KA 7 Flashcards
What is the mass attenuation coefficient (u/p) dependence of Z for (1) photoelectric and (2) Compton interactions?
(1) proportional to Z^3
(2) independent of Z
where remember mass attenuation coefficient is a constant describing the fraction of photons removed from a beam by a absorber per unit mass. So here, when Compton interactions dominate, the fraction of photons removed from the beam is independent of Z.
What is the typical Pb thickness on lead-lined doors for a CT room?
4mm
What is the approx Pb thickness for barrier shielding in a CT room?
2 mm
What is the following equation commonly referred to? BLANK= 2/3x CTDI(surface/peripheral)+ 1/3 x CTDI(centre)
CTDI_weighed, represents the average radiation dose in the x-y direction (scan plane).
What does CTDI_vol represent?
Average dose over the x-y (scan plan) and z direction. It is a useful indicator or the dose for a specific exam protocol as it takes into account such specific information like pitch.
What is the equation for DLP?
DLP (mGy cm)= CTDI_vol (mGy)x scan length (cm)
How two different scan protocols have different DLP values when they have the same CTDI_vol?
differences in scanned volume
What is the phantom dimensions used for CTDI measurements?
Body protocols: 15 cm long, 32cm diameter acyclic cylinder
Head protocols: 15 cm long, 16 cm diameter acyclic cylinder
What are the contributors of dose to secondary barriers?
Patient scatter and x-ray tube leakage
What are the two dose parameters are commonly available to view on the CT sim computer both before and after a scan?
CTDI_vol (avg dose in standard phantom) and DLP
What is the dosimetric quantity used in shielding design for kV x-ray units?
air kerma
What are the relevant shielding documents used for kV shielding?
NCRP 147 (+ NCRP 151 for MV)
What is the relevant TG document for CT simulator QA?
TG 66 (2003)
Per TG 66, What is the % tolerance for measured CTDI values on CT vs manufacturer specification?
20%
List types of imaging artefacts in CT?
- Patient-related: motion
- beam hardening
- metal artefact
- partial volume averaging
- hardware-based: ring artefact as a result of defective or miscalibrated detector
- photon starvation in shadow of a high density object
According to TG 66, what are 5 laser QA tests? Give tolerances and frequency performed
- orthogonality and parallelism to imaging plane
- Movement reproducability and linear accuracy for sagittal laser
- centre of lasers coincides with centre of imaging plane (checked DAILY)
- laser thickness
- laser coincidence with one another
everything else checked monthly, tolerance 2mm
Give the equation for weighted CTDI? I.e. dose over the 2D plane of the phantom
CTDI_w= 1/3x CTDI_100)central)+ 2/3 x CTDI_100 (peripheral)
What is the relationship between CTDI_w and CTDI_vol? Give equation
CTDI_vol= CTDI_w/pitch
Accounts for volume exposed
Give equation for DLP ?
DLP=CTDI_vol x scan length
What is the units of CTDI, CTDI_w and CTDI_vol?
mGy
What is the units of DLP?
MGy. mm
Give description of what pitch is? What are the units of pitch?
Pitch = Distance table travels during one revolution / collimated slice thickness
Pitch= unitless
What does it mean when:
(1) pitch=1
(2) pitch >1
(3) pitch <1
(1) When pitch =1, the distance the table travels during one revolution of the x-ray tube equals the slice thickness or beam collimation, the pitch ratio is 1:1. This is the ideal pitch.
(2) when pitch <1, table distance travelled is less than the slice thickness, so overlapping slices will exist. Results in greater dose to patient.
(3) pitch >1, gaps between slices exist, results in lower patient dose
What are three different photon interactions that may happen in CT energy range?
- photoelectric: Complete absorption of incoming photons. Interaction involves incident photon and inner shell electron of the atom. The energy of a photon has to be similar to the binding energy of that electron. Photon energy is transferred to an electron, and that electron is ejected. A vacant electron shell is filled by an electron from an outer shell, and this produces a characteristic x-ray (E=diff in electron binding energies of the two electron shells). If the incoming photon energy <binding energy of the electron, the photoelectric effect cannot occur. Likelihood of photoelectric absorption proportional to Z^3/E^3. Interaction dominates with higher Z and lower energies.
- Compton: photon interaction with free/loosely bound outer electron… hence the probability of interaction not related to Z. Transfer of energy to electron= ejected electron+ scattered photon.
- Rayleigh: incident photon deflected/scattered from interaction with an electron. Elastic scattering==no loss of energy. Not a major interaction in radiology since it is dominant <10 keV. Probability of interaction proportional to Z/E^2
What do we use DLP for?
Approximate of effective dose
What do we use CTDI for?
radiation dose OUTPUT
Which photon interaction deposits energy locally as a result of complete absorption of the incoming photon?
Photoelectric
What quantity is used to represent output of an x-ray machine?
Khan 3.6m The output of an x-ray machine can be expressed in terms of ionization it produces in air. This quantity, which is a measure of ionization per unit mass of air, is known as exposure
What does exposure represent?
Exposure=ionization per unit mass of air
units: Coulomb/kg
What is the order of CT scanner regions? starting from x-ray tube
x-ray tube-> pre collimation-> bow-tie filter->filter->couch->post-collimation->detector array
what is the purpose of the collimators in CT scanners?
to minimise scatter radiation and localise the beam. Width of collimators determines slice thickness
Why do we need CT to ED curve? Think of what CT scan primarily represents
CT image= matrix of attenuation coefficients. Conversion to electron density is required for accurate dose calculation in the TPS
what is the shape of a half- bow-tie filter? What does it do
bow-tie with flat edge closest to x-ray tube. Attenuates lateral portions of the beam prior to the patient, where lateral components pass through less tissue than central. This reduces patient dose and yields uniform fluence at the detector
what material is the half bow-tie filter usually constructed from?
Al
What material is anode material commonly made of? What is common angle of anode for CT scanner?
Tungsten, 7 deg
Why does anode rotate in CT scanners?
to dissipate heat (remembering x-ray generation: 99% heat, 1% bremsstrahlung). If it didn’t rotate, it could deform the target and hence reduce the efficiency of the tube.
Pre collimation defines BLANK whilst post-collimation defines BLANK
- pre collimation defines beam dimensions
- post collimation defines slice thickness
what is an important feature of x-ray tubes to prevent leakage?
lead lined
what is the relationship between mAs and dose?
linearly dependent
what is the dependence of kV on dose?
dose proportional to kV^2
What does sinogram represent?
graph of CT projections data from the detector plane as a function of angle.
How do we go from sinogram to image?
back projection, where projection is spread evenly across the projection line. Filtered back projection in the sinogram space, reduces blurring of the reconstructed image
What are the three types of image reconstruction algorithms used in CT?
- Simple back projection: the sum of a single projection is evenly distributed along the projection line profile. Creates blurred image
- Filtered back projection: designed to overcome issues related to blurring. Applies convolution filter to projections prior to back projection.
- Iterative: iterative comparison of measured raw data with FBP of raw data. Updated image of altered data back projected to compare. iterative process until converge to solution.
how do you get from image space to projection space (i.e sinogram)?
Forward projection
how do you get from projection space (i.e sinogram) to image space?
back projection
Primarily what is difference between analytic algorithms of back projection vs iterative algorithms? How does this change computational time?
FBP= single reconstruction
Iterative= multiple repetitions, converging to solution. iterative is more computationally demanding
what is meant by image artefact?
inaccurate representation of anatomy.
What is typical Aluminium filter thickness of half bow-tie filter?
1-10mm
list factors affecting quantity of x-rays?
- target material
- mAs
- kV
- filter material type and thickness
What does DRL stand for?
diagnostic reference level for common procedures in Australia, ARPANSA provides values. Not regulatory limit but guide to help optimise and avoid over-exposure of patients.
What is approx CTDI_vol values for head vs chest? Also assuming scan length of 20cm for head and chest, 40 cm what would the DLP be?
head, CTDI_vol= 50mGy. DLP=50mGyx20cm=1000 mGy cm
chest, CTDI_vol=10 mGy. DLP= 400 mGy cmm
Given 5 couch tests for CT QA? what are tolerances per TG 66?
- couch movement accuracy in long and vert motion. 1mm
- couch movement is orthogonal to imaging plane
- couch remains in same ant-post plane (i.e no sag) throughout longitudinal movement under load, 2mm
what is meant by low contrast resolution?
ability to resolve objects that have small difference in HU compared to nearby/background object. Since the difference is small, noise is significant factor in this test.
What are two types of radiation (definitions given in shielding)
primary radiation (main useful beam)
, secondary= scatter, leakage
Why does primary radiation not considered for CT shielding?
primary beam intercepted by patient and detector array housed in the gantry.
give equation for transmission, B, in NCRP 147?
B= P*d^2/(TxKxN)
where P is design goal
d= distance from source to POV
t= occupancy factor
k= avg unshielded air kerma @ 1 m from source per patient
n= number of patients per week
Given air kerma defined at 1m per patient (mGy/patient), provide equation of weekly unshielded air kerma at a POV different from 1m
k= reference air kerma*number of patients in week/d^2
In case of CT, give equation for scattered air kerma at 1m based off CTDI_100
ks= kdose line integral= k x NrTb*CTDI_100
where k is constant given body or head
Nr=# of rotations
Tb= beam width
This is also equivalent to,
Ks=k mAs L/P *CTDI
where L = length of scan (cm)
p=pitch
What is true isodose distribution of CT scanner? Give the shape
hour-glass
list different ways shielding calculations can be performed?
- using CTDI to determine secondary air kerma
- using DLP to determine secondary air kerma
- isodose map, provided.
what are the two contributions of secondary radiation?
scatter and leakage
When doing DLP to determine scattered air kerma, what is the equation?
ks= kappa (constant) x DLP
as L and pitch already taken into account
Note for body ks=1.2 xkx DLP
what type of chamber is used for CTDi measurements?
small volume cylindrical pencil ionization chamber, length =100mm. 3cc
What is AEC?
automatic exposure control, determined from scout image to modulate the mAs to limit patient dose during CT imaging
for radiation survey, what scatter material size should be used for CT? what body part and give approx dimensions of phantom per TG 66
abdo, 20cm diameter. 40cm length
list 4 key generator tests of a CT scanner?
kVp
mA
linearity and reproducibility of mAs
HVL
timer accuracy
what is spatial resolution also referred to as?
high contrast resolution, contrast of 12% or greater
At what % of the MTF curve, is the limiting spatial resolution specified?
5%
what is the generator test tolerance for KVp measurement per TG66?
+-2 kV
