While the clinical utility and use of CT in this situation is a bit controversial, it is good to understand that excellent diagnostic CT information can be obtained in the very low radiation dose sub-millisievert range. Particularly in times of poor PCR test availability or delayed PCR results, very low dose Chest CT is quick and may have a valuable clinical function. [See this article in Radiology: Cardiothoracic Imaging or Aunt Minnie]
This article clearly shows that taking responsibility for community education and publishing optimized protocols has considerable impact on CT patient dose – and without compromising the diagnostic power of CT. Of course, the best people to execute that responsibility are radiologists – we know the facts and we are motivated toward quality and safety in ways no other group is. Optimization, education, and motivation are the functions of good consultants.
In a recent article published online (1), the authors investigated CT utilization and cumulative radiation dose in adult stone patients over a period of 3 years. In their analysis, patients were classified as “active” (≥ 2 diagnosis codes for nephrolithiasis, or receipt of stone surgery) or “inactive” (one stone diagnosis) and compared to age- and gender-matched controls. The authors concluded that CT use and non-surgical radiation exposure for active stone patients is significant, with over 10% estimated to exceed occupational limits (50 mSv annually) in the first year. They further mentioned that for active stone patients, mean 3-year estimated cumulative CT-related radiation was 28.3 ± 28.5 mSv for operative patients and 22.0 ± 24.4 mSv for non-operative patients. As has been previously mentioned in a blog I wrote in 2016 (2), there is support for tracking cumulative dose (3) as well as thought that cumulative dose should not be given any importance when making decisions about individual patients (4, 5). The linear no-threshold relationship implies that irrespective of which CT scan a patient is receiving, the absolute risk is the same. There is no increase in sensitivity from the increasing dose received from repeated CT scans, only an accumulation of probability. The linear no-threshold model would break down and not make any sense if there was an increase in sensitivity from repeated scans. Low dose techniques should be used for repeated CT scans to minimize dose to the patients. Educating our colleagues about the benefits of CT as well as its utilization and use of low dose KUB scans for repeat stone assessment would reduce dose to these patients, but using cumulative dose as a reason for not using CT is not appropriate.
Jessica C. Dai, Helena C. Chang , Sarah K. Holt , Jonathan D. Harper , National trends in CT utilization and estimated CT-related radiation exposure in the evaluation and follow-up of stone patients, Urology (2019), doi: https://doi.org/10.1016/j.urology.2019.07.030.
Researchers at Virginia Mason Medical Center in Seattle launched a hospital-wide quality improvement project, resulting in lowering CT radiation dose by nearly 20%.
In this article, they detailed three initiatives which enabled them to achieve this dose reduction:
“(1) More frequent use of existing low-dose protocols; (2) Development of new protocols specific to patient body size; and (3) Improved patient positioning.”
The combined efforts of radiologists, residents, and CT technologists at this institution “to promote the use of low-dose protocols and improve patient positioning led to statistically significant reductions in radiation dose for all types of CT exams they assessed. On average, the reduction in radiation dose was 18.3% for contrast-enhanced chest, abdomen, and pelvis CT scans and 11.8% for CT pulmonary angiograms, as reported in this article on AuntMinnie.
This article discusses how researchers assessed the use of low tube potentials for CCTA in worldwide clinical practice and the resulting influence on radiation exposure and image quality.
As reported by AuntMinnie staff writer Abraham Kim, “CCTA exams performed using low tube voltages (either 90 to 100 kVp or ≤ 80 kVp) were associated with reductions exceeding 50% for CT dose index (CTDIvol) and dose-length product, compared with the conventional tube-voltage range of 110 kVp to 120 kVp. These reductions led to statistically significant decreases in median radiation dose and volume of contrast agent required.”
Our experience at UW Medicine Radiology mirrors that of the authors in this article. DECT image quality is very much better with the current reconstruction software. It now rivals SECT in image quality and is the same in radiation dose. But tissue characterization is better and iodine contrast is much brighter – you may need much less injected contrast (up to 70% less).
This article highlights the wide variation in CT patient radiation dose between similar institutions for similar exams. Recent analysis of ACR dose registry data also suggests there is wide variation amongst different regions of the country.
Such variations suggest that attention to the details of CT technique and technology can produce CT exams at much lower dose – presumably without compromising diagnostic power.
Study concludes that ultralow-dose CT may substitute for standard-dose CT in some COPD patients
There are at least three different generations of iterative reconstruction, all of which enable substantial CT dose reductions without compromise of diagnostic power. While earlier versions of IR yielded 30% dose reductions, those with model-based IR or some blend thereof can result in 50-80% patient radiation dose reductions – with even better spatial and low contrast resolution. Access the full article on this study.
As this article demonstrates, iterative reconstruction is a very powerful way to reduce dose without impacting diagnostic ability. Key points of the authors include, “To reduce patient and operator radiation dose involves optimization of medical imaging equipment and best control of the equipment by the operator. … The results of our study confirm in a large patient number reflecting the routine clinical setting that the image noise reduction technology allows a significant reduction in radiation dose. … The substantially lower radiation dosage achieved in a routine clinical setting with the image noise reduction technique, provide further evidence of the substantial impact of the new technology. They indicate potential reduction in radiation dosage in invasive and interventional cardiology with more diffusion of newer radiation technology in clinical practice.”
This article pretty well confirms what many have felt: model-based iterative reconstruction (MBIR) lowers radiation dose by 70-80% compared to adaptive statistical iterative reconstruction (ASIR), without loss of diagnostic power/information. While the images do indeed look different because there is much less noise and because of a slightly different pattern in the remaining noise, all the findings are there. Further, the anatomy and the findings are displayed as well or better.
So, in a young patient (under age 45) – especially if they are likely to be getting multiple exams – use of model-based iterative reconstruction is well worth the longer reconstruction time.
(To read more about CT enterography, Radiologyinfo.org is a great resource for patients.)
Paying attention to limiting Z axis coverage yields big dose saving dividends! See this article for results of this study designed to assess the safety and efficacy of radiation dose reduction in hospitals lacking iterative reconstruction.
This article highlights that it is possible to achieve much lower radiation dose CT scans for commonly employed types of CT studies – the CT for urinary tract stones is one of the most common.
While not done everywhere, attention to detail can produce remarkable reductions in patient radiation without compromising diagnostic power.
Use of a lower kVp will actually make stones a bit brighter.
Careful attention to patient centering in the gantry can make a difference of up to 40% in dose.
And the use of iterative reconstruction techniques is now widely accepted to not compromise detection, yet with marked dose reduction – whether it be statistical iterative reconstruction, model based iterative reconstruction, or some blend of the two.
Radiologists and technologists both need to understand the importance of these tricks and the physics behind each.