MPR Window

The MPR Window

In PS version 6.8.6 you must create a folder named "Raw" into which 16-bit signed image data and DICOM meta-data have been exported from OsiriX in the .raw and .xml file formats. PS7 is expected to support direct import of most DICOM files.

The Multi-Planar Reconstruction (MPR) window is used to manage the loading, display, and reconstructions derived from a 3D CT (or MRI) image series. The 3D image series is expected to consist of a head-first, supine (HFS) sequence of thin axial slices covering the orbits and including both eyes plus a few cm superior and inferior to the orbits. It is important that both eyes are included because ocular symmetry will be leveraged to help compensate for head rotations in the scanner. Slice spacing must be consistent and slice thickness as thin as possible (e.g. < 1mm). Image data must be 16-bit monochrome. Individual image size is anticipated to be 512x512 pixels.

The status line just below the toolbar provides information about the currently selected layout pane.

In addition to the buttons in this window, other reconstruction parameters and access to DICOM meta-data are adjustable from the MPR menu which is added to the menu bar any time this window is frontmost.

The MPR window defaults to single pane layout mode. In this layout you can load an image series by dragging and dropping a folder of images. In version 6.8.6 the image series must be prepared using OsiriX. The folder must be named Raw and it must contain all of the image data in RAW file format using the Export->Raw... menu of OsiriX. There must also be a file containing the DICOM meta-data, exported from OsiriX's Meta-Data window in .xml file format (e.g. PatientName.xml).

Planar reconstructions are created by switching to 6 pane layout using the Layout control in the toolbar. The six planar reconstructions are:

Upper left pane (Sa): Sagittal reconstruction.
Lower left pane (Ax): Axial reconstruction.
Upper middle pane (Eq): Equator reconstruction.
Lower middle pane (TM): Tumor-meridian reconstruction.
Upper right pane (Ne): Nerve-coronal reconstruction.
Lower right pane (TC): Tumor-coronal reconstruction.

The meridian angle and coronal planar offsets from the equator automatically track those planes as they are defined and set in the Planar Dosimetry window. The crosshair displayed in the axial, sagittal and equator is used to find the geometric center (as defined by Plaque Simulator) of the eye. Fine tuning of the eye model will be accomplished by capturing these MPR window reconstructions (using the Auto Capture control) to Plaque Simulator's Image window and proceeding as usual in that window.

The orthogonal MPR crosshairs includes 1 mm tick marks and an optional histogram of the gray levels of the pixels beneath the histogram axes. The outer sclera is designated by a dashed green oval. The surface for oblate-spheroid reconstructions (e.g. the inner surface of the sclera to map the tumor base) is designated by a dashed red oval. The basic dimensions of the eye and spherical reconstruction surface can be adjusted using the control handles found on the crosshair axes. In the example below you can see in the nerve-coronal reconstruction (upper-right pane) that the center of the optic nerve connects to the eye about 1 mm superior to the geometric axial bisecting plane.

Toolbar controls

The toolbar buttons near the top of the window include controls for 8-bit and 16-bit enhancement of reconstruction images, select mouse cursor function, window layout and which eye is being modeled. The screens used with personal computers may be able to display millions of colors, but are limited to only 256 levels of gray because each pixel is represented using a 32-bit format which is composed of an 8-bit value for each color component (red, green, blue) and an 8-bit value for opacity (alpha). To display 16-bit monochome CT (and MRI) data on a screen capable of only 8-bits of monochome brightness, the 16-bit data is initally passed through a window and level function (WW and WC) which is adjusted using the mouse cursor. Once the 8-bit representation of the image is created, it may be further enhanced just as in the Image window. Plaque Simulator supports two methods of 8-bit image tonal adjustment: (1) Brightness & Contrast (BC), and (2) Gamma, which has controls for Bias, Gain and Level. In Gamma mode, slightly decreasing Gain and shifting Level is often very effective for enhancing scleral contrast in a CT image.

Gamma (vs Brightness & Contrast) - Toggles between the BC and Gamma methods of tonal adjustment.
Bias/Brightness - Turn knob CW to increase the Bias or Brightness, CCW to decrease.
Gain/Contrast - Turn knob CW to increase the Gain or Contrast, CCW to decrease.
Level - Turn knob CW to shift the Gamma Level setting to the right towards white, CCW to shift left towards black.
Reset - Resets the image tonal controls to their default settings.
Mouse Cursor - Sets the mouse cursor function.
- Information - In this mode, clicking the mouse button will display a crosshair cursor with information about the crosshair coordinate and the voxel at that coordinate.
- Drag image - In this mode, click the mouse button to drag the image set with respect to the crosshair.
- Zoom - Magnify the image. Zoom is automatically synchronized for all panes. The information displayed in a pane varies automatically depending upon the current zoom control magnification. For example, you may need to "zoom-in" to reveal the image histogram.
- Rotate crosshair - In this mode, position the cursor over a crosshair axis and click the mouse button to drag the crosshair around its center. If you position the cursor over the crosshair center you can click the mouse button to drag the crosshair center on the image plane. When you release the mouse button, the cursor will return to image center and the image set will shift to remain centered on the crosshair.
- 16-bit window & level - In this mode, click and drag to adjust the window center (WC) and window width (WW) parameters which are used to rescale the raw 16-bit data for display on an 8-bit gray level screen.
Previous - displays the previous (lesser z coordinate) slice when in single pane layout.
Slider - Slide to display the image series axial slices. In the example above, the image set consists of 196 slices, numbered 0..195. Slice 0 will be the slice with the lowest z coordinate, slice 195 will have the greatest z coordinate.
Next - displays the next (greater z coordinate) slice when in single pane layout.
Lock - Locks the eye center coordinate within the image data set. Lock to prevent accidentally translating the image or crosshair center by dragging.
Layout - Switches between single pane (raw image set functions) and 6-pane (planar reconstruction functions) modes.
Eye - Select left or right eye for treatment and 3D modeling. The eye laterality context will change automatically as you drag the image set or crosshair depending upon the eye center coordinate in the data set assuming the patient was scanned HFS with the head centered in the scanner. To prevent automatic eye laterality, manually select left or right eye and then lock the Eye laterality.
Reset rotations - Resets all crosshair rotations to zero.

Meta-data sheet

This modal sheet displays meta-data extracted from DICOM file meta-data tags. If using a series of image files exported from OsiriX in Raw.nnn file format you may need to manually override the 16-bit pixel representation to the Signed setting. PS uses the value stored in the hexadecimal slice thickness tag (0x0018,0x0050) and the decimal filename tag (.nnn) to calculate the z coordinate of Raw.nnn files. For some image series, you might also need to manually set the slice thickness to instead use the value found in the DICOM slice spacing tag in order to create properly scaled 3D reconstructions.

Dosimetry window reconstructions

In the Planar Dosimetry window, set the dosimetry plane to overlay the appropriate image (e.g. T-Mer) as usual. If a tumor-meridan image has been loaded or captured, it will be used. To "live" recalculate the planar reconstruction every time the image window is updated, such as when rotating or translating the dosimetry plane, enable the Use MPR control. Live recalculation of the dosimetry plane reconstruction adds about 3 msec per pane (e.g. when in 2-pane layout mode, both meridian and coronal reconstructions will need to be recalculated) to the window update time.

Retinal diagram spheroidal reconstruction

In the Retinal Diagram window you may optionally map a reconstruction of the oblate-spheroidal surface to the retinal diagram which is an azimuthal equidistant projection. The reconstructed surface should be slightly inset from the inner scleral surface in order to best visualize the tumor base. Insets in the range 0.5 to 1.5 mm work well. The inset can be adjusted either in the Retinal Diagram window or in the MPR window by dragging the red control handle in one of the orthogonal reconstruction panes. Below is a comparison of spheroidal surface reconstruction (inset = 1 mm) and fundus imaging. The reconstruction is displayed using custom spheroidal pseudo coloring to highlight the tumor base. Spheroidal surface reconstructions can be very useful when portions of the tumor base are located too anterior to photograph using optical technologies such as fundus photography.