CT Imaging for Treatment Planning

Control the mechanical parameters of CT table:

1. Measurement of the CT tabletop’s inclination with digital spirit level.
2. Controlling the horizontal movement with laser. (Figure1.)

Figure 1: Controlling the horizontal CT table movement with laser.

When planning on CT base the determination of the CT numbers is important since for the inhomogeneity correction of the 3D TPS’s we need to know the electron density of the different tissues; so we need the CT number – relative electron density calibration curve.
The HU can be estimated using the following formula:

 
Where: and - are the attenuation coefficient of the material for the specific voxel and the water, respectively. When on the above equation one replaces the µm =µwater, then will receive a HUwater = 0. In addition the air as material corresponds to –1000HU, since µair =0. The Hounsfield units have no upper limit but usually for medical scanners a range between –1024 to +3071 is provided.
The CT number measurements can be performed with different CT phantoms like (Figure 2.): MINI CT QC Phantom, 76-430 (Inovision Company), RMI 467 CT electron density phantom (Gamex), CIRS 062 type electron density refer¬ence phantom and CIRS Thorax IMRT phantom. During the measurements, we could evaluate the calibration of the CT.

• MINI CT QC FANTOM, 76-430 modell, Inovision Company
• RMI 467 CT elektronsűrűség fantom, Gamex
• CIRS elektronsűrűség referencia fantom – 062 modell
• CIRS Thorax IMRT fantom 74-007 modell

"Figure 2.: a. MINI CT QC Phantom, b., RMI 467 CT Electron Density Phantom, c., CIRS Electron Density Phantomd., CIRS Thorax IMRT Phantom

 
The phantom has a body made of plastic water with holes to hold interchangeable rod inserts.
For each selected inhomogeneity, water and air, the CT numbers should be averaged over a fixed area (the diameter of averaged region of interest should be close to 0.5 radius of the insert). The region of interest for which the CT numbers are averaged should not be close to the edge of the selected area. The averaged values should be compared to the CT numbers used in the CT numbers conversion curve stored in the RTPS. Agreement within 0.02 is acceptable for conversion curves, i.e. CT numbers for a given object should not vary by more than +/- 20 CT numbers. If a significant change to CT numbers is observed and cannot be eliminated by recalibration of the CT scanner, new CT numbers to conversion curves data need to be entered into the RTPS. CT images of CT phantoms are shown on Figure 3.

Figure 3.: The CT images of CT phantoms

In Figure 4. we can see the relationship between the relative electron density and CT number of material in the RMI 467 Electron Density CT Phantom in two different tube voltage (110kVp and 130kVp)

Figure 4.:The relationship between the relative electron density and CT numbers of material in the RMI 467 Electron Density CT Phantom with two different tube voltages.

 
Some CT phantoms are good for controlling of the geometric distortion of the CT images as well. We can derive the distortion of the CT images by measuring the given distances on the phantom (Figure7.).

Figure 5.: Determination of CT image distortion with a. CIRS Thorax IMRT Phantom, b. RMI 467 Electron Density CT Phantom

 
For the safe work is essential the QA/QC protocol, which has to include:
- the quality control of positioning leaser set of CT
- the workflow of CT simulation
- the constant tube voltage applied during the CT image acquisition
- patient set-up (head/foot, supine/prone patient positioning)
- patient fixation and documentation of process
- CT data set of the region to be treated, with a suitable slice spacing (typically 0.5 cm for the thorax, and the pelvis and 0.3 cm for the head and neck), is required.