With .NET API you can read 64-bit values like this:
$key = [Microsoft.Win32.RegistryKey]::OpenBaseKey([Microsoft.Win32.RegistryHive]::LocalMachine, [Microsoft.Win32.RegistryView]::Registry64)
$subKey = $key.OpenSubKey("SOFTWARE\Microsoft\.NETFramework")
$root = $subKey.GetValue("InstallRoot")