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
