Image Production Flashcards
Focal spot size range
0.5 and 1.2 mm
mA range
30 - 800 mA
Photon fluence
Quantity of x-ray photons passing through a specified area (mAs value directly controls photon fluence)
Photon flux
The rate at which a quantity of x-ray photons (fluence) passes though a unit area over a unit of time
Effective mAs
Calculated mAs per acquired slice with a MSCT system
Effective mAs formula
mAs / pitch
kVp range
70 - 150 kVp
Beam pitch formula
table feed per rotation / total collimation
Crystal materials
- Cesium iodide
- Cadmium tungstate
- Ceramic gadolinium
- Oxysulfide
- Scintillating gemstone
- Bismuth germanate
First generation
- Pencil beam
- Two detectors
- Translate (1 degree) and rotate (rectilinear or translate-rotate scanning)
- Head imaging only
Second generation
- Fan beam
- Increase in number of detectors
- Greater increments of rotation
- Translate and rotate (rectilinear or translate-rotate scanning)
Third generation
- Fan beam
- Expanded curvilinear array of detectors rotates with tube
- Rotate-rotate geometry
- Rotate only
Fourth generation
- Fan beam
- Stationary circular detector array
- Rotate-stationary geometry
- Only tube rotates
Linear attenuation coefficient
Value assigned to ability of an object to attenuate x-ray beam
Examples for cone beam reconstruction algorithms (Used to overcome beam divergence)
- Feldkamp-Davis-Kress (FDK)
- Advanced single-slice rebinning (ASSR)
CT number formula
((ut - uw)/uw) x K
K - contrast factor, 1000
HU Dense bone
3000
HU Muscle
50
HU White matter
45
HU Gray matter
40
HU Blood
45
HU CSF
15
HU Water
0
HU Fat
-100
HU Lungs
-200
HU Air
-1000
Max number of pixel CT numbers
2^k
k is bit number
Method of externally viewing a 3-D reconstructed object
Orthographic volume rendering
Viewpoint of being within the lumen of the object
Perspective volume rendering (also referred to as immersive rendering)
3 general types of noise
- Quantum noise
- Electronic system noise
- Artifactual noise
2 main types of digital image compression
- Lossless (reversible) compression
- Lossy (irreversible) compression
The average energy of photons in the x-ray beam is represented in units of:
kilo-electron volts (keV)
kVp also known as:
Potential difference
Intensity also known as:
Quantity
Energy also know as:
Quality or hardness
Contrast resolution also known as:
Low-contrast detectability or sensitivity of the system
What is the most important factor influencing contrast resolution in CT?
Image noise
Aliasing is cause by:
Under-sampling
Rotation time
0.2 - 1 sec
Gantry includes
- X-ray tube
- Detectors (detector array)
- Collimators
- High-voltage generator
- ADC
- DAS
- Slip ring
Angle of the tube and detectors in relationship to the patient position during scout acquisition
Azimuth
Overranging or z-axis overscanning
Applying radiation dose before and after the acquisition volume to ensure sufficient data collection for the interpolation algorithms of helical CT reconstruction
Isotropic
Voxels with equal dimensions along the x-, y-, and z-axes
CT system’s response to a spatial frequency
Modulation transfer function (MTF)
In-plane spatial resolution of modern MDCT system can reach levels of:
25 lp/cm or more
Able to resolve objects smaller than 1 mm
MDCT systems are capable of differentiating adjacent objects with attenuation differences as small as:
3 HU
Spatial resolution is improved by:
- Small focal spot size
- Small detector size
- Edge enhancing reconstruction algorithm (bone or lung)
- Small DFOV
- Large matrix size
- High sampling frequency
- Thinnest beam collimation (acquisition slice thickness)
- Decreased pitch
- Reconstruct with thinnest detector collimation
- Increased rotation time
- Thinnest acquisition and reconstruction slice thickness
Contrast resolution is improved by:
- Use of contrast to augment inherent subject contrast
- Smaller part
- Increased technical factors
- *Thin beam collimation (to reduce scatter)
- Soft tissue reconstruction algorithm
- Narrow WW
- Thicker detector collimation (to reduce noise due to decrease in photon flux in decreased section width)
- Decrease noise
- Thicker acquisition and reconstruction slice thickness
Noise is improved by:
- Increase photon flux (increase technical factors)
- Increase voxel dimension (but risk partial volume effect)
- Decrease pitch
- Better detector sensitivity and efficiency
- Smaller part (or utilize ATCM)
- Soft tissue reconstruction algorithm
Temporal resolution is improved by:
- Decrease rotation time
- Increase acquisition slice thickness
- Increase pitch
- Segmenting data acquisition process
- Physiologic gating
Nyquist theorem
Data sampling frequency must be at least twice the object’s spatial frequency in order for the object to be resolved by the CT system (related to spatial frequency)
SSP, PSF, FWHM
Longitudinal spatial resolution
MTF
In-plane spatial resolution
Average photon energy of primary beam used in CT
Approximately 70 keV
Analytic methods of CT image reconstructions include:
Filtered back-projection and Fourier transform method
Linear attenuation coefficient of water
0.206
Effective section width is at:
FWHM of the SSP
Io
Incident intensity
I
Transmitted intensity (intensity of radiation passing through the tissue )
A form of parallel processing used by a computer to improve computation speed
Pipelining
CT system power output
60-100 kW
