Section 2 M Vision Geometry Calibration V Mc 062707 V Rjo062807

Page 1 July 2007 Copyright © Siemens AG 2007. All rights reserved.

Section 2MVision Geometry CalibrationSiemens Medical Solutions, Inc.Oncology Care Systems Group4040 Nelson AvenueConcord, CA 95420

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MVision Physicist Self-Led Training

81 40 258 Rev. C


Section 2: Table of Contents

Objectives Scale Factor

Overview MVision Image Reconstruction

Geometry Calibration Phantom MVision Protocol

Geometry Calibration Phantom Positioning

Projection Matrices

MVision Gain Videos:

MVision Gain Field Creation Geometry Calibration field creation

Geometry Calibration Phantom positioning


Section 2: Table of Contents

MVision Control Console Display Labs:

Geometry Calibration Window MVision Gain

Geometry Calibration Results MVision Protocol

Video: Geometry Calibration MVision Geometry Calibration

Fail to Pass Geometry Calibration

Videos:

Section 2 Review

Geometry Calibration Phantom

Setup (p. 40)

Section 2 Quiz

MVision Geometry Calibration (p.

50)


Objectives

At the completion of this section, you will be able to:

Identify the fundamentals of MVision Geometry calibration

Understand the function of the Geometry Calibration Phantom

Describe the MVision Reconstruction Process

Describe the MVision Protocol parameters and how they influence the image quality

Perform the MVision gain calibration

Perform the MVision Geometry Calibration


Overview

Figure 2.0 Geometry Calibration phantom


Overview

Figure 2.1 Projection image of the phantom



CradleGantry

Side

Figure 2.2

Geometry Calibration Phantom Perspective View

Shell



Cradle Base Plate

Figure 2.3

End and Side Views of the Phantom

Small Tungsten BeadsLarge Tungsten Beads

Level screw



Phantom Physical Dimensions

Physical Parameter Dimensions

Diameter of the Phantom Cylinder 140 mm

Diameter of the small Ball Bearings 3.2 mm

Diameter of the large Ball Bearings 6 mm

Number of Ball Bearings 108

Table 2.0


Geometry Calibration

Suggested Calibration Frequency:

Every 6 months or when it is required

Examples of when Geometry Calibration has to re-done:

Every time Flat Panel mechanical alignment is performed

Adjustments to the LINAC mechanical isocenter are performed

XRETIC Calibration is performed



Figure 2.4PROJECTION MATRICES

PHANTOM MODEL: 3D INPUT PHANTOM IMAGES : 2D INPUT



2D Projection Image ProcessingEach phantom projection image is processed to determine the

ball bearing (bb’s) 2D position and size regarding the image coordinate system (u,v).

Figure 2.5

u

V

0 0 0 0 0 0 0 1 0 0 0 1 0 1 0 0 1 1 0 0 0 1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 0 0 1 1 1 0 1 0 1 1 0 1 1 0 0 0 0 1 0 1 1 0 01 0 0 1 0 1 0 1 0 1 1 1 1 0 0 1 0 1 1 1 0 1 1 1 1 1 1 1

Encoded ball bearing list

from 2D projection images



Figure 2.5

u

V

0 0 0 0 0 0 0 1 0 0 0 1 0 1 0 0 1 1 0 0 0 1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 0 0 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 0 0 1 1 1 0 1 0 1 1 0 1 1 0 0 0 0 1 0 1 1 0 01 0 0 1 0 1 0 1 0 1 1 1 1 0 0 1 0 1 1 1 0 1 1 1 1 1 1 1

Encoded ball bearing list

from 2D projection images

2D Projection Image ProcessingEach phantom projection image is processed to determine the

ball bearing (bb’s) 2D position and size regarding the image coordinate system (u,v).



3D Phantom Model Phantom Model provides the 3D input used in the calculation of the

projection matrices

Bead X,Y,Z Coordinates

Bead SizeBead TypeBead ID

Figure 2.6



Coordinate System

Figure 2.8

r

Flat Panel

Gantry

X-ray Source

World (IEC)

coordinate system 1

Gantry

coordinate system 2

Camera

coordinate system 3

X-ray receptor

coordinate system4

Pixelised Imaging

coordinate system5

Y

Z

X

V

u



Coordinate System

Figure 2.8

r

Flat Panel

Gantry

X-ray Source

World (IEC)

coordinate system 1

Gantry

coordinate system 2

Camera

coordinate system 3

X-ray receptor

coordinate system4

Pixelised Imaging

coordinate system5

Y

Z

X

V

u


Projection Matrix

Projection MatrixHas 12 coefficients representing geometrical parameters such as

rotation angles and translation vectors

Provide the 3D (voxel) to 2D (pixel) geometrical mapping

Provides the input for the Backprojection step

Is computed for each MVision projection angle

When computed:

Allows 3D reconstruction of 2D MVision projection images

Can be used only in the LINAC where it was computed


Projection Matrix

The 2D points are related to the 3D points by the projection matrix P

Figure 2.9

34333231

24232221

14131211

pppp

pppp

pppp

P

),,(),( zyxRvuR

P

Projection matrix coefficients

2D Ball bearing position on

the phantom projection images

3D Ball bearing position on

the phantom model


Projection Matrix

Projection Matrix Naming Convention

The projection matrices are stored in a single file named:

ProjectionMatrices_<SID>_<StartAngle>_<EndAngle>_<AngleInc>_<CW|CCW>_<BAD>.xml

Example filenames:

ProjectionMatrices_1450_2700_1100_10_CW.xml (successful calibration)

ProjectionMatrices_1450_2700_1100_10_CW_BAD.xml (calibration failed)


Projection Matrix

Projection Matrix Naming Convention

The projection matrices are stored in a single file named:

ProjectionMatrices_<SID>_<StartAngle>_<EndAngle>_<AngleInc>_<CW|CCW>_<BAD>.xml

Example filenames:

ProjectionMatrices_1450_2700_1100_10_CW.xml (successful calibration)

ProjectionMatrices_1450_2700_1100_10_CW_BAD.xml (calibration failed)


Projection Matrix

Projection Matrices and Phantom Model Storage

Figure 2.10


Scale Factor

A I0 - MU Scale Factor is computed from the low MU MVision gain and used in the image reconstruction process Once determined is used to obtain non-attenuated beam data (I0) for

any MVision monitor unit setting

The Scale Factor is computed as:

Figure 2.11

MU

I0FactorScale

non-attenuated beam

Intensity from gain images

Monitor units used to

acquire gain images


MVision Image Reconstruction

Reconstruction VolumeIt is a cube made of voxels and centered at the machine isocenter

It is used for the backprojection algorithm as the template to reconstruct MVision images using the projection data

Figure 2.12



The backprojection process uses the projection matrices to reconstruct 3D MVision images

LINAC Radiation Source

Reconstructed

MVision Slice

b)

LINAC Radiation Source

Reconstruction

Volume

a)Figure 2.13

Flat panel



Backprojection Process

Figure 2.14

)),(

ln(),(0I

vuivuN

X-ray source

Projection Image

Reconstruction

Volume

i(u,) - pixel intensity

at u,v positionu

v



Backprojection Process

Figure 2.14

X-ray source

Projection Image

Reconstruction

Volume

i(u,) - pixel intensity

at u,v positionu

v

)),(

ln(),(0I

vuivuN



Image filtering is necessary in the Backprojection process

Figure 2.15

e)

Filtered Image

1 Projection

a)

2 Projections

b)

4 Projections

c)

256 Projections

d)


MVision Gain

Acquired like in a MVision acquisition

Used to:Correct differences in the Flat Panel diodes behavior (as in 2D

gain)

Applied to MVision imaging only

Compute a Scale Factor used in the image reconstruction process

Suggested Calibration Frequency: Every 2 Weeks


MVision Gain

Two MVision Gain fields are delivered in free airHigh MU Gain used on MVision Geometry Calibration projection

data

Low MU Gain for regular MVision projection data

Two hundred gain images are acquired and averaged

Two MVision Gain files are createdThe software “knows” whether to save the gain as “High MU

MVision Gain” or “Low MU MVision Gain”


MVision Gain

Gain Storage location

Figure 2.16


MVision Gain Field Creation

Log in on the Practice Database and Service SoftwareUnder the Service Patient, the Site ‘Calibration Other’ on TxDelivery task card is selected

Figure 2.17



Gain is selected as type of MVision field

Figure 2.18



The Table eccentric is set to either 90˚ or 270˚ to prevent field override message

Figure 2.19



MVision protocol selection

The 8MU or 15MU protocol is used for low MU gain

The 60MU protocol is used for High MU gain

Figure 2.20


MVision Protocol

Used to set the MVision acquisition and reconstruction parameters

Protocol creation application is launched from Coherence RTT

Acquisition Parameters

Only the Monitor Units can be edited/changed

Arc Angle, Gantry direction, Sampling and SID are fixed

Reconstruction Parameters

Slice Size (pixels)

Slice thickness (mm)

Kernels (filters)


MVision Protocol

Creating a MVision ProtocolUser must be logged on the Service Software prior to create/modify MVision protocols

Figure 2.21

Cone Beam Protocol icon


MVision Protocol

Protocol configuration

Figure 2.22

Reconstruction KernelSmoothing-Pelvis (Default)

Protocol Monitor Units

Slice Size (pixels)128 x 128

256 x 256 (default)

512 x 512

Slice Thickness (default 1mm)

MVision protocol name


MVision Protocol

Effect of the Slice Size on the image quality

Figure 2.23


MVision Protocol

Effect of the Slice Thickness on the image quality

Figure 2.24


MVision Protocol

Effect of the Kernels on the image quality

Figure 2.25


MVision Protocol


Figure 2.26


MVision Protocol


Figure 2.27


MVision Protocol

Effect of the amount of monitor units on the image quality

Figure 2.28


MVision Protocol

Effect of the amount of monitor units on the image quality

Figure 2.29


MPR Thickness

Effect of the Mutiplanar Reformat Thickness (MPR) on the image quality

Figure 2.30


Phantom Positioning

The Geometry Calibration Phantom is aligned with the room lasers

Figure 2.31

a)

c)b)


Video: Geometry Calibration Phantom Positioning


Geometry Calibration Field Creation

Geometry Calibration is selected as type of MVision field

Figure 2.32


Geometry Calibration Field Creation

The 60MU protocol is selected for the Geometry Calibration

Figure 2.33


MVision Control Console Display

Figure 2.34


Geometry Calibration Window

The Geometry Calibration is a sub task under the Calibration task card

Figure 2.35

Acquired Projection Images are displayed

here

Images that have a valid

Projection Matrix after

interpolation are displayed here

Images that have an invalid

Projection Matrices are displayed here

Images that have

Valid Projection Matrices are

displayed here


Geometry Calibration Results

Successful Calibration display

Figure 2.36

Dog Ear



Successful Calibration display

Figure 2.37



Calibration Fails due Invalid projection matrices

Figure 2.38


Video: Geometry Calibration


Troubleshooting

Errors during Geometry Calibration Geometry Calibration failed due missing projection

Figure 2.39


Troubleshooting

Errors during Geometry Calibration Mismatch between expected and actual gantry position Gantry position and delivered dose of a projection image were not

received by the control console

Figure 2.40


Troubleshooting

Geometry Calibration not performed

Figure 2.41


Troubleshooting

Fail to Pass the Geometry Calibration

Phantom misalignment: Align the phantom properly and re-acquire the images Wrong Phantom Orientation: Align the phantom with label “Gantry Side” toward the Gantry Phantom position in the table: Place the phantom following the recommendations in the Lab 4 MVision

Geometry Calibration Object in the Image: Make sure there is no object other than the phantom in the beam path Wrong MVision Protocol: MVision calibration images should be acquired using the 60MU protocol


Section 2 Review

Now that you have completed this section, you will be able to: Identify the fundamentals of MVision Geometry calibration

Understand the function of the Geometry Calibration Phantom

Describe the MVision Reconstruction Process

Describe the MVision Protocol parameter and how they

influence the image quality

Perform the MVision gain calibration

Perform the MVision Geometry Calibration


Section 2 Quiz


1) The MVision Geometry Calibration is required in order to:

A) Correct flat panel sag

B) Map the geometry between a 3D object and its 2D

projections

C) Correct image offset caused by flat panel sag

D) A and C are correct


2) In order to perform the Geometry Calibration one should:

A) Place the Geometry Calibration phantom at the

LINAC isocenter

B) Acquire MVision projection images of the phantom

C) Calibrate the XRETIC

D) A and B are correct

E) A, B and C are correct


3) The Geometry Calibration phantom model and the phantom projection images provide the input necessary to:

A) Reconstruct the MVision images

B) Find out the LINAC isocenter coordinates

C) Compute the projection matrices



4) With regards the projection matrices it is correct to state that :

A) Provide voxel to pixel geometrical mapping

B) Do not depend on the flat panel alignment

C) Disable 3D reconstruction

D) All the above


5) In the MVision Protocol configuration window the protocol parameters that can be changed are:

A) Monitor units, SID and Kernels

B) Kernels, Slice Size and Slice Thickness

C) Arc length, SID and Slice Size

D) Matrix size, monitor units and sampling

E) A and D are correct


6) Increasing a slice size of a MVision image will cause the image spatial resolution, image noise and low contrast resolution respectively to:

A) Increase, decrease, decrease

B) Decrease, increase, decrease

C) Increase, increase, decrease

D) Decrease, decrease, increase


7) A 5mm increase in the MVision slice thickness will:

A) Increase reconstruction time

B) Reconstruct images 5mm thick

C) Decrease image spatial resolution

D) Reconstruct images every 5mm

E) All the above is correct


A) Smoothing Head-Neck kernel and 5mm MPR

B) 1mm MPR and Smoothing kernel

C) Edge Enhancing Head Neck kernel and 2mm MPR

D) 1mm MPR and Edge Preserving kernel

8) A Head and Neck patient of standard size will be imaged with MVision. The Kernel and MPR combination that will provide better soft tissue contrast and less cupping artifact:


9)Overall an increase on the monitor in a MVision acquisition will:

A) Increase the acquisition time and image noise

B) Decrease image noise and increase soft tissue

contrast

C) Increase the visibility of high density objects



10) Regarding the Geometry Calibration which one of the following is correct:

A) The 60MU protocol should be used

B) Phantom alignment is critical to get accurate projection

matrices

C) Image reconstruction is not possible without Geometry

calibration



11) Which of the following is correct about the MVision gain calibration:

A) Is acquired in the same way as the 2D gains

B) Should always be acquired at 15MU

C) Table should be moved out of the beam path

D) The 200 gain images are averaged

E) C and D are correct


Answer Key

1. B2. D3. C4. A5. B6. C7. C8. A9. B10. D11. E

Technology

Section 2 M Vision Geometry Calibration V Mc 062707 V Rjo062807