CT Design and Simulation PP In-Class

Question 1

What is CT and how did it evolve?

Answer 1

CT stands for Computerized Axial Tomography (CAT or CT), derived from the Greek tomos, meaning section or slice.

CT was first used clinically in 1972, significantly improving radiotherapy accuracy by aiding in tumor staging, treatment planning, and verification.
(Slide 4)

Question 2

What are the basic principles of CT scanning?

Answer 2

Detector elements are arranged in an arc and rotate around the isocenter along with the x-ray tube.

The fan-shaped x-ray beam is collimated to specific body areas.
(Slide 5)

Question 3

How do CT detectors work?

Answer 3

Scintillation crystals in the detectors emit light proportional to beam attenuation.

This light is converted to an electrical signal, forming a 2D image after computerized reconstruction.
(Slide 6)

Question 4

What is Spiral CT, and how is it achieved?

Answer 4

Spiral CT is achieved by moving the patient through the scanner during image acquisition.

This results in a spiral of data with small gaps, which are interpolated by software.
(Slide 7)

Question 5

What is pitch in CT scanning, and how does it affect image quality?

Answer 5

Pitch is the ratio of table movement to tube rotation.

A higher pitch reduces scan time and dose but increases the gaps between slices, which can degrade image quality.
(Slide 8)

Question 6

How do multi-slice CT scanners differ from single-slice scanners?

Answer 6

Multi-slice scanners use multiple rows of detectors to capture multiple slices per rotation, allowing greater tissue coverage and faster scan times.

Example: Capturing 4 slices per rotation.
(Slide 8)
Note: Review the image on this slide showing a visual comparison between single-slice and multi-slice scanning.

Question 7

In which planes can CT images be viewed?

Answer 7

CT images are most commonly viewed in the axial (transverse) plane but can also be reconstructed into sagittal and coronal planes.
(Slide 10)

Question 8

What are the three main body planes used in CT imaging?

Answer 8

Sagittal: Parallel to the midline.

Coronal: Perpendicular to the midline.

Axial (Transverse): Horizontal plane.
(Slide 10)

Question 9

How can sectional anatomy help in CT interpretation?

Answer 9

Identify structures by correlating them with their location in a specific section of the body.

(Slide 11)
Note: Review the image on this slide showing the relationship between 2D and axial CT views.

Question 10

What is the difference between pixels and voxels in CT?

Answer 10

CT images are made of pixels, but since each slice has thickness, the pixels have volume and are referred to as voxels.
(Slide 12)

Question 11

What is slice thickness in CT imaging, and how does it impact resolution?

Answer 11

Typical slice thickness for radiotherapy planning is 3mm-5mm.

Thicker slices worsen resolution, a problem known as the partial volume effect.
(Slide 13)

Question 12

What are Hounsfield Units (HU)?

Answer 12

Hounsfield Units (HU) represent the x-ray attenuation coefficient of tissue.

Water is assigned a value of 0, with denser tissues having positive values and less dense materials having negative values.
(Slide 14)

Question 13

What are the typical Hounsfield Unit (HU) ranges in CT?

Answer 13

CT systems can measure HU values ranging from +1000 (dense structures) to -1000 (less dense materials like air).
(Slide 15)

Question 14

How many shades of grey can a CT monitor display, and how does this affect image interpretation?

Answer 14

A CT monitor can display 250 shades of grey, but the human eye can only distinguish 50-100 tones.
(Slide 16)

Question 14

What is windowing in CT, and how does it improve image contrast?

Answer 14

Windowing is the process of selecting a range of CT numbers to enhance contrast, optimizing the visibility of specific tissues.
(Slide 17)

Question 15

What are contrast agents in CT, and how are they used?

Answer 15

Contrast agents are substances with distinct HU values that highlight specific structures in CT images.
(Slide 18)

Question 16

What are some common contrast agents used in CT?

Answer 16

Positive agents (e.g., barium, iodine): Denser than soft tissues, appear brighter.

Negative agents (e.g., water, air): Less dense, appear darker.
(Slide 19)

Question 17

What are CT artifacts, and how are they caused?

Answer 17

Artifacts are distortions in CT images, such as noise, which can be minimized, or motion blur, often caused by patient movement.
(Slide 20)

Question 18

What are metallic artifacts in CT imaging?

Answer 18

Metallic objects (e.g., prostheses, dental fillings) can cause streaking artifacts, interfering with image quality due to the absorption of x-rays by the metal.
(Slide 21)

Question 19

What is Image-Guided Radiation Therapy (IGRT), and how does it benefit patients?

Answer 19

IGRT uses diagnostic-quality CT images to assist in localization, allowing for smaller, more accurate treatment volumes and reducing healthy tissue exposure.
(Slide 22)

Question 20

Typical slice in radiation therapy

Answer 20

2 to 2.5cm

Question 21

Examples of Negative contrast agents

Answer 21

Air & water

Question 22

The purpose of CT simulation?

Answer 22

so the dosimitrist can make a plan for treatment.

also first contact with patient, opportunity to answer questions, find most comfortable position.

Question 23

The abbreviation CAX means

Answer 23

central axis

Question 25

Localization

Answer 25

geometric definition of the position and extent of the tumor or anatomic structures by reference of surface marks that can be used for treatment setup purposes.

example: carina at t4 & t5 aka 2cm below sternal notch

Question 26

Verification

Answer 26

final check to ensure that each of the planned beams cover the tumor and avoid critical structures.

Question 27

Radiopaque markers

Answer 27

material with a high atomic number, i.e. lead, copper, solder wire. Used on skin surface or inside patient

example: you are treating a skin cancer or a scar from prior surgery, the only way that you can delineate it from normal skin tissue.

there is also internal radiopaque markers, like surgical clips left inside body, or the markers left of biopsied tumors that are benign.

Question 28

Simulation

Answer 28

process of determining the patient treatment position, the volume to be treated, and the normal structures in or surrounding that volume.

Question 29

Contrast media

Answer 29

compound or agent used as an aid in visualizing internal structures, i.e. barium (Z=56), iodine (Z=53)

Question 30

DRR

Answer 30

2D image reconstructed from CT data – beam’s eye view

reference guide, exact position everyday that we want the patient to look

Question 31

Interfraction motion

Answer 31

change in target position from one fraction to another

patient set up differently due to being bloated, etc.

Question 32

Intrafraction motion

Answer 32

movement

Question 33

Organs at risk (OAR)

Answer 33

critical structures that may limit the amount of radiation delivered to the tumor volume

Question 34

Clinical target volume

Answer 34

GTV + surrounding volume of tissue that may contain subclinical or microscopic disease

Question 35

Planning target volume

Answer 35

CTV + margins for geometric uncertainties such as patient motion, beam penumbra, and treatment setup differences

Question 36

Internal target volume

Answer 36

CTV + internal margin that accounts for tumor motion

Question 37

Gross tumor volume

Answer 37

gross palpable or visible tumor

Question 38

Simulation Requirements

Answer 38

Effective patient care skills
CT scanner with fast acquisition time, axial and helical scanning, wide bore larger than 75 cm
External laser marking system

Options: gating technology (4DCT) means 4 dimensional CT, would be used to differentiate a tumors movement or if it remains still during the regular motion of the body, aka breathing.

surface-guided radiation therapy (SGRT)

Question 39

Benefits of CT Simulation

Answer 39

Accurate delineation of 3D volumes

Isocenter can be placed quickly and accurately in any location

Flexibility to create or change any factor of the treatment plan

More data for measurements postsimulation

Cone down or boost fields can be accomplished w/o patient present

BEV display allows anatomy to be viewed

Field shaping

Allows comparison of beams and construction of DRRs

Allows for downstream calculation and viewing of dose distribution based on patient anatomy (all calcs are done in ct)

Ability to mitigate intrafraction motion with 4DCT (you can make your ptv bigger because you can mitigate things due to 4DCT)

Reduction in procedure time

Digital archives (ten years later when they get treated for a different cancer because it spread, you can send those records to a different radiation center, to avoid going over the dose limits)

CT exposure can be quantified and recorded

Question 40

Considerations of CT Simulation

Answer 40

Size of bore must be large enough to accommodate patients in the treatment position with immobilization devices

Scanning and display fields of view must be large enough so that the entire external contour can be visualized. (problem of the patient is very large)

Couch must be flat and level so position can be replicated

External laser marking system

Speed and accuracy are considerations to minimize patient movement

Treatment machine parameters, beam-shaping devices, and treatment accessories (MLCs) cannot be verified on CT. (just because the patients arm clears the bore in CT does not mean it will clear the gantry)

Interfraction variability reduced through patient comfort and reproducibility. (cant weird things that aren’t reproducible, like positions dependent on certain clothing, because patient wont be wearing the same clothes.)

Monitor CT dose

Question 41

Steps for CT Simulation

*This is just a basic overview of steps. Refer to Chapter 21 for more detailed information.

Answer 41

1)Presimulation planning (before they get there)

2)Room preparation (writing down everything that you are setting patient up with)

3)Explanation of procedure (education tab to make sure you are educating patient)

4)Patient positioning and
imobilization

5)CT data acquisition (we pretty much stop here and hand of to dosimetrist)

6)Target and normal tissue
localization

7)Virtual simulation of treatment fields

8)Generation of dose distributions

9)Documentation of pertinent data

Question 42

Patient Immobilization

Answer 42

Positioning aids

Simple immobilization devices: restrict some movement and provide comfort

Complex immobilization devices: restrict patient movement and ensure reproducibility

Question 43

Head and Neck

Answer 43

Remove jewelry
Dentures may be removed or kept in, depending on department protocol
Headrest and/or fabricated custom cushion
Bite block
Thermoplastic mask
Shoulder straps

Question 44

Thorax

Answer 44

Arms overhead on a wingboard with a headrest
Alpha cradle or vaclock
Knee sponge
Straighten patient
Table pad not recommended

Question 45

Supine Pelvis

Answer 45

Bladder and rectum instructions
Comfortable head support
Foam ring
Alpha crale, vaclock, or rubber band
Frog legs - Which sites? Why?
Contrast agents – small bowel, large intestine, bladder, or rectum

Question 46

Prone Pelvis

Answer 46

Belly board
Prone pillow
Indexed position on belly board

Question 47

Breast

Answer 47

One or both arms up
Breast board or wingboard - angle
Vaclock optional
Head turned
Breast tissue and scars outlined
Knee sponge
Large, pendulous breasts – alternative methods

Question 48

Methods of administering contrast agents

Answer 48

Intravascularly (we never do this)
Orally
Intrathecally
Intraarteriorly
CT simulation: oral, IV, intracavitary

Question 49

the type of contrast agent you would use for each type of administration

Answer 49

Oral or rectal:
Barium sulfate
Gastrograffin (diatrizoate meglumine and diatrizoate sodium)

IV – ionic or nonionic iodine

Common CT sites using contrast:
Barium swallow for GI tract
Gastrograffin
Bladder fill
Vaginal Q-tip
Rectal enema