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.