5. Measurements with the Piranha System
CT
Piranha & QABrowser Reference Manual
2014-06/5.5C
134
CT5.7
5.7.1 CT kVp
To measure kVp on a CT is many times difficult since with most meters it is required to
stop the tube in the top position at the same time as the table is not moving. This can
normally not be obtained using an available standard clinical program. Instead a service
mode must be used. Another problem is to "find" the beam, especially when using a
small slice width. All these problems are minimized when using the Piranha since it can
"move with the table" through the beam while the tube is in the top position. This is can
easily be obtained by measuring while a topogram (scout/pilot image) is taken. A
topogram is obtained with a moving table and a stationary tube, normally in the top
position. The topogram is normally used to provide information for the actual CT scan. It
is recommended to use a slice width of 3 mm or wider. That is, if selectable use as large
slice width as possible.
You may also want to use the Timed mode to allow measurements on moving CT
machines, see section Update Modes .
To measure CT kVp:
1. Set up the Piranha and the handheld computer according to the description in
Setting Up the Piranha .
2. Place the detector on the patient bed in a
region that is irradiated during the topogram
process. Place the Piranha in the direction
indicated by the figure below. That is, the
Piranha detector surface rectangle should be
placed perpendicular to the scanning
direction.
You may use the lasers to align the Piranha
correctly.
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