Low dose CT for imaging of the extremities: How low is “low”?

By Linde Pollet¹, Sarah Loeys², Niki Fitousi³, Wayne Warfield⁴, Steve Nzitunga⁵, Jurgen Jacobs⁶

Conventional radiography has always been the standard imaging method to demonstrate fractures of the extremities. However, subtle and complex fractures are often a challenging diagnostic problem in daily clinical practice, which makes the correct diagnosis, and consequently the proper treatment, a difficult task. As reported in literature, fractures continue to be the most common type of missed injuries, with some of the reasons being mal-positioning, insufficient views, or commonly missed locations [1, 2]. In some cases, an obvious alternative would be CT imaging, which contributes much to the diagnosis of fractures by depicting subtle fracture lines and depressed or distracted articular surfaces. Additionally, it offers 3D reconstructed images and better spatial resolution [3] than plain radiography. On the other hand, one could argue that the radiation dose is much higher, making it harder to justify the selection of this imaging method. Recent studies have indicated that for this imaging task, CT can be used with much lower doses [4]. So, when using low dose CT for extremity imaging, the question is: how low can it go?

In a study recently performed at UZ Leuven in Belgium, the radiation dose of extremity imaging using a low dose CT protocol was evaluated against conventional radiography. The study comprised three parts:

In the first part of the study, a retrospective analysis of extremity trauma cases that had used conventional x-ray imaging was performed. All the relevant dosimetric data were acquired using the radiation dose management system, DOSE (Qaelum NV), that is installed at the hospital. Overall, 2189 examinations, performed in a period of one and a half years were considered; specifically, 602 hand examinations, 88 wrist examinations, 1326 foot examinations and 173 ankle examinations. The average effective dose per body part was 0.36 µSv, 0.20 µSv, 0.24 µSv and 0.54 µSv respectively. This was an overall evaluation of all the extremity examinations showing a large variation in technique and number of images, probably due to the complexity and difficulty of this imaging task.

In the second part of the study, experimental dose measurements using TLDs on a cadaver were performed for both conventional x-ray imaging and CT. For conventional radiography, the main protocol for each body part was applied on the cadaver exposures, in contrast with the variability of the patient dosimetry performed in the first part of the study. The average effective dose measured with the TLDs was 0.50 µSv, 0.55 µSv, 0.70 µSv and 2.05 µSv for the four body parts respectively.

For CT, 16 new low dose protocols were created on a Toshiba Aquilion One Vision scanner, starting from the trauma protocol, in order to identify the protocol showing ‘best performance’ (i.e. diagnostic images with the lowest possible dose). Three parameters were altered for each new protocol; tube voltage (80 kVp to 135 kVp), tube current (10 mA and 50 mA) and rotation time (0,275 s and 0,5 s). At this point, the study focussed solely on ankle examinations. The results are listed in table 1.

In the third and final part, a clinical image quality assessment was performed for the ankle CT images of the cadaver. The goal was to identify the CT protocol showing ‘best performance’ among all of the low-dose protocols that were evaluated. Two radiologists from the department scored the images based on the ability to assess fractures. The criteria were the following:

• Sharpness of contour of each bone structure
• Sharpness and noise of trabeculated bone tissue internally in the bone
• Outline of fat plans between the muscles and around the tendons
• Mineralization in the soft tissue

The scoring was performed on a 5-level scale with a score of at least 3 in each criterion being deemed as acceptable image quality.

From analysis of the aforementioned data (dose values and image quality assessment), it was found that the effective dose of the best performing CT protocol for the ankle was 4 µSv (100 kVp, 10 mA, 0.5 s). This value is approximately 5 times lower than that reported in the literature for a ‘standard’ Multi Slice CT protocol for the same body part [4]. Conventional radiography of the ankle, as described in the second part of the study, gives an approximate effective dose of 2.05 µSv, which is almost 2 times lower than that from the equivalent CT examination.

The study concluded that there can be a shift from conventional radiography to low dose CT for extremity imaging, especially when it comes to occult, subtle and complex fractures. The slightly higher dose can be justified by taking into account the diagnostic advantages and the faster, more appropriate treatment of the patient.

Having an advanced radiation dose management system provides you with all the necessary tools to monitor not only the radiation dose but also the image quality of each protocol. In this way, the user can easily perform optimization tasks and evaluate the feasibility of changing a protocol or imaging methods. This allows the provision of the best care to patients with a constant focus on quality.

For more information on our monitoring and management tools, please visit our website: https://qaelum.com

References

1. S. Ha, J. A. Porrino, F. S. Chew. “Radiographic Pitfalls in Lower Extremity Trauma”, AJR, 203; 492-500, 2014.
2. Huang, C. Chang, B. Thomas, P. MacMahon, en W. Palmer, “Using cone-beam CT as a low-dose 3D imaging technique for the extremities: initial experience in 50 subjects”, Skeletal Radiol., vol. 44, nr. 6, pp. 797–809, 2015, doi: 10.1007/s00256-015-2105-9.
3. M. Osgood e.a., “Image quality of cone beam computed tomography for evaluation of extremity fractures in the presence of metal hardware: visual grading characteristics analysis”, Br. J. Radiol., vol. 90, nr. 1073, p. 20160539, mei 2017, doi: 10.1259/bjr.20160539.
4. W. Yi e.a., “Radiation dose reduction in multidetector CT in fracture evaluation”, Br. J. Radiol., vol. 90, nr. 1077, p. 20170240, jul. 2017, doi: 10.1259/bjr.20170240.
5. Koivisto, T. Kiljunen, N. Kadesjö, X.-Q. Shi, en J. Wolff, “Effective radiation dose of a MSCT, two CBCT and one conventional radiography device in the ankle region”, J. Foot Ankle Res., vol. 8, nr. 1, p. 8, mrt. 2015, doi: 10.1186/s13047-015-0067-8.

Disclosure statement

1. Linde Pollet: currently studying for a Master’s Degree in industrial engineering in nuclear technology at UHasselt, Belgium.
2. Sarah Loeys: works as a Radiographer in AZ Damiaan in Ostend, Belgium and studied Medical Imaging in Odisee in Brussels.
3. Niki Fitousi, Ph.D: a certified Medical Physicist from Greece, currently working as Head of Research at Qaelum, focusing mostly in the fields of dosimetry, image quality and optimization in medical imaging.  
4. Wayne Warfield DCR (R): a Diagnostic Radiographer, currently working for Qaelum as a Business Development Manager in the UK and Ireland.
5. Steve Nzitunga: worked as a Radiographer for 5 years in AZ Monica in General radiology, CT, MRI and Cone beam CT for ankle and feet (pedCAT) currently working as Application Specialist with Qaelum.
6. Jurgen Jacobs: Co-founder and CEO, Qaelum