Computed tomography Flashcards
what is the attenuation coefficient
a measure of how easily a material can be penetrated by the x-ray beam.
Describe simply how x-rays are generated in the x-ray tube
x-ray photons are produced through the bremsstrahlung process.
What voltage is a ct scanner
140-150kV
residual x-ray beams after some has been absorbed by the patient are picked up by a
x-ray detector
Most detector modules are scintillating, what does this mean?
when the detector absorbs ionising radiation visual light is emitted which is detected by photodiodes which produce an electrical signal which is converted into a digital signal.
what are the two types of collimators in a ct scan, describe these.
pre-patient - the amount of radiation the patient receives in the z-direction. Shape the beam to a “cone” shape and is normally the same as the width of the detector (40mm detector = 40mm coverage)
post-patient - prevents scattered photons from hitting the detector as this would degrade the image and add to the skin dose.
What are bowtie filters and why are they needed
ensure a homogenous beam, there are multiple that can be switched out to optimise image quality/dose and they have different levels of filtration
what are the two types of acquisition modes and describe them
- helical - most common where the patient constantly moves through the gantry (the x-rays are always on), producing a HELIX over the patient.
Some areas are irradiated twice (not good) but provide good-quality data/images where the parts overlap.
- axial - the patient moves in increments i.e. 1mm, the x-rays are only on for one slice at a time.
what are the pre-processing scan parameters in CT scanning and describe them (hint there’s 4)
- x-ray tube voltage - controls how penetrative the photons are. The higher the kVp the more the photons penetrate (important for a larger patient)
- mA - controls the tube current, usually 650mA and is a measure for the number of photons in the beam, can be FIXED or MODULATED. Scout image is used in modulated mA by increasing mA for where a patient is wider and decreasing mA where a patient is thinner.
- rotation time - the time it takes for the gantry to make one full rotation (normally 0.28s-1s). The slower the time = the more dose is deposited.
- pitch - describes the relationship between table speed, gantry rotation and collimation. It is expressed as a ratio (1:1) – the table is moving at the same time as the gantry therefore there will be NO gaps in the x-ray data. A higher helix will double dose but have no gaps = greater image quality.
what is scout image
measures the width of a patient to determine dosage important considering IM(ME)R - dose limitation in areas of smaller widths but increased in larger widths (not subjecting the patient to any unnecessary radiation). OPTOMISED TREATMENT.
what are the post-processing scan parameters in CT scanning and describe them (hint there’s 4)
- algorithms - change the way the raw data is processed which can make images softer or sharper
- Windowing: using Hounsfield units to change the colour of a CT scan.
a. window width - controls the number of HU (& thus shades of grey) represented to demonstrate different tissues, controls contrast.
b. Window level - controls the brightness of the image by altering where the window sits on the HU scale
What is IV contrast, when is it used and what are safety concerns of it?
iodine is used as an IV contrast as it has a high atomic number and thus high attenuation especially for blood as blood is a poor attenuator.
safety:
1. can cause issues with patients with poor renal function it must be assessed through serum creatinine or an estimated glomerular filtration rate.
- Patients on metformin will need to stop that medication for a couple of days.
- iodine is a nephrotoxic drug.
what is the HU value of
air
water
bone
air -1000
water 0
bone +1000
what is image regristation
comparing CBCT to simulation ct.
