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
What are the negatives with pan-oral (OPG) dental radiography machines?
Hard QA, Optimisation can be a low
priority for these low dose
procedures, tendency to
not vary acquisition
parameters
What is cephalometry used for in dental imaging?
Skull radiography used mainly in orthodontics to observe the
facial structure (not common)
How are images usually reconstructed for dental CBCT?
Filtered back projection
What considerations are there for how much shielding is required?
Required dose constraint, type of radiation present, quantity of radiation present
What describes the ICRU sphere for the ICRU dose operational quantities?
Soft-tissue equivalent
30cm diameter sphere
1g/cm3
What depth do we use for the personal dose equivalent for weakly penetrating radiation (skin dose measurements)?
0.07 mm
What depth do we use for the personal dose equivalent for strongly penetrating radiation (effective dose measurements)?
10 mm
What is the personal dose equivalent H_p(d)?
Dose equivalent in ICRU sphere at depth d below where dosimeter is worn
What is the ambient dose equivalent H*(d)?
Dose free in air that would be measured in ICRU sphere at depth d, typically measured at d = 10 mm
In the DR energy range, does kerma over or underestimate the effective dose?
Kerma overestimates effective dose
According to the ACoP, are exposures received as a result of natural background radiation at normal levels considered in determining compliance with dose limits?
No
Do we need to consider background dose correction for shielding considerations and why?
Yes because normal levels do not need to be considered in determining compliance with dose limits. However, these are usually negligible on short time scales
What factors should we consider when carrying out shielding calculations?
Occupancy factors, pre-existing shielding, controlled areas, workload, use factors, energies used, types of radiation, room dimensions (distance corrections), future proofing
Do occupancy factors for shielding calculations consider an individual or a whole staff group/all public?
An individual
What types of radiation do we need to consider in shielding calculations?
Primary (emitted and transmitted - only incident on walls behind potential detector positions) and secondary (scatter in patient, tertiary scatter and leakage)
What do we have to consider for the transmitted primary beam in shielding calculations?
Lots of attenuation (from mGy at entrance dose to micro Gy at exit dose) but considerable exit dose
When do we need to consider primary emitted radiation in shielding calculations?
If we cannot guarantee that there is no primary emitted radiation hitting the wall (detector not fixed over the x-ray tube like it is in CT)
What does Compton scatter depend on in patients at DR energies?(shielding calculations for secondary scattered radiation)
Fluence and energy of primary beam, patient thickness, Beam position on patient, Beam field size, Scattering angle
Is the x-rays from leakage harder or softer than the primary beam?
Harder (only high energies make it through x-ray tube shielding)
What are the common materials used for shielding?
Lead, concrete, barium plaster, steel (less common), bricks, lead glass
What considerations should there be for materials used for shielding?
Cost, weight, thickness required, availability, structural attributes, visibility (transparency required?), ease of fitting, new or existing construction
For shielding, materials are calculated in terms of what material’s equivalence?
Lead equivalence
Roughly what order of dose does a CT give in Sv units?
Several mSv
Which generation of CT do we typically use now?
3rd
What is the typical image matrix and minimum possible slice thickness in current CT?
512 x 512 and 0.4 - 0.6 mm
What is the design of the 3rd generation CT system (typically used now)?
Larger fan beam, 500-1000 detectors in x-y direction in an arc, and the x-ray tube and detectors rotate together
What is the differences between a normal X-ray tube and a CT tube?
Long exposure times (need to dissipate heat well), high filtration (more complex for harder beam to get a mostly monochromatic beam for HU), flying focal spot (for double sampling)
What does the bow-tie filter do in CT systems?
It compensates for non-uniform thickness of patient in x-y direction to get consistent beam intensity across detector and it hardens the beam to be more monochromatic
What are the ideal detector attributes in CT?
Fast response and minimal afterglow, high geometric efficiency (little dead space), high absorption efficiency (sensitive), high dynamic range, uniform response and good stability, minimal crosstalk between detectors, construction feasibility, low cost
What type of detectors are used in CT scanners now?
Scintillation detectors with scintillation crystal and photodiode (historically was ionisation chambers)
What are UFC scintillators and why do CT scanners use them nowadays?
Ultra-fast ceramic scintillators. Fast response and not long afterglow. They use photodiodes than PMTs because they are smaller and not prone to many issues
Why is the number of data channels not necessarily equal to the number of rows (slices?) for CT detectors?
The detectors can be grouped together
How many data channels are there in CT scanners?
4
What types of arrays can exist for CT scanners?
Matrix (fixed) and adaptive
What are manufacturer specific names for a scan plan radiograph (SPR)?
Scout, topogram, surview, scanogram
What is the purpose of scan plan radiograph (SPR)?
Used to start and end of CT scan range. Used for dose modulation
What are the CT parameters that can be changed?
Tube potential (kVp), mAs, slice width/ field of view and contrast medium
Does the peak kilovoltage (kVp) applied in x-ray tubes affect quality or quantity of the x-ray spectrum generated?
Both
Is peak kilovoltage (kVp) linear or non-linear with radiation dose in CT?
Non-linear
Is the peak kilovoltage (kVp) manual or automatically chosen?
Could be either
What is a high pitch in CT scanners?
A stretched helical scan (more space between lines in helix)
Is the tube current (mAs) linear or non-linear with radiation dose?
Linear
For faster rotations of the CT scanner, what other parameter has to change to maintain image quality?
Increase mA
What is the typical rotation time of a CT scanner? (time to complete 360 degrees)
0.3 - 1 s
Why would you want a smaller reconstructed field of view?
Smaller pixel sizes
What is the typical slice thickness used in CT?
0.6 mm - 10 mm
What is the other name of axial CT scan? (mostly helical now)
Step and shoot
What is the effective mAs in CT scanning?
mAs / pitch
What is the equation for beam pitch in CT scanning?
Table feed (bed movement) divided by beam collimation (number of rows times width of detector row)
What is used in CT to make up for the difficulty of interpolating at the extremes of the z range (missing data)?
An additional scan may be used (another part of a rotation) called an overscan
What is AEC also called in CT?
Dose modulation
Which parameter is generally modulated in AEC in CT?
mA
What is longitudinal dose modulation in CT?
The mA is changed per slice in the z-direction of the patient to maintain the image quality slice by slice
What is angular dose modulation in CT and how is it done?
Dose modulation (AEC) in the x-y direction and mA varies as the tube rotates around the patient
What does angular dose modulation in CT improve in terms of artefacts?
Streaking artefacts due to photon starvation
What is AEC used for in CT?
To compensate for variation in patient size/density and both within a single patient and between patients
Is the kV changed during a CT scan for AEC and why?
No because then it would be hard to obtain HU values for pixel
For what specific organs could dose modulation be used in AEC CT?
Heart (ECG prospective gating), breast and eye
What is the issue with the ramp filter in FBP?
It also amplifies noise (very high spatial frequencies)
In which planes can image reconstruction be displayed for CT scans?
Traditionally transverse but it can be any of the three cardinal planes (some scanners can do any in between as well)
What is the typical CT number (HU) for air, water and bone?
Air: -1000
Water: 0
Bone: 700 to 3000
CT numbers are stored as how many bits and how many values does this mean it can have?
12 bit so 4096 values
Why is windowing necessary?
Human can only differentiate between a certain number of levels of grey and a monitor display can only show 256 shades
Why is CT used in radiotherapy planning?
HU can be related to electron density via mapping for treatment planning
Does noise increase or decrease in CT for increased mA and why?
Decrease (SNR decreases but actual noise decreases as well) because it is normalised with HU anyway
What is CTDI_w?
Weighted CTDI to account for patients not being air, so a phantom is used
What is CTDI_vol?
CTDI corrected for pitch for helical scans (CTDI defined for axial scans), so CTDI_vol = CTDI_w / pitch
What is the DLP equation?
CTDI_vol * length of scan
Is DLP a patient dose?
No but it can be used to calculate patient dose. Patients are not homogenous or phantom shaped.
What are some of the artefacts in CT?
Partial volume artefacts (sampling issue), beam hardening artefacts (includes cupping, streaking), ring artefacts (detector issue), shading artefacts, metal artefacts, out of field artefacts (truncation. Material outside of FOV not accounted for in reconstruction)
Why is the x-ray tube below the couch in fluoroscopy?
There is a lot of backscatter that can then be directed downwards
Is phosphorescence or fluorescence faster? (very similar processes)
Fluorescence
What typically replaces image intensifiers in fluoroscopy nowadays?
Flat-panel detectors
What is the order of these in an image intensifier: input phosphor, CCD camera, collimator, output phosphor, electron focussing cup, photocathode
Collimator, input phosphor, photocathode, electron focussing cups, output phosphor, CCD camera
Which exposure parameters in fluoroscopy affect both the dose and image quality? * is next to the ones that the operator can change in all systems (other systems can change some others)
kVp, mA, mm copper filtration, mm aluminium filtration, detector dose per pulse, *low/medium/high dose mode, *x-ray field size, *magnification, *FID (focus to image receptor distance), *pulse-rate
Which exposure parameters in fluoroscopy affect only image quality (not dose)?
Frame averaging and edge enhancement
What is the difference between fluoroscopy/screening and acquisition? (nomenclature)
Fluoroscopy/screening is lower dose and mA with more filtration and lower dose rate compared to acquisition. Acquisition images are typically archived
What is ABC in fluoroscopy and the other names for it?
Automatic brightness control. Also known as loading curves or automatic dose rate control
What does ABC do in fluoroscopy?
It is a feedback loop that maintains the image intensifier dose rate or output brightness. It does this by changing exposure parameters with kV-mA curves
What is subtraction angiography? (advanced fluoroscopy technique)
Non-contrast images are acquired then subtracted (mask) then any new image is anything new on top of that with the contrast added
What are the common dose metrics for x-ray projection radiography (planar, fluoroscopy and dental)?
Dose area product (DAP) and entrance surface dose (ESD)
What are the common dose metrics for CT?
CT dose index (CTDI) and dose length product (DLP)
Is it effective dose if considering the dose for a specific organ and why?
No you have to do a summation of all organ doses that have been irradiated.
Is DAP dependent or independent of the distance to source?
Independent
What uncertainty is there on a DAP meter?
+- 25%
Does entrance surface dose (ESD) include back-scatter and what does this mean for measurements?
Yes so you can’t use certain detectors (e.g. solid state detectors with a metal backing) as they don’t all include back-scatter
What is the entrance surface dose (ESD) measured in?
Gy (mGy is standard)
What is the erythema dose threshold?
2 Gy
What is the units for CTDI?
mGy
What is the units for DLP?
mGy cm
Are the dose metrics (e.g. DLP, DAP, CTDI, ESD) the patient dose or a metric of the equipment output?
A metric of the equipment output but we can use it in patient dose calculations
What are the methods that could be used to obtain dose metrics for a scan?
Imprinted on image, DICOM headers, Secondary capture dose report, Computed radiology Information system (CRIS), Log books, Dose management system, At the modality
If using look up tables for effective dose conversion factors (e.g. Shrimpton et al), is this specific to an individual patient or a mean?
Mean as not representative of the real patient and other uncertainties
What are some of the reasons that effective dose calculations are not the same as the reality? (i.e. areas of uncertainty)
Anatomy exposed, Patient motion, Beam quality, Dose-metric calibration, Patient size, shape, and composition, Tissue attenuations, Geometry of exposure, Simulation software assumptions / inaccuracies, Modulation during exposure, Beam perturbations (physical measurements)
What is DICOM (Digital Imaging and COmmunication in Medicine)?
A set of standards which lay out the format of files and transfer processes to ensure
compatibility and accuracy
What is a dose management system (DMS)?
A system which typically collects RDSRs
What are dose management systems used for?
Allows the user to look at parameters associated with the exposure. Overview of dose distributions for modalities and
specific exams. Invaluable for audits
If n is the number of assigned bits, how many options are there?
2^n
Why do we need to be cautious with windowing when taking digital measurements?
It can change the displayed size of the object
Does windowing change the raw pixel values (HU)?
No, HU is fixed and a property of the material (some software might do this but most shouldn’t)
What is a dose detector indicator and what is it for?
A measure of dose to the detector and it is useful for the radiographers as a quick check of under or overexposure
What is the signal transfer property?
Kerma-pixel conversion
Where do the most common forms of breast cancer originate?
Within the lobes and ducts (ductal carcinomas: 75%, lobular carcinomas: 10%)
What are the challenges in mammography?
Sufficient image contrast, dose to patients (asymptomatic in screening), overlapping structures, resolution
Are images better or worse in mammography if the breast is dense?
Worse because of contrast and may obscure small masses
Why do we use low energy photons in mammography but what is the issue with this?
For better contrast but low energy photons result in increased radiation dose
What 3 anode materials are used in mammography?
Tungsten (W), Rhodium (Rh), and Molybdenum (Mo)
What 3 filter materials are used in mammography?
Rhodium (Rh), Molybdenum (Mo), Silver (Ag)
How do you get a greater energy at the chest wall edge relative to the nipple edge in mammography?
Half-field imaging technique and the Heel effect is used to decrease the dose towards the nipple (max dose at chest wall where it is thicker)
What methods help with resolving issues with overlapping structures in mammography?
Compression, multiple views and 3D tomosynthesis
What is digital breast tomosynthesis (DBT)?
Pseudo-3D imaging of the breast that uses a series of projection images over a small angular range (15 - 50 degrees) and reconstructed with FBP or IR
Why is a good spatial resolution a requirement in mammography?
Micro-calcifications can be < 0.05 mm but the size and amount of calcification in a cluster determines the harmfulness of the calcium deposits
What are phantoms made from in mammography that represent different compressed breast tissue (CBT)?
Perspex (PMMA). It may contain aluminium to be equivalent to calcifications for CNR calculations
What is the phantom name that can be used in mammography for a subjective image quality test?
TORMAM
What methods are used in mammography to improve spatial resolution?
Compression (< scatter), small focal spot size, anti-scatter grid, small detector pixel sizes
What is the phantom name that can be used in mammography for a non-subjective image quality test?
CDMAM
What is dose parameter is used in mammography dosimetry?
Mean Glandular Dose (MGD)
Can mean glandular dose be measured directly and how is it measured?
No and the surface entrance dose is measured and correction factors are applied
How is the mean glandular dose (MGD)calculated?
K (air kerma) * g (g-factor: convert air kerma to MGD for a 50% glandularity) * c (c-factor: correction for change in breast composition from 50% glandularity) * s (s-factor: spectral correction factor if not Mo/Mo)
What contrast is used in contrast enhanced mammography (CEM)?
Iodine-based contrast agent (k-edge of iodine 33.3 keV)
Why is contrast enhanced mammography (CEM) used to detect tumours?
Growing tumours require increased blood supply and areas of high activity will be highlighted in CEM i.e. contrast agent preferentially taken up by growing tumour
What are the priorities for imaging in radiotherapy?
Clearly defined tissues. Include all tissues and objects required for the planning system. Reduce metal artefacts to not affect HU values. Reproducible patients position. All images of the same person should be comparable. Identifiable key features for matching.
What is the differences between the planning CT in radiotherapy versus diagnostic CT?
External laser system. Flat tables. Facility for positioning systems. Larger bore. The slice thickness matches the planning systems capabilities. Typically one kV setting
How many bits do planning CT systems have and why? (12 bit in diagnostic CT)
16 bit to have a larger range of characterisation
Why is the beam fanned out a lot in CBCT in radiotherapy kV imaging?
Linacs cannot move very fast and patients cant stay still for that length of time
What is the x-ray tube used for on a linac (3 ways)?
Planar X-ray imaging system (simple positioning matching). Fluoroscopy imaging system (usually as part of a
gated exposure). Cone beam CT system (for complex matching)
What type of scan is related to bone mineral densitometry?
DEXA scans
What does DEXA stand for?
Dual-Energy X-ray Absorptiometry
What are the clinical indications for a DEXA scan?
Osteoporosis, osteopenia (lower than normal bone density and can develop into osteoporosis), body composition analysis (lean muscle, fat, used in sports imaging)
Why can we only know the density per area (areal density) in DEXA scans?
DEXA is 2D so can’t know the true density
What is the bone mineral content (BMC) and its units? (DEXA)
Measure of calcium based hydroxyapatite in bone, units of g
What type of anode is typically in DEXA x-ray tubes?
Tungsten
What are the two methods used in DEXA to obtain two distinct spectra (high and low)?
Voltage switching and K-edge filtering
How is the voltage switching method done for DEXA to obtain two distinct spectra?
Pulsed switching between 70 kVp and 140 kVp, with additional filter (copper or brass) to harden higher energy beam
What type of beam shape is typically used in DEXA scans?
Fan beam
What anatomical region is it recommended by WHO to DEXA scan for diagnosing osteoporosis?
The femoral neck (top of femur)
In DEXA scans, what metrics (clinical scoring) are used to compare a patient scan to the general population for osteoporosis?
T-score (primary tool) and Z-score
The T-score for osteoporosis diagnosis is effectively a measure of how many standard deviations away from what value?
Young reference adult mean (YMD)
What are the limitations of DEXA scanning?
Only solve for 2 materials at a time. Lack of standardisation between manufacturers (difficult to compare). Limited image quality for qualitative diagnosis. Unknown bone volume (grams per unit area). Radiation dose (although small)
Why is it difficult to QA DEXA scanners?
Limited options for changing settings and not much guidance on what QA to perform
How do we QA test the aBMD measurement accuracy for DEXA scans?
Regular scan of a phantom with a known aBMD and compare to known values
What are some alternatives to DEXA scanning?
Quantitative ultrasound and quantitative CT
