Module 1 Flashcards
Allows the superimposition structure to be lurred and thus leaving the area of interest in maximum focus.
Axial tomography
Axial tomography is also called
Conventional tomography
2 types of axial tomography
sagittal and coronal
hounsfield unit also reffered to as
ct number
Goals of CT:
Overcome superimposition if structures
Improve image contrast
Measure small differences in tissue contrast
1st mathematically principle developed
It is possible to build up an image of two or three dimensional object from a large number of projections from different directions.
1917 by J. Radon
cross section portion of body scanned is called
slice/ cut
two dimensional array of numbers arranged in rows and columns is called
Matrix
Each square in matrix is called
Pixel or picture element
3D
represents tiny block of tissue called
voxel
numbers in the image matrix is called
ct number or hounsfield unit
restricts xray beam to specific area
controls slice thickness
reduce pt dose
improve image quality
Functions of Collimator
Advantage of this CT detector is to remain stable under pressure, cheaper, and easier to calibrate.
Can be used in third gen scanner.
60 - 87% absorption
Xenon gas detector
This gas must be kept under pressure in an aluminum casing
Xenon gas
Advantage of this CT detector is it has a high coefficient of 100%
Solid state crystal (Scintillation Detectors)
determines the radiation dose profile & pt radiation dose
Prepatient Collimator
determines the sensitivity profile & improves contrast
Postpatient/ Predetector Collimator
captures radiation beam from the patient & converts it into electrixal signals then be converted into binary information
CT Detector
Causes of ring artifacts
single detector
multiple detector
malfunction
The smaller the pixel size, the better the:
spatial resolution
Factors affecting Noise:
kVp & Filtration
Pixel size
Slice thickness
Detector efficiency
Pt dose
describes the amount of the CT number of a material is exactly proportional to the density of this material
must be calibrated frequently so that water is consistently represented by CT number 0
Linearity
EMI Mark 1
1st Generation Scanner
T/F
5th generation scanner uses translate-rotate principle
F
1st & 2nd generation
for head ct scan only because abdomen or chest has motion
1st Generation
more throughput, more information can be scanned
20 second image acquisition
able to rotate at 6°
2nd Generation
complete rotation of XRT and detectors
scan time about .5 seconds or less
3rd Generation
complete rotation of XRT around stationary detectors ring
fan beam geometry
4th Generation
no XRT
uses electron gun
EBCT & DSR
5th Generation
DSCT Scanner
Image moving structures such as heart
Minimize motion artifacts
Have multiple XRT and detectors
6th Generation
number of line pairs per unit length is called
spatial frequency
1 bar & its interspace of equal width
line pair
Factors affecting Image Quality
Spatial resolution
Contrast resolution
Temporal resolution
Noise
degree of blur or the ability to see the difference between two objects that are close together
Spatial resolution
ability to differentiate between small differences in density within the image
contrast resolution
Factors that improve Temporal resolution
Multidetector
Tube/Gantry rotation time
Dual source CT
good for cardiac CT
freeze any motion of a scanned object
Temporal resolution
scan parameters affecting contrast
slice thickness
reconstruction algorithm
image display (window width)
scan parameters affecting spatial
SFOV
DFOV
Matrix
Reconstruction slice thickness
Detector aperture width
Slice thickness
made of carbon filter
strong, rigid
doesn’t absorbs xrays coming from pt
patient couch/table
quick heat transfer but heavy
all metal disk design
base body is graphite
high heat storage capacity and faster anode cooling
brazed graphite anode disk
can accommodate large, lightweight disks with large heat storage capacity and fast cooling rates
chemical vapor deposition graphite disk
removes long wavelengths xrays
shapes the energy distribution across the radiation beam by using shaped filter
Filtration
residual gas molecules present within the evacuated envelope
tube arcing
topography
Allesandro Volta
made many advances in rotational scanning
David E. Kuhl & Roy Q. Edwards
derived a mathematical theory for image reconstruction
he also applied reconstruction technique in NucMed
Allan Mcleod Cormack
inventor of clinical Computed tomography
Godfrey Hounsfiled 1967
obtained readings from a specimen of Human brain
James Ambrose 1971
1st whole body ct scanner
Robert Ledley 1974
Major limitation of Radiography
Superimposition of anatomical structure
Radiography is qualitative rather than quantitative
reduction of energy of xrays
attenuation
