Chapter 19: Image Quality Constancy (Simulator)

portal verification systems, such as EPI, OBI, etc. must be calibrated___. Why?

regularly

Ensure quality images are produced= limit the need to repeat imaging

A simulation can take place in the treatment room using treatment equipment. Why do we bother with a dedicated simulator?

1. (Main reason) To reduce the burden on the tx machines.

2. To deliver a more accurate tx plan.

What is more widely used?

1. Conventional Simulators

2. Flouro Simulators

3. CT Simulators

CT Simulators

Why are CT simulators more widely used?

They give more detailed info on tissue:

(a) density, and

(b) geometry

What do the QA procedures for CT sim focus on?

The interpretation and transfer of:

1. data and

2. geometry

What CT items require Weekly QA?

1. Safety Switches

2. Alignment of Gantry Laser w. Center of Imaging Plane

3. Image Performance Checks:

List the Image Performance Checks:

1. CT # Accuracy for Water

2. Image Noise

3. In-Plane Spatial Integrity

What should the CT # for water be?

0 HU

Tolerances for the following:

1. ± 5HU

2. Manufacturer Specifications

3. ± 1mm

4. ± 2mm

CT items that require Monthly QA:

1. Orientation of gantry lasers

2. Spacing of lateral wall lasers in respect to lateral gantry lasers

3. Orientation of wall & ceiling lasers in respect to imaging plane

4. Mechanical Table Functions

5. Image Reconstruction

6. Exposure Quality

OSO’s MTF i(photoshop) exposed

CBCT stand fore

Cone Beam CT

What images does CBCT take?

verification films

CBCT provides imaging in __ dimensions.

1. 1

2. 2

3. 3

4. 4

3

CBCT uses __ beam.

1. MV

2. KV

3. either

3. either

CBCT shape of the beam:

cone-shaped

How does CBCT capture images?

Revolves around the patient, capturing a broader volume of data at each revolution around the patient.

What is the advantage of the Cone-Shaped beam?

easier for the image translator to reconstruct anatomic data = clearer more accurate images

Where is the KV CBCT mounted on a linac? Why is it on a linac?

On gantry at 90 degrees to tx head (KV arms)

Gives 3D image of a 2D detector for: target localization verification before tx deliver

OBI stand for

Onboard Imaging

True or False: CBCT is an OBI system

True

Purpose of OBIs:

Because the reproducibility of a setup can fluctuate or the patient’s external/internal anatomy can fluctuate during the course of the tx…

OBI helps therapists make adjustments to tx fields more readily.

Types of EPID/OBI Systems

1. Cone Beam CT (CBCT)

2. Fluoroscopic Portal Imaging Detector

3. Ionization Chamber Detector

4. Amorphous Silicon Flat Panel Detector

A. Fluoroscopy screen

B. metal plate

C. 45° high reflectance mirror

D. Camera

How does the Fluoroscopic Portal Imaging Detector work?

The digital processor in a Fluoroscopic Portal Imaging Detector enhances the poor contrast MV images by replacing them with a high x-ray energy digital fluoroscopy image system. Resulting in instant, better quality images.

How does the Fluoroscopic Portal Imaging Detector produce an image?

Image is formed on a fluoroscopy screen and transferred to the camera via a high reflectance mirror.

Where is the high reflectance mirror positioned?

45° under the fluoroscopic screen.

How often should you calibrate the (Fluoroscopic Portal Imaging Detector) camera?

Every few days

How often should you calibrate the Fluoroscopic Portal Imaging Detector?

1. Monthly

2. Whenever the FPID was bumped or moved

OBI systems need ___ calibration.

Regular

Some OBI systems send a ___ message indicating that the fluoroscopic screen needs to be exposed, so that adjustments can be made to the detector.

Weekly