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Using image reconstruction techniques in computed tomography

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The following case was submitted by a participant for Radiographer of the year 2017 contest sharing his best practices for radiation dose management.

Describe your focus of study

As it stands pulmonary angiography conducted in computed tomography, is often done using a dual energy scanner. This results in a data set known as a 'mixed energy data set’.

The attenuation of the pulmonary trunk in a pulmonary angiography is a key factor in its diagnostic value, the higher the attenuation, the better quality the scan. The attenuation is due to the use of iodinated contrast.

The K-shell binding energy for the absorption of X-rays, of iodine, is 33.2 keV, recent advances in image reconstruction technology have to lead to the ability to reconstruct images as a monoenergetic data set using the already completed scan, in this instance, a data set closer to the K-shell binding energy of idone.

By using this form of image reconstruction, iodinated structures will have a higher attenuation, a factor that is critical to a diagnostic pulmonary angiogram.

The project aimed to formalize the benefit in using monoenergetic image reconstruction as part of a general post-processing work up of a dual energy pulmonary angiogram, at no expense of radiation dose to the patient.

Indicate the purpose and objective of the case

Retrospectively measure the use of monoenergetic reconstruction of dual-energy computed tomographic pulmonary angiography and document the increase in attenuation of the pulmonary artery when compared to the conventional data set, using approximately 160 patient scans.

Develop a protocol that will save repeated computed tomographic pulmonary angiography based on the low attenuation of the pulmonary arteries.

Using this image reconstruction
technique as standard can potentially save dose down the line with CT scans being repeated less and less”.

Provide brief clinical and patient background

The results of this study formalized the use of monoenergetic reconstruction in CTPA's using dual energy CT as a viable tool to avoid repeated scans based on the 'rule of attenuation' in the pulmonary trunk.

Describe the dose management methods/techniques used

The purpose of this study is to use image reconstruction techniques in computed tomography to harness the monoenergetic data set as close to the K-edge of iodine as possible, in doing this the pulmonary trunk will have a higher attenuation-avoiding those repeated scans that patients receive due to 'sub optimal' enhancement.

Explain the results and conclusions reached

After running a permutation test using a custom script we found that the real difference between the mixed data set and the monoenergetic data set was an average of 833 HU.

When looking at the random differences generated from the permutation test, it can be seen that the differences are in the range of [-300,300]. Meaning, the real difference (833) is significantly higher than the random differences.

Taking this into consideration, this retrospective analysis of a mixed data set compared to a monoenergetic data set of the same patient can help improve the attenuation of the CTPA, using this image reconstruction technique as standard can potentially save dose down the line with CT scans being repeated less and less.

Discuss the case outcome(s), future implications, etc.

This difference in attenuation that has been observed, suggests that any dual-energy computed tomographic pulmonary angiogram protocol should include monoenergetic reconstruction as a standard element to the dual energy scan.

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