Physical principles of MRS
X-ray production
X-ray beam
X-ray attenuation
Scatter
Exposure factors
Image quality and image contrast
Physical Principles of MRS
Section 6: Image Quality
ALARA Principles
1) Justification
a. Only expose a patient if the benefit (to patient or society) outweighs the harm
2) Optimization
a. Keep exposure settings as low as possible while also maximizing benefit
3) Dose limits
a. Doses shall not exceed the recommended limits
Image Quality Factors
o
o
o
Contrast
a. The ability to perceive changes in density
Spatial resolution
a. Sharpness/detail (how well an image is seen)
Noise
a. Salt and pepper appearance resulting from low photon numbers
Spatial Resolution
 Can be assessed subjectively and objectively
o Assessed objectively using line pair phantoms
o
Ur = unsharpness caused by image receptor
o
Single IR = more detail than double
o
Un = unsharpness caused by noise/mottle
o
Ui = unsharpness caused by image receptor and noise (combined effect)

o
Ug = unsharpness caused by geometric unsharpness/penumbra (EFSS, OID, SID, SOD)

o
Ug =
𝐸𝐹𝑆𝑆 𝑥 𝑂𝐼𝐷
𝑆𝐼𝐷− 𝑆𝑂𝐷
Um = unsharpness caused by patient movement (voluntary or involuntary)

o
Ui = √𝑈𝑟 2 + 𝑈𝑛2
Um = velocity x time
Ut = total unsharpness (combined effect)

Ut = √𝑈𝑖 2 + 𝑈𝑔2 + 𝑈𝑚2
For 2 objects to appear
separate, the width of
separation between
the objects must be
greater than the
Full Width Half
Maximum
Measuring Spatial Resolution
Edge spread function
Line spread function
Point spread function
for
for
for
sharp-edged objects
narrow slits
point objects
Use transect plot for digital images
Use microdensitometer for film/screen
Modulation Transfer Function (MTF)
 Measurement of degradation when transferring an object to become an image
 For an image to be a perfect representation of the object, the transmitted signal wavelength
should match the recorded signal wavelength
 MTF = recorded frequency / original frequency
 Frequency domain:
o Small objects = high frequency transmitted
o Large objects = low frequency transmitted
o Measured in Hz of line pairs per mm (lp/mm)
 Spatial domain:
o The image
Signal to Noise Ratio (SNR)
(uncommon in med imaging)
 Measures the amount of signal reaching the IR
o In CR, this is the number of photons reaching the IR
 SNR measures the pixel value vs the amount of noise
 SNR is the log of the image variance divided by the noise variance
o
𝜎𝑖 2
10 log10 𝜎𝑛2
 Area of signal measurement = human anatomy
 Area of noise measurement = background
Weiner Spectrum (WS)
 The relationship between noise and spatial frequency
Detective Quantum Efficiency (DQE)




The most useful evaluation tool for image systems
Combines the effects of noise, contrast and spatial resolution
Measures how well the system converts x-rays to an image
Measures detector efficiency
𝑆𝑁𝑅𝑜𝑢𝑡 2
)
𝑆𝑁𝑅𝑖𝑛
 DQE = (
 DQE increases as object detail (frequency) increases
 DQE depends on the number of photons reaching the IR and then being converted
o DQE depends on the mAs and kVp
Subject Contrast
Radiographic Contrast
Contrast Resolution
Is inherent in the patient
Our ability to see differences in
film densities/shades of grey
The ability of the imaging system
to resolve differences in subject
contrast/density
Is affected by object
thickness, density, beam
energy and the object’s
linear attenuation
Is easier to visualize small
differences in density in larger
objects
Is dependent on object size
Can be altered using
contrast
Contrast-to-Noise Ratio (CNR)


It is only possible to visualize an image if there is a difference in contrast
Greater object size and/or depth = increased object contrast
o Smaller objects need greater depth to be seen
o Larger don’t need as great of contrast to be seen
Subjective
Assessment:
An observer’s
judgement of quality
Objective
Assessment:
Does not rely on
human opinion
Observer Performance Methods
Lesion Detection:
ROC
Visibility of Anatomical Structures:
VGA, VGC, IC
Visual Grading System (VGA)
o
An observer assesses image quality based on an absolute or relative rating scale
o
Relative rating:
worse
comparing a region on 2 images on a scale from -2 to +2 (much
than - much better than)
o
Absolute rating:
(not
and 4 (very
no reference image, only the observer’s opinion from a scale of 1
visible), 2 (poorly reproduced), 3 (adequately reproduced)
well reproduced) to determine overall image quality
Receiver Operator Characteristics (ROC) Analysis

Used to compare one imaging modality to another
o Often used to assess the quality of an imaging modality

Involves analysis of equipment, processing and observer’s performance
o Observer’s performance is based on knowledge, experience and perception factors
e.g., time of day, alcohol, sleep, eyesight etc.)

An expert is asked if a condition is present on an image or not
o The result is measured against a ‘gold standard’
Expert said present
Expert said not present
Truly present
True positive
False negative
Truly not present
False positive
True negative

Sensitivity
measure of how accurately a test detects positive (abnormal) findings
True positive fraction (TPF)

Specificity
measure of how accurately a test detects negative (normal) findings
True negative fraction (TNF)
TPF
=
𝑇𝑃
x 100
𝑇𝑃+𝐹𝑁
TNF
=
𝑇𝑁
x 100
𝑇𝑁+𝐹𝑃
False Negative Fraction (FNF) = 1 – TPF
False Positive Fraction (FPF) = 1 – TNF
Predictive Value Positive = true pos ÷ total pos
Predictive Value Negative = true neg ÷ total neg
ROC Curves
A test with high sensitivity and high specificity will have a plot close to perfect (top
left)
Lax Threshold:
increased sensitivity
increased number of true and false positives
decreased number of true negatives
Strict Threshold:
increased specificity
increased number of true and false negatives
decreased number of true positives
ROC limitation:
Does not ask the
observer for the location
or number of
abnormalities
(Could misidentify the
location of
abnormalities)
Quality Assurance (QA)



An organized process to establish and monitor the performance of a diagnostic imaging
system
Ensures consistently high-quality images with minimal unnecessary radiation and costs
Helps make consistently good x-rays and limit patient and staff dose
Basic QA tests




LBD alignment
CR accuracy
Lead apron checks
Magnification ratios
LBD Congruence Test:
Coins are placed within the
irradiation field and two
radiographs at different
exposures are taken (one for
DR) to determine if the coins
appear in the correct spot
Quality Control
CR Alignment Test:
A glass tube with 2 small
holes in the top and bottom
are placed on the CR and a
radiograph is taken to
determine whether the holes
on the image align
o An aspect of QA
o Technical tests of imaging equipment for maintenance/monitoring
o Ensures that equipment is safe and efficiently produces high quality images
Acceptance
Routine testing
Error
Preventative
testing
maintenance
Maintenance
Performed after
Periodically done to
Reject analysis
Performed biannually
installation
check system
Exposure analysis
by manufacturer
Ensures equipment
performance
Ensures compliance
performs properly
Invasive: undertaken by
with license and
physicist or engineer
registration conditions
Non-invasive:
undertaken by
radiographers
Basic QA Tests
LBD congruence test
CR alignment test
Lead apron checks
Magnification ratio
tests whether irradiation field aligns with the light beam (not for DR)
small holes in a glass on the CR line must superimpose on an image
make sure there are no holes in the apron
QA of the QA


Used to evaluate the effectiveness of the QA program
Assesses performance of image quality, reject/repeat rate and patient doses (exposure
creep)
Radiation Limits Imposed
kVp accuracy, kVp reproducibility, exposure linearity, exposure reproducibility, timer accuracy,
timer reproducibility, filtration/HVL, collimation, tube leakage
Reject Analysis

An acceptable standard of practice for quality assurance


Reject rates should not exceed 5%
Measures and monitors reject rates, identifies reason for rejects and enables comparisons of
reject rates to be made across departments
 Reject rate = number of rejects / total films
Reject Film
Repeat Film
Image deemed useless due to poor quality
Another image is taken to provide
Another film is taken to replace it
extra/missing info
Is used with the original to make a diagnosis
Computed and digital
Physical Principles of MRS: Section 7
Computed and Digital Radiography
Computed Radiography (phosphor plate radiography)
- Phosphor crystals are sensitive to photons in the x-ray range
Latent images are formed through phosphorescence
1. X-ray photon is absorbed by a phosphor crystal
2. Energy is transferred to Eu⁺² to become Eu⁺³
(Eu atoms are ionised)
3. A photoelectron is ejected from the valance band to the conduction band
4. The electron travels to the f-center (metastable higher energy state)
Readout occurs when phosphor crystals are exposed to low energy (red) light
The number of
trapped electrons is
proportional to the
amount of radiation
absorbed
1. The phosphor layer is scanned by a laser to encourage photo-stimulated excitation
2. The laser light is absorbed by the f-center
3. Trapped electrons are released to the conduction band and then back down to the
valance band (the light energy is insufficient to create more Eu⁺³ ions)
4. Photomultiplier tube (MTP) collects the light and converts it to an
The voltage of the
electrical signal
electrical signal is
5. The electrical signal is amplified and converted to a digital value
proportional to
the amount of
equivalent to its optical density
light received
Plate erasing removes any latent image remaining after image readout
1. The imaging plate is exposed to high-intensity white light
2. Imaging plate can now be reused
Digital Radiography (flat panel detectors)
1.
2.
3.
-
-
Characterized by being built into a table/bucky OR being wirelessly connected to a PC
Are based on thin-film transistors (TFT) arrays
In-direct DR
Direct DR
uses cesium iodine (CsI) to convert x- uses amorphous selenium (a-Se) to
rays to light
directly convert x-rays to an electrical
CsI is laid over amorphous silicon (a-Si)
signal
X-rays interact with CsI
Visible light interacts with a-Si
a-Si converts light to an electrical
signal
DR advantages
better spatial resolution than CR
(Produces less noise, therefore
sharper)
higher contrast resolution than CR
lower dose than CR and F/S
(theoretically)
quicker image capture
Digital Data
- one binary value = 1 bit (0 or 1)
- one byte = 8 bits
Pixels
-
digital images are made of a matrix of pixels
each pixel has a spatial location and value
o 255 = white
o 0 = black
Look-up table (LUT)
-
-
Needed for image display and for changing
brightness and contrast
Allows changes to be applied to
all pixels at once
E.g., an operation of x1.5
multiplies the input pixel value by
1.5 to give a new output value
Change in window level =
brightness alteration
Change in window width (slope) =
contrast alteration
-
DR disadvantages
older units have fixed IP positions
more expensive to buy than CR
Increased slope = increased contrast
Decreased slope = increased detail
transect histogram
-
Dose Limitations
o CR/DR doses have higher noise than F/S
o SNR (signal noise ratio) = limiting factor for image quality
o (Increased kVp and mAs = increased SNR = increased quality but also increased patient dose)
o Low SNR = noisy/low quality image (more scatter)
Dose in CR/DR
-
Exposure is measured using a histogram and anatomical region
o Cannot base exposure quality on brightness because kVp and mAs will change the
histogram, but will not change the image appearance
- EI is the most common measurement tool
Determining EI
 ROI method (take mean values of the
center 25% of the image)
 Histogram method (use histogram to
determine regions of anatomy and no
attenuation)
EI (exposure index)
Measures the detector
response to radiation
Image Receptor Artifacts
Dust/dirt
Scratches
Plate not fully erased
EIT (target exposure index)
The expected EI value for a set
anatomical region & projection
Software Artifacts
Pre-processing Post-processing
Dead pixels
Image
compression
DI (deviation index)
A number quantifying the
deviation of the EI from the EIT
Object Artifacts
Histogram analysis error
(due to incorrect collimation)
Physical Principles of MRS: Section 9
Other X-ray Equipment
Mobile X-ray Units




Mainly used in bed-side radiography and operating theatres
Have an extendable/retractable arm
Use high-frequency generators
Some are linked to DR plates to immediately display and send an image to PACs
Capacitor Discharge




Old and illegal in Australian states
Use low voltage power supply to charge a high kV capable capacitor
Residue is always left in the capacitor (incomplete discharge)
Often used in veterinary imaging
Mobile Image Intensifier
Used in operating theatres to visualize static or dynamic images
Use high frequency generators
Have a last image hold function
Tomography
 Used to view anatomical structures without overlying
anatomy
 Requires synchronous movement of the tube and
receptor in opposite directions
 The desired anatomical structure must be centered to
the fulcrum
 Smaller tomographic angle = more anatomy shown
(thicker object plane)
 The amount and quality of blurring depends on exposure
time
 Can only use an anti-scatter grid when the tube moves
linearly
Tomosynthesis






Creates very thin slices of anatomy at various depths
The x-ray tube moves linearly while the receptor remains stationary
Images can be reconstructed into a 3D image
Shift and add method is used to determine the anatomy location
More images = greater angle of movement and thinner slices
Is nearly equivalent to MRI for showing rheumatoid arthritis
Intra-oral X-ray Equipment
Uses a stationary anode
Use low kVp and mAs (used on small anatomical area)
Bite wing, peri-apical etc.
Extra-oral X-ray Equipment
 Use higher kVp and mAs (used on larger anatomical area)
Orthopantomography
o
o
o
o
o
Tomography of mandible and teeth
X-ray tube travels 150° posteriorly in a circular motion (x-ray tube behind the head)
CR has a small angle superiorly
X-ray beam is collimated as a vertical slit
Focal plane is semi-circular and thick to include the mandible
Cephalography





A lateral radiograph of the anterior head and face
A head holder and ruler are required (to measure size of head without
magnification)
Often use a modified OPG unit with a cephalic head holder
Used for reconstructive procedures (e.g., braces)
Must show the soft tissue of the face