CT - Chapter 2 Flashcards
Describe pros/cons of iterative reconstruction
- Reduces image noise
- improved SNR
- Reduced radiation doses
- Requires longer processing (reconstruction) times than traditional FBP
- Improves image quality
- Can potentially be used with any CT system
What are advantages of sequential (prospectively triggered) scan mode compared to conventional retrospective gating?
- Reduced scan time
- may be prolonged if HR is faster
- Accommodating functional reconstructions
- Reducing the radiation dose
****** Does not improve temporal resolution
Describe the relationship between:
- image noise (quantum noise)
- exposure
Inversely proportional
- not in a one-to-one relationship
- quantum noise is inversely proportional to the square root of the exposure
- quadruple the exposure –> half the noise
- Examples:
- XR exposure that is too short (uses too little tube current) –>
- poor quality, noisy XR
What will be the result of increasing peak tube voltage on a MDCT?
- Decrease in image noise
- Higher radiation dose
- Lower contrast resolution
Describe the effects of increasing tube voltage
Decrease in image noise
- image noise is inversely related to the square root of radiation exposure
- increased peak tube voltage (exposure) –> higher energy XR’s –> decrease the noise of the image
What does the “p” stand for?
- typical XR voltage for cardiac CT is 120 kVp
“Peak”
- referring to the peak (maximum) energy of the XR spectrum
What material has a Hounsfield unit of zero?
Water
- HU scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature is defined as zero HU units
How can the quality of the image be improved?
- Widening the window level
- Thin slices
- both above improve image quality
- Sharp reconstruction kernels/filters
- improve image resolution
What adjustments to acquisition or reconstruction will result in reduced image noise?
- Higher tube current or voltage
- Iterative reconstruction
What adjustments to acquisition or reconstruction will result in increased image noise?
- lower tube current or voltage
- increased pitch
- FBP reconstruction
What is the temporal resolution of the scan?
- dual-soucre CT scanner
- single-segment reconstruction
- 360 degree gantry rotation time of 280 ms
280 ms / 4 = 70 ms
- dual-source scanners use 2 XR tubes and 2 detectors, arranged at 90 degree angles to each other, to simultaneously acquire data
- 180 degrees of attenuation data that are necessary to reconstruct one image can be collected by only a qurter turn of the gantry, rather than the usual-half turn required in single source systems
- data can be gathered in one-half the time
Describe how to calculate temporal resolution
- Single-source scanner
- Dual-source scanner
- Single-source scanner
- Rotation time / 2 –> temporal resolution
- Dual-source scanner
- Rotation time / 4 –> temporal resolution
After reviewing the images of a CCTA, you decide to reconstruct the images using a smoother filter. Where can this be done?
At the scanner
- raw attenuation data are usually stored on a dedicated server attached to the scanner but separate from the CT workstation
- can be performed using raw data acquired via any acquisition mode
CCTA requires using thin detector collimation to avoid partial volume averaging, but the use of thin collimation comes at what cost?
Increased image noise
- very thin sections –> avoids partial volume averaging
- absolutely required for high-resolution coronary imaging
- comes at the cost of increased image noise
- can be overcome with:
- higher tube current
- special reconstruction algorithms for coronary arteries
What matrix size would make full use of the spatial resolution of the scanner at this field of view?
- CT scanner spatial resolution = 0.25mm
- Rows of detectors = 512
- Detector width = 0.25 mm
- Manufacturer’s protocol for CCTA = reconstructed field of view 250 mm
1024 x 1024
- main goal of image reconstruction is to properly match:
-
image pixel size = spatial resolution of scanner
- one cannot resolve something smaller than one pixel
- pixels much larger than 1/1 are essentially losing information
-
image pixel size = spatial resolution of scanner
- 250 mm (FOV) / 1024 (matrix size) = 0.243 mm pixels
- closets match to CT scanner spatial resolution = 0.25 mm
What is the utility of multi-cycle reconstruction (multi-segment reconstruction)?
Improve temporal resolution
- occurs at the expense of “averaging” data from several cardiac cycles into one single image
- requires that each slice position is scaned during several consecutive heart beats ( at least 2 )
- typically done in helical mode + low-pitch value
- “oversampling” typically occur in prospective, ECG-triggered axial scanning
What can be done to improve visualization of the proximal LAD?
Increase Peak Tube Voltage
- “burns through” the dense calcium and decrease this contrast
- calcium (partial volume averaging or “blooming” artifact) obscures the underlying contrast column within the vessel lumen –> very high photodensity gradient (contrast between the calcium and adjacent structures
- however, gain in visualization is not justified by increased radiation dose to patient.
What are the two ways in which dual-energy CT is performed?
Two raw datasets are collected for each scan –>
representing attenuation data from two different photon energies –>
differential attenuation (calcium and iodine) will allow for new differential (subtracted) images to be created
- layered detector array (which can differentiate):
- high energy photons
- low energy photons
- two XR-tubes (dual-source scanner) to scan simultaneously at two different XR energies (e.g.)
- 80 kVp
- 120 kVp
Describe the difference in regards to photons for each:
- tube voltage (kV)
- tube current (mA)
- tube voltage (kV)
- energy level of the photons coming out of the XR tube
- tube current (mA)
- quantity (output) of photons
*****Lowering both –> reduced radiation exposure
What is the definition / formula for pitch?
table feed (mm) per 360 degree gantry rotation
collimated beam width (mm) (or coverage)
What is the principle determinant of spatial resolution in CT?
Gantry rotation time
- determines how long it takes to acquire the attenuation data used to reconstruct a single axial slice
What do scintillation crystals in photodetectors convert?
X-ray photons to light photons
What is the size, in the z-dimension, of the voxels in the resultant reconstructed data set?
- Prospectively ECG-triggered, 256-slice scaner
- Detector collimation of 256 x 0.625 mm
- FOV = 250 mm
- Gantry rotation time = 270 ms
- Image matrix = 512 x 512
0.625 mm
- In prospective, axial scanning, the minimum voxel size in the z-axis dimension –>
- determined by minimum slice thickness –>
- determined by detector collimation –> 256 x 0.625 mm
What are negative aspects of utilizing high resolution setting on CT scanners?
Higher heat load to the X-ray tube
- may result in maximum heat capacity of the anode –>
- and limit the available peak tube voltage and tube current
- which may be needed in scanning obese patients
- may limit scan duration
If a scanner’s spatial resolution is reported to be 12.5 line pairs per cm, what is the equivalent spatial resolution expressed in mm?
0.4 mm = equivalent spatial resolution
- 10 / (resolution in line pairs x2)
- 10 / (12.5 x 2)
- 10 / 25 = 0.4 mm
How does increasing peak tube voltage decrease image noise?
Increases photon penetration
- increases the number of photons that pass through the patients (photon penetration)
- higher energy photons are less likely to be attenuated by the patient’s body
- more photons reach the detector –>
- better SNR
- reduced image noise
What must be present for multi-cycle reconstruction to occur?
gantry rotation time and HR must be asynchronous
What effect will increasing peak tube voltage have on soft tissue contrast?
lower soft tissue contrast
- as photon energy increases –>
- more photons pass through the body unattenuated –>
- tissue contrast is decreased
- example:
- XR in which photon energy is so high that the field is extremelly overexposed and “burned out”
What is the effect of increasing the gantry rotation time of a CT system from 1 sec to 2 sec without changing kVp or mA?
Increasing the mAs by 100%
- “tube current-time product” –> expressed in mAs, refers to the product of X-ray tube current multiplied by X-ray exposure time
- If Gantry rotation time is doubled (increases by 100%) –>
- time required to collect attenuation data is doubled –>
- exposure time will be doubled –>
- TCT product (mAs) will incerase by 100%
What filter results in the highest spatial resolution in the setting of coronary calcifications?
Sharp reconstruction filter
- improves spatial resolution by reducing “partial volume averaging” or “blooming artifact”
What variables influence attenuation of a tissue?
- density of tissue
- thickness of the tissue
-
energy of incident photons
- higher energy photons –> more likely they will pass through the tissue without interacting with it
What are factors that determine temporal resolution?
- gantry rotation speed
- number of X-ray sources
- ability to perform multi-cycle reconstruction
Which CT acquisition (using 64-row scanner) will have higher/better spatial resolution? Assume HR, Rhythm and Body habiuts are the same for both
- detector collimation = 64 x 0.625
- detector collimation = 32 x 1.25
64 x 0.625
- thinner detector collimation –> better ability to resolve small details because of less volume averaging
- Both scans have:
- collimated beam width (40 mm) –>
- same scan time
What is the conventional CT value (in Hounsfield Units) for air?
-1,000 HU
How is pixel size calculated?
Field of View (FOV)
Matrix size
