Physics

Front 1

Describe the photoelectric effect.

Back 1

When photon collides with electron, is can eject from its shell (eg. K shell) if its energy exceeds the binding energy.

The photon dissapears

Any remaining energy becomes kinetic energy of the ejected electon (photoelectron)

An electron from a shell further out falls to replace the ejected one -> characteristic x-ray photon (energy of which = difference in binding energies of two shells)

Front 2

Describe bremsstrahlung (continuous spectrum).

Back 2

Interaction of bombarding electron with nucleus.
Approaches and is deflected and slower down producing an x-ray photon emitted in any direction.
Loses part of energy up to a maximum - thus continuous spectrum.
Peak of continuous spectrum normally between one-third to one-half kV.

Front 3

What does this Figure illustrate. (1.8 pg 8)

Back 3

Characteristic x-rays (line spectrum) superimposed on Bremsstahlung (continuous spectrum)

Maximum energy depends on kV and minimum on filtration added to the tube.

Front 4

What are the possible fates of x-ray photons passing through matter?

Back 4

Transmitted: pass through unaffected
Absorbed: transfer all energy to the matter
Scattered: diverted with or without loss of energy to matter

Front 5

What are deterministic effects of radiation?

Back 5

Effects which have a threshold dose below which they will not occur and a dose-incidence curve up to a level at which the effect will invariably occur.

Front 6

What weeks are the highest risk for fetal abnormalities secondary to radiation?

What are the threshold doses?

Back 6

3-8 weeks

100-500mGy
(CXR 0.15mGy, CT pelvis 10-30mGy)

Front 7

Define stochastic effects of radiation.

Back 7

Effects that arise by chance.
Linear no threshold theory: no threshold dose for stochastic effects and risk increases linearly with dose.

Front 8

Describe the theory behind x-ray filtration.

Back 8

Aim is the absorb low energy photons which would contribute to patient dose but not reach the film.

This is done with interposing metal sheets (usually aluminium) of differing widths.

Front 9

What does increasing filtration do to the continuous x-ray spectrum?

Back 9

Increases minimum and effective photon energies, does not effect max.
Reduces area of spectrum and total output of x-rays.
Increases exit dose: entry dose
Figure 1.18

Front 10

In respect to fluroscopy:
Explain gain.

Back 10

Gain is the extent to which the output screen is intensified by the image intensifier in comparison to the input screen. = flux gain x minification gain

Flux gain: each electron causes many light photons to be emitted for the output phosphor following acceleration by the focusing field.

Minification gain: intensification caused by reducing the image size from the input to output screen. Equal to ratio of the areas of the two screens.

Overall brightness gain is a product of the two.

Front 11

How does a fluoroscopy input screen work?

Back 11

Has two components
1. Outer side is the input phosphor layer (usually caesium iodide)
2. Thin layer of a material which acts as a photocathode - a material that emits electrons when irradiated by light.

Front 12

Describe the fluoroscopy output screen.

Back 12

25-35mm diameter, no more than a few mm thick.
Thin phosphor layer converts pattern of electron intensities into light.

Front 13

How does electron focusing in fluoroscopy work?

Back 13

Focusing electrodes are metal rings within the tube at positive voltages cf photocathode.
Constrains electrons to travel directly to output screen -> minified replica of input screen.

Front 14

Why are automatic brightness control (ABC) systems necessary in fluoroscopy?

Back 14

x-rays require a certain amount of radiation to give appropriate brightness of image.
With a moving region of interest and administration of contrast the amount of radiation required changes rapidly - this can not be manually controlled.

Front 15

How do automatic brightness control (ABC) systems work?

Back 15

Brightness is measured either as light intensity of the output screen or the signal from the camera.

Adjustment in brightness is then made by changing kV and/or mA. These changes follow brightness curves programmed by the manufacturer.

Front 16

What is the effect of altering kV or mA on the dose control curve?

Back 16

Figure 6.1
A: anti-isowatt curve - increase in both kV and mA
B: Increased mA. kV maintained between 60-65kV to provide optimal spectrum for imaging iodine. Maximizes image quality as expense of dose (low kV)
C. minimizes dose at expense of image quality by increasing kV rapidly as radiological thickness increases.

Front 17

What are the ways electrons lose their energy when they arrive at the target of the x-ray tube?

Back 17

1. Large number of very small energy losses from interaction with outer shell electrons -> heat

2. Large energy losses producing x-rays from interaction with inner shell electrons (line spectrum, continuous radiation) or the nucleus (bremsstrahlung, continuous spectrum radiation).

Front 18

What is the half-value layer?

Back 18

The thickness of stated material that will reduce the intensity of a narrow beam x-ray by 50%.
This is a measure of the penetrating power or effective energy of the beam.
The HVL is inversely proportional to the linear attenuation coefficient

eg. HVL 30mm in tissue, 12mm in bone.

Front 19

List three things which would decrease the half-value layer?

Back 19

1. Increased material density
2. Increased material atomic number
3. Reduced photon energy

Front 20

Explain absorbed dose.
How does it differ to Kerma?

Back 20

Energy deposited per unit mass of stated material (J/kg).
Standard unit is Gy.
Commonly used to define the quantity of radiation delivered at a specific point.
Subtle difference to Kerma which is energy converted and released in the medium (instead of deposited). Essentially interchangable.
Used to describe deterministic effects.

Front 21

Describe Air Kerma.

Back 21

Similar to absorbed dose.
Kinetic energy released in the medium.
Unit is Gy.
Sometimes used if medium is air.
To convert to absorbed dose tissue to air ratio is 1.06. (time air kerma x 1.06)

Front 22

Describe equivalent dose.

Back 22

Derived from absorbed dose x radiation weighting factor (Wr). Summed over a range of tissues.
Wr values: 20 for alpha particles, 1 for electrons and x-rays.
Used in radiation protection.
Unit is Sv.

Front 23

Describe Linear energy transfer (LET)

Back 23

Energy lost by a particle per unit length (keV/um)
High LET = high conc ionization = more damage.
Alpha particles (two protons, two neutrons) are heavy, not scattered easily, deposit energy over a short range -> high LET.

Front 24

Describe effective dose.

Back 24

Average equivalent dose to each organ and tissue in the body multiplied by a tissue weighting factor (Wt).
Thus takes into account variable radiosensitivities of organs.
Used to describe stocastic risk.
Unit is Sv.

Front 25

What are the tissue weighting factors for
a) breast
b) skin
c) gonads
d) stomach, colon
e) thyroid

Back 25

a) 0.12
b) 0.01
c) 0.08
d) 0.12
e) 0.04

RBM, lung, breast 0.12
Gonads 0.08
Bladder, liver, oesophagus, thyroid 0.04
Skin, bone surface, brain, salivary glands 0.01
Remainder 0.12

Front 26

Describe genetically significant dose.

Back 26

Equivalent dose to gonads, weighted for age of exposed individuals, over entire population.
It is the dose that, if received by every member of the population, would result in the same genetic injury to the entire population

Front 27

What are indirect and direct actions of radiation.

Back 27

Ionization processes cause majority of immediate chemical change in tissue.

Indirect (more common): free radicals created that damage DNA

Direct: radiation ionizes atoms in the DNA, breaking chemical bonds directly.

Front 28

What is the difference between stochastic and deterministic effects at a DNA level

Back 28

Stochastic: DNA damage with misrepair/ non repair

Deterministic: DNA damage leading the cell death

Probability of stochastic increases with dose, severity of deterministic.

Front 29

Dose required for deterministic effects?
a) skin erythema
b) cataracts
c) temporary sterility
d) invasive fibrosis/atrophy skin
e) dermal necrosis

Back 29

a) 2Gy
b) 5 Sv
c) 0.15Sv (perm 3.5)
d) 10 Gy
e) 20 Gy

Front 30

What are the risks of radiation to the foetus, what doses?

Back 30

<3 weeks: failure of implantation
3-8 weeks (organogenesis): organ malformation
8-15 weeks: mental retardation
Any time: cancer risk (2-3 x higher than general population)
Dose threshold 100mSv)
Dose limit is 1mSv/yr (2mSv to abdomen)

Front 31

What are the 3 principle of dose limitiations?

Back 31

1. Benefit outweights risk. Case by case.
2. ALARA.
3. Dose equivalent shall not exceed limits recommended.

Front 32

What is the average annual population radiation exposure/ sources?

Back 32

2.3mSv (varies with geographic location)
- radon 1.3mSv
- cosmic rays 0.3mSv
- gamma rays 0.4mSv
- terrestial 0.5mSv
- Internal sources (diet) 0.3mSv

Front 33

What are the occupational/ public dose limits (mSv) for
a) effective dose
equivalent dose
b) lens
c) skin
d) hands/ feet

e) patient limit

Back 33

a) 20/ 1
b) 20/ 15 (recently reduced)
c) 500/ 50
d) 500
e) none

Front 34

What are the 3 sources of radiation.

Back 34

Leakage: from x-ray tube
Primary beam
Scatter: patient is the source of scatter. At 1m away = 1/1000 of primary beam

Front 35

What are the 3 main avenues for exposure minimisation?

Back 35

1. Time
2. Distance
3. Shielding
Personal: gloves, aprons, collars, skin guards
Fixed: wall shielding, ceiling and table shields

Front 36

Discuss options for dose reduction and their advantages/ disadvantages

Back 36

1. Increase kV - decrease dose (mA reduction), reduce contrast.

2. Filtration - decrease dose, decrease contrast (beam hardening)

3. Collimation - decrease dose, improve IQ (less scatter)

4. Increase FFD, decrease SID - reduce deterministic risk, no change to stochastic (DAP the same)

5. Collimate - less dose, less noise.

6. Magnification - increase
dose, may improve IQ.

7. Remove grid - less dose, more noise.

Front 37

Efficiency of various shields
a) lead gown
b) thyroid collar
c) eye wear
d) ceiling shields

Back 37

a) 90-99%
b) 90%
c) 35-80%
d) 85%

Front 38

Dose estimation equations

Back 38

ESAK = K. mAs. (dA/SSD)2
K is output eg. 40uGy/mAs at 75cm in which case 75cm is dA)
ESD = ESAK. BSF
(BSF = 1.35)
Depth dose estimate = ESD. PDD/100
PDD = % depth dose - look up in a table

Front 39

Ultrasound equations
a) period
b) velocity
c) Io to I
d) acoustic impedence
e) Q factor

Back 39

a) 1/f
b) wavelength x frequency
c) 10log(10) (I/Io) dB
d) Z= density x velocity of sound in medium
e) fo/ change in f

Front 40

What is intensity?
What units?

Back 40

The rate at which ultrasound is applied to a specific tissue location in the body.

W/cm2

dB scale allows comparison of one signal level to another.

Front 41

Describe resonant frequency

Back 41

Natural frequency of oscillation based on properties of object.
2t (thickness of piezoelectric disc) will create maximal output - most constructive interference.
Will continue at resonant frequency unless damping applied.

Front 42

Describe Quality Factor (Q-factor)

Back 42

Ratio of mean frequency to bandwidth (fo/change in f).
Increasing damping will increase bandwidth and decrease Q.
High Q = less damping = pure note = long pulse -> good for continuous wave ultrasound.
Low Q -> good for pulsed wave.

Front 43

Describe the design of an ultrasound transducer.

Back 43

Components
- Earthed metal case
- Trasducer lead
- Acoustic insulator
- Backing block - provides damping
- Positive electrode
- Peizoelectric crystal
- Matching layer
- Plastic slip
-

Front 44

Describe near and far fields.

Back 44

Apply to unfocused transducers (not used anymore).
Near field (Fresnel) - parallel part of beam. =fD2.
Far field *Fraunhofer) - beam diverges + side lobes.
Smaller divergence angle with larger fD.

Front 45

Describe
a) absorption
b) reflection
c) refraction
d) scatter

Back 45

a) material absorbs ultrasound energy -> heat
b) occurs at interface where acoustic impedence changes
c) Angle between beam and interface -> deflection
d) occurs if object dimension are similar or smaller than wavelength, or at rough interface

Front 46

Define
a) linear attenuation coefficient
b) mass attenuation coefficient

Back 46

a) fraction of photons lost from x-ray beam/ unit distance (/mm)

b) u/ density

Front 47

What are the steps in CT image reconstruction?

Back 47

1) signal measured by detectors (up to 800 taking measurement every 1/3 degree) depends on attenuation coefficients of voxels traversed by pencil/fan beam. For each trans-axial projection a single profile is obtained.
2. Back projection - image back projected over many angles and summed together. Assumption of uniform attenuation causes blurring.
3. Filtered back projection - applies negative attenuation around edges to reduce blurring.
4. Recon takes place in frequency domain - Fourneir transform applied first.
5. Inverse Fournier transform -> back projection applied.

Front 48

What is the equation for
a) CT number
b) pitch

Back 48

a) 1000 x (u of m x u water)/ u water
b) tabletop mvt per rotation/ collimation

Front 49

Describe role of
a) bow tie filter
b) anti-scatter collimator
c) flying focal spot

Back 49

a) thin centre and thick edges of fam beam -> equalize transmitted intensities.
b) At detectors to reduce scatter
c) wobbling focal spot, gives more projections - improves spatial resolution

Front 50

What is collimator width?

Back 50

The width of the x-ray beam (not slice thickness)
eg. 5mm collimation can have 1x5mm slice or 2x2.5mm slices.
Collimation can not exceed detector width.

Front 51

Define
a) linear attenuation coefficient
b) mass attenuation coefficient

Back 51

a) fraction of photons lost from x-ray beam/ unit distance (/mm)

b) u/ density

Front 52

What are the steps in CT image reconstruction?

Back 52

1) signal measured by detectors (up to 800 taking measurement every 1/3 degree) depends on attenuation coefficients of voxels traversed by pencil beam.
2. Back projection - image back projected over many angles and summed together. Assumption of uniform attenuation causes blurring.
3. Filtered back projection - applies negative attenuation around edges to reduce blurring.
4. Recon takes place in frequency domain - Fourneir transform applied first.

Front 53

What is the equation for
a) CT number
b) pitch

Back 53

a) 1000 x (u of m x u water)/ u water
b) tabletop mvt per rotation/ collimation

Front 54

Describe role of
a) bow tie filter
b) anti-scatter collimator
c) flying focal spot

Back 54

a) thin centre and thick edges of fam beam -> equalize transmitted intensities.
b) At detectors to reduce scatter
c) wobbling focal spot, gives more projections - improves spatial resolution

Front 55

What is collimator width?

Back 55

The width of the x-ray beam (not slice thickness)
eg. 5mm collimation can have 1x5mm slice or 2x2.5mm slices.
Collimation can not exceed detector width.

Front 56

Explain why an 8 slice is more geometric efficient than a 4 slice scanner.

Back 56

Penumbra is consistently about 1mm regardless of collimation. Therefor % smaller with increased collimation size.
(8x2.5 is 76% vs 4x2.5 is 67%)

Front 57

What are the benefits and challenges with MSCT?

Back 57

Benefits
- more efficient use of tube/generator (wider collimation)
- can acquire thick and recon thin
- faster acquisition with high z resolution

Challenges
- scatter
- over beaming and over ranging
- cone beam (more so if thin slices, more slices). Data collected from two cones instead of ideal flat plane, more pronounced on outer detectors -> more artefact. Complex recon.

Front 58

Name eight (8) factors which effect spatial resolution on CT.

Back 58

1. Focal spot size - unavoidable magnification/ limited by heat loading
2. Detector size = size of smallest object detectable (lp/cm - max 20) typically 800 detectors in arc.
3. Sampling frequency - detector spacing, number of projections
4. Slice thickness and pitch - z axis resolution
5. Recon filter/ kernel - edge enhancement, smoothing
6. Pixel size - depends on FOV and display matrix. (eg. 512x512 with 40cm FOV =40/512 = 0.8mm with 6lp/cm as need to sample twice thusb1/0.16)
7. Patient motion
8. Detector afterglow

Front 59

What factors effect noise in CT

Back 59

1. Detector QDE
2. Patient size/ attenuation
3. Voxel size - increase size = less noise. If increase matrix size or reduce FOV there is a smaller detector area defining each pixel and thus more noise.
4. Increase mAs
5. Increase kV - beam hardening, can offset with reduced mA
6. Recon filter
7. In MSCT increase pitch = increase noise.

Front 60

What is the definition if modulation transfer function? How is it used?

Back 60

Ratio of output to input signal as a function of spatial frequency, used to quantify imaging systems.
It is derived from the LSF, 1D Fournier transform proportional to ratio is info recorded to available.
Ideal system = 1, however edge enhancement can >1.
Use MTF curve to define spatial resolution.

Front 61

List 6 CT artefacts.

Back 61

1. Detector failure - ring
2. Patient movement - blur/ streaks
3. Beam hardening - cupping or dark bands due to photon starvation. Increasing kV/ bow tie filter reduces this. Eg. BOS
4. Partial volume effects - rapidly changing density in z-direction - streak artefact. decrease by reducing pitch/ slice thickness.
5. High-attenuation objects cause beam hardening, partial volume, undersampling
6. Cone beam effect - pronounced outer detectors. Blur between high contrast boundaries.

Front 62

List factors affecting dose in CT

Back 62

1. mAs - mA modulation protocol (may be based on scanogram) alter dose dependant on slice thickness/ attenuation. Three features are Patient size AEC, z-axis modulation, rotational modulation (thinner AP)
2. Pitch - dose proportional to 1/pitch
3. kVp - can reduce mA for overall dose reduction
4. Scanner geometry/ filtration
5. QDE
6. No phases
7. Over ranging - 1 x collimator thickness, over-beaming - geometric efficiency, penumbra
7. Patient size

Front 63

Explain DRLs.

Back 63

Dose reference levels.
Used as a guide for dose limits based on 75th centime of institutions around Australia.
Not a dose limit, but if > need to justify or adjust.

Front 64

What are the dose related equations for CT?

Back 64

CTDIw = 2/3 periphery + 1/3 centre (based on phantoms)
CTDIvol = CTDIw/ pitch (considers non contiguous scanning)
DLP = CTDIvol x L
L= scan length
Can the use conversion coefficients to estimate effective dose

Front 65

What are the differences between continuous and pulsed wave ultrasound?

Back 65

Continuous wave - AC voltage of desired frequency applied. Each travels a wavelength before next applied.

Pulsed - DC current on and off. Will ring at resonant frequency, varying degrees of damping will affect Q-factor.

eg. Doppler - continuous has excellent velocity determination but poor depth (long SPW) and requires a second crystal to receive.

Front 66

Expain A, B and M mode in ultrasound and how they differ.

Back 66

A mode (amplitude): pulsed wave, plots amplitude of echoes from different depths. Largely superceded, may be used for eyes, midline shift, depth of stone for ESWL.

B mode (brightness): array of transducers (linear, phased) give 2D depiction of echoes converted into grey scale. Brightness decreases with depth, TGC used to compensate. Uses pulsed ultrasound.

M- mode (motion): quick succession of pulses used to a tract of a moving object in respect to time - allows quantitative assessment (eg. heart valves)

Front 67

Ultrasound scanners - discuss
a) mechanical
b) electronic

Back 67

a) not used any more, moving element within transducer
b) Linear array - linear or curvilinear - elements pulsed in sequence to focused beam. Change speed of pulse sequence to change focal depth (point where waves arrive and reinforce)
Phased array: shorter transducer with fewer element. Energized in sequence (across instead of outside in) -> constructive in one direction and destructive in all others. Produces a 'steered' beam.

Front 68

What steps are applied to the received beam before it is displayed?

Back 68

1. Amplification
2. TGC (auto or selective)
3. Logarithmic signal compression. Compress signals with large difference (bone/air) and maintain those with small difference (soft tissue/ soft tissue)
4. Demodulation
5. Rejection of weak echoes (electronic noise)
6. Digital scan converter -> apply analogue digital converter.

Front 69

Define axial and lateral resolution.

Back 69

Axial - ability to distinuish objects along the axis (depth) = 1/2 SPW (wavelength x wavelenth/pulse)

Lateral: ability to distinguish two objects at same depth. Poorer than axial resolution. Affected by transducer diameter (lines per frame), frequency, focusing, distance, power output, dynamic range.

Front 70

What are the different ways to produce a radionuclide?

Back 70

1. Neutron excess. Addition of neutron in nuclear reactor. Same atomic number (element), mass number +1

2. Neutron defect. Addition of a proton in a cyclotron -> knocks out a neutron. Atomic number +1, mass number (proton+neutron) unchanged.
As are different element can be obtained carrier free.

3. Nuclear fission products

4. Daughter products obtained from generators such as the case with Tc99.

Front 71

How to radionuclides undergo decay?

Back 71

1. Neutron excess -> B- decay.
Neutron changes into an electon and a proton. Atomic number +1.
Still has excess energy -> emission of gamma rays.
Isometric transition - considered 'metastable' prior to gamma emission.

2. Neutron defecit -
B+ decay - proton into neutron + positron. Atomic number -1. Daughter nucleus emits gamma rays
K-electron capture: capture k shell electron. A/Z conserved. Emit characteristic +/- gamma rays.

Front 72

Define physical, biological and effective half life and how they are related.

Back 72

Physical = time taken to decay to half its original value. Fixed value.

Biological = time taken for pharmaceutical (with which a radionuclide is labelled) to be metabolised within the body to half its original. This is an oversimplification as biological excretion is not adequately described by simple exponential decay.

Effective half life = time taken for a radiopharmaceutical to reduce to half within body
= 1/biol HL + 1/phys HL

Front 73

List desirable qualities of a radionuclide.

Back 73

Physical
1. Emit only gamma rays. Isometric transition or electron capture.
2. Energy of 50-300keV - sufficient to leave patient but low enough to be easily collimated and detected.
3. Half life of a few hours
4. No daughter radiations
5. Monoenergetic emission so scatter easily eliminated by PHA.

Chemical
1. Pure chemical form
2. Chemically stable
3. Able to be attached to several pharmeceuticals with high specific activity

Pharmacology
1. Sterile
2. Pyrogen free
3. neutral pH
4. excretable

General
1. Easy to produce from readily available source
2. Easy and safe to transport - helps if can be supplied with longer lived parent in a generator
3. Not expensive

Front 74

Describe how a Tc generator works.

Back 74

Using 99Mo and 99Tcm as an example.

Generator within lead housing.
Contains parent in an exchange column (Mo with HL of 67hrs)
Daughter is decaying with its own HL (6hrs) as quickly as it is formed.
Elution (wash off column) of the daughter.
Elution daily, change generator weekly.

Front 75

Types of quality control testing for radionuclides

Back 75

1. Purity testing - contamination with parent
2. Sterility testing

Front 76

What are the components of a gamma camera.

Back 76

1. Patient emits gamma rays in all directions.
2. Collimator - close to patient. Acts like grid. Lead, 0.3mm septa, 2.5mm thick. Scatter reduction. Typically <1% of primary beam.
3. Phosphor crystal - gamma into light. 5000:1. High Z - photoelectric effect predominates.
4. Photomultiplier (around 90) - contains photocathode (light to electron) and dynodes (increase electrons 10x6)
5. Pulse arithmetic - equations applied to give horizontal and vertical coordinates (x,y) and energy (z).
6. PHA - applied to z for scatter rejection. Photopeak with PHA window of about 10%.
7. Analogue to digital converter.
8. Computer. 512x512 matrix.

Front 77

What effects spatial resolution in gamma imaging?

Back 77

Spatial resolution is inversely proportional to sensitivity - determined by collimator type. Multiple small holes = good SR, poor sensitivity.
Can reduce FOV to improve SR.
Closer patient is the collimator the better the SR.

Front 78

How is SPECT acquired.

Back 78

SPECT = single photon emission computed tomography.
Camera moves in 360 degree arc around patient stopping to acquire images every 6 degrees.
Photon/ noise -limited. Poor SR (18mm), matrix limited to 64x64, thick slices.
Filtered back projection applied.
3D or series of slices.
MPS main use.

Front 79

How does PET work?

Back 79

Utilises positron emitters - positron travels short distance (2mm) before annihilated by an electron -> 2 x 511keV photons in opposite directions.
Normally use 18F (HL 110 mins) labelled with FDG.
Camera is a ring of 10000 scintillation detectors which detect coincident events along lines of response (LOR) as well as scatter and random coincidence. Ideally high detection efficiency, short scintillation decay time, good energy resolution.
No collimator
Photomultiplier
Data acquisition - sinogram, arc and tissue attenuation correction
May perform 3D recon
Integrated PET-CT with image fusion software.

Front 80

Discuss use of filters.
What are the limited required filtation?

Back 80

Filtered are employed to remove low energy x-rays which do not contribute to the image and increase patient dose.
K-edge filters are used in mammography to remove x-rays above the k-edge (Bremsstrahlung) which reduce subject contrast, whilst retaining characteristic x-rays.
Predominantly photoelectric effect.
Ideal filter does not produce characteristic x-rays (Cu backed with Al).
Measure HVL - relationship to linear attenuation coefficient = u/0.693.

Minimum total filtration (inherent + added)
kVp <50 = 0.5 (0.3 Mo)
kVp 50-70 = 1.5 mmAl
kVp >70 = 2.5mmAl

Front 81

Write notes about CR plates.

Back 81

Operate via photostimulable luminescence = release of stored energy within a phosphor by stimulation with visible light.
Use photostimulable phosphor - plate illuminated twice. First by radiation to 'write' image and secondly with visible wavelength laser to 'read' image.
Europium used to create ímpurity centres'- when irradiated electrons are excited with some 'stuck' in immediate impurity centres or traps. Laser light (usually infrared light reading blue-green) causes electrons to jump out of traps and back to baseline 2+ with emission of light.
Can read on both sides of plate to improve spatial resolution.
Spatial resolution limited by
1) diameter of laser
2) thickness of phosphor
3) speed of laser
CR more efficient than rare earth at 37-50keV.
Emitted light is fibre optically filtered and then amplified, compressed, sampled and digitalized by analogue digital converter (ADC). Image built up in typical raster line by line fashion.

Front 82

What are the limitation and advantages of CR cf film screen?

Back 82

LImitations
- Spatial resolution is inferior - limited by laser beam diameter, PSP thickness and readout time.
- Poorer radiation dose efficiency (zone of advantage at 35-50keV)
- Increased sensitivity to scatter than rare-earth screens
- viewing monitor needs required resolution and brightness
- demands on digital storage
Advantages
- Main benefit linear characteristic curve (measure of log exposure vs optical density) giving broad dynamic range
- digital manipulation of image
- electronic storage
- reusable phospor
- post-processing of image (windowing, LUTs, brightness control)
- dose reduction -> less re-takes

Front 83

What pre-processing is applied to CR/DR images?

Back 83

Inverts appearance of image.
Removes collimated areas.
'Scales' to mid range brightness.
Grey scale transformation - use look up tables (LUTs) to map across pixel values and convert to sigmoid curve of film screen. Apply steeper curve to improve contrast resolution (narrow window, less grey scale).

Front 84

Structure of
1. CCD detector
2. Direct DR plate
3. Indirect DR plate

Back 84

1. Charge couple detector. Light sensitive silicon detector elements.
2. Selenium. Photoconductor (xray -> charge)
3. CsI phosphor -> Silicon photodiode
Detector = TFT (thin - film transistor arrays) for both direct and indirect. Contain charge collection electrode and charge storage capacitor -> signal out in raster pattern readout.
Flat panel fill factor = light sensitive area/ area of detector element. Small pixels give good SR, poor contrast as less light sensitive area -> reduced absorption efficiency.

Front 85

1. What is the equation for optical density.

2. If 1% of light transmitted what is the OD?

3. How is OD expressed with a characteristic curve?

Back 85

1. D = log10 (Io/Ii)
Io = incident light intensity
Ii = transmitted light intensity.

2. log10(100/1) = 2

3. Log of exposure vs OD
Range of useful OD 0.2-2.5

4. Maximum gradient of the slope.

5. Range of Log(exposure) for densities (0.25-2.5) - inversely proportional to gamma. Narrow latitude - good contrast, poor tolerance of exposure range.