Overall Flashcards
What are the techniques for radiation measurement?
Heating (usually too low energy), physical effects (thermolumiscence, X-ray film etc), biochemical and biochemical changes, and ionisation
What are the requirements for radiation measurement techniques? (most techniques don’t cover all but ionisation in air methods covers most)
Measurable, accurate and unbiased by the measurer. Reproducible. Sensitive to small amounts. Independent of intensity. Robust to changes in energy. can be converted into a biological response
What is the linear attenuation coefficient and its unit?
Fraction of incident beam that is absorbed or scattered per unit thickness of medium and cm^-1
What is the Mass Attenuation Coefficient (μ/ρ) and units?
Linear attenuation coefficient normalised to density and cm^2/g
What is the Mass Energy Transfer Coefficient (μtr/ρ)?
What is the equation for the radiation exposure (X) and its units?
The sum of all electrical charges of one sign (only positives or negatives as they would cancel out otherwise) divided by the sum of mass of air. C/kg
At diagnostic energies, the total kerma is equal to what?
Collision kerma
At diagnostic energies, what is the equation for the energy imparted (epsilon)?
Radiant energy in - radiant energy out (changes of rest mass energy is zero at diagnostic energies)
What are the units for absorbed dose and kerma?
Gray
What are kerma and absorbed dose used to quantify?
Kerma is used to quantify the radiation field (transfer of energy from photons to charged particles) whilst absorbed dose is used to quantify the effects of radiation (there is a change of energy = energy imparted)
At diagnostic energies and in low Z materials, are absorbed dose and kerma effectively the same or different?
Effectively the same
Why are absorbed dose and kerma basically the same for diagnostic energies in low z materials?
Minimal bremsstrahlung and charged particle range is short
When does charged particle equilibrium break down? (kerma not equal to absorbed energy)
Interfaces between different materials (e.g. absorbed skin dose and incident air-kerma)
What is the typical backscatter factor?
30-40%
Why does backscatter affect the measurements of kerma and absorbed dose?
What defines the size of the detectors?
Compared with the range of the charged (secondary) particles in the cavity
For diagnostic energies, what size detectors are we mostly using?
Large detectors (as charged particle range is small)
What is the Fano theorem?
What is the Bragg-Gray cavity theorem?
Ratio of dose in medium to dose in cavity is equal to the ratio of the stopping powers
What type of detector are personal electronic dosimeters (EPDs)?
Ion chamber or solid state
What quantity represents the risk of exposure (IRR or IRMER)?
Effective dose
Is effective dose measurable?
No
What is the population dose?
Summation of average effective dose per person over all population (sum of N x E)
Why is effective dose not defined on an individual level?
Because we use ICRP factors for organ weighting factors for a standard patient rather than individualised factors
What are the 4 interaction of photons with matter at diagnostic energies?
Transmitted unaffected (no interaction), photoelectric absorption, Rayleigh scattering (elastic), Compton scattering (inelastic)
Which photon interaction is most responsible for the contrast between different material due to its dependence on Z?
The photoelectric effect
Why is there usually not the L characteristic x-rays on the x-ray spectrum out of a clinical tube?
They are absorbed in the filtration
The probability of producing bremsstrahlung radiation is proportional to the atomic number (Z) to the power of what?
2
What is the approximate relationship between kVp and dose when changing between two kVp values?
The ratio of doses is equal to the ratio of kVp values squared
What is inside the housing of an electron tube and what does it do?
The envelope and it maintains the vacuum to increase the production efficiency, reduce unwanted deflections of the electrons, protects the tube from degradation
What is the x-ray generating component of a rotating anode mounted on?
Graphite and molybdenum
What type of x-rays are used a lot more during mammography?
Characteristic x-rays
What is the tube current for an x-ray tube referring to and what is it not?
It refers to the electrons travelling between the cathode and anode. It is not the current set across the filament (dependent on each other)
For a smaller focal spot, is the maximum tube current larger or smaller than a larger focal spot?
Smaller
For high kVp, the tube current is pretty independent of kVp until what point?
Until we get close to the filaments saturation current, which is a limit based the filament where it cant physically produce more electrons.
What does the focal spot determine?
The limiting spatial resolution of the system and the cooling efficiency (and therefore the output capability)
Is the actual focal spot always larger or smaller than the effective focal spot?
Larger
What are the factors that determine the size of the focal spot?
The anode angle, the bias on the focusing cup and the filament size
What creates the Heel effect?
The difference of self attenuation of the photons in the cathode to anode direction
What is the metric to quantify the heating of the tube?
Heat units
What are the cassettes made from for computed radiography (CR)?
Photo-stimulable phosphor (PSP) materials
What limits the sensitivity of the CR cassettes?
Efficiency of the PMT, amount of light that is captured by the light guide, dwell time of the LASER releasing all of the captured electrons, efficiency of electron capture, stability of the electron traps or the latent image delay
What type of detector is indirect x-ray capture?
Digital Radiography (DR)
What type of detector is direct x-ray capture?
Direct Digital Radiography (DDR)
Are direct or indirect x-ray capture detectors better in digital radiography?
Direct x-ray capture
What does STP stand for?
Signal Transfer Property
What are some of the corrections to the raw data to reach the Q values for x-ray digital detectors?
Defective pixel mapping, geometric distortion and flat field correction
What is the dose detector index (DDI)?
Rough indicator to show how much radiation was delivered for radiographers (median air kerma to a specific region)
What is the deviation index?
A ratio between the indicated air kerma and the target air kerma. Over +1 or -1 means an over or underexposure by 25%
What are the problems with digital imaging?
A continuous spectrum of data is reduced to discrete steps (affect image quality), post processing can hide mistakes and lead to dose creep
What is a linear shift invariant system in imaging theory?
The contribution to the image from any point is proportional to the strength of the signal at that point (linear) and the point spread function depends only on the relative displacements of the points in the image and object (shift invariant)
What does the contrast of an image depend on?
The image subject, photon spectrum and detector
What are the two definitions for contrast and when are they useful?
Weber (local) for small features on a large uniform background and Michelson (modulation) for images with similar amounts of dark and bright
Does background information increase or reduce observable contrast?
Reduce
What is the equation for the observed contrast with terms of original contrast (no background) and the ratio of background to average signal (R)?
The original contrast multiplied by 1/(1+R)
How can we reduce scatter in x-ray imaging?
Decrease the kVp of the beam (less forward direction towards the detector), reduce patient thickness, use an anti-scatter grid, air gap so that scatter misses the detector , reduction in field size, digital grid
Why do we add contrast in radiology (particularly CT)?
Enhance soft tissues (increases PE effect)
What is the requirements for adding contrast in radiology?
High Z (typically iodine/barium/gadolinium), wary of patient adverse affects (allergies, sensations of warmth) and check that kVp is around the K-edge for the contrast type
What are the ways of quantifying spatial resolution?
Frequency of an equally spaced repeating object (e.g. lp/mm), FWHM of PSF, square wave contrast (standard deviation of ROIs of different frequencies) and MTF
Place the following modalities in order of highest spatial resolution to lowest: Fluoroscopy, CT, mammography, general radiography
Mammography, general radiography, fluoroscopy and CT
What are the causes for unsharpness (poor spatial resolution)?
Geometric blur (focal spot not infinitesimally small), patient effects (movements and variation in edge boundary)
What limits noise in a good imaging system?
Quantum mottle (Poisson noise) as we try to optimise images and use the least amount of photons possible
Poisson noise is related to what quantity?
The square root of N (the signal)
What are the measurement techniques for noise in images?
Standard deviation of pixel value in a uniform image
Is poisson noise itself correlated or uncorrelated and why?
Uncorrelated because photon production is independent of past or future production
Is there spatial uncorrelation in x-ray detectors and why?
No because one photon detection may involve secondary carriers that are correlated
What is the dynamic range?
Measure of useful signal response to varying magnitudes of incident X-ray radiation
What is magnification in imaging?
Distortion where each area is equally affected
Why is there magnification in x-ray imaging and how much is the magnification (M)?
The x-ray beam is divergent and 1.1
Why are chest x-rays usually done PA?
To reduce the affect of heart magnification (and other anatomical distortion) and reduce breast and thyroid dose
What is distortion in x-ray imaging? (shape distortion often referred to as just distortion)
Unequal magnification of all areas within
the image (e.g. pincushion effect)
What is the detector quantum efficiency (DQE) and equation?
Measure of fidelity of a system or exactness of reproducibility. Equation: (SNR_out squared divided by SNR_in squared)
What is the DQE of an ideal detector?
1
What is the DQE dependent on?
Radiation exposure (system output may be non-linear with exposure), Radiation quality (Different efficiency of detection), Detector material (Quantum efficiency of detector),
Spatial frequency/MTF (High frequencies might not be transferred through to image)
What is the noise equivalent quanta (NEQ)?
Effective number of photons per unit length of detector to achieve specific SNR in an ideal detector
What is the difference between point, local and global operators in imaging?
Point operators work on a pixel by pixel basis, local operators mean pixels are affected by nearby pixel values and global works on image as a whole
Why is scatter a problem in general radiography?
Radiation protection and image quality (reduces contrast)
Which two interactions do we want in general radiography: penetration (transmission with no interaction), absorption, scatter
Penetration and transmission
How does an anti-scatter grid reduce scatter?
Reduces the number of detections from photons at oblique angles (like collimators in nuclear medicine)
What are the possible disadvantages of anti-scatter grids?
Reduced signal detection and may show as a grid on the image (designed to prevent this)
What are the two main ways of constructing anti-scatter grids?
Focussed grids and parallel grids
What quantity is given with focused grids to make them useful?
Focus-grid distance (small range of distances)
What quantities define an anti-scatter grid in general?
Strip frequency (number of lead strips per cm) and the grid ratio (grid height divided by the distance between the strips)
What are the advantages of digital anti-scatter grids?
Potential reduction in repeat exposures, dose reduction possible and potential improvement in image quality
What do Automatic Exposure Control (AEC) systems do?
Terminate the exposure once a sufficient dose as reached the detector
What are the benefits of AEC systems?
Limits dose to patient by normalising the amount of photons that reach the detector. It homogenises the image quality. Reduces difficulties associated with switching
manufacturers or equipment
What piece of equipment is typically used in AEC systems now and where are they positioned?
Ionisation chambers positioned in front of the detector (can be multiple with slightly different settings potentially)
What is the AEC density setting used for?
The operator can change this to increase the detector dose if the patient is larger or smaller than a standard size
What are the safety features of AEC systems?
Ceiling mAs (max mAs the system allows), back-up timer, low signal termination (uses AEC to check that the detector is actually measuring something, useful to prevent the tube being aligned to the wrong detector)
What equation characterises magnification?
Image size divided by object size or source to image distance divided by source to object distance
Why does it make a large difference to the image and dose if there is alignment to the correct anatomical area and detector?
Flat field corrections (created with beam centralised), anti-scatter grids, AECs (if excluded from beam, safety system may trigger) and AECs may be misaligned (system may cut of at an unsuitable point)
What are the different types of dental equipment?
Intra-oral, pan-oral (OPG - orthopantomogram), cephalometry and dental CBCT
What is different about the x-ray tubes for intra-oral dental radiography?
Static anode, wall mounted or hand held, fixed kV and mA or small adjustable range
What are the different views that can be performed with intra-oral dental radiography?
Periapical - single tooth view
Bitewing - shows upper & lower teeth crowns
Occlusal - to look at skeletal anatomy
What are the positives and negatives of intra-oral dental radiography?
Positives: easy QA, use film, easy to get a hold of, very low dose, can be optimised
Negatives: hard to engage with radiation protection as low dose, are optimisation settings even used, standards are very relaxed