CT Flashcards
what is CT used for
localisation NOT verification
higher HU units have a … density
higher Z/density
what value is HU for soft tissues
close to 0
what is the HU value for bone
middle hundreds - low thousands
what is the HU value for air
- middle hundreds - low thousands
what type of anode does CT use
rotating
why is CT used
- identification of structures (anatomical transverse slicing)
- identification of extent and surrounding tissues
- visualisation of targets with respect to other 3D structures
- outlines tumours
- BEV
- symmetrical and asymmetrical images
follow a radiation path and anatomical position - beam shaping
- adding beams
- dose volume histogram (DVH)
- DICOM: transfers info between computers
- monitor unit calculations
- export plan to linac
- export DRR for online assessment or treatment accuracy
- information of radiation dose and how the tissue absorbs it
what are the advantages of HU
- contrast resolution
- windowing
- planar images and localisation
- 3D fast image acquisition
- reconstruction of non standard planes
what is HU
a quantitive measure of radio sensitivity in evaluating CT scans
what are the advantages in diagnosis
- appropriate management
- CT-sim
- reduction in hospital times
- determining treatment
- reducing exploration surgery
- improvement in diagnosis and treatment
- follow up treatment
what are the disadvantages in diagnosis
- cost
- access
- radiation dose
why is MRI better than CT
with soft tissue contrast, so can differentiate between tumour boundaries and normal tissue
CT scanners:
- only treatment area is imaged
- X-ray tube is in the gantry as well as detectors, mechanical supports and scanner housing
- xray tube converts electrical input into x-rays, the accelerated electrons hit the tungsten target, emitting x-rays. if it has insufficient energy, majority will turn into heat
- long continuous exposure, which needs to be energised throughout
why is CT-MRI used?
- excellent spatial resolution
- geometrical reproducibility
- shorter imaging times
- accurate electron density
why is a metal ceramic tube used
ceramic is used due to the electrical arcing from the tungsten increasing the tubes lifespan. It insulates the high voltage parts for a more compact design. Enabling higher currents reduces off focus radiation. As well an alloy of chromium and iron is used.
describe the max rotalic ceramic x-ray tube
the design contains spiral grooves with a metal alloy which acts as a lubricant. It has a long life and the high output with the rotating anode being cooled directly. The continuous rotation prevents waiting time during and between examination
describe the aquiline X-ray tube
it is a high capacity multi-slice tube which has a high heat storage capacity/ cooling rate. Which has a built in grounded hood
what are detector properties
- high efficiency
- small physical size
- cheap
- narrow gaps between active elements
- high dynamic range
- fast response
describe the Stratton x-ray time
- zero heat capacity storage
- the entire tube rotates around the anode axis
- bearing is locates outside the tube - cooling faster
- uses magnetic deflection coil which shapes and controls the beam
what is the role of a photon diode
it collects the flash of light which is converted to an electrical impulse which is then turned into an image. The solid scintillator coverts x-rays into visible light photons.
- high detection efficiency
- high geometrical efficiency
- small size
what is dose determined by
exposure factors and volume
dose = total mAs x volume irradiated
what is back projection
collects data which combines it and plays it back
what is a filter
a maths process which adds, subtracts and multiplies
a basic filter adds data (density) to one side of the boundary and subtracts it from the other
what is the attenuation coefficient
reduction in intensity by the material
what does edge enhancement do?
maximises small details
describe the 3rd gen scanner
x-ray hits a row of detectors, which rotate together to image at a different angle. This is repeated until a single slice is taken, which continuously rotates. It has an arc of detectors opposite the tube
describe the 4th gen scanner
it has a fixed rotation, with a complete ring of detectors. The source rotates to capture a slice
describe the spiral CT
it has a spiral movement which is achieved simultaneously by the tube rotation and the linear movement of the patient
describe the multi-slice CT
it has more than one bank of detectors, which produces two or more slices in one rotation, however this has a negative impact.
what is window level
tissue which occupies the midpoint of the available grey levels
what is window width
the range of tissues which occupies a range of grey levels
describe windowing
- it changes the density relationship between tissues
- short range = high contrast
- changes tissue types
what is the conventional bore size and what is the bore size used
conventional = 70cm (to minimse scatter)
85cm is used reducing the quality due to scatter
what is the role of collimators
to produce a narrow beam, with no divergence and little scatter
what can cause a change in view
metal artefacts
what is the role of PET
to visualise the spatial distribution between the metabolic and biochem activity
