Logo of the Physikalisch-Technische Bundesanstalt

Determination of conversion factors for dose-relevant measuring quantities in digital volume tomography


Over the past few years, digital volume tomography (DVT) has increasingly gained in importance in various fields, such as interventional radiology and post‑operative diagnostics. Unfortunately, it is difficult to compare the radiation exposure of DVT with that of computer tomography (CT) as their parameters – the dose area product (DAP) for DVT and the dose length product (DLP) for CT – are based on different concepts. For that reason, a joint research project (initiated by the Federal Office for Radiation Protection – BfS) of PTB and the Städtisches Klinikum Braunschweig was launched in July 2019. This project is aimed at allowing the dosimetry parameters for DVT and for CT to be converted in such a way that they can be compared for examinations which can be carried out by both methods.

Three‑dimensional imaging is playing an increasingly important role in diagnostic and therapeutic patient care. Whereas in the past, operations were planned on the basis of projection images, three‑dimensional data sets are now used which make it possible to plan operations more precisely and in such a way that patients have fewer side effects. However, the way in which interventions and operations are carried out has also undergone changes. More and more often, post‑operative checks of interventions are no longer carried out by means of a computer tomography (CT) scanner after the actual operation has taken place. They are instead already performed in the operating room by means of a digital volume tomography (DVT) system. In other cases, DVT images are taken prior to an intervention (instead of afterwards, as in the case of CT images), for example, within the scope of stroke treatment in order to reduce the time until the intervention will start.

The current development has led to the fact that examinations for the same – or at least a similar – clinical purpose are carried out using both conventional CT systems and DVT systems. This has created a challenge with regard to dosimetry. In the case of conventional CT, radiation exposure is measured by the volume computed tomography dose index (CTDIvol) and the dose length product (DLP). In DVT systems, however, the dose area product (DAP) is used as the parameter.

The DAP is a plausible parameter for DVT. The reason for this is that DVT devices are also used to carry out screening examinations, for which the DAP is the usual measuring quantity in Germany. The same applies to the CTDIvol developed for CT as well as to the DLP. However, the parameters differ from each other as they are based on different concepts, and it is thus not possible to convert one parameter trivially into the other. The CT parameters measure the dose in a phantom. The dose can then be converted into a patient dose by using conversion factors. A lateral overexposure of the phantom (or of the patient) therefore has no direct influence on the measuring value. The DAP on the other hand is calculated from the incident dose, multiplied by the size of the exposed field. A lateral overexposure of the patient thus has an influence on the measuring quantity (although it is insignificant for the amount of patient exposure). It is therefore not possible to compare the radiation exposure of similar examinations with DVT and CT without difficulty.

One way dose values can be compared is with the aid of diagnostic reference levels (DRLs). These are published no later than every third year by the Federal Office for Radiation Protection – BfS. According to section 1, para. 4 of the Radiation Protection Ordinance, they are defined as “dose limits when using ionizing radiation on humans for typical examinations, related to standard phantoms or patient groups for individual device categories”. They are calculated from data which are given to the BfS by the medical bodies via competent authorities on an annual basis. DRLs do not serve as limit values but as a means of quality assurance in order to be able to detect and prevent values which exceed a mean dose unjustifiably over a longer period of time.

Within the scope of a research project commissioned by the BfS, PTB’s Working Group 6.25 – in collaboration with the Städtisches Klinikum Braunschweig – is investigating possibilities of converting one dose‑relevant measuring quantity into the other. For this purpose, examinations are being studied for which applications exist with both CT and DVT systems and for which the DRLs are published by the BfS. The aim is to include these conversion factors when the DRLs are published.

The project started in July 2019. First, the current state of research was determined by means of a review of the respective literature, and the concepts for determining the conversion factors were worked out in detail. After that, measurements on ten different DVT systems and two CT systems are planned. First, the parameters will be measured: the DAP on the digital volume tomograph; and the DLP and CTDIvol on the CT scanner. The parameters will then be compared with the dose that is measured on different parts of the body. This dose will then be determined by means of two anthropomorphic phantoms which are equipped with dose sensors which are very small and thus measure precisely to the point. The conversion factors can then be determined by comparing the parameters via the effective doses or the doses to which an organ of a human body is exposed.

Parallel to that, calculations are performed by means of ImpactMC simulation software. For validation, these calculations can be compared to the measurements. The simulations then offer the option of investigating in how far the conversion factors vary if different variables change, for example, as a function of the tube voltage, of different filtrations or of collimations.


Opens local program for sending emailS. Ketelhut, Department 6.2, Working Group 6.25

Opens local program for sending emailLudwig Büermann, Department 6.2, Working Group 6.25


Head of Press and Information Office

Dr. Dr. Jens Simon

Phone: +49 531 592-3005


Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig