Ct Imaging Flashcards
are being used to create a gradient in the intensity of the X-ray beam
They are sometimes called “bow-tie” filters
They are mounted close to the X-ray tube.
The purpose of is to ⚫ reduce the dynamic range of the signal recorded by the CT
detector • Reduce the dose to the periphery of the patient
• Attempt to normalise the beam hardening of the beam - to aid with calibration
Beam shaping filters
CT scanner detectors
⚫___-__ detector elements along the detector arc •
800-1000
___-___detectors along z-axis
1-320
CT detectors are ___ in the axial plane (x-y plane), and ____ along the longitudinal axis (z-axis)
curved
rectangular
2 Detector type of CT & THEIR EFFICIENCY
Xenon gas filled-70%
Solid state- approaching 100%
Better detection efficiency
Solid state detector
Solid state detectors are generally
____
the photons interact with the detector and generate light.
scintillators
The light is converted into an electrical signal by ____
photodiodes
The septa and the strips of the anti scatter grid should be as __ as possible, since they reduce the effective area of the detector and thus reduce the detection of X-rays.
small
Detector sizes are the effective size at the ____
iso-centre
The minimum number of detector elements should be approximately ___to achieve a spatial resolution of d in the reconstructed image
2 FOV)/d
→___- detector elements are required to achieve aspatial resolution of 1 mm within a reconstructed image at a field of view of 400 mm
800
Spatial resolution can be improved by use of the ______ or_____
quarter detector shift
Dynamic focal spot
By shifting the detector elements by a distance of a quarter of the size of the detector elements, the theoretical achievable spatial
resolution becomes twice as good. It is generally implemented in detectors of all CT scanners.
Quarter detector shift
• Focal spot position on anode is rapidly oscillated during gantry rotation, doubling the number of projections
Dynamic or flying focal spot
With the current detector rows and/or quarter detector shift and/or flying focal spot a spatial resolution of ~___-___mm in the axial plane can be achieved
0.6-0.9
4 active detector rows -19__
16 active detector rows-20__
64 active detector rows in- 20__
320 active detector rows - 20__
98
01
04
07
A typical acquisition with a single detector row scanner covered ___.
5 mm
In clinical practice the CT scanners with __active detector rows were primarily used to enhance longitudinal
could also be used for enhanced longitudinal coverage, for example by selecting a 4 x 2 = 8 mm, or even a 4x4 = 16 mm coverage.
Enhanced longitudinal coverage would allow for shorter scan times but without the benefit of improved longitudinal resolution.
4
• These scanners provided excellent longitudinal spatial resolution high quality 3D reconstructions, and at the same time reduced scan times.
16 or 64
The CT scanners with up to __ active detector rows generally cover a scan volume with a helical acquisition with multiple rotations.
64
The __ detector row CT scanner covers 160 mm on one
rotation, for organs such as the brain or the heart within one rotation.
320
An x ray tube (with a rotating __ anode) and high voltage generator are used for generating the x ray beam.
The beam is collimated to create the ‘dose slice’ (or’__’)
tungsten
cone
In fast CT scanners with rotation times in the order of magnitude of __ seconds, rotating parts are exposed to
several tenths of g forces
0.35
For multi-slice scanners where the longitudinal beam width
is no longer small the X-ray beam is often referred to as __
‘
cone beam
between detectors to
prevent photons crossing to
neighbouring detectors
septa
Resolution in the reconstructed images depends on
• ___ - along the detector arc and the z-axis
• The ___
The size of detector elements
angular separation of the projection
