Specifying the Primary Stacked Image Set

Each plan must be associated with a stacked image set. The stacked image set is chosen when the plan is created. This stacked image set is the primary image set and provides the skin boundary and pixel to density conversion. The dose may be shown on other fused image sets, but the calculation of the dose is based on the skin boundary and density defined by the primary stacked image set. Normally this is a CT data set since CT pixel numbers can be translated to density values.

Setting the Skin Boundary

Under the Contouring toolbar there is a Body Surface Contouring tool. If this tool is used to create contours, those contours will by default define the skin boundary. The skin boundary defines what is inside the patient and what is outside. If this tool is not used or is used more than once, or the resultant volume is copied, or any ambiguity exists as to what defines the skin boundary, than the user must formally select the volume that represents the skin boundary. From the main menu under Stacked Image Sets, choose Options and then choose Skin to select the skin toolbar.

The skin toolbar.

The toolbar shows the name of the current stacked image set which was chosen with the prior Options toolbar and provides a pulldown menu to select the volume that is to be the skin boundary volume. The current choice, if any, is shown to the right of the pulldown menu. If you have not yet created a volume that represents the skin boundary you must first do so using any of the contouring tools provided under Contouring on the main tool bar.

Image Pixel to Density Conversion

There must be a means to convert the pixel values in the primary stacked image set to density in either electron density relative to water or gm/cc. This is accomplished by creating a conversion curve that is used to convert pixel value, the independent variable, to density, the dependent variable. A polynomial fit of pixel value versus density data pairs is done to generate this curve. The curves are stored and may used repeatedly by different stacked image sets.

To select or create a density conversion curve, go to the Stacked Image Sets pulldown menu on the main toolbar. Select Options and be sure to select the stacked image set. Then hit the Density button and the Density toolbar will appear.

The Density Toolbar.

The name of the current stacked image set is shown to the right of the Return button on the density toolbar. Below we will describe the options on the Density toolbar.

Show Scanner

The show scanner button will display a popup with the manufacturer’s name of the imaging device, normally a CT scanner, the image modality (a two character code from the Dicom image file (0008,0060)), and the KVP if known for CT.

Show scanner popup

The software does not restrict from using imaging modalities other than CT for the primary stacked image set, but conversion of pixel values to density is generally not possible with other modalities. In those cases it is not possible to correct the dose for inhomogeneities in tissue density. For those cases the user must create and assign a constant curve whereby all pixel values are mapped to water density.

Select CT Number to Density Curve

The select curve button will display a list of the existing density conversion curves for the selection of one to use with the stacked image set. These curves are stored in the subdirectory DenCur.d under the directory for data specified by the file DataDir.loc in the program resources directory. The user assigns the file name when the curve is created. Once selected, the curve is permanently associated with the stacked image set. A copy of the curve is not made from the data directory and if the file is ever deleted, the association will be lost.

Default CT Number to Density Curve

After selecting a curve, the user may designate the curve as the default curve. Future new stacked image sets will default to this curve until the user chooses some other. The default curve name is stored in the file “DefaultPixeltoDensity” in the same directory DenCur.d.

Create Curve

Create pixel to density conversion curve popup.

The creation of a curve consists of two functions. First a list of data pairs, pixel value and density, must be created. Second, a curve must be fitted to the data pairs. This process is managed by the create density curve popup tool. The show pixel value popup will also appear and may be used to measure pixel values on a calibration scan. Phantoms can be purchased that contain tissue equivalent inserts for generating this calibration curve (Gammex RMI, Middleton, WI 53562, www.gammex.com; CIRS, Norfolk VA 23513, www.cirsinc.com; and Nuclear Associates, Carle Place, NY 11514, www.nucl.com also offers the CIRS phantom).

The density range should start at 0 for air, and go to the highest bone value that is obtainable. Because a polynomial fit is used, the density values will not be extrapolated beyond the range that is entered here. The highest bone value that is entered here will be the largest density possible. For pixels with higher values the largest bone value will be returned. The same is true at the other end of the curve, smaller pixel values will return the end value at the low end of the curve, which is why the curve should start at zero.

Pixel values refer to the values in the images that make up the stacked image set. Hounsfield numbers are not used here. For CT scans, a value of 1024 generally corresponds to water density or Hounsfield number of zero. A Hounsfield number of value –1000 would then correspond to a pixel value of 24.

The pixel values may be measured with the show pixel value popup, or may be typed in the text field provided. The corresponding density must be typed in. The density is specified relative to water with a value of 1.0 for water. Compton scatter is generally proportional to electron density and you may want to enter electron density relative to water instead of density in gm/cc. Either hit the Enter key in the density text field or hit the “Add to Data List” button with the mouse to transfer the current data pair to the list that is shown in the scrolled text area. Items in the list may be selected to be deleted. A description for the curve must be entered. The data may be sorted in increasing pixel value by hitting the sort button. When all the data is entered hit the Fit Data button to fit the polynomial.

Fit Pixel Density Data Pairs to a Polynomial

Fit Pixel Versus Density Popup

The fit pixel versus density popup provides the tool to fit the data to a polynomial. This popup is invoked from the above tool to enter the data. A slider allows one to select the order of the polynomial from 0, a constant, to a limit of order 9. You must have more data pairs than the order. You should select the lowest order that puts a smooth curve through the data. Try different orders and hit the Fit Curve To Data button. The curve is drawn through the plotted data pairs in the lower part of the popup tool. The standard deviation of the fit is also shown. You may notice that as you increase the order of a polynomial, the curve may begin to oscillate to pass through data points. You will have to lower the order of fit so that does not happen. If specifying a constant curve, simply enter data pairs that all have the same density of 1.0 and select zero order, such as (0.0,1.0) followed by (4096.0,1.0). To specify a straight curve for a CT scan simulated by program CreateSquareCTScan provided for testing purposes, enter (0.0, 0.0), (1024.0, 1.0), and (4096.0, 4.0).

A title for the display may be entered in the provided text field. Be sure to hit the enter key at the end of entering in the text. The Copy Plot to Frame button may be used to display the curve and data in a frame on a regular screen.

When satisfied with the fit, hit the Dismiss button.

Saving the Density Conversion Curve

Hit Save As button to save the curve. You will be prompted to enter a file name for the curve. Enter a name that will be descriptive of the curve that you will recognize in the future. Be sure to hit the enter key in the file name text field. Characters illegal in a file name will be stripped out and you need to see the result. Once you have saved the curve, you must select the curve for the stacked image set using the Select button on the Density toolbar.

Show Density Conversion Curve

Show Pixel to Density Curve Popup

Use the Show Curve button to display the currently used curve for the currently selected stacked image set selected under Options on the prior toolbar.

Show Density

The show density tool will allow you to determine the density value for points visible on images of the primary stacked image set. You may use the Show Pixel Value popup to measure the pixel values on images from the stacked image set. The density will be computed from the currently selected pixel to density curve for the stacked image set.