This page describes a feature that does not exist in the Weasis version selected in the sidebar.
PET/CT Image Fusion with SUV
Functional series over anatomy, without registration
Image fusion overlays a functional series — PET (PT) or SPECT (NM) — on top of an anatomical CT or MR base, so metabolic uptake can be read against the underlying anatomy. The overlay is a registration-free geometric fusion: Weasis aligns the two series from their DICOM spatial metadata alone, with no manual or algorithmic registration step. Available since v4.7.1.
Fusion is a pure spatial overlay driven by each image’s position in the patient coordinate system. It does not deform or re-register the images, so it is only offered when the two series are genuinely co-located (see Requirements below).
Requirements
A series can be fused onto the displayed base only when all of the following hold:
| Condition | Detail |
|---|---|
| Modality pairing | A functional overlay (PT, NM) or an RT dose (RTDOSE) since v4.8.0 on an anatomical base (CT, MR). An RT dose is usually shown with the Colorwash option of the RT Tool. |
| Same study | Both series share the same StudyInstanceUID. |
| Volume geometry | Both are cross-sectional volumes (≥ 2 slices with Image Position / Orientation Patient and Pixel Spacing). |
| Same coordinate system | Either the FrameOfReferenceUID matches, or — as is common for separately reconstructed PET/CT — the two volumes overlap in patient space. |
When no compatible overlay is found in the study, the fusion controls stay disabled.
Enable fusion
The fusion controls live in the Fusion section of the Image tool panel. The section is collapsed by default — click its header to expand it.
- Enable Fusion A — turns the overlay on or off. The other controls become active only while fusion is enabled.
- Series B — the functional series to overlay. The list contains every compatible overlay found in the current study (see Requirements).
- LUT C — the color lookup table applied to the overlay. PET (a hot-metal palette tuned for functional data) is the default; since v4.8.0 a PET or SPECT overlay starts with PET SUV, the same ramp graduated in SUVbw, and an RT dose overlay with Isodose. The list gathers the maps meant for overlays and those tied to
PT,NMandRTDOSE; see Color Maps for the whole library and for the editor that creates your own since v4.8.0. The values are mapped to it through a series-wide display window, not through the window/level of the functional series. - Opacity D — two sliders control the blend at composite time:
result = baseOpacity·base·(1 − a) + overlay·a, whereais the overlay’s own transparency at that pixel — it grows with the value and reaches the Opacity setting within the lower tenth of the display window, so background stays invisible while everything above it is blended at the chosen opacity. Each slider is labeled with the modality of the layer it controls (e.g. CT for the base, PT for the overlay). Defaults are 100 % base and 50 % overlay. Setting the base to 0 % leaves the colorized overlay alone over a black background. - Scale maximum E since v4.8.0 — the top of the overlay’s display window, in its display unit (
SUVbwfor a PET with SUV). It moves the top only, the bottom stays fixed; dragging the color bar on the view does the same. See Adjust the scale maximum.
The overlay follows the base view as you scroll, zoom, window, or reslice it. Because the fusion is resampled from the functional volume, it stays correct on oblique, coronal and sagittal planes, not only the native acquisition plane.
The Reset action turns fusion off and returns to the plain base image, like any other display setting it restores.
Display window and color scale
Available since v4.7.2.
The overlay is mapped to its LUT through a window derived from the whole functional series, so a given uptake keeps the same color on every slice. Its initial value is measured from the data; its top can then be adjusted since v4.8.0:
| Overlay | Window |
|---|---|
| PET whose SUV factor can be computed | 0 SUVbw to the next whole SUVbw above the series maximum, the top kept between 4 and 30. Before 4.8.0 the scale started at 1 SUVbw. |
| Overlay with a DICOM pixel value unit | 0 to the series maximum, in that unit (BQML, CNTS, …). |
| Overlay without any unit | 0 to 100 % of the series maximum since v4.8.0. |
A color map anchored to physical values carries its own scale, and then it is the map that sets the window: PET SUV displays 0 to 10 SUVbw, and Isodose 0 to 120 % of the prescribed dose when the RT plan gives one. The measured window of the table above applies to every other map — the maps that are relative to the displayed window, and the anchored maps whose unit cannot be expressed for the series (no SUV factor, a different pixel value unit, an unknown prescription).
The upper bound is a high percentile of the active voxels rather than the plain maximum: physiologic excretion (bladder) and the injection site are several times hotter than any diagnostic structure, so they saturate at the top of the palette instead of pushing the rest of the study into its first tenth. The lower bound is 0: the scale is read from zero, with no background subtraction. The lowest activity is still kept off the anatomy, because the overlay only fades in over the first tenth of the window (see Opacity above).
The resulting scale is drawn as a color bar F on the fused view, beside the LUT bar of the base image when both are shown:
- The value at the top of the bar is the window maximum, the one at the bottom its minimum.
- The unit written above it is
SUVbwwhen SUV could be computed, the DICOM pixel value unit (BQML,CNTS, …) otherwise, or a percentage of the series maximum when the series carries no unit. OnlySUVbwvalues can be compared with another acquisition: the others depend on the injected dose, the patient weight and the uptake time. - The band the overlay paints as fully transparent is left blank, so the activity that is deliberately not shown reads as such instead of being mistaken for a color.
The bar is toggled with Fusion Color Scale in the Display panel and is enabled by default. It is not drawn on views shorter than 350 pixels, where it would overlap the corner annotations.
Adjust the scale maximum
Available since v4.8.0.
The top of the overlay scale can be changed while reading, independently of the window/level of the base image. The bottom stays where it is (0 SUVbw for a PET), so the scale is always read from zero:
- Drag the color bar — press the left mouse button on the color bar of the fused view and drag up to raise the maximum, down to lower it. The cursor turns into a vertical arrow over the bar. The drag works whatever action the left button carries elsewhere on the image, so the CT or MR keeps its usual window/level with the mouse while the PET is adjusted on the bar. The change is relative: each 128 pixels of drag doubles (or halves) the scale, so it feels the same on a 0–5 scale as on a 0–20 one.
- Scale maximum slider — in the Fusion section of the Image tool, for a precise value. It shows the maximum in the display unit and follows the drag.
The maximum can go up to the series maximum and, for a PET in SUVbw, to at least 30 SUVbw, so a fixed tracer scale can be kept on a quieter study.
For scales you use every day, create a color map with an absolute SUVbw range — for example FDG 0–5, DOTATOC 0–10, PSMA 0–20 — and mark it as a favorite. Selecting the map applies its scale; you can then still raise or lower the maximum for the study at hand. To return to a map’s scale after an adjustment, select another map and then that one again.
The adjustment applies to the selected view (to all three planes in the MPR viewer). When fusion is enabled, the window is first measured on a single slice and refined once the whole series is loaded; a maximum you set in the meantime is kept.
SUV statistics on a region of interest
When the overlay is a PET series carrying the required metadata, a closed measurement drawn on the fused base image also reports the overlay’s SUV values inside the region — Min, Max and Mean, in SUVbw, g/ml.
These rows are sampled from the original PET voxels (no resampling loss on the maximum) and are tagged with the overlay’s modality — for example Max (PT) — so they read alongside the base-image statistics.
SUV is computed with the body-weight method (SUVbw), following the vendor-neutral QIBA definition. The same values can be read directly on the PET series itself — see SUV measurements.
Fusion in the MPR viewer
The screenshot above shows a fusion in the MPR viewer, with the color bar on each plane. Opening the MPR viewer from a fused 2D view carries the fusion over: the three MPR planes start with the same overlay series, LUT, opacities and scale you set in the 2D view. The three planes are views of one volume, so they share one fusion setting: a change made from any plane — enabling or disabling fusion, another series or LUT, an opacity, the scale maximum — applies to all three at once, and the overlay reads the same on every plane. The MPR fusion is independent of the 2D view it came from: changing one does not affect the other.
Inheritance only applies when the overlay is still compatible with the reconstructed MPR base. If you open MPR from a non-fused view, the planes start with fusion off, and you can enable it from the same Fusion section as in the 2D viewer.
See also
- DICOM 2D Viewer — the base viewer hosting the Image tool.
- View Synchronization — displaying a PET and its CT side by side instead of overlaid.
- Draw & Measure — SUV and other measurements.
- MPR Viewer — multiplanar reconstruction.