ELECTRONIC PORTAL IMAGING DEVICES AND CONE BEAM COMPUTED TOMOGRAPHY IN RADIATION THERAPY

1. ANJU.K.V
MSC.RADIATION PHYSICS
CALICUT UNIVERSITY

2. All radiotherapy involves risk because even a small error in
treatment planning , delivery or dosimetry can lead to negative
consequences.
This is because human body is a complex organism and tumors are
often located close proximity to the normal tissue and critical
organs.
So ensuring that the right radiation dose has been given to the right
place.

3. set-up difficulties
soft tissue deformation
tumor growth/shrinkage
irregular respiration
spontaneous motion

4. ELECRONIC PORTAL IMAGING
CONE BEAM COMPUTED TOMOGRAPHY

5.  EPID = Electronic portal imaging device
 Daily imaging for treatment localization and verification may become feasible
 Acquisition, analysis, storage and distribution of portal images can be performed
with relative ease
 Mount on the linear accelerator

6.  Electronic portal imaging devices-EPID can used for daily
imaging for treatment localization and verification.
 The portal image can be analyzed with computer software
and before the treatment, no significant additional dose is
required for patient.
 The image can be saved in DICOM file format and it can
be reviewed at any time.

7.  The first EPID system was video based.
 The beam transmitted through the patient excite a metal fluorescent
screen.
 A front-silvered mirror , placed diagonally, reflects the fluorescent
light by 90 degree into the video camera.
 The analog output of the video camera is converted into a digital array
with a ADC known as “ frame grabber ”.
 The special resolution depends on phosphor thickness.

8.  Another kind of EPID has a matrix ionization chamber system that
consist of 256*256 liquid ionization chamber(scanning liquid
ionization chamber-SLIC).

9. Fluoroscopy based system (Video based)
Matrix ionization chamber based system
Amorphous silicon based system

10. Detector : Amorphous silicon flat panel imager
Active imaging area : 40 x 30 cm2
Resolution : 1024 x 768 pixels
Spatial resolution : 0.391 mm
Max frame acquisition rate : 9.574 frames/second
Energy range : 4 - 25 MV
Dose rate range : 50 - 600 MU/min
SPECIFICATIONS:

11.  Within this unit a scintillator converts
the radiation into visible light
 The light is detected by an array of
photodiodes implanted on an
amorphous silicon panel
 The photodiodes integrate the light
into charge captures
 Resolution and contrast greater than other system

12. EPID

14.  CBCT is an imaging modality that
provide high imaging quality .
 It has ability to provide submillimeter
resolution in terms of images.

15.  Its called 3D cone beam imaging . It is fast , simple and
completely painless.
 3D cone beam imaging system is basically a digital x ray
scanner mounted on a rotating arm . Like a digital
camera. Its called “CONE BEAM”.
 Because the scanner projects x rays in a carefully
controlled ,cone shaped beam.

16.  In CBCT planar projection images are obtained from multiple directions as
the source with the opposing detector panel rotates around the patient through
180 degree or more .
 These multidirectional images provide sufficient information to reconstruct
patient anatomy in three dimensions , including cross- sectional , sagittal and
coronal planes.

17.  Conventional CT scanner has circular ring of detectors , rotating
opposite to X-ray tube . However , it is possible to perform CT scans
with detectors embedded in a flat panel instead of circular ring . CT
scanning that uses this type of geometry is known as CBCT.

19. CBCT system can be implemented either by using
 kilovoltage x ray source or
 The megavoltage therapeutic source

20.  KILOVOLTAGE x rays for a KVCBCT
system are generated by a conventional x
ray tube that is mounted on a retractable
arm at 90 degree to the therapy beam
direction .
 A flat panel of x ray detectors is mounted
opposite to the x ray tube.

21. Its an ability to produce volumetric CT images with good contrast and
submillimeter spatial resolution.
Acquire images in therapy room coordinates
Use 2-D radiographic and fluoroscopic modes to verify portal accuracy ,
management of patient motion and making positional and dosimetric
adjustments before and during treatment.

22.  Megavoltage cone beam (MVCBCT) uses the megavoltage x ray
beam of the linear accelerator and its EPID opposite to the source .
 EPIDs with a- Si flat panel detectors are sensitive enough to allow
rapid acquisition of multiple , low dose images as the gantry is
rotated through 180 degrees or more . From these multidirectional
2D images , volumetric CT images are constructed.

23. MVCBCT system has a good image quality for the bony anatomy .
MVCBCT is a great tool for on-line or pretreatment CT, avoidance of
critical structures such as spinal cord, and identification of
implanted metal markers if used for patient setup.

24.  Less susceptibility to artifacts due to high objects such as metallic
markers in the target , metallic hip implants , and dental fillings.
 No need for extrapolating attenuation coefficients from KV to
megavoltage photon energies for dosimetric corrections.

25.  Portal dosimetry using EPID for IMRT plans can be performed in
less time compared to other existing tools.
 High density data points and direct to digital data capture makes it
easier to evaluate complex IMRT fields.
 CBCT has high ability to provide submillimeter resolution in
terms of images.
 In radiation therapy treatment verification is an important
process . And at that time make sure that right amount of
radiation will given to the right place.