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All radiotherapy involves risk because even a small error in
treatment planning , delivery or dosimetry can lead to negative
This is because human body is a complex organism and tumors are
often located close proximity to the normal tissue and critical
So ensuring that the right radiation dose has been given to the right
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
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.
The first EPID system was video based.
The beam transmitted through the patient excite a metal fluorescent
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.
Another kind of EPID has a matrix ionization chamber system that
consist of 256*256 liquid ionization chamber(scanning liquid
Fluoroscopy based system (Video based)
Matrix ionization chamber based system
Amorphous silicon based system
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
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
CBCT is an imaging modality that
provide high imaging quality .
It has ability to provide submillimeter
resolution in terms of images.
Its called 3D cone beam imaging . It is fast , simple and
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.
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
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.
CBCT system can be implemented either by using
kilovoltage x ray source or
The megavoltage therapeutic source
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
A flat panel of x ray detectors is mounted
opposite to the x ray tube.
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.
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.
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.
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.
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.