Quality Check with Portal Vision Imaging
Thomas Troff European Technical Support
January 27-28, 2012 Aarhus
Agenda



Portal Vision Imager

Imaging arm

Acquisition System
Calibration

Positioning of the Cassette

Imaging Calibration
Imaging problems
Imaging arm

3 Generations of Arms

S-Arm (swing arm) >16 Years

R-Arm (retractable arm) Design 15 Years back

Exact arm
latest design
R-Arm
Exact-Arm
OBI
Acuity
Acquisition System

3 Generations of Imaging Systems

PV Mark 1 with LC250

PV Mark 2 with LC250/aS500 (IAS2 System)

IAS3 aS500-II/aS1000 (IDU11 or IDU20)
IAS2 Complete System Overview
Image Detection Unit
Image Acquisition
System 2 (IAS 2)
Arm
Controller
varian
Image Acquisition System 2
Clinac
R Arm or E-ARM
RS-422
Clinac
Controller
Dose Rate
Servo
Clinac Stand
Dedicated LAN
(10Base-T)
PortalVision
Workstation
VARiS / VARiS Vision
Network
Electronics Cabinet
Image Acquisition system 2 controller
1.
Communicates with treatment task
2.
Performs self tests and initialization
3.
Verifies clinac setup and generates timing
4.
Acquires and processes signal from IDU
IAS3: System Overview
Different System names
System IAS Resolution
IDU
Channel
04
1
LC250
2
256 x 256
aS500
2
512 x 384 11/20
aS500-II
aS1000
3
3
512 x 384 11/20
1024 x
768
20
2
4
4
Speed
1.27MHz
1.27MHz
2MHz
2MHz
IDU 11 / 20
40x30 cm2
512,384
Gate Drivers
a-Si Panel
IDU Type 11 Top View
1,1
Read-out Electronics
IDU Type 11 Bottom View
IDU Construction: X-Section of aS500 and aS1000
MeV x-ray
copper metal plate 1mm
Phosphor 0.4 mm
photodiode
TFT switch
e
x-ray converter
e- to light converter
a-Si:H 1.5um
glass substrate 1mm
scatter photon
• Conversion of a x-ray to optical signal through the phosphors
• Conversion of optical signal from the phosphor to charge
• Storage of charge on pixel (photo-diode) capacitance
aS500 and aS1000 Pixel Schematic & Microscopic View
Bias Voltage Line
Data Line
TFT
Gate Line
Data Line
TFT
Gate Line
Photodiode
Array Pixel Schematic
V_bias (-5v)
Light makes
the photodiode
conductive
Data
Photodiode
Voltage is
decreasing on
Cap
Cap has to be
on a predefined
Level in
advance
TFT
Gate
Acquisition Techniques
Agenda



Portal Vision Imager

Imaging arm

Acquisition System
Calibration

Positioning of the Cassette

Imaging Calibration
Imaging problems
Calibration: Iso-calibration
2
3
1
4
Front pointer / optical distance
indicator measurement to surface of
detector
LIC
1,000.0 mm - 11.0 mm = 989.0 mm
a-Si
1,000.0 mm - 12.5 mm = 987.5 mm
Figure not to scale
1. Isocenter
2. LIC-detector surface
3. Sensitive layer
4. a-Si-detector surface
Image Correction

Dark field image

Flood field image

No radiation image (beam OFF)

Radiation image (beam ON)

Measured 60 times and averaged

Measured 30 times and averaged

Reflects

Reflects


Chamber imperfections

Field homogeneity
Electrical leakage of the photo
diodes

Individual pixel sensitivity

Amplifier gains
Gate 1
Object Image
BEAM ON
Gate= 384
DF
FF
When to Calibrate

Complete calibration DF – FF – Pixel correction


After IDU change, DU Change, imaging problem
Darkfield only


First 3 month every 2 to 3 weeks after IDU
replacment
After this > Dosimetry cal check
What modes to calibrate

Last image Hold no Calibration

Rad shot mode (low dose / high quality)


Continuous mode


Dark field / Flood field for 1 RepRate per Energy
@ 140cm or most clinically used position
Every RR that is clinically used per Energy
Integrated mode

Every RR that is clinically used per Energy
@ isocenter
RadShot Calibration

Low dose / High quality
Integrated mode
Agenda



Portal Vision Imager

Imaging arm

Acquisition System
Calibration

Positioning of the Cassette

Imaging Calibration
Imaging problems
Noise image-Good Image

2 test images subtracted from each other



Mean value
+- 5 counts
aS500
Std dev. <6
aS1000
Std dev. <10
Drift image-Good Image

Showing defect lines, pixels etc…
Image testing
Thank you for your interest
and attention
Discussion