CT REVIEW Flashcards
(35 cards)
u values scaled to water to give CT number known as hounsfield scale, what is the value for water, our and bone
air = -1000
water = 0
bone = +1000
what is filtered back projection
To create a 3d (CT), we create multiple projections. We combine this by back projection, smearing the signal per direction back on the slice/image matrix.
- filtering is done/added to reduce the noise, different filters for different purposes are available
- filters can be smooth, fine etc
(simple back projection gives streaky image)
what is iterative reconstruction
Iterative reconstruction starts with an estimated image (let’s say a circle (which is almost a human ;)), then compares the circle to the collected data and creates a correction (with each correction, it generates a better guess), with each iteration (comparison of estimate with data) the image becomes better untill it doesn’t improve further
- slow
why is IR better than filtered back projection
The advantage of IR is that less data (and thus radiation and dosage) is needed to create an accurate not-noisy image
multiple views must be taken in planar radiography in order to give an idea of depth in an object.
why is a softer kernel (filter) preferred over a high-pass sharpening filter
high pass filter creates an image that is too grainy (accentuates the noise)
what term is used to refer to how ‘soft’ the kernel is?
reconstruction kernel
how can the strength of kernel help visualise target anatomy?
if u want to view more finer detail/discrete features e.g nodules and fractures, you want to use a HARD kernel (sharp) to accentuate those features.
if you want to see larger features/compare larger features in soft tissue or find mild differences in lesions, you want to use a SOFT kernel
- soft kernel smoothes image
- sharp kernel shows edges better but with more noise
iterative reconstruction is much more computationally intensive than FBP, why
it accounts for noise and other artefacts as well as undergoing its refining process of guessing, comparing and correcting
why does iterative reconstruction work better in high noise (low dose) or high attenuation (metal implants) environments
as the process of IR already attempts to model real world noise and attenuation process
what does CTDI stand for
computed tomography dose index
what is CTDI
represents the total dose deposited at a point within a single scan slice during a complete examination
- distribution of radiation dose in single CT
what distance is CTDI measure at and with what and where
100mm length
- pencil chamber or
- PMMA phantom (head 16cm, body 32cm)
- in air
what is CTDI100
measure of dose over a pencil chamber (100mm in length)
what does NT define/represent
number of detector rows x width of detector row (irradiated slice width)
what is weighted CTDI
similar to regular CTDI but CTDIw also takes into account the different tissue densities encountered in the body.
- closer to the human dose profile as compared with the CTDI100
what is DLP + definition/explanation
dose length product
- CTDI (dose deposited at a point within a single scan slice) but also factors in the length of the scan to show OVERALL DOSE OUTPUT
- CTDI vol x scan length
what is CTDI volume
obtained by dividing CTDIw by pitch factor
pitch factor is the table distance travelled in one 360° gantry rotation divided by beam collimation
define effective dose and what its measured in
Also referred to as the equivalent dose, the effective dose is the measure of radiation calculated with the radiosensitivity of specific organs taken into account. It is measured in Sievert (Sv).
how does slice thickness affect image quality
thin slice = MORE noise, better z-axis resolution
thick slice = LESS noise, worse z-axis resolution
why is there less noise with thicker slices
more xray photons contribue to image
what is pitch (found in helical ct scanning)
pitch factor is the table distance travelled in one 360° gantry rotation divided by beam collimation
what is the isocenter in ct
the central point around which the scanner rotates and where the X-ray beams converge …
explain how the value of pitch being less than, equal to or more than the value of 1 affects the dosage, image quality, time of scan etc
pitch = 1
- similar to axial scan at isocenter
pitch less than 1
- slices overlap
- higher dose
- better image quality
- slow scan
pitch more than 1
- gaps in slcies
- lower dose
- faster scan
