CT Flashcards
How do filters affect the HVL?
Increase HVL (due to increased energy of electrons)
What does iterative reconstruction do?
Alloweds for increased noise which decreases patient dose
What determines minimal thickness of a slice?
Minimal detector aperture width
How do you determine how many shades of gray are in a pixel?
2^bit
What is Pitch?
Distance Table Moved/Beam Width
What does Pitch <1 Mean?
- More Overlap
- More Dose
- Better spatial resolution
What is the relationship between X Ray attentuation and Houndsfield units?
Increase in HU by 10 attenuates X rays by 1%
How does kEV affect HU?
Closer you are to the k edge, more PE, which means more attenuation and higher HU
How does Filtration affect HU?
Decreases HU: Increase filtration= increased average energy of the beam= less PE effect (less aborption, more transmitted)
What is Axial Acquisition?
Table is stationary and the Tube Steps and Shoots
- Better Spatial resolution in Z dimension
- Less Partial Volume averaging
What is helical acquisition?
Tube is constantly on and the Table Moves
-Less stair step artifact
Sharp versus smooth kernal?
Sharp: More spatial resolution and more noise
Smooth: Less spatial resolution and less noise
What is cardiac imaging best acquired?
Diastole
Prospective versus Retrospective
Prospective: Always Axial, less dose, no functional imaging
Retrospective: More dose, functional imaging
Contraindications to giving B Blocker for Coronary CTA
- Asthma
- Severe COPD
- 2nd or 3rd degree heart block
- Active Bronchospasm
- Decompensated Heart Failure
- BP <100 and HR <60
Contraindications to giving Nitoglycerin
- Severe Aortic Stenosis
- HOCM
- PDE inhibitor with last 48 hours
How does X Ray Flux effect SNR?
- Doubles Signal
- Increases the noise by square root of N
-Need to increase Flux 4 X to double the SNR
How do you increase SNR?
- Increase mA
- Increase KVP
- Increase slice thickness
- Increase rotation time
- Larger Pixel Size
- Decrease Pitch
How does Pixel Size affect Spatial and Contrast Resolution?
Small Pixel
- Increases spatial resolution
- Decreases contrast resolution (less photons in a nox)
How does slice thickness affect spatial and contrast resolution?
Increased slice thickness
- Decreases Spatial resolution
- Increased contrast resolution (less noise)
What is CT Dose index (CTDI)?
Radiation dose Normalized to Beam Width
What is Weighted CTDI?
2/3 Peripheral CTDI + 1/3 Central CTDI
What is Volume CTDI?
Weight CTDI/Pitch
What is Dose Length Product?
Volume CTDI X Length of Scan in cm
What is effective dose? (Sv)
DLP X K (body part constant)
How does CTDI Volume affect small and large patients?
Based on a phantom of 32 cm If smaller (kids)= Underestimates the dose If larger (fat)= Overestimates the dose
What is the risk of radiation induced cancer in adults and Children?
5% per Sv in Adults
15% per Sv in Children
How to you adjust CT parameters for obese patients?
- Increase mA (will increase Dose)
- Increase mAs (Increase scan time will increase Dose and motion(blur))
- Lower Pitch (increases Scan time and dose)
- Increase kVP (increases Scatter which decreases contrast resolution)
How do you decrease beam hardening?
- Filter the low level photons out (prehardening)
- Corrective Software
- Calibration correction
- Avoidance (Tilting gantry)
How do you fix partial volume averaging?
Thinner slices
How do you fix photon starvation?
Automatic Tube modulation (increase the kVP and mA)
How do you fix metal artifact?
- Increase kVP
- Thinner Slices
How do you fix stair step artifacts?
Thinner slices
