Visage 7 Clinical Training Brain Perfusion

Contents

Overview ...... 3 Perfusion Details ...... 4 Loading Data / Perfusion Protocol ...... 5 Calculate Temporal MIP ...... 7 Define Artery and Vein ...... 9 Calculate Perfusion ...... 10 Define Centerline & Compare ROIs ...... 12 Save Perfusion Results ...... 14 MR Perfusion ...... 15 Advanced: Perfusion Tool Card ...... 16 Value Range ...... 17 Images before Contrast Application ...... 18 Hematocrit ratio ...... 19 Vessel Radius ...... 19

V02.00 – Jul 2018 2 of 20 Overview

Overview

Load Study

Visage 7 supports many different Study loads into a Perfusion workflows. This document hanging protocol. describes calculating perfusion results for CT and MR brain perfusion studies.

The workflow consists of the following steps:

 Loading data Calculate tMIP  Calculating tMIP  Defining artery and vein With one click, the registration  Calculating perfusion results and temporal MIP are calculated. Optionally, you can also do the following:

 Define center line  Define ROI for mirrored comparisons Define Artery and Vein

Define an artery and vein. Uptake for each position is shown to confirm good placement.

Calculate Perfusion Results

Working with screenshots, tables, and report addendums in the Export window.

Overview 3 of 20 V02.00 – Jul 2018

Perfusion Details

• Implements linear system model using deconvolution approach

– Uses all phases of the perfusion data

– Very fast computation (few seconds)

– Robust even for variation of bolus volume, injection rate, cardiac output, etc.

• Calculates parametric maps for regional blood volume, regional blood flow, mean transit time, Tmax, and time to peak

• Requires an arterial region to be set by the user

• Reduce partial volume effects by optional use of venous region to scale arterial time density curve

• Air and bone pixels are excluded from computation

• Corrects for differences between tissue to artery hematocrit ratio

V02.00 – Jul 2018 4 of 20 Perfusion Details

Loading Data / Perfusion Protocol

The perfusion data normally consists of multiple volumes within one series of a study. Each volume holds one phase of acquisition. After loading a study with CT or MR brain perfusion data, it should match a Visage Brain Perfusion hanging protocol. This protocol has additional layouts for displaying the time series and the perfusion results, as well as additional toolbars related to perfusion.

Temporal MIP

Define Artery

Define Vein

Calculate Perfusion

Define Centerline

Compare ROI

Toggle Centerline

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All important functions for perfusion imaging are included in the Perfusion toolbar. For a complete analysis, just follow the buttons from left to right. The functions provided are as follows:

• Temporal MIP (tMIP) – Instead of showing one phase at a time, show a Maximum Intensity Projection (MIP) over the time domain.

• Define Artery – This function lets you define a region for the artery function, e.g. the change of intensity in this region is taken as the arterial input.

• Define Vein – Similar to “Define Artery”, with this function you can define the venous function. This is not available for MR perfusion.

• Calculate Perfusion – Once artery and vein are defined, you can start the calculation with this button. This will compute the result volumes.

• Define Centerline – In order to compare the two brain hemispheres, you can use this tool to define a symmetry line along the fissura longitudinalis.

• Compare ROI – Use this tool to define regions of interest (ROI) that you want to compare. The ROI is automatically mirrored along the centerline.

• Toggle Centerline – Use this tool to toggle the centerline on and off.

V02.00 – Jul 2018 6 of 20 Loading Data / Perfusion Protocol

Calculate Temporal MIP

In the axial viewer you can see the original images of the acquisition. Use the previous / next phase buttons in the dataset navigation toolbar or the left and right cursor keys to navigate between the phases. When you hit “c”, you can turn on the cine mode which will cycle through all available phases in a cine loop.

(Depending on your keyboard shortcuts, it’s possible that the cine tool is not mapped to “c”.)

The temporal MIP button in the Perfusion toolbar takes all phases into account and shows the maximum intensity for each voxel over all phases. In CT perfusion, this makes it easier to distinguish contrast filled arteries as they are only filled during some phases.

At this point the phases will be registered to each other. The registration is defined in the perfusion preset and can be viewed or changed in the Perfusion tool card.

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Depending on patient movement, you can perform the tMIP and perfusion calculations with

 No registration  2D registration  3D registration

These values are defined in presets. Visage support personnel can configure presets that best match your acquisition protocols.

The resultant tMIP is shown on the left monitor.

V02.00 – Jul 2018 8 of 20 Calculate Temporal MIP

Define Artery and Vein

In order to compute the perfusion, a region within an artery has to be defined. The change of intensity in this region is then taken as the arterial function for the computation of the perfusion in the surrounding tissue. To define the arterial function, click on the define artery button and select a point. A graph showing the change of intensity over all phases in this region is then shown. You can adjust the point by grabbing it with the mouse and dragging it to a different location. The graph will update as soon as the mouse button is released.

Defined Artery

Verify that the arterial function has one articulate peak

Defined Vein

For CT perfusion, the definition of a venous function is needed in order to be able to compute absolute values for blood volume and blood flow. If no venous function is defined, it is still possible to compute all result volumes, but the scale of blood volume and blood flow will be undefined.

The definition of the venous function is similar to the definition of the arterial function. Click on the define vein button and select a vein in the axial image. The resulting function is then displayed in the same graph as the arterial function.

It can be helpful to adjust the window level to a wider window so that you can more easily distinguish venous structures from bone. Depending on the vein you pick, the resulting function is usually larger than the arterial function.

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Calculate Perfusion

After both vessel functions have been defined, you can click on the “Calculate Perfusion” button. This will trigger the perfusion calculation on the server. Processing time is dependent upon the size of the series or acquisition. After the calculation, the four viewers are filled with the color-coded results:

• Mean Transit Time (MTT) – For each tissue voxel, the mean time that the contrast agent needs to transit this point is calculated. A higher value indicates slower blood transport. The unit measured is seconds.

• Time to Peak (TTP) – For each tissue voxel, the time that the contrast agent needs to reach its peak is calculated. Higher values indicate delayed blood transport. The unit measured is seconds.

• Blood Volume (BV) – For each tissue voxel, the amount (volume) of blood that is perfused through the tissue is calculated. Higher values indicate a stronger blood supply in this region. The unit measured is ml / 100g.

• Blood Flow (BF) – For each tissue voxel, the amount of blood per time unit that is transported through the tissue is calculated. Higher values indicate a better perfusion of the tissue. The unit measured is ml / min / 100g.

If no venous function is defined, the units for blood volume and blood flow are undefined, the display changes to rBV (relative Blood Volume) and rBF (relative Blood Flow), respectively.

V02.00 – Jul 2018 10 of 20 Calculate Perfusion

The window level for each result viewer is automatically set. You can adjust the window level automatically for each viewer individually. Press the left and right mouse button simultaneously and adjust the window level. While holding down the mouse buttons, you will see the histogram of the values and the range of the current window level in the lower left corner. In order to get back to the original window level, select the default from the viewer control or press 1.

Adjust Window/Level with both mouse buttons. Each viewer can be adjusted individually.

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Define Centerline & Compare ROIs

In order to use the Visage ROI comparison tools, you have to define the line of the fissura longitudinalis. When you define ROIs in a later step, they will be mirrored along this line, ensuring that the size and position of the ROIs are the same.

When you click on the define symmetry line button, a line will be drawn in the middle of the current viewer. You can use any of the axial viewers for the definition. The position and angle of the line can be changed by dragging the endpoint of the line to a new position or by shifting the complete line.

Define Symmetry Line Define Elliptic ROI Regions of Interest (ROI) are mirrored along the Symmetry Line

Once a symmetry line is defined, you can draw a ROI around the region that you want to compare. There are two ways to initially define a ROI:

• Elliptic ROI – With this tool, you start with an elliptic region

• Freehand ROI – Using the freehand tool, you can draw any shape as basis for the ROI

If you draw a contour that crosses the defined symmetry line, it will not be mirrored.

V02.00 – Jul 2018 12 of 20 Define Centerline & Compare ROIs

Both elliptic and freehand contours can be changed after the initial definition using the nudge tool. The size of the nudge tool depends on the distance to the contour line when the mouse button is pressed. If the mouse is far away from the contour, the diameter of the nudge tool will be larger. You can use this refinement from the outside in order to shrink the contour or from the inside in order to make the contour bigger. Editing can be undone with the undo function from the context menu that appears in the lower right corner of the active viewer.

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Save Perfusion Results

You can save various perfusion results with the “Save Perfusion Results” tool.

Here you can specify which series to save, including the original maps as DICOM and/or as RGB (for downstream non-Visage systems), the artery and vein placement as a screenshot and as a DICOM presentation state, and the result maps in RGB stored as one tiled image.

This is useful for workflows where a technologist does the initial perfusion calculation. The reading radiologist still has the ability adjust the window/level, or to move the artery or vein and recalculate the perfusion results.

The export will use the slice thickness of the viewers, so if you would prefer thick slices for export, you can simply make the slices thick in the viewers.

V02.00 – Jul 2018 14 of 20 Save Perfusion Results

MR Perfusion

When working with MR Perfusion data, the workflow is in general the same as when working with CT Perfusion data. A few differences apply:

• We consider only T2 Perfusion measurements where the contrast agent in MR Perfusion data causes a lower signal, resulting in darker voxels when the contrast agent is present. The software currently does not support T1 weighted perfusion measurements.

• The resulting arterial function can be observed as a sharp, negative peak.

• The input for the vessel function is exactly one image voxel and not a region as in CT perfusion.

• Because of the small image matrix (usually 128x128 pixel), the bilinear filter can smooth the image too much so that the artery is not clearly visible. In order to switch the filtering off, right-click into the viewer, select Properties from the context menu and de-select the entry “Bilinear Filtering”. This will result in a coarser image where each pixel is represented as a rectangle.

• The tMIP function is only of limited help in defining the arterial function. It is easier to use the cine display to define the artery.

• No venous function needs to be defined.

• Blood flow and blood volume are relative.

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Advanced: Perfusion Tool Card

More advanced parameters for Perfusion calculation can be adjusted using the Brain Perfusion tool card. You can change the values either by using the sliders or by directly typing in the value in the designated field.

Each time a value has been changed, you have to click on “Calculate Perfusion” in order to trigger the calculation that applies the new values to the result images. There are two defaults settings for all values, one for adults and one for small children, that can be set with the respective default button.

You can also perform either a 2D or 3D registration here if there is patient movement in the time series.

Advanced Feature Settings

Registration

Re-calculate Perfusion

V02.00 – Jul 2018 16 of 20 Advanced: Perfusion Tool Card

Value Range

The value range determines which voxels are considered for the perfusion calculation. Voxels whose value does not lie within the value range will result in black areas in the result image. If you experience black areas inside the brain tissue where you would expect non-zero values, try to include a bigger value range for the computation and trigger the calculation again.

As default, for MR data the range is set between 10% and 100% of the maximal value, for CT data it is fixed between 0 and 120 HU.

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Images before Contrast Application

In order to calculate the perfusion, first a baseline without contrast agent has to be established. For this, by default the first six phases are taken into account. You can adjust the number of phases with the “Images before contrast application” slider. This can be necessary if the contrast uptake start very early so that you have to choose fewer phases for the baseline or if there is an imaging artifact during the baseline phases.

Note that the baseline always starts with the first phase and that the unit in the vessel graph is seconds, not the number of phases. In order to get the number of phases, you have to count the dots in the graph which mark the measured value for one phase.

When to adjust: • Early start of contrast inflow • Presence of an imaging artifact

V02.00 – Jul 2018 18 of 20 Advanced: Perfusion Tool Card

Hematocrit ratio

The cerebral hematocrit value affects the Mean Transit Time (MTT) that is calculated by the perfusion measurements. There is also a difference depending on the size of the vessel, since the smaller vessels show a smaller hematocrit value.

These settings should only be adjusted using the defaults for adults and small children, respectively. Vessel Radius

For CT images, the arterial and venous functions are not taken from the intensity values of one pixel, but from the intensity values of a region around the defined seed point. With the two sliders “Artery radius” and “Vein Radius”, you can adjust the size of this region. This can be useful if you have extraordinary large or small vessel in the image data.

Another factor that can be adjusted is the Density factor. This is the factor that is used to convert tissue volume into tissue mass. This factor should normally not be changed.

When to adjust: • If the vessels are unusually large or small • Research purposes

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