Diagnostic Imaging and Applications in RTT part 1 Flashcards
What are the five types of photon interactions?
Rayleigh, compton scattering, photoelectric effect, pair production, and photodisintegration
This type of photon interaction is where the incident photon excites the whole atom which emits a photon with the same energy as the incident photon but in a slightly different direction. No energy is transferred.
It also accounts for less than 5% of interactions at diagnostic energies.
Rayleigh (coherent or classical) scattering
Rayleigh scattering is very probable in high atomic number materials with photons of what kind of energy?
low energy (15-30keV)
This type of photon interaction is where the photon interacts with an atomic valence electron (free/loosely bound), which receives energy from the photon and is emitted at an angle. The photon is scattered with reduced energy.
Compton scattering
Compton scattering is the main interaction in what energy range?
diagnostic (between 30keV and 30MeV)
With Compton interactions, the conservation of energy and momentum allows us to figure out:
what the energy of the scattered photon and ejected electron is
The probability of compton interaction decreases with increasing _______ ______
photon energy
The probability of compton interactions increases with increasing _______ _______
electron density (this causes more free electrons to interact with)
The probability of compton interaction is nearly independent of atomic number (z) because the number of free electrons is what matters.
Compton interaction happen more frequently with high _______ content.
hydrogen
This type of photon interaction is where an incoming photon transfers all its energy to an inner shell electron of an atom of the absorbing medium. The electron is ejected, leaving a vacancy (KEpe=hv-Ebe). This causes an electron cascade where the vacancy is filled.
Photoelectric Effect
In photoelectric effect, this is when an inner shell vacancy is filled by higher energy electrons, producing characteristic x-rays and Auger electrons
Electron cascade
Photoelectric effect is common when the energy of the incident photon is slightly greater than the _______ ______ of electron.
binding energy (10-100keV)
Higher Z material and lower photon energy means higher probability of photoelectric effect. The probability of photoelectric effect is
Z^3/E^3
where z is atomic number of absorbing material and E is energy of incident photon
What are the radiographic consequences of the photoelectric effect?
The probability is proportional to Z^3 so between bone and tissue, the absorption in bone is more. Thus, the contrast between bone and tissue in radiographs should increase at lower kV, but at lower kV, patient dose increases since photoelectric effect results in total absorption of incoming photon
With this photon interaction, the photon interacts strongly with electric field of nucleus and gives up all energy in process of creating a pair of particles: a negative electron and positive electron (positron). Most probable distribution is each particle acquires half available kinetic energy although any energy distribution is possible.
Pair production
What must the incident photon energy be greater than in pair production?
1.022MeV
This photon interaction occurs at very high energies that actually allow photons to eject neutrons from the nucleus. This is responsible for neutron contamination therapy beams of greater energy than 10MV.
Photodisintegration
As the thickness, x, of the attenuation increases, transmission falls very quickly (exponentially). I(x)=Ioe^-ux
where u is the linear attenuation coefficient and Io is the number of photons
Exponential Attenuation
The fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness is the
linear attenuation coefficient
The linear attenuation coefficient is dependent upon
incoming photon energy
z of the attenuator
density of the attenuator
A larger attenuation coefficient number indicates that ____ photons will be removed from the beam. Z is normally close to the same for various tissues, but density is not. So there is another way of writing the attenuation coefficient that takes out the density variability
more
This occurs when the primary electron interacts with positively charged nucleus. The particle’s trajectory is altered, causing deceleration. The particle’s deceleration results in release of an x-ray of varying energy.
Bremsstrahlung
Imaging technique that synthesizes images from digitized data using a computer
computed
Imaging technique that generates cross sectional images or slices of a 3D object
Tomography
The detector in a CT unit does not form the image. The patient is scanned with a narrow x-ray beam. The transmitted radiation along the ray is measured. ______ takes the data and reconstructs it.
computer
In CT, this is a circle in the x-y plane but can extend along the x-axis thus creating a cylindrical FOV
Scanner field of view
In CT, this is the data collected at a specific angle of interrogation of the object. This is also a collection of rays
projection
In CT, these are the individual attenuation measurements that correspond to a line through the object
Rays
All rays in a projection parallel to one another
Parallel beam projection
Rays at a given projection angle diverge
Fan beam projection
Uses both a fan and cone angle
cone beam projection
Basics of CT: The 2D CT image corresponds to a 3D section of the patient. The CT slice thickness is very thin (1 to 10mm) and is approximately uniform
The 2D array of pixels in the CT image corresponds to an equal number of 3D voxels (volume elements) in the patient. Each pixel on the CT image displays the average x-ray attenuation properties of the tissue in the corresponding voxel.
This type of beam geometry was used in the early years. The x-ray tube and detectors used a linear scanning trajectory. The x-ray tube translated.
Parallel Beam
This beam geometry refers to the 2D geometry where there is minimal divergence of the x-ray beam trajectory. True fan beam on top; narrow cone beam on bottom.
Fan beam
This beam geometry is used is many modern CT units. Most systems angle < 2 deg; True CBCT > 10 deg
Cone Beam
Challenges of CBCT
x-ray scatter and cone beam artifact
Advantages of CBCT
whole organ imaging with no table movement
CT x-ray tube and detector stay stationary while patient is translated through the scanner.
Digital radiographic image is obtained
CT techs use this image to make sure region of interest is included in the field of view, to check the exposure technique or as a baseline prior to administration of contrast material
Scout view, topogram, scanogram, localizer
This mode of acquisition is the basic step and shoot of a CT.
The CT tube is turned on for 360 degrees, then table is translated, then tube is turned back on for 360 degrees. etc.
Each rotation contributes to only one image.
Slow
basic axial/sequential
This mode of acquisition in Ct is where the table moves at a constant speed while gantry rotates around patient. It results in a helix forming around patient. Interpolating data that will contribute to one slice. This affects image quality.
Advantage is speed.
Helicial (spiral)
This describes relative advancement of CT table per rotation of the gantry
=Ftable/nT, where Ftable is distance table travels during one 360 revolution and nT nminal collimated beam width or slice thickness
Pitch
A pitch greater than 1 results in:
undersampled data, less dose, and interpolation is needed which affects image quality
A pitch less than 1 results in:
oversampled data and more dose
This mode of acquisition allows the whole organ to be imaged. The challenges include increased x-ray scatter and increased cone beam artifacts.
cone beam
What are some challenges of Cone beam CT?
Increased x-ray scatter and increased artifacts
This type of filter reduces beam intensity at periphery of the beam where the attenuation part through the patient is thinner. The body typically has a round cross section that is thicker in the middle than the periphery.
Bowtie filter
Why is the bowtie filter shaped the way it is?
The x-ray fluence produced at the detector is high at the periphery and low at the center.
List the order of advancement of CT
1st-rotate: pencil beam geometry
2nd- rotate: narrow fan beam w/more detectors
3rd- rotate: detector and x-ray system rotate together
4th- rotate: stationary; rotating x-ray source but detector array is stationary
Helical scanning
slip ring
multiple detector array CT
Cone beam CT
This generation of CT has a parallel ray geometry with pencil beam. The single pencil beam translates across patient, and rotates. There are only 2 x-ray detectors used (2 different scales).
Translated linearly to acquire 160 rays across a 24 cm FOV
Rotated slightly between translations to acquire 180 projections at 1‐degree intervals
1st generation
This type of beam is good for scatter reduction and image quality, but not for time. It was used in first generation CT scanners.
Pencil beam
How long did it take 1st generation CT scanners to scan?
about 4.5 minutes
(image reconstruction ran overnight)
This generation of CT scanners used a fan beam (more detectors were added). Linear array of 30 detectors.
More scattered radiation was detected.
Shorter scan time
2nd Generation CT
This type of beam had more scattered radiaiton detected
Narrow fan beam
This generation of Ct scanners had the X-ray tube and detector mounted across from each other in a fixed position. It had a wide fan beam that hit a large array of detectors. The fan beam covered whole patient so no translation needed. Most modern CT scanners are this type.
3rd generation
what was the scanning of 3rd generation ct scanners?
0.3 to 0.5 sec/rotation
What is one drawback to 3rd generation CT scanners?
ring artifacts
These allow the rotating gantry to have an electrical connection to the stationary components.
They have concentric metal bands that are connected to a series of gliding contacts.
Eliminated bulky cables connecting stationary to rotating components
Enables helical scanning – led to major reductions in time
3rd generation
Slip rings
With slip rings, there was a major reduction in scan time. What did slip rings enable that allowed for this?
helical scanning
This generation of CT scanner is where ony the tube moves. It has a stationary ring of detectors (~4,800) which dramatically increased cost. It was designed to reduce ring artifact
Expensive
4th generation
These CT scanners acquire data while the table is moving.
By avoiding the time required to translate the patient table, the total scan time required to image the patient can be much shorter.
Allows the use of less contrast agent and increases patient throughput.
In some instances, the entire scan be done within a single breath hold of the patient.
We can also acquire multiple set of scans in the same position for breathing patients
Helical scanners
These CT scanners have multiple detector array scanner, slice thickness is determined by detector size, not by the collimator.
Collimator spacing is wider and more of the x-ray that are produced by the tube are used in producing image data.
Multiple Detector Array CT (MDCT)
In these CT scanners, the x-ray beam forms a conical geometry between the source (apex) and the detector (base). These systems acquire an entire volumetric dataset with a single rotation of the gantry.
Cone Beam CT
I= Ioe^-ux
Where “u” ,linear attenuation coefficient, is a function of density, atomic number, and photon energy.
X is the thickness photon travels.
Therefore, the intensity “I” is a function of original intensity I0, u, and x.
Process of estimating an image from a set of projections
Reconstruction
Types of reconstruction algorithm
Brute Force
Simple back projection
Filtered back projection
Iterative reconstruction
Fourier-Based
Feldkamp reconstruction
This reconstruction algorithm has a projection set that defines a system of simultaneous linear equations (can be solved using algorithms from linear algebra)
It is not practical for real systems (can have hundreds of simultaneous equations for a single slice)
Brute force
Why is brute force not practical for real system?
it can have hundreds of simultaneous equations for a single slice
This reconstruction algorithm is where the image matrix is initially set to 0.
Smearing back the projection data onto the image matrix at the angle it was acquired without any manipulation
Transmission value is added to each pixel that falls in the path through the object corresponding to the measurement
Image looks similar to the object being reconstructed but is very blurry
1/r blurring
Simple Back Projection
This reconstruction algorithm alters the projection data via convolution before we back project.
Convolution is simply a type of mathematical multiplication
Filtered back projection
This reconstruction algorithm consists of three steps:
Make an initial guess at the solution
Compute forward projections based on the guess and compare with CT acquired projections to produce error matrix
Next iteration of the image is updated
Iterative process continues until error matrix is minimized and an accurate reconstructed image is produced
Iterative reconstruction (also known as algebraic reconstruction technique)
This reconstruction algorithm is used for CBCT reconstruction.
Similar to fan beam reconstruction algorithms, but keeps track of beam divergence in the z-direction (the cone angle direction)
Computes backprojection angles in both the fan and cone angles
Reconstructs the entire volume data set simultaneously
Feldkamp reconstruction
Gray scale values are called
Hounsfield units
In CT, what photon interaction is most likely?
Compton Process
What is compton process dependent on?
Electron density
A measure of the attenuation of the material in each voxel
CT numbers
CT numbers range from about:
-1000 to +1000
HU of water is:
HU of air is:
0
-1000
How do we use CT technology in the clinic?
CT simulation, Daily set up checks, PET/CT
What are some differences between Simulation CT vs. Diagnostic CT?
Sim has a flat top table instead of cradle shaped.
Larger bore size
External lasers used for patient marking
High capacity MHU capacity- more images are taken in short time for treatment planning, which needs high heat capacity.
CT Sim/TP setup steps
BBs get placed on surface
Defines marked location for setup in linac
Dosimetrist uses this location as a reference for their treatment plan
Often the final iso-center is placed on the tumor
Therapist sets up pt to first marked location, then shifts to final iso
Some type of verification is performed to ensure tumor is at correct position
This is a simulation of a 2D x-ray image created from a CT image.
are produced by tracing ray lines from a virtual source position through the CT data of the patient to a virtual film plane. The sum of the attenuation coefficients along any one ray line gives a quantity analogous to optical density on a radiographic film.
Digitally reconstructed radiograph (DRR)
EPID portal film vs DRR
EPID is compared to the DRR that is constructed during the CT simulation
