
Longer rotation time
Higher kVp
Larger slice thickness
Larger pixel size
Decrease pitch
How does increasing the mA improve the signal to noise?
You need 4x the mA to double the SNR (or a 4x the Ma to cut the noise in half) it decreases quantum mottle
What it the typical spatial resolution for CT?
0.5-0.7 Lp/mm (ex. mammo is 7- 11Lp/mm)
What determines transaxial spatial resolution in CT?
- Focal spot Smaller the better!
- Smaller pixels, better spatial resolution Pixel= DFOV/Matrix size
- Increasing the matrix size or decreasing the FOV will make your pixels smaller
What determines spatial resolution in the z plane?
- Detector size Smaller the better!
- Thicker slices= less spatial resolution - as pitch increases, so does the slice sensitivity profile… when that widens, so does slice thickness
- Reconstruction slice thickness
Holding matrix size constant, you decrease the FOV. What is the effect on pixel size, contrast and spatial resolution?
Pixel size = FOV/matrix size You decrease FOV, you decrease Pixel size. This improves spatial resolution but worsens contrast resolution (less photons per box)…. and vice versa
Five factors that improve spatial resolution
- Smaller focal spot
- Smaller detector width
- Once the limitations of focal spot blur and detector blur have been reached, larger matrix sizes will not improve resolution
- More projections- Oversampling the data
- Sharp Reconstruction filter (i.e. bone)
- Reducing x-ray tube rotation times will minimize motion blur
- Zoom reconstruction which improves central image region resolution
- Smaller pixels (assuming above is true) Pixel size = DFOV/Matrix size
Four factors that improve contrast resolution
- Number of xrays (mAs or kVp, pitch)- more dose (less mottle) will improve resolution 2. Thicker slice thickness (thicker- more xray quanta= less noise) 3. Reconstruction method (Iterative > Filtered back) 4. Soft tissue reconstruction Filter
What is the matrix size for CT?
512 x 512 with each pixel representing 4096 possible shades of gray (12 bits) 2^12= 4096
What happens if you turn down the mAs?
MORE NOISE! no good
What is the image time with CT fluoro? What is different?
constantly updated 6 frames per second is common LOW TUBE CURRENT (20-50 mA) to minimize radiation dose
ACR limit on CTDIvol for Adult abdomen? Ped abdomen? Head CT?
Adult abdomen = 25 mGy PED abdomen = 20 mGy Head CT = 75 mGy
What is the weighted CTDI?
This is 1/3 central CTDI + 2/3 peripheral CTDI (expressed in mGy) *note this is underestimated for peds and overestimated for larger patients*
What is Volume CTDI?
weighted CTDI/pitch
How big is the body phantom?
16 cm (head) or 32 cm(abdomen) if the patient is larger than the phantom (obese), the dose is overestimated… if the patient is smaller than the phantom (peds), the dose is underestimated
What are the average doses in CTDI (mGy) for adult head? adult abdomen? peds abdomen?
Adult head= 58-60 mGy (effective dose 1-2 mSv)
Adult abdomen = 15-18 mGy (effective dose 8-11 mSv)
[this may be increased to 20 mGy if the purpose is detecting soft tissue lesions]
Peds abdomen = 15 mGy (embryo dose in CT AP is 30 mGy)
What is the DLP?
Dose Length Product DLP= CTDIvol x the length of the scan in cm mGy*cm
What is the effective dose?
effective dose = k x DLP k= body part constant Sv
What is the risk of radiation induced cancer per Sv?
- 5% risk per Sv in adult
- 15% risk per Sv in child
- 1/10 of that for someone older than 50
- There is an average risk of 0.05% for a uniform whole body dose of 5 mSv in a 25 year old individual
What is the DLP for a standard abdomen and pelvis CT?
Chest CT?
AP: 500 mGy*cm
Chest: 300 mGy*cm
What is the kV for a neonate on a non-contrast CT?
80 kV
What is the best achievable temporal resolution for a standard CT?
For dual-source CT?
half the tube rotation time (~150 msec)
What is the standard spatial resolution for CT versus radiology versus mammography?
CT = 0.7 lp/mm
Radiology = 3 lp/mm
Mammography = 7 lp/mm
What is partial volume artifact?
Result of averaging the linear attenuation coefficient in a voxel that is heterogenous in composition.
What causes cone beam artifacts?
Inadequate anatomical coverage
What causes aliasing artifact in CT?
inadequate data sampling when edges are imaged
What are the five most dose sensitive organs and what doses are they sensative to?
- Red Bone Marrow (25 mGy)
- Lung (35 mGy)
- Colon (30 mGy)
- Stomach (30 mGy)
- Breast (25 mGy)
What is the embryo dose in a standard CT AP of a pregnant patient?
What about chest CT?
30 mGy (mostly due to scatter so an apron will not help)
Chest CT= 0.1 mGy
Contraindications to beta blocker therpay
- Asthma
- Severe COPD
- Hypotension
Treament for bradycardia or hypotension with beta blockers?
Treatment for bronchospasm?
- ABS
- fluids
- atropine 0.5 mgIV
- glucagon (5 mg IV bolus or 50 micrograms/kg IV)
- For bronchospasm– albuterol
What is CTDIvol?
CTIDIvol= CTDIw/pitch
Mechanism of nitroglycerine? What are the contraindications?
Mechanism: dilates coronay arteries (vasodilator)
Contraindications:
- Aortic stenosis (severe)
- HOCM or RV infarctoin
- severe bradycardia
- PDE-5 inhibitor for up to 48 hours prior to the exam
What is the best way to increase contrast?
Decrease kVp
Decreases the number of photons that make it to the receptor, making it a grainier image unless the mAs is also increased.
What is the best way to increase contrast-to-noise ratio?
Increase mAs or decrease kVp while increasing mAs
May increase dose to patient
How are scan characteristics usually changed for pediatric CT?
- Decrease kVp (i.e. from 120 to 80)
- Decrease mA
- Decrease sFOV
- Sometimes slight increase in mA to decrease noise
- Reduces dose while increasing contrast. Since no phototimer in CT, decreased kVp with a fixed mA and scan time = decreased overall # xray produced, decreasing CTDIvol despite more PE absorption.
How are scan characteristics changed for obesity?
- kV is increased to 140 for regular imaging and 120 for angiography
- Improves photo-starved (grainy images) because higher energy xrays penetrate the patient more easily
How do you change the scan characteristics for Angiography or post contrast CT?
kVp = 100
Increases vessel and tissue contrast by choosing kVp so that the kVaverage is just above the kedge of iodine (33 keV) but even higher than DSA’s 70 kVp to avoid grainy images
What is the best way to remedy stair step artifact?
Cuased by reformatting algorithm
- decrease beam width (collimation)
- overlap reconstruction intervals
What is equivalent dose? How does it compare to Absorbed dose and effective dose?
mSv
Accounts for radiation type.
What is the csL?
collimated slice length= width of the xray beam
What are three benefits of a bowtie filter?
- Compensate for uneven filtration
- Reduce scatter
- Reduce dose
- (Note they use a low Z material such as teflon)
If you increase the beam width does the dose change?
No
Compared to radiograph xray tubes, those used in CT most likely have higher?
A. tube voltage
B. tube currents
C. electrial power
D. Anode capacity
D. Anode capcity
Anode heat capcities are 5 MJ which is 10x higher than normal radiographic and fluoroscopic xray tubes
Xray tubes lose heat at a rate of 10 kJ per second (10kW)
Ther percentage (%) of incident xrays that are absorbed by a CT detector is __%?
>90%
CT detecros are usually thick scintillators with high Z that absorb most of the incident xray photons
For CT xray tubes, maximum power loadings (kW) on the small and large focal spots are __ and __, respectively.
25 kW and 100 kW
For CT xray tubes, 25 kW can be applied to the small focal spot (0.6 mm) and 100k W to the large focal spot (1.2 mm)
Increasing the size of antiscatter lamellae in CT detector arrays would most likely result in better:
A. Contrast (i.e. increased)
B. Resolution (i.e. increased)
C. Mottle (i.e. reduced)
D. Artifacts (i.e. less)
A. Contrast (i.e. increased)
Longer antiscatter lamellae will reduce scatter and increase image contrast
Geometric magnification at the isocenter in acquiring CT projections i most likely?
- 1.1
- 1.5
- 2
- 3
2
Geometric magnification is 2, with a typical focus (i.e. xray tube) to isocenter distance of about 60 cm, and double this distance between the focus and the CT detector array.
A CT scanner with x-ray tube rotation time of 0.5 seconds, beam width of 40 mm, and table speed of 160 mm/second has a pitch of?
- 0.5
- 1
- 1.5
- 2
2
In one xray tube rotation (0.5 seconds), the table moves 80 mm, twice the xray beam wdith (40 mm), so the pitch is 2.
What reconstruction method is mot likely to result in the best CT images?
Iterative reconstruction
**Model-based***
Difference in x-ray attenation coefficient between gray and white matter is most likely (%)?
- 0.2
- 0.6
- 2
- >2
0.6%
HU of gray matter is about 6 HU higher than that of white matter, and where each increase in 1 HU corresponds to an increase of 0.1% in attenuation.
Pixel dimensions (mm) in head CT are most likely?
- 0.1
- 0.2
- 0.5
- 1.0
0.5 mm
A head is about 250 mm and a CT image is typically a matrix size of 512^2 so the pixel dimension is 0.5 mm (i.e. 250 mm divided by 500).
Body CT FOV is 400 mm
What does 1 CT image need for storage space?
0.5 MB
A head CT examination with 20 images most likely requires a storage space (MB) of about:
- 1
- 10
- 100
- >100
10
8 bit in 1 byte.
1 pixel requires 12 bits but you must add on an extra byte to hit twelve…. so need 2 bytes per pixel)
1 Image is 0.5 mB of storage space
Which CT image most likely uses the greatest dispay window width?
- Head
- Chest (Mediastinum)
- Chest (lung)
- Abdomen
Chest lung
Widow width 1500, level -500
what is the ACR reference dose CTDIvol (mGy) for
Adult head (small phantom)?
Adult abdomen (large phantom)?
Pediatric abdomen (small phantom)?
Pediatric abdomen (large phantom)?
Adult head 75 mGy
Adult abdomen 25 mGy
Pediatric abdomen 20 mGy
Pediatric abdomen 10 mGy
These are the refrence doses and anything above them is bad
The HVL (mm Al) of a CT xray beam (120 kV) is most likely?
9 mm
The DLP for a routine chest CT in an adult is most likely (mGy-cm)?
- 3
- 30
- 300
- 3000
300
The examination would use a CTDIvol 10 mGy x scan length of 30 cm
Limiting spatial resoluition in CT is most likely (lp/mm)?
0.7 mm
Contrast HU between fat and soft tissues in a CT scan is?
150 HU
Difference betwene fat (HU = -100) and soft tissue (HU=50) is 150
Contrast in CT images is most likely to _____ with increasing mAs?
- Increase
- Be unchanged
- Decrease
Be unchanged
When mAs quadruples, the amount of CT noise (mottle) is most likely??
Havled
Increasing the CT xray tube voltage at constant mAs most likely ____ contrast and ____ image noise.
- increases; increases
- increases; decreases
- decreases; increases
- decreases; decreases
Decrease, decrease
Higher voltages (photon energies) reduce contrast, but high voltages increase x-ray tube output as well as patient penetration, so image mottle will be decreased as well.
What is the multiple scan average dose (MSAD)?
It is the average dose to a slice in the central portion of a scan that also takes scatter from adjacent slices into account.
