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The Extras - Collimation, filtration, and grids
Artifacts
 Read Fauber pg 222 - 225
 These may show up on the final practical and/or
exam
 Be able to identify cause based on appearance
Scatter Radiation
 Results from Compton interactions (deflection)
 Increases with kVp
 Increases with volume of tissue exposed
o Thickness
o Area
 Scatter photons hit film, produce density
o FOG
Collimation
 AKA - Beam limiting device
 Limits the area exposed to film size or less
 Reduces patient exposure
 Old style - cones, cylinders
 New style - variable aperture collimator box
 Produces round collimation field
 Cylinders still used in dentistry
 Cumbersome to change cones for body imaging
Grids
 Post-patient scatter reduction
 Increases patient dose
 Lead strips separated by aluminum or carbon
fiber
 Focused (most common) or parallel
 How will grids affect the film?
o Density? Decrease
o Contrast? Increase
o Artifacts? Gridlines
Grid Characteristics
 Grid ratio (10:1, 12:1)
 Grid frequency (lines per inch)
Grid Cutoff
 Read page 156 to 162 in Fauber
 This will be on the test.
Filtration
 Changes beam quality and quantity
o Patient exposure
o Image quality
 Types:
o Total Filtration
 Inherent
 Added
o Compensating
Total Filtration
 Total filtration required is 2.5 mm of Al or
equivalent
 Reduces patient exposure
Compensating Filters
 Improve image quality
 Evens image density for uneven thickness
anatomy
 What else can we do to help accomplish this?
Beam Hardening
 Weaker photons preferentially removed
 Beam quantity decreases
 Beam quality increases
Half-value Layer (HVL)
 Defined: thickness of material required to
attenuate half the photons in the beam
 Permits equivalency of different materials
 Al HVL = Pb HVL = Cu HVL
 First HVL removes 50% of the initial photons
 Second HVL removes 50% of what's left
 Third HVL removes 50% of what's left
 What percentage of beam remains after 3 HVL's?
12.5%
Reading Assignment
 Chapter 5 in Fauber
 Grid cutoff, pgs 156-162
The Extras - Penumbra, Magnification, Distortion and
the Inverse Square Law
Geometric Properties
 Sharpness
o Geometric unsharpness
o Receptor unsharpness
o Motion unsharpness
 Distortion
o Size
o Shape
Vocabulary
 FFD (Film focal distance)
o Distance from tube to film
o AKA: SID - source imaged distance
 OFD (Object film distance)
o Distance from object to film
o AKA: OID - object image distance
 SOD (Source object distance)
o Distance from tube to object
Sharpness
 Penumbra - edge unsharpness
 Caused by not having a point source
 Different parts of focal spot produces different
shadows
 Different shadow overlapping cause blurry edges
Penumbra vs. Focal Area
 Larger focal spot produces more penumbra
 Larger filament = large focal spot
 Large anode angle = large focal spot
Penumbra vs. FFD
 Longer FFD = less penumbra
Penumbra vs. OFD
 Smaller OFD = less penumbra
Receptor Unsharpness
 Screen Speed
o Increase phosphor crystal size = increase
penumbra
o Increased phosphor thickness = increased
penumbra
 Film
o Increased film grain size = increased
penumbra
Motion Blur
Magnification
 Increased FFD = less magnification
 Decreased OFD = less magnifcation
Distortion
 Caused by unequal magnification, irregular
shapes, off center anatomy and tube not
perpendicular to film (tube angle)
 Minimized by reducing magnification, placing
anatomy-of-interest in the middle of the film and
avoiding tube angles when possible
Does any of this affect density and contrast?
FFD vs. optical density
 Photons spread out
 Less photons per unit area
 What will that to do to the image?
Inverse Square Law
 Intensity = 1/d^2
 Double FFD = ¼ density
 How do we compensate?
OFD vs. optical density
 Scatter from object diverges and spreads out
similar to primary beam
 What happens as OFD increases?
 How will that affect film density?
OFD vs. Contrast
 Fewer scatter photons hit film as OFD increases
 How does this affect fogging? Decreases
 How does this affect contrast? Increases
Reading Assignment
 Read Appendix D in Fauber pgs 360-362
Radiobiology
ALARA (As Low As Reasonably Achievable)
 Guiding principle
 Assumes any and all radiation is harmful
 Only expose when necessary
 Must weigh benefits and risks
 Worst offender for overexposure is retakes
Terminology – Units
 Roentgen (R) - measures radiation intensity
 Rad (rad) – measure of absorbed dose (used in
class)
 Rem (rem) – measure of absorbed dose in humans
(effect on human tissue)
Tissue Sensitivity
 Law of Bergonie and Tribondeau
 Radiosensitivity decreases with cell maturity
 Radiosensitivity decreases with tissue age
 Radiosensitivity increases with metabolic rate
 Radiosensitivity increases with proliferation rate
Sensitive Tissues
 High sensitivity – lymphoid tissue, bone marrow,
gonads
 Moderate sensitivity – skin, eyes, GI lining, liver,
kidney, thyroid, growing bone
 Low sensitivity – muscle, nerves, mature bone
Tissue effects
 Cells/tissues damaged by direct effects (DNA hit
by photon) or indirect effects (free radical
formation)
 Early effect – occurs within minutes to days
o Erythema (skin redness), death

Late effect – occurs after 6 months
o Cancer, death
Acute Radiation Syndrome
 Prodromal period – few hours to days
o Symptoms present
 Latent period – few days to weeks
o No outward symptoms
 Manifest illness
o Rapid decline usually ending in death
(hematologic, GI, nervous system)
Decreasing Tissue Effects
 Protraction
o Slower rate of exposure over longer
period of time
 Fractionation
o Same rate but broken into short session
spread out over time
 Reduce area exposed
o More dose needed for smaller area
Prenatal Exposure
 Embryo/fetus is more sensitive
 Avoid exposure in non-ife-threatening cases
 10 day rule
o Women of child-bearing age can only be
imaged between the 1st day of LMP and
10th day
o No imaging between 10th and 28th day
o After 28th day, must fail pregnancy check
Population Effects
 Exposing most in a population to a little bit of
radiation is bad for the population (20-30 mrad
GSD)
 Exposing a few in a population to large amounts of
radiation is bad for those few
 Exposing a few in a population to a little bit of
radiation has a minimal impact
Dose Response  TQ
Death Risk  TQ
Dose Limits (Annual)  TQ
Limiting Exposure
 Only x-ray when information gained will alter
patient management
 Utilize shields (lead aprons) whenever possible
 Collimate beam as much as possible
 Avoid retakes (worst offenders)
o Measure/proper factor selection
o Proper patient positioning
o Dark room protocols
Typical Exposures  TQ
Take home message
 X-rays can provide important clinical information
 X-rays cause cellular and subcellular damage
 X-rays are very low risk when used sparingly and
appropriately
 X-rays should not be used as a routine method of
examination
 Childhood and prenatal exposure should be
avoided
 Use the 10-day rule to avoid exposing
embryo/fetus
 Practice ALARA always
Reading Assignment
 Read Chapter 12 in Fauber, pgs 293-327
Digital Radiography
Why digital?
 Greater dynamic range
 Greater contrast resolution
 Easier to store, easier to send
 Can be in two places at once
 Post-processing (clean up image, enhance edges)
 Manipulate image at will in real-time
Why not?
 Digital is “slower speed” and requires higher
exposure
 Expensive (upfront)
So how does it work?
 Everything is the same up to the point where
photons hit the receptor
 Two types
o Computed Radiography (CR)
 Cassette-based
o Direct Digital Radiography (DR)
 Cassetteless
CR
 Uses an imaging plate instead of film/screen
 Utilizes phosphorescence
 Stores photon energy for release during
“processing”
 Laser stimulation causes stored energy to be
released as visible light
 Light detector determines brightness (density)
DR
 CCD capture
o Take a digital picture of a glowing
phosphor plate
 Flat Panel capture
o Indirect converstion
 Convert x-ray photons to light,
convert light to electrons,
measure electrons
o Direct conversion
 Convert x-ray photons to
electrons, measure electrons
CR vs. DR  TQ
Displaying Digital Imaging
 Very high resolution display needed to show all
the detail that is recorded
o Typically 1500x2000
o Long scale of gray (1024 shades)
o $$$
Window/Level
 Window
o How many shades of gray to display
o Essentially changes contrast
 Level (Center)
o How bright is middle shade of gray
o Essentially changes density
Storing Digital Imaging  TQ
PACS
 Picture Archival and Communication System


Stores, sends and displays images
On-site and off-site components ensure data
protection
 Passwords and encryption ensure data security
Computed Tomography (CT) – An Introduction to
Cross-sectional Imaging
Review
 5 Primary Radiographic Densities
o Air (Black)
o Fat (Dark Gray)
o Water (Light Gray)
o Bone (White)
o Metal* (Bright White)
A Brief History  TQ
I thought this was a CAT scan
 Computed Axial Tomography (CAT) was the
original name
 Advances have to led to high resolution images
(reconstructions) in non-axial planes
Body Planes
 Axial – Divides into top and bottom
o Superior/Inferior
 Sagittal – Divides into right and left
 Coronal – Divides into front and back
How is this the same?
 Uses ionizing radiation (x-ray photons)
 Photons generate the same way
 Photons behave the same way
 Physics is physics
So, then what’s new?
 Novel application of math to attenuation data
 Gives us new info
 Give us new perspective
 Gives us more sensitive measurements
What do you mean by cross-sectional?
 X-ray 2D
 Slide 3D
Slice and Dice
How the heck does that work?
 Take lots of pictures from different viewpoints
 Computer takes all the data and figures out what
the middle looks like
Ok, now I’m confused
 How do we turn those seemingly useless strips of
data into that sweet picture of the dude’s chest?
 The simplest method is back-projection
How do we speed it up?
 Fourier analysis – crazy math that makes
calculations super fast
 Same method is used in MR, although in a slightly
different way
 We’ll look in more detail in the MR section
Where are we at today?
 Super fast helical CT
 Electron bea, CT (heart scans)
Computed Tomography – Basic of Image
Interpretation
What does it all mean?
 Each picture element (pixel) in the image
represents a chunk of tissue

An attenuation value is calculated for each pixel
o Hounsfield unit
o -1000 to +1000
 A shade of gray is assigned based on the
window/level and the attenuation value
The Physics of MRI – An Introduction
Key Terms
 Field strength (T)
 Field direction (B0)
 Dipole
 Precession
 RF pulse
 Lamor frequency
 Resonance
 Signal
 Noise
 Signal-to noise ratio (SNR)
 T1
 T2
 TR
 TE
 Echo
 Spin echo (SE)
 T1-weighted image (T1WI)
 T2-weighted image (T2WI)
 Proton density image (PDWI)
Magnetism
 Types of magnets
o Permanent
 Kitchen magnets, bar magnets
o Electromagnet
 Electric motors, solenoids
o Superconducting
 Special type of electromagnet
 Field strength
o Varies with location within the field
o Measured in tesla (T)
 Field direction
o Varies with location within field
o Typically referred to as B0
o Usually aligned with longitudinal axis of
patient
Paramagnetic properties
 T1
 T2 (T2*)
 Relaxation time is defined as time required to
relax 63% of the way
 5x T1 or T2 is effectivelt 100% relaxed
T1 relaxation
 Relaxation back towards the magnetic field
direction B0
 Signal decays exponentially
 Fat relaxes faster than water
 On the order of 200 to 2000 ms
T2 relaxation
 Dephasing of precessing atoms
 Decays exponentially
 Much faster than T1 relaxation
 Affected by local field inhomogeneities (T2*)
 Water dephases slower than fat
 On the order of 20 to 200 ms
It’s all about the HYDROGEN
 Why hydrogen?
o Dipole – acts like a mini-magnet
o Plentiful – very important
Precession
 Atomic wobble
 Occurs about the main magnetic field
 Can be perturbed
How do we perturb the precession?
 Knock it over using a radio frequency (RF) pulse
o RF pulse – short burst radio wave
 Radio frequency needed to knock over termed the
Lamor frequency (resonance)
Precession
 Length of RF pulse determines how far the atoms
gets knocked over
 Flip angle – how far the atom is knocked over
o 90’, 180’ (RF pulses)
Why does it get knocked over?
 RESONANCE!
Resonance
 Natural frequency of a system
 Input into a system at the response frequency
causes unbounded perturbations
Signal
 THIS is what it’s all about!
 No image without a signal
 Listen for signal after pulsing
 It is the currency of MRI
Noise
 This is signal what we don’t want
o Comes from the background (lights,
electronics, etc.)
o Adjacent tissue
 Reduces image quality
Signal-to-Noise Ratio (SNR)
 Measure of signal quality
 Want to maximize this
o Increase signal
o Minimize noise
What causes the signal?
 Rotation in transverse plane cause RF signal
 In reality, signal strength decays to quickly to be
usuable due to dephasing (T2*)
Signal
 So how do we cause tissue to produce usable
signal
o Use RF pulses in a specific sequence to
cause an echo
o Echo – RF pulse given off by tissue due to
rephrasing of processing nuclei
o The echo is the signal we’re looking for
The Pulse Sequence
 Spin echo technique
o 90’ pulse cause precession in transverse
plane
o 180’ pulse causes rephrasing of the
precessing nuclei
o Echo produced
The Echo
 90’ RF pulse starts the nuclei processing in the
transverse plane but they quickly diphase and
signal is not generated
 180’ RF pulse flips the spins causing them to
temporarily rephrase and give off an RF pulse (the
echo)
Tissue contrast
 Contrast created by differences in T1 and T2
between different tissue types
 The greater the difference, the greater the
contrast
 Pulse timing is modified to weight the image for
T1 or T2 or a mix (proton density)
Imaging Parameters
 TR- repetition time (time between 90’ pulses)
 TE- echo time (time between 90’ & 180’ pulses)
Parameters vs. Image contrast
Short TR
Long TR
Short TE
T1-weighted Proton density weighted
T2-weighted
Long TE
X
Tissue Appearance
 T1-weighted image (T1WI)
o Fluid is low signal
o Fat is high signal
 T2-weighted image (T2WI)
o Fluid is high signal
o Fat is variable
 Proton density image (PDWI)
o Fluid is generally intermediate signal, fat
is generally higher signal
o Generally used for anatomical detail
 Bone (cortical) and ligaments low on all sequences
 Marrow depends on red vs. yellow and T1 vs. T2
 Muscle intermediates on all sequences
Constructing an Image – The Crazy Math
Key Terms
 Voxel
 Gradient
 Frequency
 Phase
 Slice selection gradient
 Spatial encoding
 Frequency encoding
 Phase encoding
 Fourier transform
 Fat suppression
 STIR
 Gadolineum
 Fast spine echo (FSE)
 Echo train
 Gradient echo (GE)
Vocabulary
 Voxel – 3D extenstion of a pixel; represents a
volume of tissue being imaged
 Gradient – variable field strength (B0) over
distance
 Frequency – rate of repetition
 Phase – offset
How do we select a slice?
 Remember precession and Larmor frequency
o Precessing nuclei only knock over if RF
pulse frequency matches the Larmor
frequency, which varies with field
strength
 Apply a gradient to the magnetic field
o Slice selection gradient
o Only nuclei within that slice will be
pushed to precess in the transverse plane
so only they will be able to generate
signal
Slice Selection  TQ
How do we get an image?
 Must have a way of determining where echoes are
coming from
 We use spatial encoding to encode within the echo
information which can be used to figure out where
it came from
o Uses gradient to encode the information
into the frequency and the phase of the
echo
Phase encoding
 Apply a gradient during precession that causes
nuclei within a particular row to precess at slight
offset position
 By working back from that offset, we can
determine which row a particular echo came from
Frequency encoding
 Apply a gradient during the echo that causes all
the nuclei within a particular column to echo at a
particular column to echo at a particular
frequency
o Remember Larmor frequency varies with
field strength (B0)
 By working back from the frequency of an echo we
can determine which column a particular echo
came from
How do we work back?
 Fourier transform
o Convert a wave function from the time
domain to the frequency domain
Making an image
 When we apply the Fourier process to the signals
received, we are left with essentially 3 pieces of
information for each voxel in the image
o Its signal intensity
o Its ‘x’ coordinate (frequency)
o Its ‘y’ coordinate (phase)
Pulling it all together  TQ
Typical Spin Echo Sequence  TQ
Advanced techniques
 Signal suppression
o Fat suppression
 STIR
 Chemical suppression
o Fluid suppression
 FLAIR
 Signal enhancement
o Gadolinium
Other echo techniques
o Fast spin echo
o Gradient echo
Fat suppression
 STIR – Short tau inversion recovery
o Takes advantage of the short T1 of fat
o Add 180’ pulse to begin sequence
o Remaining pulses timed to catch fat when
it can’t produce a signal
o Results in uniform suppression of fat
o Only works in T1 and T2
o Can’t be used in Gadolinium
Fat Suppression
 Chemical suppression (fat saturation)
o Takes advantage of the slightly different
resonance frequency of lipid molecules
o Uses extra pulses and gradients to ‘spoil’
the fat signal
o Can be used with any sequence type to
suppress fat signal
Fluid Suppression
 FLAIR  TQ
 Same principle as STIR but pulsed timed to
suppress water signal instead
 Provides enhanced contrast on T1WI
 Especially important in fast spin echo and ultrahigh field imaging where contrast is normally
decreased
 Used mostly in brain and spine imaging
Signal Enhancement
 Gadolinium
o Paramagnetic compound
o Reduces the T1 of the surrounding tissues
causing hyper-intense signal
o Distributed in the blood and rapidly
enters the interstitial space
o Does not cross the intact blood-brain
barrier
o Pattern of signal changes tells us
something about the lesion
o Often used with fat-saturation technique
o Always compared to unenhanced T1WI
Fast Spin Echo (FSE)  TQ
Gradient echo (GRE)  TQ
The Future  TQ
Nuclear Imaging – Scintigraphy and SPECT
Radioactive Isotopes
 Atomic isotopes – same number of protons,
different number of neutrons
 Some isotopes are radiotive and give off photons
(14C example)
 These atoms can be included into complex
molecules
o Termed radiolabeled molecule
Radiotracers
 Radiation (photons) given off by radioactive
molecules can be detected
 Detecting the radiation allows us to track where
the molecule is


Tracking radiolabeled metabolically active
molecules (radiotracers) can tell us physiological
information
What can it tell us?
 Where certain biochemical reactions are occurring
 Where certain type of cells are at or are going
 Where certain molecules are being stored
 Where blood is flowing
 Whether certain molecules are being absorbed by
the body
 Whether certain biological barriers are intact
 The list could go on
How does it work?
 As radioisotopes decay, they give of gamma rays
 Gamma ray photons hit a scintillator and is
recorded by the computer
 There is usually an anterior and a posterior
detector
 Resulting image shows where gamma photons
were detected
 Photons generally detected in close proximity to
where they were generated
Multi-phase scans  TQ
What isotopes are used?
 Technetium-99m – Bone
How about 3D?
 SPECT – Single photon emission computed
tomography
o Similar concept to CT, take pictures from
different angles
o Detector rotates around the patient
o Computer calculate where photons where
coming from in 3D space
o Maybe combined with a CT machine
 CT for anatomy
 SPECT for physiology
o Images can be fused together
o SPECT data can be fused with separately
acquired MR
Image Fusion
 Images are registered and then overlaid
o Automatic for SPECT/CT
o Manual for SPECT/MR
 Shows anatomy and physiology simultaneously
Nuclear Imaging – PET
Positron Emission Tomography
 AKA – PET imaging
 Provides similar information to SPECT
 Uses fluorodeoxyglucose (18F) or 18F-FDG
 18F gives off a positron as it decays
 FDG is a glucose analog and its transported in the
body just like glucose
 More glucose = more metabolically active tissue
What is a positron?
 It’s anti-matter
 Just like an electron but with a positive charge
 Matter and anti-matter don’t mix
 Causes an annihilation event that gives off 2
gamma photons
 2 photons travel in opposite directions
How it works?
 The computer looks for gamma photon pairs that
strike 180’apart at nearly the same time
 Information is processed to produced slice data
displaying local tissue uptake
Image Fusion
 This data can fused with CT or MR data to provide
anatomical info
 Together we can learn something about anatomy
and physiology
 Currently dual PET/CT scanners are in use and
PET/MR are being developed