Diagnostic Imaging and Applications in RTT part 1

Question 1

What are the five types of photon interactions?

Answer 1

Rayleigh, compton scattering, photoelectric effect, pair production, and photodisintegration

Question 2

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.

Answer 2

Rayleigh (coherent or classical) scattering

Question 3

Rayleigh scattering is very probable in high atomic number materials with photons of what kind of energy?

Answer 3

low energy (15-30keV)

Question 4

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.

Answer 4

Compton scattering

Question 5

Compton scattering is the main interaction in what energy range?

Answer 5

diagnostic (between 30keV and 30MeV)

Question 6

With Compton interactions, the conservation of energy and momentum allows us to figure out:

Answer 6

what the energy of the scattered photon and ejected electron is

Question 7

The probability of compton interaction decreases with increasing _______ ______

Answer 7

photon energy

Question 8

The probability of compton interactions increases with increasing _______ _______

Answer 8

electron density (this causes more free electrons to interact with)

Question 9

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.

Answer 9

hydrogen

Question 10

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.

Answer 10

Photoelectric Effect

Question 11

In photoelectric effect, this is when an inner shell vacancy is filled by higher energy electrons, producing characteristic x-rays and Auger electrons

Answer 11

Electron cascade

Question 12

Photoelectric effect is common when the energy of the incident photon is slightly greater than the _______ ______ of electron.

Answer 12

binding energy (10-100keV)

Question 13

Higher Z material and lower photon energy means higher probability of photoelectric effect. The probability of photoelectric effect is

Answer 13

Z^3/E^3
where z is atomic number of absorbing material and E is energy of incident photon

Question 14

What are the radiographic consequences of the photoelectric effect?

Answer 14

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

Question 15

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.

Answer 15

Pair production

Question 16

What must the incident photon energy be greater than in pair production?

Answer 16

1.022MeV

Question 17

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.

Answer 17

Photodisintegration

Question 18

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

Answer 18

Exponential Attenuation

Question 19

The fraction of photons removed from a monoenergetic beam of x-rays or gamma rays per unit thickness is the

Answer 19

linear attenuation coefficient

Question 20

The linear attenuation coefficient is dependent upon

Answer 20

incoming photon energy
z of the attenuator
density of the attenuator

Question 21

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

Answer 21

more

Question 22

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.

Answer 22

Bremsstrahlung

Question 23

Imaging technique that synthesizes images from digitized data using a computer

Answer 23

computed

Question 24

Imaging technique that generates cross sectional images or slices of a 3D object

Answer 24

Tomography

Question 25

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.

Answer 25

computer

Question 26

In CT, this is a circle in the x-y plane but can extend along the x-axis thus creating a cylindrical FOV

Answer 26

Scanner field of view

Question 27

In CT, this is the data collected at a specific angle of interrogation of the object. This is also a collection of rays

Answer 27

projection

Question 28

In CT, these are the individual attenuation measurements that correspond to a line through the object

Answer 28

Rays

Question 29

All rays in a projection parallel to one another

Answer 29

Parallel beam projection

Question 30

Rays at a given projection angle diverge

Answer 30

Fan beam projection

Question 31

Uses both a fan and cone angle

Answer 31

cone beam projection

Question 32

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

Answer 32

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.

Question 33

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.

Answer 33

Parallel Beam

Question 34

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.

Answer 34

Fan beam

Question 35

This beam geometry is used is many modern CT units. Most systems angle < 2 deg; True CBCT > 10 deg

Answer 35

Cone Beam

Question 36

Challenges of CBCT

Answer 36

x-ray scatter and cone beam artifact

Question 37

Advantages of CBCT

Answer 37

whole organ imaging with no table movement

Question 38

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

Answer 38

Scout view, topogram, scanogram, localizer

Question 39

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

Answer 39

basic axial/sequential

Question 40

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.

Answer 40

Helicial (spiral)

Question 41

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

Answer 41

Pitch

Question 42

A pitch greater than 1 results in:

Answer 42

undersampled data, less dose, and interpolation is needed which affects image quality

Question 43

A pitch less than 1 results in:

Answer 43

oversampled data and more dose

Question 44

This mode of acquisition allows the whole organ to be imaged. The challenges include increased x-ray scatter and increased cone beam artifacts.

Answer 44

cone beam

Question 45

What are some challenges of Cone beam CT?

Answer 45

Increased x-ray scatter and increased artifacts

Question 46

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.

Answer 46

Bowtie filter

Question 47

Why is the bowtie filter shaped the way it is?

Answer 47

The x-ray fluence produced at the detector is high at the periphery and low at the center.

Question 48

List the order of advancement of CT

Answer 48

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

Question 49

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

Answer 49

1st generation

Question 50

This type of beam is good for scatter reduction and image quality, but not for time. It was used in first generation CT scanners.

Answer 50

Pencil beam

Question 51

How long did it take 1st generation CT scanners to scan?

Answer 51

about 4.5 minutes (image reconstruction ran overnight)

Question 52

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

Answer 52

2nd Generation CT

Question 53

This type of beam had more scattered radiaiton detected

Answer 53

Narrow fan beam

Question 54

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.

Answer 54

3rd generation

Question 55

what was the scanning of 3rd generation ct scanners?

Answer 55

0.3 to 0.5 sec/rotation

Question 56

What is one drawback to 3rd generation CT scanners?

Answer 56

ring artifacts

Question 57

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

Answer 57

Slip rings

Question 58

With slip rings, there was a major reduction in scan time. What did slip rings enable that allowed for this?

Answer 58

helical scanning

Question 59

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

Answer 59

4th generation

Question 60

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

Answer 60

Helical scanners

Question 61

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.

Answer 61

Multiple Detector Array CT (MDCT)

Question 62

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.

Answer 62

Cone Beam CT

Question 63

I= Ioe^-ux

Answer 63

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.

Question 64

Process of estimating an image from a set of projections

Answer 64

Reconstruction

Question 65

Types of reconstruction algorithm

Answer 65

Brute Force Simple back projection Filtered back projection Iterative reconstruction Fourier-Based Feldkamp reconstruction

Question 66

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)

Answer 66

Brute force

Question 67

Why is brute force not practical for real system?

Answer 67

it can have hundreds of simultaneous equations for a single slice

Question 68

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

Answer 68

Simple Back Projection

Question 69

This reconstruction algorithm alters the projection data via convolution before we back project. Convolution is simply a type of mathematical multiplication

Answer 69

Filtered back projection

Question 70

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

Answer 70

Iterative reconstruction (also known as algebraic reconstruction technique)

Question 71

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

Answer 71

Feldkamp reconstruction

Question 72

Gray scale values are called

Answer 72

Hounsfield units

Question 73

In CT, what photon interaction is most likely?

Answer 73

Compton Process

Question 74

What is compton process dependent on?

Answer 74

Electron density

Question 75

A measure of the attenuation of the material in each voxel

Answer 75

CT numbers

Question 76

CT numbers range from about:

Answer 76

-1000 to +1000

Question 77

HU of water is: HU of air is:

Answer 77

0 -1000

Question 78

How do we use CT technology in the clinic?

Answer 78

CT simulation, Daily set up checks, PET/CT

Question 79

What are some differences between Simulation CT vs. Diagnostic CT?

Answer 79

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.

Question 80

CT Sim/TP setup steps

Answer 80

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

Question 81

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.

Answer 81

Digitally reconstructed radiograph (DRR)

Question 82

EPID portal film vs DRR

Answer 82

EPID is compared to the DRR that is constructed during the CT simulation