Yes. You did not specify if you are using MacOS, Windows or Linux. You can use the "Mouse Gestures" item of the "Help" menu to see the required modifier key. On macOS, this is "Option-Drag: Measuring tool".

When you use the measuring tool, the start point, end point and length are reported in the title bar. The graph on the bottom samples along the line, revealing the relative brightness. For example, in the attached screenshot the white matter has an intensity around 125, while the ventricles dip to about 30.