Elekta iViewGT Electronic Portal
Imaging Device
Elekta iViewGT Electronic Portal Imaging Device
Version 3.1 of Elekta’s (Elekta, Norcrcoss, GA, IviewGT referred to here is a trade marked product of Elekta) imaging system supports integration. By integration we mean that the pixel values are summed over the entire beam on time. Hence a 100 monitor unit (mu) exposure will result in twice the pixel values of a 50 mu exposure.
When the iViewGT system is put into integration mode, one must select the option of having each segment written out as a single file. As of this writing, an available composite mode does not correctly add up the individual segments, although a final image is available. We have also noted that one may have to interrupt the accelerator between segments to give the iViewGT system time to write out the segment file.
We are providing two utility programs here. The first, IviewToDicom, is to be used in the general case. This utility will read the iViewGT data base and assemble all the segment files for each beam. This utility will then write each IMRT beam out in a single dicom file coded so that program ConvertEPIDImages can read the files, and the program ReadIviewGT provided here below is not to be used. The manual ConvertEPIDImages is then to be referred to.
ReadIviewGT is provided for the case when the user might want to export a single integration file and will require the user to record and reenter the scaling factor for the file.
Program IviewToDicom
This utility program will read the iViewGT data base and assemble the segement files for each beam, adding up the segments while applying the scaling factors. Beam files will then be written out in dicom format to the new images directory, in sub-directories by the patient name then by the plan name. The files are coded so that utility program ConvertEPIDImages can read them, and ReadIviewGT below is not used. IviewToDicom is licensed separately.
IviewGTDirectory.loc
A text file is read in the program resources directory that contains necessary information. A sample file is shown below:
/* file
format version */ 1
/*
number of systems to follow */ 1
/* for each system: */
// description of system.
<*Elekta IviewGT on 1*>
//
path to the iViewGT directory where PATIENT.DBF is:
g:\iview\db
//
translation of L, M, H, energy designations:
L
6
M
10
H
15
For each
iViewGT system there must be an entry consisting of a description text field,
the path to the system from the computer IviewToDicom is running on, and
translations for the letter L, M, and H to energy in mV. /* to / are comments not read as well as //
to the end of a line. Text fields are
set off with <* to *> if there are spaces in the text field. One would do an NFS mount which need only be
for reading, between the host computer and the IviewGT system computer.
Select
image system, patient
The main
toolbar is shown below:
Use the
option menu to select the iViewGT system if you have more than one (see above
file). There is also an option menu to
select IMRT cases (more than one segment per beam) and Convectional cases (only
one segment per beam). The IviewGT
system marks those image files different (IMRT is of type ‘T’, conventional is
type ‘P’). However, a file for a beam
name with letters “cal” in the name will be read with the IMRT choice so that
the conventional calibration file can also be included in the list of found
beams.
Then select
the patient. The program will then find
all files that were integrated for the patient. If more than one plan, you will be prompted to select the
plan. The program will then list all
the segment files found, sorted by beam.
A very simple example appears below:
One can
remove from the list selected items.
The list is constrained to consider only segments measured on the latest
day. Hitting the “Write Dicom Files”
button will write out a composite dicom file for each beam in the new images
directory. Run program ConvertEPIDImges
to read in these files along with a calibration file and proceed, using a
deconvolution kernel for the iViewGT system.
The “Write out the input files” button is for testing purposes and will
copy the data base files read on the IviewGT system into the new images
directory.
Calibration
File
To include a
calibration file, include the text “cal” in the port name. Otherwise the file may be excluded as files
of only type 84 (‘T’) are picked up for the IMRT mode. A non-IMRT field is marked type 80 (‘P’).
Program ReadIviewGT
Program ReadIviewGT has been provided to read in images from the Elekta iViewGT portal imaging system and to convert them to fluence for input to Dosimetry Check. This utility is provided for the case of exporting a single integration file in a dicom format. When the Elekta system displays an image, they are renormalized so that all images display with the same amount of brightness and darkness. A scaling factor is needed to restore the pixel values to their integrated value. This scaling factor is available on the screen and the user has to write it down. The scaling factor must then by typed into text field provided below when the images are read. The images are to be exported in Dicom format and are read in that format. Pixel values are divided by the scaling factor, which therefore cannot have the value of zero.
This conversion consists of three steps:
- Convert the pixel values to relative monitor units (RMU) by which every pixel is mapped to the monitor units that would produce the same pixel value at the center of a 10x10 cm field size.
- Deconvolute the image with a kernel that removes the effects of the imaging device, restoring the image to fluence.
- Multiply the image by the measured in air off axis ratio to restore the in air shape of radiation fields. Deviation from a flat distribution is mainly caused by the flattening filter.
Each of these steps requires data. The first step requires a calibration curve that can be generated from two or more images of a 10x10 cm field at different monitor units. This curve may have to be regenerated depending upon the stability of the imaging system. Note: if you calibrate to a different field size, use that field size.
The kernel generation for deconvolution is covered in a separate document: “Fitting Deconvolution Kernels for Electronic Portal Imaging Devices (EPID).” A link to the kernel fitting toolbar is provided in ReadIviewGT as shown below.
The in air off axis ratio curve that is part of the accelerator data base for a particular energy is used directly. This requires that the accelerator and energy be selected at the time of conversion. This curve is a circularly symmetric curve as a function of radius (actually equivalent as a function of angle with the central ray). It is measured with a detector in air but with a build up cap. We recommend you use something like brass instead of plastic. Generally you would scan the diagonals of a 40x40 field at any convenient distance, and take an average of each of the four legs.
The image files are written out in “Field Dose” format for direct input by Dosimetry Check (see the Dosimetry Check manual). In Dosimetry Check for each treatment portal (beam) the user must select the corresponding Field Dose file. The image files may here be converted as a group.
Choose Accelerator
The use must first select the accelerator and energy for which the images were obtained. The choose accelerator and energy toolbar is shown below:
Simply pick the accelerator from the pulldown list and then the energy, and then hit the “Continue…” button.
EPID Toolbar
The EPID toolbar is shown below:
We will cover each option below.
Patient
The patient must first be selected or a patient entry created. The other buttons on the pulldown will be grayed out until a patient entry is defined. Resulting Field Dose files are written to the patient directory under the patient’s name in sub-directory FieldDose.d. The treatment plan and stacked image set should be stored under the same patient entry.
A patient entry is actually a directory name made out of the patient’s name in the patient directory. The patient directory is defined by the file PatientDirectory.loc in the program resources directory (which is in turn defined by the file rlresources.dir.loc in the current directory).
Convert Images
Use this option to convert images once a calibration curve and kernel is available. You will first get a file selection box to select the iViewGT files to be converted:
Select Input Files
Select all the fields for the same machine and energy for the case. Then hit the OK button. The Move File and Copy File selections will have no effect as the input files are only read.
Convert to Field Dose files
The selected files will then appear in the next popup:
You must hit the “Select Calibration Curve” button to select a calibration curve. Once selected, the same curve will be the default entry here for this machine and energy. Calibration curves are stored in the data sub-directory CalDCur.d. The data directory is located by the file DataDir.loc in the program resources directory rl.dir specified by file rlresources.dir.loc in the current directory. An entry will be made in the data directory for each treatment machine to store the name of the last calibration file used here.
Likewise hit the “Select Deconvolution Kernel” to select the kernel to use. Again thereafter the program will default to the selection for the same machine and energy. An entry will be made in the data directory for each treatment machine to store the name of the last kernel file used.
Do not deconvolute images that are to be used to fit the kernel. Unselect the toggle button if the files are to be used for fitting the kernel. The kernel fitting routine won’t accept files that have already been put through a deconvolution process. Unselecting also means that the in air off center ration function will not be multiplied in.
The user must now type in the scaling factor for each image file selected. The scaling factor is available on the iViewGT system. The image label is in the third column and is made from the file name in the first column. The label can be changed. In the last column is the file name that will be used for the resultant Field Dose file that will be written into the particular patient’s directory. The user may change this file name.
We suggest leaving the pixel size at 1 mm for the resultant Field Dose file.
When finished, hit the “Convert to RMU” button. The images will be processed and written out. Then run Dosimetry Check, and for each beam select the appropriate Field Dose file created here.
Create Calibration Curve
Use this option to create a calibration curve. You must integrate at least two 10x10 cm field size at two different monitor units. We would recommend at least three fields, with the monitor units well spread out to cover the effective range of monitor units used in patient cases. The effective range is not necessarily the total monitor units for a modulated field but is generally considerably less.
We are assuming your machine calibration is performed for a 10x10 cm field. If you calibrate to a different field size, then use that field size.
The calibration popup is shown below:
You first hit the “Get Pont From Image File” button at which you will get the file selection popup shown below.
The program will read the signal value at the center of the field and write this number into the first text box as the signal value. You must then type in the scaling factor and then the monitor units used to expose the field.
When all fields have been entered, hit the “Fit Data” button which is described below. After fitting a straight line, save the calibration file by selecting the “Save As” button.
Files are saved to the subdirectory CalDCur.d in the data directory (located by file DataDir.loc in the program resources directory).
Get point from image file
Hitting the “Get Point from Image File” will bring up file selection box as describe above.
Here you can only select one file at a time. Hit the “Apply” button and use the “OK” button on the last file to take down the popup, or hit the Cancel button.
Fit Data
This button will bring up the Field Dose curve fitting popup control. But the control will be defaulted to a linear fit. We recommend that you not change it from a linear fit. Only a linear fit can be extrapolated. Otherwise you would have to provide a zero point (which you could type in). Results of using the any fit will always truncate RMU < 0 to 0.
The functions of this control are described in detail in the Field Dose section of the Dosimetry Check manual. Here it is only necessary to hit the “Fit Curve to Data” button.
And you may want to enter a title for the plot.
After fitting the line, you can dismiss this popup. Then save the calibration curve.
Field Dose
Access to the Field Dose toolbar as described in the Dosimetry Check manual is provided here for the convenience of using some of those functions. In particular, under the Dose pulldown is the “Show Dose” function which can be used here to display the RMU of a converted file. Use this function to check the current calibration of a 10x10 cm field. You might want to include a 10x10 field with each case to test the stability of the iViewGT system for example. The center should hold the monitor units used to make the image.
Fit Deconvolution Kernel
Hit this button to get the kernel generation tool described in “Fitting Deconvolution Kernels for Electronic Portal Imaging Devices (EPID).” But you first need to convert iViewGT images to RMU files (Field Dose files) without the deconvolution process. Use those files along with water phantom scans to fit the kernel.