Discussion
In this study, we found a significant increase in the mean value of CE in HU of the DDFT in all scans. Specifically, 150 mg I/mL of CM concentration and 2 mL/s of rate in conjunction with 80 kV yielded significantly higher CE of the DDFT than other conditions, corresponding to the fact that iodine makes higher CE at low voltage by its nature (Bae, 2010). In horses, voltages and CM concentrations for intra-arterial CECT in distal limbs were reportedly 120 or 140 kV and over 150 mg I/mL, respectively (Puchalski et al. , 2009 van Hamelet al. , 2014, Puchalski et al ., 2007, Vallance et al ., 2012a and b, Pauwels et al. , 2021). We scanned intra-arterial CECT using the same or lower voltage and concentration of CM, resulting in the CE of the DDFT not being significantly different from each other except for the scans using 80 kV and 150 mg I/mL. Considering these results, we suggest that 120 mg I/mL of 90 mg I/mL of CM might be applied for intra-arterial CECT of the distal limbs to yield CE of the DDFT in horses.
In the first phase, CE of the DDFT was positively correlated with CM concentration. However, in fixed IDR and total iodine dose (TID), volume and rate of CM negatively correlated with CE of the DDFT, indicating high CM concentration and low volume and rate efficiently yield CE of the DDFT in fixed IDR. Previously, Benrendt et al. reported that the volume and rate of CM administered intravenously did not affect a significant change in CE in abdominal soft tissues in fixed IDR (Behrendt et al., 2008). Additionally, in experimental models using a constant CM concentration with a fixed injection duration, hepatic enhancement proportionally increased with an elevation of rate ≤ 2 mL/s (Bae et al., 1998), demonstrating that an increase in the velocity of injection over 2 mL/s slightly increased the CE of the liver. Although we administered CM intra-arterially in this study, the CE of the DDFT revealed a negative correlation with the rate and volume of CM at a fixed IDR. We assumed that it could be caused by the administration route and CM concentration; or because CE of tendons mainly occur through extravasation or neo-vasculization. Therefore, in fixed IDR or TID, we suggest that CE of the DDFT could efficiently be achieved at a high CM concentration with a slow rate (< 4 mL/s).
CM volume reportedly influences the CE of vessels (Ahmed et al. , 2009, Van Cauteren et al., 2018). Van Cauteren et al.reported that the combination of high volume and low CM concentration yielded significantly higher HU with constant IDR in intravenous CECT (Van Cauteren et al,. 2018). Additionally, in intra-arterial CECT of canine models, CM volume was the most influential scan parameter for contrast enhancement (Ahmed et al. 2009). In this study, with 180 mg I/mL of IDR, the combination of 30 mg I/mL of concentration and 150 mL of large volume resulted in a significant decrease in CE of the arteries. The mean HU of arteries showed no decline in the condition of 45 mg I/mL of CM concentration with 100 mL of volume. Considering these results, large volumes might not result in higher CE of the arteries when CM concentration is < 45 mg I/mL in intra-arterial CECT of equine distal limbs.
Volume and rate positively correlated with the CE of arteries; conversely, CM concentration negatively correlated with the CE of arteries during maintaining 300 mg I/s of IDR, coordinating with a previous study (Van Cauteren et al., 2018). However, with 180 mg I/s of IDR, all parameters showed negative correlations with the CE of arteries. We presumed that 30 mg I/mL of low CM concentration yielded significantly low CE, causing the negative correlations. Van Cauterenet al. reported that in 0.64 g/s of constant IDR, CM concentration negatively correlated with CE of the aorta, central vena cava, and hepatic parenchyma in intra-venous CECT (Van Cauteren et al,. 2018). Our results also suggested that low CM concentration in conjunction with volume or rate adjustment with a certain fixed IDR could generate adequate CE in the arteries in the equine distal limbs
Concerning CM administration, the max pressure is reportedly below 150 psi (Pollard and Puchalski, 2011, Indrajit et al. , 2015). Previously, under constant IDR and TID, the rate had a greater impact on maximal pressure than viscosity (Van Cauteren et al. , 2018), indicating that in 180 mg I/s of IDR, the pressure positively correlated with the rate in this report. Pollard and Puchalski reported that for intra-arterial CECT of the equine distal limbs, the rate of CM injection is usually set at 2 mL/s using an 18-gauge catheter to minimize hemodynamic changes (Pollard and Puchalski, 2011). In this study, CM was administered at 2, 4, and 6 mL/s of rate. In a horse during the scan with 6 mL/s of rate and 300 mg I/s of IDR, the maximal pressure was 118 psi, causing no failure of the CE of the scan. However, during the following scan, an ultrasound revealed the damaged arteries; the horse showed no lameness or swelling in the region. Although 118 psi of the maximal pressure contributed to outcome, we considered that repeated catheterization, the patient’s physiologic status, the integrity of the vascular wall, or vascular spasm could also have been potential causes.
In neck CT using phantom models, 80 kV of CT resulted in a decrease in radiation dose while maintaining the subjective image quality (Hoanget al. , 2012). Additionally, Gnannt et al. reported that 70 kV of CT showed no significantly different image quality of soft tissue in a cervical scan compared to that of a 120 kV scan (Gnanntet al. , 2012). Despite not evaluating the image quality of 80 kV and 120 kV CT scans, CTDIvol in 80 kV scans was lower than that of 120 kV scans. In equine imaging, standing head CT scans are widely used, and little research on standing distal limb CT has been reported (Brounts et al. , 2022, Mageed 2020, Mathee et al. , 2023). Considering the future of CT scans without general anesthesia, human intervention during CT scans is inevitable, raising concerns against radiation doses. Therefore, further research on radiation dose reduction is required while maintaining diagnostic image quality in horses.
This study had several limitations. First, the number of animals was small for parametric statistical analysis. Second, the animals had no lesions in the distal limbs; thus, further research on CECT in horses with lesions is required. Third, we only conducted a CT scan of Jeju horses, which may have influenced CE. However, considering that we injected CM via intra-arterial catheterization, body size hardly affected CE. Fourth, we scanned the right limb only five times instead of the left due to the stationary facility and compromised left medial palmar artery.
We demonstrated contrast attenuation of the DDFT and palmar digital artery in the equine forelimb, depending on the scanning parameters, concentration, rate, and volume of CM administration. We found that the contrast attenuation of DDFT was significantly higher at low voltage (80 kV) and high CM concentrations (150 mg I/s); > 900 HU contrast attenuation of the artery was statistically lower at a low IDR (180 mg I/s) with a high CM rate (6 mL/s). Most horses did not show complications associated with intra-arterial injection following CT scans with a ≤ 4 mL/s rate. This study provides practical information on the scan parameters and CM injection in equine intra-arterial CECT, the evaluation of equine distal limbs, and the anatomical features of CECT in Jeju horses.