MPR basics

The Multi-Planar Reconstruction or Reformatting (MPR) function projects a desired image volume onto three orthogonal planes. I.e. irrespective of the orientation of the image volume an axial, a coronal and a sagittal projection from the volume is displayed.

When MPR is activated the axial projection is in the upper left panel, coronal projection in the upper right panel and sagittal projection in the lower left panel. The three orientations are color coded in the crosshairs and frames around each projection image. The axial plane is blue, the coronal plane red and the sagittal plane is color coded green.

The line surrounding each projection image shows its active state. If the line is solid, the surrounded projection image is the current active image, while the other two are passive. A projection image is activated by clicking on it with any of the mouse buttons.

MPR implements visualization with respect to two coordinate systems, the Image volume coordinate system and the Magnet coordinate system . The image volume system has axes that coincide with the edges of the image volume. Its origin is located in the upper left hand corner of the first image in the series as seen in the image window. The magnet system coincides with what is sometimes called the patient system or the DICOM system. It is centered in the center of the magnet, with x direction from right to left, y direction from anterior to posterior, and z direction from inferior to superior.

Interpolation and geometric information

Projections are interpolated in order to account for the difference in in-plane to through-plane resolution. This resolution information as well as orientation is obtained from the header of DICOM images. For SPM/Analyze data the resolution is obtained from the header file (if available) and orientation is obtained from the matrix file (if available). If no resolution information is available default resolution as set in the Default voxel size field of the Settings' Image Format tab is used. And if the image orientation may not be obtained, it is assumed that the images are axial.

Labels

When hovering the mouse pointer over any of the projection images a label with the current coordinates of the mouse pointer is shown in the lower right corner of the corresponding projection panel. The position label is continuosly updated as the mouse pointer changes position.

Each projection panel has an orientation label. The nomenclature is the same as with the octant label in the status bar. The axial and coronal panels contain the letter L indicating the patient's left side. The sagittal panel displays the letter P pointing to the patient's posterior side. When planar views are updated by moving crosshairs or rotation transformation the shade of the orientation labels indicate their position relative to the projection plane. A greyed label indicates that the orientation label is positioned behind the projection plane while a white label indicates that it is positioned above the plane.

When performing transformations additional labels occur in the lower left corner of each projection panel. The label describes the amount of transformation such as translation distance, angular rotation and scaling percentage.

Resizing window and panels

The MPR window may be maximized to cover the full extent of the screen, or resized to dimensions chosen by the user. The projection images are automatically resized accordingly.

Also the relative size of the image panels may be modified by dragging the split lines that separate the panels in the MPR window. By dragging the horizontal split line the vertical dimension of the panels is updated, and vice versa for the vertical split line. Both dimensions are updated my dragging the intersection of the split lines. The relative panel dimensions may be reset by double-clicking any of the split lines or the intersection.

If any of the three projection images are double-clicked its size increases to cover the whole client area of the MPR window. Double-clicking the image again resets its relative size. Note, however, that double-clicking also means that the crosshairs are repositioned leading to updates of the two other projection images.