Principles of Imaging

Question 1

The benefits of CT for RT planning:

Answer 1

● Provides geometrically accurate 3D data
● Provides electron density information
○ Used to calculate attenuation information
○ Relative linear attenuation coefficient

Question 2

CT scans provide electron density information, what can be calculated from this?

Answer 2

○ Used to calculate attenuation information

○ Relative linear attenuation coefficient

Question 3

What is the Hounsfield unit a measure of?

Another name for this unit in 1 common application.

Answer 3

Radiodensity. It is frequently used in CT scans, where its value is also termed CT number.

Question 4

Calculation of Hounsfield unit:

Answer 4

u = a voxel’s average linear attenuation coefficient:

In HUs:

CT number = (u - uH2O)/(uH2O-uAIR)

Question 5

Steps in acquiring a CT scan

Answer 5

Helical versus step and shoot.

1) Fan beam scans across a patient as bed is moved through scanner (i.e as opposed to step and shoot). Speed at which patient moved relative to 1 beam rotation is controlled by specifying the pitch factor.
2) Detectors (single or multiple row) measure beam intensity (e.g at every one of 360 views) PROFILE of attenuated radiation passing through body.
3) Image slices formed by combining (BACKPROPAGATION) the profiles obtained over the width spanned by the slice. These images are divided into VOXELS. Image filtering typically occurs at this stage.
4) The linear XR beam attenuation within a voxel is converted to a CT number.

Question 6

X-ray attenuation depends on both the ………. and …….. of materials and the energy of the x-ray photons.

For CT imaging a high KV (like?) and heavy beam filtration is used. Why?

Answer 6

X-ray attenuation depends on both the density and atomic number of materials and the energy of the x-ray photons.

For CT imaging a high KV (120-140) and heavy beam filtration is used. This minimizes the photoelectric interactions that are influenced by the Z of a material, therefore removing this dependence and leaving CT numbers to reflect tissue density.

Question 7

CT numbers for:
■ Air =
■ Water =
■ Bone =

Answer 7

■ Air = -1000 HU
■ Water = 0 HU
■ Bone = 1000 HU

Question 8

For CT imaging windowing does what?

Answer 8

● Windowing maximizes image contrast
○ Level- median HU value
○ Width- total number of HU from black to white

Question 9

Multislice CT uses

Answer 9

Multislice CT uses cone-shaped beam with multiple rows of detectors

Question 10

Common fundamental property of CT contrasts.

Two forms available (and their uses):

Answer 10

● Agent with very high HU to differentiate between soft tissues
● IV
○ Blood vessels and lymph nodes
○ Enhancing lesions that break down the blood brain barrier
● Oral
○ Differentiate bowel wall from adjacent soft tissues

Question 11

Describe the XR heel effect:

Answer 11

Heel effect: Beam intensity lower on the anode side due to increased anode material needed to pass through. ‘self attenuation’

Question 12

2 contributions to the x-ray spectrum produced by the tube (give percent and what they reflect):

Answer 12

Bremsstrahlung radiation (80%, due to kV) and characteristic x-rays (20% for tungsten, due to Z of material).

Question 13

The dominant (by far) form of attenuation of XR diagnostic beams?

This attenuation is proportional to?

Answer 13

Photoelectric effect - photoelectric absorption is proportional to (Z/E)^3

Question 14

Typical beam energies for diagnostic XR?

Why does this suit Iodine and barium contrast

Answer 14

Around 80KV

A beam at 80 kV will have an average x-ray energy near 30 keV(i.e 1/3 peak) - exactly the k-edge of iodine.

Question 15

The dominant (by far) form of attenuation of XR diagnostic beams?

The implications for choosing KV? And the central trade-off of XR imaging?

Answer 15

The photoelectric effect is highly accentuated at the k-edge of a material; materials with k-edges in the relevant range include iodine and barium. The k-edges are typically at low energies, and the closer your x-ray energy is to the k-edge (as long as it is above it), the more likely a photoelectric event is to occur.

Thus, to improve contrast, decrease your x-ray energy (kV). However, this increases dose because more x-rays are absorbed by the patient so you need to send more x-rays through from the tube. This represents the central trade-off in x-ray imaging.

Question 16

X-ray penetration is an exponentially decreasing function of patient thickness. Large patients get much larger doses; you can improve this by? By at the cost of?

Answer 16

X-ray penetration is an exponentially decreasing function of patient thickness. Large patients get much larger doses; you can improve this by increasing the x-ray energy but at the cost of contrast (i.e low energies accentuate differences in PE attenuation).

Question 17

For diagnostic XRs, the number of photons sent through a patient is determined by?.

Changing kV changes the number of photons?

Answer 17

For diagnostic XRs, the number of photons sent through a patient is determined by the current across the tube (mA) and the imaging time (s), which are sometimes multiplied together as mA*s = mAs.

Changing kV changes the number of photons in a non-linear fashion ~ (kV)3.

Question 18

Diagnostic XRs interact with film to produce …… proportional to the intensity of the beam.

Answer 18

Darkening of the film is proportional to radiation intensity incident on the film

Question 19

USS uses sound waves in the freq range:

It exploits

Answer 19

1 to 20MHz

Exploits differences in the speed of sound transmission through different mediums (dependent on medium’s density and stiffness). Stiffness = bulk modulus.

Question 20

As an USS beam penetrates, energy is lost as:

Answer 20

1) Reflection: at interfaces of different densities (Acoustic impedance) - bigger the diff, the bigger the echo.
2) Refraction: change of direction, with loss of speed but not frequency.
3) Scattering: different organs have defined scatter pattern.

Question 21

Scan modes of USS:

Answer 21

A = Amplitude mode - a line through the medium on which echoes are plotted
B = Brightness mode - Linear array scans across a 2D plane to make grayscale fan image
M = A or B modes as a function of time
D = Doppler - ie exploits the red shift in frequency (like the expanding universe) to determine if moving towards or away from probe.

Question 22

4 steps in explaining how MRI works:

Answer 22

1) Strong magnetic field (e.g 1.5-3T) aligns proton spin axis with field (parallel or anti-parallel) B0 axis/direction. Slightly more parallel than anti makes detectable field/
2) Body region targeted by increasing magnetic field strength to ROI and once increasing precessional frequency (Larmour frequency).
3) RF pulse resonant with Larmour frequency applied 90 degrees to B0, this B1 transverse field measured by coil. Changes over time recored, fransormed in Fourier domain, and used to create images.
4) Decay time (of B1 strength, time to 63% decay = T2) or Recovery time (B0 strength, time to 63% recovery of B0 strength) used to create T1 or T2 images.

Question 23

In physical (not tissue) terms, what is bright on T1 compared with T2

Answer 23

On T2, long relaxation time is bright (fluids)

On T1, long relaxation times are dark (solids and fluids), fat is bright.

Question 24

What tissues are bright on T1, what dark

What can be done to “tune out” the bright component?

Answer 24

solids and fluids are both dark on T1 (Slower relaxation times)
Fat is bright (quick relaxation time)

STIR (short T1 inversion recovery) take image at time when half of fat has recovered to B0, half is in b1 = net 0 magnetic field.

Question 25

Besides T1, and T2, what are some other common MRI imaging techniques:

Answer 25

● STIR (short T1 inversion recovery) is a fat suppression technique. Image is taken at the fat ‘null point’ on the T1 curve
● FLAIR (fluid attenuation inversion recovery) is water suppression technique.
● DWI (diffusion weighted imaging) is based on diffusion of water molecules within a voxel. Stationary water retains signal hence appears bright, moving water loses signal and appears black.
● Spectroscopy allows measurements of concentrations of various metabolites
○ eg. in radiation necrosis choline will be low but high in tumours.

Question 26

How does PET work?

Answer 26

1) Tracer (e.g FDG) labelled with positron emitter administered
2) Tracer accumulates at target.
3) Positrons annihilate with electrons releasing two 511 keV photons in opposite directions
4) Parallel detectors determine spatial position of annihilation events

Question 27

What is FDG:

Answer 27

● F18- fluorodeoxyglucose (FDG) is taken into cells and trapped intracellularly.
● Accumulates in metabolically active cells, eg. inflammatory tissue, tumours, bone marrow, GI epithelium.

Question 28

In PET scanning, how is uptake measured?

Answer 28

Standardised Uptake Value (SUV)
i.e normalises the value of the radioactivity in a tissue by dividing by the averaged radioactivity (injected amount/body weight)

SUV(t) = concentration at POI (at time t)/(total radioactivity/body weight)

Question 29

What is SPECT?

What are some advantages and disadvantages?

Answer 29

● Uses gamma emitting tracer (eg. technetium-99m)
● Gamma camera acquires multiple 2D images from multiple angles, 3D image reconstructed.
● Compared to PET, contrast and spatial resolution is lacking, but it is less expensive.

Question 30

Port films have been replaced by?

Both are produced by?

Answer 30

Port films have been replaced by electronic portal imaging devices (EPID).

Question 31

Disadvantages of EPID?

Answer 31

Disadvantages:

1. Poor contrast between tissues due to mainly Compton interactions (i.e MV rather than kV)
2. Dose to patient

Question 32

You have a radical intent (i.e IMRT) prostate cancer patient with a metal hip replacement. How does this effect your choice of CBCT?

Answer 32

Choose MV CBCT, since compton attenuation is relatively independent of Z (only electron density).

(Note: Not all linacs have kv CBCT - which needs to be mounted as an accessory at 90o to the treatment gantry)

Question 33

Advantages of CBCT:

Answer 33

Advantages:

1. Available in MV or kV CBCT
2. Good spatial resolution and in kV CBCT, good resolution between soft tissues.
3. For MV CBCT, there is less metal artifacts
4. 3D image produced, enabling more accurate patient positioning.
5. Mounted to gantry to reproduce treatment fields.

Question 34

Disadvantages of CBCT:

Answer 34

Disadvantages:

1. kV CBCT’s are mounted as an accessory at 90o to the treatment gantry, requiring additional maintenance work.
2. Significant more dose compared to EPID (this is usually calculated into the total dose given with each treatment)
3. Time to obtain images slightly longer than for EPID.
4. Additional QA.
5. Higher cost
6. Additional skills required to read and interpret images.

Question 35

What do you always forget to draw when drawing the stomach?

Answer 35

Angular incisure, also point out the cardia