Cone Beam CT Solutions

Along with the development of Computed Tomography (CT), new procedures and tools for testing must be developed.

For the past ten years, the main topic in this area has been the testing on Cone Beam CT (CBCT). With the wider beam, the standard CTDI100 formalism is no longer sufficient.

The IEC standard IEC 60601-2-44, Ed 3 Amendment 1 is recommending new procedures for CBCT testing. The IAEA has produced a free, downloadable report that guides you through the standard: http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1528_web.pdf

A simplified definition of CBCT is that the beam width is larger than 40 mm. If the beam is 40 mm or less, the standard CTDI100 formalism can be employed and measured using a CT Dose Profiler with helical scans or a Pencil Ion Chamber with axial scans.

With a beam width (NxT) larger than 40 mm, the IAEC propose you use this formula to get the CTDI(w,NxT):

where (ref) is a reference beam width. This should be 20 mm (if that option is possible for the CT). If you want to measure a 160 mm beam, and you can use a 20 mm beam, the equation will look like this:

The CTDI(w,20) should be measured in the standard way, using the CT Dose Profiler or a 100 mm Pencil Ion Chamber in standard CTDI Phantoms.

To get the CTDI(free-in-air,20) is also simple; place your detector in the isocentre, without any Phantom, and either measure the CTDI100 with the CT Dose Profiler or the Pencil Ion Chamber.

It becomes complicated when you should measure the .

It is not possible to measure the ∞ length, but the recommendation is that you should at least measure a distance that is NxT mm + 40 mm. For these situations there are three possibilities:

1. Using an RTI CT Dose Profiler and LoniMover™

Place the RTI LoniMover™ on the table and make sure the CT Dose Profiler is in the isocentre.

Enter the scan parameters you want to use into the software. When you push the start button, the measurements and movements will be performed automatically.

Now you can easily scan the CT Dose Profiler through the beam of any length up to 300 mm (see picture 1A below).

A result will appear within a few seconds, and you will get the full CTDI(free-in-air,NxT) as well as a visible view of the dose profile (see picture 2).

Picture 2. 160 mm dose profile measured with CT Dose Profiler and LoniMover™. The dose between the two red dotted lines is the CTDI100.

2. Using a 300 mm Pencil Ion Chamber

Place a 300 mm Pencil Ion Chamber (free-in-air) in the isocentre of the CT and the middle of the Pencil Ion Chamber in the centre of the CT.

Now you can do any axial scan with beam widths up to 260 mm, for example 160 mm, and directly measure CTDI(free-in-air,160) (after dividing with 16=NxT, if the Ion Chamber is calibrated in mGycm).

3. Using a 100mm Pencil Ion Chamber and LoniMover™

A CBCT cannot be measured with one scan over the 100 mm Pencil Ion Chamber. Depending on the beam width, you must try to position the “Chamber” so that it will “catch” the whole dose profile in two or three exposures by stepping it through the beam for each exposure (see picture 3).

Remember, you must have an active length of NxT + 40 mm; so for a 160 mm beam, there is no real margin to do the measurement in two scans.

(If the Chamber is calibrated in mGycm, the sum of the measurements should be divided with 16=NxT to get the CTDI(free-in-air,160)).

Picture 3. A Pencil Ion Chamber in two or three steps to measure the CTDI(free-in-air,160)

It is difficult to move the Pencil Ion Chamber by hand in exact steps of 100 mm. The latter measurement is very important to get good, reproducible values. However, it is very easy to do it with the help from the LoniMover™ (see picture 1B), which can “step” the detector in exact steps.

The LoniMover™ can be used with any 100 mm Pencil Ion Chamber. It does not have to be from RTI.

No matter what you want to measure in X-ray, from the easiest to the most advanced, RTI has the solutions for you!

Björn Cederquist, RTI Pte. Ltd. Managing Director in Singapore

Björn is a Singapore-based Swedish medical physicist. In 2008, he started his career at RTI in the company’s R&D department where he, among other things, developed the CT Dose Profiler and T20 Dose Probe. In the middle of 2009, Björn started to help the Sales Team, and after some months he started to work with sales full time. His sales and technical support areas today include the whole of South East Asia as well as Australia, Japan, South Korea and New Zealand.

Published in EFOMP News, Issue 02/2019/Summer, page 31-32