Computed Tomography I Flashcards
how many generations of CT scanner have been produced?
four
which generation CT scanner do we use today? Why?
we use the third generation CT scanner nowadays, fourth was developed but increased patient dose so was abandoned
what does CT involve?
sectional imaging of the patient in the axial plane
name equipment required for CT?
X-ray tube, high voltage generator, collimator, detector array mounted on a rotating gantry
what is the axis of rotation referred to in CT?
The Z-axis
is there collimation in CT?
yes
what is the aim of collimating in CT?
to produce a wide fan beam to cover the full cross section of the patient
what does the gantry contain?
the x-ray tube, x-ray generator, detector
can the gantry be tilted and what conditions would this be done?
gantry can be tilted, this is often done during brain CT
one complete rotation of the gantry = ?
one complete rotation of the gantry = one complete data set
what is used by the computer to reconstruct the images?
the complete data set
what is the anode-cathode parallel to?
the anode-cathode axis is parallel to the Z-axis
CT uses high ___ and __
CT uses high kVp and high mA
what types of radiation does CT scanners have?
bremsstrahlung and characteristic radiation
what cools the oil around the CT scanner and why?
the heat exchangers, this is because CT scanners have a high heat capacity
what is the function of slip rings?
to provide electric current
what is a feature of slip rings?
copper or graphite brushes that rub against the slip rings to produce electric current
what type of x-ray beam do you need for CT to produce a reasonably monochromatic image for reconstruction?
you need a monochromatic beam!
how does CT achieve a mono-energetic beam?
by using high filtration
what type of filter is commonly used in CT?
Copper filters, removes low energy x-ray photons
what is the purpose of the bow tie filter?
the bow tie filter equalises noise levels, allowing for varying thicknesses of the patient
where is noise greatest in a CT image?
noise greatest at the centre of the imaged area and lowest at the periphery
what does the bow tie ensure?
bow tie filter ensures that all detectors receive the same exposures
what is the bow tie filter made of ?
teflon
purpose of beam-shaping/bow tie filter?
the purpose of the bow tie filter is to reduce the dynamic range of signal recorded by the detector, reduce dose, normalise beam hardening, aid calibration
what can be adjusted to produce a range of slice widths
the z-axis dimension of the fan beam
what determines slice thickness?
collimator
what sets the upper limit of slice thickness?
the detector width
if kVp and mAs stay the same, what increases linearly with slice thickness
the number of photons increases linearly with slice thickness if kVp and mA stay the same
what does the Slice Sensitivity Profile depend on?
the SSP depends on the finite size of the focal spot, penumbra of the collimator, use of multiple projections to produce image, helical scanning further increases SSP
How many projections does a full rotation of the x-ray tube need
1000 projections
desirable properties of detector
high DQE, small size, fast response, negligible dead time, stable, noise free response and a wide dynamic range
what is involved in a helical CT scanner?
the patient is moved continuously through the scanner as the gantry rotates around the patient
what is the scan pitch?
table movement per rotation
/
nominal slice thickness
eg: patient moves 10mm/rotation, slice thickness is 2mm, therefore scan pitch is 2
what is the effect of an increased scan pitch?
decrease scan time, decrease dose, decrease image quality due to Z-axis blurring
