CT Ion Chamber design for high precision and low uncertainty

A range of parameters have an influence on the uncertainty in the measurement on axial scans. Calibration uncertainty, dosimeter sensitivity, ion chamber energy dependence and effective length of the ion chamber are all parameters that have to do with the measuring equipment. Other influence factors are measuring geometry and CT machine behaviour.

For wide beams, it is advised to measure free-in-air in the isocenter in two to three positions, along the z-axis, using a 10 cm pencil-type ion chamber. The purpose of this method is to achieve the dose integral over the entire beam width, see figure 2.

It has been shown that the effective length of different chamber models may vary. This variation will have a direct influence on the uncertainty on further calculations of CTDI and DLP.

Various pencil type of ion chambers, currently available from different manufacturers, show variation in effective length from 97 mm to 117 mm – with an estimated uncertainty of ±1 mm. The difference in length in comparison to how the chamber is positioned will directly influence the calculation of the dose integral.

Figure 3 illustrates the effect for a chamber with an effective length of 110 mm when measuring in an 80 mm wide beam.

The result of the over-lapping of 10 mm, renders a dose overestimate of 10/80 mm = 12.5%.

The new 10 cm RTI CT Ion chamber is designed with this effect in mind with high precision of effective length close to 10 cm.

The precision in the positioning of the 10 cm chamber will of course also have influence on the uncertainty, but may also take more or less time.

To increase efficiency and reduce uncertainty, a 30 cm ion chamber can be used for the free-in-air measurement. Alternatively, a mover, like the LoniMover by Lonitech, can be used for high precision in positioning. The LoniMover has a precision of fewer than 0.1 mm and is controlled via USB or Bluetooth from a laptop.

The energy response of the ion chamber is another factor for uncertainties. For the free-in-air measurements, most types of chambers have quite a good energy response. However, when it comes to high attenuated beam qualities (high HVL) and soft radiation from scatter in a CTDI phantom the energy, linearity will play a more significant role.

To avoid having to play with energy corrections, the CT ion chamber should have a flat energy dependence for soft radiation (low HVL), as well as high attenuated radiation (high HVL). The graph below shows the extremely flat energy response of the 10 cm RTI CT Ion Chamber.

The new RTI CT Ion chamber has an outstanding energy linearity, within 0.5% in the range 70 – 150 kV for the IEC 61267 radiation qualities RQR 5 to 10, RQA 5 to 10, and RQT 8 to 10, and ISO N-150. With an effective length that is very precise (100 ±0.5 mm), dosimetry at wide beams when the chamber needs repositioning, becomes very accurate. With the optional LoniMover the positioning becomes even more accurate and efficient.

The rubber O-rings make the positioning in the CTDI phantom safe and precise. The length of the chamber and the flat ends match the edges of a 150 mm wide CTDI phantom for a perfect positioning, and eliminates the need of adapter to match the diameter of the holes.

 

Sören Sturesson, M.Sc. Medical Physics. Product Specialist and Technical Manager Calibrations at RTI Group, Mölndal, Sweden

Sören is a Sweden-based medical physicist, with 25 years of experience within medical X-ray. Since 1995, he has been involved in development of X-ray detector technology, and applications for X-ray quality control. With the long experience of dosimetry in medical X-ray, Sören plays an important role at the ISO 17025 accredited calibration laboratory in Mölndal. He has also played an influential role in the design and the physical characteristics of the new RTI CT Ion Chamber.

Published in EFOMP News, Issue 01/2020/Spring, page 37-38