Introduction to imaging

Question 1

What imaging are we referring to when using the term “on- treatment”?

Answer 1

Patient in room on linac bed having image taken. Image being taken by linac or equipment in linac room.

Question 2

Do you know why we use it? (imaging)

Answer 2

Make sure radiotherapy is given in correct place

May be anatomical changes

Question 3

Right amount of radiotherapy to patients

Answer 3

Not too much or too less (destroying tumour without damaging too many healthy cells)
Diagnostic tests
Protocols
Previous experience
Trials

Question 4

Right place

Answer 4

Scan
Planning
On treatment imaging

Question 5

What are the 2 types of error?

Answer 5

Random and systematic

Question 6

What is random error?

Answer 6

Patient movement

All measurements will be different every time

Tend to be large

Tend to be different every time

Unpredictable

Question 7

What is systematic error?

Answer 7

More compromising

Plans based on a single session

Error for whole of treatment

Same every time

Question 8

What are expected errors?

Answer 8

Simulator geometry		
Plan transfer				
Lasers					
Radiographers			
Patients
Usually systematic

Question 9

What are unexpected errors?

Answer 9

Staff error
Computer error
Patient issues!
Usually random

Question 10

Which is most important? Random or systematic?

Answer 10

Systematic as the most compromising as happens everyday.

Question 11

Random error CTV example

Answer 11

Random error- different part of CTV outside of treatment beam for some treatments

Question 12

Systematic error CTV example

Answer 12

Systematic error-same part of CTV outside of treatment beam for all treatments

Question 13

Reference image examples

Answer 13

Simulator image, DRR, DCR

Question 14

On treatment image examples

Answer 14

MV, KV, CBCT

Question 15

Simulator films

Answer 15

Simulator film/ digital image
Imported into the imaging systems
Consider the quality of the image – too light / too dark renders the image useless
If digital – use the windowing tools to ensure best contrast achieved

Question 16

What does DRR stand for and what do they do?

Answer 16

Digitally Reconstructed Radiographs

Imported electronically into the imaging systems from the planning computer
Produced at the same time as the plan, from the original CT planning scan, based on the plan produced

Radiograph = x-ray image

They are generally quite fuzzy due to the computer filling in the blanks between the CT slices

Question 17

What does DCR stand for and what are they used for?

Answer 17

Digital Composite Radiograph

Imported directly in to imaging system

Produced at the same time as the plan, from the original CT planning scan, based on the plan produced

The content of the image has been altered in some way.

Due to the energy taken allows us to see the soft tissue more easily but removes ability to see hard structures such as the ribs.

Settings have been altered and weighted down the bone (changed weightings)

Question 18

Choosing reference image

Answer 18

Simulator verification images compare treatment position with a single set-up on the simulator, not directly to the plan.
DRR/DCR images are produced with the plan, therefore compare treatment position with the planned position
Simulator images are used if the patient is planned directly from simulation not a scan. i.e. Some palliative patients. Not commonly used in UK anymore.

Question 19

Imaging on set

Answer 19

EPIs (electronic portal imaging)

Electronic image of treatment portal, however can be used to describe both MV and kV on-treatment images

Automated, thus fast

Facilitates intra-fractional and inter-fractional analysis

Good resolution but varies between manufacturers

Some operator subjectivity

Costly on introduction, maintenance and replacement

Question 20

MV - on treatment imaging

Answer 20

Uses a detector, mounted on the gantry at 180* to the treatment head

MV beam from treatment heads is used to produce the image

Ability to use treatment fields for imaging, therefore no extra dose

Treatment field image v’s isocentre check image

Higher radiation dose if treatment field not suitable

Not diagnostic quality

Question 21

KV - on treatment imaging

Answer 21

kV tube and detector are mounted to the side of the linac, each at 90* to the treatment head

kV beam from the separate kV tube is used to produce the image

Kilovoltage energy – less dose

Better image contrast and definition

Automated registration software

Intra-fractional analysis most accurate method

Better anatomical matching process

Always isocentre check image- no MLC

Cone beam CT, most common method

Facilitates IGRT

Question 22

CBCT

Answer 22

Requires full CT planning dataset to be imported into ARIA
Visualise all tissue in FOV, allows soft tissue registration
Facilitates accurate anatomical registration and correction
Use of CBCT continuously developing
CBCT can be taken (depending on equipment) using MV or kV beam

Question 23

Active Matrix Flat Panel Imagers (AMFPI) detector

Answer 23

Incoming x-rays, exiting from patient

X-rays interact with the metal plate to produce high energy electrons

High energy electrons interact within the converting phosphor/scintillator to produce optical light photons

Optical photons interact with the photodiodes => photoelectrons which discharge the capacitors in each pixel

Electrical signals from each pixel are readout in rows and fed to a computer to form an image; the electrical signal is proportional to the x-ray intensity over each pixel