Physics questions 5
Diagnostic Imaging Physics
MPHY-501
Bushberg Chapter 7
Radiography Part 1
During the next two years you will learn the basics of Diagnostic Imaging. Ultimately after this period you will enroll in a Medical Physics Residency Program and finally take the Board examination that, once you pass, will certify you as a medical physicist.
1
Magnification
a
b
L object
L image
Source to Image Distance (SID)
183 cm (72 in)
Chest X-Ray
Source
a
b
L object
L image
SID
100 cm (40 in)
Chest X-Ray
Source
Source to Image Distance = SID
Source to Object Distance = SOD
Setup Geometry
Cause of Partial
Volume Artifact
Focal Spot Blurring
Focal Spot blurring limits resolution in magnification mode…
Lg = Edge Gradient Length
Lf = Focal Spot Length
Image Receptor
Object
Target
Image Receptor
Object
Target
b
a
a
b
Intensifying Screens
Screen-film radiography
Film is sandwiched between two intensifying screens
Light tight cassette encapsulates screens and film
intensifying screens use rare earth materials as scintillators
High Z material that has high absorption efficiency for X-Rays
CaWO4
Gd2O2S
LaOBr
YTaO4
Thick intensifying screens yield more light but
Thick intensifying screens blur more due to light diffusing more within
Film
Thin plastic base
Coated on one or both sides with a layer of light-sensitive emulsion
Silver chloride (AgCl)
silver bromide (AgBr)
silver iodide (AgI)
*Photochemistry:
Ag+ + Cl- + photon = Ag+ + Cl + e-
Ag+ + e- = Ag (metal)
Silver metal forms as a result of the electron capture, it forms a dark image on film
Chemically, we say that:
Ag+ has been ”REDUCED" to Ag (metal)
At the same time, Cl- is said to be "oxidized"
Silver Halide (95% AgBr and 15% AgI) in water-soluble gelatin
silver "halide" compounds used in photography
*https://chlorine.americanchemistry.com/Science-Center/Chlorine-Compound-of-the-Month-Library/Silver-Chloride-Helping-Us-Get-the-Picture/#:~:text=When%20exposed%20to%20light%2C%20a,denoted%20Ag%2B%20and%20Cl%2D.
Reduction
X-Ray Quality
Albert von Kölliker’s hand: X-ray of Kölliker's hand, made by Röntgen on 23 Jan 1896
Anna Bertha Ludwig’s (Wife of Röntgen) hand Dec. 22, 1895
One Month
Digital x-ray of hand, 3/30/2009. https://pixels.com/featured/normal-hand-digital-x-ray-du-cane-medical-imaging-ltd.html
114 Years
Film Processing Part I: Exposure
Total Exposure: Photon Intensity * Time
Reciprocity Law: film response is determined by total exposure
Reciprocity is valid with good accuracy over a range of values of exposure duration.
¨Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form. In particular, if the surface of the silver halide crystal has a cluster of at least four or five (but usually a few dozen) photons, reduced silver atoms are rendered and are developable.
This creates the “Latent Image”
Reciprocity Law Failure (Schwarzschild effect)
¨As the light level decreases out of the reciprocity range, the increase in duration, and hence of total exposure, required to produce an equivalent response becomes higher than the formula states
Coefficients used to correct this effect are called Reciprocity Factors
Reciprocity also breaks down at extremely high levels of illumination with very short exposures. with very high shutter speeds (1/10,000 sec or faster)
¨https://en.wikipedia.org/wiki/Reciprocity_(photography)
Film Processing Part II: Developer
After exposure the film is taken to a liquid bath of reducing agent called “developer”
The developer reacts with the Silver Salt grains that have had some of the silver atoms reduced to Silver Metal (i.e. the latent image). Those grains act as a catalyst for the developer to further reduce many more of the silver atoms on that grain preferentially over grains that have not already had some silver atoms reduced by exposure to light
This turns those grains appear as black specks on the film.
Film Processing Part III: Fixer
After developing the film in reducing agent the film is washed in a Fixer
The fixer dissolves the Ag+ from the film and all other underexposed grains through oxidation.
Film Processing Part IV: Rinse
The film is then rinsed to remove residual developer and fixer.
Film is then dried.
The image thus produced has many grains reduced and developed in areas where more photons were incident on the film and fewer where photons were in less abundance
The density of developed grains lends to dark areas on the image
Hence in areas where there was bone fewer light photons reached the film as opposed to for example where light was shown directly on the film or where photons only passed through soft tissue.
Thus Bone appears bright (few photons), tissue less bright (more photons) and air dark (max photons for that protocol).
*Hurter and Driffield Curve (H&D curve)
*Ferdinand Hurter (1844-1898) and Vero Charles Driffield (1848 – 1915)
H&D curve
Low Contrast
Low Contrast
High Contrast
Low Contrast areas: change in Kerma (number of x-ray photons) results in little or no change in optical density.
Low contrast is evidenced in the “toe” and “shoulder” regions of this H&D response graph.
A radiograph will require more photons than reflected in the Kerma of the toe region. Photons in excess of the number found in the high contrast “linear” region will not contribute to the image and could result in “over exposure” thereby “washing out” the image.
Kerma = Kinetic Energy Released in Matter
Film Speed/Dynamic Range
Because film that has a larger latitude requires more Kerma it is considered a “slower speed” film
High speed film has larger grain size hence lower resolution, also less latitude = less dynamic range.
Computed Radiography: The Process
Photostimulable Phosphor Detector Systems (PSP)
Uses Scintillators: Gd2O2S or CsI, and BaFBr “Barium Fluorohalide”
X-rays cause prompt light emissions in the scintillator however some of the x-ray energy is trapped in the same scintillator.
Later a red laser (700nm) is passed over PSP and stimulates the trapped energy.
Energy cascades emit blue – green visible light
Light is collected through fiberoptic light guide & sent to photomultiplier tube
Electronic signal is digitized and stored
PSP is exposed to bright white light to reset PSP and ready it for next x-ray
CR’s Excitation and Emission Light
X-Ray
energy absorbed
in media
2) e- moved to
and trapped in
conduction band
3) Laser adds
energy to trapped e-
trapped e- moves
to valence band
4) Blue/Green
visible light emitted
CR Readout
Imaging plate is translated through the mechanism between rollers
Laser beam scans horizontally across the plate.
Rotating multifaceted mirror causes the laser beam to scan the imaging plate in a raster fashion (Rasterized).
Light released by laser stimulation is collected by the light guide and produces a signal in the PMT.
Laser’s red light filtered out before reaching PMT.
Filtered Out
Sent to PMT
Charged-Coupled Devices (CCD)& Complimentary Metal-Oxide Semiconductor (CMOS) Detectors
CCD Chip is crystalline Silicon integrated array (like CPU chips)
An array of discrete detector electronics etched on surface of CCD
Each del (Detector Element) must be ≤ 20mm to achieve charge conduction efficiency of 99.99%
Surface of dexel is photo electric. As visible lights hit the surface electrons are liberated and accumulate in the dexel.
The electronics about the dexel form a voltage barrier that contain the electrons
After exposure the dexels are read out one by one
CCD Chip Image Capture and Readout
Shift of a given pattern of exposure down one column in a CCD chip in four (t1-t4) successive clock cycles.
High-Resolution CCD chip
From 2.5cm X 2.5cm to 6cm X 6cm
After exposure, electrodes in the chip shift the charge packets for each detector element by switching voltages, allowing the charge packets to move down by one detector element at a time. Charge from the bottom element of each column spills onto the readout row, which is rapidly read out horizontally. This process repeats itself until all rows in each column are read out.
≤ 20mm
Digital Radiography
One CCD Based DR System
Quantum Limited Detector
Quantum limited means that the quantum noise is at the same level or greater than the quantum signal being detected
Detectors have several layers where quantum limited issues can happen
Ideally the quantum sink stage is the stage where the x-rays impinge on the detector
When other stages are quantum limited more exposure to the patient is required to overcome and is inefficient (negative gain)
X-rays
Visible Light
Electrons
Conversion Step 1
Conversion Step 2
X-ray Quantum Limit
Visible Light Quantum Limit
Electrons Quantum Limit
Quantum Sink: Stage where the number of quanta is lowest!
If the X-Ray stage is the quantum Sink then the system is an:
“X-Ray Quantum Limited Detector”
Detector Technologies
Scintillators
NaI (Sodium Iodide) (high efficiency is stage I conversion
Gd2O2S cost less and are less efficient
Large Field CCD detectors have secondary Quantum Limit,
lens of system is not efficient
Demagnification not efficient
Linear arrays with fiberoptic individual channels
uses less demagnification than 2D array systems
Does not require anti-scatter grid
more efficient
reduced secondary quantum limit
Time Delay Integration (TDI)
Pros
Excellent dose efficiency
Good signal to noise ratio
Can reduce quantum limited noise
Excellent in chest (sternum, ribs & air) and full body scanning (integrates point by point until S/N is reached per dexel)
Cons
Slow
May increase motion artifact
Increased tube loading
CMOS
Random access memory (RAM) with
build in scintillator
storage capacitors
readout electronics
Pros
Built-in Automatic Exposure Control (AEC)
Con
High electronic noise
Storage
Readout
Reset
(Currently) limited to FOV 100mm X 150mm
Flat Panel Thin-Film-Transistor (TFT) Array Detectors
Flat amorphous silicon
TFT array is divided into individual detector elements (dexels), arranged in a row and column matrix.
series of horizontal and vertical electrical lines, combined with appropriate readout logic, address each individual display element
2000 connections between the display and electronics are required for a 1000 × 1000 display, instead of 1,000,000 individual connections
TFT is an electronic switch comprised of three connections:
Gate
Source
Drain
Gate is transistor's “on” - “off” switch attached to the gate conductor line along each row of the array.
Source is attached to the storage capacitor
Drain is attached to the drain conductor line running along each column of the array.
Charge collection electrode captures charge produced by incident x-ray energy deposited over the area of the dexel (either by indirect or direct conversion)
Storage capacitor stores the charge
During x-ray exposure, the TFT switch is closed
charge in each dexel to be accumulated and stored.
When exposure is completed,
sequential activation of the TFT array occurs one row at a time, by sequentially turning on the gate line to every dexel in the row
Allows the accumulated charge in each dexel capacitor to flow:
through the transistor to the drain line,
and subsequently to the connected charge amplifier.
Charge amplifiers
positioned outside of the panel active area
amplify the charge,
convert charge to a proportional voltage,
digitize the voltage level (gray scale value for each dexel in row)
Sequence is repeated row by row, to fully read out the array.
Direct and Indirect Detection TFT Arrays
Scintillator
Semiconductor
To reduce this spread scintillation elements are grown in crystals that form “light Pipes”
Indirect Detection:
x-rays impinge on scintillator. CsI requires thinner layer than Gd2O2S. CsI reduces blurring and subsequent loss of resolution
Direct Detection: x-rays impinge on semiconductor material instead of a scintillator. X-rays are directly converted into electrons. These electrons follow the electric field imposed via a bias to the semiconductor material
Technique Factors
Technique Factors are those user adjustable elements that control the x-ray beam quality, production and image acquisition.
Technique Charts charts are required for every instrument.
Typical Adjustable Elements:
kV (Contrast is reduced at higher kV, penetration reduced at lower kV)
mA (Increase to overcome quantum limits)
Seconds (longer times mean more motion blurring)
mAs (= mA * Seconds, Increase to overcome quantum limits)
SID (100cm for table based radiographs, for chest x-ray 183 cm)
SOD (Source Object Distance, short SOD increases magnification)
Grid (in or out. In reduces x-rays to detector but decreases blur, Out for small thickness such as pediatric, hand etc.)
Filter (Al, Cu, …)
Equalizing Wedges (ER heal effect, compensate for geometric differences, etc.)
Spread Sheet Item:
“Is there a Technique Chart
For this equipment?”
kV Ranges
kV increases with increasing thickness and or density of image object
Examples:
56 kV for wrist (low kV increase contrast for high Z elements like Ca Z=20, I Z = 53, Ba Z = 56)
75 – 90 kV abdomen
120 kV for chest imaging (to reduce the ribs’ [bone] conspicuity)
mA Considerations
mA is adjusted to increase fluence
Higher mA increases tube loading (Heat)
High mA reduces blurring due to motion
High mA can lead to reciprocity failure
In AEC mA is set to 500 or higher and image is “Photo Timed”
Seconds
The number or fraction of seconds is adjusted to:
Have correct exposure to film or fill dexel
Reduce motion
Standard Configuration for Radiography
X-ray collimator has a light bulb and mirror assembly, and (when activated) casts a light beam onto the patient allowing technologists to position and size the x-ray beam relative to the patient's anatomy.
There is either an integrated tape measure to determine the distance for table top exposures, and indent to set SID for table Bucky, or a laser based measurement tool.
The light beam is calibrated to be congruent with the x-ray beam. X-rays passing through patient must pass through the anti-scatter grid and the photocell (part of the AEC system) to then strike the x-ray detector.
Most table-based radiographic systems use a SID of 100 cm.
Automatic Exposure Control (AEC)
The (air-ionization) photo cell is typically set behind the grid and in front of x-ray detector [AEC should be seen in the image however they are nearly x-ray transparent]
Placement of AEC and help or hurt image quality.
Typically there are at least three photo cells in a chest Bucky mechanism.
Each photo cell can be individually turned on or off
If photocell is placed under lead shield this could drive up the dose to the patient quite a bit.
Similarly if photocell is not placed under sternum/spinal column image may become under exposed
End of Lecture 5