Author: Kate Yip, Radiation Therapist, Clinical Engagement Manager at Leo Cancer Care

Equine imaging has made real advancements in the last 20 years as software and hardware have improved. Computed tomography or CT, specifically standing computed tomography (sCT) is increasingly the preferred method for imaging equine patients. The equine head anatomically is very complex and CT images are better able to visualize the structures when compared to plain radiographs which typically superimposes structures making diagnosis of fractures, dental disease, infection and neoplasia more difficult.  

Standing CT is also the preferred imaging modality for limbs as it can allow for a more complete non-invasive evaluation of an affected area. It can be argued that it is quicker and more cost effective than other imaging modalities and can support care providers to better diagnose, determine prognosis and plan for necessary interventions.

New research out of the University of Wisconsin-Madison by Veitch et al. (2024) was undertaken with the helical fan-beam CT, Equina, developed by Asto CT which can image either the head and neck or limbs of a sedated horse. This scanner is able to do both as the gantry is able to move both vertically and horizontally. Fan-beam CT compared to cone-beam CT has been shown to provide superior image quality for the evaluation of soft tissues. While cone-beam volumetric imaging can have faster image acquisition it also has increased motion susceptibility which can heavily affect image quality. Another significant advantage is that it provides a lower dose to the patient.

Even with the number of clear clinical and economic advantages sCT hosts, what must also be considered is the need for personnel to remain in the room for the duration of the scan to ensure the safety of the sedated patient and the equipment. While fan-beam sCT decreases the need for repeat scans there is still potential exposure to ionizing radiation to those personnel required to remain in the room. This was the focus of Veitch’s research. They looked into the radiation doses at typical personnel locations during a scan of the head and neck and then during a scan of a limb. Previous research in the area of scatter radiation dose to occupational workers is limited so this research is vital in supporting the use of sCT in equine healthcare.

The production of scatter radiation is affected by tube current. A study by Davies et al (2020) determined that lower tube current could increase worker safety by reducing scatter radiation. This research was able to reduce the tube current from 300 to 150 mAs without compromising on image quality. The Equina sCT unit used during this research uses a fixed kVp and mAs at 160 kv/8 mAs, which is significantly lower than other conventional helical fan beam CT units.

The study placed ionization chambers at three different locations to represent the control operator, horse handler and lead rope handler during a head and neck scan. Followed by the 2 different locations of personnel during thoracic and pelvic limp image acquisition. The measurements were taken multiple times with and without the use of a lead apron (0.5mm lead equivalent thickness) in front of the ionization chamber.

Results

Use of a lead apron and personnel position directly impacted the scattered radiation dose measured.

For a typical head and neck scan lasting 60 seconds wearing a lead apron:

• Control operator dose was 13.3 µGy

• Horse handler dose was 3.5 µGy

• Lead rope handler dose was 6.8 µGy

For a typical limb sCT scan time of 30 seconds and personnel wearing a lead apron,

• Control operator location dose was 1.29 µGy

• Lead rope handler location dose was 0.15 µGy for a simulated pelvic limb

• Lead rope handler location dose was 5.36 µGy for a simulated thoracic limb

Simulated head sCT scans resulted in the highest scatter radiation dose to all personnel locations. These findings are consistent with a study by Chen et al (2019) who found that the highest doses in the room are at the angle of the gantry opening or isocenter. With head and neck scan lengths being typically longer due to the extended field of view.

The higher dose to the lead handler in thoracic limb imaging can be attributed to the closer proximity to the CT gantry.

Recommendations

The acquisition of clearly defined images is so important in equine healthcare for effective decision-making regarding a horse's health. Standing CT is able to provide this but in order to ensure the safety of the patient, workers need to be present. For this to be a viable option, efforts must be made to ensure their safety also. The radiation principle ‘as low as reasonably achievable’ is essential when taking into consideration that some facilities will image as many as 10 horses a day and up to 250 horses a year with personnel present for everyone.

The findings of this research can be used to help shape effective working protocols that support the personnel and handler's health by limiting their exposure to radiation. Key areas include limiting exposure time, maximizing distance from the primary beam and scattered radiation, and using appropriate personal protective equipment and shielding.

• With the Equina system the location of the control operator is not fixed as it has a flexible, tethered hand control and so the operator should increase their distance from the CT gantry as much as possible.

• The lead rope handler should also increase their distance from the gantry opening as much as possible and position themselves to the side of the gantry when working with more compliant patients.

• Have the minimum number of personnel present and rotating the role of control operator and equine handler between staff.

• All personnel are advised to wear radiation safety glasses (0.75-mm lead equivalent thickness), lead gowns and a thyroid shield (0.5mm lead equivalent thickness). The use of radiation safety glasses is supported by the International Commissions on Radiological Protection’s decision to lower the threshold dose of equivalent exposure to the lens of the eye from 150 to 20 mSv in a year and no more than 50 mSv over a 5-year period following increased incidences of radiation-induced cataracts among radiologists and cardiologists.

• The scan should be aborted as soon as it has been determined that the scan will not be diagnostic which can occur with non-compliant patients.

• Reduce tube current where possible without compromising on image quality.

The authors of this research highlighted the disadvantages of other sCT systems which cannot take images in a natural weight-bearing position and instead take scans of individual limbs secured in place with ropes requiring increased risk of repeat scans due to motion artifacts. The Equina system used during this research acquires images with the patient in a weight-bearing position so has a reduced likelihood for needing repeat scan.

Veitch confirmed that if the control operator during a head and neck scan were to wear a lead apron and radiation glasses then that individual would have to scan more than 1500 horses before they exceeded the recommended radiation dose to the eye.

The authors of this research concluded that when using a standing CT system such as the Asto CT Equina system and where personnel are taking every precaution possible to limit their own radiation exposure with the use of personal protective equipment and consideration of the position during the scan, then it is possible to scan a large number of horses without personnel exceeding the recommended safe threshold for occupational exposure.

References

Veitch, K. E., Szczykutowicz, T. P., Brounts, S. H., Ergun, D. L., Muir, P., & Loeber, S. J. (2024). Radiation exposure during simulated equine head and limb fan beam standing computed tomography appears safe for personnel using lead shielding. Journal of the American Veterinary Medical Association (published online ahead of print 2024). Retrieved Nov 19, 2024, from https://doi.org/10.2460/javma.24.06.0424

Davies T, Skelly C, Puggioni A, D’Helft C, Connolly S, Hoey S. Standing CT of the equine head: reducing radiation dose maintains image quality. Vet Radiol Ultrasound. 2020;61(2):137-146. doi:10.1111/vru.12823

Chen KS, Chou YH, Wu RS, Lee WM, Tyan YS, Chen TR. Radiation dose distribution of a plain radiography room and computed tomography room in a veterinary hospital. Radiat Prot Dosimetry. 2019;187(2):243-248. doi:10.1093/rpd/ncz158