Can ultra-low-dose CT replace x-ray in the diagnosis of sacroiliitis?

The rising availability of medical imaging means increased radiation exposure for some populations. Research in the field of radiation dose regulation is necessary, especially for CT, which is a common part of diagnostic algorithms for most diseases. The use of radiation must be in line with the ‘as low as reasonably achievable’ (ALARA) principle. There are many options to reduce dose and get images with acceptable quality. Restrictions on the extent of exposure, adaptation of the scanning parameters manually or automatically, and post-processing image adjustments are commonly used.

Tin filtration, which dramatically reduces the radiation dose, has recently been introduced. The principle is to filter low-energy photons between the x-ray generator and the patient’s body. Low-energy photons are responsible for image contrast and noise reduction. The tin filter significantly reduces the dose, but at the cost of increased noise and reduced image contrast. Therefore, the indications for the use of tin filtration are limited to distinguishing structures with significantly different density, such as bone and soft tissue; soft tissue and air; or bone and air.

One possible indication of tin filtrated ultra-low-dose CT (TFULDCT) is the examination of sacroiliac joints (SIJ) in patients with suspected sacroiliitis. Assessment of bone changes is very important in patients suffering from inflammatory rheumatoid disease. In the subacute stage of the disease, there are minor deteriorations, joint irregularities and small bone bridges. At this early stage, x-ray findings are often negative or uncertain, while on CT, which is highly sensitive to bone changes, they are easily visible.

Tin filtrated ultra-low-dose computed tomography, minor bone changes typical for sacroiliitis – joint irregularities, minor deteriorations and reactive sclerotisation around the joint surfaces (provided by Eva Korčáková, University Hospital Pilsen).

MRI is considered to be the most sensitive method for the diagnosis of sacroiliitis. But in the subacute stage, when bone oedema is no longer present and bone changes are not yet advanced, MRI may not be sensitive enough and may cause false negative results. The reason why CT has not yet been widely used in this indication is the high radiation dose. TFULDCT provides a convenient alternative where CT sensitivity is maintained but the dose decreases below the level of x-ray exposure.

Our pilot study evaluated the effective radiation doses that patients received from TFULDCT and from x-ray of SIJ. In our cohort, the TFULDCT doses were about half that of the x-ray doses (TFLDCT median 0.11mSv (0.06 – 0.40mSv), x-ray 0.25mSv (0.08 – 1.16 mSv)). We also examined the diagnostic usefulness of the method. Three evaluators independently evaluated the TFULDCT and x-ray images and decided whether the findings are clearly positive, clearly negative or uncertain for the diagnosis of sacroiliitis. The overall diagnostic benefit (ability to decide on the presence or absence of the disease correctly and with certainty) of TFULDCT was 80%, as opposed to only 30% for x-ray. Our results confirm that TFULDCT is able to more accurately decide the presence or absence of inflammatory rheumatoid disease of SI joints compared to x-ray.

TFULDCT is a very promising method in the diagnosis of rheumatoid diseases of SIJ. It is significantly more accurate than x-ray and is associated with lower radiation dose. The only limitation of TFULDCT in this indication is that it is an innovation and it is not yet widespread.

Dr. Eva Korčáková, PhD, works as a radiologist at the Department of Imaging Methods, University Hospital in Pilsen, Czech Republic. She is also a researcher at the Biomedical Centre of Charles University Faculty of Medicine in Pilsen, and does research in radiology, surgery and oncology.

Research Presentation Session
RPS 1410a Spine and inflammatory disorders

How tin filtration affects the value of an effective radiation dose in CT of the sacroiliac joints: can CT replace x-ray in patients with suspected sacroiliitis
E. Korčáková, J. Štěpánková, D. Suchy, P. Hosek, K. Bajcurova, J. Pernicky, H. Mirka; Plzen/CZ

Read the full abstract in the ECR 2020 Book of Abstracts
Korčáková E, et al. (2020) How tin filtration affects the value of an effective radiation dose in CT of the sacroiliac joints: can CT replace x-ray in patients with suspected sacroiliitis? Abstract RPS 1410a-4 in: ECR 2020 Book of Abstracts. Insights Imaging 11, 34 (2020). DOI 10.1186/s13244-020-00851-0