Chapter 10: CT scan Flashcards
type of scanning pattern where scanning pattern corresponds to attenuation meausrements that are aprallel to each other
Parallel Beam Projection
individual attenuation measurements that corresponds toa line thru the objects defined at one end of the xray source and at the other end by the detector
Rays
data collected at specified angle of interogation of the object
projection/profile/view
individual rays each correspond toa line integral that spans between the source and the detector
fan beam projection
minimal divergence of xray beam trajectory
true fan beam
non negligible beam divergence in the orthogonal direction
nrrow cone beam geometry concept
fan angle and cone angle for fan beam projection
fan angle: >60 degrees
full cone angle 2.4 degrees
seen in flat panel detector array
cone angle is almost equal to fan angle
full cone beam
correction methods needed for the different distance from source to detector
planar detector system
center of rotation of the CT gantry
isocenter
defined by the physical extent of the curved detector arrays (fan angle)
Max FOV
xray tube and detector are fixed rigidly to a rotating gantry
rotate rotate - 3rd gen
gantry rotation speed
5 rotation per second
HU of water, air, adipose, most organs, CM/Bone
Water = 0
air = -1000
Adipose = -80 to - 30
most organs = 30-220
contrast, bone = 3095
reduces the intensity of the incident xray beam in the periphery of the xray field where the attenuation path thru the px is generally thinner
bow tie filter
modern multidetector CT scanner uses _____ solid state detectors
indirect
in a single array CT, what determine slice thickness
xray beam collimation
in MDCT, what determines slice thickness and xray beam width
slice thickness = detector configuration
xray beam width = collimator
table moves at a constant speed without stopping while the gantry rotates
Helical/spiral acquisition
describe the relative advancement of CT table per rotation of the gantry
pitch
pitch used for thoracic and pedia where speed is imp
1.5 pitch
1.0 = continguous axial CT <1 = overscanning, higher rad dose >1 = underscanning, lower rad dose
challenges related to cardiac CT
temporal resolution
projection data are acquired over the enetire cardiac cycle using ECG gating info
retrospective construction
ECG triggers the xray tube such that data are acquired only during END DIASTOLE
Prospective gating
used to identify blockage in the coronary artery
retrospective gating
Designed primarily for cardiac imaging
kV can be switched during acquisitionto achieve dual energy data
Dual energy CT
allows a CT scanner to repeatedly image a volume of tissue wider than the detector array
Shuttle mode CT
identifies regions in teh data with low signals
adaptive noise filtering algorithm
projected value thru each line passing thru the patient is linear sum of the voxel values that each ray intersects
simple back projection
creating projection from central data
forward projection
computing the image matrix from the projection values can be achieved
back projection
a CT reconstruction method used in nuclear medicine application, such as SPECT, and produce images with higher SNR at the same dose.
Iterative reconstruction
presence of densestructures such as bones on xray path cause the lower energies in the xray spectrum to preferentially attenuate compared to the higher energy photons
beam hardening
attenuation levels of a region in the patient are excessibe, which can exceed the dynamic range of the detector systems or the effective linear range of detectors
Streak artifact
use of few projection images acquired to reconstruct high frequency objects in the image
View aliasing
CT voxels are large enough to encompass several types of tissues, such as bone and tissue or tissues from different organs
Partial volume
stack of alternating disks separated by low density material which can be used to evaluate cone beam artifacts
Defrise phantom
angular sampling due to the relationship of the detectors and xray source
ring artifact
Determine the generation translate-rotate pencil beam fan beam rotate-rotate rotate-stationary
1st gen
2nd gen
3rd gen
4th gen
CT tubes has a power rate of
5-7MJ
Sintered to increase physical density and light output
Intensifying screen:Gd2S2O
Heart is most stationary
End diastole
Allow the CT gantry to rotate freely and eliminates need to deal with gantry inertia between scans.
Eliminated cable connections
Slip ring
